Digital Subscriber Line Access Multiplexer

Digital Subscriber Line Access Multiplexer
Siemens DSLAM SURPASS hiX 5625

A Digital Subscriber Line Access Multiplexer (DSLAM, often pronounced dee-slam) is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer Digital Subscriber Line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques.[1] By placing additional remote DSLAMs at locations remote to the telephone exchange, telephone companies provide DSL service to locations previously beyond effective range.


Path taken by data to DSLAM

  1. Customer premises: DSL modem terminating the ADSL, SHDSL or VDSL circuit and providing LAN interface to single computer or LAN segment
  2. Local loop: the telephone company wires from a customer to the telephone company's central office or to a Serving area interface, often called the "last mile" (LM).
  3. Central Office (CO):
    • Main Distribution Frame (MDF): a wiring rack that connects outside subscriber lines with internal lines. It is used to connect public or private lines coming into the building to internal networks. At the telco, the MDF is generally in proximity to the cable vault and not far from the telephone switch.
    • xDSL filters: DSL filters are used in the Central Office (CO) to split voice from data signals. The voice signal can be routed to a POTS provider or left unused whilst the data signal is routed to the ISP DSLAM via the HDF (see next entry).
    • Handover Distribution Frame (HDF): a distribution frame that connects the last mile provider with the service provider's DSLAM
    • DSLAM: a device for DSL service. The DSLAM port where the subscriber local loop is connected converts analog electrical signals to data traffic (upstream traffic for data upload) and data traffic to analog electrical signals (downstream for data download).

Role of the DSLAM

xDSL Connectivity diagram

The DSLAM equipment at the telephone company (telco) 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 PTM-TC [Packet Transfer Mode - Transmission Convergence]) 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 Layer 2 of the OSI model. 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 company's central office, and may also serve multiple data and voice customers within a neighborhood Serving Area Interface (SAI), 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 an external device like a traditional computer modem. Like traditional voice-band modems, a DSLAM's integrated DSL modems 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, 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 is more costly).

Bandwidth versus distance

Balanced pair cable has higher attenuation at higher frequencies, hence the longer the wire between DSLAM and subscriber, the slower the maximum possible data rate. The following is a rough guide to the relation between wire distance (based on 0.40 mm copper and ADSL2+ technology) and maximum data rate. Local conditions may vary, especially beyond 2 km, often necessitating a closer DSLAM to bring acceptable bandwidths:

  • 25 Mbit/s at 1,000 feet (~300 m)
  • 24 Mbit/s at 2,000 feet (~600 m)
  • 23 Mbit/s at 3,000 feet (~900 m)
  • 22 Mbit/s at 4,000 feet (~1.2 km)
  • 21 Mbit/s at 5,000 feet (~1.5 km)
  • 19 Mbit/s at 6,000 feet (~1.8 km)
  • 16 Mbit/s at 7,000 feet (~2.1 km)
  • 8 Mbit/s at 3 km
  • 1.5 Mbit/s at 15,000 feet (4.5 km)
  • 800 kbit/s at 17,000 feet (~5.2 km)

Additional features

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 DSLAMs 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.

Hardware details

Customers 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 has multiple aggregation cards, and each such card can have multiple ports to which the customers lines are connected. Typically a single DSLAM aggregation card has 24 ports, but this number can vary with each manufacturer. The most common DSLAMs are housed in a telco-grade chassis, which are supplied with (nominal) 48 volts using DC. Hence a typical DSLAM setup may contain power converters, DSLAM chassis, aggregation cards, cabling, and upstream links. The most common upstream links in these DSLAMs use gigabit ethernet or multi-gigabit fiber optic links.


IP-DSLAM stands for Internet Protocol Digital Subscriber Line Access Multiplexer. User traffic is mostly IP based.

Traditional 20th century DSLAM used Asynchronous Transfer Mode (ATM) technology to connect to upstream ATM routers/switches. These devices then extract the IP traffic and pass it on to an IP network. IP-DSLAMs extract the IP traffic at the DSLAM itself. Thus, it is all IP from there. The advantage of IP-DSLAM over a traditional ATM DSLAM is in terms of lower capital expenditure and operational expenditure and a richer set of features and functionality.

See also


External links

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