R22 regulat Computer Networks UNIT 2.ppt

DATA LINK LAYER
Presented by
Mr. N. Balaraman
Assistant Professor
Department of Computer Science and Engineering
St. Martin’s Engineering College
COMPUTER NETWORKS
B.Tech: III Year – I Sem
UNIT 2
Mr. N. Balaraman(CN-Unit 2)
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UNIT 2 - Data link layer
• Data link layer:
• Design issues
• Framing
• Error detection and correction
• Elementary data link protocols:
• simplex protocol
• A simplex stop and wait protocol for an error free channel
• A simplex stop and wait protocol for noisy channel
• Sliding Window protocols:
• A one-bit sliding window protocol
• A protocol using Go-Back-N
• A protocol using Selective Repeat
• Example data link protocols
• Medium Access sub layer:
• The channel allocation problem
• Multiple access protocols:
• ALOHA
• Carrier sense multiple access protocols
• Collision free protocols.
• Wireless LANs, Data link layer switching
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Data Link Layer
• In the OSI model, the data link layer is a 4th layer from the top and 2nd layer from the
bottom.
• The communication channel that connects the adjacent nodes is known as links, and in
order to move the datagram from source to the destination, the datagram must be moved
across an individual link.
• The main responsibility of the Data Link Layer is to transfer the datagram across an
individual link.
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(DL)Cont…
• The Data link layer protocol defines the format of the packet exchanged across the nodes
as well as the actions such as Error detection, retransmission, flow control, and random
access.
• The Data Link Layer protocols are Ethernet, token ring, FDDI and PPP.
• An important characteristic of a Data Link Layer is that datagram can be handled by
different link layer protocols on different links in a path.
• For example, the datagram is handled by Ethernet on the first link, PPP on the second link.
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Following services are provided by the Data Link Layer:
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DLL (Cont…)
Framing & Link access:
• Data Link Layer protocols encapsulate each network frame within a Link layer frame
before the transmission across the link.
• A frame consists of a data field in which network layer datagram is inserted and a number
of data fields.
• It specifies the structure of the frame as well as a channel access protocol by which frame
is to be transmitted over the link.
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DLL (Cont…)
Reliable delivery:
• Data Link Layer provides a reliable delivery service, i.e., transmits the network layer
datagram without any error.
• A reliable delivery service is accomplished with transmissions and acknowledgements.
• A data link layer mainly provides the reliable delivery service over the links as they have
higher error rates and they can be corrected locally, link at which an error occurs rather
than forcing to retransmit the data.
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DLL (Cont…)
Flow control:
• A receiving node can receive the frames at a faster rate than it can process the frame.
• Without flow control, the receiver's buffer can overflow, and frames can get lost.
• To overcome this problem, the data link layer uses the flow control to prevent the sending
node on one side of the link from overwhelming the receiving node on another side of the
link.
Error detection:
• Errors can be introduced by signal attenuation and noise.
• Data Link Layer protocol provides a mechanism to detect one or more errors.
• This is achieved by adding error detection bits in the frame and then receiving node can
perform an error check. 8

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DLL (Cont…)
Error correction:
• Error correction is similar to the Error detection, except that receiving node not only detect
the errors but also determine where the errors have occurred in the frame.
Half-Duplex & Full-Duplex:
• In a Full-Duplex mode, both the nodes can transmit the data at the same time.
• In a Half-Duplex mode, only one node can transmit the data at the same time.
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OVERVIEW OF DLL
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• The data link layer transforms the physical layer, a raw transmission facility, to a
link responsible for node-to-node (hop-to-hop) communication.
• Specific responsibilities of the data link layer include framing, addressing, flow
control, error control, and media access control.
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DATA LINK LAYER DESIGN ISSUES
 Providing a well-defined service interface to the network layer.
 Dealing with transmission errors.
 Regulating the flow of data so that slow receivers are not swamped by fast
senders
For this, 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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SERVICES PROVIDED TO THE NETWORK LAYER
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2
 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.
 The data link layer can be designed to offer various services. The actual services offered
can vary from system to system. Three reasonable possibilities that are commonly
provided are
1) Unacknowledged Connectionless service
2) Acknowledged Connectionless service
3) Acknowledged Connection-Oriented service
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UNACKNOWLEDGED CONNECTIONLESS SERVICE
1
3
 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
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ACKNOWLEDGED CONNECTIONLESS SERVICE
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4
 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.
 Adding Ack in the DLL rather than in the Network Layer is just an optimization
and not a requirement.
 If individual frames are acknowledged and retransmitted, entire packets get through
much faster.
 On reliable channels, such as fiber, the overhead of a heavyweight data link
protocol may be unnecessary, but on wireless channels, with their inherent
unreliability, it is well worth the cost.
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ACKNOWLEDGED CONNECTION-ORIENTED 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.
 When connection-oriented service is used, transfers go through three distinct
phases.
 In the first phase, the connection is established by having both sides initialize variables
and counters needed to keep track of which frames have been received and which ones
have not.
 In the second phase, one or more frames are actually transmitted.
 In the third and final phase, the connection is released, freeing up the variables, buffers,
and other resources used to maintain the connection
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FRAMING
 DLL translates the physical layer's raw bit stream into discrete units
(messages) called frames.
How can frame be transmitted so the receiver can detect frame boundaries?
That is, how can the receiver recognize the start and end of a frame?
 Character Count
 Flag byte with Byte Stuffing
 Starting and ending flag with bite stuffing
 Encoding Violations
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FRAMING – CHARACTER COUNT
 The first framing method uses a field in the header to specify the number of characters in
the frame.
 When the data link layer at the destination sees the character count, it knows how many
characters follow and hence where the end of the frame is.
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The trouble with this algorithm is that the count can be garbled by a transmission error. 17

