Comparison of DSL types
Different examples of DSL technology have been implemented so far:
Asymmetric DSL (ADSL). In the above model, the word “asymmetric” is used due to the difference in the speed of receiving and sending information. The nature of operations performed by Internet users is such that the amount of information received is always many times greater than the information sent.
High bit-rate DSL (HDSL). The speed of the above model is at the limit of T1 lines (1.5 megabits per second). The speed of receiving and sending information is the same in the above method, and in order to provide services, two separate lines are needed compared to the existing normal telephone line.
ISDN DSL (ISDL). The above model was initially available to ISDN users. ISDL has the lowest speed compared to other DSL models. The speed of these lines is 144 kilobits per second (two directions).
Multirate Symmetric DSL (MSDSL). In the above model, the speed of sending and receiving information is the same. The rate of data transmission speed is regulated by the DSL service provider center.
Adaptive Rate (RADSL). It is the most common ADSL model and it will give the modem the possibility to determine the communication speed according to factors such as distance and line quality.
Symmetric DSL (SDLS). The speed of sending and receiving information is the same. In the above model, unlike HDSL which uses two separate lines, only one line will be needed.
Very high bit-rate (VDSL). The above model is “asymmetrical” and is used in short distances with copper telephone lines.
Voice-over DSL (VoDSL). It is a special type of IP telephone. In the above model, several telephone lines are combined and turned into one telephone line.
VDSL standards
Two different consortia are trying to standardize VDSL. The VDSL Alliance consortium uses a carrier system called Discrete MultiTone (DMT). Most of the ADSL equipment used uses the DMT method.
DMT divides the relevant signal into 247 separate channels. Each channel has a 4 kHz band. (The above situation is similar to the fact that the relevant telephone company has divided the existing copper line into 247 separate 4 kHz lines and connected each of the above lines to a modem. The simultaneous use of 247 modems, each of which has a 4 kHz band each of the channels is controlled and if the quality of one channel drops, the signal will shift to another channel.
The process of shifting signals between different channels and searching to find the best channel will be done continuously. Some channels are used bi-directionally (sending and receiving information), controlling and sorting information in bi-directional channels and maintaining the quality of each of the 247 existing channels has made the implementation of the DMT standard much more complicated than CAP. DMT standard has much more flexibility in relation to the quality of lines and channels.
The second consortium is called VDSL Coalition. In the standard provided by the above consortium, two Carrier systems are used. Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude Phase (CAP) are two examples of Carrier system.
The CAP standard divides signals into three separate bands: telephone conversations with a band of 0 to 4 kHz. The channel for receiving information from the user for the server has a band between 25 and 160 kHz (Upstream) and the channel for sending information from the server for the user has a range that starts from 240 kHz.The maximum band above depends on different factors such as: line length, the number of users in a particular telephone company, etc. However, the maximum range of the above band will not exceed 1.5 MHz. The above system (CAP) will be able to send relevant signals using the above three channels.
QAM is a special modulation method that, depending on the type of version used, triples the information sent along a line. In the above method, modulation (changing the shape of the carrier wave) and phase shift (changing the angle of the carrier wave) are used. An unmodulated signal simply presents two states of zero and one. (only one bit of information will be sent in each cycle) by sending the second wave that has shifted by 90 degrees (compared to the first) and the modulation of each of the waves, there will be two points in each wave.
(Eight possible modes). In this way, it will be possible to send three bits in each cycle: having two states is equivalent to one bit (21=2), four states is equivalent to two bits (22=4) and eight states is equivalent to three bits (23=8). With the addition of four waves and a phase shift of 15 degrees each, 16 different modes can be presented and the possibility of sending 4 bits per cycle will be provided.
