Module 4 Data Communication Part II

1
Module 4Data Communication Part II
Multiplexing, SONET, and CDMA
2

• Textbook sections
• LG section 4.1 Multiplexing
• LG section 4.2 SONET
• LG section 6.5 Channelization
• Topics
• Multiplexing
• Frequency Division Multiplexing (FDM)
• Time Division Multiplexing (TDM)
• Wavelength Division multiplexing (WDM)
• SONET
• Introduction
• Physical Configuration
• STS-1 Frame Structure
• Frame Synchronization
• Payload Mapping
• SONET Topologies
• Channelization
• Time-division Multiple Access (TDMA)
• Frequency-division Multiple Access (FDMA)

3
1. Multiplexing

• Multiplexing
• The process of combining signals from multiple
  sources for transmission across a single data
  link
• Sharing the bandwidth of a data link
• Measured in Hertz for analog transmission systems
• Measured in bits/second for digital transmission
  systems

4
1. Multiplexing
LG Figure 4.1 Multiplexing
(a)
(b)
A
A
A
A
Trunk group
B
B
B
DMUX
MUX
B
C
C
C
C
5
1. Multiplexing - FDM
LG Figure 4.2 Frequency division multiplexing
(a) Individual signals occupy W Hz
(b) Combined signal fits into channel bandwidth
6
1. Multiplexing - TDM
Figure 4.3 Time division multiplexing

• (a) Each signal transmits 1 unit every 3T
  seconds

Note In this example, 1 unit 1Tsecond
(b) Combined signal transmits 1 unit every T
seconds
7
1. Multiplexing - TDM

• Typically, the transmission line is organized
  into frames that in turn are divided into
  equal-sized slots
• Added-digit framing
• One control bit is added to each TDM frame
• An identifiable pattern of bits, from frame to
  frame, is used on this control channel
• A typical example is the alternating bit pattern,
  101010 This is a pattern unlikely to be
  sustained on a data channel
• T1 uses added-digit framing

8
1. Multiplexing - TDM

• T-1 Line
• Digital line consisting of 24, 64-Kbps channels,
  with a total of 1.544 Mbps of throughput
  available per second.
• Each channel can be configured to carry voice or
  data traffic. Most providers will allow you to
  buy just some of these individual channels, known
  as fractional T-1 access.
• T1 also called Digital Signal-1 (DS-1)
• Components of the 1.544 Mbps throughput
• Each voice signal occupies 4 kHz of bandwidth -gt
  sampling a speech waveform 8000 times/second and
  by representing each sample with eight bits. (64
  Kbps)
• Carries 24 digital telephone connections
• The beginning of each frame is indicated by a
  single bit that follows a certain periodic
  pattern
• T-1 has a speed of
• (1 248)bits/frame 8000 frames/second
  1.544 Mbps

9
1. Multiplexing - TDM
Figure 4.4 T-1 carries system
1
1
2
DMUX
MUX
2
. . .
. . .
22
24
b
1
2
23
24
. . .
b
24
frame
24
Note b is the control bit used in Added-digit
framing
8 bits frame
10
LG Figure 4.5 Basic digital hierarchies
Note In North America and Japan, the digital
signal 1 (DS1), which corresponds to the output
of a T-1 multiplexer, became the basis building
block
North American Digital Hierarchy
11
1. Multiplexing - WDM

• Wavelength Division Multiplexing (WDM)
• A relatively new technology, which uses FDM
  techniques by dividing the optical spectrum into
  multiple channels, called wavelengths. These
  wavelengths are transported across one fiber. In
  contrast, conventional TDM optical links
  transport only one wavelength (one channel).
• Multiple information signals modulate optical
  signals at different optical wavelengths
• Resulting signals are combined and transmitted
  simultaneously over the same optical fiber
• Why use WDM
• Currently technology to convert between
  electrical and optical media have maximum speed
  in the tens of Gbps
• The bandwidth of a single fiber is about 25,000
  GHz
• Great potential for multiplexing many channels
  together over long-haul routes.

