DSU's

In telecom, a data service unit (DSU) is a device used for interfacing data terminal equipment (DTE) to the PSTN (Public Switched Telephone Network). It is also a type of short-haul, synchronous-data line driver, usually installed at a user location, that connects user synchronous equipment over a 4-wire circuit at a preset transmission rate to a serving CO (Central Office). This service can be for a point-to-point or multipoint operation in a digital network.
A DSU is a two or more port device; one port is call the WAN port and each other is called a DTE port. The purpose of the DSU is to transfer serial data synchronously between the WAN port and the DTE ports. If more than one DTE port is used, the DSU assigns the DTE data according to time slots (channels) on the WAN side.

On the WAN side, the DSU interfaces with a digital carrier such as DDS (Digital Data Service), DS1 (T1) or DS3 (T3). To accomplish that it generates the proper bit rate, and generates and consumes the proper framing. The WAN port also generates and consumes line coding as necessary. On the DTE side, the DSU provides control lines, timing lines and appropriate physical and electrical interface. To maintain the synchronous relationship between the ports, the DSU manages timing by slaving ports to the bit rate of another or to its internal clock. Typically, the DTE port provides timing to the data terminal equipment while the WAN port dictates the rate.

Constraints on the WAN side may require a minimum "one's density" of the WAN data. In those cases, the DSU may be called on to throttle the rate of the DTE traffic and to insert "1" bits in one bit position of each time slot. The bit "stuffed" in that way is usually #7, but sometimes bit #2 is stuffed instead.

DSUs usually include some maintenance capabilities. At minimum, they can loop data back at either the WAN or DTE ports, or at both. When only one port is looped back, the data received at that port is simultaneously sent back toward the port and passed in normal fashion to the other port. Most DSUs also allow various data patterns to be generated and monitored to measure error rate of the communication link.

Channel Service Unit (CSU)

A line bridging device for use with T-carriers that:

Is used to perform loopback testing - a method of performing transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal, OR a method of testing between stations (not necessarily adjacent) wherein two lines are used, with the testing being done at one station and the two lines interconnected at the distant station.

May perform bit stuffing - the insertion of noninformation bits into data. Bit stuffing is used for various purposes, such as for bringing bit streams that do not necessarily have the same or rationally related bit rates up to a common rate, or to fill buffers or frames. Bit stuffing may be used to synchronize several channels before multiplexity or to rate-match to single channels to each other.

May also provide a framing and formatting pattern compatible with the network.
Is the last signal regeneration point, on the loop side, coming from the CO (Central Office), before the regenerated signal reaches a multiplexer or data terminal equipment.

CSUs can be categorized by the class of service they support (DDS, DS1, DS3, etc.) and by the capabilities within that class. For example, basic DS1 (T1) CSUs support loopback of each interface and will produce an alarm indication signal (a signal transmitted by a system that is part of a concatenated telecommunications system to let the receiver know that some remote part of the end-to-end link has failed at a logical or physical level, even if the system it is directly connected to is still working) to the provider's network interface device in the event of loss of signal from the customer premises equipment (CPE). More advanced units will include internal monitors of the performance of the carrier in both directions and may have test pattern generation and monitor capabilities.

CSUs are required by PSTN (Public Switched Telephone Networks) providers at digital interfaces that terminate in a DSU (Data Service Unit) on the customer side. They are not used when the service terminates in a modem, such as the DSL (Digital Subscriber Line) family of services. The maintenance capabilities of the CSU provide important guidance as to whether the provider needs to dispatch a repair technician to the customer location.

CSU/DSUs

Channel service units (CSUs) and data service units (DSUs) are required to interface with digital T-1, T-3, fractional T-1 and T-3, and 56 Kbps lines. CSU/DSUs are supplied in one integrated piece of equipment. The CSU plugs into the network jack. The DSU connects to the customer's equipment, such as the T-1 or E-1 multiplexer. CSU/DSUs are generally cards within multiplexers, IADs (Integrated Access Devices), and PBXs rather than standalone, external devices. A CSU/DSU operates at Layer 1 (the physical layer) of the OSI model.

Maintenance and performance tests are done from the CSU/DSU to determine if a repair problem is in the equipment, the CSU/DSU, or the telephone line. The CSU also provides clocking and signal reshaping. The clocking function is responsible for sending out bits in an evenly timed fashion. If the clocking is off, the transmission will not work. In this case, the technician might say, "the line is slipping," or "the timing is off." The CSU also provides framing, in which the starting and ending points for each channel are set and monitored. The DSU makes sure the correct positive and negative voltages are present on the signals from the multiplexer to the CSU.

