15CS46 - Data communication or computer networks 1_Module-3.ppt

Error Detection and Correction, Data
link control
Module-3
1

Data can be corrupted during transmission.
For reliable communication, errors must be detected and
corrected.
Types of Error
Single-Bit Error
Burst Error
2

In a single-bit error, only one bit of a given data unit is
changed from 1 to 0 or from 0 to 1.
3

A burst error means that 2 or more bits in the
data unit have changed from 1 to 0 or from 0 to 1.
A burst error does not necessarily mean that the
errors occur in consecutive bits. The length of the burst is
measured from the first corrupted bit to the last corrupted
bit
4

Redundancy
To detect or correct errors we need to send
redundant bits with data.
These redundant bits are added by sender and
removed by receiver.
Error detection uses the concept of redundancy,
which means adding extra bits for detecting errors at the
destination.
5

Detection Vs Correction
Error Detection: Process of observing whether error
has occurred or not. We are not even interested in the
number of errors
Correction: The correction of errors is more difficult
than the detection. Finding exact number of bits that
are corrupted and their location. The number of errors
and size of the message are important
7

Forward Error correction
And Retransmission
There are two methods of error correction
Forward error correction: It is the process in which the
receiver tries to guess the message by using redundant
bits.
Correction by retransmission: It is a technique in
which the receiver detects the occurrence of an error and
asks the sender to resend the message.
8

Coding
Redundancy is achieved through various coding
scheme.
The sender adds redundant bits through a process
that creates a relationship b/w the redundant bits and
the actual data bits
The receiver checks the relationship b/w two sets
of bits to detect or correct the errors
Block coding and Convolution coding
9

Structure of encoder and decoder
10

Modular Arithmetic
• Only limited range of integers are used.
• Upper limit N is used-Modulus.
• Allowed integers are 0 to N-1
• In addition and subtraction – carry is ignored
• This operation is similar to XOR operation.
1 0 1 0
1 1 0 0
XOR 0 1 1 0
11

Block Coding
• Message is divided into blocks, each of k bits called
data words.
• r –redundant bits are added to each block to make
length n=k+r
• The resulting n-bit blocks are called code words.
12

Error Detection
Conditions:
• The receiver has list of valid code words
• The original code word has changed to an invalid code words.
• In detection , the receiver needs to know only that the received
code word is invalid
13

Example
• Data words Code words
00 000
01 011
10 101
11 110
Assume sender encodes the data words 01 as 011 and
sends it to receiver, receiver received data 111. find
out error
14

Consider the following cases
1. The receiver receives 011. it is valid codeword. The
receiver extracts the data word 01 from it
2. The code word is corrupted during transmission and
111 is received. This is not a valid code word and it is
discarded
3. The code word is corrupted during transmission and
000 is received. This is a valid code word. The
receiver incorrectly extracts the data word 00
15

• An error-detecting code can detect only the
types of errors for which it is designed; other
types of errors may remain undetected.
16

Error Correction
• The receiver needs to find the original code word sent.
• We need more redundant bits for error correction than
for error detection
17

Table for error correction
Data words Codeword
00 00000
01 01011
10 10101
11 11110
The data word is 01. the sender create the code word
01011. the code word is corrupted during transmission and
01001 is received.
First, the receiver finds that the received code word is not
in the table. This means error has occurred.
18

The receiver uses the following strategy to guess the
correct data word
1. Compare the received code word with the first code word in
the table , the receiver decides that the first code word is not
the one that was sent because there are two different bits
2. By the same reasoning, the original code word can not be the
third or fourth in the table
3. The original code word must be the second one in the table
because this is the only one that differs from the received code
word by 1 bit. The receiver replaces 01001 with 01011 and
consult the table to find the data word 01
19

Hamming Distance
• Hamming distance between two words is the number
of difference between corresponding bits.
• It can easily be found by applying XOR operation on
the two words and count the number of 1s in the
result.
• Example: d(000,011) is 2
20

