ch08 1

Multiplexing

• Many to one/one to many

• Types of multiplexing

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Multiplexing

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link i.e. Dividing link into channels

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Multiplexing vs. No Multiplexing

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

FDM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

FDM

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FDM

• FDM is an analog technique that can be applied when the bandwidth of a link is greater than the combined bandwidth of the signals to be transmitted.

• Channels can be separated by strips of unused bandwidth guard bands to prevent signals from overlapping.

• Applications: Telephone System, AM& FM radio broadcasting, TV Broadcasting

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Multiplexing process

• Each source generates a signal of a similar frequency range.

• Inside a MUX, these similar signals modulates different carrier frequencies

• The resulting modulated signals are the combined into a single composite signals that is sent over a media link

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

FDM, Time Domain

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Multiplexing, Frequency Domain

Demultiplexing process

• The demultiplexer uses a series of filters to decompose the multiplexed signals into its constituent signals.

• The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Demultiplexing, Time Domain

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Demultiplexing, Frequency Domain

TDM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

TDM

• TDM is a digital multiplexing technique for combining several low rate channels into one high rate one.

• It is a digital process which allows several connections to share the high bandwidth of a link.

• Instead of sharing a portion of the BW as in FDM, time is shared

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Synchronous TDM• In this the data flow is divided into units where each input connection is divided into

units where each input occupies one input time slot. A unit can be 1 bit, I character or 1 block of data

• If the input time slot is Ts then o/p time slot will be T/n where n is no of connections• TDM can be visualized as two fast rotating switches, one on the multiplexing side

another on demultiplexing side

Synchronous TDM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

TDM, Multiplexing

Figure 8-11

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TDM, Demultiplexing

Framing Bits

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Asynchronous TDM• In this TDM slots are dynamically allotted to improve bandwidth efficiency.• Multiplexer checks each input in round robin fashion. It allocates a slot for input line if the line

has data to send, otherwise it skip the data and check the next time.• Addressing: In asynchronous multiplexing there is no fixed relationship between input & output.• We need to include the address of the receiver inside each slot to show where it is to be delivered.

• The addressing can be n bits to define N different output lines with n =log2 N

Asynchronous TDM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Frames and Addresses

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

a. Only three lines sending data

Frames and Addresses

b. Only four lines sending data

Frames and Addresses

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

c. All five lines sending data