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to previous page - Coursework / Teaching AssignmentsAny communication network has two distinct entities - switching and transmission. While switching allows many users to share a limited communication resource, transmission deals with maximising the communication channels available using multiplexing / multiple access methods. The transmission system can use any kind of channel - HF/VHF/UHF radio, co-axial cable, line-of-sight microwave and the latest and most promising medium - optical fiber. Each channel / medium has its own inherent advantages / disadvantages in terms of many factors such as bandwidth, utilisation efficiency, complexity of equipment and of course, cost. And there is the question of whether the transmission should be analog or digital. But this question has been resolved quite some time back and the preferred choice is digital transmission, whereby all information to be transmitted is converted into a sequence of ones and zeros.
An important advantage of digital transmission is that allows the achievement of the 'so far theoretical' channel loss of 0 db. An analog transmission system cannot achieve 0 db channel characteristic. This is because in a transmission system with a number of repeaters ( both co-ax cable and microwave links require repeaters every 60km or so ), there is some noise added in the message band by the amplifiers in each repeater. This noise becomes part of the signal at the next repeater in the route, and thus at the final destination, there will always be some noise. But, in digital transmission, the transmitted symbol is either a one or a zero, and this knowledge makes it possible to reconstruct the exact original signal at each repeater before transmission to the next repeater. This of course assumes that the clocks at both the sending end and the receiving end are in sync, but that is possible with advanced line coding techniques. Therefore each repeater does not add noise to the signal, and therefore we can achieve 0db channel loss in reality. The term 'repeater' is a misnomer. The actual repeater in digital transmission is called a regenerator or a regenerative repeater.
The Southern Railway started the use of microwave transmission with an analog frequency division multiplexing microwave system, that had a capacity of 24 voice channels. But that was long ago. Now, the microwave system is state-of-the-art and completely digital. The underlying concept of digital transmission is maximising the usage of the transmission path by dividing it in the time domain ( time division multiplexing - TDM ), as opposed to FDM, which divides the band in the frequency domain, alloting each user a slice in the available frequency spectrum.
Frequency division multiplexing - some of the total bandwidth all of the time.
Time division multiplexing - all of the total bandwidth some of the time.
In TDM, the channel is divided into time slots, each user being alloted one time slot. The duration of a time slot depends on the number of channels being multiplexed and the bandwidth of the transmission medium. And TDM has a pre-requisite that the all the inout signals ( the messages being transmitted ) are digital in nature. So, use of any TDM equipment to multiplex voice signals requires that the signals be digitised ( A-D converted ) first.
This A-D conversion is performed using hardware that implements the ITU-standard G.711 Pulse Code Modulation with 8000 samples / second and 8 bits per sample, thus coverting the voice signal to a 64kbps digital bit stream.
In commercial multiplexing systems, the primary rate is defined as a 2.048 Mbps digital stream that is a time-division multiplexed combination of 30 basic 64kbps channels. The 30-channel primary rate is the European E1 electrical standard, while in the North America and Japan, the primary rate contains 24 64 kbps channels, thus giving a bit rate of 1.544 Mbps and is called the T1 carrier. In addition to the number of basic channels in one multiplexed stream, the T1 and E1 also differ in that in E1, the A-D conversion is done using the A-law, and in T1, the conversion uses mju-law. These are just the functions used for the non-linear companding of the VF signal, and both these PCM systems can work with each other through well-defined interfaces. Yet another difference is that 30-channel CEPT E1 uses HDB3 ( high-density bipolar of order 3 ) line coding, while 24-channel ANSI T1 uses AMI ( alternate mark inversion ). The ITU standards for these PCM systems are G.732 ( E1 ) and G.733 ( T1 ).
Primary rate multiplexing is not the end. 4 such primary rate streams multiplexed together give rise to the 8 Mbps higher order multiplexed stream. And again, 4 such 8 Mbps streams are multiplexed to yield a 34 Mbps stream. This 34 Mbps stream actually contains information from 16 E1 streams, and therefore a 34 Mbps stream actually contains 480 voice channels. This sort of multiplexing hierarchy allows for standardised interfaces between different equpiment manufacturers, and also allows operators to design their systems based on available bandwidth and resources.
Examples:
The Southern Railway Digital Microwave Transmission System
The Southern Railway Optical Transmission System
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