CSUs use mainly extended superframe (ESF) so that T-1 and T-3 circuits can be monitored while a line is in service. Network service providers have extended superframe CSU/DSUs in their networks. Earlier CSUs used superframe (SF), which required taking the circuit out of service for many maintenance functions.

CSU/DSUs are also made as separate physical products; CSUs and DSUs. Either or both may be part of a T-1 WAN card inserted into a data terminal equipment such as a router.

*In the picture below, the RJ-45 cable connects the CSU to the "Telcom Demarc" which would be the data service jack (network jack). The v.35 cable connects the DSU to the customer's equipment (in this case the router).

[Source: http://www.more.net/technical/netserv/routers/cisco1720/images/dsu-router-connection-w.jpg]

More on T-1 and Time Division Multiplexing

All T carrier signals (e.g., T-1, T-3, E-3) are based on time division multiplexing. Each device that communicates over a T-1 line is assigned a time slot. If there are eight telephones contending for a T-1 circuit, a time slot is saved for each telephone for the duration of the particular telephone call. For example, telephone 1 might be assigned slot A; telephone 2, slot B; and so forth. During pauses in voice conversations, the slot is not assigned to another computer. The assigned time slot is transmitted without any bits. This is why time division multiplexing is not an efficient way to use a WAN. Pauses in data transmission result in idle time slots. In a network with millions of time slots, this can result in many idle time slots and wasted bandwidth.
More on time division multiplexing: http://en.wikipedia.org/wiki/Time_division_multiplexing

Sale of Individual Wavelengths

As technologies mature and manufacturing costs decrease, technologies initially used only by carriers become cost effective for enterprises. The sale of individual wavelengths is an example. Providers now offer individual wavelengths of fiber-optic capacity to customers at speeds of, for example, 1.5, 2.5 and 10 gigabits per second. One application for wavelengths is disaster recovery for between two to five sites. If one site fails, the other sites will still be in service for functions such as call centers and financial transactions. Enterprises also use individual wavelengths for connections to data centers and SANs in the same metro area.

Because access to LD (Long Distance) networks often represents up to 40% of customers' LD costs, providers with fiber to large enterprises now offer individual wavelengths from enterprises to their LD provider. The enterprise leases the fiber and is able to send Ethernet or optical carrier (OC) type traffic to its LD provider. If fiber is available, this is often a less-costly option than purchasing access from incumbent telephone companies.

In addition, service providers used wavelengths between their metro fiber ring and the customer to provide Ethernet service. Because of distance limitations in the hundreds of kilofeet, wavelength service is a metro area service.

DS-1 & DS-3 Transmission Media

DS-1 is media agnostic. It can operate over any medium, including fiber, copper and microwave. DS-3, however, because of its higher speeds, requires fiber, terrestrial microwave, or satellite-based microwave.

Telephone companies use T-1 and T-3 over microwave for hard-to-cable areas such as across rivers and canyons, and cellular providers use microwave as one choice between antennas and mobile COs (Central Offices) in cellular carriers' networks.

T-1 on fiber is more reliable than T-1 over copper. Because of decreasing costs of fiber and the lower cost to maintain it, telephone companies deploy fiber to new sites. Fiber is now commonly in place directly to medium and large business premises. When fiber is brought into a user's premises, the end-user must supply the electricity for the equipment on which the fiber terminates. If there is no backup power, customers lose their T-1s and T-3s when they lose power.

Channel Banks

Channel banks are multiplexing devices that connect digital T-1 circuits to older analog private branch exchanges (PBXs) and key systems. Decoders within the channel bank perform coding and decoding functions of converting analog voice to digital and vice versa. All new PBXs and key systems are digital, as are the vast majority of central office switches, and channel banks are not required for T-1 connections on these systems.

Integrated Access Device (IAD)

An Integrated Access Device or IAD is a customer premise device that provides access to wide area networks and the Internet. Specifically, it aggregates multiple channels of information including voice and data across a single shared access link to a carrier or service provider POP (Point-of-Presence). The access link may be a T-1 line, a DSL connection, a CATV network, a broadband wireless link or a metro-Ethernet connection.

At the POP, the customer's aggregated information is typically directed into a multiservice provisioning platform (MSPP) which is a complex and expensive device that sits between customers and the core network. It manages traffic streams coming from customers and forwards those streams to the PSTN (Public Switched Telephone Network) or appropriate WAN (Wide Area Network e.g. ATM, FR or the Internet).

