The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
OUTLINE
1 INTRODUCTION
2 DATA LINK LAYER DESIGN ISSUES
3 ERROR DETECTION AND CORRECTION
4 ELEMENTARY DATA LINK PROTOCOLS
5 SLIDING WINDOW PROTOCOLS
6 EXAMPLE DATA LINK PROTOCOLS
7 CONCLUSIONS

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
INTRODUCTION I
The goal is to study the design principles for the data
link layer.
To study the algorithms for achieving reliable, efficient
communication of whole units of information.
To understand communication with frames as opposed
to bits.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
DATA LINK LAYER DESIGN ISSUES I
The data link layer uses the services of the physical layer to
send and receive bits over communication channels. It has
a number of functions, including:
1 Providing a well-defined service interface to the
network layer.
2 Dealing with transmission errors.
3 Regulating the flow of data so that slow receivers are
not swamped by fast senders.
To accomplish these goals, the data link layer takes the
packets it gets from the network layer and encapsulates
them into frames for transmission.
Each frame contains a frame header.
And a payload field for holding the packet.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
2. Dealing with transmission errors.
3. Regulating the flow of data so that slow receivers are not swamped
DATA LINK LAYER DESIGN ISSUES II
by fast senders.
To accomplish these goals, the data link layer takes the packets it gets from the
network layer And and a frame encapsulates trailer.
them into frames for transmission. Each frame
contains a frame header, a payload field for holding the packet, and a frame
trailer, as Frame illustrated management in Fig. 3-1. Frame forms management the heart forms of the the functionality
heart of what the
data link of layer the does. data In link the layer.
following sections we will examine all the above-mentioned
issues in detail.
Header Payload field Trailer
Frame
Sending machine
Receiving machine
Packet Packet
Header Payload field Trailer
Figure 3-1. Relationship between packets and frames.
FIGURE: Relationship between packets and frames.
Although this chapter is explicitly about the data link layer and its protocols,
many of the principles we will study here, such as error control and flow control,
are found in transport and other protocols as well. That is because reliability is an

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
SERVICES PROVIDED TO THE NETWORK LAYER I
The function of the data link layer is to provide services
to the network layer.
The rincipal service is transfering data from the network
layer on the source machine to the network layer on the
destination machine.
On the source machine is an entity, call it a process or
a program, in the network layer that hands some bits to
the data link layer for transmission to the destination.
The job of the data link layer is to transmit the bits to
the destination machine so they can be handed over to
the network layer (of the destination machine).

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
transmit the bits to the destination machine so they can be handed over to the net-work
layer there, as shown in Fig. 3-2(a). The actual transmission follows the
SERVICES PROVIDED TO THE NETWORK LAYER
II
path of Fig. 3-2(b), but it is easier to think in terms of two data link layer proc-esses
communicating using a data link protocol. For this reason, we will impli-citly
use the model of Fig. 3-2(a) throughout this chapter.
4
3
2
1
4
3
2
1
4
3
2
1
4
3
2
1
Host 1 Host 2 Host 1 Host 2
Virtual
data path
Actual
data path
(a) (b)
Figure 3-2. (a) Virtual communication. (b) Actual communication.
FIGURE: (a) Virtual Communication. (b) Actual communication.
The data link layer can be designed to offer various services. The actual ser-vices