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 This technique allows data frames to contain an arbitrary number of bits and allows character codes with an
arbitrary number of bits per character. It works like this. Each frame begins and ends with a special bit
pattern, 01111110 (in fact, a flag byte).
 Whenever the sender's data link layer encounters five consecutive 1s in the
data, it automatically stuffs a 0 bit into the outgoing bit stream.
 This bit stuffing is analogous to byte stuffing, in which an escape byte is stuffed into the outgoing character
stream before a flag byte in the data. When the receiver sees five consecutive incoming 1 bits, followed by a
0 bit, it automatically destuffs (i.e., deletes) the 0 bit
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FRAMING – BIT STUFFING

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BIT STUFFING EXAMPLE
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Bit stuffing and unstuffing
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 Use reserved characters to indicate the start and end of a frame. For instance, use the two-character sequence
DLE STX (Data-Link Escape, Start of TeXt) to signal the beginning of a frame, and the sequence DLE ETX
(End of TeXt) to flag the frame's end.
 The second framing method, Starting and ending character stuffing, gets around the problem of
resynchronization after an error by having each frame start with the ASCII character sequence
DLE STX and end with the sequence DLE ETX.
 Problem: What happens if the two-character sequence DLE ETX
happens to appear in the frame itself?
 Solution: Use character stuffing; within the frame, replace every occurrence of DLE with the two-character
sequence DLE DLE. The receiver reverses the processes, replacing every occurrence of DLE DLE with a
single DLE.
 Example: If the frame contained ``A B DLE D E DLE'', the characters transmitted over the channel would be
``DLE STX A B DLE DLE D E DLE DLE DLE ETX''.
 Disadvantage: character is the smallest unit that can be operated on; not all
architectures are byte oriented.
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FRAMING – BYTE STUFFING
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Byte stuffing and unstuffing
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 This Framing Method is used only in those networks in which Encoding on the Physical Medium
contains some redundancy.
 Some LANs encode each bit of data by using two Physical Bits
i.e. Manchester coding is Used. Here, Bit 1 is encoded into high-
low(10) pair and Bit 0 is encoded into low-high(01) pair.
 The scheme means that every data bit has a transition in the middle, making it easy for the receiver
to locate the bit boundaries. The combinations high-high and low-low are not used for data but are
used for delimiting frames in some protocols.
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PHYSICAL LAYER CODING VIOLATIONS

