ADSL (Asymmetric Digital Subscriber Line) is the first technology of DSL technologies. It is just like 56K modem that is able to provide a higher speed of data in the downstream direction than in the upstream direction. That is the reason why its name is Asymmetric DSL. It divides bandwidth unevenly to meet the need of residential customers. As we know that business purposes require higher bandwidth so it is not suitable for them.
Using Existing Local Loops :
It uses existing local loops. It is capable to reach data rate that is not possible by traditional modem it is just because of the use of twisted pair local loop i.e. capable of handling bandwidths up to 1.1 MHz, but there are the installations of filters at the end offices of the telephone company where the bandwidth limits of each loop are up to 4KHz. Without the filter, the 1.1 MHz is available for both data and voice communication.
Adaptive Technology :
Unfortunately, 1.1 MHz is just a theoretical bandwidth. The factors like distance between residence and switching offices will affect the bandwidth due to reasons like the size of the cable, signal used and so on. Before setting up the data rate, the developers of this technology uses adaptive technology for this purpose. Thus, we can say that the data rate of ADSL technology us not fixed, it changes according to the conditions and the type of local loop used.
Discreate Multitone Technique :
The standard modulation technique for ADSL is Discreate Multitone Technique which combines both QAM and FDM. There is the availability of 1.104 MHz bandwidth. It is divided into 256 channels. The following is the distribution of bandwidth –
- IDLE –
1 to 5 channels are not used and these will provide a gap between voice and data communication.
- Upstream Data and Control –
Channel 6 to 30 are used for upstream data transfer and control. One channel is used for control and other 24 are used for data transfer.
- Downstream Data and Control –
Channels 31 to 255 are used for downstream data transfer and control. Like upstream in this also one channel is used for control and other 224 are used for data transfer.
Customer Site : ADSL Modem
The ADSL Modem is installed at a customer’s site. In this local loop connects to a splitter which is used to separate voice and data communication. Modulation and Demodulation are done using DMT and it creates separate downstream and upstream channels.
The splitter must be installed at the customer premises, from the telephone company. The voice line can be installed by the existing wiring in the house but the data line need to be installed by the professional. Due to this, the ADSL line becomes expensive.
Telephone Company Site : DSLAM
The view is different from the telephone company site. Instead of using ADSL modem, they are using DSLAM i.e. Digital Subscriber Line Multiplex. In addition to ADSL features, it has an additional feature to packetizes the data to is be sent to the internet. The following is the configuration of the DSLAM
Features Of ADSL :
- High speed internet access.
- Both internet and calling facilities.
- Fair prices.
- Real time access to information.