Chapter 7 Data Link Control

1
Chapter 7Data Link Control
Read Chapter 7 pay attention to the reasons why
the Data Link Layer exists (regulates data rate
among other functions).
2
Physical Layer
3
Data Link Layer

• Provides for reliable transfer of information
  across physical link
• Includes
• transmission of blocks of data (frames)
• synchronization
• error control
• flow control

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Data Link Layer Functions

• Frame Synchronization - Create abstraction of
  frame-at-a-time channel for higher layer (start
  end of each frame must be recognizable)
• Addressing - Needed when many nodes share
  transmission link
• Flow Control - Control rate of transmission to
  prevent overflow of receiver's buffers
• Error Control - Correct transmission errors (by
  retransmission) or by error correction schemes
• Sequence Control - Receiver must be able to
  distinguish control information from data
• Link Management - Initiation, maintenance,
  termination of connections

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Flow Control

• Necessary when data is being sent faster than it
  can be processed by receiver
• Computer to printer is typical setting
• Can also be from computer to computer, when a
  processing program is limited in capacity

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Stop-and-Wait Flow Control

• Simplest form of Flow Control
• Source may not send new frame until receiver
  acknowledges the frame already sent
• Very inefficient, especially when a single
  message is broken into separate frames

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Sliding-Window Flow Control

• Allows multiple frames to be in transit
• Receiver sends acknowledgement with sequence
  number of anticipated frame
• Sender maintains list of sequence numbers it can
  send, receiver maintains list of sequence numbers
  it can receive
• ACK (acknowledgement) supplemented with RNR
  (receiver not ready)

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Error Control Process

• All transmission media have potential for
  introduction of errors
• Error control process has two components
• Error detection
• Error correction

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Error Detection

• Parity Check
• Cyclic Redundancy Check (CRC)

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Error Correction

• Two types of errors
• Lost frame
• Damaged frame
• Automatic Repeat reQuest (ARQ)
• Error detection
• Positive acknowledgment
• Retransmission after time-out
• Negative acknowledgment and retransmission

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Stop-and-Wait ARQ

• One frame received and handled at a time
• If frame is damaged, receiver discards it and
  sends no acknowledgment
• Sender uses timer to determine whether or not to
  retransmit
• Sender must keep a copy of transmitted frame
  until acknowledgment is received
• If acknowledgment is damaged, sender will know it
  because of numbering

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Go-Back-N ARQ

• Uses sliding-window flow control
• When receiver detects error, it sends negative
  acknowledgment (REJ)
• Sender must begin transmitting again from
  rejected frame
• Transmitter must keep a copy of all transmitted
  frames

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Flow Control

• Technique for controlling data rate so that
  sender does not over-run receiver
• Two approaches exist
• 1. Stop-and-wait
• 2. Sliding-window

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Stop Wait Flow Control

• 1. Sender sends a frame
• 2. Receiver receives frame acknowledges it
• 3. Sender waits to receive ack before sending
  next frame (If receiver is not ready to receive
  another frame it holds back the ack)
• One frame at a time is sent over the
  transmission line

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Utilization (Efficiency) of Stop Wait

• Propagation time time taken for signal to travel
  from S to R. Thus first bit transmitted at t0
  arrives at R at t T p d / V.

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Utilization (Efficiency) of Stop Wait

• Transmission time time taken to emit all bits of
  frame at sender T t L / B.

