
A digital subscriber line access multiplexer (DSLAM, often pronounced ''DEE-slam'') is a network device, often located in
telephone exchange
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
s, that connects multiple customer
digital subscriber line
Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...
(DSL) interfaces to a high-speed digital communications channel using
multiplexing techniques. Its
cable internet (
DOCSIS) counterpart is the
cable modem termination system.
Path taken by data to DSLAM
# Customer premises:
DSL modem
A digital subscriber line (DSL) modem is a device used to connect a computer or Router (computing), router to a telephone line which provides the digital subscriber line (DSL) service for connection to the Internet, which is often called ''DSL ...
terminating the
ADSL
Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over Copper wire, copper telephone lines than a conventional voiceband modem ...
,
SHDSL or
VDSL circuit and providing a
LAN or interface to a single computer or LAN segment.
#
Local loop: the telephone company wires from a customer to the telephone exchange or to a
serving area interface, often called the "
last mile" (LM).
#
Telephone exchange
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
:
#*
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 telephone exchange to split voice from data signals. The voice signal can be routed to a
plain old telephone service
Plain old telephone service (POTS), or publicly offered telephone service, is basic Voice band, voice-grade telephone service. Historically, POTS has been delivered by Analog signal, analog signal transmission over copper loops, but the term also d ...
(POTS) provider, digital
telephone exchange
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
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

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
Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and International Telecommunication Union Telecommunication Standardization Sector (ITU-T, formerly CCITT) for digital trans ...
(ATM),
Frame Relay
Frame Relay (FR) is a standardized wide area network (WAN) technology that specifies the Physical layer, physical and data link layers of digital telecommunications channels using a packet switching methodology.
Frame Relay was originally devel ...
, and/or
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
IP ...
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 access network is a type of telecommunications telecommunications network, network which connects subscribers to their immediate telecommunications service provider, service provider. It is contrasted with the core network, which connects l ...
(AN), also called a
Network Service Provider (NSP), at up to 10
Gbit/s data rates.
The DSLAM acts like a
network switch
A network switch (also called switching hub, bridging hub, Ethernet switch, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destinat ...
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
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
.
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
A digital subscriber line (DSL) modem is a device used to connect a computer or Router (computing), router to a telephone line which provides the digital subscriber line (DSL) service for connection to the Internet, which is often called ''DSL ...
. This modem functionality is integrated into the DSLAM itself instead of being done via individual external devices like 20th-century voiceband
modem
The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
s 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
Twisted pair cabling is a type of communications cable in which two conductors of a single Electronic circuit, circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a Single-ended signaling, sin ...
(UTP)
Ethernet
Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
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
Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
) 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.
Bandwidth versus distance
Balanced pair cable has higher
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...
at higher frequencies. Therefore, the longer the wire between DSLAM and subscriber, the slower the maximum possible data rate due to the lower frequencies being used to limit the total attenuation (or due to the higher number of errors at higher frequencies, effectively lowering the overall frequency/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 10,000 feet (~3 km)
* 3 Mbit/s at 15,000 feet (4.5 km)
* 1.5 Mbit/s at 17,000 feet (~5.2 km)
Hardware details
Customers connect to the DSLAM through
ADSL modem
A digital subscriber line (DSL) modem is a device used to connect a computer or Router (computing), router to a telephone line which provides the digital subscriber line (DSL) service for connection to the Internet, which is often called ''DSL ...
s or DSL
routers, which are connected to the
PSTN
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists ...
network via typical
unshielded twisted pair
Twisted pair cabling is a type of communications cable in which two conductors of a single Electronic circuit, circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a Single-ended signaling, sin ...
telephone lines. Each DSLAM has multiple aggregation cards, and each such card can have multiple
ports Ports collections (or ports trees, or just ports) are the sets of makefiles and Patch (Unix), patches provided by the BSD-based operating systems, FreeBSD, NetBSD, and OpenBSD, as a simple method of installing software or creating binary packages. T ...
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
DC. Hence a typical DSLAM setup may contain power converters, DSLAM chassis, aggregation cards, cabling, and upstream links.
On the upstream trunk (ISP) side many early DSLAMs used
ATM—and this approach was standardized by the
DSL Forum—with
Gigabit Ethernet
In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use in ...
support appearing sometime later.
Today, the most common upstream links in these DSLAMs use
Gigabit Ethernet
In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use in ...
or multi-gigabit
fiber optic links.
IP-DSLAM
IP-DSLAM stands for ''
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
IP ...
Digital Subscriber Line Access Multiplexer''. User traffic is mostly IP based.
Traditional 20th century DSLAMs used
Asynchronous Transfer Mode
Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and International Telecommunication Union Telecommunication Standardization Sector (ITU-T, formerly CCITT) for digital trans ...
(ATM) technology to connect to upstream ATM routers/switches. The DSLAM simply extracted the ATM signals from the DSL signal, and passed the ATM signal to ATM routers, which then extract the IP traffic and pass it on to an IP router in an IP network. This division of work was thought to be sensible because DSL itself is based on ATM, and could theoretically carry data other than IP in that ATM stream. In contrast, an IP-DSLAM extracts the IP traffic in the DSLAM itself and passes it on to an IP router. The advantages of IP-DSLAM over a traditional ATM DSLAM are that the merged equipment is less expensive to make and operate and can offer a richer set of features.
See also
*
Asymmetric digital subscriber line (ADSL)
*
Internet access
Internet access is a facility or service that provides connectivity for a computer, a computer network, or other network device to the Internet, and for individuals or organizations to access or use applications such as email and the World Wide ...
*
Broadband remote access server (BRAS)
*
ISDN digital subscriber line (IDSL)
*
Multi-service access node (MSAN)
*
Symmetric digital subscriber line (SDSL)
*
Single-pair high-speed digital subscriber line (SHDSL)
*
Triple play (telecommunications)
*
Very-high-bit-rate digital subscriber line (VDSL)
*
Very-high-bit-rate digital subscriber line 2 (VDSL2)
References
{{DSL technologies
Multiplexing
Digital subscriber line