1. DATA
LINK
LAYER
TOPIC: Error control & Flow control
Submitted to : Submitted by:
Prof Manraj Singh Pattar Alisha Korpal
Nancy Jain
Nivea Jain
Sharuti Jain
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2. INDEX
S no Topic Page no
1 Introduction to data link layer 3
2 Services provided to network layer 4
3 Services 6
4 Framing 7
5 Error detection 8
6 Error correction 15
7 Flow control 16
8 Error control 19
9 References 24
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3. INTRODUCTION
Data Link Layer Design Issues
The data link layer has a number of specific functions it can carry out. These functions
include
1. Providing a well-defined service interface to the network layer.
2. Dealing with transmission errors.
3. Regulating the flow of data so that slow receivers are not swamped by fast senders.
To accomplish these goals, the data link layer takes the packets it gets from the network layer
and encapsulates them into frames for transmission. Each frame contains a frame header, a
payload field for holding the packet, and a frame trailer.
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4. Services Provided to the Network Layer
The function of the data link layer is to provide services to the network layer. The principal
service is transferring data from the network layer on the source machine to the network layer
on the destination machine. On the source machine is an entity, call it a process, in the
network layer that hands some bits to the data link layer for transmission to the destination.
The job of the data link layer is to transmit the bits to the destination machine so they can be
handed over to the network layer there.
1. Unacknowledged connectionless service.
2. Acknowledged connectionless service.
3. Acknowledged connection-oriented service
Unacknowledged Connectionless Service
Unacknowledged connectionless service consists of having the source machine send
independent frames to the destination machine without having the destination machine
acknowledge them. No logical connection is established beforehand or released afterward. If a
frame is lost due to noise on the line, no attempt is made to detect the loss or recover from it
in the data link layer. This class of service is appropriate when the error rate is very low so
that recovery is left to higher layers. It is also appropriate for real-time traffic, such as voice,
in which late data are worse than bad data. Most LANs use unacknowledged connectionless
service in the data link layer.
Acknowledged Connectionless service
When this service is offered, there are still no logical connections used, but each frame sent is
individually acknowledged. In this way, the sender knows whether a frame has arrived
correctly. If it has not arrived within a specified time interval, it can be sent again. This
service is useful over unreliable channels, such as wireless systems.
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5. Connection Oriented Service
With this service, the source and destination machines establish a connection before any data
are transferred. Each frame sent over the connection is numbered, and the data link layer
guarantees that each frame sent is indeed received. Furthermore, it guarantees that each frame
is received exactly once and that all frames are received in the right order. With
connectionless service, in contrast, it is conceivable that a lost acknowledgement causes a
packet to be sent several times and thus received several times. Connection-oriented service,
in contrast, provides the network layer processes with the equivalent of a reliable bit stream.
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6. SERVICES
1. Framing
2. Error control
3. Flow control
4. Access Control
5. Physical Addressing
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7. Framing
To provide service to the network layer, the data link layer must use the service provided to it
by the physical layer. What the physical layer does is accept a raw bit stream and attempt to
deliver it to the destination. This bit stream is not guaranteed to be error free. The number of
bits received may be less than, equal to, or more than the number of bits transmitted, and they
may have different values. It is up to the data link layer to detect and, if necessary, correct
errors.
The usual approach is for the data link layer to break the bit stream up into discrete frames
and compute the checksum for each frame. (Checksum algorithms will be discussed later in
this chapter.) When a frame arrives at the destination, the checksum is recomputed. If the
newly-computed checksum is different from the one contained in the frame, the data link
layer knows that an error has occurred and takes steps to deal with it (e.g., discarding the bad
frame and possibly also sending back an error report).
Breaking the bit stream up into frames is more difficult than it at first appears. One way to
achieve this framing is to insert time gaps between frames, much like the spaces between
words in ordinary text. However, networks rarely make any guarantees about timing, so it is
possible these gaps might be squeezed out or other gaps might be inserted during
transmission. Since it is too risky to count on timing to mark the start and end of each frame,
other methods have been devised. In this section we will look at four methods:
1. Character count.
2. Flag bytes with byte stuffing.
3. Starting and ending flags, with bit stuffing.
4. Physical layer coding violations.
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8. Error Control
Having solved the problem of marking the start and end of each frame, we come to the next
problem: how to make sure all frames are eventually delivered to the network layer at the
destination and in the proper order. Suppose that the sender just kept outputting frames
without regard to whether they were arriving properly. This might be fine for
unacknowledged connectionless service, but would most certainly not be fine for reliable,
connection-oriented service.
