8/8/2019 TDC462 Multiplexing
1/49
Data Communications
Multiplexing
8/8/2019 TDC462 Multiplexing
2/49
Multiplexing
8/8/2019 TDC462 Multiplexing
3/49
Frequency Division Multiplexing
FDM
Useful bandwidth of medium exceeds requiredbandwidth of channel
Each signal is modulated to a different carrierfrequency
Carrier frequencies separated so signals do notoverlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
8/8/2019 TDC462 Multiplexing
4/49
Frequency Division Multiplexing
Diagram
8/8/2019 TDC462 Multiplexing
5/49
FDM System
8/8/2019 TDC462 Multiplexing
6/49
FDM of Three Voiceband Signals
8/8/2019 TDC462 Multiplexing
7/49
Analog CarrierSystems
AT&T (USA)
Hierarchy of FDM schemes
Group
12 voice channels (4kHz each) = 48kHzRange 60kHz to 108kHz
Supergroup
60 channel
FDM of 5 group signals on carriers between 420kHzand 612 kHz
Mastergroup
10 supergroups
8/8/2019 TDC462 Multiplexing
8/49
Synchronous Time Division
Multiplexing
Data rate of medium exceeds data rate of digitalsignal to be transmitted
Multiple digital signals interleaved in time
May be at bit level of blocksTime slots preassigned to sources and fixed
Time slots allocated even if no data
Time slots do not have to be evenly distributedamongst sources
8/8/2019 TDC462 Multiplexing
9/49
Time Division Multiplexing
8/8/2019 TDC462 Multiplexing
10/49
TDM System
8/8/2019 TDC462 Multiplexing
11/49
TDM Link Control
No headers and trailers
Data link control protocols not needed
Flow control
Data rate of multiplexed line is fixedIf one channel receiver can not receive data, the othersmust carry on
The corresponding source must be quenched
This leaves empty slots
Error controlErrors are detected and handled by individual channel
systems
8/8/2019 TDC462 Multiplexing
12/49
Data Link Control on TDM
8/8/2019 TDC462 Multiplexing
13/49
Framing
No flag or SYNC characters bracketing TDMframes
Must provide synchronizing mechanism
Added digit framingOne control bit added to each TDM frame
Looks like another channel - control channel
Identifiable bit pattern used on control channel
e.g. alternating 01010101unlikely on a data channelCan compare incoming bit patterns on each channel
with sync pattern
8/8/2019 TDC462 Multiplexing
14/49
Pulse Stuffing
Problem - Synchronizing data sources
Clocks in different sources drifting
Data rates from different sources not related by
simple rational numberSolution - Pulse Stuffing
Outgoing data rate (excluding framing bits) higher thansum of incoming rates
Stuff extra dummy bits or pulses into each incomingsignal until it matches local clock
Stuffed pulses inserted at fixed locations in frame andremoved at demultiplexer
8/8/2019 TDC462 Multiplexing
15/49
TDM ofAnalog and Digital
Sources
8/8/2019 TDC462 Multiplexing
16/49
Digital CarrierSystems
Hierarchy ofTDM
USA/Canada/Japan use one system
ITU-T use a similar (but different) system
US system based on DS-1 formatMultiplexes 24 channels
Each frame has 8 bits per channel plus one
framing bit193 bits per frame
8/8/2019 TDC462 Multiplexing
17/49
Data Communications and Computer Networks
Chapter 5
The T-1 multiplexor stream is a continuous series of frames.
