chapter 8: multiplexing
DESCRIPTION
COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal. Chapter 8: Multiplexing. Where are we:. Chapter 7: Data Link: Flow and Error control. Data Link. Chapter 8: Improved utilization: Multiplexing. Physical Layer. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 8:
Multiplexing
COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal
2
Where are we:
Physical Layer
Transmission Medium
Data Link
Chapter 4: Transmission Media
Chapter 3: Signals and their transmission over
media, Impairments
Chapter 5: Encoding: From data to signals
Chapter 7: Data Link: Flow and Error control
Chapter 6: Data Communication: Synchronization,
Error detection and correction
Chapter 8: Improved utilization: Multiplexing
3
Contents
1. Introduction
2. Two Multiplexing Techniques1. FDM
2. TDM1. Synchronous
2. Statistical
3. Application: ADSL
(Asymmetric Digital Subscriber Line)
4
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
5
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. Large number of channels between cities over large
distances
6
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
7
Multiplexing Types
Synchronous Statistical
FDM: Frequency Division MultiplexingTDM: Time Division MultiplexingWDM: Wavelength Division Multiplexing (a form of FDM)
Analog Signals Digital SignalsRepresentingdigital or analog data
ModulationOr shift keying
Separation in Frequency
Separation in time by interleaving
Encoding
8
Frequency Division Multiplexing (FDM)
• Channels exist on the same line (space) at the same time:• Must be separated in frequency!
f
t
9
FDM Useful bandwidth of medium exceeds required
bandwidth of 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)
10
FDM
Different Frequencies
Same time
11
FDM Multiplexing Process: Time-Domain View at TX
Fc (Different for each channel)Modulator
Qty: 3
12
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
Restoration at RX:3 different pass-band filters,each bracketing a channel
13
FDM De-Multiplexing Process: Time-Domain View at RX
f1
f2
f3
fcDemodulator
Low Pass Filter
Qty: 3
Demultiplexing Filters
Qty: 3
Filter pass bands
(Different for each channelSame as those used at TX)
14
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
15
FDM System – Transmitter
Subcarriers
Main CarrierAny type of modulation:AM, FM, PM
Group of channels
To meet Transmission Requirements
Individual base band channels
16
FDM System – Receiver
fc
Composite base band signal mb(t)
recovered
Individual base band channels
recovered
Subcarriers
Main Carrier
f1
f2
f3
17
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
18
Analog Carrier Systems One-go Vs Hierarchical:
…. Channel ….
Group ….
Master super group
Super group ….
4000 channels 4000 channels
... ...
• Modular approach• Easier to implement• Also, not all channels may be available at one place
Stages
….
….
19
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
20
Analog Carrier Systems
12
8 KHz
GroupChannel Super group Master super group
3084
21
Analog FDM Hierarchy
0.24 x 10 Vs 2.52
22
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 gurdbands
Inter modulation noise:
- Nonlinearities in amplifiers ‘mix’ channels
- This generates spurious frequency components (sum, difference) which fall within channel BWs!
23
Waveform Division MUXing (WDM) A form of FDM used with optical fibers Lasers of different colors (different wavelengths)
are used simultaneously in the same fiber Each beam carries a separate data channel 256 such beams @ 40 Gbps each 10 Tbps
over 100 km
24
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)
25
Time Division Multiplexing
300 Hz Channels occupy the same frequency band(Base band)
Channels must go on the link atdifferent times
3400 Hz
26
Time Division Multiplexing (TDM)
BasebandSignals
Ts = 1/2fmax
Channel sampling interval
Note: MUXing and DeMUXingare transparent to the end stations.Each pair ‘think’ they have a dedicated link !
Time
27
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 Interval
28
Interleaving may be: At bit level: Suitable for synchronous transmission At block level (e.g. bytes): Suitable for asynchronous
transmission 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 Could use both synchronous and asynchronous transmission
Time Division Multiplexing (TDM)
29
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
30
TDM System – Receiver
Bit stream
31
Data Link Control with Sync TDM Frames on the link consist of interleaved channel frames
They will not have headers and trailers of their own 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
32
Data Link Control with TDM
Channel 1 Frame
This is what goes on the linkEverything is mixed up, even FCS bytesFCS applies only to channel framesChannel frames get reassembled at RX
Start of “1” Frame
MUXed stream can not be considered as an HDLC frames!
33
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”
34
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
35
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!