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ERROR CONTROL
 Error control is concerned with insuring that all frames are eventually delivered (possibly in order) to a
destination. How? Three items are required.
 Acknowledgements: Typically, reliable delivery is achieved using the “acknowledgments with
retransmission" paradigm, whereby the receiver
returns a special acknowledgment (ACK) frame to the sender indicating the correct receipt of a frame.
o In some systems, the receiver also returns a negative acknowledgment (NACK) for incorrectly-
received frames. This is nothing more than a hint to the sender so that it can retransmit a frame right
away without waiting for a timer to expire.
 Timers: One problem that simple ACK/NACK schemes fail to address is recovering from a frame that
is lost, and as a result, fails to solicit an ACK or NACK. What happens if an ACK or NACK becomes
lost?
 Retransmission timers are used to resend frames that don't produce an ACK. When sending a frame,
schedule a timer to expire at some time after the ACK should have been returned. If the timer goes o,
retransmit the frame.
 Sequence Numbers: Retransmissions introduce the possibility of duplicate frames. To suppress
duplicates, add sequence numbers to each frame, so tha1t 9a receiver can distinguish between new
frames and old copies.
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FLOW CONTROL
 Flow control deals with throttling the speed of the sender to match that of the receiver.
 Two Approaches:
 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
 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.
 Various Flow Control schemes uses a common protocol that contains well-defined
rules about when a sender may transmit the next frame. These rules often prohibit frames
from being sent until the receiver has granted permission, either implicitly or explicitly.
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 It is physically impossible for any data recording or transmission medium to be 100%
perfect 100% of the time over its entire expected useful life.
 In data communication, line noise is a fact of life (e.g., signal attenuation, natural
phenomenon such as lightning, and the telephone repairman).
 As more bits are packed onto a square centimeter of disk storage, as communications
transmission speeds increase, the likelihood of error increases-- sometimes geometrically.
 Thus, error detection and correction is critical to accurate data transmission, storage and
retrieval.
 Detecting and correcting errors requires redundancy -- sending additional information
along with the data.
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ERROR CORRECTION AND DETECTION

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TYPES OF ERRORS
 There are two main types of errors in transmissions:
1. Single bit error : It means only one bit of data unit is changed from 1 to 0 or from 0 to 1.
2. Burst error : It means two or more bits in data unit are changed from 1 to 0 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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ERROR DETECTION VS ERROR CORRECTION
 There are two types of attacks against errors:
 Error Detecting Codes: Include enough redundancy bits to detect
errors and use ACKs and retransmissions to recover from the errors.
 Error Correcting Codes: Include enough redundancy to detect and correct errors. The use of
error-correcting codes is often referred to as forward error correction.
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ERROR DETECTION
Error detection means to decide whether the received data is correct or not without having a
copy of the original message.
Error detection uses the concept of redundancy,which means adding extra bits for
detecting errors at the destination.
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VERTICAL REDUNDANCY CHECK (VRC)
 Append a single bit at the end of data block such that the number of ones is even
 Even Parity (odd parity is similar)
0110011  01100110
0110001  01100011
 VRC is also known as Parity Check. Detects all odd-number errors in a data block
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EXAMPLE OF VRC
 The problem with parity is that it can only detect odd numbers of bit substitution errors, i.e. 1 bit,
3bit, 5, bit, etc. errors. If there two, four, six, etc. bits which are transmitted in error, using VRC
will not be able to detect the error.
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LONGITUDINAL REDUNDANCY CHECK (LRC)
 Longitudinal Redundancy Checks (LRC) seek to overcome the weakness of simple, bit-oriented, one-
directional parity checking.
 LRC adds a new character (instead of a bit) called the Block Check Character (BCC) to each block of data.
Its determined like parity, but counted longitudinally through the message (also vertically)
 Its has better performance over VRC as it detects 98% of the burst errors (>10 errors) but less capable of
detecting single errors
 If two bits in one data units are damaged and two bits in exactly the same positions in another data unit are
also damaged, the LRC checker will not detect an error.
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11100111 11011101 00111001 10101001
11100111
11011101
00111001
10101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
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TWO DIMENSIONAL PARITY CHECK
 Upon receipt, each character is checked according to its VRC parity value and then the entire block
of characters is verified using the LRC block check character.
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ANOTHER EXAMPLE OF TWO DIMENSIONAL PARITY CHECK
Consider the following bit stream that has been encoded using VRC, LRC and even parity.
Locate the error if present
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CYCLIC REDUNDANCY CHECK (CRC)
 The cyclic redundancy check, or CRC, is a technique for detecting errors in digital data, but
not for making corrections when errors are detected.
 It is used primarily in data transmission
 In the CRC method, a certain number of check bits, often called a checksum, are appended to
the message being transmitted.
 The receiver can determine whether or not the check bits agree with the data, to ascertain
with a certain degree of probability whether or not an error occurred in transmission
 The CRC is based on polynomial arithmetic, in particular, on computing the remainder of
dividing one polynomial in GF(2) (Galois field with two elements) by another.
 Can be easily implemented with small amount of hardware
 Shift registers
 XOR (for addition and subtraction).
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GENERATOR POLYNOMIAL
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 A cyclic redundancy check (CRC) is a non-secure hash function designed to detect
accidental changes to raw computer data, and is commonly used in digital networks and
storage devices such as hard disk devices.
 CRCs are so called because the check (data verification) code is
a redundancy (it adds zero information) and the algorithm is based on cyclic codes.
 The term CRC may refer to the check code or to the function that calculates it, which
accepts data streams of any length as input but always outputs a fixed- length code
 The divisor in a cyclic code is normally called the generator polynomial or simply the
generator. The proper
1. It should have at least two terms.
2. The coefficient of the term x0 should be 1.
3. It should not divide xt + 1, for t between 2 and n − 1.
4. It should have the factor x + 1.