Minimum Hamming Distance
• It is the smallest hamming distance between all possible pairs in a
set of words.
Find minimum hamming distance between code words:
00 000
01 011
10 101
11 110
d(000,011)=2; d(000,011)=2;
d(000,101)=2; d(000,110)=2
d(011,110)=2; d(101,110)=2
d min is 2
21

Simple Parity Check Code
In this scheme , a k-bit data word is changed to an n-
bit code word where n=k+1.
This extra bit is called parity bit.
The parity bit selected to make the total number of 1‘s
in the codeword even.
A simple parity check code is a single bit error
detecting code in which n=k+1 with dmin=2
23

Simple parity check code –Encoder
• The encoder uses a generator that takes a copy of a 4-
bit data word(a0,a1,a2,a3). It generates a parity bit r0
• Data word bits and the parity bit creates 5-bit
codeword
• The parity bit that is added makes the number of 1s
in the codeword even
• r0=a3+a2+a1+a1 Modulo 2
24

Encoder and Decoder for simple parity check code
25

Simple parity check code –decoder
• The receiver receives a 5-bit word
• The checker performs addition on all 5 bits received.
The result is called syndrome-just 1 bit.
• The syndrome is 0 when the number of 1s in the
received codeword is even ;otherwise it is 1.
• s0=b3+b2+b1+b0+q0 modulo 2
26

Assume the sender sends the dataword 1011. the code word
created from this data word is 10111, which is sent to the
receiver.
1. No error occurs, the received code word is 10111. the syndrome is
0. the data word is created
2. One single bit error changes a1. the received code word is 10011. the
syndrome is 1. no data word is created
3. One single bit error changes r0. the received code word is 10110. the
syndrome is 1. no data word is created
4. An error changes r0 and a second error changes a3. the received code
word is 00110. the syndrome is 0. the data word 0011 is created at the
receiver
5. Three bits a3, a2, a1 are changed by errors. The received code word is
01011. the syndrome is 1. the data word not created.
A simple parity check code can detect an odd number of errors
27

Calculation of row and column
parities
28

Two errors affects two parities
30

Hamming Codes
• Minimum distance is 3. Expressed as C(n,k); where n=2m-1
• Data words : a3,a2,a1,a0 and Code words : a3,a2,a1,a0,r2,r1,ro
• r0=a2+a1+a0; r1=a3+a2+a1; r3=a1+a0+a3
• Checker in the decoder creates a 3-bit syndrome (s2s1s0); where
s0=b2+b1+b0+q0;s1=b3+b2+b1+q1;s3=b1+b0+b3+q2
Syndrome 000 001 010 011 100 101 110 111
Error None q0 q1 b2 q2 b0 b3 b1
33

Structure of Encoder and Decoder Hamming code
34

• r0=a2+a1+a0 modulo-2
• s0=b2+b1+b0+q0 modulo-2
35

Let us trace the path of three data word from the sender
to the destination
1. The data word 0100 becomes the code word 0100011. the
code word 0100011 is received. The syndrome is 000, the
final dataword is 0100
2. The data word 0111becomes the codeword 0011001 is
received. The syndrome is 011. according to table b2 is in
error. After flipping b2, the final data word is 0111
3. The dataword 1101 becomes the code word 1101000. the
code word 0001000 is received. The syndrome is 101, which
means that b0 is in error. After flipping b0, we get 0000, the
wrong data word. This shows that our code can not correct
two errors
36

Cyclic Codes
• In this scheme, if a code word is cyclically shifted, the
result is another codeword
• b1=a0;b2=a1;b3=a2;b4=a3;b5=a4;b6=a5;b0=a6.
37

Binary division in a CRC generator
41

Two cases – with /without Error
42

A polynomial and its representation
43

Table 10.1 Standard polynomials
Name Polynomial Application
CRC-8 x8 + x2 + x + 1 ATM header
CRC-10 x10 + x9 + x5 + x4 + x 2 + 1 ATM AAL
ITU-16 x16 + x12 + x5 + 1 HDLC
ITU-32
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10
+ x8 + x7 + x5 + x4 + x2 + x + 1
LANs
44

