2. Introduction
 To achieve unnecessary control over data communication link
 A logic is added above the physical interface which is referred
as Data Link Control/ Protocol
 To see the need of data link control, some of the requirements
and objectives for data communication are listed as follows
 Frame synchronization- start and end should be recognizable
 Flow control- the sender and receiver data rate
 Error control- check the error and correct it
 Addressing- identify the address
 Control and Data on same link- should have control information
 Link Management- the initiation, maintenance and termination
 We will study the three mechanisms that are a part of data link
control
 Flow control, error detection and error control

3. Flow Control
Flow control is used to make sure that the
transmitted data is not over whelming the receiver
The receiver allocates a data buffer for the
processing before passing it to higher level software
In the absence the system may bottle neck

4. Ideal Flow Control
In the absence of errors, the model in
figure depicts a vertical time
sequence
It has advantage of showing time
dependencies
Each arrow represents a single frame
(data + control info)
It was an errorless communication
However, transmitted frame suffers a
variable amount of delay

5. Stop and Wait Flow Control
Simplest form of flow control
Source transmit a frame, after reception the
destination indicates its willingness to accept more
data by sending back an acknowledgment of the
frame just received
The source waits for the acknowledgment
The destination can thus stop the data flow by with
holding acknowledgment
If a larger data is broken down into smaller blocks,
this is done for the following reasons

6. Stop and Wait Flow Control
 Limited buffer size
 Longer the transmission, more likely there will be an error
 Smaller frames, errors are detected sooner and
retransmitted faster
 On LANs not to permit a station to hold a medium for longer
time
With multiple frames of a message, stop and wait
may be inadequate
We know about transmission time and propagation
delay

7. Stop and Wait Flow Control

8. Stop and Wait Flow Control
As per previous figure transmission time is normalized to 1
and propagation is expressed as variable ‘a’
When a<1 the propagation time is less than transmission time
The frame is long and before it is transmitted it arrives destination
When a>1 the propagation time is greater than transmission
time
The sender completes transmission before the frame arrives at
receiver
Note that for a>1 line is always underutilized and for a<1 line is
inefficiently utilized
In essence for high data rates or for long distances stop and
flow control provides inefficient line utilization

9. Sliding Windows Flow Control
As per the previous flow control the efficiency can be improved
by allowing multiple frames to be in transit at same time
A and B are connected via full duplex link (A -> B)
B allocates a buffer for n frames to be received from A
To keep track of the frames, they are labeled
B ack a frame by sending ack that includes sequence number of
next expected
The scheme can be used to ack multiple frames
A and B both maintains a list; sent sequence numbers and
received sequence numbers
Each of these lists can be thought of as a window or frame
Additional comments need to be made

10. Sliding Windows Flow ControlTo represent a sequence number, it occupies a field
in a frame
For K-bit field, the range of sequence number is O
through 2k
-1

11. Sliding Windows Flow Control
Receiver can control the size of the sending window
By limiting the size of the sending window data flow from
sender to receiver can be limited
Let 3 bits sequence has max of 7 frames, A->B
After transmitting some frames without ack A shrunk its
window to 4 frames
B then transmits RR3 (receive ready) which is ack of receive
frames
With this ack A is back up to permission of transmitting 7
frames starting from 3 onwards
RNR (receive not ready) a station can cut off a flow of frames
if it exceeds its buffer size

12. Sliding Windows Flow Control

13. Sliding Window Transmission
If 2 stations exchange data, each has to maintain 2 windows
(two directional)
One for transmit and one for receive
And each one has to send data and ack to other
Piggybank- provides efficient support for above requirement
Each frame holds sequence number of frame + field that holds
sequence number for ack
Thus a station sends data and ack both in one frame, saving
communication capacity
If station has ack but no data, it sends a separate ack frame
Similarly no ack but data to send, it must repeat the last ack it
sent
 Because data frame includes a field for ack number and some
number has to be put in that field
 Duplications are simply ignored
 Pipe line concept

14. Error detecting
We talked about transmission impairments and the effect of
data rate and signal-to-noise ratio on bit error rate.
Regardless of the design of the transmission system, there will
be errors, resulting in the change of one or more bits in a
transmitted frame.
So, how can a receiver know if the data received is error free?
 There are several methods that can be implemented to
accomplish this.
Some of these methods are simple and somewhat effective –
others are complicated and very effective.

