Firewalls and Token IDs

Firewalls - Virus Protection
A firewall is a device or set of devices at carriers, enterprises, and homes that screens incoming and internal traffic to prevent hackers' access to files. Firewalls are designed to keep out hackers by allowing only designated users to access networks. In organizations' networks, firewall software is installed on routers and on remote access switches called VPN gateways. Organizations that use carriers' firewall protection have onsite firewall protection as well.
Firewalls use various techniques including address filtering, which looks at a user's IP address and accepts or rejects messages based on the IP address. Important applications might contain their own firewalls for extra protection. Firewalls can also restrict communications to certain addresses. New firewalls can also filter by port. In addition, they can be programmed to recognize applications and content. Acting as an agent for and screening traffic for applications is referred to intermediation or proxy-type functions.

Because employees use their laptops at home to surf the Web and then bring them into work, corporations monitor internal transmissions as well as communications from the Internet. The goal is to avoid contamination from these laptops.

Firewalls do not protect against viruses and other threats. Corporations often subscribe to security services that keep them posted about new software attacks, monitor their networks for unusual amounts or types of traffic, and download protection against new types of attacks.

Token ID Security - Identify Verification
Token identification, which adds an additional layer of user authentication in addition to passwords, is used in most remote access services. Tokens are small devices that generate new six- to eight-digit numbers every 60 seconds. When prompted, users type in the token-generated number. These numbers are generated by a combination of factory set matching numbers in the user's device and a central server combined with the time. To be authenticated, the number the user types in must match that generated by the central computer. RSA is the leading supplier of token IDs. If a person's password is stolen, the hacker will not be able to access the network unless he or she has the token as well as the password.

SSL VPNs

The attraction of SSL VPN service is that the service works from within standard browsers on laptops, desk computers, and personal digital assistants. This makes VPNs easier to use, with less administrative support required from IT staff. The business or commercial enterprise is not required to supply special software to each laptop computer used from remote access. The simplified login results in fewer user login errors.

SSL is a newer technology used for VPNs, however, simplified access and improvements in SSL are expected to spur growth. Employees using SSL-type security can only access applications such as email supported by the SSL gear. Software in the SSL appliances is adapted to enable access to particular applications. An appliance is a specialized computer dedicated to a particular task. These appliances prompt users for their names, passwords, and (if used) token number. They apply encryption and a secure tunnel and allow or deny access to enterprise applications. They have other features such as scanning users' PCs and automatically downloading software patches to computers that do not have the latest security corrections loaded.

Because SSL is a higher-level security protocol, it has the benefit of allowing or denying access to particular applications based on privileges granted to classes of employees. As it is possible for remote computers to pass viruses to corporate networks, some SSL appliances have the capability to scan remote computers for antivirus software and operating systems with the latest security patches. Other appliances have the capability to wipe out passwords and corporate data from computers used for remote access. This eliminates the possibility of computers in public areas, such as kiosks, storing and passing on private information and passwords.

IPSec VPNs

Public Internet-Based VPNs for Intersite Connections
Some organizations save money by using the public Internet for VPN service rather than MPLS (Multi-Protocol Label Switching) VPN or frame relay service. Companies using the public Internet mix intracompany and public Internet traffic on the same access lines. They provide their own security, usually IPSec as described below, as well as firewalls and antivirus software. Alternatively, they contract with their carrier to manage their security devices, which are onsite or at the carriers' POPs.

While the public Internet does not guarantee speeds, companies are finding that providing a high-speed access line gives them adequate site-to-site service at a lower price than frame relay and MPLS VPNs. This is because many Internet backbone providers overbuilt their networks, expecting a larger increase in traffic than occurred. Moreover, the costs for T-1 and T-3 have been decreasing, making them affordable for many more organizations.

Network-Based IPSec VPNs- Over Carriers' Private IP Networks
These IPSec VPN-based services operate over carriers' private IP networks instead of the public Internet. The carrier provides security in its network. It encapsulates (creates tunnels around) packets routed between its points of presence (POPs).

** Both of these IPSec VPN-type offerings don't offer the classes of service for voice and video. In addition, they do not provide service level agreements with statistics on traffic levels and network reliability. Customers are responsible for monitoring traffic flows through their own routers.

IPSec VPNs for Remote Access
To support VPN remote access, IT staff distribute client software to each person's computer or laptop. Users click on the client software, which is a special program that contains IPSec security, to launch remote access. It can be used with dial-up or broadband access. A shortcoming is that employees can only access their e-mail when they have their computers with the client software with them. This service does not work at public computers such as those at airports or Internet cafes.

IPSec establishes a secure connection between the corporate local area network and the remote user by scrambling and tunneling the bits and hiding the IP header in each packet. This ensures privacy. Tunneling prevents hackers from learning corporate LAN IP addresses. To stop remote users from passing viruses from the Internet to corporate networks, the client software will often not function if there is an open connection to the Internet while the user is logged in remotely.

Multiprotocol Label Switching (MPLS) VPNs - Everyone-to-Everyone Links

When customers sign up for MPLS VPN service they give their provider a list of the Internet protocol (IP) addresses associated with each site they want included in the VPN. The carrier uses this list to define a closed group of users allowed to communicate with each other using the VPN service.

Classes of Service - To Prioritize Particular Traffic
The customer chooses from a list of four or five classes of service. These classes of service are used to define the priority given to traffic for each class. For example, there may be two or three classes for data, one for voice, and another (the most expensive) for video. Voice and video have higher priorities than data. Some organizations use the lowest priced class of service for most data and higher-priced classes of service for database lookups. Often customers choose MPLS for its capability to treat voice differently than data. They initially use the network exclusively for data but plan to add voice traffic at a later time. Examples of voice traffic include:
Worldwide voice mail functionality such as broadcasting lists made up fo staff at diverse sites

Audio conferences
Sending call center traffic to remote sites based on time-of-day or staffing levels
Transmitting voice calls between international and domestic sites
Electronic Tags on MPLS Packets

MPLS attaches electronic tags to packets. Routers read the tags and assign levels of priority. The tags also enable routers to forward packets more quickly because they don't have to look up addresses in tables for each packet.

Most carriers offer service level agreements (SLAs) for an additional fee in conjunction with MPLS VPNs. These agreements offer guarantees on issues like the following:

Uptime, the percentage of time that the service operates
Latency, the amount of delay in milliseconds between when packets are sent and when they are received. This is important for voice and video
Restoral time per failure
Packet loss
Access line (the line from the customer to the carrier) uptime
Carriers that do not meet these SLAs generally give agreed-upon credits to customers.
Service Components

Customers that order MPLS use access lines between their network and the carrier. These lines are typically T-1, 1.54 Mbps or less. Most customers have a separate access line for MPLS traffic and a different line for their Internet traffic. They feel their MPLS traffic is from trusted sources at branches. The public Internet traffic requires higher levels of security.

They also specify the following:

A port speed at the provider's point of presence, often at a lower speed than their access line, perhaps 1 Mb

A committed access rate (CAR) - also referred to as committed data rate (CDR), and committed information rate (CIR). The bandwidth charge is the fee charged by many carriers for guaranteeing a particular speed between the carrier's edge and the carrier's high-speed core network. Some carriers charge a higher rate for international traffic. These speeds vary from 64 kilobits to 44 megabits (T-3)

Access charge for the circuit connecting the customer to the provider's network
They can "burst" send data at up to the speed of the port and access line they lease
Service level agreements
Classes of service; classes with a lower priority cost less than those with a higher priority (see above)

Managed Service - Provider Monitor Onsite Routers
Customers have the option of managing their own router or paying their provider to manage it. Carrier management of the router is referred to as managed service. With managed service, carriers monitor the router 24 hours per day, 7 days a week for service disruptions, denial of service attacks indicated by unusual traffic levels, and viruses. For medium-size companies, it may cost less to depend on a pool of specially trained provider technicians than to train and hire their own technical staff for these functions. As part of the service, carriers provide activity reports that track the level of traffic so that customers can ensure there is adequate capacity.

MPLS Advantages for Carriers - Revenue Sources and Administrative Efficiency
Carriers are eager to migrate customers to MPLS to save money on administration and as a platform for new services. Administratively, carriers have the capability to add classes of service for higher-priced voice and videoconferencing. These changes can be made in real time by programming requested modifications. Making changes to frame relay service is more complex because each path between sites must be programmed separately. Other potential sources of revenue for carriers are hosting, and access to hosted data storage (backup storage of customer files on the network).

However, carriers still have investments in asynchronous transfer mode (ATM) network infrastructure that is not fully depreciated. The transition to MPLS as the single network will take place gradually.

VPN Technology

Improvements in routing and security protocols and increased capacity in the Internet led to the capability of IP networks to differentiate different types of corporate traffic and to improvements in secure remote access. The following are newer VPN services carried on IP networks:

VPNs for Site-to-Site Communications Within Organizations:
• Multiprotocol label switching (MPLS) VPNs provide any site-to-any site connectivity. This is referred to as meshed service. MPLS service is more flexible than frame relay to configure and is more suitable for intersite voice traffic. MPLS VPN traffic is carried separately from public Internet traffic to guarantee levels of service.

• IP-VPNs are for site-to-site data communications using the public Internet and mixing Internet traffic with site-to-site email and other applications with Internet protocol security (IPSec). IPSec creates a tunnel for each packet. The tunnel hides the destination IP address by surrounding it with a different address. IPSec also scrambles data by encrypting it.