12
1. Multiplexing - WDM
The WDM Configuration
Multiple Fibers
Multiple Fibers
W D M
W D M
Single Fiber
SDH/ SONET
SDH/ SONET
LG Figure 4.18 Wavelength Division Multiplexing
Optical MUX
Optical deMUX
?1
?1
?2
?2
?1
?2.
?m
Optical fiber
?m
?m
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2. SONET - Introduction

• Introduction
• Synchronous Optical Network (SONET)
• A standard developed by ANSI for fiber-optic
  networks
• The dominant standard for long-distance
  transmission of data over optical networks.
• Set industry standard in U.S. for
  telecommunication
• Defined a hierarchy of signals (similar to DS
  hierarchy) called synchronous transport signals
  (STS)
• Optical carries (OC) levels are the
  implementation of STSs
• Synchronous Digital Hierarchy (SDH)
• Developed by International Telegraph and
  Telephone Consultative Committee (CCITT)
• SDH is a world standard, and, as such, SONET can
  be considered a subset of SDH

14
2. SONET - Introduction

• Date rate
• The lowest level or base signal is referred to as
  synchronous transport signal level 1 or STS-1.
• STS-1 operates at 51.84 Mbps
• Higher-level signals are integer multiples of
  STS-1, creating the family of STS-N
• An STS-N signal is composed of N byte-interleaved
  STS-1 signals.

Table 4.1 SONET digital hierarchy
15
2. SONET - Physical Configuration

• Physical configuration
• SONET devices
• STS Multiplexer/Demultiplexer
• Regenerator
• Similar to a repeater
• However, it replaces some of the existing
  overhead information with new information. It
  functions at the data link layer.
• Add/drop multiplexer (ADM)
• A major benefit of the SONET is the ability to
  perform add-drop multiplexing.
• ADM can add signals coming from different sources
  into a given path or remove a desired signal from
  a path and redirect it without demultiplexing the
  entire signal.
• Instead of relying on timing and bit position,
  ADM use header information such as addresses and
  pointers to identify individual streams.

16
2. SONET - Physical Configuration
LG Figure 4.9 SONET Add-Drop Multiplexing

• Note
• The benefit of an ADM on a wide-area network
  (WAN) is to drop (de-multiplex) only those
  portions of the optical stream required for a
  location and let the rest pass through without
  the de-multiplexing. It would be extremely
  inefficient to have to de-multiplex an entire
  OC-12 stream, only to drop out one DS-1.
• ADM allows carriers to bundle many lower-speed
  communications channels onto a single OC-1 or
  OC-3 to carry the information back to the central
  metropolitan area.

(a) pre-SONET multiplexing
insert tributary
17
2. SONET - Physical Configuration
LG LG Figure 4.8 SONET multiplexing
STS Multiplexer
DS1
Low-Speed Mapping Function
DS2
STS-1
CEPT-1
51.84 Mbps
DS3
Medium Speed Mapping Function
STS-1
44.736
? ? ?
? ? ?
OC-n
STS-n
STS-3c
STS-1
E/O
Mux
Scrambler
CEPT-4
High- Speed Mapping Function
STS-1
STS-1
139.264
STS-3c
STS-1
STS-1
High- Speed Mapping Function
ATM
Note Scrambling maps long sequences of 1s or 0s
into sequences that contain a more even balance
of 1s and 0s to facilitate bit-timing recovery
STS-1
150 Mbps
18
2. SONET - Physical Configuration

• SONET layers
• Photonic layer
• Corresponding to the physical layer of the OSI
  model
• Includes physical specifications for the optical
  fiber channel, etc.
• Use NRZ encoding with the presence of light
  representing 1 and the absence of light
  representing 0
• Section layer
• Responsible for the movement of a signal across a
  physical section
• Handles framing, scrambling, and error control
• Line layer
• Responsible for the movement of a signal across a
  physical line
• STS multiplexers and ADMs provides line layer
  functions

