CEN 342Introduction to Data Transmission
Chapter 8Multiplexing
Dr. Mostafa Hassan DahshanComputer Engineering DepartmentCollege of Computer and Information SciencesKing Saud University
Multiplexing
Communication links are expensive
Using one link/party is inefficient
Many applications require modest data rates
Sharing link is more cost efficient
Link sharing requires multiplexing
Multiplexing Types
Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
Statistical Time Division Multiplexing
Frequency Division Multiplexing
Possible with large bandwidth
Multiple signals carried simultaneously
Each signal modulated onto different carrier frequency
Carrier frequencies must be sufficiently separated
Bandwidths should not overlap
Frequency Division Multiplexing
Frequency Division Multiplexing
Example: FDM of TV Signals
Example – FDM of Voice Signals
Bandwidth of voice signal (300-3400 Hz)
Generally taken as 4 kHz
Using AM with carrier frequency 64 kHz
Spectrum of modulated signal is 8 kHz60 kHz – 68 kHz
To make efficient use of bandwidthtransmit only lower sideband
FDM Problems
Crosstalkspectrum overlap between adjacent component signals
guard band should be added
e.g. voice 4 kHz instead of 3100 Hz
Intermodulation noiseamplifiers produce frequency components of other channels
Wavelength Division Multiplexing
FDM used in optical fiber
Multiple signals use different frequency
WDM is the commonly used term
Each wavelength carry channel of data
More channels, closely spaceddense wavelength division multiplexing DWDM
160 channels, each 10 Gbps now available
Time Division Multiplexing
Multiple digital signals carried on single path
Portions of each signal interleaved in time
Interleaving: bit level, blocks of bytes, larger
Data from each source is buffered
Buffers scanned sequentially
Data organized into frames
Time Division Multiplexing
Rate of mc(t) must be ≥ Σni mi(t)
Time Division Multiplexing
Synchronous TDM
Time slots pre-assigned to sources
Slot transmitted even if source has no data
May waste capacity but simple to implement
Different data rates are possible
Fast source can be assigned multiple slots
Slots dedicated to source called channel
Framing
Frame sync to identify frame boundaries
Added-digit framingone control bit added to each TDM frame
effectively another channel (control channel)
bit pattern 101010… unlikely on data channel
Synchronizing Multiple Sources
Most difficult problem in TDM design
If each source has separate clockany variation among clock loss of sync
Input data rates not related by simple rational number
Pulse stuffing is used to solve this problem
Pulse Stuffing
Outgoing data rate of multiplexer > sum of max instantaneous incoming rates
Extra capacity used to stuff extra bits
Dummy bits/pulses added to each input until rate matches local clock
Stuffed pulses added at fixed locations
Removed by demultiplexer
Example
Input
Source 1: Analog, 2 kHz
Source 2: Analog, 4 kHz
Source 3: Analog, 2 kHz
Sources 4-11: Digital, 7200 bps
Example
For analog sources
Sources 1, 3 sampled at 4000 samples/s
Source 2 at 8000 samples/s
PCM, quantized using 4 bits/sample
2 sample / scan for source 2 (8 bits)
1 sample / scan for sources 1, 3 (8 bits)
Total sources 1-3 =16 × 4000 = 64 kbps
Example
For digital sources
Pulse stuffing raise each source to 8 kbps
For aggregate data rate = 64 kbps
Frame bit allocation
Suppose frame = 32 bits
16 bits for PCM sources 1-3 (1:4, 2:8, 3:4)
2 bits for each source from 4-11 = 16 bits
Digital Carrier SystemsSynchronous TDM transmission structure
TDM performed at multiple levels
Hierarchy of TDM structuresUS, Canada, Japan use AT&T system
Other countries use ITU-T system
DS-1 Transmission Format
Voice Transmission
Voice is PCM digitized 8000 samples/s
Frame rate must be 8000 frames/s
Frame length = 24 × 8 + 1 = 193 bits
Data rate = 8000 × 193 = 1.544 Mbps
