 In this module, we shall discuss various aspects of transmission system. In particular, we'd look at what is the role of multiplexers vis-a-vis the transmission systems. We'd also appreciate the concept of network budgeting in terms of service provisioning to a certain number of users. And then we'd also look at some design trade-offs which are always carried out by telecommunication investment companies. The transmission systems are essentially media which carry signals over distances, either short distance or long distance. These signals are carried either as electrical signals, optical signals or radio signals. Especially in long-haul transmission, there's a requirement for these signals from various users to be multiplexed onto a limited number of medium. So the concept of multiplexing is essential to a transmission system. Principally speaking, these multiplexers take inputs from each user one by one and then pass it on to the transmission system in an interleaved bit stream or a byte stream. Doing so allows multiple users traffic to be carried simultaneously onto a single medium or more than a single medium. The concept of multiplexing can be best understood using this diagram. Here you can see we start from the left-hand side. We have sources numbered from 1 to n which are interested in sending their transmissions to their respective destinations as shown in the form of users from 1 to n. On the sending end, multiplexing takes place thereby taking traffic from every user in a round-drop in kind of manner and putting it on the transmission system. The transmission system carries it on a communication channel from point A to point B where a de-multiplexer takes this single long stream of data and de-multiplexers or extracts individual user data and passes it on to the right destination. These multiplexers are deployed in the transmission system at some cost. Usually speaking, the transmission system is only a part of the complete telecom system, the other one being the network elements. Network elements including the multiplexer, the switching elements such as routers and the switches, all these comprise the transmission system. Here through empirical and physical deployments, a thumb rule has been coined that is almost double the investment is required for the transmission system because you can understand laying the cables by doing a lot of digging and passing it over the head in the form of poles needs mega investment. If you look at the price, it is usually double as compared to the network elements. So a smart choice is for the network designers to think about ways where more investment on the end network infrastructure could be done to compensate and give an overall saving by reducing the costs incurred in the transmission system both in terms of the capital cost and the recurring cost. This leads us to a very important concept of doing a design trade-off. Let's look at the design trade-off from the technology perspective first. Sometimes providing a service means compromising on certain technical parameters like if you really want to minimize the delay you may be required to reduce the error detection mechanism for an outgoing stream from the sender to the receiver. So this is a technological design trade-off. In all practical sense the design trade-off for technology is not a big issue because in real world making a design choice is heavily dependent upon the business perspective it is concerned with. So it is an important question to ask whether or not a service could be provided to a majority of population using technology A or technology B and so forth. It means making the choice that allows the minimum capital investment and minimum recurring investment should be the choice in terms of the investment.