Multiplexing

1
Multiplexing

• Multiplexing Combining multiple data (voice)
  channels for transmission on a common medium.
  Multiple devices sharing one physical link.
• Demultiplexing Recovering the original separate
  channels from a multiplexed signal.
• Multiplexing and demultiplexing are performed by
  a multiplexer.
• The two common forms of multiplexing are
  frequency-division multiplexing (FDM) and
  time-division multiplexing (TDM).

2
Frequency-Division Multiplexing (FDM)

• Useful bandwidth of the medium exceeds the
  required bandwidth of the channel.
• Each signal is modulated to a different carrier
  frequency.
• Carrier frequencies are separated so signals do
  not overlap (guard bands).
• E.g. broadcast radio, television, cable
  television, etc.
• Channel allocated even if there is no data to be
  sent.

3
Frequency-Division Multiplexing (FDM)
4
FDM System- Transmitter

• Analog or digital inputs mi (t), i 1,n.
• Each is modulated onto a subcarrier fi.
• Signals summed to produce a composite baseband
  mb(t).
• fi chosen such that there is no overlap.

5
FDM System- Band
6
FDM System- Receiver

• mb(t) is passed through n bandpass filters with
  response centered on fi.
• Each si(t) component is demodulated to
• recover the original analog/digital data.

7
FDM of Three Voiceband Signals

• Effective spectrum of voice 300-3400Hz.
• Amplitude modulation at 64-kHz carrier produces
  an 8 kHz bandwidth (60-68kHz).
• For efficiency use of bandwidth, only transmit
  lower sideband.
• Three carriers at 64, 68 and 72 kHz.
• Crosstalk (overlap) use guard band to avoid.
• Intermodulation noise nonlinear effects of
  amplifiers in one channel can produce frequency
  components in other channels.

8
FDM of Three Voiceband Signals
9
Analog Carrier Systems

• Long distance voiceband signals over
  high-capacity links (coaxial cable, microwave).
• ATT (USA) designated a hierarchy of FDM schemes.
• Group
• 12 voice channels (4kHz each) 48kHz.
• Range from 60kHz to 108kHz.
• Supergroup
• 60 channels.
• FDM of 5 group signals on carriers between 420
  kHz and 612 kHz.
• Each group is treated as a separate signal
  with 48 kHz bandwidth.
• Mastergroup
• 10 supergroups.
• 600 voice channels with a bandwidth of 2.52
  MHz
• so original signal can be modulated many times

10
Analog Hierarchy
11
North American and International FDM Carrier
Standards
12
Example
Five channels, each with a 100-KHz bandwidth, are
to be multiplexed together. What is the minimum
bandwidth of the link if there is a need for a
guard band of 10 KHz between the channels to
prevent interference?
For five channels, we need at least four guard
bands. This means that the required bandwidth is
at least 5 x 100 4 x 10 540
KHz, as shown in the Figure.
13
Example
Four data channels (digital), each transmitting
at 1 Mbps, use a satellite channel of 1 MHz.
Design an appropriate configuration using FDM
The satellite channel is analog. We divide it
into four channels, each channel having a 250-KHz
bandwidth. Each digital channel of 1 Mbps is
modulated such that each 4 bits are modulated to
1 Hz. One solution is 16-QAM modulation. The
figure shows one possible configuration.
14
Example
The Advanced Mobile Phone System (AMPS) uses two
bands. The first band, 824 to 849 MHz, is used
for sending and 869 to 894 MHz is used for
receiving. Each user has a bandwidth of 30 KHz in
each direction. The 3-KHz voice is modulated
using FM, creating 30 KHz of modulated signal.
How many people can use their cellular phones
simultaneously?
Each band is 25 MHz. If we divide 25 MHz into 30
KHz, we get 833.33. In reality, the band is
divided into 832 channels.
15
Wavelength Division Multiplexing

• Multiple beams of light at different frequency
• Carried by optical fiber
• A form of FDM
• Each color of light (wavelength) carries separate
  data channel
• 1997 Bell Labs
• 100 beams
• Each at 10 Gbps
• Giving a total 1 terabit per second (Tbps)
• Commercial systems of 160 channels of 10 Gbps now
  available
• Lab systems (Alcatel) 256 channels at 39.8 Gbps
  each
• Total 10.1 Tbps
• Over 100km

16
WDM Operation

• Same general architecture as other FDM
• Number of sources generating laser beams at
  different frequencies
• Multiplexer consolidates sources for transmission
  over single fiber
• Optical amplifiers amplify all wavelengths
• Typically tens of km apart
• Demux separates channels at the destination
• Mostly 1550nm wavelength range
• Was 200MHz per channel
• Now 50GHz

17
WDM
18
Dense Wavelength Division Multiplexing

• DWDM
• No official or standard definition
• Implies more channels more closely spaced that
  WDM
• 200GHz or less

19
Synchronous Time-Division Multiplexing

• Data rate of the medium exceeds the data rate of
  the digital signal to be transmitted.
• Multiple digital signals interleaved in time May
  be at the bit level or blocks of bytes.
• Time slots preassigned to the data sources and
  fixed.
• Time slots allocated even if no data is sent
  (like FDM).
• Time slots do not have to be evenly distributed
  amongst sources.