The usual way to ensure reliable delivery is to provide the sender with some feedback about
what is happening at the other end of the line. Typically, the protocol calls for the receiver to
send back special control frames bearing positive or negative acknowledgements about the
incoming frames. If the sender receives a positive acknowledgement about a frame, it knows
the frame has arrived safely. On the other hand, a negative acknowledgement means that
something has gone wrong, and the frame must be transmitted again.
An additional complication comes from the possibility that hardware troubles may cause a
frame to vanish completely (e.g., in a noise burst). In this case, the receiver will not react at
all, since it has no reason to react. It should be clear that a protocol in which the sender
transmits a frame and then waits for an acknowledgement, positive or negative, will hang
forever if a frame is ever lost due to, for example, malfunctioning hardware
• Error Detection
• Error Correction
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9. Error Detection
When data is being transmitted from one machine to another, it may possible that data
become corrupted on its way. Some of the bits may be altered, damaged or lost during
transmission. Such a condition is known as error.
The error may occur because of noise on line, attenuation and delay distortion. For reliable
communication, it is important that errors are detected and corrected.
Types of errors
• Single bit error
• Burst error
Single bit error
It means only one bit of data unit is changed from 1 to 0 or from 0 to 1. Single bit error can
happen in parallel transmission where all the data bits are transmitted using separate wires.
Single bit error is least likely type of errors in serial transmission.
Burst Error
It means two or more bits in data unit are changed from 1 to 0 or from 0 to 1. In burst error, it
is not necessary that only consecutive bits are changed. The length of burst error is measured
from first changed bit to last changed bit.
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10. Techniques
Error detection means inspecting the data so as to check whether or not data is changed during
the transmission. An error detection system is needed at the receiver end that should check the
incoming data and report the number and type’s errors that define have occurred. The most
common technique used for error detection is redundancy.
Redundancy
Redundancy is the method in which some extra bits are added to the data so as to
check whether the data contain error or not.
These redundant bits are added by the sender and removed by the receiver as soon as
the accuracy of the transmission has been determined
These redundant bits are then added to the data and data + redundant bits are
transmitted to the receiver
Types
1. Vertical Redundancy check/parity check
2. Longitudinal Redundancy check
3. Cyclic Redundancy check
4. Checksum
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11. Vertical Redundancy Check/Parity Check
This method is also known as parity check. In this method, a redundant bit called parity bit is
added to each data unit. This method may include even parity or odd parity. Even parity
means that total number of 1are even. Odd parity means the total no of 1 are odd.
Example
If source wants to transmit a data unit 1100111 using even parity to the destination. The
source will first pass this unit to even parity generator.
The parity generator will count the number of 1 in data and will add parity bit. In this
example, since the number of 1 in data unit is five, the parity generator appends a parity bit 1
to this data unit making the total number of 1 even.
1 1 0 0 1 1 1
Even Parity generator
1
(Counts 1 in data)
1 1 0 0 1 1 1 1
VRC can detect all single bit error
It can also detect burst error but only in those cases where number of bits changed is
odd.
The major disadvantage of this method is that it is not able to detect burst error if the
numbers of bits are even.
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12. Longitudinal Redundancy Check
In this method block of bits is divided into table or matrix of rows and columns.
In order to detect the error, a redundant row of bits is added to the whole block and this
block is transmitted to the receiver.
The receiver uses this redundant row to detect error. After checking the data for errors,
receivers accept the data and discard the redundant row of bits.
Sender data
11001010 10101010 11001100 11100011
11001010
10101010
11001100
11100011
01001111
11001010 10101010 11001100 11100011 01001111
Receiver data
When this data reaches the destination, receiver uses LRC to detect error in data.
LRC is used to detect burst errors. For e.g. suppose 32 bit data plus LRC that was being
transmitted is hit by burst error of length five and some bits are corrupted.
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13. Cyclic Redundancy Check
CRC is more powerful than VRC and LRC in detecting errors.
It is not based on binary addition like VRC and LRC. Rather it is based on binary
division.
At the sender side, the data unit to be transmitted is divided by a predetermined divisor
(binary number) in order t obtain the remainder. This remainder is called CRC.
The CRC has one bit less than the divisor. It means that if CRC is of n bits, divisor is
of n+ 1 bit.
The sender appends this bit CRC to the end of data unit such that the resulting data
unit becomes exactly divisible by predetermined divisor i.e. remainder become zero.
1011
1011 1001000
1011
0100
0000
1000
1011
0110
0000
110
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14. Checksum
Checksum is the error detection method used by upper layer protocols and is
considered to be more reliable than LRC, VRC & CRC.
This method makes use of checksum generator on the sender side &checksum checker
on the receiver side.