IADs have multiple functions, including T-1 service, security protection via firewalls, a point for remote monitoring, routing and frame relay access. IADs that are based on ATM (Asynchronous Transfer Mode) technology provide dynamic bandwidth allocation on the T-1. Dynamic bandwidth allocation dynamically shifts traffic on the T-1 so that customers don't have to permanently allocate a set number of channels for voice or data. If there are only five channels used for voice, the IAD automatically uses the rest of the "pipe" for data. However, if a sixth outgoing or incoming call is placed, data is shifted to the other channels. Traffic is dynamically reallocated within the entire T-1's capacity. Therefore, the customer uses the T-1 line more efficiently by, for example, having more capacity for data during times of the day or night when voice traffic is low.

CLECs (Competitive Local Exchange Carriers) were the first telephone companies to offer IADs to small organizations. This innovation greatly increased the affordability of T-1 for small customers by eliminating the need for a separate broadband connection for Internet access in addition to voice lines. RBOCs (Regional Bell Operating Companies) and smaller incumbents later adopted this strategy. Integrated access for voice and data on one T-1 benefits telephone companies that only need to provision a single four-wire (two-pair) copper line or fiber rather than multiple copper pairs for voice and data.

T Carrier Signaling and Framing

The total bandwidth of all DS-1 and higher circuits is higher than the sum of all of the channels, or 24 X 64,000, which equals 1,536,000. The total bandwidth of DS1 is 1,544,000 bps. On DS-1s, these extra 8,000 bits are used for signaling and framing. A frame is a grouping of bits with samples of user data from each of the 24 channels. Data from devices connected to the T-1 are samples 8,000 times per second, put into frames and sent sequentially on the T-1. Multiplexing on T-1 depends on strict adherence to timing - time division multiplexing.

T-3 or DS-3

DS-3 speed differ internationally:

The T-3 North American speed is 44 mb with 672 channels (28 x 24 = 672) - the equivalent of 28 T-1s

The Japanese flavor, J-3, has 32 mb with 480 channels - the equivalent of 20 T-1s

The E-3 speed is 34 mb over 480 channels - the equivalent of 16 E-1s

DS-3 is also offered at fractional speeds, however customers that need speeds of, for example, 10 mb or 100 mbps are starting to opt for Gigabit Ethernet because of its flexibility in upgrading speed without changing equipment and lower prices for higher speeds.

DS-3 applications: Internet Access and Private Networks
DS-3 service is used mainly for private networks and Internet access. An organization's small sites often access the Internet using the T-3 circuits located at headquarters. This is an example of a hub and spoke configuration in which the headquarters acts as the hub for smaller sites.
DS-3 is also used in large, private networks operated by conglomerates for data communications between sites. They use these links for access to corporate databases and updates on transactions in, for example, large organizations that supply transportation and delivery services. ISPs and small carriers also use T-3 to access the Internet and for connections to other carriers.

T-3 services, the equivalent of 28 T-1 circuits, start to cost less than multiple T-1s when customers have eight to ten T-1s at the same site.

T-1

T-1 - a multiplexing scheme that enables one circuit to carry 24 voice, video or data conversations at 64 kilobits per channel. T-1 is the most prevalent technology for Internet access and access lines connecting customers to their carriers' POPs (Points of Presence) for LD (Long Distance).

T-1 circuits operate at 1.54 mbps. The letters "DS" stand for digital signal level. DS-1 refers to the entire 1.54 mb T-1 line. The terms DS-1 and T-1 are used interchangeably. The US, Canada and Japan use the 1.54 flavor of T-1. The rest of the world used E-1, which operates at 2.048 mb with 32 channels - 30 channels for voice or data, one channel for signaling, and one channel for framing and remote maintenance. Organizations that run a T-1 from the US to an office in Europe need rate-adaptation equipment so that the carrier the US can connect the domestic T-1 to the European T-1.

DS-0 is the 56 kbps or 64 kbps speed of each of the 24 individual channels of the T-1 or E-1 circuit. The DS-0 speed of 64 kb is the same worldwide.

Fractional T-1
Customers who require more than 64 kbps but less than a full 1.54 mb T-1 or E-1 often opt for lower-priced, fractional T-1/E-1. Fractional T-1/E-1 is sold at speeds of, for example, 4 channels at 256, 8 channels at 512, and 12 channels at 768 kbps. Customers don't purchase fractional DS-1 at higher than 8 or 12 DS-0 channels because the price becomes equivalent to the cost of a full T-1.

