Various forms of Digital Subscriber Line service are broadband in the sense that digital information is sent over one channel and voice over another channel sharing a single pair of wires. Analogue modems operating at speeds greater than 600 bit/s are technically broadband. They obtain higher effective transmission rates by using multiple channels with the rate on each channel limited to 600 baud. For example, a 2400 bit/s modem uses four 600 baud channels (see baud). This is in contrast to a baseband transmission where one type of signal uses a medium's full bandwidth such as 100BASE-T Ethernet.
BT is a major player in this market
Confusing usage
While many lower rate forms of data transmission, such as analogue modems above 600 bit/s, are broadband, broadband has been more closely associated with higher data rate forms of broadband data transmission such as T-carrier and Digital Subscriber Lines. Therefore, the word "broadband" has also come to mean a relatively high rate, while the term "narrowband" is used to mean a relatively low rate. It is now quite common to hear a broadband method such as a 9600 bit/s modem described as "narrowband", while a high rate baseband transmission such as 10BASE-T is described as "broadband". The International Telecommunication Union Standardization Sector (ITU-T) recommendation I.113 has defined broadband as a transmission capacity that is faster than primary rate ISDN, at 1.5 to 2 Mbit/s. However speeds of 256 kbit/s and greater are commonly marketed as "broadband" and this convention is held to by policy makers and ISPs alike. See Broadband Internet access. Note: The term "narrowband" is also used to mean the opposite of "wideband" instead of the opposite of "broadband".
Asymmetric Digital Subscriber Line (ADSL Broadband) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide.
ADSL Broadband has the distinguishing characteristic that the data can flow faster in one direction than the other, i.e., asymmetrically. Providers usually market ADSL Broadband as a service for people to connect to the Internet 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 bandwidth in the other direction.
There are both technical and marketing reasons why ADSL Broadband 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 DSLAM end (where the wires from many local loops are close together) 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 telcos chose to make a virtue out of necessity, hence ADSL Broadband.
For conventional ADSL Broadband, downstream rates start at 256 kbit/s and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL Broadband Lite is sometimes used for the slower versions.
Note that distances are only approximations. Signal attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely independently of distance (e.g., non-copper cabling, cable diameter). Real world performance is also dependent to the line impedance, which can change dynamically either dependent on weather conditions (very common for old overhead lines) or on the number and quality of joints or junctions in a particular cable length.
A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 km (8000 ft). Higher symbol rates and more advanced noise shaping are responsible for these increased speeds. ADSL2+, also referred to as ITU G.992.5, boosts these rates to up to 25 Mbit/s for spans of less than 1.5 km (5000 feet). ADSL2+ also offers seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e., up to 50Mbit/s for two lines, etc.), as well as options in power management and seamless rate adaptation - changing the data rate used without requiring to resynchronise.
Because of the relatively low data-rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as web traffic; ATM runs widely over ADSL technology to ensure that this remains a possibility.
ADSL Broadband service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL Broadband to host a Web server.
Providers are:
AOL Broadband BT Yahoo Broadband Tiscali Broadband TalkTalk Broadband