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.