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
III
The data link layer can be designed to offer various
services.
The actual services that are offered vary from protocol
to protocol.
The reasonable possibilities are:
1 Unacknowledged connectionless service.
2 Acknowledged connectionless service.
3 Acknowledged connection-oriented service.
Unacknowledged connectionless service consists of
having the source machine send independent frames to
the destination machine without having the destination
machine acknowledge them.
Ethernet is a good example that provides this service.
No logical connection is established beforehand or
released afterward.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
IV
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 low.
The recovery is left to the higher layers.
It is also appropriate for real-time traffic, such as voice.
Or for other service in late data is worse than bad data.
In an acknowledged connectionless service there
are still no logical connections.
However, each frame sent is individually acknowledged.
The sender knows that whether a frame has been
arrived correctly or has been lost.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
V
If it does not arrive within a certain time interval, it can
be sent again.
The service is useful over unreliable channels, such as
wireless systems.
IEEE 802.11 is a good example of this class of service.
Providing acknowledgements in the data link layer is a
good thing, but not a requirement.
The network layer can always send a packet and wait
for it to be acknowledged by its peer on the remote
machine.
If the acknowledgement does not arrive within a certain
interval of time, the sender can send the message
again.
This strategy can be inefficient.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
VI
Links usually have a strict maximum frame length
imposed by the hardware, and known propagation
delays.
The network layer does not know these things.
The network layer might send a large packet that is
broken up into, say, 10 frames, of which 2 are lost on
average.
It would then take a very long time for the packet to get
through.
On the other hand, if individual frames are
acknowledged and retransmitted, then errors can be
corrected more directly and more quickly.
On reliable channels, such as fiber, the overhead of a
heavyweight data link protocol may be unnecessary.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
VII
However, it might be very useful on wireless channels
which are inherently unreliable.
Providing a connection oriented service is the most
reliable thing the data link layer can do.
The source and destination machines establish a
connection before any data are transferred.
Each frame sent over the connection is numbered.
Moreover, the data link layer ensures that each frame
sent is indeed received.
Moreover, it guarantees that each frame is received
exactly once.
And that all the frames are received in the right order.
Transfers go through three phases in a connection
oriented scheme:

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
VIII
1 Connection establishment.
2 Frame transmission.
3 Connection release.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
FRAMING I
The data link layer uses the services of the physical
layer.
The physical layer accepts a raw bitstream and tries to
deliver it to the destination.
Physical layer tries to handle noise and transmission
errors.
There is no guarantee for error-free deliveries.
The data link layer has to correct the errors.
The usual way is to break up the bit stream into frames.
The data link layer computes the checksums.
Upon arrival of a frame the checksum is recomputed.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
FRAMING II
A few strategies for framing:
1 Byte count.
2 Flag bytes with byte stuffing.
3 Flag bits with bit stuffing.
4 Physical layer coding violations.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
FRAMING III
198 THE DATA LINK LAYER CHAP. 3
Byte count One byte
5 1 2 3 4 5 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3
(a)
5 1 2 3 4 7 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3
(b)
Error
Frame 1
5 bytes
Frame 1
Frame 2
5 bytes
Frame 2
(Wrong)
Frame 3
8 bytes
Frame 4
8 bytes
Now a byte
count
Figure 3-3. A byte stream. (a)Without errors. (b)With one error.
FIGURE: A byte stream. (a) Without errors. (b) With one error.
The second framing method gets around the problem of resynchronization
after an error by having each frame start and end with special bytes. Often the
same byte, called a flag byte, is used as both the starting and ending delimiter.
This byte is shown in Fig. 3-4(a) as FLAG. Two consecutive flag bytes indicate
the end of one frame and the start of the next. Thus, if the receiver ever loses syn-chronization

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
FRAMING IV
SEC. 3.1 DATA LINK LAYER DESIGN ISSUES 199
FLAG Header Payload field Trailer FLAG
Original bytes After stuffing
A ESC FLAG B
A ESC ESC B
A ESC ESC ESC FLAG B
A ESC ESC ESC ESC B
A FLAG B
A ESC B
A ESC FLAG B
A ESC ESC B
(a)
(b)
Figure 3-4. (a) A frame delimited by flag bytes. (b) Four examples of byte se-quences
before and after byte stuffing.
The third method of delimiting the bit stream gets around a disadvantage of
byte stuffing, which is that it is tied to the use of 8-bit bytes. Framing can be also
be done at the bit level, so frames can contain an arbitrary number of bits made up
of units of any size. It was developed for the once very popular HDLC (High-level
Data Link Control) protocol. Each frame begins and ends with a special
bit pattern, 01111110 or 0x7E in hexadecimal. This pattern is a flag byte. When-ever
the sender’s data link layer encounters five consecutive 1s in the data, it
FIGURE: (a) A frame delimited by flag bytes. (b) Four examples of
byte sequences before and after byte stuffing.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
FRAMING V
200 THE DATA LINK LAYER CHAP. 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 0
0 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 0 0 1 0
Stuffed bits
(a)
(b)
(c) 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 0
Figure 3-5. Bit stuffing. (a) The original data. (b) The data as they appear on
the line. (c) The data as they are stored in the receiver’s memory after destuf-fing.
FIGURE: (Bit stuffing. (a) The original data. (b) The data as they
appear on the line. (c) The data as they are stored in the
receiver’s memory after destuffing.
With both bit and byte stuffing, a side effect is that the length of a frame depends on the contents of the data it carries. For instance, if there are no bytes in the data, 100 bytes might be carried in a frame of roughly 100 bytes. however, the data consists solely of flag bytes, each flag byte will be escaped the frame will become roughly 200 bytes long. With bit stuffing, the increase