15. Error Detection
Data transmission can contain errors
Single-bit
Burst errors of length n
How to detect errors
If only data is transmitted, errors cannot be detected
 Send more information with data that satisfies a
special relationship
 Add redundancy

16. Error Detection Methods
Parity bit
The simplest error-detection scheme is to append a parity bit
to the end of a block of data. A typical example is ASCII
transmission, in which a parity bit is attached to each 7-bit
ASCII character.
The value of this bit is selected so that the character has an
even number of 1s (even parity) or an odd number of 1s (odd
parity).
If one bit (or any odd number of bits) is erroneously inverted
during transmission then the receiver will detect an error.
Note, however, that if two (or any even number) of bits are
inverted due to error, an undetected error occurs.
Typically,
Even parity is used for synchronous transmission and
Odd parity for asynchronous transmission

17. Error Detection Methods
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
Performance:
 Detects all odd-number errors in a data block

18. Longitudinal Redundancy Check
(LRC)
Organize data into a table and create a parity for
each column
11100111 11011101 00111001 10101001
11100111
11011101
00111001
10101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC

19. Checksum
A checksum error detection method give us more accuracy if
used with a parity bit method.
The checksum field can be one or more bytes
A checksum method will basically check the sum of all the '1's
in a message and check that value against the checksum
value added by the sender to the message.
A checksum method can provide more precision to your error
detection efforts, there are still limitations.
A simple checksum cannot detect an even number of errors
which sum to zero, an insertion of bytes which sum to zero (add
0), or even the re-ordering of bytes in the message.
While there are some more advanced implementations of the
checksum method, including Fletcher's checksum method

20. CRC (cyclic redundancy check)
A better method to detect errors in a large block of data
involves the use of CRC codes.
The hardware required to implement a CRC code is slightly
more complicated than other error detection codes but it has
an error detection effectiveness greater than 99.9%.
CRC, which can be described as follows.
Given a k-bit block of bits, or message, the transmitter generates
an n-bit sequence, known as a frame check sequence (FCS)
Making a resulting frame, consisting of k + n bits which is exactly
divisible by some predetermined number.
The receiver then divides the incoming frame by that number
and, if there is no remainder, assumes there was no error.

21. CRC Concept
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
+=

22. Cyclic Redundancy Check
Sending
1.Multiply M(x) by xn
2.Divide xn
M(x) by P(x)
3.Ignore the quotient and keep the reminder C(x)
4.Form and send F(x) = xn
M(x)+C(x)
Receiving
1.Receive F’(x)
2.Divide F’(x) by P(x)
3.Accept if remainder is 0, reject otherwise

23. Generating Polynomial
One way of considering CRC systems is to treat
the stream of transmitted bits as a representation of a
polynomial with coefficients of 1:
Let us assume k message bits and
n bits of redundancy
Associate bits with coefficients of a polynomial
1 0 1 1 0 1 1
1x6
+0x5
+1x4
+1x3
+0x2
+1x1
+1x0
= x6
+x4
+x3
+x+1

24. Generating Polynomial
CRC-16: G(x) = x16
+ x15
+ x2
+ 1
detects single and double bit errors
All errors with an odd number of bits
Burst errors of length 16 or less
Most errors for longer bursts
CRC-32: G(x) = x32
+ x26
+ x23
+ x22
+ x16
+ x12
+ x11
+ x10
+
x8
+ x7
+ x5
+ x4
+ x2
+ x + 1
Used in ethernet
Also 32 bits of 1 added on front of the message
 Initialize the LFSR to all 1s

25. Modulo 2 Arithmetic
Modulo 2 arithmetic uses binary addition with no
carries, which is just the exclusive-OR
operation. For example:

26. Example
Send
 M(x) = 110011  x5
+x4
+x+1 (6
bits)
 P(x) = 11001  x4
+x3
+1 (5 bits, n
= 4)
 4 bits of redundancy
 Form xn
M(x)  110011 0000
 x9
+x8
+x5
+x4
 Divide xn
M(x) by P(x) to find C(x)
100001
1001
11001
10000
11001
110011000011001
= C(x)
Receive
Send the block 110011 1001
00000
11001
11001
11001
110011100111001
No remainder
 Accept

27. Designing of CRC polynomial
 The most commonly used
polynomial lengths are
 9 bits (CRC-8)
 17 bits (CRC-16)
 33 bits (CRC-32)
 65 bits (CRC-64)
 CRC-8:
x8+x2+x+1
 Used in: 802.16 (along with error
correction).
 CRC-CCITT:
x16+x12+x5+1
 Used in: HDLC, SDLC, PPP
default
 IBM-CRC-16 (ANSI):
x16+x15+x2+1
 802.3:
x32+x26+x23+x22
+x16+x12+x11+x10
+x8+x7+x5+x4+x2+x+1
 Append 32 bits to the message.
 Detects all bursts of length 32 or
less.
 Used in: Ethernet, PPP option

