FIT 1005 Networks & Data Communications
Lecture 7 – Multiplexing
Reference: Chapter 8
Data and Computer Communications
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Updated / modified slides : http://users.monash.edu.au/~amkhan/fit1005
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Multiplexing
• Multiplexing is the process where multiple
channels are combined for transmission over a
common transmission path.
• Full capacity of data transmission links are not
always fully utilized
• To make efficient use of high-speed
telecommunications lines, some form of
multiplexing is used
• common application of multiplexing is done in
long-haul communications.
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Multiplexing – in long-haul communications.
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Multiplexing
• Trunks on long-haul networks are high-capacity fiber, coaxial,
or microwave links.
• These links can carry large numbers of voice and data
transmissions simultaneously using multiplexing.
• Common forms of multiplexing are:
– Frequency Division Multiplexing (FDM),
Multiplexing
– Time Division Multiplexing (TDM), and
– Statistical TDM (STDM).
– Wave Division Multiplexing (WDM)
Analog
Digital
FDM
WDM
(S)TDM
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Multiplexing
• multiple n inputs / 1 outputs on 1 physical line
• multiplexing allows several transmission sources
to share a larger transmission capacity
• the link can carry multiple channels of data
• common on long-haul, high capacity, links
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Frequency Division Multiplexing
• Division of a transmission link into multiple channels by
splitting the frequency band into multiple slots.
• Used when useful bandwidth of the link is greater than
required bandwidth of individual signals to be
transmitted.
• Each signal is modulated on a different carrier frequency.
• Carrier frequencies are centered at each signal BW and
typically separated by guard bands so that signals do not
overlap.
• An FDM receiver uses filters, one per slot, to separate the
individual channels, each of which is separately
demodulated to extract the signal.
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Frequency Division Multiplexing
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FDM
System
Overview
D to A m n(t)
FDM animation
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http://www.netbook.cs.purdue.edu/animations/fdm.html
FDM System - A conceptual illustration of the
multiplexing and de-multiplexing process.
Each source generates
a signal of a similar
frequency range.
Multiplexer modulates these
similar source signals with
different subcarrier
frequencies (f1, f2, and f3)
The de-multiplexer uses a
series of filters to decompose
the multiplexed signal into its
constituent component
signals.
at Transmitter
This results in composite
baseband modulating signal
that is sent out over a media
link that has enough bandwidth
to accommodate it.
at Receiver
The individual signals are then
passed to a demodulator
that separates them from their
carriers and passes them to the
output lines
FDM Voiceband Example
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Analog Carrier Systems
• long-distance links use an FDM hierarchy
• AT&T (USA) and ITU-T (International) variants
• Group
– 12 voice channels (4kHz each) = 48kHz - bandwidth
– in range 60kHz to 108kHz
• Supergroup
– FDM of 5 group signals supports 12 x 5 = 60 channels
– 48 x 48kHz = 240kHz - BandWidth
– on carriers between 312kHz and 552 kHz
• Mastergroup
– FDM of 10 supergroup supports 60 x 10 = 600 channels
• so original signal can be modulated many times
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Wavelength Division Multiplexing
• FDM with multiple beams of light at different
frequencies
• WDM carried over optical fiber links uses λ wavelength
– commercial systems with 160 channels of 10 Gbps
– lab demo of 256 channels 39.8 Gbps
• Architecture similar to other FDM systems
– multiplexer consolidates laser sources (1550nm)
of different λ ‘s for transmission over single fiber
– Optical amplifiers amplify all wavelengths
– Demux separates channels at the destination
• There are two types of WDM, namely:
– Coarse WDM (CWDM) - here the wavelengths are spaced well apart
– Dense WDM (DWDM) - here larger number of closely spaced wavelengths
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Synchronous Time Division Multiplexing
• Time slots on a shared medium are assigned to
devices on a fixed, predetermined basis
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TDM System
Overview
TDM animation-1
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http://www.netbook.cs.purdue.edu/animations/tdm.html
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TDM Link Control
• no headers and trailers
• data link control protocols (flow & error) not needed
• flow control
– data rate of multiplexed line is fixed
– if one channel receiver cannot receive data, the others must
carry on
– corresponding source must be quenched
– leaving empty slots
– so no flow control needed
• error control
– errors detected & handled on individual channel or Perchannel basis
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Data Link Control on TDM
HDLC frame per channel basis
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Framing
• no flag or SYNC chars bracketing TDM frames
• must still provide synchronizing mechanism
between src and dest clocks
• added digit framing
–
–
–
–
one control bit added to each TDM frame
identifiable bit pattern used on control channel
eg. alternating 01010101…unlikely on a data channel
compare incoming bit patterns on each channel with known
sync pattern
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Pulse Stuffing
• have problem of synchronizing various data sources
• with clocks in different sources drifting
• also issue of data rates from different sources not related by
simple rational number
• Pulse Stuffing a common solution
– to have outgoing data rate (excluding framing bits) higher than
sum of incoming rates
– by stuffing extra dummy bits or pulses into each incoming
signal until it matches the local clock
– stuffed pulses inserted at fixed locations in frame and are
removed at the de-multiplexer
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TDM Example with pulse stuffing
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Digital Carrier Systems
•
•
•
•
•
long-distance links use an TDM hierarchy
AT&T (USA) and ITU-T (International) two variants
US system based on DS-1 format
can carry mixed voice and data signals
24 channels used for total data rate 1.544Mbps (193 x 8000
samples/sec)
• each voice channel contains one word of digitized data (PCM,
8000 samples per sec)
• same format for 56kbps digital data
• can interleave DS-1 channels for higher rates
– DS-1 is 24 channels at 1.544 Mbps
– DS-2 is four DS-1 at 6.312Mbps
– DS-3 is eight DS-1 at 44.736 Mbps
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DS-1 Transmission Format
The first bit is used as framing bit, this is used for synchronization.