8/8/2019 TDC462 Multiplexing
18/49
Digital CarrierSystems (2)
For voice each channel contains one word ofdigitized data (PCM, 8000 samples per sec)
Data rate 8000x193 = 1.544Mbps
Five out of six frames have 8 bitPCM samples
Sixth frame is 7 bitPCM word plus signaling bit
Signaling bits form stream for each channel containingcontrol and routing info
Same format for digital data23 channels of data
7 bits per frame plus indicator bit for data or systems control
24th channel is sync
8/8/2019 TDC462 Multiplexing
19/49
Mixed Data
DS-1 can carry mixed voice and data signals
24 channels used
No sync byte
Can also interleave DS-1 channelsDs-2 is four DS-1 giving 6.312Mbps
8/8/2019 TDC462 Multiplexing
20/49
8/8/2019 TDC462 Multiplexing
21/49
Basic ISDN Interface (1)
Digital data exchanged between subscriber andNTE - Full Duplex
Separate physical line for each direction
Pseudoternary coding scheme1=no voltage, 0=positive or negative 750mV +/-10%
Data rate 192kbps
Basic access is two 64kbps B channels and one16kbps D channel
This gives 144kbps multiplexed over 192kbps
Remaining capacity used for framing and sync
8/8/2019 TDC462 Multiplexing
22/49
Basic ISDN Interface (2)
B channel is basic user channel
Data
PCM voice
Separate logical 64kbps connections o differentdestinations
D channel used for control or data
LAPD frames
Each frame 48 bits long
One frame every 250Qs
8/8/2019 TDC462 Multiplexing
23/49
Frame Structure
8/8/2019 TDC462 Multiplexing
24/49
Sonet/SDH
Synchronous Optical Network (ANSI)
Synchronous Digital Hierarchy (ITU-T)
Compatible
Signal HierarchySynchronous Transport Signal level 1 (STS-1) or Optical
Carrier level 1 (OC-1)
51.84Mbps
Carry DS-3 or group of lower rate signals (DS1 DS1CDS2) plus ITU-T rates (e.g. 2.048Mbps)
Multiple STS-1 combined into STS-N signal
ITU-T lowest rate is 155.52Mbps (STM-1)
8/8/2019 TDC462 Multiplexing
25/49
SONET Frame Format
8/8/2019 TDC462 Multiplexing
26/49
SONET STS-1 Overhead Octets
8/8/2019 TDC462 Multiplexing
27/49
Statistical TDM
In Synchronous TDM many slots are wasted
Statistical TDM allocates time slots dynamicallybased on demand
Multiplexer scans input lines and collects data untilframe full
Data rate on line lower than aggregate rates ofinput lines
8/8/2019 TDC462 Multiplexing
28/49
Statistical TDM Frame Formats
8/8/2019 TDC462 Multiplexing
29/49
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
8/8/2019 TDC462 Multiplexing
30/49
BufferSize
and Delay
8/8/2019 TDC462 Multiplexing
31/49
Asymmetrical Digital
SubscriberLine
ADSL
Link between subscriber and network
Local loop
Uses currently installed twisted pair cableCan carry broader spectrum
1 MHz or more
8/8/2019 TDC462 Multiplexing
32/49
ADSL Design
Asymmetric
Greater capacity downstream than upstream
Frequency division multiplexing
Lowest 25kHz for voicePlain old telephone service (POTS)
Use echo cancellation or FDM to give two bands
Use FDM within bands
Range 5.5km
8/8/2019 TDC462 Multiplexing
33/49
ADSL Channel Configuration
8/8/2019 TDC462 Multiplexing
34/49
Discrete Multitone
DMT
Multiple carrier signals at different frequencies
Some bits on each channel
4kHz subchannelsSend test signal and use subchannels with better
signal to noise ratio
256 downstream subchannels at 4kHz (60kbps)
15.36MHz
Impairments bring this down to 1.5Mbps to 9Mbps
8/8/2019 TDC462 Multiplexing
35/49
DMT Transmitter
8/8/2019 TDC462 Multiplexing
36/49
Wavelength DivisionMultiplexing
Wavelength division multiplexing multiplexes multipledata streams onto a single fiber optic line.
Different wavelength lasers (called lambdas) transmitthe multiple signals.
