Lecture Notes Unit III The DataLink Layer

Unit III - Data Link Layer
Computer Network
Dr. S.Murugan
Associate Professor, Department of Computer Science.
Alagappa Government Arts College,
(Affiliated by Alagappa University)
Karaikudi.
Email: muruganjit@gmail.com
1

This slides compiled from
Computer Network by
Andrew S. Tenenbaum 4th Ed.

Ch.3 The Data Link Layer
➢ This study deals with the algorithms for
achieving reliable, efficient communication
between two adjacent machines at the data link
layer.
➢ The essential property of a channel that makes
it ''wirelike'' is that the bits are delivered in
exactly the same order in which they are sent

3.1 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.

Data Link Layer Design Issues
➢ 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, as
illustrated in Fig. 3-1.

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 as shown in
Fig. 3-2(a).
➢ The actual transmission follows the path of Fig. 3-2(b).

➢ The data link layer can be designed to offer various
services.
➢ Three reasonable possibilities that are commonly
provided are
1. Unacknowledged connectionless service.
2. Acknowledged connectionless service.
3. Acknowledged connection-oriented service.
➢ The source machine send independent frames to the
destination machine without having the destination
machine acknowledgement.

➢ The acknowledged connectionless 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.

➢ 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.
➢ 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.

Framing
➢ To provide service to the network layer, the data link
layer must use the service provided to it by the
physical layer.
➢ The data link layer to break the bit stream up into
discrete frames and compute the checksum for each
frame.
➢ 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.

Framing
➢ The following four methods used to compute
checksum error.
1. Character count.
2. Flag bytes with byte stuffing.
3. Starting and ending flags, with bit stuffing.
4. Physical layer coding violations.

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.
➢ This technique is shown in Fig. 3-4(a) for four frames
of sizes 5, 5, 8, and 8 characters, respectively.

Framing - Character count
➢ In Fig 3-4 (b) character count of 5 in the second frame
becomes a 7. It is considered as an error.
➢ The destination will get out of synchronization and will be
unable to locate the start of the next frame.

Framing - Byte stuffing or character stuffing
➢ The second framing method, each frame contains start and
end with a flag byte as shown in Fig. 3-5(a).

➢ A serious problem occurs with this method when
binary data, such as object programs or floating-
point numbers, are being transmitted.
➢ To solve this problem, the sender's data link layer
insert a special escape byte (ESC) just before each
''accidental'' flag byte in the data.
➢ The data link layer on the receiving end removes
the escape byte before the data are given to the
network layer.
➢ This technique is called byte stuffing or character
stuffing. Some examples are shown in Fig. 3-5(b).

Framing - Bit stuffing
➢ 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.
➢ When the receiver sees five consecutive incoming 1
bits, followed by a 0 bit, it automatically destuffs
(i.e.,deletes) the 0 bit.
➢ This techniques is called bit stuffing.
➢ For example, If the user data contain the flag
pattern, 01111110, this flag is transmitted as
011111010 but stored in the receiver's memory as
01111110. Figure 3-6 gives an example of bit
stuffing.

Framing - Physical layer coding violations
➢ The last method of framing is only applicable to
networks in which the encoding on the physical
medium contains some redundancy.
➢ For example, some LANs encode 1 bit of data by
using 2 physical bits.
➢ Normally, a 1 bit is a high-low pair and a 0 bit is a
low-high pair.
➢ The combinations high-high and low-low are not
used for data, but are used for delimiting frames in
some protocols.

Error Control
➢ Error control determines how to make sure all
frames are eventually delivered to the network layer
at the destination and in the proper order.
➢ In connection oriented service, 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.

Error Control
➢ When the sender transmits a frame, it generally also
starts a timer.
➢ If either the frame or the acknowledgement is lost,
the timer will go off, alerting the sender to a
potential problem. The solution is to just transmit
the frame again.
➢ When frames may be transmitted multiple times
there is a danger that the receiver will accept the
same frame two or more times and pass it to the
network layer more than once.
➢ To prevent this from happening, it is generally
necessary to assign sequence numbers to outgoing
frames, so that the receiver can distinguish
retransmissions from originals.

Flow Control
➢ Another important design issue in the data link
layer is, 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.
➢ Two approaches are commonly used to solve the
above issue.