In figure 7.2 Page 197, Transmission Time1,
therefore a Propagation (Delay) Time
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Utilization (Efficiency) U
Note aTp/Tt or TpaTt
Vertical-Time Sequence Diagram
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The effect of a on utilization
This shows that for large a (Propagation
TimegtTransmission Time), the line is
under-utilized
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Utilization (Efficiency) of Stop Wait

• With stop wait scheme, for high channel
  utilization, we need a low a (since U 1/(12a))

• However, in practice it is desirable to limit
  frame length L because
• error probability increases with L
• high average delay with multi-point lines
• buffer size limitations
• So a more efficient scheme is called for,
  especially with WAN/satellite communication

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Utilization (Efficiency) of Stop Wait Examples

• 1). WAN using ATM
• L 53 bytes 424 bits, B 155.52 Mbps, d1000km
  Þ T t 424 / (155.52 x 106 ) 2.7 x 10-6
  seconds assuming optical fiber, T p 10 6 m / (3
  x 108 m/sec) 0.33 x 10-2 seconds Þ a (0.33 x
  10-2 )/(2.7 x 10-6 ) 1200 Þ U 1/2401 0.0004
• 2). LAN
• d 0.1 10 km, B 10 Mbps, V 2 x 108 m/sec
• L 1000 bits Þ a 0.005 0.5 Þ U 0.5 0.99
• 3). Digital trans. vis modem over voice-grade
  line
• B 28.8 kbps, L 1000 bits, d 1000 m Þ a
  (28.8 kbps 1000 m) / (2 x 108 m/sec x 1000
  bits) 1.44 x 10-4 Þ U 1.0, if d 5000 km, a
  (28.8kbps x 5000km) / (2 x 108 x 1000bits)
  0.72 Þ U 0.4

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Sliding-Window Flow Control

• Pipeline transmission of successive frames
• Transmit up to N frames if necessary without
  receiving acks.
• Wait for acks when N unacked frames in transit
• For full duplex transmission each station needs a
  sending window receiving window.

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Figure 7.4 Example of a sliding-window protocol
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Utilization U is a function of a and N

• Case 1 N gt 1 2a U 1
• Ack for frame 1 reaches Sender before
  transmission of Nth frame Þ continuous
  transmission possible

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Utilization U is a function of a and N

• Case 2 N lt 1 2a U N / (1 2a)
• Wasted time between N and 1 2a

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Error Detection

• Basic Principle
• Transmitter For a given bit stream M, additional
  bits (called error-detecting code) are calculated
  as a function of M and appended to the end of M
• Receiver For each incoming frame, performs the
  same calculation and compares the two results. A
  detected error occurs if there is a mismatch

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Error Detection

• Two common techniques
• Parity checks
• Cyclic redundancy checks (CRC)
• Parity Check
• One extra parity bit is added to each word
• Odd parity bit added so as to make of 1s odd
• Even parity makes total of 1s even
• Single parity is very effective with white noise
  (noise on a line without any active signals on
  it e.g., Thermal Noise, see chapter 3), but not
  very robust with noise bursts (which may extend
  over whole word duration.)

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Cyclic Redundancy Checks

• Powerful error detection method, easily
  implemented
• Message (M) to be transmitted is appended with
  extra frame checksum bits (F), so that bit
  pattern transmitted (T) is perfectly divisible by
  a special generator pattern (P) - (divisor)
• At destination, divide received message by the
  same P. If remainder is nonzero Þ error

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Cyclic Redundancy Checks

• Let
• T (kn)-bit frame to be transmitted, nltk
• M k-bit message, the first k bits of T
• F n-bit FCS, the last n bits of T
• P n1 bits, generator pattern (predetermined
  divisor)
• The concept uses modulo-2 arithmetic
• no carries/borrows add º subtract º XOR

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Cyclic Redundancy Checks

• Method
• Extend M with n 0s to the right (º 2 n M)(shift
  left by n bits
• Divide extended message by P to get R (2 n M / P
  Q R/P)
• Add R to extended message to form T (T 2 n M
  R)
• Transmit T
• At receiver, divide T by P. Nonzero remainder
  means Þ error

Note Remainder RFFCS in these examples
Note RR0 in mod-2 arithmetic
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Cyclic Redundancy Checks Examples

• M 110011, P 11001, R 4 bits
• 1. Append 4 zeros to M, we get 1100110000

-For each stage of division, if the number of
dividend bits equals number of divisor P bits,
then Q1, otherwise Q0
-Also, see example on page 204
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Cyclic Redundancy Checks

