Chapter 8: Multiplexing COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal.
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Transcript of Chapter 8: Multiplexing COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal.
Chapter 8:
Multiplexing
COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal
2
Contents
1. Introduction
2. Two Multiplexing Techniques1. FDM
2. TDM1. Synchronous
2. Statistical
3. Application: ADSL
(Asymmetric Digital Subscriber Line)
3
Introduction Multiplexing: A generic term used when more
than one application or source share the capacity of one link
Objective is to achieve better utilization of the link bandwidth (channel capacity)
Multiplexer Demultiplexer
4
Motivation High capacity (data rate) links are cost effective. i.e.
it is more economical to go for large capacity links But requirements of individual users are usually fairly
modest…e.g. 9.6 to 64 kbps for non intensive (graphics, video) applications
Solution: Let a number of such users share the high capacity channel (Multiplexing)
Example: Long haul trunk traffic: High capacity links: Optical fiber, terrestrial microwaves, etc.
carrying large number of channels between cities over large distances
5
Multiplexing Types Our three resources:
Space Time Frequency Our channels must be
separated in at least one resource (can overlap in the other two)
The resource in which they are separated is “divided” between them: SDM: Separation in space TDM: Separation in time FDM: Separation in frequency
Space
FrequencyTime
TDM:Time Division Multiplexing
To use the same circuit (line)i.e. sharing space:
Use either TDM or FDM
FDM:Frequency Division Multiplexing
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Frequency Division Multiplexing (FDM)(With Analog Signals)• Channels exist on the same line (space) at the same time:
• Must be separated in frequency!
f
t
7
FDM Useful bandwidth of medium exceeds required
bandwidth of a channel Signal of each channel is modulated on a different
carrier frequency fc
So, channels are shifted from same base band by different fc’s to occupy different frequency bands
Carrier frequencies separated so that channels do not overlap (also include some guard bands)
Disadvantage: Channel spectrum is allocated even if no data available for transmission in channel (rigid allocation)
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FDM Multiplexing Process: Frequency-Domain View at TX
All source channels are at (same) base band
ff
f1
f2
f3
0 4 KHz f1 f2 f3
Channels go on the same Link at the same time… butin different frequency bands
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FDM De-Multiplexing Process: Frequency-Domain View at RX
f1
f2
f3
0 4 KHz
All received channels restored to base band
• Guard bands prevent channel overlap• But represent wasted spectrum
f
Restoration at RX:3 different pass-band filters,each bracketing a channel
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FDM System – Transmitter
Subcarriers
Main CarrierAny type of modulation:AM, FM, PM
Group of channels
To meet Transmission Requirements
Individual base band channels
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FDM System – Receiver
fc
Composite base band signal mb(t)
recovered
Individual base band channels
recovered
Subcarriers
Main Carrier
f1
f2
f3
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FDM of Three Voice band Signals
What is the modulation type ?
3 MUXed channelUsing lower side band only
3 Subcarriers at:64, 68, and 72 KHz
Channel overlap means crosstalk!
Guard bandsTo reduce channel spectrum overlap
BW, allocated: 0 – 4000 Hz
BW, actual : 300 – 3400 Hz
fc
Assume we will keep only the lower side band
for each channel
13
Analog Carrier Systems: How to MUX 4000 Channels?
One-go Vs Hierarchical: (in stages)
…. Channel ….
Group ….
Master super group
Super group ….
4000 channels 4000 channels
... ...
• Modular approach• Easier to implement: e.g. fewer sub carriers• Also, not all channels may be available at one place
Stages
….
….
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Analog Carrier Systems Devised by AT&T (USA) Hierarchy of FDM schemes: MUXing in stages Group: AM, Lower Side band
12 voice channels = 12 x 4 kHz = 48 kHz BW 12 sub carriers: 64 kHz – 108 kHz in 4 KHz intervals Frequency range for group: 60 kHz – 108 kHz = 48 kHz (lower side band)
Super group: FM 5 groups = 5 x 48 kHz = 240 kHz BW 5 sub carriers: 420 kHz - 612 kHz at 48 KHz intervals (No GBs bet. groups) Frequency range: 312 kHz – 552 kHz = 240 kHz
Master group: FM 10 super groups = 10 x 240 kHz = 2400 kHz BW 10 sub carriers: 1116 kHz - 3396 kHz (Min of 8 KHz GBs between SGs) BW of 2.52 MHz (> 10 x 240 KHz = 2.4 MHz due to GB between SGs)
Jumbo group: FM
6 master groups i.e. total of 6 x 10 x 5 x 12 = 3600 voice channels BW of 16.984 MHz (> 3600 x 4 KHz due to gaud bands between super groups)
Each channel is 300 to 3400 = 3100 Hz. 4000 Hz provides 900 Hz guard band
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Analog Carrier Systems
12
8 KHz
GroupChannel Super group Master super group
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Analog FDM Hierarchy
0.24 x 10 Vs 2.52
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FDM characteristic problems Two potential problems characterize FDM and
all broadband applications Crosstalk:
- Due to overlap between channel spectra and the use of non-ideal filters to separate them
- Use guard bands to reduce it
Inter modulation noise:
- Nonlinearities in amplifiers ‘mix’ channels
- This generates spurious frequency components (sum, difference) which fall within channel BWs!