Secure access on VPNs for Remote Access:
• Internet protocol security (IPSec) requires client software on computers. The IPSec protocol establishes a secure, encrypted link to a security device at the carrier or enterprise. This is referred to as tunneling.

• Secure socket layer (SSL) security is a newer VPN access method. Access is embedded in browsers so that organizations are not require to install special client software in each user's computer.

IP VPN and MPLS offerings enable carriers to migrate traffic to their existing IP networks rather than older networks designed to carry frame relay traffic.

VPN's for Remote Access

In many organizations, employees assume that they will have the tools to be as productive (or more productive) away from the office as in the office. Organizations frequently supply salespeople and other remote workers with laptop computers that enable them to work offsite. Employees remotely access e-mail messages, place orders, check order status, and check inventory levels from the road and from home computers (I know this is probably shocking news to you all!). With the growth of Voice over IP, some employees also receive phone calls directed to their office extensions on their laptops or PDAs.

Without a VPN, employees dial into remote access equipment consisting of modem banks at corporate headquarters to access e-mail or other applications using toll-free numbers billed to the corporation. Organization rack up thousands of dollars in toll-free charges. In addition, calls are frequently dropped and speeds are slow. Moreover, these dial-in remote access arrangements do not support cable or DSL modems.

VPNs provide staff at remote offices or home offices to gain access to the corporate Intranet in the same manner they would if they were locally connected to files. Distributing VPNs to home, telecommuters, and small offices may put access to sensitive information in facilities not as well protected as more traditional facilities. VPNs need to be designed and operated under well-thought-out security policies. Organizations using them must have clear security rules supported by top management. When access goes beyond traditional office facilities, where there may be no professional administrators, security must be maintained as transparently as possible to end users.

Some organizations with especially sensitive data, such as health care companies, even arrange for an employee's home to have two separate WAN connections: one for working on that employer's sensitive data and one for all other uses. More common is that bringing up the secure VPN cuts off Internet connectivity for any use except secure communications into the enterprise; Internet access is still possible but will go through enterprise access rather than that of the local user.

Rationale for VPN's

Organizations use VPNs to save money on renting and managing private lines between sites. Dedicated private lines are circuits used only by the organization that leases them monthly (more on dedicated private lines in the future). In contrast, virtual private networks use shared circuits (electronic paths between points) within carriers' networks. Carriers benefit by not having to dedicate as much infrastructure to single customers.

In addition, VPN installation intervals, the time between ordering service and implementations, are shorter than for new private lines, which take weeks to engineer. Thus, customers with existing virtual private networks can quickly add locations. The biggest delays revolve around new access lines, if they are not already in place between the customer and the VPN provider. If links to the carrier are in place, sites can be added in a matter of days using spare capacity on these links.

VPNs enable businesses to avoid administering growth and day-to-day maintenance of private networks. Adding capacity to a virtual private network is simpler than adding higher-speed, dedicated lines and new hardware to each site of a private network. The customer only needs higher-speed access lines from its building to the carrier's network. The carrier is responsible for making sure there is capacity in the network for the customer's applications.

Large organizations often have a mix of private lines for routes with the highest amount of voice and data traffic and VPN services for routes with less voice and data traffic.

In summary, VPN benefits include:
• Shared facilities may be cheaper - especially in CAPEX (Capital Expenditure) - than traditional routed networks over dedicated facilities
• VPNs can rapidly link enterprise offices, as well as small-and-home-office and mobile workers (more later this week)
• Can scale to meet sudden demands, especially when provider-provisioned on shared infrastructure
• Can reduce opex (Operational Expenditure) by outsourcing support and facilities

A virtual private network (VPN)

Any arrangement that provides connections between offices, remote workers, and the Internet without requiring dedicated lines, also referred to as private networks between sites. The term "virtual" refers to the fact that these VPNs provide the features of private lines; they are virtually private.

An alternate definition: A VPN is a communications network tunneled through another network, and dedicated for a specific network. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

Digital Network Services Overview

As stated last week, we will now be shifting our attention towards networks and network services. As an introduction, the table below provides an overview of digital network services for your reference:

Storage Area Networks

As more business processes are computerized, enterprises are storing exact copies of all of their data at remote data centers and storage areas networks (SANs). A SAN is a network designed for backup and disk mirroring of large databases. These networks include those used for vital functions such as inventory, accounts receivable, order entry and accounts payable. Disk mirroring is the process of simultaneously writing data to backup and primary servers. SANs provide data restoral in the event of disasters or computer failures. They are located at the same site as the primary servers or at other locations in the metro area or across the country.

A data center is a centralized location for corporate data with special environmental controls such as air conditioning, fire alarms, and duplicate power sources. It also has special security provisions regarding who is allowed to enter the center. Data centers can support multiple enterprise sites. Some organizations hire outside organizations to manage their data center either at their site or at a remote site.

Enormous speed is required to transport the massive amounts of data between corporate sites and SANs and/or data centers. In some cases, data centers act as backups to each other. Fibre channel is a standard for high-speed connections at between 133 megabits per second and 4 gigabits per second between servers and storage devices. Fibre channel is used as a local physical channel within the data center. Because there are many devices in a SAN, SANs use either hubs or switches to distribute data to devices. Hubs provide a common path between devices, and switches establish dedicated paths. Fibre channel data is transmitted directly to devices' input/output interfaces. Fibre channel operates over most network protocols.

Because of the speed and reliability requirements, organizations typically use Ethernet VPNs, SONET, or leased fiber with an individual wavelength. Individual wavelengths have distance limitations of hundreds of kilometers but are less costly than SONET. Wavelengths are sent to data centers, customers, or SANs at speeds of 1.5 gigabits, 2.5 gigabits or 10 gigabits per second over one pair of fiber.

SONET Offerings for Enterprises

Local telephone companies sell SONET (Synchronous Optical Networking) transport for connections between local customers and interexchange carriers. The speeds offered are at OC-3 (155 megabit), OC-12 (622 megabit), and OC-48 (2.5 gigabit) rates. The local telephone companies guarantee 50 millisecond network restoration in the case of a network failure or degradation. They run the SONET service to multiple local central offices. In the case of a failure at one CO (Central Office), service is immediately available from the backup CO. Matching SONET multiplexers are required at the customer premises and at the telephone company office. Another variation of SONET service protects customers from fiber cuts. This diverse routing scheme offers fiber from separate building entrances to the same CO.

Customers often opt for point-to-point SONET rather than bidirectional rings to save money. The major impediment on sales of these services is the cost to dig trenches for additional fiber runs from the customer to the incumbent carrier's fiber ring. Because it is lower in price, newer Ethernet services at gigabit or lower speeds ranging from 10 megabits to 500 megabits are gaining in popularity for data communications. However, customers with existing SONET service have the option to add Ethernet data that runs at 10, 50 or 100 megabits per second. This uses spare capacity on the SONET multiplexer for perhaps LAN-to-LAN connections in metro areas.

Third generation SONET

Connectivity to Ethernet: Transporting IP voice and Ethernet traffic on SONET-equipped links wastes capacity on carriers' networks. This is because SONET carries traffic in "chunks" at 64 kilobits per second in fixed-size frames called cells. However, IP and Ethernet traffic bits are in variable-size packets. In addition, SONET cells have high overhead (nonuser data such as monitoring and addressing), which adds to its inefficiency because less customer traffic is carried in each cell. This mismatch between frame size results in carriers stuffing zeros into many SONET frames.

Some manufacturers have developed SONET equipment that handles packet traffic more efficiently. For example, newer multiplexers have Gigabit Ethernet ports and ports that can interface directly with telecommunications services used in storage area networks. These SONET multiplexers have the capability to pick up and drop off Ethernet and IP traffic more efficiently at Ethernet speeds. However, they transport traffic to older SONET devices in SONET frames, which wastes capacity.

Second generation SONET

Second generation SONET also referred to as multiservice platforms, achieved higher speeds (up to OC-192 [Optical Carrier-192], 10 gigabits), took up less space by supporting more ports on each card, and gave carriers the capability to increase and decrease speeds remotely without taking the ring out of service. They also enabled carriers to drop off lower optical carrier streams to customers for enterprise SONET (Synchronous Optical Networking) services such as Ethernet and storage area network services. However, next-generation multiplexers do not interface directly to MPLS (Multiprotocol Label Switching) networks. In addition, although they carry Ethernet and storage area network services, they do so inefficiently, in SONET frames. Next-generation SONET devices can have internal add and drop multiplexers and digital cross-connect systems.

ADD and Drop Multiplexers (ADM)
Add and drop multiplexers add and drop channels from fiber rings at the edge of the network. They drop off and pick up channels to a particular central office or to a small metropolitan area from rings that connect the core to the access network. Add and drop multiplexers are less complex and handle fewer streams of traffic than digital cross connects.

Digital Cross Connects
Digital cross connects rearrange channels of traffic between multiple routes. A digital cross connect system has the same functionality as a switch. For example, multiple rings may connect at a carrier's point of presence (POP) in the core network in the northeast. The digital cross connect sends some of the traffic to, for example, New York, some to Pennsylvania, and the rest to New Jersey. It also accepts traffic from these states and connects it to other routes. The newest digital cross connects are all-optical. They switch colors (channels) of traffic without converting light signals carried on fiber to electrical signals and electrical signals back to light. This eliminates the need for conversation equipment in these devices, which leads to lower prices and higher-speed switching.

