- Digital subscriber line access multiplexer
A Digital Subscriber Line Access Multiplexer (DSLAM, often pronounced "dee-slam") allows
telephone lines to make faster connections to theInternet . It is a network device, located in the telephony exchanges of the service providers, that connects multiple customerDigital Subscriber Line s (DSLs) to a high-speedInternet backbone line usingmultiplexing techniques. [citeweb|title=Digital Subscriber Line Access Multiplexer (DSLAM)|url=http://www.iec.org/online/tutorials/dslam/|publisher=iec.org|accessdate=2008-02-16] By placing "remote DSLAMs" at locations remote to the telephone company central office (CO), telephone companies provide DSL service to locations previously beyond effective range.Path taken by data to DSLAM
# Residential/commercial source:
DSL modem plugged into the customer's computer.
#Local loop : the telephone company wires from a customer to the telephone company's central office, often called the "last mile".
# DSLAM: a device forDSL service. Sending on the customer or downstream side, it intermixes voice traffic andVDSL traffic onto the customer's DSL line. Receiving on that side, it accepts and separates outgoing phone and data signals from the customer. It directs the data signals upstream towards the appropriate carrier's network, and the phone signals towards the voice switch.
#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 thecable vault and not far from the telephone switch.Role of the DSLAM
The DSLAM equipment at the telephone company ("telco") collects the
digital signal s from its many modem ports and combines them into one signal viamultiplexing . Depending on the product being used, a DSLAM would aggregate the DSL lines with some combination ofAsynchronous Transfer Mode (ATM),frame relay , orInternet Protocol networks (i.e., IP-DSLAM that uses thePTM-TC stack)(Packet Transfer Mode - Transmission Convergence).The aggregated signal is then loaded onto the telco's backbone switching equipment, traveling through an
access network (AN)—also known as a Network Service Provider (NSP)—at speeds of up to 10Gbit/s and connecting to the Internet-backbone.In terms of the OSI 7 Layer Model, the DSLAM acts like a massive
network switch since its functionality is purely Layer 2.A DSLAM is not always located in the telephone company's
central office , but may also serve customers within a neighborhood "Serving Area Interface " (SAI), sometimes in association with adigital loop carrier . DSLAMs are also used by hotels, lodges, residential neighbourhoods, and other corporations setting up their own privatetelephone exchange .Besides being a data switch and multiplexer, a DSLAM is also a collection of modems. Each modem on the aggregation card communicates with a subscriber's
DSL modem . The modem function is integrated into the DSLAM itself, rather than being separate hardware like a traditional computermodem . Like traditional, voice-band modems, the integrated DSL modem has the ability to probe the line and train itself to compensate forforward echo es and other impairments in order to move data at the maximum rate the telephone line allows. This is also whytwisted pair DSL services have a longer range than physically similar unshielded twisted pair (UTP)Ethernet .peed 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 maximum data rate. Local conditions may vary, especially beyond 2 km, often necessitating a closer DSLAM to bring acceptable speeds:*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 or ~.95 miles)
*19 Mbit/s at 6,000 feet (~1.8 km or ~1.14 miles)
*16 Mbit/s at 7,000 feet (~2.1 km or ~1.33 miles)
*1.5 Mbit/s at 15,000 feet (4.5 km or ~2.8 miles)
*800 kbit/s at 17,000 feet (~5.2 km or ~3.2 miles)Additional features
A DSLAM may offer the ability to tag VLAN traffic as it passes from the subscribers to upstream
router s. Though not a fullstateful firewall , some DSLAMs also offerpacket filtering facilities like dropping inter-port traffic and dropping certain protocols.The DSLAM also supports
quality of service (QoS) features likecontention ,differentiated services ("DiffServ") andpriority queue s.Hardware details
Customers connect to the DSLAM through
ADSL modem s or DSLrouter s, which are connected to the PSTN network via typicalunshielded 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) 48volt s 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
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. Advantage of IP-DSLAM over a traditional ATM DSLAM is in terms of lowercapital expenditure and operational expenditure and a richer set of features and functionality.ee also
*
Asymmetric Digital Subscriber Line (ADSL)
*Broadband Internet access
*Broadband Internet access worldwide
*Broadband Remote Access Server (BRAS)
*Cable modem termination system analogous device for CATV
*ISDN Digital Subscriber Line (IDSL)
*Symmetric Digital Subscriber Line (SDSL)
*Symmetric High-speed Digital Subscriber Line (SHDSL)
*Triple play (telecommunications) References
External links
*
Google Directory : [http://www.google.com/Top/Computers/Data_Communications/DSL/Vendors/ DSL Vendors]
* [http://www.convergedigest.com/blueprints/ttp03/z2critical1.asp?ID=38&ctgy=Loop Technical whitepaper on merging fiber with DSLAM]
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