36
Pulse Stuffing Other Practical Problems:
Different sources (channels) may require sampling at different rates
Different sources may be using different clocks and you would like to standardize them on a common (higher rate) clock
Solution - Pulse Stuffing Make outgoing data rate higher than the sum of incoming rates
and an exact multiple of each to allow uniform sampling Stuff extra dummy bits (pulses) into incoming channel signals to
satisfy the higher data rate Stuffed pulses inserted at fixed locations in frame by TX MUX
and removed by RX deMUX
37
Pulse Stuffing Example:
Source 1: 1 bps
Source 2: 3 bps
MUXed data rate 1+3 = 4 bps take as 6 bps
(divides both rates)
(1/6) s1 sSource 1
Sampling
Source 2Sampling (1/3) s
X X
MUXed (composite) sampling
Dummy pulses stuffed in place of blank (unused) samples
MUXedFrame
Useful data rate: 4 bps
38
Example: TDM of 11 Analog and Digital Sources
=BW=fmax
Now channel 2 is sampled uniformly and at the correct rate
221 2 3
PCM with n = 4 bits/sample
PAM Analog samples
PCM
Sys
tem
3 Analog Channels
Digital Signals:64 kbps
Analog to Digital Converter
RotationFrequency
8 D
igit
al I
nput
s
Sampling rate = 2 fmax = 4K sample/s
64 kbps
Sampling rate:2 fmax = 8K sample/s
= MUXed data rate
Satisfies the two requirements:• 64 + 8 x 8 128 kbps• Divides 64kbps, 8 kbps exactly
Rotation/s
39
Example: TDM of 11 Analog and Digital Sources
16-bit Buffer
2-bit Buffer
2-bit Buffer
2-bit Buffer
Suggested framing and buffering arrangement
64 kbps 16-bit2-bit………2-bit 2-bit
32-bit MUXed frames
64 kbps
No. of frames/s= 128 kbps/32bpframe= 4 k frames/s= Rotation rate
Time slot, enough to empty buffer8 kbps
This is also the rate of emptying any ofThe MUXed buffers: 4 k buffers/s
Rate of filling the buffers should not exceed the rate of emptying them
Rate of filling this buffer= 64 kbps/16 bpbuffer= 4 k buffers/s
4 k rotations/s
Frame bits are allocated to Scanned sources in proportion to their data rates
40
Digital Carrier Systems Hierarchy for TDM (as with FDM!) USA/Canada/Japan use one system ITU-T use a similar (but different) system 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
41
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 ?
42
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
43
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
44
DS-1 Transmission Format
Frame
(frame-to-frame)
125/193
(8000 x 7 bits = 56 kbps)
45
T1
Due to 1 framing bitPer frame
46
T1 Frames
Framing bit
1 second
47
SONET/SDH SONET: Synchronous Optical Network (ANSI) SDH: Synchronous Digital Hierarchy (ITU-T) They utilize the large channel capacity of
optical fibers They are Compatible
48
Statistical (Asynchronous) TDM In Synchronous TDM many time slots may be wasted
since not all channels will have data all the time Statistical TDM allocates time slots to channels
dynamically based on demand Multiplexer scans input lines and collects data available
from all channels to fill a MUXed frame and sends it: Skips ‘empty’ channels
Must specify source of data since MUX rotation is no longer fixed
Data rate on MUXed line can be made lower than the aggregate peak rate on input lines This saves on channel capacity (and bandwidth)
A calculated risk!
49
Statistical TDM
t1 t2 t3 t4Time slots wasted: Couldserve a higher user demand
using same link capacity!
We could use a lower data rate for sending same data Reduce channel capacity (BW requirement)!
Penalty: Should specify source generating the data. More overhead!
Automatic addressing by fixed rotation time
time
Same data rate
Lower data rate
50
Statistical TDM Frame FormatsStation
Channel
Channel Channel
• Source address and length of data (if variable) for each channel have to be specified• To reduce overhead:
- Use relative addressing (e.g. relative the previous source), or - Use a single address bit map (e.g. 10010010) indicating which channels are sending
51
Performance Issues Use a data rate that is less than peak aggregate
input rate from individual sources (channels) to improve utilization (economize)
But this may cause problems during peak periods when all channels suddenly transmit and you get peak demand!
52
Performance Issues Solution:
MUX should keep a buffer of adequate size for holding excess data from arriving during peak times
Buffer size is determined by data rate allowed for the MUXed data (on the link) in relation to the aggregate average data rate from sources: The closer the data rate used to the average demand the
more economical the link is, but the larger the buffer size required to handle the expected large backlog during peaks
Larger buffers slow down system response: increase waiting time by sources for service (MUX will be busy sending backlog in buffer first!)
Compromise between required link capacity (economy) and source waiting time (user satisfaction)!