- Limits the amount of amplification possible
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Usually uses synchronous transmission,
but asynchronous is also possible Data rate of medium exceeds data rate of digital
signals to be transmitted for one channel Digital signals of multiple channels interleaved in time Interleaving may be:
At bit level At block level (e.g. bytes)
Two types: Synchronous TDM (Fixed rotation on channels) Statistical or asynchronous TDM (More efficient utilization of
the time slots)
Time Division Multiplexing (TDM)
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Time Division Multiplexing
300 HzChannels occupy the same frequency band(Base band)and go on the same link
Channels must go on the link atdifferent times
3400 Hz
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Time Division Multiplexing (TDM)All BasebandSignals- share same Frequency band
Ts = 1/2fmax
Channel sampling interval
Note: MUXing and DeMUXingare transparent to the end stations.Each pair ‘think’ they have a dedicated link !
Time
RotateAt the channel sampling rate
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TDM Frames
Channel sampling interval = 3TFor each channel, data rate is 1 sample/3T
1
2
3
4
5
6
123456….Sample Number
On the link: Data is sent at a rate of 1 sample/TData rate is 3 times the channel data rate
Sampling Interval
Sampling IntervalChannel data rate = R Link data rate = NR
N Channels
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Synchronous TDM: (Fixed channel scan arrangement) Time slots pre-assigned to sources and fixed Disadvantage: Time slots allocated even if no data
available (channel capacity waste, as with BW waste in FDM)
But simple to implement, e.g. No need to send ID of source channel
We could assign more than time slot per scan for faster sources- but on a permanent basis
Time Division Multiplexing (TDM)
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Synchronous TDM – Transmitter
Scanning and link data rate: high enough to prevent channel buffers overflowing
N channels,Sampling rate R sample/s Minimum link
capacity = N R sample/s
Analog Signal
Digital Signalor
Transmitted frames consist of interleaved channel data
Time Slot, T Channel dwell time
T Should be enough to
empty a channel buffer
Channel 2
Bit stream
Channel Buffers
Sample data fills buffer
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TDM System – Receiver
TDM signal
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Data Link Control with Sync TDM
Data rate on the link (multiplexed line) is fixed and MUX and DEMUX must operate at it
Data link control protocol not needed on the MUXED line Flow control Channel based
If one channel receiver is not ready to receive data, other channels will carry on
Channel-based flow control would then halt corresponding source channel
This causes transmission of empty slots for that channel in the MUXED data
Error control Channel based
Errors are detected and handled by individual channel systems
26
Framing in TDM So far, no flag or SYNC characters bracketing
composite (MUXED) TDM frames on the link Must provide frame synchronization to allow RX
to keep ‘in step’ with TX Two approaches:
Frame-by-Frame: A synch pattern at the beginning of each assembled frame (similar to the preamble flag)
Frame-to-Frame: Additional control channel with a unique frame-to-frame pattern that can be easily identified by RX (can be just 1 bit, and extends across frames, so less overhead)This is called “added digit framing”
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Framing in TDM Added digit framing
One control bit added to each TDM frame as an additional “control channel”
Carries an identifiable known bit pattern in time (frame to frame) e.g. alternating 01010101…unlikely to occur on a normal data channel
RX searches frame-to-frame for this pattern until it finds it. This establishes frame sync. Will keep locked to it
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Frame-to-Frame Sync (added digit framing)
C 1 2 3 4 C 1 2 3 4 C 1
0 1 0
Four data channels
Control Channel, C010101….
…..
A data ChannelUnlikely to have 010101…. over successive frames
Once the position of this control channel is established, RX knows where the channel sequence starts and sync is established with TX
…..
MUXed frame
RX knows the size of the MUXed frame
It can check each frame bit frame-to-frame for the special pattern until it finds it!
29
Digital Carrier Systems Hierarchy for TDM (as with FDM!) US system based on DS format, for example DS-1 (similar to a group in FDM):
Multiplexes 24 PCM voice channels digitized with n = 8 bits + a framing bit (a control channel for frame-to-frame synchronization)
Frame takes a sample of each channel So, frame size is 24 x 8 +1 = 193 bits Channels must be sampled at 2 x 4000 = 8000
sample/s This gives a data rate = 8000 x 193 = 1.544 Mbps for
DS-1 Note: FDM Group needed 48 KHz for 12 channels
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The DS Hierarchy
42 x 96 = 4032 DS-0(4032 voice channels)
DS-0 is a PCM voice channel:8000 sample/s x 8 b/sample= 64 kbps
Transmission lines used should support the progressivelyincreasing data rate (channel capacity) requirement
FDM Jumbo group: 16.984 MHz for 3600 channels
Which one uses BW more efficiently ?