For Voice Channels:
1.
2.
For five of every six frames, 8-bit PCM samples are used, and
For every sixth frame, each channel contains a 7-bit PCM + 1-bit for signalling
(contains network control and routing information)
For Data Channels:
1.
2.
3.
23 Channel are used for data and Channel-24 is used for signalling.
If 7 bits used per channel i.e. Bits 1 to 7 are used for data then each channel can
achieve (7bits x 8000 frames/sec =56 kbps)
If 6 bits are used per channel i.e. Bits 2 to 7 are used for data then each channel
can achieve ( 6bits x 8000 frames/sec = 48 kbps ).
•
By multiplexing we can achieve 5 x 9.6 kbps, 10 x 4.8 kbps, or 20 x 2.4 kbps.
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Statistical TDM
• In Synch TDM many slots are wasted
• Statistical TDM allocates time slots dynamically
based on demand
• multiplexer scans input lines and collects data until
frame full
• line data rate lower than aggregate input line rates
• may have problems during peak periods
– must buffer inputs
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Statistical TDM Frame Format
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Cable Modems
• To support full duplex data transfer two cable TV
channels are reserved
• each channel is shared by a number of subscribers,
using statistical TDM
• Downstream channel
– cable scheduler delivers data in small packets
– active subscribers share downstream capacity
– also allocates upstream time slots to subscribers
• Upstream channel
– user requests timeslots on shared upstream channel
– Head-end scheduler notifies subscriber of slots to use
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Cable Modem Scheme (uses statistical TDM)
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Asymmetrical Digital Subscriber Line (ADSL)
•
•
•
•
link between subscriber and network
uses currently installed twisted pair telephone cable
It is Asymmetric - bigger downstream than upstream
uses Frequency division multiplexing (FDM)
– reserve lowest 25kHz for voice - Plain old telephone service
(POTS)
– uses echo cancellation or FDM to get two distinct bands
• has a range of up to 5.5km
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ADSL Channel
Configuration
Uses channel separation i.e. FDM



exploit the 1-MHz capacity of twisted pair
0-20KHz for voice POTS i.e. Only 0-4khz
for voice and rest for guard band
Remaining 25KHz - 1000KHz of the BW
for data upload and download streams
Uses – 2 techniques for internet access (data):
1.Echo cancellation : A signal processing
technique
– allow transmission of digital signals in
both direction, single line
simultaneously
2.FDM to allocate two bands – Upstream and
Downstream
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Discrete Multitone (DMT)
•
•
•
•
•
Uses multiple carrier signals at different frequencies
Bandwidth(BW) is divided into 4kHz of sub-channels
DMT modems tests and uses sub-channels with better SNR
DMT modem assigns more data to channels with large SNR’s
ADSL / DMT both design & employ 256 downstream sub-channels
at 4kHz (each channel 0 - 60kbps)
– in theory (256 x 60kbps)=15.36Mbps, in practice 1.5 - 9Mbps
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DMT Transmitter
Each sub-stream is then
converted to an Analog signal
using Quadrature amplitude
modulation (QAM),
Input bit stream is
Divided into multiple
sub-streams
Input bit stream
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xDSL
• High data rate DSL (HDSL)
– 2B1Q coding on dual twisted pairs (2B1Q means that two bits are
combined to form a single Quaternary line state symbol one of four signal levels on
the line)
– up to 2Mbps over 3.7km
• Single line DSL (SDSL)
– 2B1Q coding on single twisted pair (residential) with echo
cancelling
– up to 2Mbps over 3.7km
• Very high data rate DSL (VDSL)
– DMT/QAM for very high data rates
– over separate bands for separate services
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Summary
• looked at multiplexing multiple channels on a
single link
• FDM
• TDM
• Statistical TDM
• ADSL and xDSL
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Term
Line Rate
Also known as
E1
2.048 Mbps
T1
1.544 Mbps (24 x 64 kbps)
DS1
T2
6.312 Mbps
DS2
T3
Ethernet
Fast Ethernet
Token Ring
DS0
DS1
DS2
DS3
OC-1
OC-3
OC-12
OC-24
OC-48
OC-96
OC-192
ISDN Basic (2B+D)
ISDN B Channel
ISDN D Channel
ISDN Primary (24B+D)
Sonet
SDH
PDH
44.736 Mbps
10 Mbps
100 Mbps
4 or 16 Mbps
64 kbps
1.544 Mbps
6.312 Mbps
44.736 Mbps
51.84 Mbps
155.52 Mbps
622.08 Mbps
1244.16 Mbps
2488.32 Mbps
4976.64 Mbps
9953.28 Mbps
2 x 64 kbps & 1 x 16 kbps
64 kbps
16 kbps
24 x 64 kbps & 1 x 16 kbps
51.84 Mbps - 10 Gbps
155 Mbps - 10 Gbps
T1/E1 - 565 Mbps
DS3
T1
T2
T3
STS-1
STS-3 & STM-1
STS-12 & STM-4
STS-24 & STM-8
STS-48 & STM-16
STS-96 & STM-32
STS-192 & STM-64
ISDN Signalling Channel
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