20
Synchronous Time-Division Multiplexing
21
Synchronous TDM System

• Interleaving
• Can be compared to a very fast rotating switch
  which selects each device at a constant rate and
  a fixed order.
• Each device sends a fixed number of bits in
  its timeslot.
• Weakness of Synchronous TDM fixed time slot
  allocations can lead to empty slots when a device
  has nothing to send.

22
TDM System

• Transmitter
• Digital inputs, mi(t), i 1,n, are briefly
• buffered.
• Buffers are scanned sequentially to form
• a composite signal mc(t).
• Scanning is rapid enough so buffers are
• emptied before more data arrives.
• Data organized into frames of one cycle.
• Receiver
• Interleaved data is demultiplexed and
• routed to destination buffers.

23
TDM Link Control

• no headers and trailers
• data link control protocols not needed
• flow control
• data rate of multiplexed line is fixed
• if one channel receiver can not receive data, the
  others must carry on
• corresponding source must be quenched
• leaving empty slots
• error control
• errors detected handled on individual channel
• Flow control and error control can be provided on
  a per-channel basis by using a data link control

24
Data Link Control on TDM
25
Framing Bits

• There is no flag or SYNC characters bracketing
  TDM frames.
• Must provide a synchronising mechanism.
• Most common mechanism Added digit framing.
• One control bit is added to each TDM frame
• Looks like another channel - control channel
• Identifiable bit pattern used on the control
  channel.
• E.g. alternating 01010101 which is
  unlikely to be sustained on a data channel.
• Receiver can compare incoming bit patterns on
  each channel with the sync pattern.
• Once synchronized, the receiver continues to
  monitor the control channel.

26
Synchronous TDM System Example

• Each device sends 250 characters/second 2508
  2000 bps.
• Transmission is character interleaved, and each
  frame has one framing bit.
• Therefore, the devices create 200048000 bits of
  data per second, and the multiplexer adds 250
  bits of overhead per second.

27
Pulse/Bit Stuffing

• It is possible to connect devices of different
  rates using different time slot allocations.
• Since a fixed number of bits are transmitted in
  each time slot, each device must have a data rate
  which is an integer multiple of the channel
  rates.
• For example a device three times faster than the
  other devices uses three time slots.
• Problem - What do we do for a device 3.75 times
  faster.
• Solution - Pulse/Bit Stuffing
• The multiplexer adds extra dummy bits or
  pulses into the devices data stream to force the
  integer speed relationship.
• Therefore the 3.75 times faster device will be
  raised to 4 times faster.
• Stuffed bits are inserted at fixed locations
  in the frame and removed at the demultiplexer.

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Example TDM of Analog and Digital Sources
29
North American and International TDM Carrier
Standards
30
Digital Carrier Systems

• Hierarchy of TDM.
• USA/Canada/Japan use one system and ITU-T use a
  similar (but different) system.
• Digital Signal (DS) Service DS-1 format
  Multiplexes 24 channels.
• Each frame has 8 bits per channel plus one
  framing bit 8241 193 bits per frame.
• The DS service is implemented using T lines.

31
Digital Carrier Systems

• The first bit is a framing bit, used for
  synchronization.
• Voice channels
• 8-bit PCM used on five of six frames.
• 7-bit PCM used on every sixth frame bit 8 of
  each channel is a signaling bit.
• Data channels
• Channel 24 is used for signaling only in some
  schemes.
• Bits 17 used for 56-kbps service (8-bit data or
  control)
• Bits 27 used for 9.6-, 4.8-, and 2.4-kbps
  service.
• subrate multiplexing
• additional bit is robbed from each channel to
  indicate which subrate multiplexing rate is being
  provided

32
Digital Carrier Systems Format
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Digital Carrier Systems Hierarchy
34
SONET/SDH

• SONET Synchronous Optical Network is an optical
  transmission interface (proposed by BellCore and
  standardized by ANSI).
• SDH Synchronous Digital Hierarchy (defined by
  ITU-T).
• Both standards are compatible and designed for
  the high-speed transmission capability of optical
  fiber.
• Signal Hierarchy
• Lowest level Synchronous Transport Signal
  level 1 (STS-1) or Optical Carrier level
• 1 (OC-1).
• 51.84 Mbps.
• STS-1 carries a single DS-3 or a group of
  lower rate signals (DS1, DS1C, DS2) plus
• ITU-T rates (e.g. 2.048 Mbps).
• Multiple STS-1 signals combined into an STS-N
  signal.
• ITU-T lowest rate is 155.52 Mbps (STM-1) and
  corresponds to SONET STS-3.

35
SONET/SDH Signal Hierarchy
36
SONET Frame Format

• The basic SONET building block is the STS-1 frame
• STS-1 frame contains 9 rows of 90 octets 810
  octets every 125µs ?51.84 Mbps.

