Data Communications and
Networking
Chapter 7
Error Control and Data Link Control
References:
Book Chapter 6 and 7
Data and Computer Communications, 8th edition,
by William Stallings

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Data Link Layer
• Objective:
— Achieving reliable communication between two adjacent machines
• Design Issues:
— Framing: data are sent in blocks called frames, the beginning and
end of each frame must be recognized by the receiver.
— Error control: bit errors introduced by the transmission system
should be detected and/or corrected.
— Flow control: the sending station must not send frames at a rate
faster than the receiving station can absorb them.
— Addressing: on a multipoint line, such as a LAN, the identity of the
two stations involved in a transmission must be specified.
— Transmit control information and data on the same line

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Framing
• Large block of data may be broken up into
small frames at the source because:
—limited buffer size at the receiver
—A larger block of data has higher probability of
error
• With smaller frames, errors are detected sooner, and only
a smaller amount of data needs to be retransmitted
—On a shared medium, such as Ethernet and
Wireless LAN, small frame size can prevent one
station from occupying medium for long periods

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Framing
• Need to indicate the start and end of a block of data
• Use preamble (e.g., flag byte) and postamble
• If the receiver ever loses synchronization, it can just search for
the flag byte.
• Frame: preamble + control info + data + postamble
• Problem: it is possible that the flag byte’s bit pattern occur in the
data
• Two popular solutions:
— Byte stuffing
• The sender inserts a special byte (e.g., ESC) just before each “accidental”
flag byte in the data (like in C language, “ is replaced with ”).
• The receiver’s link layer removes this special byte before the data are
given to the network layer.
— Bit stuffing: each frame starts with a flag byte “01111110”.
• Whenever the sender encounters five consecutive 1s in the data, it
automatically stuffs a 0 bit into the outgoing bit stream.
• When the receiver sees five consecutive incoming 1 bits, followed by a 0
bit, it automatically deletes the 0 bit.

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Byte Stuffing
Four examples of byte sequences before and after byte stuffing

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Bit Stuffing
Bit stuffing:
(a) The original data.
(b) The data as they appear on the line.
(c) The data as they are stored in the receiver’s memory after destuffing.

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Error Detection: Types of Error
• An error occurs when a bit is altered between transmission and
reception
• Single bit errors
— One bit is altered
— Adjacent bits are not affected
— Can occur in the presence of white noise (thermal noise)
• Burst errors
— A cluster of bits with Length B
— the first and the last and a number of intermediate bits in error
(not necessarily all the bits in the cluster suffer an error)
— More common and more difficult to deal with
— Can be caused by impulse noise

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Error Detection Process
• Additional bits are added by transmitter
for error detection (called error-detecting
codes, or check bits, or checksum)
• Can be used in different network
layers

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Parity Check
• Append a parity bit to the end of a block of
data
• Value of parity bit is such that the new data
has even (even parity) or odd (odd parity)
number of ones
—E.g., original data 1110001 -> 11100011 (odd
parity)
• Even number of bit errors goes undetected
—E.g., 11100011 -> 11010011 (undetected!)

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Cyclic Redundancy Check
• For a block of k bits, transmitter generates an
(n-k)-bit sequence called Frame Check
Sequence (FCS)
• The resulting frame consisting of n bits is
exactly divisible by some predetermined
number.
• Receiver divides the incoming frame by the
predetermined number:
—If no remainder, assume no error

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• Modulo 2 arithmetic
—Binary addition with no carries
—Binary subtraction with no carries
—The same as XOR operation
1111 + 1010 = 0101
1111 − 0101 =
1010
1010 + 1010 = 0000

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• Define:
— T = n-bit frame to be transmitted
— D = k-bit block of data: the first k bits of T
— F = (n-k)-bit FCS: the last (n-k) bits of T
— P = pattern of (n-k+1) bits: the predetermined
divisor
• T = 2n-kD + F
• We want T/P to have no remainder.
• Problem: Given D and P, how to calculate F ?

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P
R
Q
P
Dkn
2
quotient
remainder
Step 1:
Step 2: F = R
Verification:
Q
P
R
P
R
Q
P
R
P
D
P
RD
P
T
RDT
knkn
kn
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2
Problem: Given D and P, how to calculate F ?

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• Example:
—Given D = 1010001101 (10 bits)
— P = 110101 (6 bits)
—Then n = 15, k = 10, (n-k) = 5
—Multiple D by 25, yielding 101000110100000
—Divide it by P: remainder R = 01110
—Transmit D+R to the receiver: 101000110101110
—The receiver divide it by P, if no remainder, there
is no error.