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CRC CALCULATION
 Given a k-bit frame or message, the transmitter generates an n-bit sequence,
known as a frame check sequence (FCS), so that the resulting frame, consisting of
(k+n) bits, is exactly divisible by some predetermined number.
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CYCLIC REDUNDANCY CHECK
 Let M(x) be the message polynomial
 Let P(x) be the generator polynomial
 P(x) is fixed for a given CRC scheme
 P(x) is known both by sender and receiver
 Create a block polynomial F(x) based on M(x) and P(x) such that F(x) is divisible
by P(x)
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CYCLIC REDUNDANCY CHECK
 Sending
1. Multiply M(x) by xn
2. Divide xnM(x) by P(x)
3. Ignore the quotient and keep the reminder C(x)
4. Form and send F(x) = xnM(x)+C(x)
 Receiving
1. Receive F’(x)
2. Divide F’(x) by P(x)
3. Accept if remainder is 0, reject otherwise
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EXAMPLE OF CRC
 Consider a message 110010 represented by the polynomial M(x) = x5 + x4 + x Consider a generating
polynomial G(x) = x3 + x2 + 1 (1101)
This is used to generate a 3 bit CRC = C(x) to be appended to M(x).
Steps:
1. Multiply M(x) by x3 (highest power in G(x)). i.e. Add 3 zeros. 110010000
2. Divide the result by G(x). The remainder = C(x).
1101 long division into 110010000 (with subtraction mod 2)
= 100100 remainder 100
3. Transmit 110010000 + 100
To be precise, transmit: T(x) = x3M(x) + C(x)
= 110010100
4. Receiver end: Receive T(x). Divide by G(x), should have remainder 0.
Note if G(x) has order n - highest power is xn, then G(x) will cover (n+1) bits
and the remainder will cover n bits. i.e. Add n bits to message. 36 42