37
SONET STS-1 Overhead Octets
38
Statistical TDM

• In synchronous TDM many slots are wasted (empty
  slots).
• Statistical or Asynchronous TDM allocates time
  slots dynamically based on demand.
• The multiplexer scans the input lines and
  collects data until the frame is full.
• The data rate on the multiplexed line is lower
  than the aggregate rates of the input lines.
• may have problems during peak periods (must
  buffer inputs)

39
Statistical TDM
40
Statistical TDM Frame Formats

• Address information is required for proper
  delivery of data in the frames greater overhead.
• Generally use a synchronous protocol such as
  HDLC.
• Include one (b) or more (c) sources of data in
  each HDLC frame (a).

41
Performance

• Output data rate is less than the aggregate input
  rates.
• Anticipate the average rate of input is less than
  the multiplexed capacity.
• May cause problems during peak periods
• Buffer inputs.
• - smallest possible buffer and smallest
  possible data rate ?
• Keep buffer size to minimum to reduce delay.

42
Performance
43
Performance

• I the number of input resources
• R data rate of each source
• M effective capacity of multiplexed line (after
  accounting for overhead).
• a mean fraction of time each source is
  transmitting 0 lt a lt1
• K M/IR fraction of multiplexed line capacity
  (compression factor, i.e K 1 synchronous TDM)
• The value of K can be bounded alt K lt1

44
Performance

• MUX as a single server queue
• Total delay time spent waiting service time
• Delay depends on the pattern of arriving traffic
  and the characteristics of the server.
• Assume poisson arrivals and const service time
• ? a I R (? average arrival rate)
• Ts 1/M ( Ts time it takes to transmit one bit)
• The utilization of total link capacity
• ? ? Ts a I R/M a /K ?/M
• Buffer size depend on ? , and not directly on M
  (look at the two cases in the book)
• As utilization rises, so do the buffer
  requirements and delay.

45
Performance
46
Performance

• Data are transmitted in 1000-bit frame
• Utilization is expressed as a percentage of the
  total line capacity.
• Utilization above 80 is not desirable

47
Cable Modems

• dedicate two cable TV channels to data transfer
• each channel shared by number of subscribers,
  using statistical TDM
• Downstream
• cable scheduler delivers data in small packets
• active subscribers share downstream capacity
• also allocates upstream time slots to subscribers
• Upstream
• user requests timeslots on shared upstream
  channel
• Headend scheduler notifies subscriber of slots to
  use

48
Cable Modem Scheme
49
Asymmetrical Digital Subscriber Line (ADSL)

• Modem technology designed to provide high-speed
  digital data transmission over ordinary telephone
  wire.
• Link between the subscriber and the network
  (local exchange) Local loop.
• Uses currently existing twisted pair cables
• Can carry broader spectrum than just voice
  bandwidth (1 MHz or more).
• ADSL Design
• Asymmetric Greater capacity downstream than
  upstream.
• Frequency-division multiplexing
• Lowest 25kHz for voice Plain old telephone
  service (POTS).
• Use echo cancellation or FDM to give two bands
  for downstream and upstream.
• Use FDM within bands each band.
• Range of up to 5.5 km.

50
ADSL Channel Configuration
51
ADSL Channel Configuration

• Advantages of echo cancellation compared to the
  use of distinct frequency bands
• The higher the frequency, the greater the
  attenuation. With the use of echo cancellation,
  more of the downstream bandwidth is in the good
  part of the spectrum.
• The echo cancellation design is more flexible for
  changing upstream capacity. The upstream channel
  can be extended upward without running into the
  downstream (overlap is extended).
• disadvantage of the use of echo cancellation is
  the need for echo cancellation logic on both ends
  of the line

52
Discrete Multitone (DMT)

• Available transmission bandwidth is divided into
  4kHz subchannels, each having its own carrier
  frequency.
• DMT modem sends out test signals on each
  subchannel to determine the signal-to-noise
  ratio.
• assign more bits to subchannels with better
  signal-to-noise ratio.
• Use quadrature amplitude modulation (QAM) easy
  to assign variable number of bits.
• Presently 256 downstream subchannels with each
  4kHz subchannel carrying 60 kbps
• 15.36 Mbps possible, impairments bring this
  down to 1.5 Mbps to 9 Mbps.

53
DMT Transmitter
54
DMT Transmitter
55
xDSL

• ADSL is one of a number of recent schemes for
  providing high-speed digital transmission of the
  subscriber line. Many scheme are used which are
  collectively referred to as xDSL.
• High-speed digital transmission of the subscriber
  line.
• High data rate Digital Subscriber Line (HDSL)
• More cost-effective means of delivering a T1
  data rate (1.544 Mbps).
• Up to 2 Mbps over two twisted pair lines
  within a bandwidth of 196 kHz.
• Range of about 3.7 km.
• Single line Digital Subscriber Line (SDSL)
• HDSL is not suitable for residential
  subscribers because of two twisted pair.
• SDSL provides same service as HDSL but over a
  single twisted-pair line.
• Very high data rate Digital Subscriber Line
  (VDSL)
• Similar to ADSL but a much higher data rate
  and shorter distance (up to 1.4 km).

56
Comparison of xDSL Alternatives
57
Problem Assignments

• Solve all the review questions
• Try the following problems
• 8.1, 8.8, 8.9, 8.10, 8.11, 8.14, 8.17