Computer Communications Networks - Introduction.- Network Architectures - Examples.- Network and User Data.- The Physical Layer.- Data Link Control Layer Functions and Procedures.- Data Link Control Layer Protocol Examples.- Multiplexing and Carrier Systems.- The Network and Higher Layer Functions.- Performance Models for Data Networks.
7. Multiplexing and Carrier Systems (p.202)
In earlier chapters, we learnt that the channel bandwidth must be greater than or equal to the signal bandwidth for a successful transmission of a data signal. The channel bandwidth sometimes limits the ability of the user equipment to transmit information. Historically, there have been three ways used to achieve higher data rates. One simple way is to use channels with higher bandwidths. The second mechanism is to design modulation and coding mechanisms to use the available bandwidth more efficiently. The bandwidth efficient modulation and coding schemes result in higher data rates per unit bandwidth, thus increasing the data rates achievable in a channel for a given channel bandwidth. This technique has resulted in gradual increase in the data rates of the telephone line MODEM.
The third important factor contributing to the increased data rates is the improvements in cable manufacturing. This has helped in many ways and can be considered as a part of the first mechanism. Not only higher bandwidth cables are available these days, but also there are ways to allow higher data rates on the already existing cable types. This is possible due to the developments in manufacturing cables with fewer impurities and ones using special circuitry to undo many channel impairments. The cable technology has influenced telecommunications to a point that even new protocols have been introduced with reduced processing. An example of such protocols is the frame relay technology that minimizes processing at layer 2 in order to achieve higher end-to-end throughputs.
Most of the long haul telecommunications transmission systems, however, use channels with much higher bandwidths than a single user signal would need. Many users share each of these high-speed channels. Multiplexing is the mechanism used for channel sharing. In this Chapter, we will look at the difference between two types of transmissions once more, the digital and analog transmissions. We will then define multiplexing schemes that can be used with either type of transmission. In the end, we will look into digital multiplexing in greater detail and discuss carrier systems using digital multiplexing. A carrier system is the term used to describe the transmission systems typically used for long-haul communications for private and public networks. These systems provide a set of standard bandwidths or data rates from which a user can choose. The equipment is designed according to the carrier system it will use, conforming to the signal and transmission formats.
Multiplexing is mostly discussed with reference to the physical layer of the OSI-RM. The fact is that all layers make use of multiplexing. It is one of the functions that can be provided at any layer to open more than one simultaneous connections. Our main focus is on multiplexing at the physical layer, but we will also include a section on multiplexing at other layers.