19
2. SONET - Physical Configuration

• Path layer
• Responsible for the movement of a signal from its
  optical source to its optical destination
• At the optical source, the signal is changed from
  an electronic form into an optical form,
  multiplexed with other signals, and encapsulated
  in a frame
• At the optical destination, the received frame is
  demultiplexed, and the individual optical signals
  are changed back into their electronic forms.
• STS multiplexers provide path layer functions
• SONET layers and their corresponding OSI layers
• SONET layers correspond to OSI physical layer
• In many SONET backbone networks, before IP is
  encapsulated into SONET, they are first
  encapsulated into an ATM cell. ATM is designed
  to run over fiber-optic cable operating the
  SONET.

20
2. SONET - Physical Configuration
A SONET System
21
2. SONET - Physical Configuration
Device-Layer Relationship in SONET
22
2. SONET Frame Structure

• SONET STS-1 frame structure
• Each frame consists of 9 rows of 90 columns of
  8-bit byte (a total of 810 bytes)
• The byte transmission order is row-by-row and
  left to right at a rate of 8000 frames/second
  (125 micro second per frame)
• STS-1 rate is 51.84 Mbps
• (90 bytes/row)(9 rows/frame)(8000 frames/s)
  51,840,000 bps 51.84 Mbps
• Frame components
• Transport overhead The first three columns of
  each STS-1 frame
• Section overhead First three rows of transport
  overhead
• Line overhead remaining six rows of transport
  overhead
• Synchronous payload envelope (SPE)
• STS path overhead (POH) first column of SPE
• Payload remaining 86 columns of SPE

23
2. SONET Frame Structure
STS-1 Frame Overhead
24
2. SONET Frame Structure
STS-1 Frame
25
2. SONET Frame Structure
STS-n

• Note
• The STS-n is formed by byte-interleaving STS-1
  modules
• The transport overhead (section overhead and
  line overhead) of
• the individual STS-1 modules are frame aligned
  before interleaving.
• The associated STS SPEs are not required to be
  aligned because each
• STS-1 has a payload pointer to indicate the
  location of the SPE.

26
2. SONET Frame Structure

• Example - User Data Rate Calculation
• Problem statement
• The user data rate for OC-3 is stated to be
  148.608 Mbps. Show how this number can be derived
  from the SONET OC-3 Gross Data rate.
• Answer
• Taking transport overhead into consideration
• 155.52 ((387)/(390)) 150.336
• Taking path overhead into consideration
• 150.336 (3(86/87)) 148.608

27
2. SONET Frame Synchronization

• Pointer
• SONET uses pointer to maintains synchronization
  of frames and SPEs in situations where their
  clock frequencies differ slightly.
• Pointer allow the transparent transport of SPE
  between nodes with separate network clocks having
  almost the same timing
• The first two bytes of the line overhead are used
  as a pointer that indicates the byte within the
  information payload where the SPE begins.
  Consequently, the SPE can be spread over two
  consecutive frames. The use of the pointer makes
  it possible to extract a tributary signal from
  the multiplexed signal. This feature gives SONET
  it add-drop capability

28
2. SONET Frame Synchronization
Figure 4.16 The synchronous payload envelope can
span two consecutive frames
first octet
Pointer
frame k
87 columns
Synchronous Payload Envelope
9 rows
Pointer
last octet
frame k1
first column is path overhead
29
Effects of Minor Difference Between STS-1 Frame
Rate and SPE Frame Rate
n bits frame
n bits frame
n bits frame
data bit stream
SPE frame rate is slower than STS-1 frame rate
n bits frame
n bits frame
n bits frame
data bit stream
STS-1 frame rate
n bits frame
n bits frame
n bits frame
data bit stream
SPE frame rate is faster than STS-1 frame rate
30
2. SONET Frame Synchronization

