Data link layer

DATA LINK LAYER
MUKESH CHINTA
Asst Prof, CSE
VRSEC

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.

 Services Provided to the Network Layer
 The network layer wants to be able to send packets to its neighbors
without worrying about the details of getting it there in one piece.
 Framing
 Group the physical layer bit stream into units called frames. Frames are
nothing more than "packets" or "messages". By convention, we use the
term "frames" when discussing DLL.
 Error Control
 Sender checksums the frame and transmits checksum together with
data. Receiver re-computes the checksum and compares it with the
received value.
 Flow Control
 Prevent a fast sender from overwhelming a slower receiver.
DLL DESIGN ISSUES

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
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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

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.
 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
 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.
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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.
 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
 Here, 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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PLACEMENT OF DATA LINK PROTOCOL
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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.

 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

 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
FRAMING – BIT STUFFING
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Byte stuffing and unstuffing

PHYSICAL LAYER CODING VIOLATIONS
 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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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.
 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 that a
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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ERROR CORRECTION AND DETECTION
 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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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
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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.

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

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
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Consider the following bit stream that has been encoded using
VRC, LRC and even parity. Locate the error if present

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) 31

GENERATOR POLYNOMIAL
 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. 32

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)
)(
0
)(
)(
)(
xP
xQ
xP
xF


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

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

CRC DIVISION IN POLYNOMIAL FORM
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CRC STANDARD POLYNOMIALS
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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.
CRC PERFORMANCE

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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46
Q) For a pattern of, 10101001 00111001 00011101 Find out whether any
transmission errors have occurred or not

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 data
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.

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

Let Receiver receives 10010100101

 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

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.
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• An Unrestricted Simplex Protocol
• A Simplex Stop-and-Wait Protocol
• A Simplex Protocol for a Noisy Channel

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
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The sender sends one frame and waits for feedback from the
receiver. When the ACK arrives, the sender sends the next
frame

 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.
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A SIMPLEX PROTOCOL FOR A NOISY CHANNEL

 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.
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STOP-AND-WAIT, LOST ACK FRAMESTOP-AND-WAIT, LOST FRAME

 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).
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FLOW DIAGRAM OF A STOP & WAIT PROTOCOL
68

 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.
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DATA FRAME TRANSMISSION
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 Unidirectional assumption in previous elementary
protocols
 Not general
 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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 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

PIGGYBACKING
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 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

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 efficiency,
complexity, and buffer requirements.
 Each outbound frame contains an n-bit sequence number
 Range: 0 - MAX_SEQ (MAX_SEQ = 2n - 1)
 For stop-and-wait, n = 1 restricting the sequence numbers to 0
and 1 only
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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
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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.
76

SENDER SLIDING WINDOW
77
• 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

RECEIVER SLIDING WINDOW
78
• Size of the window at the
receiving site is always 1
in this protocol.
• 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.

CONTROL VARIABLES
79
• 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.

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.

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

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

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

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

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 start to send 1st bit in frame
t=20 msec frame sent completely
t=270 msec frame arrives
t=520 msec 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 

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, n-2,… are
acknowledged (i.e., received correctly)
Performance of Stop-and-Wait Protocol

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

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.

GO-BACK-N ARQ WITH WINDOW=4
90

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

92
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
expires and thus improve performance.

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.

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
and then sender
resends only
frame 2 and it is
accepted as it is in
the range of the
window.

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?

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.

99
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

100
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.

101
ACKNOWLEDGEMENT TIMER
 Problem
 If the reverse traffic is light, effect?
 If there is no reverse traffic, effect?
 Solution
 Acknowledgement timer:
If no reverse traffic before timeout
send separate acknowledgement
 Essential: ack timeout < data frame timeout Why?

Data link layer

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Data link layer