• Exercise Compute the frame to be transmitted for
  message 1101011011 using P 10011
• Answer 11010110111110

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Cyclic Redundancy Checks

• Can view CRC generation in terms of polynomial
  arithmetic
• Any bit pattern º polynomial in dummy variable X
  as shown in the following example
• e.g., M 110011 º 1X5 1X4 0X3 0X2
  1X1X0 \ M(X) X5 X4 X1

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Cyclic Redundancy Checks

• CRC generation in terms of polynomial
• Append n 0s º Xn M(X)
• Modulo 2 division
• Transmit Xn M(X)R(X) T(X)
• At receiver

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Cyclic Redundancy Checks

• Commonly used polynomials, P(X)
• CRC-16 X16 X15 X2 1
• CRC-CCITT X16 X12 X5 1
• CRC-32 X32 X26 X23 X22 X16 X12 X11
  X10 X8 X7 X12 X4 X2 X1

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Can detect

• 1. All single-bit errors
• 2. All double-bit errors, as long as P(X) has a
  factor with at least three terms (as long as p
  has at least three 1s)
• 3. Any odd number of errors, as long as P(X)
  contains a factor (X1)
• 4. Any burst error for which the length of the
  burst is less than the length of the FCS
• 5. Most larger burst errors

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Why?

• Error can also be represented by polynomial,
  E(X).
• Tr(X) T(X) ? E(X)
• Error is undetectable if E(X) is divisible by
  P(X)
• P always has at least two terms, Xn , 1 (most
  least significant bits equal to 1)
• 1. Single-bit error E(X) Xi (one bit1)
• P(X) X n 1 \ Error is detectable since
  E(X) is not divisible by P(X)

TTransmitted frame EError pattern with 1s in
positions where errors happen TrReceived frame
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Why?

• 2. Double-bit error E(X) Xi Xj Xi (1Xk ),
  kj-igt0
• P(X) does not divide into Xi
• P(X) can be chosen which does not divide 1Xk up
  to the maximum value of k (i.e., up to the
  practical frame length). (e.g., X15 X14 1 will
  not divide 1Xk for any k below 32768)

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Why?

• 3. No polynomial with an odd of terms is
  divisible by (X1)
• Assume E(X) has an odd of terms and is
  divisible by (X1). Then E(X) (X1)Q(X). E(1)
  (11)Q(1) 0. However, E(1) cannot be zero since
  it has an odd of 1s
• 4. A burst error of length r lt n can be
  represented by Xi (Xr-1 1).
• P(X) does not divide into Xi
• P(X) which is a polynomial of degree n cannot
  divide into Xr-1 1 since r-1ltn.

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Implementation

• Implemented by a circuit consisting of
  exclusive-or gates and a shift register
• The shift register contains n bits (length of
  FCS)
• There are up to n exclusive-or gates
• The presence or absence of a gate corresponds to
  the presence or absence of a term in P(X)

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Implementation
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Example
NoteR01110 see pages 206-207
Shift Register Circuit for dividing by the
polynomial X5 X4 X2 1
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Error Control

• See page 208

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Error control techniques

• Forward error control
• Error recovery by correction at the receiver
  Forward Error Correction (FEC)
• Backward error control
• Error recovery by retransmission Automatic
  Repeat Request (ARQ)

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ARQ

• Based on
• Error detection
• Positive ack
• Retransmission after timeout
• Negative ack. And retransmission
• ARQ
• Stop-and-wait ARQ
• Continuous ARQ
• Go-back-N ARQ
• Selective-reject ARQ

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Stop Wait ARQ

• Simple and minimum buffer requirement, but
  inefficient
• Sender transmits message frame
• Receiver checks received frame for errors sends
  ACK/NAK
• Sender waits for ACK/NAK. NAK Þ retrans ACK Þ
  next frame

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Stop Wait ARQ

• Frame/ACK could be lost Þ Uses a timeout
  mechanism
• Possibility of duplication Þ Number frames
• Only need a 1-bit frame number alternating 1 and
  0 since they are sent one at a time

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Go-back-N ARQ

• If the receiver detects an error on a frame, it
  sends a NAK for that frame. The receiver will
  discard all future frames until the frame in
  error is correctly received. Thus the sender,
  when it receives a NAK or timeout, must
  retransmit the frame in error plus all succeeding
  frames. (Sender must maintain a copy of each
  unacknowledged frame.)

