Twisted pair copper cables

Written on 10:53 AM by ooe

The rapid growth in dial-up access to the Internet has led to customers using ‘high speed’ voiceband data modems which are capable of operation at data rates of up to 56 kbit/s over analogue copper lines. However, this technology is reaching the fundamental limits set by Shannon's Law for transmission using these types of protocol over installed twisted pair cables.

Further enhancements to data rates on copper cables requires the use of different formats of line coding, implemented in two main classes: Integrated Services Digital Network (ISDN) basic rate at 128 + 16 kbit/s and the range of standard methods known generically as Digital Subscriber Line (DSL). There are a range of DSL systems operating at a variety of data rates which are often referred to as xDSL. The first to be deployed was High speed DSL (HDSL), which provides 2 Mbit/s transmission over distances of up to several km. Transmission spans may be increased by using multiple pairs. More recently asymmetric DSL (ADSL: up to 6 Mbit/s towards the customer) has become fairly widespread in the consumer market as Telcos have moved into high speed Internet services. Very high speed DSL (VDSL) allows speed up to and in excess of 15 Mbit/s, at the expense of reductions in the transmission distance. It is still largely an emerging technology, although it has been used to deliver entertainment TV services.