Unchannelized T-1
T-1 circuits, or a portion of the T-1 circuit used for data, can be ordered from carriers in an unchannelized format. An unchannelized T-1 is a single pipe with more capacity for data than one broken up into 24 channels using T-1 multiplexing. With unchannelized service, the router performs the multiplexing. Unchannelized T-1 service is used for:

- Access to frame relay, the Internet and VPN service

- Packetized, compressed, digitized voice (i.e. VoIP)

- Dedicated lines between locations

Unlike voice carried in VoIP packets, voice carried on T-1 to the PSTN (Public Switched Telephone Network) must be channelized, put into DS-0 channels compatible with CO (Central Office) switches.

Local and Interexchange Channels

Rates for private lines are based on distance and speed. Higher-speed lines that run over longer distances cost more than slower, shorter circuits. A T-1 line at 1.544 mbps is less expensive than a T-3, 44 mbps line. Pricing for dedicated lines consists of two items:

Local channels - local channels run from a customer's premises to the carrier's equipment. One local channel is required at each end of the private line. A carrier with no fiber installed in the customer's area will lease the local channel from the incumbent telephone company or another carrier. Local channels are also referred to as local loops. Local channels are often supplied by the incumbent telephone company. Because of limited competition for this service, pricing on these short links is often close to the same price as the longer interexchange circuit.

Interexchange miles - Interexchange mileage is the portion of the circuit located within a carrier's network, to the egress point, where it leaves the carrier's switch. These are carrier's points of presence (POPs).

Network Topologies (How Sites Are Connected)

The term "topology" refers to the geometric shape of the physical connection of the lines in a network. The shape of the network, the configuration in which lines are connected to each other, impacts cost, reliability and accessibility. The following are network configurations:

• Point-to-point - one line connecting two locations


• Multipoint - one line connecting more than two sites together, also referred to as multidrop


• Star (hub and spoke) configuration - all locations connect to, or "hub into," a central site. PBXs and data switches in LANs are configured in star topologies. If the main location in a star configuration goes down, all nodes (locations) on the network are out of services.


• Mesh design - all points on the network, nodes, connect to each other in a flat or nonhierarchical manner. If one link in a mesh network is out of service, traffic can be rerouted over other links. Peer-to-peer networks for music sharing are examples of mesh networks. Most wireless community networks based on 802.11 technology use a form of mesh design called partial mesh in which access points with antennas are connected to each other. In partial mesh designs, not all end-user devised are connected to each other.

Image Source: http://upload.wikimedia.org/wikipedia/commons/9/96/NetworkTopologies.png


For more information about network topologies (and about more topologies) visit: http://en.wikipedia.org/wiki/Network_topologies.

Dedicated Private Lines

In telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between locations. In practice, dedicated, private lines may not be provided by a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guaranteed for more public systems. Such properties add a considerable premium to the price charged.

The number of costly private lines used by commercial organizations is decreasing. They are being replaced by VPNs, which are less costly to maintain and have lower monthly lease rates. However, large enterprises, utilities, and financial services organization still use high-speed private lines for high-speed, secure communications.

Factoids about private line services:

• Pricing - Private, dedicated links are priced at flat monthly fees. The fees are not based on minutes used or the amount of data transmitted

• Fixed routes - Dedicated lines are not flexible. Calls and data can only be sent between the fixed points to which the lines are connected. Thus, communications with a site not on the network is not possible

• Exclusive use - Dedicated, private lines can only be used by the organization that leases them

• Multi-service - Dedicated lines are suitable for transmission of video, voice and data. Voice, video and data can share the same dedicated services, or they can use completely different dedicated lines. Firms often lease T-3 lines that have 672 channels to tie locations together. They can use, for example, 24 of the paths for voice and the rest for data and video

• Fixed capacity - Dedicated services are leased or built with a fixed capacity or bandwidth. These speeds range from low 9,600 bps up to OC-3 (155 mbps) and ATM megabit speeds. They also include T-1 and T-3 and fractional T-1 and T-3 speeds

• Security - Dedicated lines provide secure transmissions. Organizations concerned about security may place encryption devices on both ends of dedicated services. The encryption device scrambles the transmission when it leaves the sending location and unscrambles it when it arrives at the receiving location

• Convenience of services - Private lines provide abbreviated dialing and other convenient features, e.g., 4- or 5-digit dialing between sites, one operator can answer calls for multiple locations, one voice mail system can be shared by multiple locations, one directory across locations

Metropolitan area networks (MANs) consist of private lines that connect buildings within a city or metropolitan area. Large hospitals transmit customer records, research files, and radiology images over MANs. Major univserties also use MANs. Dedicated services are available around the clock. This is cost effective for companies that use the dedicated lines for voice, video, and e-mail during the day and bulk data transmissions after hours.

