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Lecture- BS(CIS) Semester-IV

Data Communication

Today’s Lecture

• Multiplexing  Why Multiplex  Frequency Division Multiplexing

Frequency Division Multiplexing

• FDM is done when useful bandwidth of medium exceeds

required bandwidth of channel

 Sufficient carrier frequencies are available

• Basic frequency modulation is applied for each user  Each signal is modulated to a different carrier frequency (channel)  Carrier frequencies separated so that signals do not overlap (guard bands)  The composite signal is further modulated over a carrier frequency  Demodulation of composite is done first  Demodulation of individual channels is done
• Channel allocated even if no data

Synchronous Time Division Multiplexing

• Data rate of medium exceeds data rate of digital signal to

be transmitted

• Multiple digital signals interleaved in time
• May be at bit level or blocks
• Time slots preassigned to sources and fixed
• Time slots allocated even if no data
• Time slots do not have to be evenly distributed amongst

sources

Time Division Multiplexing

TDM Link Control

• No headers and trailers
• Data link control protocols not needed
• Flow control  Data rate of multiplexed line is fixed  If one channel receiver can not receive data, the others must carry on  The corresponding source must be quenched  This leaves empty slots
• Error control  Errors are detected and handled by individual channel systems

Data Link Control on TDM

Pulse Stuffing

• Problem - Synchronizing data sources
• Clocks in different sources drifting
• Data rates from different sources not related by simple rational number
• Solution - Pulse Stuffing  Outgoing data rate (excluding framing bits) higher than sum of incoming rates  Stuff extra dummy bits or pulses into each incoming signal until it matches local clock  Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

TDM Example

STDM Operation

Addressing

• Addressing is required as time slots for input sources are not fixed
• For over all multiplexed data a frame is used e.g. HDLC, in that frame a sub frame can be used for each source  Sub frame will contain data and address if only single sub frame is used  Multiple sub frames can also be used but that would require a data length field
• Addressing Schemes  Use address field and length field  Use address field and length label (e.g 00,01,10,11)  Use fixed data length and use address bits

Performance

• Output data rate less than aggregate input rates
• May cause problems during peak periods  Buffer inputs  Keep buffer size to minimum to reduce delay

Buffer Size

and Delay