31
DS & T Lines Rates
Service LineData Rate
(Mbps)No. of Voice
Channels
DS-1DS-1 T-1T-1 1.5441.544 2424
DS-2DS-2 T-2T-2 6.3126.312 9696
DS-3DS-3 T-3T-3 44.73644.736 672672
DS-4DS-4 T-4T-4 274.176274.176 40324032
Transmission line that supports it Corresponding Channel Capacity
Relevant Standards:SONET: Synchronous Optical Network (ANSI)SDH: Synchronous Digital Hierarchy (ITU-T)
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DS-1 Digital Carrier Systems For voice, each channel contains one byte of digitized
data (PCM, 8000 samples per sec) Data rate 8000 MUXed frames/s x (24x8+1) bits/frame =
1.544Mbps Five out of every six frames have 8 bit PCM user data samples
for each channel Sixth frame has (7 bit PCM user data + 1 signaling bit) for each
channel Signaling bits form a stream for each channel containing
control (e.g. error and flow) and routing info Same format for digital data
23 channels of data 7 bits per frame plus indicator bit for data or systems
control 24th channel is for signaling
DS-1 can carry mixed voice and data signals
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DS-1 Transmission Format
Frame
(frame-to-frame)
125/193
(8000 x 7 bits = 56 kbps)
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Asymmetric Digital Subscriber Line (ADSL) ADSL is an asymmetric communication technology designed for residential users over ordinary telephone twisted pair wires
High speed digital data transmission Existing subscriber lines (local loops) were installed for base band
speech (0 – 4 kHz), but can actually provide bandwidths of up to 1 MHz (short distances)
ADSL is an adaptive technology, using different data rates based on the condition of the local loop line
Ranges up to 5.5 km (95% of subscriber lines in USA) Two main technologies: - Multi-level encoding, e.g. QAM
- Discrete Multitone (DMT) by FDM
Shorterdistance,Higherdata rates
Q. What is the BE for 2.5 km lines?
35
ADSL Design
Asymmetric: Providing higher capacity down stream (to customer) than upstream (from customer)
Originally targeting the video-on-demand market Now being used for Internet traffic Uses Frequency Division Multiplexing (FDM) in a novel
way to utilize the 1 MHz BW of twisted pair wires
ServiceProvider
Video, graphics
Voice, e-mailADSLSubscriber
Downstream (download)
Upstream (upload)
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FDM is used at two levels:a. Use FDM to obtain three major bands:
1. POTS band: “Plain Old Telephone Service!” 0 - 20 kHz
(includes 16 KHz Guard band)
2. Upstream band:
25 – 200 kHz
3. Downstream band:
250 – 1000 KHz
4
b. Discrete Multitoning (DMT): Further FDM inside the upstream and the downstream bands: Single fast bit stream is split into multiple bit streams traveling at lower data rates in parallel (simultaneously) in subchannels at different subbands within the upstream and downstream bands.
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ADSL Hardware Home
Telephone Exchange
Subscriber Loop
Telephone channel operateseven is ADSL line is down or unplugged
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ADSL Frequency Bands and DMT Channels
Guard bandsBetween voice and data
Channel #
256 x 4 kHz 1 MHz
1 MHz
- 256 4KHz sub channels- DMT distributes data rate load on sub channels, non uniformly
39
Discrete Multitoning (DMT) Multiple subchannels (each 4 KHz wide) within the upstream
and downstream bands Subchannels are modulated with subcarriers
of different frequencies (FDM) (hence “multi-tone”) Bit stream to be transmitted is split into a number of streams
that travel in parallel (simultaneously) at a lower data rate on a number of these limited BW subchannels
1
1011010011Serial to Parallel
Converter
Subchannel 1
Subchannel 5
.
.
1
0
1
1
0
1
0
0
1
1
Data rate for each channel:R/5 bpsOverall data rate: R bps
Data rate R bps
(Each: 4 kHz BW)
15
40
Discrete Multitoning (DMT): Adaptive ADSL adaptive property:
Not all subchannels run at the same data rate! Each subchannel can carry data rate from 0 - 60 kbps In general, channels towards the higher frequency end
would carry lower data rates (larger attenuation lower SNR lower channel capacity!)
Moreover, DMT modem can send out test signals on various subchannels to determine SNR (expected lower for subchannels located at higher frequencies due to larger attenuation)
Then faster data rates are assigned to subchannels having better signal transmission conditions
1
1
41
Discrete Multitoning (DMT) Uses QAM (Quadrature Amplitude Modulation)
multilevel modulation allowing up to 15 bits/baud (L = 15 bits/signal level)(4 KHz B D = 4 kbauds (if filtering coefft. r = 0) R max = 4 kbauds x 15 = 60 kbps per channel)
Ideally, 256 x 60 kbps = 15.36 Mbps maximum (if uniform)
But rate is not uniform over all sub channels, and maximum is not achievable in practice due to various transmission impairments
Practical system operate at 1.5 to 9 Mbps depending on distance and line quality
42
DMT
i: data rate distributing factors (<1) over available sub channels (in either the Upstream or the Downstream bands)
R = Aggregate (total) data rate
R = i R + i R + …. + n R
43
DMT
Demodulators
Modulators