SONET Rings

SONET can run as a straight point-to-point line between sites, or in a ring topology. When fiber in a point-to-point arrangement is cut, service is lost. However, the higher speeds attainable on fiber make reliability critical. When a medium such as copper carries a conversation from one telephone subscriber, a copper cut only impacts one customer. Fiber cuts in networks can put hundreds of locations out of service. For this reason, the majority of telephone companies deploy bidirectional ring topology.

In the bidirectional SONET/SDH ring, one set of fiber strands is used for sending and receiving; the other is the protect ring (spare ring). If one set of fiber strands is broken, the spare (protect) ring reroutes traffic in the other direction. In addition, if one multiplexer on one set of fibers fails, the backup multiplexer on the fiber running in the other direction automatically takes over.

SONET continued

SONET was developed to aggregate (multiplex) and carry circuit switched traffic such as T-1, E-1, T-3 and E-3 as well as slower rates from multiple sources on fiber-optic networks. SONET transports traffic at high speeds called OC (optical carrier). The international version of SONET is synchronous digital hierarchy (SDH). SDH carries traffic at synchronous transport mode (STM) rates. See the table below for optical carrier and synchronous transport mode speeds. Interfaces in the equipment make SONET and SDH speeds compatible with each other. The same SONET equipment can be used for both OC and SDH speeds.

Europe's time division hierarchy is based on E1 (2-megabit) and E3 (34-megabit) signals. E1 circuits carry 30 channels at 64 kilobits per channel. E3 circuits carry 512 channels at 64 kilobits per channel. Traffic that is carried between cities in Europe or in undersea cables is often referred to as being carried at STM-1 or STM-16 rates.

SONET (Synchronous Optical Network):

First introduced in 1994, SONET is a North American standard for multiplexing slower streams of traffic onto fiber-optic cabling and transporting it at optical carrier (OC) speeds. The international standard for the same functions is synchronous digital hierarchy (SDH). SONET/SDH was a major innovation in enabling carriers to carry enormous amounts of voice and data traffic reliably on fiber networks. As SONET equipment prices dropped, large enterprises adopted it as well.

SONET equipment transports high-speed traffic on fiber-optic network between the following:
- Central offices in metropolitan areas (the metropolitan core)
- Remote terminals (digital loop carriers) in metropolitan networks (metropolitan access networks) and central offices
- Long-haul backbone networks and metropolitan areas
- Points of presence (POPs) in long-haul, core networks
- Enterprises and data centers where backup data is stored
- Enterprises and points of presence (POPs) that carry their long distance traffic
- Enterprises to separate central offices for redundancy in case of a central office failure or a fiber cut

SONET also can carry ATM and IP traffic and television signals. However, as increasing amounts of traffic is data rather than voice and more of the data and a growing percentage of the traffic is IP based, lower-priced gear is becoming available to transport IP traffic more efficiently and at lower costs on redundant fiber rings. These rings found in MPLS (Mulitprotocol Label Switching) networks and some metro-area networks are based on dense wavelength division multiplexing.

ATM Characteristics

When ATM was developed in the early 1990s, its speed provided a key advantage over T-1 and T-3 services, which are based on time division multiplexing. It was also faster than routers available at that time. ATM's speed is due to its fixed-size cells, switching in hardware and asynchronous technology, which does not depend on timing. Rather, cells are forwarded based on priority and arrival time.

Fixed-Sized Cells - Less Processing: ATM packages data into discrete groups called cells. These cells are of a fixed size. Handling fixed-sized cells requires less processing than older routers with variable-sized packets. The ATM switch does not have to look for bits telling it when the cell is over. Each cell is 53 bytes long. Five of the 53 bytes contain header information. This includes bits that identify the type of information contained in the cell (for example, voice, data , or video) so that the cell can be prioritized. The remaining 48 bytes are the "payload" - user data such as voice, video, or sales proposals.

Switching in Hardware - Less Address Lookup: ATM cells are switched in hardware. This means that an ATM switch does not have to look up each cell's address in software. Rather, an ATM switch sets up a route through the network when it sees the first cell of a transmission. It puts this information into its hardware and sends each cell with the same header routing information down the virtual path previously established. For example, all cells with XXX in the header use route 234. Using the same path for each cell makes ATM a connection-oriented service.

Asynchronous Switching - Improving Network Utilization: With asynchronous switching, every bit of network capacity is available for every cell. This is different than synchronous multiplexing technology such as T-1/E-1 and T-3/E-3. With T-3 multiplexing, every one of the 672 input transmissions is assigned a time slot. If device A has nothing to send, its slot is sent through the network empty. ATM has no synchronous requirements. It statistically multiplexes cells onto the network path based on quality-of-service information in the header. With ATM, network capacity is not wasted forwarding empty cells.


Source: The Essential Guide to Telecommunications, 4th Edition by Annabel Z. Dodd

ATM (Asynchronous Transfer Mode)

ATM (Asynchronous Transfer Mode): a high-speed switching service capable of carrying voice, data, video, and multimedia images. ATM is used primarily in frame relay networks, carrier networks and enterprises for private lines. The key advantage of ATM is that it enables providers and end users to carry multiple types of traffic at assigned quality-of-service levels. ATM carries parallel streams of traffic at different levels of service quality over the same circuit. In frame relay networks, carriers deploy multiplatform switches with both frame relay and ATM ports. The switch converts the frames from enterprise sites to ATM cells and transports them through the network. It converts them back to the frame relay format before sending data to the enterprise site to which the frames were addressed.

Because of improvements in IP protocols - in particular, MPLS's (Multi-Protocol Label Switching) capability to "tag" traffic so that voice and video can be prioritized - and the lower cost and easier programming of IP, ATM is becoming displaced by IP equipment. In addition, in carrier networks, IP services achieve higher speeds. On the enterprise side, Gigabit Ethernet and individual wavelengths offer lower-cost options than ATM for end users who need to send large files between sites. However, GigE and individual wavelength services are still not universally available, and wavelength service has distance limitations. (We'll elaborate on these technologies/services in the future.)

ATM is expensive and complex for carriers to install and program. As older equipment is depreciated, carriers will transition to IP with MPLS for voice, data and video traffic.

Source: The Essential Guide to Telecommunications, 4th Edition by Annabel Z. Dodd

DSL Access Multiplexers (DSLAMs)

DSLAMs (DSL Access Multiplexers) aggregate traffic from multiple DSL modems and combine it into higher speeds before sending it to the Internet of data networks. DSLAMs are located in carriers' COs or digital loop carriers, also referred to as remote terminals, in neighborhoods and in the wiring closets of large apartment and office buildings. DSLAMs combine DSL traffic into higher-speed streams. These are, for the most part, ATM speeds of optical carrier level 3 (OC-3), 155 million bits per second, but some DSLAMs use slower DS-3 44 Mbps connections.

Customers have dedicated capacity between their DSL modem and the DSLAM that they don't share with other customers. However, capacity between the DSLAM and the Internet or the ISP (Internet Service Provider) is shared by data from other customers. The connection between the DSLAM and an ISP is a potential site for network congestion. If not enough capacity is available, a customer might experience delays. DSLAMs have been manufactured by Adtran, Alcatel, Catena, Lucent, Paradne and Westell.

The links below are the pictures of various DSLAMs

http://www.pssi-us.com/DSLInfo.gif
http://www.tsninternet.com.au/webpages/Prices/images/dslam-image2lg.gif
http://www.nag.ru/2002/2309/img/dslam.jpg


MiniRAM-Mini Remote Access Multiplexer: A newer, lower cost, smaller DSLAM is being deployed to provide DSL over short copper telephone lines. These MiniRAMs are about the size of two pizza boxes stacked on top of each other. They can be located on telephone poles or in standalone boxes on the ground and serve 10 to 24 customers. Power is fed to MiniRAMs through copper telephone lines on the pole or underground.

Because they are closer to customers, MiniRAMs avoid most of the impairments found on copper lines further from COs. These impairments are caused by crosstalk, loading coils that boost signals, and bridge taps used to share copper lines among customers. The dilemma is that the closer the fiber and MiniRAMs are to customers, the higher the overall costs. As they get closer to customers, MiniRAMs serve fewer customers. Overall there are more fiber runs, more MiniRAMs, and more equipment to maintain and install.

Smaller MiniRAMs are connected to CO-based aggregation switches that packetize the data and send it to ISPs. Traffic from larger MiniRAMs is aggregated in DSLAMs. In the future, switches in the DSLAM will provide more of the aggregation function.

DSL (Expanded Definition)

Digital subscriber line (DSL) service is used primarily for high-speed Internet access. (The most commonly used types of DSL services are listed in the attached table.) Asymmetric DSL (ADSL) counts for the largest installed base. Asymmetric services have higher download speeds away from the Internet to the customer and slower uploading speeds from the consumer to the Internet. Business customer, for the most part, lease symmetric DSL with equal speeds upstream to the Internet and downstream. ADSL shares the same copper cabling already in place for voice. This made it an appealing technology for telephone companies that can, for the most part, use existing cabling to provide broadband access. However, copper cabling is not suitable for carrying video over long distances.

Now, however, newer versions of ADSL are available that support television on shorter cabling runs of 5,000 to 8,000 feet (5-8 kft). However, DSL works only on copper, not fiber. To create short copper cabling runs, telephone companies extend fiber closer to customers. They convert DSL signals to those compatible with fiber, where fiber connects to the copper cabling carrying DSL signals.

Interest in new DSL standards has been spurred by competition from cable TV, wireless, and VoIP providers. Cable TV operators are starting to steal more voice telephony along with Internet access, television, and video on demand. To compensate for lines lost each year since 2001 to competitive services, incumbent telephone companies are putting in place strategies for new infrastructure that will enable them to sell television, voice telephony, and Internet access plus enhanced services.