Each signal carried on the fiber can be transmitted at a
different rate from the other signals.Dense WDM High number of lambdas
Coarse WDM A few lambdas
8/8/2019 TDC462 Multiplexing
37/49
Code Division MultiplexingAlso known as code division multiple access
An advanced technique that allows multiple devices to
transmit on the same frequencies at the same time.Each mobile device is assigned a unique 64-bit code
To send a binary 1, mobile device transmits the uniquecode
To send a binary 0, mobile device transmits the inverseof code
8/8/2019 TDC462 Multiplexing
38/49
Code Division Multiplexing
Receiver gets summed signal, multiplies it by receiver
code, adds up the resulting valuesInterprets as a binary 1 if sum is near +64
Interprets as a binary 0 if sum is near 64
8/8/2019 TDC462 Multiplexing
39/49
Code Division MultiplexingExample
For simplicity, assume 8-chip spreading codes
3 different mobiles use the following codes:
-Mobile A: 10111001
-Mobile B: 01101110
-Mobile C: 11001101
Assume Mobile A sends a 1, B sends a 0, and C sends a1
8/8/2019 TDC462 Multiplexing
40/49
Code Division Multiplexing
Example
Signal code: 1-chip = +N volt; 0-chip = -N volt
Three signals transmitted:
-Mobile A sends a 1, or 10111001, or +-+++--+
-Mobile B sends a 0, or 10010001, or +--+---+
-Mobile C sends a 1, or 11001101, or ++--++-+
Summed signal received by base station: +3, -1, -1,+1, +1, -1, -3, +3
8/8/2019 TDC462 Multiplexing
41/49
Code Division MultiplexingExample
Base station decode for Mobile A:
Signal received: +3,-1,-1,+1,+1,-1,-3,+3
Mobile As code: +1,-1,+1,+1,+1,-1,-1,+1
Product result: +3,+1,-1,+1,+1,+1,+3,+3
Sum ofProduct results: +12Decode rule: For result near +8, data is binary 1
8/8/2019 TDC462 Multiplexing
42/49
Code Division Multiplexing
Example
Base station decode for Mobile B:
Signal received: +3,-1,-1,+1,+1,-1,-3,+3
Mobile Bs code: -1,+1,+1,-1,+1,+1,+1,-1
Product result: -3,-1,-1,-1,+1,-1,-3,-3
Sum ofProduct results: -12
Decode rule: For result near -8, data is binary 0
8/8/2019 TDC462 Multiplexing
43/49
Optical Spatial Division
Multiplexing
Improves network utilization of SONET networks
Fact data traffic is often bursty
Fact SONET is sync TDM
Sync TDM does not like bursty traffic
OSDM is not limited to multiples of 1.544 Mbpscontainers
8/8/2019 TDC462 Multiplexing
44/49
Orthogonal Frequency
Division Multiplexing
OFDM is a discrete multi-tone technology
Numerous signals of different frequencies are combinedto form a single signal for transmission
Before combining, each carrier is phase modulated torepresent bits
HomePlug technology modulates data bits on 84individual carriers ranging from 4 MHz 21 MHz
8/8/2019 TDC462 Multiplexing
45/49
Business Multiplexing InAction
XYZ Corporation has two buildings separated by a
distance of 300 meters.A 3-inch diameter tunnel extends underground betweenthe two buildings.
Building A has a mainframe computer and Building B
has 66 terminals.
List some efficient techniques to link the two buildings.
8/8/2019 TDC462 Multiplexing
46/49
Data Communications and Computer Networks
Chapter 5
8/8/2019 TDC462 Multiplexing
47/49
Possible SolutionsConnect each terminal to the mainframe computerusing separate point-to-point lines.
Connect all the terminals to the mainframe computerusing one multipoint line.
Connect all the terminal outputs and use microwavetransmissions to send the data to the mainframe.
Collect all the terminal outputs using multiplexing andsend the data to the mainframe computer using aconducted line.
8/8/2019 TDC462 Multiplexing
48/49
Review Questions
1. What is FDM used for? What are itsadvantages? Disadvantages?
2. If you FDM 20 channels together, each channel40,000 Hz, what is total bandwidth?
3. What is TDM used for? What are itsadvantages? Disadvantages?
4. If you sync TDM 40 voice channels together,what is the total data rate?
5. What is the advantage of statTDM? Anydisadvantages?
8/8/2019 TDC462 Multiplexing
49/49
Review Questions
6. In a T-1, how many frames are transmitted persecond? How many channels within oneframe? What is the sync bit used for?
7. In ISDN
, how many channels are transmittedon a basic rate service? What is the data rateof each of those channels? What is the Dchannel used for?