28. Hamming Distance Based Checks
If we want to detect d bit errors in an n bit word we can map
every n bit word into a bigger n+d+1 bit word so that the
minimum Hamming distance between each valid mapping is
d+1.
Meaning if error-correcting bits are included with a message,
and if those bits can be arranged such that different incorrect
bits produce different error results, then bad bits could be
identified.
In a 7-bit message, there are seven possible single bit errors,
so three error control bits could potentially specify not only that
an error occurred but also which bit caused the error.
Hamming studied the existing coding schemes.
To start with he developed a nomenclature to describe the
system, including the number of data bits and error-correction
bits in a block.

29. Calculating the Hamming Code
 The key to the Hamming Code is the use of extra parity bits to allow the
identification of a single error.
 Create the code word as follows:
 Mark all bit positions that are powers of two as parity bits.
(positions 1, 2, 4, 8, 16, 32, 64, etc.)
 All other bit positions are for the data to be encode
 Each parity bit calculates the parity for some of the bits in the code word.
 The position of the parity bit determines the sequence of bits.
 Position 1: check 1 bit, skip 1 bit, check 1 bit, skip 1 bit, etc.
(1,3,5,7,9,11,13,15,...)
 Position 2: check 2 bits, skip 2 bits, check 2 bits, skip 2 bits, etc.
(2,3,6,7,10,11,14,15,...)
 Position 4: check 4 bits, skip 4 bits, check 4 bits, skip 4 bits, etc.
(4,5,6,7,12,13,14,15,20,21,22,23,...)
 Position 8: check 8 bits, skip 8 bits, check 8 bits, skip 8 bits, etc.
(8-15,24-31,40-47,...)
 Position 16: check 16 bits, skip 16 bits, check 16 bits, skip 16 bits, etc.
(16-31,48-63,80-95,...)
 Position 32: check 32 bits, skip 32 bits, check 32 bits, skip 32 bits, etc.
(32-63,96-127,160-191,...)
 Set a parity bit to 1 if the total number of ones in the positions it checked is odd
and 0 if the total number of ones in the positions it checks is even.

30. Example
With Hamming, can find errors quickly by just looking at one pattern:
Let's say error in a data bit:
100 sent
111000
became: 111001
i.e. data 101, but check bits wrong
Check bit 1 - 1 - checks bits 3,5 - 1 0 - OK
Check bit 2 - 1 - checks bits 3,6 - 1 1 - WRONG
Check bit 4 - 0 - checks bits 5,6 - 0 1 - WRONG
The bad bit is bit 2 + 4 = bit 6. Data was corrupted. Data
should be 100.

31. Example
Let's say error in a check bit:
100 sent
111000
became: 011000
i.e. data 100, but check bits wrong
Check bit 1 - 0 - checks bits 3,5 - 1 0 - WRONG
Check bit 2 - 1 - checks bits 3,6 - 1 0 - OK
Check bit 4 - 0 - checks bits 5,6 - 0 0 - OK
The bad bit is bit 1. Check bit was corrupted. Data is
fine.

32. Finding and fixing a bad bit
Do a example and then add an error and check the result
For above example create a code word of 011100101010.
Insert and error suppose the word that was received was
011100101110 instead.
Then the receiver could calculate which bit was wrong and
correct it.
The method is to verify each check bit. Write down all the
incorrect parity bits.
Doing so, you will discover that parity bits 2 and 8 are
incorrect. It is not an accident that 2 + 8 = 10, and that bit
position 10 is the location of the bad bit.
In general, check each parity bit, and add the positions that
are wrong, this will give you the location of the bad bit.

33. Error Correction
There are various methods used to correct errors.
 An obvious and simple one is to just detect the error
and then do nothing, assuming that something else
will fix it.
This method is fine when something else is able to
fix the error, but is of no use if there is no something
else!
_____In computers radiation can cause the gates to
change state from time to time.
Modern computer memory corrects these errors it is
called FEC (Forward Error Control) to differentiate it
from ARQ systems.

34. Techniques For Error Control
The most common are based on some or all of the
following ingredients:
Error detection. Techniques and coding of data.
Positive acknowledgment.
Retransmission after timeout. The source
retransmits a frame that has not been acknowledged
after a predetermined amount of time.
Negative acknowledgment and retransmission.
The estimation returns a negative acknowledgment to
frames in which an error is detected. The source
retransmits such frames.