There is disagreement in the industry about whether DSL is an interim technology and whether fiber should be run to people's homes and businesses. Some telephone companies are planning to bring fiber to every customer location in their territory. They believe that bringing fiber to the premises (FTTP) is less expensive in the long run because it is more reliable, less costly to maintain, and supports higher speeds. However, in the short run, the labor involved in digging trenches for fiber and purchasing materials will cost billions of dollars. SBC and BellSouth (now combined AT&T) and Qwest have announced they will bring fiber closer to customers and use DSL for the last few thousand feet (Fiber to the Node-FTTN is SBC's, i.e. AT&T's, plan and Fiber to the Curb-FTTC was BellSouth's plan). They will build fiber to premises at new housing developments. Verizon has taken a different tack. They have announced a nationwide initiative to lay fiber to all of their residential and business customers' premises instead of using new ADSL technology to reach customers.

Although DSL modems often use the same copper cabling that carries voice, data carried on DSL service is handled separately from voice in carriers' networks. When DSL traffic hits the central office, it is routed on data networks that are separate from the PSTN. Equipment at the CO packetizes DSL traffic and sends it to Internet service providers (ISPs) or other data networks.

SDSL (Symmetric DSL) and variants

Symmetric: Same Speed Both Ways

HDSL (High Bit Rate DSL)
The most mature DSL, HDSL provides T1 transmission over existing twisted pair without the additional provisioning typically required for setting up T1 circuits, such as bridged tap removal and repeater installation. HDSL requires two cable pairs up to 12,000 feet, while HDSL-2 requires only one cable pair and spans 18,000 feet. HDSL does not allow line sharing with analog phones.

SDSL (Symmetric DSL)
SDSL is an HDSL variation that is rate adaptive, uses one cable pair and is offered in speeds from 144 Kbps to 1.5 Mbps. Like HDSL, SDSL does not share lines with analog phones.

IDSL (ISDN DSL)
IDSL is a slightly faster basic BRI ISDN service. It uses the 16 Kbps "D" channel for data rather than call setup to achieve 144 Kbps instead of 128 Kbps. It also offers the longest distance of 26,000 feet. Unlike standard ISDN, IDSL does not support analog phones, and signals are not switched through the telephone network. Since IDSL uses the same 2B1Q line coding as ISDN, ISDN customers can use existing BRI terminal adapters and routers.

ADSL (Asymmetric DSL) and variants

ADSL shares ordinary telephone lines by using frequencies above the voice band, but the higher frequencies interfere with regular telephone usage. The first versions required a visit from the phone company to install a POTS (Plain Old Telephone Service) splitter that divides the line into separate lines for DSL and telephone. Subsequent splitterless versions (also known as G.Lite, Universal ADSL and ADSL Lite) eliminate the phone company visit, but require that the user plug DSL low-pass filters into every telephone outlet that serves ordinary telephones, answering machines and faxes. ADSL is available in two modulation schemes: Discrete Multitone (DMT) or Carrierless Amplitude Phase (CAP).

ADSL Transmission: The higher frequencies of DSL have to be filtered out for regular telephones, answering and fax machines. Low-pass DSL filters split the line between phone and DSL modem and must be used wherever a telephone is plugged into the wall.

RADSL (Rate Adaptive DSL): RADSL is a version of ADSL that adjusts speeds based on signal quality. Many ADSL technologies are actually RADSL.

VDSL/VHDSL (Very High Bit Rate DSL): VDSL is used as the final drop from a fiber optic junction point to nearby customers. VDSL lets an apartment or office complex obtain high-bandwidth services using existing copper wires without having to replace the infrastructure with optical fiber. Like ADSL, VDSL can share the line with the telephone.

DSL (Digital Subscriber Line)

A technology that dramatically increases the digital capacity of ordinary telephone lines (the local loops) into the home or office. DSL speeds are based on the distance between the customer and telco central office. There are two main categories. Asymmetric DSL (ADSL) is for Internet access, where fast downstream is required, but slow upstream is acceptable. Symmetric DSL (SDSL, HDSL, etc.) is designed for connections that require high speed in both directions.

Typically, the download speed of consumer DSL services ranges from 256 kilobits per second (kbit/s) to 24,000 kbit/s, depending on DSL technology, line conditions and service level implemented. For the most part, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for the rarer Symmetric Digital Subscriber Line (SDSL).

DSL provides "always-on" operation. At the telco central office, DSL traffic is aggregated in a unit called the DSL Access Multiplexor (DSLAM) and forwarded to the appropriate ISP or data network. DSL arrived in the late 1990s with more versions and "alphabet soup" than most any other new transmission technology. We will explore the "flavors" of DSL this week.

Amplifier

When telephone conversations travel through a medium, such as a copper wire, they encounter resistance and thus become weaker and more difficult to hear. An amplifier is an electrical device which strengthens the signal. Unfortunately, amplifiers in analog circuits also strengthen noise and other extraneous garbage on the line. Cascading amplifiers, therefore, compound, or accumulate, noise. Digital systems make use of regenerative repeaters, which regenerate (i.e. reshape or reconstruct) the signal before amplifying it and sending it on its way. As a result, noise is much less prevalent and less likely to be amplified in digital systems, whether one or many repeaters are in place. The ultimate yield of a repeater in a digital environment is that of improved error performance, which also yields improved throughput, assuming that error correction involves retransmission.

Repeater Coil (and Repeater)

Repeater Coil: Also called a Repeat Coil. It's really just a transformer, which converts AC power to the voltages used to charge batteries and to power various devices such as PBXs. Repeater coils also are used for impedance matching, which serves to maximize the power transfer of a signal where two electrical circuits (e.g, twisted pair) are interconnected. The power transfer is improved through the elimination of echo, which is signal reflection back towards the signal source.

Different than a Repeater…

Repeater: Also known as a Regenerative Repeater and a Regenerator. A device inserted at intervals along a digital circuit to regenerate the transmitted signal. As the digital signal transverses the circuit, it loses its shape due to the combined effects of attenuation and noise. Attenuation is weakening of the signal as it transverses the circuit. Noise, or distortion, can be caused by EMI (ElectroMagnetic Interference), RFI (Radio Frequency Interference) frequency shifts internal to the circuit, and various other factors. At some point, the original signal becomes incoherent unless a repeater is placed on the circuit at specific intervals, which are sensitive to the specifics of the circuit design. The repeater is capable of reading the signal, even though it is somewhat attenuated and distorted, reshaping it into proper "ones" and "zeros," and repeating (i.e., retransmitting) it at the proper level of signal strength. Repeaters are used exclusively in digital circuits, whether they are metallic (e.g., twisted pair and coaxial), radio (e.g., cellular, microwave, and satellite), or optical (e.g., optical fiber). Analog circuits make use of amplifiers, which simply serve to boost the signal strength, and which cannot reshape it.

Loop Extender

Device in the CO (Central Office) that supplies augmented voltage out to subscribers who are at considerable distances. It provides satisfactory signaling and speech for such subscribers. More specifically, an ADSL (Asymmetric Digital Subscriber Line) loop extender increases the channel capacity of a DSL connection from the CO to the subscriber. ADSL repeaters are aggressively deployed by rural telephone companies trying to reach farms and small towns in areas where it is impractical to place the DSLAM (DSL Access Multiplexers) closer. The typical distance improvement with a loop extender is shown in the diagram below, with rate in Megabits per second and distance in thousands of feet. In future WotD's we will explore in more detail DSL (and ADSL) along with the market for these services.

For graph visit: http://www.strowger.com/images/moz-screenshot-8.jpg

[Note: ADSL2 and ipTV in this sense, refer to types/levels of service offered to DSL subscribers]

Load Coil

Load coils are also known as impedance matching transformers. Load coils are used by the telephone companies on long analog POTS (Plain Old Telephone Service) lines to filter out frequencies above 4 kHz, using the energy of the higher frequency elements of the signal to improve the quality of the lower frequencies in the 4 kHz voice range. Load coils are great for analog voice grade local loops, but must be removed for digital circuits to function. Load coils must be removed for DSL loops, as the frequencies required are well above 4 kHz. Today many phone companies offer broadband service, but often tell their customers that they can't get the service because "you live too far from the telephone company's office." Tell the company to remove the loading coils and any bridging taps on your local loop and it will work. Or offer to pay for commercial ADSL service. (We'll explore the flavors of DSL in future WotDs.)

Jumper (and Jumper Cable)

Jumper:
1. A wire used to connect equipment and cable on a distributing frame.
2. Single twisted pairs used for cross connecting between 66, 110 or Krone blocks.
3. A patch cable or wire used to establish a circuit, often temporarily, for testing or diagnostics.
4. Jumpers are pairs or sets of small prongs on adapters and motherboards. Jumpers allow the user to instruct the computer to select one of its available operation options. When two pins are covered with a plug, an electrical circuit is completed. When the jumper is uncovered the connection is not made. The computer interprets these electrical connections as configuration information. When errors are found on printed circuit boards, a jumper cable is sometimes soldered in to correct the problem.


Jumper Cable: A short length of conductor or cable used to make a connection between terminals or around a break in a circuit, or around an instrument.

Patch Cord (and Panel)

Patch Cord: A short length of wire or fiber cable with connectors on each end, a patch cord is used to join communication circuits at a cross connect point. A patch cord is much like an extension cord. In the context of telephony, it's much like the cords that the telephone operators in the early 1900s used to use on a manual switchboard. They would use a short cord with a plug on each end to connect to one jack for the calling party and another for the called party. Thereby, a unique physical and electrical path was established. When the call was concluded, the operator unplugged the cord from the jacks. The next call involved a repeat of the same process, and so on. Patch cords still have a very important purpose where semi-permanent and highly reliable connections must by made between links.