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ANOTHER EXAMPLE OF CRC
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CRC DIVISION IN POLYNOMIAL FORM
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CRC STANDARD POLYNOMIALS
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CRC PERFORMANCE
CRC is a very effective error detection technique. If the divisor is chosen according to the previously
mentioned rules, its performance can be summarized as follows:
CRC can detect all single-bit errors
CRC can detect all double-bit errors (three 1’s)
CRC can detect any odd number of errors (X+1)
CRC can detect all burst errors of less than the degree of the polynomial.
CRC detects most of the larger burst errors with a high probability.
For example CRC-12 detects 99.97% of errors with a length 12 or more.
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CHECKSUM
 Checksum is the error detection scheme used in IP, TCP & UDP.
 Here, the data is divided into k segments each of m bits. In the sender’s end the segments are
added using 1’s complement arithmetic to get the sum. The sum is complemented to get the
checksum. The checksum segment is sent along with the data segments
 At the receiver’s end, all received segments are added using 1’s complement arithmetic to get the
sum. The sum is complemented. If the result is zero, the received data is accepted; otherwise
discarded
 The checksum detects all errors involving an odd number of bits. It also detects most errors
involving even number of bits.
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CHECKSUM
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CHECKSUM EXAMPLE
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Q) For a pattern of, 10101001 00111001 00011101 Find out whether any
transmission errors have occurred or not
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EXAMPLE OF CHECKSUM
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CHECKSUM VS CRC
 CRC is more thorough as opposed to Checksum in checking for errors and reporting.
 Checksum is the older of the two programs.
 CRC has a more complex computation as opposed to checksum.
 Checksum mainly detects single-bit changes in data while CRC
can check and detect double-digit errors.
 CRC can detect more errors than checksum due to its more complex function.
 A checksum is mainly employed in data validation when implementing
software.
 A CRC is mainly used for data evaluation in analogue da5t0a transmission.
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ERROR CORRECTION
 Once detected, the errors must be corrected
 Two Techniques for error correction
 Retransmission (aka Backward error correction)
 Simplest, effective and most commonly used technique –
involves correction by retransmission of data by the sender
 Popularly called Automatic Repeat Request (ARQ)
 Forward Error Correction (FEC)
 Receiving device can correct the errors itself
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ERROR CORRECTION
 Messages (frames) consist of m data (message) bits and r redundancy bits, yielding an n = (m+r)-
bit codeword.
 Hamming Distance. Given any two codewords, we can determine how many of the bits differ.
Simply exclusive or (XOR) the two words, and count the number of 1 bits in the result.
 Significance? If two codewords are d bits apart, d errors are required to convert one to the other.
 A code's Hamming Distance is defined as the minimum Hamming Distance between any two of
its legal codewords (from all possible codewords).
 To detect d 1-bit errors requires having a Hamming Distance of at
least d+1 bits.
 To correct d errors requires 2d+1 bits. Intuitively, after d errors, the garbled messages is still closer
to the original message than any other legal codeword.
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HAMMING CODE
 Ex : If The Value of m is 7, the Relation will Satisfy if The Minimum Value of r is 4.
2^4 = 16 > 7+4+1
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If The Number of Data bit is 7, Then The Position of
Redundant bits Are :
2^0=1 2^1=2
2^2=4 2^3=8
HAMMING CODE EXAMPLE
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Let Receiver receives 10010100101
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 Hamming Code Cannot Correct a burst Error Directly.
 it is Possible To Rearrange The Data and Then Apply The code.
Instead of Sending All the bits in The data Unit Together, we can organize N units in a
column.
Send The First bits of Each Followed by The Second bit of each, and so on.
In This Way, if a burst Error of M bit Occurs (M<N), Then The Error does not Corrupt M
bit of Single Unit, it Corrupt Only 1 bit of Unit.
 Then We Can Correct it Using Hamming Code Scheme.
Burst Error Correction
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FUNCTIONS AND REQUIREMENTS OF THE DATA LINK PROTOCOLS
The basic function of the layer is to transmit frames over a physical communication link. Transmission may be half
duplex or full duplex. To ensure that frames are delivered free of errors to the destination station (IMP) a number of
requirements are placed on a data link protocol. The protocol (control mechanism) should be capable of performing:
 The identification of a frame (i.e. recognise the first and last bits of a frame).
 The transmission of frames of any length up to a given maximum. Any bit pattern is permitted in a frame.
 The detection of transmission errors.
 The retransmission of frames which were damaged by errors.
 The assurance that no frames were lost.
 In a multidrop configuration -> Some mechanism must be used for preventing conflicts caused by simultaneous
transmission by many stations.
 The detection of failure or abnormal situations for control and monitoring purposes.
It should be noted that as far as layer 2 is concerned a host message is pure data, every single bit of which is to be delivered to
the other host. The frame header pertains to layer 2 and is never given to the host.
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ELEMENTARY DATA LINK PROTOCOLS
The protocols are normally implemented in software
by using one of the common
programming languages.
• An Unrestricted Simplex Protocol
• A Simplex Stop-and-Wait Protocol
• A Simplex Protocol for a Noisy Channel
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AN UNRESTRICTED SIMPLEX PROTOCOL
 In order to appreciate the step by step development of efficient and
complex protocols we will begin with a simple but unrealistic protocol. In
this protocol: Data are transmitted in one direction only
 The transmitting (Tx) and receiving (Rx) hosts are always ready
 Processing time can be ignored
 Infinite buffer space is available
 No errors occur; i.e. no damaged frames and no lost frames (perfect
channel)
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A SIMPLEX STOP-AND-WAIT PROTOCOL
 In this protocol we assume that Data are transmitted in one direction
only
 No errors occur (perfect channel)
 The receiver can only process the received information at a finite rate
 These assumptions imply that the transmitter cannot send frames at a
rate faster than the receiver can process them.
The problem here is how to prevent the sender from flooding the receiver.
 A general solution to this problem is to have the receiver provide some
sort of feedback to the sender. The process could be as follows: The
receiver send an acknowledge frame back to the sender telling the
sender that the last received frame has been processed and passed to
the host; permission to send the next frame is granted. The sender, after
having sent a frame, must wait for the acknowledge frame from the
receiver before sending another frame.
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STOP & WAIT PROTOCOL
The sender sends one frame and waits for feedback from the
receiver. When the ACK arrives, the sender sends the next
frame
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 In this protocol the unreal "error free" assumption in protocol 2 is dropped.
 Frames may be either damaged or lost completely.
 We assume that transmission errors in the frame are detected by the hardware checksum.
 One suggestion is that the sender would send a frame, the receiver would send an ACK frame only if the
frame is received correctly.
 If the frame is in error the receiver simply ignores it; the transmitter would time out and would retransmit it.
 One fatal flaw with the above scheme is that if the ACK frame is lost or damaged, duplicate frames are
accepted at the receiver without the receiver knowing it.
A SIMPLEX PROTOCOL FOR A NOISY CHANNEL
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66
 Imagine a situation where the receiver has just sent an ACK frame back to the sender saying that it correctly received
and already passed a frame to its host. However, the ACK frame gets lost completely, the sender times out and
retransmits the frame.
 There is no way for the receiver to tell whether this frame is a retransmitted frame or a new frame, so the receiver
accepts this duplicate happily and transfers it to the host.
 The protocol thus fails in this aspect.
STOP-AND-WAIT, LOST FRAME STOP-AND-WAIT, LOST ACK FRAME
72

