As mentioned previously, attenuation by the cable is a major impedance to network bandwidth. DSL uses software and electronics to "fix" the problems with copper wire, creating an overall higher bandwidth data pipe.
What does DSL give us? An almost instantaneous network upgrade. DSL uses the existing copper network to give a marked increase in efficiency. A cheaper alternative to fiber or coaxial cable. DSL's algorithms make the data transmission more dependable.
DSL provides for several more upgrades to the existing network infrastructure, including increased distances, decreased installation time, immunity to normal cable problems, and the like. DSL can be used in a variety of environments. The local phone companies could use it for delivering video, audio, and increased bandwidth to both local residential and business customers.
Businesses could also use it in a multiple building or even multiple floor setting, to provide increased network bandwidth or allow for newer interoffice network technologies video on demand, conferencing, and more. The possibilities for this cheap network upgrade are as endless as the networks it can be applied to. Table DSL acronym table. The speed of this technology for use over existing mediums phone lines is astounding: up to 8. The overall downstream rate depends on, among other things, the distance covered, the size of the wire, and interference.
Upstream speeds range from 16Kbps to 64Kbps to Kbps. ADSL was designed for two major functions: high speed data communications and interactive video. Data communication functions can be Internet access, corporate telecommuting, or other specialized network applications. Interactive video covers things such as video on demand, movies, games, or any other application that requires high-speed networked video.
The biggest strength in ADSL lies in the fact that it uses existing telephone lines over which to communicate. This means that more than million existing phone customers are already cabled for ADSL and require little or no upgrade. Technologies such as ISDN or cable modems can require expensive hardware and software upgrades in both the network and in the client site. But, ADSL is still in testing around most of the world, with an anticipated niche release slated for late This release will be available mainly for Internet access and other packet-based communication applications.
The difference between the two is the way they send data down the pipe. Incoming data modulates a single carrier channel that is then sent down a telephone line. The carrier is suppressed before transmission and reconstructed at the receiving end.
Discrete Multi-Tone It separates incoming data into many sub-carrier channels. Each channel is measured for quality, and then the channel is assigned a certain number of bits based on quality of the channel. DMT uses a mechanism we are familiar with to create a modem connection.
When two DMT modems connect, they try the highest speed possible. Depending on line noise and attenuation, the modems will successfully connect at the highest rate or decrease the rate until a satisfactory connection speed can be reached.
It creates a three-channel pipe see Figure An ADSL three-channel pipe. Both the high-speed and medium-speed channels can be multiplexed to create multiple lower-rate pipes. There is no carrier frequency transmitted in CAP. Three channels are allocated: POTS, upstream data, and a high-speed downstream data. Each channel is separated in the frequency domain. Data modulates a single carrier and is transmitted down a phone line.
The carrier is removed before transmission and added again at the receiving end. Copper lines distort heavily at the high frequencies used by ADSL circuits. DMT is particularly good at overcoming this. Originally, it was intended for sending video over copper. Actually a form of frequency-division multiplexing FDM , Discrete Multitone is a multicarrier modulation that divides the available channel into individual subchannels called tones. Each of these tones has the same bandwidth but is modulated on a separate carrier.
There are different tones that range from 20KHz to 1. DMT dynamically adjusts to compensate for line noise, allocating subcarriers with a high signal-to-noise ratio to the highest spectrum. Some telephone lines are equipped with induction devices called loading coils. The loading coil is designed to improve voice transmission over POTS when the local loop is more than 18, feet the last 3 copper miles.
It does this by compensating for the capacitance of the wire and raising the frequency. This is nice for voice transmission; however, a side effect is the distortion of high frequency signals.
This distortion will annoy xDSL. Ask your telephone company about its procedure for removing loading coils from xDSL connections.
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SDSL enables equal bandwidth downstream from a network service provider CO to the customer site as upstream from the customer site to the CO. SDSL supports data only maximum of 1. HDSL is a more cost-efficient method of installing T1 service to a customer site than traditional dedicated DS1 service.
HDSL-2 also was developed as a standard by which different vendors' equipment can interoperate. The maximum downstream rate under consideration is between 51 and 55 Mbps over lines up to feet m. Downstream speeds as low as 13 Mbps over lengths beyond feet m also are in consideration.
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