Patch Panel: A device in which temporary connections can be made between incoming lines and outgoing lines. It is used for modifying or reconfiguring a communications system or for connecting devices such as test instruments to specific lines. A patch panel differs from a distribution frame in that the connections on a distribution frame are intended to be permanent.




Source: http://www.americantechsupply.com/images/CAT%206%2048%20port%20patch%20panel.jpg

Cross Connect

Cross Connect: Imagine you have an office that you need to wire up for voice and data. So you wire every desk with a bunch or wires. You punch one end of the wires into various plugs at the desk. You punch the other onto some form of punchdown block, for example a 66-block. That punchdown block may be in a closet on the same floor or it may be down in the basement. Then you bring the wires in from your telecom suppliers. The T-1s, the ATM, the FR, the local lines, the analog lines, the digital lines, etc. You punch them down on another punchdown block. Now you have two sets of blocks - one for those going to the office and those coming in from the outside world. You now have to join them in a process known as "cross-connecting" in the telecom world. You simply run wires from one punchdown device to the other. The reason you use cross-connect wires rather than just punching down an incoming phone line, for example, directly to your phone system is that moves, adds and changes would, over time, horribly confuse things, screw connections up, and eventually become a total mess. It's easier to simply have all the changes accomplished through the cross-connect wires and wiring. Follow the short wires. It's easy to see what's connected to what and provides for labeling, documentation, etc. In short, cross-connect is a connection scheme between cabling runs, subsystems, and equipment using patch cords or jumpers that attach to connecting hardware on each end. Cross-connection is the attachment of one wire to another usually by anchoring each wire to a connecting block and then placing a third wire between them so that an electrical connection is made. The TIA/EIA-568-A standard specifies that cross-connect cables (also called patch cords) are to be made out of stranded cable.

Cross Connect Equipment: Distribution system equipment used to terminate and administer communication circuits. In a wire cross connect, jumper wires or patch cords are used to make circuit connections. In an optical cross connect, fiber path cords are used. The cross connect is located in an equipment room, riser closet or satellite closet.

Cross Connect Field: Wire terminations grouped to provide cross connect capability the groups are identified by color-coded sections of blackboards mounted on the wall in equipment rooms, riser closets, or satellite closets, or by designation strips placed on the wiring block or unit. The color coding identifies the type of circuit that terminates at the field.

Inside Wiring

Inside Wiring: That telephone wiring located inside your premises or building. Inside Wiring starts at the telephone company's Demarcation Point and extends to the individual phone extensions. Traditionally, Inside Wiring was installed and owned by the telephone company but now you can install your own wiring. And most companies installing new phone systems are installing their own new wiring because of potential problems with reusing the old telephone company cable.

Inside Wire or Line Backer: names of products sold by LECs (Local Exchange Carriers) to their customers as "insurance" on their inside wire. Customers pay upwards of $5 per month in order not to have to pay the phone company a pile of cash if something goes wrong with their inside wiring.

66 Block

The most common type of connecting block used to terminate and cross-connect twisted-pair cables. It was invented by Western Electric eons ago and has stood the test of time. It is still being installed. Its main claims to fame: Simplicity, speed, economy and space. You don't need to strip your cable of its plastic insulation covering. You simply lay each single conductor down inside the 66 block's two metal teeth and punch the conductor down with a special tool, called a punch-down tool. As you punch it down, the cable descends between the two metal teeth, which remove its plastic insulation (it's called insulation displacement) and the cable is cut. The installation is then neat and secure. 66 blocks are typically rated Category 3 and as such as used mostly for voice applications, although Category 5 66 blocks are available. 66 blocks are open plastic troughs with four pins across, and the conductors are more susceptible to being snagged or pulled than conductors terminated on other types of blocks (e.g., 110, Krone or BIX).

A note on the Bell Labs numbering system… They just started with "number 1" on whatever system they were working on. TD1 radio, TD2 radio, etc., Whenever there was a "hole" in the sequence, that meant that the labs had worked on something, but it didn't pan out for some reason.

Drop (and variants of Drop)

Drop:
1. A wire or cable from a pole or cable terminal to a building.
2. That portion of a device that looks toward the internal station facilities, e.g., toward an AUTOVON 4-wire switch, toward a switchboard, or toward a switching center.
3. Single channel attachment to the horizontal wiring grid (wall plate, coupling, MOD-MOD adapter).
4. The CO (central office) side of test jacks.
5. To delete, intentionally or unintentionally, part of a signal for some reason, e.g., dropping bits.

Drop Cable:
1. The outside wire pair which connects your house or office to the transmission line coming from the phone company's CO.
2. In local area networks, a cable that connects a network device such as a computer to a physical medium such as an Ethernet network. Drop cable is also called transceiver cable because it runs from a network node to a transceiver (a transmit/receiver) attached to the trunk cable.

Drop Loop: The segment of wire from the nearest telephone pole to your home or business.

Drop Wire: Wires going from your phone company to the 66 Block (type of punchdown block used to connect sets of wires in a telephone system) or protector in your building.

Distribution Cable (OSP and ISP)

Distribution Cable, OSP (Outside Plant): The cable running from a central office or remote terminal to the side of a subscriber's lot.

Distribution Cable, ISP (Inside Plant): Cables usually running horizontally from a closet on a given floor within a building. Distribution cables may be under carpet, simplex, duplex, quad, or higher fiber count cables.

Feeder Cable

A group of wires, usually 25-pair or multiples of 25-pair, that supports multiple phones in a single cable sheath. These cables may or may not be terminated with a connector on one or both ends. A Feeder cable typically connects an intermediate distribution frame (IDF) to a main distribution frame (MDF). But the term "feeder cable" is also used in backbone wiring.

For a basic illustration, you can access the following link:

http://www.svrops.com/svrops/Images/HorizCC.gif


Bellcore defines the term slightly differently: A large pair-size loop cable emanating from a central office and usually placed in an underground conduit system with access available at periodically placed manholes. (This is a very common usage for the term.)


Feeder Route: A network of loop cable extending from a wire center into a segment of the area served by the wire center.

Serving Area Interface (SAI)

A serving area interface is part of a phone company's outside plant. It is a fancy name for a box on a pole, a box attached to a wall or a box in the ground that connects the phone company's feeder or subfeeder cables (those coming from the central office) to the drop wires or buried service wires that connect to the customer's premises. It's also called a cross-wire box.

(See below for picture of a typical grounded Serving Area Interface)




Source: http://upload.wikimedia.org/wikipedia/en/thumb/9/9d/1200_pair_SAI.jpg/180px-1200_pair_SAI.jpg

Tandem Switch

Tandem: In a telecommunications context, the term refers to switches, circuits, or other Network Elements (NEs) that serve to allow other NEs to work together. For example, tandem switches, or tandem offices, serve to interconnect other, lesser switches, (i.e. Central Offices [CO's] or lesser tandems). Tandem switches, in the purest sense of the term, serve no end users directly, as that is the responsibility of the COs. Rather, they strictly serve to interconnect the COs, which are at the lowest level of the switching hierarchy in the PSTN. Tandem tie trunks serve to interconnect tandem switches.


Tandem Switch: Tandem is a telephony term meaning to "connect in series." Thus a tandem switch connects one trunk to another. A tandem switch is an intermediate switch or connection between an originating telephone call location and the final destination of the call. The tandem point passes the call along. A PBX can often handle tandem calls from other/to other locations as well as process calls to, from and within its own location.



Source: http://img.zdnet.com/techDirectory/CO1.GIF

Remote Node

1. A remote node is a device that connects to a network from a point some distance away from the central host. For example, a CO (Central Office) in the PSTN (Public Switched Telephone Network) might support a number of remote nodes. Some of the nodes are dumb line concentrators that serve only to concentrate traffic over high-capacity trunks in order to reduce cabling costs. Other nodes are intelligent switching partitions that can switch basic local traffic within their own geographic domains, even though they rely on the CO for guidance in the delivery of more complex services, such as custom calling features.

2. Remote node software allows remote users to dial-in to the corporate LAN and work with the applications and data on the LAN as if they were "actually in the office." By dialing in, they become nodes on the LAN. Using a PC, Mac or UNIX workstation; a modem; and a remote access server, employees can connect from any location in the world that has an analog, a switched digital, or a wireless connection.

Switching Fabric

The term "switching fabric" refers to the component at the heart of a data communications switch that allows any input port to send data to any output port. Many different kinds of switching fabric have been used over the years, depending on the manufacturer, the size and type of the data communications switch, and the technology available at the time. Sometimes a switching fabric will directly connect to all ports, but usually there are a group of ports on a single card called a line card and the switching fabric connects the line cards together. There are many different types of switching fabrics available on the market today. An example of one of the most basic is the "crossbar" switching fabric, which consists of a matrix of rows and columns, where each row is connected to an input port and each column is connected to an output port. The resulting diagram looks like a fabric with threads crossing at right angles. A switch or "crosspoint" is located at each intersection between a row and a column. By closing the right crosspoints, each input port can be connected to each output port. Crossbar fabrics are very general, but expensive to create in large sizes because the number of crosspoints is equal to the number of input ports times the number of output ports. For instance, if you had a small eight port switch you would have eight potential input and output ports making a total of 64 crosspoints, but if you had a large switch with 100 ports you would need 10,000 crosspoints to allow every port to connect with each other. Other types of switching fabric use buffering, queuing, packet shaping, switching logic, and specialized application specific integrated circuits (ASIC) to enhance switching fabric performance. A well-designed switching fabric will reach switching speeds equal to the line rate of the port. For instance, a port with a theoretical speed of 100 Mbsp should be able to pass packets across the switching fabric to the destination port or ports at 100 Mbps, which is also known as line-rate or wire-speed switching. A poorly designed switching fabric has delays or other latency that will drop the data rate as packets travel through the switching fabric. The variety and performance of switching fabrics depend on many different variables as well as the manufacturer. However, one thing is for certain future trends in switching fabrics are hard to anticipate, but the switching fabric will always remain at the heart of the data communication switch.