The sender follows these steps:
•The message is divided into 16-bit words.
•The value of the checksum word is set to 0
•All words including the checksum are added using ones
complement addition
•The sum is complemented and becomes the checksum.
•The checksum is sent with the data.
Internet checksum
the internet has been using a 16-bit checksum. The
sender calculates the checksum by following these steps.
46

The receiver follows these steps:
•The message (including checksum) is divided into 16-
bit words.
•All words are added using one’s complement addition
•The sum is complemented and becomes the new
checksum.
•If the value of checksum is zero, the data are accepted:
otherwise, rejected.
47

Example 7
Suppose the following block of 16 bits is to be sent using a
checksum of 8 bits.
10101001 00111001
The numbers are added using one’s complement
10101001
00111001
------------
Sum 11100010
Checksum 00011101
The pattern sent is 10101001 00111001 00011101
48

Example 8
Now suppose the receiver receives the pattern sent in Example
7 and there is no error.
10101001 00111001 00011101
When the receiver adds the three sections, it will get all 1s,
which, after complementing, is all 0s and shows that there is no
error.
10101001
00111001
00011101
Sum 11111111
Complement 00000000 means that the pattern is OK.
49

Example 9
Now suppose there is a burst error of length 5 that affects 4
bits.
10101111 11111001 00011101
When the receiver adds the three sections, it gets
10101111
11111001
00011101
Partial Sum 1 11000101
Carry 1
Sum 11000110
Complement 00111001 the pattern is corrupted.
50

Let us calculate the checksum for a text of 8 characters
(“Forouzan”). The text needs to be divided into 2-byte (16-
bit) words. For example, F is represented as 0x46 and o is
represented as 0x6F.
51

Data link control
Functions:
> Framing
> Flow control
> Error control
> Smooth and reliable transmission of frames
54

• To implement these set of data link protocols
are needed
• Protocols are set of rules that need to be
implemented in software and run by the two
nodes involved in data exchange at the data
link layer.
55

Framing
• Data transmission in physical layer means moving bits in
the form of a signal
• The data link layer needs to pack bits into frames.
• Each frames must be distinguishable
• Framing separates a message from one source to a
destination or from other messages to other destinations,
by adding a sender address and a destination address.
56

Frame types: Fixed size, Variable size
Fixed size
• No need for defining boundaries
• Size itself is the delimiter:
• Ex: ATM network frame
Variable size
• Need to define the end of the frame and the beginning of
the next.
57

Character Oriented protocols
• Data to be carried are 8-bit characters from a coding
system-ASCII
• The header normally carries the source and
destination addresses and other information
• The trailer carries error detection or error correction
redundant bits, are also multiple of 8-bits.
• 1-byte Flag is added at the beginning and end of a
frame. 58

Character-oriented protocol, stuffing, un-stuffing
59

The flag type
• Byte stuffing strategy is used
– A special byte is added to the data section of the
frame when there is a character with the same
pattern as flag.
– This byte is called escape character (ESC)
– Whenever the receiver encounters the ESC
character; it removes it from data section and treats
the next character as data
60

Bit-oriented protocols
• The data section of a frame is a sequence of bits to be
interpreted by the upper layer as text, graphic, audio,
video, etc.
• In addition to header, delimiter is used to separate one
frame from the other
• Usually the pattern flag 01111110 used as the
delimiter.
61

• If the flag pattern appears in the data, we need to
inform the receiver that this is not the end of the
frame
• We do this by stuffing 1 single bit to prevent the
pattern from looking like a flag. The strategy is called
bit stuffing
• In bit stuffing, if a 0 and five consecutive 1 bits are
encountered, an extra 0 is added. This extra stuffed
bit is removed from the data by the receiver
62

Flow and error control
• Flow control refers to a set of procedures used to
restrict the amount of data that the sender can send
before waiting for acknowledgment.
• Error control in the data link layer is based on
automatic repeat request, which is the retransmission
of data.
63

