Multiplexing

1. Although process-to-process communication is the realm of the transport layer rather than of the physical layer, one implication of process-to-process communication is important to understanding important aspects of the physical layer -- the need for multiplexing.
   • In a situation where process A on machine 1 wants to communicate with process B on machine 2 at the same time that process X on machine 1 wants to communicate with process Y on machine 2 we basically have two choices:

     1. Drag multiple cables between the two machines. Unless the machines are very close together this can get expensive quickly.

     2. Find some way that the two pairs of communicating processes (or more!) can share a single physical link.

     In case you can't guess, we need to discuss how to make the second approach possible.

2. One approach to sharing is taking turns.

   • Since the phrase "taking turns" isnt quite impressive enough, this approach is know as time division multiplexing (TDM) in the world of data communications.

   • The idea is that as long as the total number of bits all the processes wish to send per unit time is less than the capacity of the connection, the sending computer can take turns sending data for different processes.

   • Getting this to work is a software issue and belongs to the upper layers (transport).

   • Notice, however, that it only works because it is so easy to chop digital data up and then glue it back together later.

3. The other approach is to somehow send two signals on a wire or fiber simultaneously in such a way that the reciever can distinguish one incoming signal from the other.

   • To understand how this is possible, imagine two pairs of humans trying to signal through a single fiber at the same time.

   • Suppose that at one end of the fiber we give the two humans different flashlights (red and blue) and then tell them to send signals in binary by turning on their flashlights to send a one and turning them off to send a zero.

   • The person receiving from the individual with the blue flashlight would recognize a zero when the fiber was dark or red and recognize a one when the fiber was blue or purple. Similarly, the individual whose friend had the red flashlight would identify darkness or blueness as a zero and redness and purpleness as a one.

   • This process can be automated by using prisms to combine beams of different colors at the sending end and to direct them to separate photo detectors at the receiving end.

     

   • This technique is known as frequency division multiplexing.

   • As we will see, it is also possible in electical communication systems.