cdm-fdm tdm
TRANSCRIPT
Time Division Multiplexing The concept of Time Division Multiplexing
TDM Examples
Frame Synchronization
TDM Hierarchy
Packet Transmission
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
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
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
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
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
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.
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.
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.
Time Division Multiplexing
Conceptual diagram of multiplexing-demultiplexing.Conceptual diagram of multiplexing-demultiplexing.
PAM TDM System
Illustrating 4-Channel PAM TDM Multiplexing
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.
Block diagram of TDM system.Block diagram of TDM system.
PAM TDM System
A Typical Framing Structure for TDM
Time Division Multiplexing
Frame structure of a certain TDM signal
Composite Signal Format
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
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.
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
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
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
TDM PAM for Radio Telemetry
CCITT Digital TDM Hierarchy
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.
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)