Voice on Frame Relay

Some customers replace private lines with FR networks to carry voice traffic between sites. Voice is compressed so that it requires less bandwidth. Customers either add separate PVCs (Permanent Virtual Circuit) for voice or upgrade their CIR (Committed Information Rate) for extra capacity. For large organizations, defining separate PVCs are cumbersome. It is also costly. Moreover, it does not guarantee quality on the access line between the customer and the frame network. These frames do not have fields capable of indicating priority levels.

Because of the quality issue, some customers do not use FR for customers' calls, only for employee-to-employee calls. If the FR network becomes congested, voice quality can be degraded because packets are dropped or delayed even with higher CIRs.

The desire to add voice between sites is another factor in customers' migration to MPLS (Multiprotocol Label Switching) because of this extra expense to upgrade FR service and the lack of prioritization on the access line. This is particularly true for larger organizations that want to send customer traffic between sites.

Frame Relay - Permanent Virtual Circuits and Committed Information Rate

Frame relay service is priced at fixed monthly fees based on the following elements, plus the cost of the access line used to connect each site to the carrier's frame relay equipment:

- The permanent virtual circuit (PVC) is a logical, predefined path or link through a carrier's network. If sites at Point A and Point B need to exchange data, the carrier defines a permanent virtual circuit between these two locations. PVCs are priced at fixed monthly fees.

- The frame relay port is the entry point, on a FR provider's switch, to the FR network. Multiple PVCs can use one port. Ports are available to variable speeds such as T-1, 56Kbps, 256Kbps and 512 Kbps.

- The committed information rate (CIR) is the minimum number of bits per second, perhaps half the capacity of the port, that the customer is guaranteed to be able to send from each site. Some customers save money by using low committed information rates. Customers can "burst," send data at the maximum speed of their FR port, if bandwidth is available.

Frame Relay Access to Other Networks

Frame relay is an access technology in which customers' packets are put into frames. In addition to LAN-to-LAN connectivity, it is used to access the following types of networks:

• Frame relay networks that carry traffic on asynchronous transfer mode (ATM) switches
• MPLS (Multiprotocol Label Switching) virtual private networks
• The public Internet

Equipment on customer premises that converts Ethernet local area network packets into frames is called a Frame Relay access device (FRAD). It is often a card within the router. Each frame has bits called the flag, telling the network when the user data (frame) starts and when it ends. There also are addressing and destination bits in the frame for billing and routing purposes so that the FR provider knows where to route and bill each frame.

Customers' frames are sent to ports on the carrier's network. Routers at the carrier's central office convert the customer data to a format compatible with the carrier's network and send it to the core network.

Frame Relay Access Line

The line that connects each customer to the Frame Relay network is called an access line. It provides access from the user's router to the FR network. Each site that uses the FR service leases a circuit, a telephone line, from its equipment to a port on the FR switch. Access line speeds vary from 56 Kbps to subrate T-1 speeds (that is, 128, 156, and 384 Kbps) and T-1 all the way up to T-3 (44 Mbps).

Sites at different locations in the same organizations can be configured with access lines at different speeds. Some FR vendors also offer dial-up (for example, ISDN) access to their networks, most often as a backup to their dedicated access in case the dedicated access lines to the FR network fail. Higher-speed access lines cost more than lower-speed ones.

To save money on access lines, smaller customers share their T-1 lines for voice and FR access. For example, 18 channels of the 24 T-1 channels may be connected to the telephone system for voice traffic. The other six channels carry FR traffic to the network service provider's FR port.

Frame Relay

A public network offering that enables customers to transmit data between LANs at multiple locations. It also is used to access the Internet. Frame relay (FR) was first promoted as a lower-priced substitute for private lines. By using FR, organizations do not have to plan, build, and maintain their own duplicate paths to each of their sites. Multiple users share the FR networks. It is offered by local and long distance (LD) telephone companies.

Frame relay's requirement of defining links between sites in advance, which can be cumbersome for large organizations that want to connect each site to every other site, is leading some companies to choose MPLS VPNs. Falling prices on MPLS service and frame relay's unsuitability for voice are other reasons organizations are starting to migrate to MPLS. However, FR is still a popular, cost-effective choice for organizations.

Source: http://upload.wikimedia.org/wikipedia/en/2/2e/Frame_relay.jpg