26-11-2012, 01:16 PM
Asymmetric Digital Subscriber Line (ADSL)
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A gateway is commonly used to make an ADSL connection. The model pictured is also a wireless access point, hence the antenna.
Asymmetric Digital Subscriber Line (ADSL) 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 telephone call.[1] A splitter, or DSL filter, allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office, typically less than 4 kilometres (2 mi),[2] but has been known to exceed 8 kilometres (5 mi) if the originally laid wire gauge allows for further distribution.
At the telephone exchange the line generally terminates at a Digital Subscriber Line Access Multiplexer (DSLAM) where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL are typically routed over the telephone company's data network and eventually reach a conventional Internet Protocol network.
Overview
ADSL differs from the less common symmetric digital subscriber line (SDSL) in that bandwidth (and bit rate) is greater toward the customer premises (known as downstream) than the reverse (known as upstream). This is why it is called asymmetric. Providers usually market ADSL as a service for consumers to provide Internet access in a relatively passive mode: able to use the higher speed direction for the download from the Internet but not needing to run servers that would require high speed in the other direction.
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the Digital Subscriber Line Access Multiplexer (DSLAM) end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL. On the marketing side, limiting upload speeds limits the attractiveness of this service to business customers, often causing them to purchase higher cost leased line services instead. In this fashion, it segments the digital communications market between business and home users.
Operation
Currently, most ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplex (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user.
Interleaving and fastpath
ISPs (rarely, users) have the option to use interleaving of packets to counter the effects of burst noise on the telephone line. An interleaved line has a depth, usually 8 to 64, which describes how many Reed-Solomon codewords are accumulated before they are sent. As they can all be sent together, their forward error correction codes can be made more resilient. Interleaving adds latency as all the packets have to first be gathered (or replaced by empty packets) and they, of course, all take time to transmit. 8 frame interleaving adds 5 ms round-trip-time (RTT), while 64 deep interleaving adds 25 ms RTT. Other possible depths are 16 and 32.
"Fastpath" connections have an interleaving depth of 1, that is one packet is sent at a time. This has a low latency, usually around 10 ms (interleaving adds to this, this is not greater than interleaved) but it extremely prone to errors, as any burst of noise can take out the entire packet and so require it all to be retransmitted. Such a burst on a large interleaved packet only blanks part of the packet, it can be recovered from error correction information in the rest of the packet. A "fastpath" connection will result in extremely high latency on a poor line, as each packet will take many retries.