Below is a simplified illustration of switching fabric which shows how any input port (i.e. line card) can transmit data to any output port, essentially linking all of the line cards together. Notice a "crosspoint" is depicted with a solid black dot indicating the intersection between a row and a column of the fabric.


Source: http://choonho.files.wordpress.com/2007/09/capture4.jpg

Internetworking

Communication between two networks or two types of networks or end equipment. This may or may not involve a difference in signaling or protocol elements supported. In the narrower sense - to join local area networks together. This way users can get access to other files, databases and applications. Bridges ad routers are the devices which typically accomplish the task of joining LANs. Internetworking may be done with cables - joining LANs together in the same building, for example. Or it may be done with telecommunications circuits - joining LANs together across the globe.

Two architectural models are commonly used to describe the protocols and methods used in internetworking. The Open System Interconnection (OSI) reference model provides a rigorous description for layering protocol functions from the underlying hardware to the software interface concepts in user applications. Internetworking is implemented in Layer 3 (Network Layer) of the model.

The Internet Protocol Suite, also called the TCP/IP model, of the Internet was not designed to conform to this model. Despite similar appearance as a layered model, it uses a much less rigorous, loosely defined architecture that concerns itself only with the higher level aspects of networking, i.e. it does not discuss hardware-specific low-level interfaces, other than assuming availability of a link-layer interface to the local network link. Internetworking is facilitated by the protocols of its Internet Layer.

Hub

The point on a network where circuits are connected. In local area networks, a hub is the core of a physical star configuration, as in ARCNET, StarLAN, Ethernet, and Token Ring. Hub hardware can be either active or passive. Wiring hubs are useful for their centralized management capabilities and for their ability to isolate nodes from disruption. Hubs work at Layer 1 (Physical) and 2 (Data Link) of the OSI Reference Model, with emphasis on Layer 1. Hubs aren't switches, as they have very little intelligence, if any, and don't set up transmission paths. Rather, hubs comprise a physical bus and numerous ports, to which are connected a bunch of wires, to which are connected individual terminal devices. As hubs are protocol-specific (e.g, Ethernet) and are not intelligent, they are very fast and very cheap. A 10Base-T hub is an inexpensive means of allowing LAN-attached devices to share a common, collapsed bus contained within a hub chassis. The connections are via UTP (Unshielded Twisted Pair), which is much less expensive than are the classic connections through coaxial cable. Unlike switches, hubs do nothing internally to control congestion. However, they typically are workgroup-level solutions which allow a large, logical Ethernet to be subdivided into multiple physical segments. For example, you could even use a small five-port hub on your desk to connect a couple of laptops and a desktop PC. Hubs can be interconnected directly, or through switches or routers, with the traffic being forwarded from the originating hub only if the destination address of the data packet indicates that is necessary to do so. Therefore, hubs do reduce congestion through the control of interhub traffic.

Bridge

1. In classic terms, a bridge is a data communications device that connects two or more network segments and forwards packets between them. Such bridges operate at Layer 1 (Physical Layer) of the OSI Reference Model. At this level, a bridge simply serves as a physical connector between segments, also amplifying the carrier signal in order to compensate for the loss of signal strength incurred as the signal is split across the bridged segments. In other words, the bridge is used to connect multiple segments of a single logical circuit. Classic bridges are relatively dumb devices, which are fast and inexpensive; they simply accept data packets, perhaps buffering them during periods of network congestion and forward them. Bridges are protocol-specific, e.g., Ethernet or Token Ring in the LAN domain. Bridges also are used in the creation of multipoint circuits in the WAN domain, e.g., DDS (Dataphone Digital Service).

Bridges also can operate at Layer 2 (Link Layer) of the OSI Reference Model. At this level, a bridge connects disparate LANs (e.g., Ethernet and Token Ring) at the Medium Access Control (MAC) sub-layer of Layer 2. In order to accomplish this feat, the MAC Bridge may be of two types, encapsulating or translating.

Encapsulating bridges accept a data packet from one network and in its native format; they then encapsulate, or envelope, that entire packet in the format acceptable to the target network. For instance, an Ethernet frame is encapsulated in a Token Ring packet in order that the Token Ring network can deliver it to the target device, which must strip away several layers of overhead information in order to get to the data payload, or content. In order to accomplish this process, a table lookup must take place in order to change basic MAC-level addressing information.

Translating bridges go a step further. Rather than simply encapsulating the original data packet, they actually translate the data packet into the native format of the target network and attached device. While this level of translation adds a small amount of delay to the packet traffic and while the cost of such a bridge is slightly greater, the level of processing required at the workstation level is much reduced.

Bridges also can serve to reduce LAN congestion through a process of filtering. A filtering bridge reads the destination address of a data packet and performs a quick table lookup in order to determine whether it should forward that packet through a port to a particular physical LAN segment. A four-port bridge, for instance, would accept a packet from an incoming port and forward it only to the LAN segment on which the target devices is connected; thereby, the traffic on the other two segments is reduced and the level of traffic on those segments is reduced accordingly. Filtering bridges may be either programmed by the LAN administrator or may be self-learning. Self-learning bridges "learn" the addresses of the attached devices on each segment by initiating broadcast query packets, and then remembering the originating addresses of the devices which respond. Self-learning bridges perform this process at regular intervals in order to repeat the "learning" process and, thereby, to adjust to the physical relocation of devices, the replacement of NICs (Network Interface Cards), and other changes in the notoriously dynamic LAN environment.

While bridges are relatively simple devices, in the overall scheme of things, they can get quite complex as we move up the bridge food chain. Bridges also can be classified as Spanning Tree Protocol (STP), Source Routing Protocol (SRP), and Source Routing Transparent (SRT).
Spanning Tree Protocol (STP) bridges, defined in the IEEE 802.1 standard, are self-learning, filtering bridges. Some STP bridges also have built-in security mechanisms which can deny access to certain resources on the basis of user and terminal ID. STP bridges can automatically reconfigure themselves for alternate paths should a network system fail.

SRP bridges are programmed with specific routes for each data packet. Routing considerations include physical node location and the number of hops (intermediate bridges) involved. This IBM bridge protocol provides for a maximum of 13 hops.

SRT bridges, defined in IEEE 802.1, are a combination of STP and SRP. SRT bridges can act in either mode, as programmed.



2. Bridge is also a verb, as in "to bridge." Imagine a phone line. It winds from your CO (Central Office) through the streets and over the poles to your phone. Now imagine you want to connect another phone to that line. A phone works on two wires, tip and ring (positive and negative). You simply clamp each one of the phone's wires to the cable coming in. That's called bridging. Imagine bridging as connecting a phone at a right angle. When you do that, you've made what's known as a "bridged tap." The first thing to know about bridging is that bridging causes the electrical current coming down the line to lose power. How much? That typically depends on the distance from the bridged tap to the phone. A few feet, and there's no significant loss. But that bridged tap can also be thousands of feet. For example, the phone company could have a bridged tap on your local loop, which joined to another long-defunct subscriber. The phone company technicians simply saved a little time by not disconnecting that tap. If you want the cleanest, loudest phone line, the local loop to your phone should not be bridged. Instead it should be a direct "home run" from your CO to your phone.

Bridging can be a real problem with digital circuits. Circuits above 1 Mbps (e.g, T-1) should never, ever be bridged. Because of the power loss, they simply won't work or will work so poorly they won't be worth having. ISDN BRI (Integrated Services Digital Network Basic Rate Interface) channels are also digital. But they were specifically designed to work with the existing telephone cable plant, which has a huge number of bridged circuits. Telephone companies typically will install ISDN BRI circuits with up to six bridged taps and about 6,000 feet of bridged cabling.

Router

1. As in software, router is a system level function that directs a call to an application.

2. As in hardware, routers are the central switching offices of the Internet and corporate Intranets and WANs. Routers are bought by everybody - from backbone service providers to local ISPs, from corporations to universities. The main provider of routers in the world is Cisco. It has built its gigantic business on selling routers - from small ones, connecting a simple corporate LAN to the Internet, to corporate enterprise wide networks, to huge ones connecting the largest of the largest backbone service providers. A router is, in the strictest terms, an interface between two networks.

Routers are highly intelligent devices which connect like and unlike LANs (Local Area Networks). They connect to MANs (Metropolitan Area Networks) and WANs (Wide Area Networks), such as X.25, Frame Relay and ATM. Routers are protocol-sensitive, typically supporting multiple protocols. Routers most commonly operate at the bottom 3 layers of the OSI model, using the Physical, Link and Network Layers to provide addressing and switching. Routers also may operate at Layer 4, the Transport Layer, in order to ensure end-to-end reliability of data transfer.