Flow Control
➢ In the first one, feedback-based flow control, the
receiver sends back information to the sender
giving it permission to send more data.
➢ In the second one, rate-based flow control, the
protocol has a automatic built-in mechanism that
limits the rate at which senders may transmit data,
without using feedback from the receiver.

3.2 Error Detection and Correction
➢ Error is a condition when the output information
does not match with the input information.
➢ During transmission, digital signals suffer from
noise that can introduce errors in the binary bits
travelling from one system to another.
➢ That means a 0 bit may change to 1 or a 1 bit may
change to 0.

3.2.1 Error Detecting Codes - Simple Parity check
➢ Whenever a message is transmitted, it may get
scrambled by noise or data may get corrupted.
➢ To avoid this, we use error-detecting codes which are
additional data added to a given digital message to
help us detect if any error has occurred during
transmission of the message.

➢ Blocks of data from the source are subject to a check
bit or parity bit generator form, where a parity of :
➢ 1 is added to the block if it contains odd number of
1’s, and
➢ 0 is added if it contains even number of 1’s

➢ It is a simple error-checking method used in data communication. Parity
checking helps detect errors in data transmission. If the parity bit doesn't
match the expected value, it indicates an error in the received data.
1.Sender Side:
1. The sender has a binary data sequence: 100011.
2. It computes a parity bit to ensure even parity (total number of 1s is
even).
3. Since there are three 1s (odd count), an extra 1 is added to make it
even.
4. The final transmitted data: 1000111.
2.Transmission:
1. The data is sent through the communication channel.
3.Receiver Side:
1. The receiver receives 1000111.
2. It also computes the parity to check if the number of 1s is even.
3. If even → Data is accepted.
4. If odd → Data is rejected (error detected).

3.2.2 Error-Detecting Codes (Polynomial or Cyclic
Redundancy Check)
➢ In CRC (, a sequence of redundant bits, called cyclic
redundancy check bits, are appended to the end of
data unit so that the resulting data unit becomes
exactly divisible by a second predetermined binary
number.
➢ At the destination, the incoming data unit is divided
by the same number. If at this step there is no
remainder, the data unit is assumed to be correct and
is therefore accepted.
➢ A remainder indicates that the data unit has been
damaged in transmit and therefore must be rejected.

Redundancy Check)

Redundancy Check)
CRC Workflow (Sender and Receiver)
This image outlines the process of CRC error detection through a flowchart:
Sender Side:
1. Data Preparation:
o A data message of m bits is taken.
o n-1 zeros are appended to the message (where n is the length of the CRC
generator polynomial).
2. Division:
o The modified message (with zeros) is divided by the CRC generator (divisor),
typically a fixed binary polynomial.
o This division is performed using binary modulo-2 division (similar to XOR).
3. CRC (Remainder):
o The result of the division is the remainder (n bits), referred to as the CRC.
o This CRC is appended to the original message and transmitted.

Redundancy Check)
Receiver Side:
1. Verification:
o Upon receiving the data, the receiver splits it into the original message
and the appended CRC.
o The entire received sequence is divided again by the same divisor.
2. Check for Errors:
o If the remainder (result of division) is zero, the data is considered error-
free and accepted.
If the remainder is non-zero, the data is rejected due to errors during
transmission.

3.2.2 Error-Detecting Codes (Polynomial or Cyclic Redundancy Check)
This example demonstrates the process in numbers with the following:
Given Inputs:
1. Original Message: 1010000
2. Generator Polynomial: x3+1 => 1*x3 + 0* x2*+ 0*x1x + 1 (binary: 1001)
Sender's Process:
1. Append Zeros:
o Append n-1 zeros (3 zeros for a 4-bit generator) to the original message:
1010000 → 1010000000.
2. Binary Division:
o Perform modulo-2 division of 1010000000 by 1001.
o XOR is used instead of subtraction:
▪ Step-by-step division shown in the image.
o The remainder is 011.
3. Transmit Data:
o Append the remainder (011) to the original message: 1010000 + 011 →
1010000011.

