ADSL

Written on 9:53 AM by ooe

There are a number of flavours of DSL available but the principle flavour now being deployed is ADSL (asymmetric digital subscriber line). It is called asymmetric because of the difference in capacity of the upstream and downstream channels – the upstream channel being of lower capacity than the downstream. The ADSL technology is well described in other articles [3,4].

The ADSL technology that enables this upgrade is similar to the modem technology used on 28 and 56 k dial-up modems. There is, however, one major difference in that the coding is not constrained to fit within the normal voice bandwidth. Indeed, it starts above the voice spectrum (4 kHz) and continues up to 1 MHz. The way in which the bandwidth is allocated between the voice channel, and the upstream and downstream channels is shown in. The technology and spectrum allocated is described in more detail in.

The fact that the ADSL coding is not constrained to fit within the normal voice channel means that the data channel can be used at the same time as a telephony channel, i.e. simultaneous voice and data communication. However, because the encoding is not restricted to fit within the voice bandwidth, the voice network cannot transport the signal. Hence, whereas for voice band modems, the encoded signal is transported over the voice network to another modem to decode the signal, for ADSL the signal needs to be extracted in the local exchange before it reaches the voice network.

The device that performs this extraction in the local exchange is the DSLAM (DSL Access Multiplexer). The DSLAM performs this separation and returns the voice traffic to the voice network with the ADSL encoding removed. The DSLAM also performs the task of a number of modems coding and decoding the data from the ADSL signals, and aggregates the individual customer data channels onto a single connection suitable for carriage over a data network. More detail of this will be covered in.

The way the voice and data signals are separated from each other is via the use of filters. The filters are required at both ends of the copper pair to avoid interference from the voice signals on the data equipment and vice versa. shows how the filters select the correct channel.

The ADSL equipment is capable of carrying quite significant bandwidths, e.g. up to 8 Mbit/s in the downstream direction. However, the limiting factor is the quality and length of the copper pairs. Hence, people living near to the local exchange would be able to achieve bit rates in the order of 8 Mbits/s whereas someone living many kilometres away would not be able to achieve more than say 0.5 Mbit/s.

It is then up to the network provider to determine how to work within this speed versus distance trade-off in order to offer products. Offer a high rate service and only a very limited number of people will be able to take it. At the other extreme if the speed was reduced until everyone could have it, the speed that could be offered would be so low as to be pointless. An example graph that shows how the data rates tail off as the insertion loss increases is shown in. Anyone deploying ADSL needs to understand the trade off between speed and potential market place that could be achieved for a particular network.