Control bits used in every 6th frame
Data Transmission
Same 1.544 Mbps data rate used
23 channels for data, 1 for sync byte
Within channel7 bits used for data
1 bit indicate channel is user or sys control data
7 × 8000 = 56 kbps max rate / channel
SONET/SDH
SONET (Synchronous Optical Network)optical transmission interface
proposed by BellCore, standardize by ANSI
SDH (Synchronous Digital Hierarchy)compatible version published by ITU-T
few differences from SONET
Signal Hierarchy
SONET defines hierarchy of data ratesLowest STS-1 / OC-1: 51.84 Mbps
STS-1 carry 1 DS-3 or group of DS-1
Multiple STS-1 combined to form STS-N
SDHlowest rate is STM-1: 155.52 = STS-3
Signal Hierarchy
Frame Format
Frame consists of 810 octets
Transmitted every 125 μs = 8000 frame/s
810×8 bit/frame × 8000 frame/s = 51.84Mbps
Frame logically viewed as matrix9 rows, 90 octets each
transmitted one row at time
first 3 cols (27 octets) are overhead
payload includes a column for path overhead
Statistical TDM
TDM does not efficiently utilize capacity
Many times, slots are wasted
Statistical TDM allocates slots on demand
Number of lines n < number of time slots k
Not slots are reserved for specific input line
Multiplexer collects data until frame is filled
Statistical TDM
Output data rate < sum input rates
Can take more sources than TDM at same output rate
or less output rate for same sources as TDM
More overhead than TDMslot positions must be identified
address information must be included with data
Statistical TDM
Frame Structure
Control information is needed
Two possible formats
One data source per frameneed to identify address of source
work well under light load
inefficient under heavy load
Multiple sources per frameneed to identify length of data of each source
Frame Structure
Digital Subscriber Line (DSL)
Subscriber line: customer to central office
Carry voice grade signal: 0 – 4 kHz
Wire can support 1 MHz or more
Provide high speed data over phone line
Asymmetric DSL (ADSL)more downstream rate the upstream
most home user traffic is downstream
ADSL Design
Lowest 25 kHz reserved for voiceknown as plain old telephone service (POTS)
more than 4 kHz to prevent crosstalk
FDM or echo cancellation to allocate bandssmaller upstream, larger downstream
FDM used within each bandbit stream split into multiple parallel bit streams
each portion carried in separate frequency band
Echo Cancellation
Allow simultaneous transmission in both directions on the same band
To recover received signal, transmitter subtracts echo of its own transmission
Frequency band of up/down stream overlap
Echo Cancellation
Advantagesless attenuation in low frequency range
more downstream band in good part of spectrum
more flexible allocation of up/down stream bands
Disadvantageslogic for EC must be installed both sides
more complexity
Discrete Multitone (DMT)
Used in ADSL transmission
Multiple carrier signals different frequencies
Transmission band divided to 4kHz channels
Send some bits on each sub-channel
Substream converted to analog using QAM
QAM can assign different bits / signal
Total data rate = sum of sub-channel rates
Discrete Multitone (DMT)
Initially, DMT modem sends test signals
Test signals sent on all channels to test SNR
More bits assigned to better SNR channels
Each channel carries between 0-60 kbps
ADSL/DMT Transmission
ADSL/DMT Transmission
Design uses 256 downstream sub-channels
Each sub-channel is 4 kHz
Max possible rate 60 kbps× 256 = 15.36 Mbps
In practice, limited by impairments
Actual rates from 1.5 to 9 Mbps
Rate depends on distance and quality
xDSL
ADSL = Asymmetric DSLHDSL = High data rate DSL
SDSL = Single line DSLVDSL = Very high data rate DSL
Additional References
DS0, DS1, DS3, T1, T3 Dedicated FAQ, dedicated-voice-data.alllongdistance.com/dedfaq.shtml
An introduction to ADSL, people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture13/DSL/DSL.html