At the sender side, checksum generator divides the data into equal subunits of n bit
length. This length is generally of 16 bits.
For example
If the data unit to be transmitted is 10101001 00111001, the following procedure is
used at the sender & receiver site.
Sender site:
10101001 (subunit 1)
00111001 (subunit 2)
11100010 (sum using 1 compliment)
00011101 check sum (compliment of sum )
10101001 00111001 00011101
DATA Checksum
Receiver site:
10101001 subunit 1
00111001 subunit 2
00011101 checksum
11111111 sum
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00000000 sum compliment
15. Error Correction
Error correction means rectifying all the bits that have been altered or changed during the
transmission of data. Error correction can be done as follows:
By retransmission of data from sender to receiver.
By the use of error correction code at the receiver side.
Hamming Code
Hamming code is a technique developed by R.W. hamming for error correction. This method
corrects the error by finding the state at which the error has occurred.
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16. Flow Control
Another important design issue that occurs in the data link layer (and higher layers as well) is
what to do with a sender that systematically wants to transmit frames faster than the receiver
can accept them. This situation can easily occur when the sender is running on a fast (or
lightly loaded) computer and the receiver is running on a slow (or heavily loaded) machine.
The sender keeps pumping the frames out at a high rate until the receiver is completely
swamped. Even if the transmission is error free, at a certain point the receiver will simply be
unable to handle the frames as they arrive and will start to lose some. Clearly, something has
to be done to prevent this situation.
Two approaches are commonly used. In the first one, feedback-based flow control, the
receiver sends back information to the sender giving it permission to send more data or at
least telling the sender how the receiver is doing. In the second one, rate-based flow control,
the protocol has a built-in mechanism that limits the rate at which senders may transmit data,
without using feedback from the receiver. In this chapter we will study feedback-based flow
control schemes because rate-based schemes are never used in the data link layer.
Methods
Stop and Wait
Sliding window
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17. Stop and Wait
In this method of flow control, the sender sends a single frame to receiver and waits
for an acknowledgment.
The next frame is send by sender only when acknowledgment of previous frame is
received.
This process of sending a frame & waiting for an acknowledgment continues as long
as sender has data to send.
To end up the transmission sender transmits EOT frame
The main advantage is its accuracy. Next frame is transmitted only when the
acknowledged is received.
So no chance of frame being lost
Receiver
Sender
DATA
ACK
DATA
EOT
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18. Sliding Window
In sliding window method, multiple frames are sent by sender at a time before needing an
acknowledgment. Multiple frames sent by source are acknowledged by receiver using a single
ACK frame. Sliding window refers to an imaginary boxes that hold the frames on both sender
and receiver side. It provides the upper limit on the number of frames that can be transmitted
before requiring an acknowledgment. Frames may be acknowledged by receiver at any point
even window is not full on receiver side. Frames may be transmitted by source even when
window is not yet full on sender side.
6 7 0 1 2 3 4 5
Window
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19. Error Control
Error control function of data link layer detects the error in transmitted frames and retransmits
all the erroneous frames. Therefore error control function of data link layer helps in dealing
with data frames that are damaged in transmit, data frame lost in transmit and the
acknowledgment frame that are lost in transmission. The method is used for error control is
called Automatic Repeat Request (ARQ).
Automatic Repeat Request (ARQ)
If an error is detected in any frame, the receiver sends a negative acknowledgement (NAK)
back to source and the specific frame is retransmitted.
ARQ techniques:
Stop and Wait ARQ
Sliding window ARQ
Go - back - n
Selective – reject
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20. Stop and Wait
The sending device keeps a copy of the last frame transmitted until it receives an
acknowledgment for that frame. Keeping this copy helps the sender in retransmission
of lost or damaged frames later on.
Both data frames and ACK frames are numbered alternately 0 and 1 for identification
purpose.
Criteria used by stop and wait ARQ method:
Damaged data frames
Lost data frames
Lost acknowledged frames
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21. Damaged data frames
If a data frame received by a receiver contains an error, it returns NAK frame to the
sender. On receiving NAK frame, sender retransmits the last data frame.
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22. Lost data frames
Every sending device is equipped with timer.
The sender starts this timer when it transmits data frame.
If a data frame is lost on its way, it will not be received by receiver. As a result the
receiver can never acknowledge it, positively or negatively.
The sending device waits for an ACK and NAK frame until timer goes of.
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23. Lost acknowledgment frames
When any data frame reaches the destination, the receiver acknowledges it either with
ACK or with NAK.
If ACK or NAK frame returned by the receiver is lost in transmit, sending device
retransmit the data frame that has not been acknowledged
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