ADSL and Spectrum allocation:
- The long form of a ADSL is a symmetrical DSL.
- Similar to 56 k modem, it provides higher bit rate in the downstream direction as compared to the bit rate in upstream direction. this is why it is called as asymmetrical DSL.
- ADSL divides the bandwidth of a twisted pair cable of 1MHz into 3 bands as shown in the figure.
- The first band is between 0 and 25 kHz. It is used for regular telephone (plain old telephone sets - POTS).
- This service uses only 4 kHz of this band and the rest is used as guard band to separate the voice channel from the data channels.
- The second band is from 50 kHz to 200 kHz. It is used for upstream data communication.
- The third band is from 250 kHz to 1 MHz. It is used for downstream data communication.
- ADSL technology is designed for home users. It is not suitable for business applications because such applications need larger bandwidth than the home users.
Two modulation techniques can be used for ADSL:
Carrier less amplitude / phase (CAP)
Discrete multitone (DMT)
Discrete multitone technique:
- DMT is a standard modulation technique for ADSL. It is a combination of QAM and FDM techniques.
- Typically the available bandwidth of 1.1 MHz is divided into 256 channels. However this is not a rule. The division may change from system to system.
- Figure shows the block schematic of the DMT system.
- Channel 0 is reserved for the voice communication. Channels 1 to 5 are called as ideal channels since they are not used. These channels create a guard band between the voice and data channels.
- Channels 6 to 30 are used for upstream data transfer and control. 24 channels are used for data transfer and one channel for control. For 24 Channel (4 kHz each) with QAM modulation the bandwidth is 24 x 4 kHz x 15 or 1.44 Mbps in the upstream.
- The channels 31 to 255 (255 channels) are allotted for downstream data transfer and control. Out of 225 channels, one is reserved for control and remaining 224 are reserved for data. The downstream bit rate is 224 x 4 kHz x 15 = 13.4 Mbps. Figure shows the bandwidth allocation for this (DMT) ADSL system.
Fig : Bandwidth Allocation for DMT ADSL system