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• By choosing different P, CRC can detect
different types of errors:
—All single-bit errors if P has more than one
nonzero item
—All double-bit errors if P is a primitive polynomial
—Any odd number of errors if P contain factor 11
—……

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Error Correction
• Retransmission: correction of detected errors usually
requires data block to be retransmitted
—Retransmission may not be appropriate for some wireless
applications
• Bit error rate in wireless network is high
• Result in lots of retransmissions
—Propagation delay can be long (satellite) compared with
frame transmission time
• Result in retransmission of frame in error plus many subsequent
frames (back to this issue in next chapter)
• It would be desirable to enable the receiver to
correct errors in an incoming transmission on the
basis of the bits in that transmission.
• FEC: forward error correction

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Error Correction Process
Diagram

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Line Configuration
• Link Topology
—Physical arrangement of stations on the medium (link)
—Link has two stations: a point-to-point link
—More than two stations: a multipoint link
• local area network, satellite
• Half duplex
—Of two stations, only one station may transmit at a time
—Requires one data path
• Full duplex
—Simultaneous transmission and reception between two
stations
—Digital signaling: requires two data paths (e.g., two twisted
pairs)
—Analog signaling: can use different frequencies

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Traditional Configurations

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Flow Control
• Ensuring the sending entity does not overwhelm the
receiving entity
—Preventing buffer overflow
• Transmission time
—Time taken to emit all bits into medium at the sender’s side
—Determined by the data rate
• Propagation time
—Time for a bit to traverse the link and reach the destination
—Determined by the transmission distance
• We first assume error-free transmission.

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Model of Frame Transmission

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Stop-and-Wait Flow Control
• Source transmits a frame.
• Destination receives the frame, and replies
with a small frame called acknowledgement
(ACK).
• Source waits for the ACK before sending the
next frame.
—This is the core of the protocol !
• Destination can stop the flow by not sending
ACK (e.g., if the destination is busy …).

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Performance of Stop-and-Wait
• Assumptions
—Transmission time of the data frame is 1
—Transmission time of the ACK frame is 0
—Propagation time is a
• a is the ratio of propagation time over transmission time
—Error-free transmission
• The channel utilization ratio is 1/(1+2a)
—In a time period of 1+2a, the transmitter is only busy with 1
unit of time.
• It is not efficient for long haul transmission and high
speed transmission.
—Another type of protocol called “sliding-window” is
designed for this situation.

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Stop-and-Wait Link Utilization

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Sliding-Window Flow Control
• Idea: allow multiple frames to transmit
—Receiver has a buffer of W frames
—Transmitter can send up to W frames without
receiving ACK
• Each frame needs to be numbered: sequence
number is included in the frame header
—Sequence number is bounded by the length of
“sequence number field” in the header, e.g., k
bits
• Frames are numbered modulo 2k
• ACK includes the sequence number of the
next expected frame by the receiver

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Sliding-Window Diagram
Need to buffer
them in case of
retransmission

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Example Sliding-Window
Have been
delivered to upper
layer
More spaces for
future frames
RR3 means the receiver has
received all frames up to frame 2
and is ready to receive frame 3.

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Performance of Sliding-Window
• Assumptions
—Window size is W
—Frame transmission time is 1
—ACK transmission time is 0
—Propagation time is a
—Error-free transmission
• The channel utilization ratio is
1 2 1
2 1
2 1
W a
U W
W a
a

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≥

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Error Control
• Error control: detection and correction of errors
• We consider two types of errors:
— Lost frames
• The receiver cannot recognize that this is a frame.
— Damaged frames
• The receiver can recognize the frame, but some bits are in error.
• Two approaches for error control
— ARQ: automatic repeat request, based on some or all of the
following ingredients:
• Error detection
• Positive acknowledgment
• Retransmission after timeout
• Negative acknowledgement and retransmission
— FEC: forward error correction

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Automatic Repeat Request
(ARQ)
• The effect of ARQ is to turn an unreliable
data link into a reliable one.
• Three versions of ARQ:
—Stop-and-wait
—Go-back-N
—Selective-reject (or, selective repeat)

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Stop-and-Wait ARQ
• Based on stop-and-wait flow control
• The source station is equipped with a timer.
• Source transmits a single frame, and waits for an ACK
• If the frame is lost…
— The timer eventually fires, and the source retransmits the frame.
• If receiver receives a damaged frame, discard it
— The timer eventually fires, and the source retransmits the frame.
• If everything goes right, but the ACK is damaged or lost, the
source will not recognize it
— The timer eventually fires, the source will retransmit the frame
— Receiver gets two copies of the same frame!
— Solution: use sequence numbers, 1 bit is enough, i.e., frame0 and
frame1, ACK0 and ACK1

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Stop-and-Wait
Diagram
Simple, but inefficient for
long distance and high
speed applications.
We can use sliding-window
technique to improve the
efficiency.

