cdm-fdm tdm

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Time Division Multiplexing The concept of Time Division Multiplexing TDM Examples Frame Synchronization TDM Hierarchy Packet Transmission

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Page 1: CDM-FDM TDM

Time Division Multiplexing The concept of Time Division Multiplexing

TDM Examples

Frame Synchronization

TDM Hierarchy

Packet Transmission

Page 2: CDM-FDM TDM

Frequency Division MultiplexFrequency Division Multiplex Separation of spectrum into smaller frequency Separation of spectrum into smaller frequency

bandsbands Channel gets band of the spectrum for the whole Channel gets band of the spectrum for the whole

timetime Advantages:Advantages:

no dynamic coordination neededno dynamic coordination needed works also for analog signalsworks also for analog signals

Disadvantages:Disadvantages: waste of bandwidth waste of bandwidth

if traffic distributed unevenlyif traffic distributed unevenly inflexibleinflexible guard spacesguard spaces

k3 k4 k5 k6

f

t

c

Channels ki

Page 3: CDM-FDM TDM

f

t

ck2 k3 k4 k5 k6k1

Time Division MultiplexTime Division Multiplex

Channel gets the whole spectrum Channel gets the whole spectrum for a certain amount of timefor a certain amount of time

Advantages:Advantages: only one carrier in theonly one carrier in the

medium at any timemedium at any time throughput high even throughput high even

for many usersfor many users Disadvantages:Disadvantages:

precise precise synchronization synchronization necessarynecessary

Channels ki

Page 4: CDM-FDM TDM

f

Time and Frequency Division MultiplexTime and Frequency Division Multiplex

A channel gets a certain frequency band for A channel gets a certain frequency band for a certain amount of time (e.g. GSM)a certain amount of time (e.g. GSM)

Advantages:Advantages: better protection against tappingbetter protection against tapping protection against frequency protection against frequency

selective interferenceselective interference higher data rates compared tohigher data rates compared to

code multiplexcode multiplex Precise coordinationPrecise coordination

requiredrequired

t

c

k2 k3 k4 k5 k6k1

Channels ki

Page 5: CDM-FDM TDM

Code Division MultiplexCode Division Multiplex

Each channel has unique codeEach channel has unique code All channels use same spectrum at same timeAll channels use same spectrum at same time Advantages:Advantages:

bandwidth efficientbandwidth efficient no coordination and synchronizationno coordination and synchronization good protection against interferencegood protection against interference

Disadvantages:Disadvantages: lower user data rateslower user data rates more complex signal regenerationmore complex signal regeneration

Implemented using spread spectrum Implemented using spread spectrum technologytechnology

k2 k3 k4 k5 k6k1

f

t

c

Channels ki

Page 6: CDM-FDM TDM

Multiplexing Two basic forms of multiplexing. Two basic forms of multiplexing.

(a)(a) Frequency-division multiplexing Frequency-division multiplexing (F(FDMDM)) (with guardbands). (with guardbands).(b)(b) Time-division multiplexing Time-division multiplexing ((TDMTDM)); no provision is made here for ; no provision is made here for

synchronizing pulses.synchronizing pulses.

FDMTDM

Page 7: CDM-FDM TDM

TDM Composition of one frame of a multiplexed PAM signal incorporating four voice-Composition of one frame of a multiplexed PAM signal incorporating four voice-signals and a synchronizing pulse. signals and a synchronizing pulse.

Page 8: CDM-FDM TDM

Frequency Division Multiplexing (FDM) Block diagram of FDM system, showing the important constituents of the transmitter and Block diagram of FDM system, showing the important constituents of the transmitter and receiver.receiver.

Page 9: CDM-FDM TDM

Time Division MultiplexingDefinition: Time Division Multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel.

At the Transmitter

Simultaneous transmission of several signals on a time-sharing basis.

Each signal occupies its own distinct time slot, using all frequencies, for the duration of the transmission.

Slots may be permanently assigned on demand.

At the Receiver

Decommutator (sampler) has to be synchronized with the incoming waveform Frame Synchronization

Low pass filter

ISI – poor channel filtering

Feedthrough of one channel's signal into another channel -- Crosstalk

Applications of TDM: Digital Telephony, Data communications, Satellite Access, Cellular radio.

Page 10: CDM-FDM TDM

Time Division Multiplexing

Conceptual diagram of multiplexing-demultiplexing.Conceptual diagram of multiplexing-demultiplexing.

PAM TDM System

Page 11: CDM-FDM TDM

Illustrating 4-Channel PAM TDM Multiplexing

Page 12: CDM-FDM TDM

Digital Time Division Multiplexing

Digit Interleaving

WORD or Byte Interleaving

Interleaving channel with different bit rates

Interleaving channel with different bit rates using two

multiplexers

Time Division Multiplexing (TDM) can be accomplished at bit or byte (word) level.

Channhels having different data rates can also be TDM multiplexed but must be interleaved accordingly.

Page 13: CDM-FDM TDM

Block diagram of TDM system.Block diagram of TDM system.

PAM TDM System

A Typical Framing Structure for TDM

Page 14: CDM-FDM TDM

Time Division Multiplexing

Frame structure of a certain TDM signal

Composite Signal Format

Page 15: CDM-FDM TDM

Time Division Multiplexing

Pulse width of TDM PAM:

s

s

f

T

3

1

3

n

Ts

3

Pulse width of TDM PCM:

s

1 satisfies Nyquist rates

s

f fT

Page 16: CDM-FDM TDM

Pulse Stuffing in TDM Stuff bits, which are dummy bits are inserted in the TDM output data when the different inputs are not completeley synchronized or the different input rates are not related by a ratinal number.

Page 17: CDM-FDM TDM

Pulse Stuffing in TDM Stuff bits, which are dummy bits are inserted in the TDM output data when the different inputs are not completeley synchronized or the different input rates are not related by a ratinal number.

Multiplexing of two data streams with bit stuffing

Page 18: CDM-FDM TDM

TDM Example (Multiplexing Analog and Digital)

Source 1: 2 kHz bandwidth.

Source 2: 4 kHz bandwidth.

Source 3: 2 kHz bandwidth.

Source 4-11: Digital 7200 bits/sec.

64 kb/s

16 ksam/s

128 kb/s

8x7.2=57.6 kb/sUse stuff bits to complete 7.2 to 8 kb/s.Now 8 and 64 rates are complete multıples

Page 19: CDM-FDM TDM

Frame Synchronization

To sort and direct the received multiplexed data to the appropriate output channel

Two ways to provide frame sync to the demultiplexer circuit

- Over a separate channel

- Deriving from the TDM signal itself

Frame sync (unique k-bits) +Information words of an N-channel TDM system

Page 20: CDM-FDM TDM

TDM PAM for Radio Telemetry

Page 21: CDM-FDM TDM

CCITT Digital TDM Hierarchy

Page 22: CDM-FDM TDM

Packet Transmission System

TDM is Synchronous Transfer Mode (STM) technology

- Data source is assigned a specific time slot – fixed data rate

- More efficient when sources have a fixed data rate

- Inefficient to accommodate bursty data source

Solution?

Packet Transmission System

- Partitions source data into data packets (destination address, header)

- Efficiently assigns network resources when the sources have bursty data

- Examples : Internet TCP/IP technology and the Asynchronous Transfer Mode (ATM) technology.

Page 23: CDM-FDM TDM

Summary

How information in analog waveforms can be represented by digital signaling

How to compute the spectra for line codes

How filtering of the digital signal, due to the communication channel affects our ability to recover the digital information at the receiver [ISI]

How we can merge information from several sources into one digital signal by using time division multiplexing (TDM)