• Positive stuffing mechanism
• Used when the frame rate of the SPE that is being
  placed in the STS-1 frame is is too slow in
  relation to the frame rate of the STS-1.
• The H1 and H2 bits of the header will eventually
  reveal that the SPE is approximately one byte
  slower than the STS-1 frame. A byte is stuffed
  into the STS-1 frame, which allows the SPE to
  slip back.
• The stuffed byte contains no useful information.
  It follows the H3 byte in the header. The
  pointer is incremented by 1 in the next frame,
  and the subsequent pointer contain the new value

31
2. SONET Frame Synchronization

• Negative stuffing mechanism
• Used when the frame rate of the SPE that is being
  placed in the STS-1 frame is is too fast in
  relation to the frame rate of the STS-1.
• If the SPE frame is traveling more quickly than
  the STS-1 frame, ever now and then pulling an
  extra byte from the flow and stuffing it into the
  overhead capacity (the H3 byte) gives the SPE a
  one-byte advance. The pointer is decremented by
  one byte value in the next frame, with all
  subsequent pointers containing the new value. n
  essence, the SPE has been speeded up to match
  the alignment with the STS-1 frame.

32
2. SONET Payload Mapping

• Virtual Tributary (VT)
• A SONET STS-1 signal can be divided into virtual
  tributary signal that accommodate lower-bit-rate
  stream
• Current digital hierarchy data rates (DS-1 to
  DS-3) are lower than STS-1. To make SONET
  backward compatible with the current hierarchy,
  SONET frame design includes a system of virtual
  tributaries .
• Definition of VT
• A VT is a partial payload that can be inserted
  into an STS-1 and combined with other partial
  payloads to fill out the frame.
• Instead of using all 86 payload columns of an
  STS-1 frame for data from one source, we can
  subdivide the SPE and call each component a VT.

33
2. SONET Payload Mapping

• Virtual Tributary (VT)
• In each SPE, 84 columns are set aside and divided
  into seven groups of 12 columns each.
• Two columns (column 30 and 59) are not used for
  payload but are designated as the fixed-stuff
  columns.
• Some numbers associated with SONET STS-1 Frame
  9, 90, 87, 86, and 84.
• Each group constitutes a virtual tributary and
  has a bit rate of 12988000 6.912 Mbps.
• A VT can accommodate four T-1 carries signals or
  three CEPT-1 signals

34
2. SONET Payload Mapping

• Types of VTs
• Four types of VTs have been defined. Notice that
  the number of columns allowed for each type of VT
  can be determined by doubling the VT
  identification number.
• VT1.5 accommodates the U.S. DS-1 service
  (1.544Mbps)
• VT2. accommodates the European CEPT-1 service
  (2.048Mbps)
• VT3 accommodates the DS-1C service (fractional
  DS-1, 3.152 Mbps)
• VT6 accommodates the DS-2 service (6.312 Mbps)

35
2. SONET Payload Mapping
Virtual Tributaries
36
2. SONET Payload Mapping
VT Types
37
2. SONET - SONET Topologies

• SONET Rings
• Because SONET can carry large amount data, a cut
  in a fiber span or problems in timing can cause
  disruptions for many customers. In order to plan
  for such catastrophic failure, SONET is usually
  deployed in rings.
• 2-fiber ULSR (Unidirectional Line Switched Ring)
• One ring is called the protection ring, which
  normally does not carry any traffic, and the
  other ring is the working ring, which carries all
  of the traffic
• Entire capacity of one fiber can be transferred
  from one ring to the other, whenever there is a
  fault or maintenance is needed.
• 4-fiber BLSR (Bidirectional Line Switched Ring)
• There are four rings present. Traffic travels on
  only tow rings during normal operation while the
  other two rings are on protection mode.
• With twice as many fibers as 2-fiber rings, these
  rings provide twice as much protection.

