11-Mar-16Physical Layer Multiplexing Multiplexing is the set of techniques that allows multiple signal transmission across a single medium at the same time (simultaneously). It can be applied when the bandwidth of a medium connecting two devices is greater than the bandwidth needs of the devices. As such, the use of the medium is maximised.

Figure 6.1 Dividing a link into channels

11-Mar-16Physical Layer Multiplexing (Continued…) Multiplexing techniques can be divided into three categories: Frequency division multiplexing (FDM) Wave division multiplexing (WDM) Time division multiplexing (TDM)

11-Mar-16Physical Layer FDM FDM is used when the bandwidth of the medium is greater than the combined bandwidths of the signals to be transmitted across it. 1. Signals generated by each device modulate different carrier frequencies. 2. The modulated signals are combined into a single composite signal and transported across the medium (link).

11-Mar-16Physical Layer FDM (Continued…) To ensure successful recovery of data at the receiver, the following need to be followed. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. Channel must be separated by strips of unused bandwidths (guard bands). Carrier frequencies must not interfere with the original data signal frequencies.

Figure 6.3 FDM

Figure 6.4 FDM process

Figure 6.5 FDM demultiplexing example

Figure 6.6 Example 1

Figure 6.7 Example 2

11-Mar-16Physical Layer FDM (Continued…) FDM are used in the following applications: AM (amplitude modulation) and FM (frequency modulation) radio broadcasting – filtering at the receiving end is done by tuning. TV broadcasting – filtering is done by choosing the desired channel. First generation mobile phones.

11-Mar-16Physical Layer WDM WDM is designed to make use of the high data rate capability of the fiber-optic cable. Its operation is similar to the FDM, except that The multiplexing and demultiplexing involve optical signals transmitted through the fiber- optic channels and The frequencies involved is very high compared to those used in FDM. Application - SONET

Figure 6.10 WDM

Figure 6.11 Prisms in WDM multiplexing and demultiplexing

11-Mar-16Physical Layer TDM TDM is a digital process that allows several connections to share the high bandwidth of a link. In TDM, a portion of a time is shared instead of bandwidth. The data flow in each connection is divided into units. The link combines one unit of each connection to make a frame. The size of the unit can be 1 or several bits.

11-Mar-16Physical Layer TDM (Continued…) For n connections, n time slots are at least required in each frame. The data rate of the combined link = n x data rate of each connection. Therefore, the duration of a unit in a connection is n times longer than its duration in the frame/combined link. The operation of a TDM can be visualised as a a pair of rotating switch operating at the same speed but opposite direction.

Figure 6.12 TDM

Figure 6.13 TDM frames

Figure 6.14 Interleaving

11-Mar-16Physical Layer TDM (Continued…) Synchronisation is required to ensure that the demultiplexer is able to sort out the incoming data accordingly. Framing bits, which are alternately numbered, are included in each frame to provide this synchronisation.

Figure 6.17 Framing bits