Digital subscriber line access multiplexer
Encyclopedia
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s of the telecommunications operators. It connects multiple customer digital subscriber line
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
(DSL) interfaces to a high-speed digital communications channel using multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
techniques. By placing additional DSLAMs at locations remote to the telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
, telephone companies
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
provide DSL service to locations previously beyond effective range.
Path taken by data to DSLAM
- Customer premises: DSL modem terminating the ADSL, SHDSL or VDSL circuit and providing LAN interface to single computer or LAN segment
- Local loopLocal loopIn telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
: the telephone company wires from a customer to the telephone exchange or to a serving area interfaceServing area interfaceThe serving area interface or service area interface often called B-box, cross-connect box, cross box, ped , or access point is an outdoor telecommunications cabinet usually mounted on the ground on cable right-of-ways, but sometimes also on telephone poles...
, often called the "last mileLast mileThe "last mile" or "last kilometer" is the final leg of delivering connectivity from a communications provider to a customer. The phrase is therefore often used by the telecommunications and cable television industries. The actual distance of this leg may be considerably more than a mile,...
" (LM). - Telephone exchangeTelephone exchangeIn the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
:- Main distribution frameMain distribution frameIn telephony, a main distribution frame is a signal distribution frame for connecting equipment to cables and subscriber carrier equipment . The MDF is a termination point within the local telephone exchange where exchange equipment and terminations of local loops are connected by jumper wires...
(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 filtersDSL filterA DSL filter is an analog low-pass filter installed between analog devices and a plain old telephone service telephone line, in order to prevent interference between such devices and a digital subscriber line service operating on the same line...
: DSL filters are used in the telephone exchange to split voice from data signals. The voice signal can be routed to a POTSPlain old telephone servicePlain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
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 loopLocal loopIn telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
is connected converts analog electrical signals to data traffic (upstreamUpstream (networking)In computer networking, upstream refers to the direction in which data can be transferred from the client to the server . This differs greatly from downstream not only in theory and usage, but also in that upstream speeds are usually at a premium...
traffic for data upload) and data traffic to analog electrical signals (downstream for data download).
- Main distribution frame
Role of the DSLAM
The DSLAM equipment collects the data from its many modem ports and aggregates their voice and data traffic into one complex composite "signal" via multiplexingMultiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
. Depending on its device architecture and setup, a DSLAM aggregates the DSL lines over its Asynchronous Transfer Mode
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM), frame relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
, and/or Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
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
Backbone network
A backbone network or network backbone is a part of computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different...
switch, via an access network
Access network
An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...
(AN) also called a Network Service Provider
Network service provider
A network service provider is a business or organization that sells bandwidth or network access by providing direct backbone access to the Internet and usually access to its network access points...
(NSP) at up to 10 Gbit/s data rates.
The DSLAM acts like a network switch
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
since its functionality is at Layer 2
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. 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
Broadband Remote Access Server
A broadband remote access server routes traffic to and from broadband remote access devices such as digital subscriber line access multiplexers on an Internet service provider's network....
where the end user traffic is then routed across the ISP network to the Internet. Customer-premises equipment
Customer-premises equipment
Customer-premises equipment or customer-provided equipment is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication channel at the demarcation point...
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
Serving area interface
The serving area interface or service area interface often called B-box, cross-connect box, cross box, ped , or access point is an outdoor telecommunications cabinet usually mounted on the ground on cable right-of-ways, but sometimes also on telephone poles...
, sometimes in conjunction with a digital loop carrier
Digital loop carrier
A digital loop carrier is a system which uses digital transmission to extend the range of the local loop farther than would be possible using only twisted pair copper wires...
. DSLAMs are also used by hotels, lodges, residential neighborhoods, and other businesses operating their own private telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
.