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 To overcome this problem it is required that the receiver be able to distinguish a frame that it is
seeing for the first time from a retransmission.
 One way to achieve this is to have the sender put a sequence number in the header of each frame
it sends.
 The receiver then can check the sequence number of each arriving frame to see if it is a new
frame or a duplicate to be discarded.
 The receiver needs to distinguish only 2 possibilities: a new frame or a duplicate; a 1-bit sequence
number is sufficient.
 At any instant the receiver expects a particular sequence number.
 Any wrong sequence numbered frame arriving at the receiver is rejected as a duplicate.
 A correctly numbered frame arriving at the receiver is accepted, passed to the host, and the
expected sequence number is incremented by 1 (modulo 2). 67 73

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FLOW DIAGRAM OF A STOP & WAIT PROTOCOL
68 74

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 After transmitting a frame and starting the timer, the sender waits for something exciting to
happen.
 Only three possibilities exist: an acknowledgement frame arrives undamaged, a damaged acknowledgement
frame staggers in, or the timer expires.
 If a valid acknowledgement comes in, the sender fetches the next packet from its network layer and
puts it in the buffer, overwriting the previous packet.
 It also advances the sequence number.
 If a damaged frame arrives or no frame at all arrives, neither the buffer nor the sequence number is
changed so that a duplicate can be sent.
 When a valid frame arrives at the receiver, its sequence number is checked to see if it is a duplicate.
 If not, it is accepted, passed to the network layer, and an acknowledgement is generated.
 Duplicates and damaged frames are not passed to the network layer.
69 75

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SLIDING WINDOW PROTOCOLS
76

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DATA FRAME TRANSMISSION
77
 Unidirectional
assumption protocols
 Not general
in previous elementary
 Full-duplex - approach 1
 Two separate communication channels(physical circuits)
Forward channel for data
Reverse channel for acknowledgement
 Problems: 1. reverse channel bandwidth wasted
2. cost

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72
 Full-duplex - approach 2
 Same circuit for both directions
 Data and acknowledgement
are intermixed
 How do we tell acknowledgement
from data?
"kind" field telling data or
acknowledgement
 Can it be improved?
 Approach 3
 Attaching acknowledgement to
outgoing data frames

DATA FRAME TRANSMISSION
78

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PIGGYBACKING
73
 Temporarily delaying transmission of outgoing acknowledgement so that they
can be hooked onto the next outgoing data frame
 Advantage: higher channel bandwidth utilization
 Complication:
 How long to wait for a packet to piggyback?
 If longer than sender timeout period then sender retransmits
 Purpose of acknowledgement is lost
 Solution for timing complexion
 If a new packet arrives quickly
 Piggybacking
 If no new packet arrives after a receiver ack timeout
 Sending a separate acknowledgement frame 79

12/10/2025
SLIDING WINDOW PROTOCOLS
 The next three protocols are bidirectional protocols that belong to a class called sliding window
protocols. (max sending window size, receiving window size)
 One-bit sliding window protocol (1, 1)
 Go back N (>1, 1)
 Selective repeat (>1, >1)
 The three differ among themselves in terms of complexity, efficiency,
and buffer requirements.
74 80

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SENDING & RECEIVING WINDOWS
 At any instance of time
 Sender maintains a set of sequence numbers of frames permitted to send
 These frames fall within sending window
 Receiver maintains a set of sequence numbers of frames permitted to accept
 These frames fall within receiving window
 Lower limit, upper limit, and size of two windows need not be the
same - Fixed or variable size
 Senders Window contains frames can be sent or have been sent but not yet acknowledged –
outstanding frames
 When a packet arrives from network layer
 Next highest sequence number assigned
 Upper edge of window advanced by 1
 When an acknowledgement arrives
 Lower edge of window advanced by 1 75 81

