What is broadband

Understanding DSL


To understand DSL, you first need to take into account what you have learned about a normal telephone line, the kind that telephone professionals call POTS. This stands for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry.
Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 hertz, or cycles per second. This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 hertz to 20,000 hertz. The telephone wires themselves have the potential to handle frequencies up to several million hertz in most cases. The use of such a small portion of the wire's total bandwidth is historical; remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century.
By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference from one line causing problems on another. Modern equipment that sends digital, rather than analogue, data can safely use much more of the telephone line's capacity. DSL does just that.
DSL utilises the frequencies above 4000hz, this untapped resource is used to carry data. The phone line is split at 4000hz, everything below is dedicated to POTS and everything above is dedicated to DSL.This means DSL and POTS can operate simultaneously on the same copper wires. Consider a conversation in a room; if while I was talking, another person were blowing a dog whistle in the background, it would not interfere because the whistle operates at a frequency way above the range we can hear. Therefore if a person could talk at the same frequency of the dog whistle then it would be possible to have two people talking in the same room without interfering with each other.
Most home and small business users are connected to an Asymmetrical DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is 3-4 times faster than the connection from the user back to the Internet, then the user will see the most benefit, most of the time.

The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the exchange have the potential for seeing very high speeds in the future.

ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64-640 kbps.

As can be seen from the diagram, the upstream and downstream channels use different frequencies; this is called Frequency Division Multiplexing (FDM). This allows both directions of transmission to co-exist with each other, without interfering with each other.
You might wonder, if distance is a limitation for DSL, why it's not also a limitation for voice telephone calls. The answer lies in small amplifiers called repeaters that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the exchange will disqualify you from receiving ADSL.
DSL technology uses existing phone lines, commonly known as the twisted copper pair, this means no civil works are required, this is DSL strongest selling point. DSL technology fits in to the local loop, which is also referred to as the last mile and the access network. Local loop, the last mile and the access network all refer to the wires connecting the customer premises to the local exchange, DSL fits in to the last mile.
ADSL comes from a family of technologies referred to as XDSL, all are closely related and basically slight variations on the same theme. The different members of the family are suited to different needs, for instance ADSL would not be ideal for hosting a server because this would require quite a lot of information to be uploaded.

DSL Family
ADSL: Asymmetric DSL ADSL is capable of delivering 8mbps downstream and 1mbps upstream. It’s called Asymmetric because the upload and download speeds are in different proportions.
CDSL: Consumer DSL CDSL (Consumer DSL) is a trademarked version of DSL that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) but has the advantage that a "splitter" does not need to be installed at the user's end.
G.Lite or DSL Lite G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company.
HDSL: High Bit Rate DSL The earliest variation of DSL to be widely used has been HDSL, which is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. For this reason, the maximum data rate is lower than for ADSL.
IDSL: ISDN DSL IDSL is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.
RADSL: Rate Adaptive DSL RADSL is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly.
SDSL: Singe Line DSL SDSL is apparently the same thing as HDSL with a single line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line.
VDSL: Very High Rate DSL VDSL is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments are not yet determined. A number of standards organizations are working on it.


Accessing the Internet using ADSL

By following the map below, you can trace a customer’s connection from their pc using a DSL modem to the Internet via their local exchange and ISP.

The MDF (Main Distribution Frame)

The MDF allows us to identify where customers phone lines terminate within the exchange so we can provide the service they subscribe to such as PSTN, ISDN etc.

The ASAM 7300 Server (ADSL Subscriber Access Multiplexer)

The ASAM is the piece of equipment with the exchange that makes it ADSL enabled.

The ASAM does three things: · It splits up the POTS and DSL services, the POTS service is directed to the PSTN network.· It multiplexes DSL lines before feeding the traffic in to the ATM access node.· It acts as a PPPoE server, stripping or adding PPPoE layers.

ATM Access Node

ATM is a fibre network with connection points around the country, the ATM network is used from carry aggregated ADSL traffic from remote exchanges back to Crown Alley so it can be routed on to the Internet through eircom net’s network.


The Broadband Access Server (BAS)

The purpose of the BAS is to unwrap the various protocols inside which your data travels over the ADSL connection. It also makes your connection to the ISP appear exactly as if you had connected using a dial-up modem or ISDN.

The result of this is that there are at least five different ways in which the data can be carried between the PC and the BAS. The method used by the PC and the modem must be the same as that used by the BAS for the connection to work.

See full manual in the attachment. http://www.seekin.co.cc/showthread.php?t=38