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DSLAM Systems

A Digital Subscriber Line Access Multiplexer (DSLAM) allows telephone lines to make faster connections to the Internet.

It’s a network device that connects multiple Digital Subscriber Lines (DSL) to a high-speed Internet backbone line using multiplexing techniques.

DSLAM is a device for DSL service:

  • Sending to the customer or downstream side, it intermix voice and data traffics on the customer’s DSL line.
  • Receiving from the customer or upstream side, it accepts and separates outgoing phone and data signals.
  • It directs the data signals upstream to the appropriate carrier’s network, and the phone signals to the voice switch.

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.

DSLAM systems are used by hotels, lodges, and residential neighborhoods and also by other businesses that operates their own private telephone exchange system.

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.

Like traditional voice-band modems, a DSLAM integrated DSL modem usually have the ability 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 connection rate capability of the subscriber’s physical line.

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 – in fact, a balanced line type is generally required for LAN connections longer than 90 m 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 is more costly).

A DSLAM may offer the ability to tag VLAN traffic as it passes from the subscribers to upstream routers. Though not a full stateful firewall, some DSLAM also offer packet filtering facilities like dropping inter-port traffic and dropping certain protocols.

A DSLAM may also support quality of service (QoS) features like contention, differentiated services (“DiffServ”) and priority queues.

Users connect to the DSLAM through ADSL modems or DSL routers, which are connected to the PSTN network via typical unshielded twisted pair telephone lines.

Each DSLAM system has multiple aggregation cards, and each such card can have multiple ports to which the customer’s lines are connected.

Typically a single DSLAM aggregation card has 24 ports, but this number can vary with each manufacturer.