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ERROR CONTROL I
How to make sure all frames are eventually delivered to
the network layer at the destination and in the proper
order.
Use of acknowledgements, retransmissions, timers and
sequence numbers.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
FLOW CONTROL I
Sender transmitting faster than the receiver accepting.
Feedback based flow control.
Rate based flow control.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ERROR DETECTION AND CORRECTION I
For wireless and very old networks error rates can be
high.
Network designers add redundant data to deal with
errors.
Error Correcting Codes: Help in figuring out the
specific error that has occurred.
Error Detecting Codes: Help in figuring out if some
error has occurred or not.
The use of error correcting codes is often referred to as
FEC (Forward Error Correction).
For reliable channels error detecting codes are
normally employed.
For noisy channels, error correcting codes are used.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ERROR DETECTION AND CORRECTION II
Error correcting codes are also implemented in physical
layer.
Applied mathematics: Galois fields, properties of
sparse matrices.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ERROR-CORRECTING CODES I
Four different error-correcting codes:
1 Hamming codes.
2 Binary convolutional codes.
3 Reed-Solomon codes.
4 Low-Density Parity Check codes.
All of these codes add redundancy to the information
that is sent.
A frame consists of m data (i.e. message) bits and r
redundant (i.e. check) bits.
In a block code, the r check bits are computed solely
as a function of the m data bits with which they are
associated.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ERROR-CORRECTING CODES II
It is as if the m bits were looked up in a large table to
find their corresponding r check bits.
In a systematic code, the m bits are sent directly,
along with the check bits, rather than being encoded
themselves before they are sent.
In a linear code. the r check bits are computed as a
linear function of the m data bits.
XOR or modulo 2 addition is a popular choice.
This means that encoding can be done with operations
such as matrix multiplications or simple logic circuits.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ERROR-DETECTING CODES I
They are used for reliable channels.
Linear systematic block codes:
1 Parity.
2 Checksums.
3 Cyclic redundancy checks (CRCs).
Fletcher’s checksum.
CRC (Cyclic redundancy check).
Polynomial code.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
ELEMENTARY DATA LINK PROTOCOLS I
A Utopian Simplex Protocol.
A Simplex Stop-and-Wait Protocol for an Error-Free
Channel.
A Simplex Stop-and-Wait Protocol for a Noisy Channel.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
SLIDING WINDOW PROTOCOLS I
A One-Bit Sliding Window Protocol.
A Protocol Using Go-Back-N.
A Protocol Using Selective Repeat.

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
EXAMPLE DATA LINK PROTOCOLS I
Packet over SONET.
ADSL (Assymetric Digital Subscriber Loop).

The Data
Link Layer
Muhammad
Adil Raja
Introduction
Data Link
Layer Design
Issues
Error
Detection
and
Correction
Elementary
Data Link
Protocols
Sliding
Window
Protocols
Example
Data Link
Protocols
Conclusions
REFERENCES
The inspiration and figures for these slides have been taken
from, Computer Networks, Andrew S. Tanenbaum, 5th
Edition.

The Data Link Layer

More Related Content

What's hot

Viewers also liked

Similar to The Data Link Layer

More from adil raja

Recently uploaded

The Data Link Layer