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 If the maximum window size is n, the sender needs n buffers to hold the unacknowledged
frames.
 If the window ever grows to its maximum size, the sending data link layer must forcibly
shut off the network layer until another buffer becomes free.
 The receiving data link layer's window corresponds to the frames it may accept. Any
frame falling outside the window is discarded without comment.
 When a frame whose sequence number is equal to the lower edge of the window is
received, it is passed to the network layer, an acknowledgement is generated, and the
window is rotated by one
 Unlike the sender's window, the receiver's window always remains at its initial size.7
82

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SENDER SLIDING WINDOW
77 83
• At the sending site, to hold the outstanding
frames until they are acknowledged, we use
the concept of a window.
• The size of the window is at most 2m -1 where
m is the number of bits for the sequence
number.
• Size of the window can be variable, e.g. TCP.
• The window slides to include new
unsent frames when the correct ACKs are
received

12/10/2025
RECEIVER SLIDING WINDOW
78 84
• Receiver is always looking for a specific frame
to arrive in a specific order.
• Any frame arriving out of order is discarded
and needs to be resent.
• Receiver window slides as
shown in fig.
• Receiver is waiting for frame 0 in
part a.

12/10/2025
CONTROL VARIABLES
79 85
• Sender has 3 variables: S, SF, and SL
• S holds the sequence number of recently sent frame
• SF holds the sequence number of the first frame
• SL holds the sequence number of the last frame
• Receiver only has the one variable, R, that holds the sequence number of the frame it expects to receive.
If the seq. no. is the same as the value of R, the frame is accepted, otherwise rejected.

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80 86

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A ONE BIT SLIDING WINDOW PROTOCOL
81
A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received. 87

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82
(a) Case 1: Normal case. (b) Case 7: Abnormal case.
The notation is (seq, ack, packet number). An asterisk indicates
where a network layer accepts a packet.
A ONE BIT SLIDING WINDOW PROTOCOL
88

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83
ONE BIT SLIDING WINDOW PROTOCOL
 Case 1: no error
A B
Time (0,1,A0)
(0,0,B0)
Case 2: data
lost A
Time (0,1,A0)
X
Timeout
(1,0,A1)
(1,1,B1)
(0,1,A2)
(0,0,B2)
(0,1,A0)
(0,0,B0)
*
*
*
*
*
*
*
*
Exp=0
Exp=1
Exp=0
Exp=1
Exp=0
Exp=1
Exp=0
Exp=0
Exp=1
B
Exp=0
Exp=1
89

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84
Case 3: data error  Case 4: ack.
lost
Timeout
A
Time
Error
Timeout
(0,1,A0)
(0,1,A0)
(0,0,B0)
(0,1,A0)
(0,1,A0)
(0,0,B0)
(0,0,B0)
X
duplicate,
discarded
*
*
* *
B
Exp=0
Exp=1
Exp=0
B
A
Exp=0 Time
Exp=0
Exp=1
Exp=1 Exp=1
90

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85
Case 6:
outgoing
frame timeout
A
Time
Timeout
Case 5: early
timeout
A
Time
Timeout
(0,1,A0)
(0,1,A0)
(0,0,B0)
(0,1,A0)
(1,1,A1)
(0,1,B0)
duplicate,
discarded
(1,1,B1)
ACK 0
Exp=0
B
Exp=0
Exp=0
B
Exp=0
Exp=0
*
Exp=
*
1
Exp=1
*
Exp=1
*
Exp=1 (1,0,A1)
*
Exp=0
*
Exp=0
*
91

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86
PERFORMANCE OF STOP-AND-WAIT PROTOCOL
 Assumption of previous
protocols:
 Transmission time is negligible
 False, when transmission time is long
 Example - satellite communication
 channel capacity: 50 kbps, frame size:
1kb round-trip propagation delay: 500
msec
 Time: t=0
t=20 msec
t=270 msec
t=520
msec
start to send 1st bit in
frame frame sent
completely frame arrives
best case of ack. Received
 Sender blocked 500/520 = 96% of time
 Bandwidth utilization 20/520 = 4%
t
0
20
270
520
Conclusion:
Long transit time + high bandwidth + short frame length  92

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87
• In stop-and-wait, at any point in time, there is only one frame
that is sent and waiting to be acknowledged.
• This is not a good use of transmission medium.
• To improve efficiency, multiple frames should be in transition
while waiting for ACK.
 Solution:
 Allowing w frames sent before blocking
 Problem: errors
 Solutions