Routers are much more capable devices than are bridges, which operate primarily at Layer 1, and switches, which operate primarily at Layer 2. Routers send their traffic based on a high level of intelligence inside themselves. This intelligence allows them to consider the network as a whole. How they route (also called routing considerations) might include destination address, packet priority level, least-cost route, minimum route delay, minimum route distance, route congestion level, and community of interest. Routers are unique in their ability to consider an enterprise network as comprising multiple physical and logical subnets (subnetworks). Thereby, they are quite capable of confining data traffic within a subnet, on the basis of privilege as defined in a policy-based routing table. In a traditional router topology, each router port defines a physical subnet, and each subnet is a broadcast domain. Within that domain, all connected devices share broadcast traffic; devices outside of that domain can neither see that traffic, nor can they respond to it. Contemporary routers have the ability to define subnets on a logical basis, based on logical address (e.g., MAC or IP address) information contained within the packet header, and acted upon through consultation with a programmed routing table. In addition to standalone routers developed specifically for that purpose, server-based routers can be implemented. Such routers are in the form of high-performance PCs with routing software. As software will perform less effectively and efficiently than firmware, such devices generally are considered to be less than desirable for large enterprise-wide application, although they do serve well in support of smaller remote offices and less-intensive applications. Routers also are self-learning, as they can communicate their existence and can learn of the existence of new routers, nodes and LAN segments. Routers constantly monitor the condition of the network, as a whole, in order to dynamically adapt to changes in network conditions.

Characteristics of routers can include: LAN extension, store and forward, support for multiple media, support for multiple LAN segments, support for disparate LAN protocols, filtering, encapsulation, accommodation of various and large packet sizes, high-speed internal buses (1+Gbps), self-learning, routing based on multiple factors, route length, number of hops, route congestion, traffic type, support for a community of interest (VLAN), redundancy, and network management via SNMP (Simple Network Management Protocol).

Router protocols include both bridging and routing protocols, as they perform both functions. These protocols fall into 3 categories:

1. Gateway protocols establish router-to-router connections between like routers. The gateway protocol passes routing information and keep alive packets during periods of idleness.

2. Serial Line Protocols provide for communications over serial or dial-up links connecting unlike routers. Examples include HDLC (High-level Data Link Control), SLIP (Serial Line Interface Protocol) and PPP (Point-to-Point Protocol).

3. Protocol Stack Routing and Bridging Protocols advise the router as to which packets should be routed and which should be bridged.

Switch

A mechanical, electrical or electronic device which opens or closes circuits, completes or breaks an electrical path, or select paths or circuits. Switches work at Layers 1 (Physical) and 2 (Data Link) of the OSI Reference Model, with emphasis on Layer 2. A switch looks at incoming data (voice data, or data data) to determine the destination address. Based on that address, a transmission path is set up through the switching matrix between the incoming and outgoing physical communications ports and links. Data switches (e.g., LAN (Local Area Network) switches and packet switches) also typically contain buffers, which can hold data packets in temporary memory until the necessary resources are available to allow the data packets to be forwarded. Voice switches, of course, don't, because you can't delay voice. Switches work link-by-link, with multiple switches typically being involved in complex networks; each switch forwards the data on a link-by-link (hop-by-hop) basis. Routers are highly intelligent data switches which are capable of setting up paths from end-to-end, perhaps in consideration of the level of privilege of the user and application. Routers commonly are used at the edges of complex data networks, where intelligence is required to set up appropriate network paths. Although such intelligent decisions impose some delay on the packet traffic, they are made only at the ingress and egress edges of the network. The routers often instruct switches in the core of the network, where speed is of the essence-switches aren't as intelligent as routers, but they are faster and less expensive.

OSI (Open Systems Interconnection):

*We are going to spend the next several days learning about various communication network equipment. Before we do that it will be helpful to understand the OSI Reference Model.

OSI (Open Systems Interconnection): A Reference Model developed by the ISO (International Organization for Standardization, as translated into English). The OSI Reference Model is the only internationally accepted framework of standards for communication between different systems made by different vendors. ISO's goal is to create an open systems networking environment where any vendor's computer system, connected to any network, can freely share data with any other computer system on that network or a linked network. Most of the dominant communication protocols used today have a structure based on the OSI model.

Although OSI is a model and not an actively used protocol, and there are still very few pure OSI-based products on the market today, it is still important to understand its structure. The OSI model organizes the communications process into seven different categories and places these categories in a layered sequence based on their relation to the user. Layers 7 through 4 deal with end to end communications between the message source and the message destination, while layers 3 through 1 deal with network access.

Layer 1 - The Physical Layer deals with the physical means of sending data over lines (i.e. the electrical, mechanical and functional control of data circuits). Examples include EIA-232 , T-Carrier and SONET.

Layer 2 - The Data Link Layer is concerned with procedures and protocols for operating the communications lines. It also has a way of detecting and correcting message errors. Examples include Frame Relay, PPP, and SLIP (Serial Line Internet Protocol). ATM runs at Layers 1 & 2, as do LANs.

Layer 3 - The Network Layer determines how data is transferred between computers. It also addresses routing within and between individual networks. The most visible example is IP (Internet Protocol).

Layer 4 - The Transport Layer defines the rules for information exchanges and manages end-to-end delivery of information within and between networks, including error recovery and flow control. TCP (Transmission Control Protocol) is an example, as is the OSI Transport Protocol (TP), which comprises five layers of its own. Layer 4 protocols ensure end-to-end integrity of the data in a session. The X.25 packet-switching protocol operates at Layers 1, 2, 3 and 4.

Layer 5 - The Session Layer is concerned with dialog management. It controls the use of the basic communication facility provided by the Transport layer. If you’ve ever lost your connection while Web surfing, you've likely experienced a session time-out, so you have some sense of the Session Layer.

Layer 6 - The Presentation layer provides transparent communications services by making the differences of varying data formats (character codes, for example) between dissimilar systems. Conversion of coding schemes (e.g., ASCII to EBDCIC to Unicode) and text compression and decompression exemplify Presentation Layer functions.

Layer 7 - The Applications layer contains functions for particular applications services, such as file transfer, remote file access and virtual terminal. TCP/IP application protocols such as FTP (File Transfer Protocol), Simple Mail Transfer Protocol (SMTP), SNMP (Simply Network Management Protocol) and TELNET (TELecommunications Network) take place at Layer 7.

The OSI Model is an important concept to understand, at least at a high level. I have attached one of Novell's Network Tutorials which goes into the OSI Model in more detail and also provide some very useful diagrams, including one which illustrates where different protocols operate in the OSI model. The tutorial can be found at the following link:

http://www.novell.com/info/primer/prim05.html.

Switch Access Line Service, etc.

Switched Access Line Service (Switched Access): All residential and most businesses use this type of telephone access. It refers to the connection between your phone and the long distance companies' switch (POP or Point-of-Presence) when you make a regular local or LD (long distance) telephone call over standard phone lines.

Switched Access: A method of obtaining test access to telecommunications circuits by using electromechanical circuitry to switch test apparatus to the circuit.

Switched Local Service: You pick up the phone. You dial a local number. Bingo, you have switched local phone service. The reason this trivial definition is even in Newton's dictionary is because many states in the US now, finally, allow companies to offer local switched telephone service in competition with the established company, e.g. United Telecom (Embarq). Previously, they had only allowed competition in leased lines. And then previous to that they had not allowed any competition in any area of local phone service.

Reciprocal Compensation

(Recip Comp) - A form of financial compensation that occurs when a local or LD (Long Distance) service provider terminates a call on another provider's facilities. Imagine a phone call from New York to Los Angeles. It may start with the customer of a new phone company, then proceed to a local phone company (e.g., New York Telephone, part of Verizon). Then it may proceed to a LD company before ending in Los Angeles and going through another one or two local phone companies before reaching the person dialed. Under the existing rules, all the companies carrying these phone calls have to be paid in some way for their transmission and switching services. There are programs in place such that the company doing the billing and collecting the money pays over some of those monies to the other phone companies in the chain. One such program is called "reciprocal compensation." The opposite of recip comp is called "Bill and Keep." Under this program, the company billing the call gets to keep all the money. The others in the chain (or most of the others in the chain) get nothing.

IXC (IntereXchange Carrier)

Also less commonly known as IEC (InterExchange Carrier) and IC. Long-haul long distance carriers, IXCs include all facilities-based inter-LATA carriers. The largest IXCs are AT&T (acquiredby SBC in 2005), MCI (merged with Verizon in 2006) and Sprint; a huge number of smaller, regional companies also fit this definition. The term generally applies to voice and data carriers, but not to Internet carriers. IXC is in contrast to LEC (Local Exchange Carrier), a term applied to traditional telephone companies which provide local service and intraLATA toll service. IXCs also provide intraLATA toll service and operate as CLECs (Competitive Local Exchange Carriers) in many states. Once upon a time the non-AT&T IXCs were called OCCs (Other Common Carriers), a status which they resented for understandable reasons.

Below is a link to an annual FCC publication (pdf format), "Statistics of Common Carriers." This document provides a wealth of data on common carriers. The FCC publishes a wide range of useful documents which are available at http://www.fcc.gov. The most up-to-date version is the 2005-2006 annual report released 6/08.

