Broadband definition & technologies overview

Introduction

ADSL allows high speed data transfer down existing twisted copper pair wiring which is used for telephone services. The upstream speed is slower than the downstream speed hence the Asymmetric name. This, for technical reasons, is explained later. 

Broadband is generally defined as a connection which is greater than 128kbs (kilo-bits per second). Care should be taken when calculating transfer speeds as many people confuse kilo-bits with kilobytes. There are 8 kilobits to each kilobyte.

Use of the Telephone System

Nearly every home in the UK has copper wire to the door in the form of a telephone line. Analogue voice traffic runs at 3.3 KHz (3300 Cycles per second) which is extremely slow in modern electronics, the latest computers run at 2 GHz (2,000,000,000 cycles per second). This limitation is to prevent signal degradation over large distances. Because of all this, an analogue modem will only transmit and receive at low speeds - a V34 modem will only achieve 28.8 Kbps. However the weakness of the system is also its main strength, an analogue internet connection is treated as voice traffic and therefore is reliable and requires no configuration at the exchange and in nearly every possible case can connect wherever a phone line exists.

DSLAM - Thank you Ma'am

DSLAM is a multiplexing system that must be installed at your local telephone exchange to separate voice traffic and data traffic on your telephone line. Without DSLAM on your local exchange its NOT ADSL. The DSLAM equipment means that the ADSL modem or router can exchange data at up to 80kHz, which means that data transfer is very rapid compared to that of a modem.

Further Reading on DSL

The structure of the DSL network is quite complex, different types of DSL system are available and are used in different stages of data delivery and transmission. For the majority of us in the UK, we rely on BT to provide broadband with a simple yes or no answer. If you are interested in the technology behind ADSL, have a look at the links on the right hand side of the page.