Acknowledge n means frames
n, n-1, acknowledged (i.e.,
received correctly)
n-2,… are
Performance of Stop-and-Wait
Protocol
93

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GO BACK N PROTOCOL
 Improves efficiency of Stop and Wait by not waiting
 Keep Channel busy by continuing to send frames
 Allow a window of upto Ws outstanding frames
 Use m-bit sequence numbering
 Receiver discards all subsequent frames following an error one, and send no
acknowledgement for those discarded
 Receiving window size = 1 (i.e., frames must be accepted in the order they were
sent)
 Sending window might get full
If so, re-transmitting unacknowledged frames
 Wasting a lot of bandwidth if error rate is high
94

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89
GO BACK N PROTOCOL
Frames 0 and 1 are correctly received and acknowledged. Frame 2, however, is damaged or
lost. The sender, unaware of this problem, continues to send frames until the timer for frame
2 expires. Then it backs up to frame 2 and starts all over with it, sending 2, 3, 4, etc. all over
again.
95

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GO-BACK-N ARQ WITH WINDOW=4
96

GO-BACK-N ARQ, SENDER WINDOW SIZE
• Size of the sender window must be less than 2 m. Size of the
receiver is always 1. If m = 2, window size = 2 m – 1 = 3.
• Fig compares a window size of 3 and 4.
Accepts as
the 1st
frame in
the next
cycle-an
error
97

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SELECT REPEAT PROTOCOL
 Receiver stores correct frames following the bad one
 Sender retransmits the bad one after noticing
 Receiver passes data to network layer and acknowledge with the highest number
 Receiving window > 1 (i.e., any frame within the window may be accepted and buffered
until all the preceding one passed to the network layer. Might need large memory
 ACK for frame n implicitly acknowledges all frames ≤ n
 SRP is often combined with NAK
 When error is suspected by receiver, receiver request retransmission of a
frame
 Arrival of a damaged frame
 Arrival of a frame other than the expected
 NAKs stimulate retransmission before the corresponding timer 92
expires and thus improve performance.
98

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SELECTIVE REPEAT WITH NAK
99

SELECTIVE REPEAT ARQ, SENDER AND RECEIVER WINDOWS.
• Go-Back-N ARQ simplifies the process at the receiver site. Receiver only keeps
track of only one variable, and there is no need to buffer out-of-order frames,
they are simply discarded.
• However, Go-Back-N ARQ protocol is inefficient for noisy link. It bandwidth
inefficient and slows down the transmission.
• In Selective Repeat ARQ, only the damaged frame is resent. More bandwidth
efficient but more complex processing at receiver.
• It defines a negative ACK (NAK) to report the sequence number of a damaged
frame before the timer expires.
100

SELECTIVE REPEAT ARQ, LOST FRAME
• Frames 0 and 1
are accepted when
received because
they are in the
range specified by
the
receiver window.
Same for frame 3.
• Receiver sends a
NAK2 to show
that frame 2 has
not been received
resends
and then sender
only
frame 2 and it is
accepted as it is in
the range of the
window.
101

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SELECTIVE REPEAT DILEMMA
97
 Example:
 seq #’s: 0, 1, 2, 3
 window size=3
 receiver sees no difference
in two scenarios!
 incorrectly passes duplicate data as
new in (a)
 Q: what relationship between seq #
size and window size?
102

SELECTIVE REPEAT ARQ, SENDER WINDOW SIZE
• Size of the sender and receiver windows must be at most one-half of 2 m.
• If m = 2, window size should be 2 m /2 = 2. Fig compares a window size
of 2 with a window size of 3. Window size is 3 and all ACKs are lost, sender
sends duplicate of frame 0, window of the receiver expect to receive frame 0
(part of the window), so accepts frame 0, as the 1st frame of the next cycle –
an error.
103

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SELECT REPEAT PROTOCOL - WINDOW SIZE
 Problem is caused by new and old windows overlapped
 Solution
 Window size=(MAX_SEQ+1)/2
 E.g., if 4-bit window is used, MAX_SEQ = 15
 window size = (15+1)/2 = 8
 Number of buffers needed
= window size
104

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00
1
SELECT REPEAT PROTOCOL
(a) Initial situation with a window size seven.
(b) After seven frames sent and received, but not
acknowledged.
(c) Initial situation with a window size of four.
(d) After four frames sent and received, but not
acknowledged.
105

R22 regulat Computer Networks UNIT 2.ppt

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R22 regulat Computer Networks UNIT 2.ppt