3.2.2 Error-Detecting Codes (Polynomial or Cyclic Redundancy Check)
Receiver's Process:
1. Received Data:
o Receiver gets 1010000011.
2. Verification:
o Perform modulo-2 division of 1010000011 by 1001.
o The division results in a remainder of 000.
3. Conclusion:
o A remainder of zero indicates the data is error-free and accepted.
Summary:
• CRC ensures data integrity by appending a calculated remainder (CRC) to the
transmitted data.
• At the receiver, division is repeated to verify correctness.
• Advantages:
o Efficient error detection for large datasets.
o Simple to implement with hardware or software.
• Limitation: Cannot correct errors; it only detects them.

Hamming Code
➢ Hamming code is a set of error-correction code s
that can be used to detect and correct bit errors that
can occur when computer data is moved or stored.
➢ To enable this, a transmitting station must add extra
data (called error correction bits ) to the
transmission.

3.3 Elementary Data Link Protocols
➢ Assume the following…
➢ The physical layer, data link layer, and network layer
are independent processes that communicate by
passing messages back and forth.
➢ Machine A wants to send a long stream of data to
Machine B, using a reliable, connection-oriented
service.
➢ B also wants to send data to A Simultaneously.
➢ Machines do not crash.

Elementary Data Link Protocols

➢ Figure 3-9 shows some declarations (in C) common to
many of the protocols.
➢ Five data structures are defined: boolean, seq_nr,
packet, frame_kind, and frame.
➢ A boolean is an enumerated type and can take on the
values true and false.
➢ A seq_nr is a small integer used to number the frames.
These sequence numbers run from 0 up to and
including MAX_SEQ.

➢ A packet is the unit of information exchanged
between the network layer and the data link layer.
➢ A frame is composed of four fields: kind, seq, ack,
and info, the first three of which contain control
information and the last of which may contain actual
data to be transferred. These control fields are
collectively called the frame header.
➢ The kind field tells whether there are any data in the
frame. The seq and ack fields are used for sequence
numbers and acknowledgements, respectively. The
info field of a data frame contains a single packet.

➢ A number of procedures are listed in Fig. 3-9.
➢ The procedure wait_for_event waiting for something
to happen.
➢ The procedures from_physical_layer and
to_physical_layer pass frames between the data link
layer and physical layer.

➢ The procedures to_network_layer and
from_network_layer pass packets between the data
link layer and network layer.
➢ The procedures start_ack_timer and stop_ack_timer
control an auxiliary timer used to generate
acknowledgements under certain conditions.
➢ The procedures enable_network_layer and
disable_network_layer are used in the network layer
always has packets to send.
➢ Frame sequence numbers are always in the range 0 to
MAX_SEQ.

3.3.1 An Unrestricted Simplex Protocol
➢ A protocol that is as simple as it can be. Data are
transmitted in one direction only.
➢ Both the transmitting and receiving network layers
are always ready.
➢ Processing time can be ignored. Infinite buffer space
is available.
➢ The communication channel between the data link
layers never damages or loses frames.
➢ This thoroughly unrealistic protocol, which we will
nickname ''utopia,'' is shown in Fig. 3-10.

➢ The protocol consists of two distinct procedures, a
sender and a receiver.
➢ The sender runs in the data link layer of the source
machine, and the receiver runs in the data link layer of
the destination machine.
➢ The sender is in an infinite while loop just pumping
data out onto the line as fast as it can.
➢ The body of the loop consists of three actions: go fetch
a packet from the (always obliging) network layer,
construct an outbound frame using the variable s, and
send the frame on its way.

➢ The receiver is equally simple. Initially, it waits for
something to happen, the only possibility being the
arrival of an undamaged frame.
➢ The call to from_physical_layer removes the newly
arrived frame from the hardware buffer and puts it in
the variable r, where the receiver code can get at it.
➢ Finally, the data portion is passed on to the network
layer.

3.3.2 A Simplex Stop-and-Wait Protocol
➢ The sender sends one frame and then waits for an
acknowledgement before proceeding the next frame is
called stop-and-wait. Figure 3-11 gives an example of
a simplex stop-and-wait protocol.
Figure 3-11. A simplex stop-and-wait protocol.

3.3.2 A Simplex Stop-and-Wait Protocol

3.3.3 A Simplex Protocol for a Noisy Channel
➢ The simplex protocol, follow an existing stop and
wait protocol mechanism with a timer.

