Apr 1999 AN215: Advanced Networking (Question 3)

(a)	What is multiplexing? Does multiplexing typically apply to point-to-point or to broadcast communication? Explain your answer.	[4]
	Multiplexing is the technique of combining several lower speed communication channels into a single high speed channel to share communication facility among several users. Multiplexing typically applies to point to point communication than Broadcast Communication. Because broadcasting typically means a message spread through a network any one can get hold of it, but normally in multiplexing , what it achieved is taking message from one user to another from one point to another over long distance using a portion of a high speed line.
(b)	Explain the operation of synchronous Time-Division Multiplexing (TDM).	[4]
	Synchronous TDM : This means the high speed channel is shared among users by means of allocating a time slot to transmit in a round robin fashion. Typically, its a fixed time slot and will be assigned to all the users in a order even if they have nothing to transmit.
(c)	Statistical TDM usually performs better than synchronous TDM. Explain why this is so, and under what circumstances it will not be the case.	[5]
	Statistical TDM is also the operation is quite similar to the above (synchronous TDM) but some overheads will be incurred to control the assigning that is a time slot is allocated to a user. Only if he has something to transmit. It performs better than synchronous TDM because their won't be any empty time slots with users have nothing to transmit. So the channel is best utilized and propagation delay is low. But in a situation where all the users have something to transmit most of the situation this will not be a good choice because of the applied overhead and will increase the propagation delay. (i.e the time a certain user has to wait till his chance comes to transmit the data)
(d)	Polling is sometimes used in broadcast type of network. Why is it inappropriate for general-purpose computer communications? Explain the difference between the balanced and unbalanced approaches to polling.	[7]
	Polling is inappropriate for general purpose communications because generally all the users does not have something to transmit always, and when he need arise they might need to transfer large files. So if a polling scheme is used, most of the time slots allocated to users will be wasted and people who have large files to transfer have to wait longer time until their chance comes, will increase propagation delay. Unbalanced approach : This is a central controller to control the communication that is primary station. It is primaries responsibility to poll and assign users the authority to communicate. Balanced approach : All stations are taken as combined stations and their is no central controller. All the functions and responsibilities share among combined stations equally so they transmit if they want to if no other station is transmitting at that time.