Communications
Author: Mr D Wilson
Source of document: Open University
Date: 6/8/97
The analogue telephone network began development at the end of the 19th century. Improvements in dialling and switching lead to the Public Switched Telephone Network (PSTN) in the late 1960's. In the mid 1970's the Integrated Services Digital Network (ISDN) was introduced. This enables digital signals to be sent directly from the home to a system X exchange. The system X exchanges now provide the backbone to the UK's telecommunications network, however the majority of local loop connections are still analogue. As ISDN is increasing in popularity, the network is transforming from analogue/digital to a fully digital network.
A basic rate ISDN connection, 2 B channels each 64 kbps and 1 D channel 16 kbps, is capable of providing a bandwidth of 144 kbps. A modem is capable of producing 28.8 kbps. The increased bandwidth provided by ISDN enables the use of video conferencing at 128 kbps which current modems can only provide with small jerky pictures. Both digital and analogue signals are susceptible to noise, however over long distance an analogue signal will suffer more than a digital signal due to the noise being amplified along with the signal during the transmission. With digital, a repeater rather than an amplifier is used resulting in an exact copy of the original being repeated. For a digital signal some of the noise will be due to quantisation error. This error will remain constant each time the signal is repeated.
An ISDN signal can benefit from the use of compression. Both lossy and loss less compression can be used depending on the end application. The use of compression can increase the bandwidth of a media. The physical bandwidth or bit rate will remain constant depending on media, distance, conditions etc but taking into account the original file size and the time taken to send the compressed file, the overall through put will increase by using compression.
Although ISDN is capable of 144 kbps, modern local loops that are less than 7-8 km are capable of 2 Mbps. The ISDN standard limits the bit rate. This prevents basic rate ISDN providing full screen animation at 1 Mbps, even though the local loop may have the bandwidth to provide it. ISDN is also more expensive. A basic rate ISDN line from BT is around £400 as opposed to £116 for analogue. Primary ISDN lines can provide 2.048 Mbps which will provide full screen animation but these are more expensive. The European primary rate ISDN line consists of 30 B channels and 1 D channel.
Some digital networks use packet switching as the basis for transmission of data. The data is split into packets containing information about the intended recipient and the position of the packet in the data. Packets are routed by computer nodes to the recipient by the best route. They may also be stored on route at switching nodes. ISDN uses a variant of packet switching known as a virtual circuit. The data is split into packets through time division multiplexing. The user appears to get a dedicated connection but in reality the message is split into packets.
Various technologies can utilise the local loop in a better way than ISDN. Through the use of very large scale integration High bit rate Digital Subscriber Lines (HDSL) are now commercially available. HDSL can use non loaded local loops to provide 2 Mbps in Europe. It needs 2 twisted pair lines to operate. This is full duplex and can provide the business community with cheap high bandwidth communications. The system is symmetrical providing equal bandwidth in both directions. The HDSL system requires one more twisted pair line than ISDN but provides approximately 7 times the bandwidth of two ISDN lines.
For the home market a symmetrical system is not required. For services aimed at the home, the digital link needs high bandwidth into the home but only requires a low bandwidth out of the home. For example, browsing the internet requires simple commands sent to a server by a browser. The server then responds by send text, graphics, animation, audio or even video to the browser which require a high bandwidth. The asymmetrical nature of this communication lends itself to Asymmetrical Digital Subscriber Line (ADSL) services. ADSL is suitable for the home user as it only requires 1 twisted pair to provide 1.5 Mbps unidirectional and 16 kbps in the reverse direction. The ADSL system uses the same amount of twisted pair lines as ISDN ie one, but can provide approximately 10 times the bandwidth unidirectionally.
Digital networks are not confined to the telephone companies' twisted pair local loops. Cable companies have been laying co-axial cable into the home to provide cable TV. These cables have a bandwidth of 60 MHz as opposed to 100 kHz at 1 KM for twisted pair. The networks are capable of providing 1.5 Mbps compressed video on demand 9.6 kbps selection and 64 kbps bi-directional data along with 10 kHz analogue voice. A set top box containing an MPEG decoder decompresses the video which is then converted to an analogue TV signal. A cable network can provide both analogue TV multiplexed with ADSL digital services. This flexibility coupled with the high bandwidth can provide the same services as ISDN and more.
For the mobile user digital satellite communications would seem to be the way ahead. The GSM service will provide a world wide digital network. However the gain in mobility must be set against the loss of bandwidth. GSM currently only provides 9.6 kbps which is 15 times less than ISDN can provide.
The benefits of ISDN are encouraging the change from analogue
to digital. The ultimate aim would be cheap high bandwidth
optical fibre directly to the home. This is currently not
feasible due to the costs of laying the fibre to the home and
linking to fibre optic exchanges. Until this is practically
feasible, the installed local loops must be utilised to their
full potential. By providing digital services to the home
user, an increase in demand will result in reduced cost of high
bandwidth connections due to the mass market.