➢ The sender could send a frame, but the receiver
would only send an acknowledgement frame if the
data were correctly received.
➢ If a damaged frame arrived at the receiver, it would
be discarded.
➢ After a while the sender would time out and send
the frame again.
➢ This process would be repeated until the frame
finally arrived intact.

➢ During the data transmission, the following
situation may occur.
Situation 1:
➢ The network layer on A gives packet 1 to its data
link layer.
➢ The packet is correctly received at B and passed to
the network layer on B.
➢ B sends an acknowledgement frame back to A. It
means there is no error in data transmission.

Situation 2:
➢ The acknowledgement frame gets lost completely.
➢ This problem solved by introducing sequence
number for data.

Situation 3:
➢ The data link layer on A eventually times out. i.e.,
The data is lost.
➢ The data should be sent again.

Situation 4:
➢ The duplicate frame also arrives at the data link
layer on B.
➢ This problem solved by introducing sequence
number for Acknowledgement also.

3.4 Sliding Window Protocols
➢ Less efficient in stop and wait protocol. Stop and
wait protocol send data (one frame) only one
direction at a time. Sender to receiver for data
(Forward) and receiver to sender (backward) for
acknowledgement.
➢ To overcome this issue, here introducing sliding
window protocol for sending data (several frames)
in both directions at a time.
➢ In stop and wait protocol the sender sends the next
packet after receiving the acknowledgement. In
sliding window protocol the sender sends the data
before receiving acknowledgement.

➢ When a data frame arrives, instead of immediately
sending a separate control frame, the receiver
restrains itself and waits until the network layer
passes it the next packet.
➢ The acknowledgement is attached to the outgoing
data frame (using the ack field in the frame header).
This technique known as piggybacking.
➢ Sliding window protocol primarily used for error
control protocol. There are two types of sliding
window protocol used one is GO Back N and
another one is selective repeat protocol.

➢ Sliding window protocol mainly used for, how the
data is transmitted.
➢ It follows two parameters.
➢ One is the sequence number and another one is a
sliding window.
➢ The sequence number ranges from 0 to 2n – 1
frames.
➢ The Sliding window is a buffer which holds the
temporary data.

3.4.1 One Bit Sliding Window Protocols
➢ The sequence numbers within the sender's window
represent frames that have been sent or can be sent
but are as yet not acknowledged.
➢ Whenever a new packet arrives from the network
layer, it is given the next highest sequence number,
and the upper edge of the window is advanced by
one (Fig 3.13 (b) of the sender ).
➢ When an acknowledgement comes in, the lower
edge is advanced by one (Fig 3.13 (d) of the
sender). In this way the window continuously
maintains a list of unacknowledged frames. Figure
3-13 shows an example.

3.4.1 One Bit Sliding Window Protocols

Lecture Notes Unit III The DataLink Layer

➢ N represents the number of frames to be sent.
➢ In GO BACK N mechanism, several frames are
sent before receiving the acknowledgement.
➢ Here N should be greater than one. If N=1 means
simple stop and wait protocol.
3.4.2 A Protocol Using Go Back N

➢ If the data or acknowledgement is lost, then the
action is performed by the sender.
➢ For example, the sender sends the several frames
at a time.
➢ During the transmission the data2 may be damaged.
➢ Whenever the data is damaged or lost the sender
simply sends the data continuously.
➢ But the receiver discards the data after the damage
and the receiver sends the Negative
acknowledgement (NAK) to the sender.
➢ Now the sender sends the data from data2 onwards.

➢ In Go Back N mechanism, It retransmits all the
frame after the frame was damaged.
➢ In the selective repeat protocol, it retransmits only
that frame which is damaged or lost.
➢ The receiver is capable of sorting the frames in a
proper sequence. The sender must be the capable of
searching the frame for which the negative
acknowledgement has been received.
➢ It requires less window size as compared to GO
Back N protocol.
3.4.3 A Protocol Using Selective Repeat

➢ If the data or acknowledgement is lost, then the
action is performed by the sender.
➢ For example, the sender sends the several frames at
a time.
➢ During the transmission, the data2 may be
damaged.
➢ Whenever the data is damaged or lost the sender
simply sends the data continuously.
➢ The receiver discard the damaged data data2 and
sends the negative acknowledgement to the sender.
➢ The sender checks the buffer and resend the
damaged data data2 only.

Lecture Notes Unit III The DataLink Layer

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