Multiplexing

•Many to one/one to many•Types of multiplexing•Telephone system

Many to One/One to Many

• Multiplexing provides a mechanism to share the use common channel/circuit by 2 or more users.

• Multiplexing devices include:– Multiplexers on the sending side

– Demultiplexers on the receiving end

– path(physical link) composed of n channels.

• Multiplexing techniques are employed for both digital & analog transmissions.

Multiplexing

Frequency Division

Multiplexing (FDM)

Time Division

Multiplexing (TDM)

Wave Division

Multiplexing (WDM)

Synchronous Asynchronous

Types of Multiplexing

FDM - Frequency Division Multiplexing• FDM is an analog tech. used when the bandwidth of

medium exceeds the required bandwidth of the signals to be transmitted.

• FDM was used by telephone companies.• Each modulated signal requires a certain bandwidth

centered around its carrier frequency called a channel.

• How it works?– FDM translates the frequencies of several voiceband

channels to different frequency channels.– Then, combines the frequencies.– And transmit them together.

• Figure 8.3 Forouzan

Figure 8.3

•Bandwidth is actually 3 times the bandwidth of each signal.•Carrier frequencies are sufficiently separated by unused bandwidth(guard bands) so that they do not overlap.

Demultiplexing:

• At the receiving end, the demultiplexing process involves:– Slitting the received signal into three frequency bands.

– Demodulating each signal back to the original voice band.

– Passing the signals onto the receivers.

– Figure 8.6 Forouzan

TDM - Time Division Multiplexing

• TDM is digital process used when the data rate capacity of the transmission medium exceeds the data rate required by the sending and receiving devices.

• Each source connected to a TDM . TDM has the entire bandwidth for a portion of the time called a “time slot”.

• TDM constructs a “frame” consisting of one or more time slots for each input source.

• TDM can be either synchronous TDM and asynchronous TDM.

• Figure 8.10 Forouzan.

TDM

Synchronous TDM

• A method of TDM in which time slots on a shared transmission line are assigned to devices on a fixed, predetermined basis.

• Synchronous , that the multiplexer allocates the same time slot to each device at all times whether or not it has anything to transmit.

• Figure 8.10 Forouzan.

Frames• Framing provides a method of synchronization of

transmission.• The data are organized into frames, each of which contains

a cycle of time slots.

Synchronous TDM

In each frame, one or more slots is dedicated to each data source.Transmission consists of the transmission of a sequence of frames.The set of time slots dedicated to one source, from frame to frame is called “channel”Since time slot order is fixed, frame size does not vary.Hence, the demultiplexer requires no additional addressing.Framing bit are added to the beginning of each frame to help the demultiplexer synchronize the incoming data stream & separate the time slots.If a device has no data to transmit, the time slot is wasted.Figure 8.11 Forouzan

InterleavingSynchronous TDM can be compared a very fast rotating switch.As the switch opens in front of a device, that the device has the opportunity to send a specified amount of data onto the path.The switch moves from device to device at a constant rate and in a fixed order.This process is called interleaving.Figure 8.12 Forouzan, shows interleaving and frame building.

Asynchronous TDM

A method of TDM in which time slots on a shared transmission lined are assigned to devices on demand.“Asynchronous” means flexible or not fixed.In synchronous TDM, capacity is wasted if not all time slots in frame are used.Based on the principle that not all input channels are ready to transmit when scanned. ATDM supports n channels but its frame contains only m slots where m < nFigure 8.16 Forouzan.Every cycle ATDM scans the inputs buffers and fills up the frame up to m slots and then sends the frame across the link.

Asynchronous TDM

Note that, the ATDM bandwidth required is smaller than synchronous TDM channel.Figure 8.17 shows 3 examples ATDM frames.

c. All five lines sending data

b. Only four lines sending data

a. Only three lines sending data

Addressing and OverheadSince slot assignments are not fixed, header information identifying the data with their perspective source are added.This creates additional overhead.

Variable length time slotsATDM can accommodate traffic of varying data rates by varying the length of the time slots.Control bits are appended to the beginning of each time slot to indicate the length of the incoming data portion.

WDM - Wave Division Multiplexing

WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve light signals transmitted through fiber optic channels.The idea is the same, combining different signals of different frequencies.The difference is that the frequencies are very high.Figure 8.8 give a conceptual view of a WDM multiplexer and demultiplexer.Combining and splitting of light sources are easily handled by a prism.

Multiplexing and Inverse Multiplexing

Self - Reading - Telephone Network

Services

Analog Services Digital Services

Switched Leased Switched/56 DDS DS