Document: http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-282813A1.pdf

InterLATA Service

As defined by the Telecom Act of 1996, the term interLATA service means telecommunications between a point located in a local access and transport area (LATA) and a point located outside such area. InterLATA services, traffic or facilities originate in one LATA and cross over and terminate in another LATA. This can be either Interstate or Intrastate service, traffic or facilities. Under provisions of Divestiture, the BOCs (Bell Operating Companies) were restricted from provided interLATA services, but could provide intraLATA services (since changed, right?). Reminder, the term LATA means a contiguous geographic area: (a) established before the date of enactment of the Telecommunications Act of 1996 by a BOC such that no exchange area includes points within more than 1 metropolitan statistical area, consolidated metropolitan statistical area, or State, except as expressly permitted under the AT&T Consent Decree; or (b) established or modified by a BOC after such date of enactment and approved by the Commission. IntraLATA services originate and terminate in the same LATA.

Long Distance

Now that we've learned all about LATA's and numbering plans, we'll shift our focus to long distance. Although today's word is rather succinct, there is plenty to learn in this area (which we will dive into later). Stay tuned in future weeks for more information on the various companies spawned from the original AT&T and terminology pertaining to the architecture of the local loop!

Long Distance: Any telephone call to a location outside the local service area. Also called a toll call or trunk call.

NANP (North American Numbering Plan)

Invented in 1947 by AT&T and Bell Telephone Laboratories (now Lucent). The NANP assigns area codes and sets rules for calls to be routed across North America (i.e. the US and Canada). The new one, put into effect in January, 1995 has one major change: The middle number in a North American area code no longer is required to be a 1 or a 0; rather, it can range between 0 and 9. NANP numbers are 10 digits in length, in the format NXX-NXX-XXXX. The first three digits are the NPA code (i.e., area code). The second three are the central office code or central office prefix, and the last four are the line number. NANP numbers conform to E.164, which is the ITU-T (International Telecommunication Union) international standard for numbering plans. NANP administration was shifted from Bell Labs to Bellcore, when it was formed in 1986. Due to Bellcore's obvious conflict of interest, responsibility was shifted to NANC (North American Numbering Council) in 1995; it was shifted again in 1997 to Lockheed Martin. In November, 1999, it was shifted to NeuStar Inc., when it was discovered that Lockheed Martin had a conflict of interest. NeuStar originally was an independent business unit of Lockheed Martin, but was spun off in order to resolve the conflict.

Section 271

Section 271 of the Telecommunications Act of 1996 describes the conditions by which a BOC (Bell Operating Company) may enter the market to provide interLATA (Local Access and Transport Area) services, long distance in particular, within the region where they operate as the dominant local telephone service provider. The Act mandates that BOCs must open their local telephone markets to competition as a precondition to entry into the long distance market. The term 271 has come to be used as shorthand for referring to the strategic efforts of the BOCs to prove competition exists, and thereby gain FCC approval to provide interLATA long distance service. Although final authority to approve a BOC's entry into the LD (Long Distance) market is given to the FCC (Federal Communications Commission), Congress provided in Section 271 a checklist to guide the FCC's assessment of local market competition.

The checklist points are (summarized):
1. Interconnection for any requesting telecommunications carrier with the BOC's network that is at least equal in quality to that provided by the BOC to itself.
2. Non-discriminatory access to network elements.
3. Nondiscriminatory access to the poles, ducts, conduits, and rights-of-way owned or controlled by the BOC at just and reasonable rates.
4. Local loop transmission from the central office to the customer's premises, unbundled from local switching or other services.
5. Local transport from the trunk side of a wireline local exchange carrier switch unbundled from switching or other services.
6. Local switching unbundled from transport, local loop transmission, or other services.
7. Non-discriminatory access to 911, directory assistance and operator call completion services.
8. White pages directory listings for customers of the other carrier's telephone exchange service.
9. Nondiscriminatory access to telephone numbers for assignment to the other carrier's telephone exchange service customers.
10. Nondiscriminatory access to databases and associated signaling necessary for call routing and completion.
11. Telecommunications number portability.
12. Nondiscriminatory access to services or information to allow the requesting carrier to implement local dialing parity (the ability to complete a connection without the use of additional access codes).
13. Reciprocal compensation arrangements.
14. Telecommunications services available for resale.

U.S. Telecommunications Act of 1996

(Summarized from Newton's Telecom Dictionary) A federal bill signed into law on Feb. 8, 1996 "to promote competition and reduce regulation in order to secure lower prices and higher quality services for American telecommunications consumers and encourage rapid deployment of new telecommunications technologies." The Act is widely reputed to be among the worst pieces of legislation ever passed by Congress. There were 3 key requirements/objectives related to the ILECs (Incumbent Local Exchange Carriers): 1) required local service providers in the 100 largest MSAs (Metropolitan Service Area) to implement Local Number Portability by the end of 1998, 2) allowed local RBOCs (Regional Bell Operating Company) into long distance once they had met certain conditions about allowing competition in their local monopoly areas, and 3) forced the local phone companies to rent their local copper loops to new telecommunications carriers (Competitive Local Exchange Carriers). President Clinton signed the Telecom Act of 1996 into law using the same pen President Dwight D. Eisenhower used in 1957 to authorize the interstate highways. "We will help create an open marketplace where competition and innovation can move quick as light," Clinton said. The telecom boom began, tons of new CLECs were born and billions of dollars were invested in the telecom sector. By mid-2001 the whole telecom boom had pretty much went bust.
Congress is considering a major overhaul of the 1996 Act.
See Wikipedia for a description of the 7 titles and more info (worth a quick read, it's not that long):

http://en.wikipedia.org/wiki/Telecommunications_Act_of_1996

Unbundled Network Element

UNE (Pronounced you nee): The Telecommunications Act of 1996 requires that the ILECs (Incumben Local Exchange Carriers) unbundle their NEs (Network Elements), which must be made available to the CLECs (Competitive Local Exchange Carriers) on the basis of incremental cost. This means that CLECs will pay the additional costs the ILECs incur in making these facilities available. The words "incremental cost" are meant to signal to the ILECs that they are not to inflate the price of these facilities by adding overhead costs (e.g., the salary of the ILEC's people in the charge of investor relations). UNEs are defined as physical and functional elements of the network, e.g., NIDs (Network Interface Devices), local loops, switch ports, and dedicated and common transport facilities. When combined into a complete set in order to provide an end-to-end circuit, the UNEs constitute a UNE-P (UNE-Platform). Unbundled Network Elements is a term used in negotiations between a CLEC and the ILEC to describe the various network components that will be used or leased by the CLEC from the ILEC. These components include such things as the actual copper wire to the customers, fiber strands, and local switching. The CLEC will lease these UNEs with pricing based on the previously-signed Interconnection Agreement between the CLEC and the ILEC. Typically, a CLEC will collocate a switch at the ILEC's wire center, then pay for the "unbundled" local loop to make a connection to the customer. Alternately, a CLEC might lease both an unbundled local loop and an unbundled switch, and make a connection to their network at the LEC's switch.

Subloop

As defined by the Telecommunications Act of 1996, a subloop is a portion of a local loop that is accessible to terminals at any point in the ILEC's (Incumbent Local Exchange Carrier) outside plan, including inside wire. An accessible terminal is any point on the loop where technicians can access the wire or fiber within the cable without removing a splice case. Such points can include a telephone pole or pedestal, the minimum point of entry (MPOE), the single point of interconnection, the main distribution frame (MDF), and the feeder/distribution cable interface. Subloops are one of the categories of Unbundled Network Elements (UNEs), which the ILECs must make available to the CLECs (Competitive Local Exchange Carrier).

*Note: Tomorrow we will explore the Telecommunications Act of 1996 in more detail

Demarc

(Pronounced "D-Marc") The demarcation point is the physical point at which the separation is made between the carrier's responsibilities for the circuit and those of the end user organization. The carrier is responsible for the local loop, which connects the user organization's premises to the carrier's CO (Central Office) or POP (Point of Presence) at the edge of the network. In a residential or small business application, the demarc is at the NIU (Network Interface Unit), which typically is on the side of the house or inside the garage. In a larger business application, it is at the MPOE (Minimum Point of Entry), which is the closest practical point to where the carrier facilities cross the property line or the closest practical point to where the carrier cabling enters a building. While the MPOE typically is in the form of a physical demarc, in older installations it may simply be in form of a tag hung on the entrance cable to identify a point of logical demarcation. There are exceptions. In some older Centrex installations in some states, the demarc is at the jack for each individual voice or data terminal. In some older campus environments, there may be a demarc for each of several cables coming from various directions, and the demarc may be well inside the property line. In either case, it is the responsibility of the carrier to install and maintain the local loop and the demarc device, which includes some form of protector against lightning and other electrical anomalies, and some form of intelligence to support loopback testing. It is the responsibility of the end user organization or building owner to install and maintain the inside cable and wire system, which typically terminates in the demarc through a plug-and-jack arrangement. A demarc for voice services might be in the form of a simple RJ-11C jack (one line or trunk) connection, an RJ-14C (two trunks), an RJ-21X (up to 25 trunks), or a 66-block. A demarc for data services typically supports an RJ-48 (used primarily with a T1 or a Primary Rate Interface) termination.

Local Loop

The physical connection from the subscriber's premise to the carrier's Point of Presence (POP). The local loop can be provided over any suitable transmission medium, including twisted pair, fiber optic, coax, or microwave. Traditionally and most commonly, the local loop comprises twisted pair or pairs between the telephone set, PBX or key telephone system, and the LEC (Local Exchange Carrier) CO (Central Office). As a result of the deregulation of inside wire and cable in the US, the local loop typically goes from the demarc (demarcation point) in the phone room closet, in the basement or garage, or on the outside of the house, to the CO. The subscriber or building owner is responsible for extending the connection from the demarc to the phone, PBX, key system, router, or other CPE device.