Digital subscriber line access multiplexer
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A Digital Subscriber Line Access Multiplexer (DSLAM, often pronounced dee-slam) allows telephone lines to make faster connections to the Internet. It is a network device, located in the telephone exchanges of the service providers, that connects multiple customer Digital Subscriber Lines (DSLs) to a high-speed Internet backbone line using multiplexing techniques.[1] By placing remote DSLAMs at locations remote to the telephone exchange, telephone companies provide DSL service to locations previously beyond effective range.
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[edit] 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 loop: the telephone company wires from a customer to the telephone company's central office, often called the "last mile" (LM).
- DSLAM: a device for DSL service. Sending on the customer or downstream side, it intermixes voice traffic and VDSL traffic on 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 to the appropriate carrier's network, and the phone signals to 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 the cable vault and not far from the telephone switch.
- Handover Distribution Frame (HDF): a distribution frame that connects the last mile provider with the service providers DSLAM
[edit] Role of the DSLAM
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 on 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. Therefore it cannot re-route traffic between multiple IP networks, only between ISP devices and end-user connection points. 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).
[edit] Speed 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)
[edit] 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.
The DSLAM also supports quality of service (QoS) features like contention, differentiated services ("DiffServ") and priority queues.
[edit] 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 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.
[edit] 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 lower capital expenditure and operational expenditure and a richer set of features and functionality.
[edit] See also
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- 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)
[edit] References
- ^ "Digital Subscriber Line Access Multiplexer (DSLAM)". iec.org. http://www.iec.org/online/tutorials/dslam/. Retrieved on 2008-02-16.