38
2. SONET - SONET Topologies
Survivability in a 2-fiber SONET ring
Working Ring
a
a
NE
NE
Protection Ring
NE
NE
NE
NE
b
d
b
d
NE
NE
c
c
(a) 2-fiber ring
(b) Loop-around in response to fault
Note It takes only one span for network element
(NE) a to communicate with NE b. However, it
takes three spans for NE b to communicate with NE
a. These types of rings are usually used in a
metropolitan area or within cities where the
difference in delays are negligible.
39
2. SONET - SONET Topologies
Survivability in a 4-fiber SONET ring
a
a
NE
NE
NE
NE
b
d
NE
NE
b
NE
NE
c
(a) 4-fiber ring
(b) Loop-around in response to fault
Note The 4-fiber bi-directional rings are more
popular with the wide area carriers.
40
3. Channelization

• Frequency-division Multiple Access (FDMA)
• Frequency is divided into a number of frequency
  channels and each user accesses a particular
  channel for the length of the call.
• Time-division Multiple Access (TDMA)
• Each user accesses all the frequency but only a
  short period of time.
• Code Division Multiple Access (CDMA)
• Each user accesses all the frequency for all the
  time but distinguishes the transmission through
  the use of a particular code.

41
LG Figure 6.36 Frequency-division Multiple Access
(FDMA)
42
LG Figure 6.37 Time-division Multiple Access
(TDMA)
43
Different multiple access schemes
44
3. Channelization- CDMA

• Code Division Multiple Access (CDMA)
• Background
• Used for military communication for many years
• The first commercial CDMA service was launched in
  Hong Kong in 1995
• CDMA has rapidly become the primary choice of
  wireless communications technology
• Description
• In CDMA the transmission from different stations
  occupy the entire frequency band at the same
  time.
• The transmissions are separated by the fact that
  different codes are used to produce the signals
  that are transmitted by the different stations.
  The receivers use these codes to recover the
  signal from the desired station.

45
3. Channelization- CDMA

• Code Division Multiple Access (CDMA)
• Description
• CDMA is an application of the spread-spectrum
  communication technique. A spread spectrum is a
  means of transmission in which the signal
  occupies a bandwidth in excess of the minimum
  necessary to send the information the band
  spread is accomplished by means of a code which
  is independent of the data, and a synchronized
  reception with the code at the receiver is used
  for de-spreading and subsequent data recovery.

46
3. Channelization - CDMA

• Concept
• In CDMA, each users narrowband signal is spread
  over a wider bandwidth. This wider bandwidth is
  greater than the minimum bandwidth required to
  transmit the information
• Each users narrowband signal is spread by a
  different wideband code. Each of the codes are
  orthogonal to one another, and channelization of
  simultaneous users is achieved by the use of this
  set of orthogonal codes.
• All the spread wideband signals (of different
  users) are added together to form a composite
  signal, and the composite signal is transmitted
  over the air in the same frequency band.
• The receiver is able to distinguish among the
  different users by using a copy of the original
  code. The receiver sifts the desired user out of
  the composite signal be correlating the composite
  signal with the original code. All other users
  with codes that do not match the code of the
  desired user are rejected.

47
3. Channelization - CDMA

• Algorithm
• CDMA assigns a different code to each station.
• Each station uses its unique code to encode the
  data bits its sends. Each bit being sent by the
  sender is encoded by multiplying the bit by the
  code that changes at a much faster rate (known as
  the chipping rate) than the original sequence of
  data bits
• CDMA allows different stations to transmit
  simultaneously. A station transmits over all the
  frequency band all the time.
• At the receiver end, a receiver can extract its
  message by multiplying the incoming signal by the
  correct code.
• Figure 6.38 Code-division multiple access

48
3. Channelization - CDMA
LG Figure 6.38 Code-division multiple access
49
3. Channelization - CDMA

• Code
• Also called pseudo-noise code
• Each channel has a spreading sequence (the high
  data rate bit patterns which spreads the signals
  bandwidth)
• Orthogonal spreading sequences
• Let a (a1,a2,an) and b (b1,b2,bn) be two
  spreading sequences.
• Two sequences are orthogonal if their inner
  product is zero.
• a b a1b1 a2b2anbn 0
• Because the spreading sequence consist of 1s
  and 1s,
• aa a12 a22an2 n
• bb b12 b22bn2 n

