DSLAM

A digital subscriber line access multiplexer (DSLAM, often pronounced DEE-slam) is a network switch often located in telephone exchanges, that multiplexes multiple downstream links from digital subscriber line (DSL) customers interfaces to an upstream interface. Its cable internet (DOCSIS) counterpart is the cable modem termination system.

The DSLAM equipment collects the data from its many modem ports and aggregates their voice and data traffic into one complex composite "signal" via multiplexing. Depending on its device architecture and setup, a DSLAM aggregates the DSL lines over its Asynchronous Transfer Mode (ATM), Frame Relay, and/or Internet Protocol network, i.e., an IP-DSLAM using Packet Transfer Mode - Transmission Convergence (PTM-TC) protocol(s) stack.

The aggregated traffic is then directed to a telco's backbone switch, via an access network (AN), also called a Network Service Provider (NSP), at up to 10 Gbit/s data rates.

The DSLAM acts like a network switch since its functionality is at the link layer. Therefore, it cannot re-route traffic between multiple IP networks, only between ISP devices and end-user connection points. The DSLAM traffic is switched to a Broadband Remote Access Server where the end-user traffic is then routed across the ISP network to the Internet. Customer-premises equipment that interfaces well with the DSLAM to which it is connected may take advantage of enhanced telephone voice and data line signaling features and the bandwidth monitoring and compensation capabilities it supports.

A DSLAM may or may not be located in the telephone exchange, and may also serve multiple data and voice customers within a neighborhood serving area interface, sometimes in conjunction with a digital loop carrier. DSLAMs are also used by hotels, lodges, residential neighborhoods, and other businesses operating their own private telephone exchange.

In addition to being a data switch and multiplexer, a DSLAM is also a large collection of modems. Each modem on the aggregation card communicates with a single subscriber's DSL modem. This modem functionality is integrated into the DSLAM itself instead of being done via individual external devices like 20th-century voiceband modems used for dial-up internet.

Like traditional voice-band modems, a DSLAM's integrated DSL modems are usually able to probe the line and to adjust themselves to electronically or digitally compensate for forward echoes and other bandwidth-limiting factors in order to move data at the maximum possible connection rate.

This compensation capability also takes advantage of the better performance of "balanced line" DSL connections, providing capabilities for LAN segments longer than physically similar unshielded twisted pair (UTP) Ethernet connections, since the balanced line type is generally required for its hardware to function correctly. This is due to the nominal line impedance (measured in Ohms but comprising both resistance and inductance) of balanced lines being somewhat lower than that of UTP, thus supporting 'weaker' signals (however the solid-state electronics required to construct such digital interfaces are more costly). Many early DSLAMs required a separate splitter rack to split POTS service from ADSL service, but newer DSLAMs can be connected directly to phone lines as they have built-in splitters.