• Connectionless Protocol
• In a connectionless protocol, frames are sent from one node to
the next without any relationship between the frames; each
frame is independent.
• The frames are not numbered and there is no sense of ordering.
• Most of the data-link protocols for LANs are connectionless
protocols.
• Connection-Oriented Protocol
• In a connection-oriented protocol, a logical connection should
first be established between the two nodes (setup phase).
• the frames are numbered and sent in order.
• Ex: wired in LANs, Point to point protocols , Wireless LANs and
wired WANs
64

Protocols
• Noiseless Channel:
– Simplest,
– Stop and Wait
• Noisy Channel:
– Stop and Wait ARQ,
– Go-Back N ARQ,
– Selective Repeat ARQ
* ARQ: Automatic Repeat Request
65

Assumptions, Terminologies
• Data frames travel from one node(sender) to another
node (receiver)
• Some special frames are used: Acknowledgement and
Negative Acknowledgement frames
• These flow in the opposite direction for flow and
error control purposes
• Data flow in only one direction
• Piggybacking: ACK and NAK is included in the data
frames.
66

Noiseless channels-Simplest Protocol
• It has no flow control or error control.
• It is a Uni-directional protocol.
• At sender side: gets data, makes frame, sends
• At receiver side: receives data, extracts data,
delivers data to N/W layer
67

• The sender cannot send until its network layer has a data
to send
• The receiver cannot deliver until a frame arrives.
• Introduce the idea of events in the protocol
• The procedure at the sender site is constantly running: no
action until there is a request from network layer
• The procedure at the receiver site is constantly running:
no action until notification from physical layer
68

Stop and Wait protocol
• Frames must be stored when receives frames faster its
processing capacity.
• There should be some method to slowdown the speed of
sender.
• There must be feedback from the receiver to the sender.
• Sender sends one frame, stops until it receives
confirmation, and then sends next frame.
71

Flow diagram of Simplest protocol
73

Noisy Channels-Stop and Wait Automatic Repeat Request
• It adds a simple error control mechanism to the Stop-and –Wait
protocol.
• It needs to add redundancy bits to data frame. When frames
arrives at the receiver site, it is checked and if it is corrupted, it is
silently discarded
• The corrupted and lost frames need to be resent in this protocol.
• Frames needs to be numbered-using sequence number. Sequence
number must be known size to keep frame size minimum.
74

Sequence number
• The protocol specifies that frames need to be
numbered.
• A field is added to the data frame to hold the
sequence number of that frame
• We want to minimize the frame size, the smallest
range that provides unambiguous communication
• We want x and x+1 sequence numbers
75

• Sequence numbers must be suitable for both data and
acknowledgement frames. It also prevents retaining of
duplicate frames.
• Numbered acknowledgments are needed if an
acknowledgment is delayed and the next frame is lost.
• Acknowledgement frame always announces next
sequences to be received from sender.
• In Stop-and –Wait ARQ, the acknowledgement number
always announces in modulo 2 arithmetic the sequence
number of the next frame expected.
76

HDLC
High Level Data Link Control (HDLC):
– It is bit-oriented protocol for communication over
point-to-point and multipoint links.
79

Configuration and Transfer Modes
• HDLC provides two common transfer modes that
can be used in different configurations:
– Normal Response Mode (NRM): the station
configuration is unbalanced. We have one
primary station and multiple secondary
stations. A primary station can send
commands, a secondary station can only
respond
– Asynchronous Balanced Mode(ABM): the
configuration is balanced. 80

NRM
NRM is used for both point-to-point and multiple point links.
81

ABM
The link is point-to-point and each station can function as
a primary and a secondary.
82

Frames
• HDLC defines three types of frames:
– Information frames (I-frames)
– Supervisory frames (S-frames)
– Unnumbered frames(u-frames)
Each type of frame serves as an envelope for the
transmission of a different type of message
I-Frame: used to transport user data and control
information relating to user data
S-Frame: used only to transport control information
U-Frame: Reserved for system management
83