35. ARQ (Automatic Repeat reQuest)
In data communication protocols, it is common to just ignore
errors that are received, while acknowledging correct data.
If an error is received, the lack of an acknowledgement eventually
leads to a retransmission after some timeout period.
The effect of ARQ is to turn an unreliable data link into a reliable
one
This technique is called ARQ (for Automatic Repeat reQuest).
Three versions of ARQ have been standardized:
Stop-and-wait
Go-back-N
Selective-reject

36. Stop N Wait ARQ
Based on the stop-and-wait flow-control technique
The source station transmits a single frame and then must
await an acknowledgment (ACK).
No other data frames can be sent until the destination station's
reply arrives at the source station.
Error Control - Two sorts of errors could occur.
First, the frame that arrives at the destination could be
damaged; the receiver detects this by error detection
technique
referred to earlier and simply discards the frame.
To account for this possibility, the source station is equipped
with a timer.
After a frame is transmitted, the source station waits for an
acknowledgment. If no acknowledgment is received by the
time the timer expires, then the same frame is sent again,
keeping a copy at transmitter

37. Stop N wait
 Second, damaged acknowledgment.
 Station A sends to station B, which
responds with an acknowledgment (ACK).
 The ACK is damaged in transit and is not
recognized by A, which eventually ends
timer & generate frame
 This duplicate frame arrives and is
accepted by B, which has therefore
accepted two copies of the same frame as
if they were separate.
 To avoid this problem, frames are
alternately labeled with 0 or 1
 ACK are of the form ACKO and ACK1.
 In keeping with the sliding-window
convention, an ACKO acknowledges receipt
of a frame numbered 1 and indicates that
the receiver is ready for a frame numbered
0.
 The sliding-window flow control technique
can be adapted to provide more efficient
line use.

38. Go back N ARQ
The form of error control based on sliding-window flow control
In go-back-N ARQ, a station may send a series of frames
sequentially numbered
The number of unacknowledged frames outstanding is determined
by window size, using the sliding-window flow control technique.
While no errors occur, the destination will acknowledge (RR =
receive ready)
If the destination station detects an error in a frame, it sends a
negative acknowledgment (REJ = reject) for that frame.
The destination station will discard that frame and all future incoming
frames until the frame in error is correctly received.
Thus, at source station, when it receives an REJ, must retransmit the
frame in error plus all succeeding frames that were transmitted in the
interim.

39. GO Back N
 When A is sending to B the following errors could
occur which is called Damaged frame.
 There are three sub cases:
 A transmits frame i. B detects an error and has
previously successfully received frame (i - 1). B
sends REJ i, indicated that frame i is rejected.
 When A receives the REJ, it must retransmit frame
i and all subsequent frames, since the original
transmission of i.
 Frame i is lost in transit. A subsequently sends
frame (i + 1). B receives frame (i + 1) out of order
and sends an REJ i
 Frame i is lost in transit, and A does not soon send
additional frames.
 B receives nothing and returns neither an RR nor
an REJ.
 When A's timer expires, it transmits an RR frame
that includes a bit known as the P bit, set to =1
 B interprets the RR frame with a P bit of 1 as a
command that must be acknowledged by sending
an RR indicating the next frame that it expects.
 When A receives the RR, it retransmits frame i.

40. Go Back N
 Damaged RR. There are two sub cases:
 B receives frame i and sends RR (i + I), which is lost in transit.
 Because acknowledgments are cumulative (e.g., RR 6 means that all frames
 through 5 are acknowledged),
 A will receive a subsequent RR to a subsequent frame and that it will arrive
before the timer associated with frame i expires.
 If A's timer expires, it transmits an RR command as in case of pervious slide. It
sets another timer, called the P-bit timer.
 If B fails to respond to the RR command, or if its response is damaged, then A's
P-bit timer will expire. At this point
 A will try again by issuing a new RR command and restarting the P-bit timer.
 This procedure is tried for a number of iterations.
 If A fails to obtain an Ack after some maximum number of attempts, it initiates a
reset procedure.
 Damaged REJ. If an REJ is lost, this is equivalent to the case lost in transit

41. Selective-reject ARQ
With selective-reject ARQ, the only frames retransmitted are those
that receive a negative acknowledgment, in this case called SREJ,
or that time-out.
This would appear to be more efficient than go-back-N, because it
minimizes the amount of retransmission.
On the other hand, the receiver must maintain a buffer large enough
to save post-SREJ frames until the frame in error is retransmitted
and it must contain logic for reinserting that frame in the proper
sequence.
The transmitter, too, requires more complex logic to be able to send
a frame out of sequence.
Because of such complications, select-reject ARQ is much less used
than go-back- N ARQ.
The window-size limitation is more restrictive for selective-reject than
for go back- N. Consider the case of a 3-bit sequence-number size
for selective-reject.