53
Example A system serves:
10 sources, each with a peak data rate of 1000 bps But on average, data from the sources will be produced
at 50% of the maximum rate Examine system performance and determine
minimum buffer size for: A link capacity = average aggregate input data rate
(5000 bps) A link capacity > average aggregate input data rate
(= 7000 bps) We are given the following information on actual
aggregate input data rate at twenty 1ms time intervals:
54
Performance IssuesA
ctua
l agg
rega
te in
put (
bits
) ov
er tw
enty
1 m
s in
terv
als
Average = 5 bits/ms = 5000 bps
MUXed link capacity = 5000 bpsMin buffer size = ?
MUXed link capacity = 7000 bpsMin buffer size = ?
(= Average I/P) (> Average I/P) Actual Aggregate I/P, bits
55
Statistical Performance I = number of (identical) input sources R = maximum data rate for each source, bps
(when a source sends, it sends at this maximum rate) Peak data rate from all sources combined = R I = mean fraction of time over which a source
transmits (0 < <1 ) Average input data rate from all sources combined () = R I
M = effective capacity of multiplexed line, bps (excluding overhead)
K = M / (IR)= ratio of multiplexed line capacity to the maximum input data rate= measure of compression achieved by multiplexer (=1 for synch TDM)
(link capacity reduction over synchronous TDM)
For Statistical TDM Average< M < Peak < K < 1 If K = 1, this is synchronous TDM! (no longer statistical TDM) If K < , Capacity is below the average input data rate (Avoid) i.e.< K < 1
56
Performance A queuing theory model: Data sources queue for service
by the MUX Event (request for service): A bit generated by a source Service: MUX sends that bit Assume random (Poisson) arrivals and fixed service time Average event arrival rate = Rate of requesting service, I R bit arrivals/s (Demand rate) Rate of providing the service = M bits sent/s (Service
rate) Service time Ts:
Link utilization, (fraction of total line capacity utilized): = Average rate of sources requesting service/Rate of MUX
providing it
MTs
1
1 be will ,K with , KM
IRT
M s
57
Choice of M for a statistical TDM
RI
MK
= R I
Average Demand
Peak Demand
R I
Less synchronousGreater utilization Larger BuffersPoorer quality of service
M
< K < 1 K = 1K =
K
M
More synchronousLower utilization Smaller buffersBetter quality of service
< < 1 =
Synchronous
Minimum Utilization
Utilization:
Larger Values
58
The Poisson Distribution of random arrivals
e-1
!)(
a
meaP
am
59
A measure of the buffer size needed (in frames)
Average delay suffered by a request
Function of only ( has M)
Function of both and M
(MUX)
What happens as approaches 1?
= /M = Utilization
= /M
60
Buffer Size and Delay
Increasing utilization increases Buffer size required Delay encountered
Utilization > 0.8 is undesirable
Frame size: 1000 bits
,
Average Input load = 8,000 bps,Link Capacity = 10,000 bps
Increasing link capacity, M reduces delay time for same
Frame size: 1000 bits
N
Tr
,
Frames
ms
N does not depend on Mdirectly
61
Probability of Buffer overflow Vs Buffer Size
For a given buffer size, higher utilization increases probability of overflow
For a given utilization , Increasing buffer size drastically reduces probability of overflow, particularly for low
Again, utilization > 0.8 is highly undesirable
62
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?
63
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)
64
FDM is used at two levels:
Use FDM to obtain three major bands:
1. POTS band: “Plain Old Telephone Service!” 0 - 20 kHz
2. Upstream band:
25 – 200 kHz
3. Downstream band:
250 – 1000 KHz
4
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.
65
ADSL Using Echo Cancellation Echo cancellation is a signal
processing method that allows overlapping the upstream and downstream bands
Advantages: Allows more of the downstream
band to fall in the lower frequency region Lower attenuation and larger distances
Gives flexibility in defining the width of the upstream band to suit user requirements
66
ADSL Hardware Home
Telephone Exchange
Subscriber Loop
67
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
68
Discrete Multitone (DMT) Multiple subchannels (each 4 KHz wide) within the upstream
and downstream bands Subchannels are modulated with subcarriers
of different frequencies (FDM) (hence “multitone”) Bit stream to be transmitted is split into a number of streams
that travel in parallel 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
69
Discrete Multitone (DMT): Adaptive ADSL adaptive property:
Not all subchannels run at the same data rate! Each subchannel can carry from 0 to 60 kbps DMT modem sends 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
70
Discrete Multitone (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)
Not uniform, not maximum in practice due to various transmission impairments
Practical system operate at 1.5 to 9 Mbps depending on distance and line quality
.
.
1
1
71
DMT
72
DMT
Demodulators
Modulators