Fields
• Flag field:
– 8-bit sequence, with bit pattern 01111110
– Identifies beginning and end of a frame
– Serves as a synchronization pattern for the receiver
• Address field:
– Contains address of the secondary station
– If primary created the frame: contains to address
– If secondary created the frame: contains from address
– It can be one byte or several byte long depending on
network
– If the address field is only 1 byte, the last bit is always a
1. if the address is more than 1 byte, all bytes but the
last one will end with 0: only the last will end with 1
85

• Control field:
– 1 or 2 byte segment of the frame used for flow and
error control
• Information field:
– Contains the user’s data from network layer or
management information
• FCS: (Frame check sequence)
– Used for error detection
– Can contain either a 2- or 4 byte ITU –T CRC.
86

Control field
• The control field determines the type of
frame and define its functionality
87

Control field for I-frame
• Bit 1: defines the type. If it is 0 then I
• Bit 2-4: called N(S) defines the sequence number of the
frame
• Bit 5: Dual purpose bit P: Poll(0), Final (1)
– Poll frame sent by primary, Final the frame sent by
secondary.
• Bit 6-8: called N(R) corresponds to the
Acknowledgement number
88

Control field for S-frame
• S-frames are used for flow and error control.
This frame does not have information fields. If
the first 2 bits of the control field is 10, this
means the frame is an S-frame.
• The last 3-bits called N(R), corresponds to the
acknowledgement number or NAK
• The 2-bits called code is used to define the type
of S-frame itself
89

• Receive ready(RR): the code subfields is 00, it is an RR
S-frame. This frame acknowledges the receipt of a safe
group of frames. The value of N(R) fields defines the
ACK number
• Receive not ready(RNR): the code subfields is 10, it is
an RNR S-frame. This frame acknowledges the receipt
of a frame or group of frames and it announces that the
receiver is busy and can not receive more frame. The
value of N(R) fields defines the ACK number
90

• Reject (REJ): the code subfields is 01, it is a REJ S-
frame. This is a NAK frame. It is a NAK that can be used
in Go-Back-N ARQ to improve the efficiency of the
process by informing the sender, before the sender time
expires. The value of N(R) fields defines the NCK
number
• Selective reject(SREJ): the code subfields is 11, it is an
SREJ S-frame. This is a NAK frame used in selective
reject instead of selective repeat ARQ. The value of N(R)
fields defines the NCK number
91

Control field for U-frames
Unnumbered frames are used to exchange
session management and control
information b/w connected devices.
U-frame codes are divided into two sections: a
two bit prefix before the P/F bit and a 3-bit
suffix after the P/F bit
92

Table 11.1 U-frame control command and response
Command/response Meaning
SNRM Set normal response mode
SNRME Set normal response mode (extended)
SABM Set asynchronous balanced mode
SABME Set asynchronous balanced mode (extended)
UP Unnumbered poll
UI Unnumbered information
UA Unnumbered acknowledgment
RD Request disconnect
DISC Disconnect
DM Disconnect mode
RIM Request information mode
SIM Set initialization mode
RSET Reset
XID Exchange ID
FRMR Frame reject
93

Point to Point Protocol
• Services:
– Defines the format of the frame to be exchanged between devices
– Defines how two devices can negotiate the establishment of the link
and exchange of data
– Defines how network layer data are encapsulated in the data link layer
– Defines how two devices can authenticate each other
– Provides multiple network layer services supporting a variety of
network layer protocols
– Provides connections over multiple links
– Provides network address configuration
• Missing :
– Does not provide flow control
– Very simple mechanism for error control
– Does not provide a sophisticated addressing mechanism
94

Framing
• Flag: Starts and ends with a 1-byte flag 01111110
• Address: Constant and set to 11111111
• Control: Set to constant value; 11000000
• Protocol: Defines what is being carried in the data field
• Payload field: Carries either user data or other information
• FCS: 2 or 4 byte standard CRC.
95

• Dead: In the dead phase the link is not being used
• Establish: Connection goes into this state when one of the
nodes starts the communication
• Authenticate: For authentication purpose; optional
• Network: Negotiation takes place in this phase.
• Open: Data transfer takes place
• Terminate: Connection is terminated in this phase.
97

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