数字用户线接入复用设备

目录·数据通往DSLAM的路线
·DSLAM扮演的角色
·Speed versus distance
·Additional features
·Hardware details
·IP-DSLAM
·See also
·References
·External links








Siemens DSLAM SURPASS hiX 5625


数字用户线接入复用设备(DSLAM,英文全称Digital Subscriber Line Access Multiplexer )通过电话线提供高速因特网接入服务。DSLAM是一种安装在近客户端的网络设备,利用多工技术将大量用户的DSL线路连接至高速骨干网络。[1] 电信公司通过将DSLAM安装在离电话交换机较远距离上,让那些原本因为离电话交换机的距离超出DSL最大工作距离的客户,得以使用DSL技术。



数据通往DSLAM的路线


住宅/商业 数据源:客户的电脑连接了DSL调制解调器。
本地环线:电信公司的电线从本地连接至电话总局,这段距离经常被称为“最后一公里”
DSLAM:从客户(下行)发送数据,这些数据可能同时混杂语音和VDSL信号,DSLAM负责将数据信号上行至适当的载波网络、将语音信号传送到交换机。
主配电板 (MDF):将外部DSL线路与内部线路连接在一起的配线架。在通信业,主配电板基本上与电缆室相同,距离电话交换机亦不远。


DSLAM扮演的角色

The DSLAM equipment at the telephone company (telco) collects the digital signals from its many modem ports and combines them into one signal via multiplexing. Depending on the product being used, a DSLAM would aggregate the DSL lines with some combination of Asynchronous Transfer Mode (ATM), frame relay, or Internet Protocol networks (i.e., IP-DSLAM that uses the PTM-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 10 Gbit/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 a digital loop carrier. DSLAMs are also used by hotels, lodges, residential neighbourhoods, and other corporations setting up their own private telephone 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 computer modem. Like traditional, voice-band modems, the integrated DSL modem has the ability to probe the line and train itself to compensate for forward echoes and other impairments in order to move data at the maximum rate the telephone line allows. This is also why twisted pair DSL services have a longer range than physically similar unshielded twisted pair (UTP) Ethernet.

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 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)
Based on 0.40mm copper.


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.

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.

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.

See 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



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External links


Google Directory: DSL Vendors
Technical whitepaper on merging fiber with DSLAM





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