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
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
. 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 echo
Forward echo
Forward echo: In a transmission line, a reflection propagating in the same direction as the original wave and consisting of energy reflected back by one discontinuity and then forward again by another discontinuity. Forward echoes can be supported by reflections caused by splices or other...
es 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
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
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 attenuationAttenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
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 VLANVirtual LAN
A virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
traffic as it passes from the subscribers to upstream routers. Though not a full stateful firewall
Stateful firewall
In computing, a stateful firewall is a firewall that keeps track of the state of network connections traveling across it. The firewall is programmed to distinguish legitimate packets for different types of connections...
, some DSLAMs also offer packet filtering facilities like dropping inter-port traffic and dropping certain protocols
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
.
A DSLAM may also support quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(QoS) features like contention
Contention
Contention may refer to:* The main contention, in rhetoric, the main point being argued* Resource contention, a general concept in communications and computing, is competition by users of a system for the facility at the same time:...
, differentiated services
Differentiated services
Differentiated Services or DiffServ is a computer networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying and managing network traffic and providing Quality of Service on modern IP networks...
("DiffServ") and priority queue
Priority queue
A priority queue is an abstract data type in computer programming.It is exactly like a regular queue or stack data structure, but additionally, each element is associated with a "priority"....
s.
Hardware details
Customers connect to the DSLAM through ADSL modems or DSL routers, which are connected to the PSTNPublic switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
network via typical unshielded twisted pair telephone lines. Each DSLAM has multiple aggregation cards, and each such card can have multiple ports
Computer port (hardware)
In computer hardware, a port serves as an interface between the computer and other computers or peripheral devices. Physically, a port is a specialized outlet on a piece of equipment to which a plug or cable connects...
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
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
-grade chassis, which are supplied with (nominal) 48 volts DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
. 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
Gigabit Ethernet
Gigabit Ethernet is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second , as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks where it performed...
or multi-gigabit fiber optic
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
links.
IP-DSLAM
IPInternet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
-DSLAM stands for Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
Digital Subscriber Line Access Multiplexer. User traffic is mostly IP based.
Traditional 20th century DSLAM used Asynchronous Transfer Mode
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(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
Capital expenditure
Capital expenditures are expenditures creating future benefits. A capital expenditure is incurred when a business spends money either to buy fixed assets or to add to the value of an existing fixed asset with a useful life extending beyond the taxable year...
and operational expenditure
Operating expense
An operating expense, operating expenditure, operational expense, operational expenditure or OPEX is an ongoing cost for running a product, business, or system . Its counterpart, a capital expenditure , is the cost of developing or providing non-consumable parts for the product or system...
and a richer set of features and functionality.
See also
- Asymmetric digital subscriber lineAsymmetric Digital Subscriber LineAsymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
(ADSL) - Broadband Internet accessBroadband Internet accessBroadband Internet access, often shortened to just "broadband", is a high data rate, low-latency connection to the Internet— typically contrasted with dial-up access using a 56 kbit/s modem or satellite Internet with inherently high latency....
- Broadband Remote Access ServerBroadband Remote Access ServerA broadband remote access server routes traffic to and from broadband remote access devices such as digital subscriber line access multiplexers on an Internet service provider's network....
(BRAS) - Cable modem termination systemCable modem termination systemA cable modem termination system or CMTS is a piece of equipment typically located in a cable company's headend or hubsite, and used to provide high speed data services, such as cable Internet or voice over Internet Protocol, to cable subscribers...
analogous device for CATV - ISDN digital subscriber line (IDSL)
- Symmetric digital subscriber lineSymmetric Digital Subscriber LineSymmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...
(SDSL) - Symmetric high-speed digital subscriber line (SHDSL)
- Triple play (telecommunications)Triple play (telecommunications)In telecommunications, triple play service is a marketing term for the provisioning of two bandwidth-intensive services, high-speed Internet access and television, and a less bandwidth-demanding service, telephone, over a single broadband connection. Triple play focuses on a combined business...
- Very-high-bit-rate digital subscriber line (VDSL)
- Very-high-bitrate digital subscriber line 2 (VDSL2)