42. Selective reject
 Allow a window size of seven, and consider the following scenario
1. Station A sends frames 0 through 6 to station B. Frame
(0,1,2,3,4,5,6,7,0,1,2,3,4,5)
2. Station B receives all seven frames and cumulatively Ack with RR 7.
3. Because of some noise burst, the RR 7 is lost/corrupted
4. A’s times out and retransmits frame 0 - 6.
5. B has already advanced its receive window to accept frames
7,0,1,2,3,4, and 5
6. Thus, B assumes that frame 7 has been lost and that this is a new
frame 0,which it accepts.
 The problem with this scenario is that there is an overlap between the
sending and receiving windows.
 To overcome the problem, the maximum window size should be no more
than half the range of sequence numbers.
 In this above scenario, if only four unacknowledged frames may be
outstanding, no confusion can result.
 In general, for a k-bit sequence number field, which provides a sequence
number range of 2k, the maximum window size is limited to 2k-1.

43. OTHER DATA LINK CONTROL
PROTOCOLS
In addition to HDLC, there are a number of other
important data link control protocols.
LAPB
LAPD
Logical Link Control (LLC)
Frame Relay
Asynchronous Transfer Mode (ATM)

44. Frame structure

45. LAPB
LAPB (Link Access Procedure, Balanced) was
issued by ITU-T as part of its X.25
packet-switching network-interface standard. It is a
subset of HDLC that provides
only the asynchronous balanced mode (ABM); it is
designed for the point-to-point
link between a user system and a packet-switching
network node. Its frame format
is the same as that of HDLC.

46. LAPD
LAPD (Link Access Procedure, D-Channel) was issued by ITU-T as
part of its set
of recommendations on ISDN (Integrated Services Digital Network).
LAPD provides data link control over the D channel, which is a
logical channel at the user-ISDN interface.
There are several key differences between LAPD and HDLC.
Like LAPB, LAPD is restricted to ABM. LAPD always uses 7-bit
sequence numbers; 3-bit sequence numbers are not allowed.
The FCS for LAPD is always the 16-bit CRC.
Finally, the address field for LAPD is a 16-bit field that actually
contains two sub-addresses:
 one is used to identify one of possibly multiple devices on the user side
of the interface, and
 The other is used to identify one of possibly multiple logical users of
LAPD on the user side of the interface.

47. LLC
LLC is part of the IEEE 802 family of standards for
controlling operation over alocal area network (LAN).
LLC is lacking some features found in HDLC and
also has some features not found in HDLC.
The most obvious difference between LLC and
HDLC is the difference in frame format.
Link control functions in the case of LLC are actually
divided between two layers:
A medium access control (MAC) layer, and
The LLC layer, which operates on top of the MAC
layer.

48. LLC
Above Figure shows the structure of the combined MAC/LLC
frame;
The shaded portion corresponds to the fields produced at the
LLC layer.
The unshaded portions are the header and trailer of the MAC
frame.
The MAC layer includes source and destination addresses for
devices attached to the LAN.
Two addresses are needed as there is no concept of primary
and secondary in the LAN environment; therefore, both the
sender and receiver must be identified.
Error detection is done at the MAC level, using a 32-bit CRC.
Finally, there are some control functions peculiar to medium-
access control that may be included in a MAC control field.

49. Frame Relay
Frame relay is a data link control facility designed to provide a
streamlined capability for use over high-speed packet-
switched networks.
It is used in place of X.25,which consists of both a data link
control protocol (LAPB) and a network-layerprotocol (called
X.25 packet layer).
The data link control protocol defined for frame relay is LAPF
(Link Access Procedure for Frame-Mode Bearer Services).
There are actually two protocols: a control protocol, which has
similar features to HDLC, and a core protocol, which is a
subset of the control protocol.
There are several key differences between the LAPF control
protocol and HDLC.

50. Asynchronous Transfer Mode
(ATM)
Like frame relay, ATM is designed to provide a
streamlined data-transfer capability across high-
speed networks.
Unlike frame relay, ATM is not based on HDLC.
Instead, ATM is based on a completely new frame
format, known as a cell, that provides minimum
processing overhead.