Introduction to Asymmetric Digital Subscriber Line (ADSL)

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.

Bandwidth division in ADSL

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 is not fixed, it changes according to the conditions and the type of local loop used.

Discrete Multitone Technique :
The standard modulation technique for ADSL is Discrete 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.

Asymmetric Digital Subscriber Line (ADSL) is a technology that allows high-speed digital data transmission over existing copper telephone lines. It is called “asymmetric” because it provides higher download speeds than upload speeds, which is suitable for applications such as web browsing, email, and streaming video.

1. ADSL technology works by dividing the available bandwidth of a telephone line into separate channels, with each channel carrying a different type of traffic. The majority of the bandwidth is allocated to downstream traffic, which is data traveling from the internet to the user’s device, while a smaller portion is allocated to upstream traffic, which is data traveling from the user’s device to the internet.
2. ADSL uses a modulation technique called Discrete Multitone (DMT) to divide the available bandwidth into channels. DMT uses a complex algorithm to divide the bandwidth into hundreds of separate channels, each carrying a different frequency range. This allows ADSL to transmit data at high speeds while minimizing interference from other devices on the same telephone line.
3. One of the key advantages of ADSL is that it can provide high-speed internet access over existing telephone lines, which means that it is widely available in many areas where other types of broadband internet services may not be available. It also allows users to use their phone line for voice calls while simultaneously using the internet, without interfering with each other.
4. However, ADSL has some limitations. The maximum distance between the user and the telephone exchange can affect the speed and quality of the connection. In addition, the available bandwidth is shared among all users connected to the same telephone exchange, which can result in slower speeds during peak usage periods.

Overall, ADSL is a widely used technology for providing high-speed internet access over existing telephone lines, and it has paved the way for other broadband technologies such as VDSL and fiber-optic internet.