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Go-back-N ARQ
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Selective-reject ARQ

• The only frames retransmitted are those that
  receive a NAK or which timeout
• Can save retransmissions, but requires more
  buffer space and complicated logic
• See Figure 7.9b Page 212

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Maximum window size (with n-bit sequence number)

• Go-back-N 2n - 1
• Selective-reject 2n-1

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High-Level Data Link Control (HDLC)

• A synchronous data link protocol
• Widely used, and basis for many other data link
  control protocols
• Connections can be multipoint or point-to-point
• Can be used in half-duplex or full-duplex

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Three types of stations

• Primary station - (e.g., Mainframe) Controls the
  operation of the link, issues commands, and
  receives responses
• Secondary station - (e.g., Terminal) Usually only
  communicates (responds) to a primary station
• Combined station - Can be both a primary and a
  secondary

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Two link configurations

• Unbalanced configuration - One primary and one or
  more secondary stations
• Balanced configurations - Two combined stations

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Three data transfer modes

• Normal response mode (NRM) - Unbalanced
  configuration Primary station always dictates who
  sends and receives
• Asynchronous balanced mode (ABM) - Two combined
  stations. Either can initiate transmission
• Asynchronous response mode - Unbalanced
  configuration Secondary station can send at any
  given time, but only one secondary can be active
  at a time

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HDLC Frame Structure
The 1st 3 fields are the header and the last 2
fields are the trailer
0
1
The first one or 2 bits identify the frame type
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Flag fields

• 8 bits (01111110)
• Bit stuffing is used for data transparency
• Bit stuffing whenever five 1s are transmitted,
  extra zero is inserted

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Flag fields
An inverted bit could split a frame in two
An inverted bit merges 2 frames
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HDLC Frame Structure

• Address field
• 8 bits. If needed longer address, the MSB can be
  set to zero and the address field is then assumed
  to be 8 bits longer.
• All 1s indicates this is a broadcast frame

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HDLC Frame Structure

• Control field
• 8 bits. If the 1st bit is 0, then information
  frame, Otherwise, 10 indicates supervisory
  frame and 11 indicates unnumbered frame
• Information frame
• two 3 bits sequence numbers, P/F bit
• sequence number can be extended to 7
• used to send data

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All of the control field formats contain the
Poll/final (P/F) bit. Its use depends on
context. Typically, in command frames, it is
referred to as the P bit and is set to "1" to
solicit (poll) a response frame from the peer
HDLC entity. In response frames, it is referred
to as the F bit and is set to "1" to indicate the
response frame transmitted as a result of a
soliciting command.
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HDLC Frame Structure

• Control field
• Supervisory frame bits 3 and 4 determine types
• 00 Receive Ready. Used to ACK
• 01 Reject. Essentially a NACK in Go-back-N ARQ
  (Automatic Repeat Request)
• 10 Receive Not Ready. Indicates busy condition
• 11 Selective Reject. Request for a single
  frame retransmission
• Bit 5 is P/F. Bits 6,7,8 are sequence number

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HDLC Frame Structure

• Control field
• Unnumbered frame
• More control functions. Has a variety of
  purposes, but most often used for establishing
  the link setup and disconnect.
• Sets up the data transfer mode, sequence number
  size. Also used to reset the link and other
  miscellaneous stuff.

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HDLC Commands and responses
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Examples of Operation