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Go-Back-N ARQ
• Based on sliding-window flow control
• Use window size to control the number of unacknowledged
frames outstanding
• If no error, the destination will send ACK as usual with next
frame expected (positive ACK, RR: receive ready)
• If error, the destination will reply with rejection (negative ACK,
REJ: reject)
— Receiver discards that frame and all future frames, until the
erroneous frame is received correctly.
— Source must go back and retransmit that frame and all succeeding
frames that were transmitted in the interim.
— This makes the receiver simple, but decreases the efficiency

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Go-Back-N: Damaged Frame
• Suppose A is sending frames to B. After each
transmission, A sets a timer for the frame.
• In Go-Back-N ARQ, if the receiver detects
error in frame i
—Receiver discards the frame, and sends REJ-i
—Source gets REJ-i
—Source retransmits frame i and all subsequent
frames

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Go-Back-N: Lost Frame (1)
• Assume receiver has received frame i-1. If
frame i is lost
—Source subsequently sends i+1
• Receiver gets frame i+1 out of order
—At data link layer, this means the lost of a frame!
• Receiver sends REJ-i
• Source gets REJ-i, and so goes back to frame
i and retransmits frame i, i+1, …

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Go-Back-N: Lost Frame (2)
• Assume receiver has received frame i-1
• Frame i is lost and no additional frame is sent
• Receiver gets nothing and returns neither
acknowledgement nor rejection
• Source times out and sends a request to
receiver asking for instructions
• Receiver responses with RR frame, including
the number of the next frame it expects, i.e.,
frame i
• Source then retransmits frame i

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Go-Back-N: Damaged RR
• Receiver gets frame i and sends RR-(i+1)
which is lost or damaged
• Acknowledgements are cumulative, so the
next acknowledgement, i.e., RR-(i+n) may
arrive before the source times out on frame i
• If source times out, it sends a request to
receiver asking for instructions, just like the
previous example

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Go-Back-N: Damaged REJ
• It is equivalent to the case of lost frame (2).
• Source times out and sends a request to
receiver asking for instructions
• Receiver responses with RR frame, including
the number of the next frame it expects
• Source then retransmits

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Go-Back-N
Diagram
Remark:
RR(P bit = 1) is a special RR which
is used by the source to ask for
instructions.
For a k-bit sequence number, the
window size can be at most 2k-1,
otherwise RR 0 is ambiguous (e.g.,
first sends frame 0 and gets back an
RR1, and then sends frames
1,…,7,0, and gets another RR1).

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Selective-Reject ARQ
• Also called selective repeat
• Pros:
—Only rejected frames are retransmitted
—Subsequent frames are accepted by the receiver
and buffered
—Minimizes the amount of retransmissions
• Cons:
—Receiver must maintain large enough buffer, and
must contain logic for reinserting the retransmitted
frame in the proper sequence
—Also more complex logic in the source

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Selective Reject -
Diagram
Remark:
For a k-bit sequence number, the window size
can be at most 2k-1, because the sending and
receiving windows overlap.
Assume k=3, and window size is 5.
1. A sends frames 0, 1, …, 4 to B.
2. B receives all 5 frames, and cumulatively
acknowledges with RR5.
3. RR5 is lost.
4. A times out, and retransmits frame 0.
5. B is expecting a new set of frames 5, 6, 7,
0, 1. So it will accept the retransmitted frame
0 and regard it as a new frame, which is
wrong.

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High-Level Data Link Control
• HDLC (ISO 33009, ISO 4335)
—Modified from SDLC (IBM)
• Used in X.25
• Basis for other data link control protocols

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Frame Structure
• Synchronous transmission
• All transmissions in frames
• Single frame format for all data and control
exchanges

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Frame Structure
•Flag: delimit frame at both ends
•Address: identify the frame receiver
•Control: specify different frame types
•FCS: frame check sequence (error detecting code)

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KEY POINTS
• Framing is performed by breaking the
information into small frames. Each frame
uses preamble and postamble to indicate the
start and end.
• Error detection is performed by calculating an
error-detecting code that is a function of the
bits being transmitted.
• Error correction operates in a fashion similar
to error detection but is capable of correcting
certain errors.

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KEY POINTS
• Data link control protocol provides functions such as
flow control, error detection, and error control.
• Flow control enables a receiver to regulate the flow
of data from a sender so that the receiver’s buffers
do not overflow.
• In a data link control protocol, error control can
achieved by retransmission of damaged frames that
have not been acknowledged or for which the other
side requests a retransmission.

Dc ch07 : error control and data link control

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