50
3. Channelization - CDMA

• Code
• Three conditions that must be met by a set of
  orthogonal spreading sequences
• The cross- correlation should be zero or very
  small
• The dot product of each code divided by the order
  of the code should equal to 1. The order of the
  code is effectively the length of the code.
• Each sequence in the set has an equal number of
  1s and -1s, or the number of 1s differs from the
  number of -1s by at most one.
• Walsh-Hadamard matrix
• A way to generate orthogonal sequences
• Wnc is obtained by taking the complement of the
  element of Wn
• Figure 6.43 shows the construction of
  Walsh-Hadamard matrices

51
3. Channelization - CDMA
LG Figure 6.43 Construction of Walsh-Hadamard
matrices
0
1

• Note
• Orthogonal spreading sequences are obtained by
  changing the 0s to -1s in each of the rows in the
  matrix.
• The first row does not satisfy the conditions of
  orthogonal spreading sequence, and is not used
  for channelization.

52
3. Channelization - CDMA

• Example (LG Section 6.5.3 Orthogonal Spreading by
  Using Walsh Functions)
• The following four orthogonal spreading sequences
  are used
• Channel 1 (-1, -1, -1, -1)
• Channel 2 (-1, 1, -1, 1)
• Channel 3 (-1, -1, 1, 1)
• Channel 4 (-1, 1, 1, -1)
• Figure 6.41 shows how the orthogonal spreading
  sequences are applied
• Figure 6.42 shows how the signal from channel 2
  is recovered from the composite signal.

53
Figure 6.41 Example of orthogonal coding for
channelization
Note Correction Channel should be 1 1 1.
54
LG Figure 6.42 Example of channel signal recovery
using orthogonal coding

• Note For channel 4
• The sequence is (-1,1,1,-1)
• Each value of the correlator output pulse is
  equal to the value of the sum signal pulse
  multiplied by the corresponding value of the
  sequence pulse. Therefore the correlator output
  are
• (-1,-1,-1,3) (1,1,-3,1) (-1,-1,3,-1)
• Each integrator output is the sum of its
  corresponding sequence pulse values. Therefore
  the integrator output values are
• (0) (0) (0)

55
3. Channelization - CDMA

• Considerations of CDMA Implementation
• Synchronization
• The ability to channelize depends heavily on the
  orthogonality of the code sequences during all
  stages of the transmission.
• For example, if due to multi-path delay one of
  the users codes is delayed by one chip, then the
  delayed code is no longer orthogonal to the other
  (non-delayed) codes in the code set. For
  example, the two Walsh codes
• w2 (-1,-1,1,1) and w3 (-1,1,1,-1)
• are orthogonal. However, if w3 is delayed by
  one chip, that is
• w3 (-1,-1,1,1)
• then w2 and w3 are no longer orthogonal.
• Therefore, synchronization is essential for
  using Walsh codes for
• multiple access.

56
3. Channelization - CDMA

• The Near-far problem of CDMA
• In the algorithm, the signal level for each
  received signal was assumed to be the same at a
  given receiver.
• A powerful transmission from a nearby station
  could overwhelm the desired signal from a distant
  station.
• To help eliminate the near-far problem, CDMA
  uses power control. The base station rapidly
  samples the radio signal strength indicator
  levels of each mobile and then sends a power
  change command over the forward radio link. This
  sampling id done 800 times per second and can be
  adjusted in 84 steps of 1 DB. The purpose of
  this is so that the received power from all users
  are roughly equal.

57
3. Channelization - CDMA

• Length of the codeword
• The length of the codeword, and hence the chip
  rate, is a fundamental design parameter of a CDMA
  system.
• Spreading with a large codeword results in a
  large transmitted bandwidth, which, if there are
  only a small number of users, will prove
  inefficient.
• Spreading with a smaller codeword yields a
  smaller transmitted bandwidth, which may not be
  able to accommodate sufficient users
• The spreading factor also is influenced by the
  size of the frequency assignment available to the
  operator.