Datalink control(framing,protocols)

BrAinY 4
Presented to:-
Ma’am Sumbal and class
Group Members
ZUPASH
SANA
MARHABA
Group Leader
HiRA

DATA LINK CONTROL:-
Framing
Error control &flow controls
Protocols
Noiseless channel
Noisy channel
Topics to be discussed:-

FRAMING
The data link layer needs to pack bits into
frames. So that each frame is distinguishable
from another.
TYPES
Fixed size framing
Variable sized framing

•Data to be carried are of 8 bits character from
a coding system such as ASCII.
•The header carries the source and destination
addresses and other control information .
•Trailer which carries error detection or error
correction redundant bits, are also multiple of
8 bits.
•To separate one fame from the next an 8bits
frame is added at the beginning and at the end
of a frame.
Character oriented protocol:

A frame in character oriented protocol

11.10
Byte stuffing and unstuffing

Byte stuffing is the process of adding 1
extra byte whenever there is a flag or
escape character in the text
Note:

11.12
Bit stuffing is the process of adding one extra 0 whenever
five consecutive 1s follow a 0 in the data, so that the
receiver does not mistake
the pattern 0111110 for a flag.
Note

Bit oriented protocol:
the data section of a frame is a sequence
of bits to be Interpreted by the upper
layer as text, graphic, audio, video and
so on.
Figure shows a frame in a bit oriented
protocol.

 ERROR CONTROL AND FLOW
CONTROL
 PROTOCOLS
SANA AYUB

Data link control
 The most important responsibilities of the
data link layer are flow control and error
control. Collectively, these functions are
known as data link control.
 Data link control=flow +error control

Flow 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.
 Receiving device has
 Limited speed
 Limited amount of memory.
 So the sending device must not send frame at
a rate faster than a receiving station can
absorb them.

Error control
 Error control includes both error control
and error detection.
 Allows reciever to inform the sender if a
frame is lost or damaged during
transmission and coordinates the
retransmission of those frames by the
sender.
 Whenever an error is detected,specified
frames are retransmitted.

Protocols
 Now let us see how the data link layer can
combine framing, flow control, and error
control to achieve the delivery of data from
one node to another. The protocols are
normally implemented in software by using
one of the common programming languages.
To make our discussions language-free, we
have written in pseudocode a version of each
protocol that concentrates mostly on the
procedure instead of delving into the details of
language rules.

 NOISELESS CHANNEL
 simplest protocol
 stop-and-wait protocol
MARHABA WAZIR

1. NOISELESS CHANNEL
lets assume that we have an ideal
channel in which no frames are lost,
corrupted or duplicated.
We introduce two protocols for this
type channel:
a. Simplest protocol
b. STOP-and-WAIT protocol

The design of the simplest protocol with no
flow or error control
Figure shows an example of communication using this protocol

a.Simplest protocol
 It is very simple.
 The sender sends a sequence of frames
without even thinking about the receiver.
 To send three frames, three events occur at
the sender site and three events at the
receiver site.

 Note
that the data frames are shown by tilted
boxes; the height of the box defines the
transmission time difference between the
first bit and the last bit in the frame.

Flow diagram for simplest protocol

Stop-and-Wait Protocol
 Simple tokens ofACK and flow control added

Stop-and-Wait Protocol flow diagram

Stop-and-Wait protocol
 At most one packet in flight at any time.
 Sender sends one packet.
 Reciver sends acknowledgment packet when
it receive data.
THINKING CORNER
What type of a link we need here?
(Simplex, half duplex, full duplex)

NOISY CHANNEL
1) Stop-and-waitARQ
2) Go-back-N ARQ
3) Selective repeatARQ
HIRA ANWAR

Noisy channel
 Realistic
• error can and will happen.
• needs error control.
 Mechanisms
1) Stop-and-waitARQ
2) Go-back-N ARQ
3) Selective repeatARQ

Frames
Packet Error Free Packet
Transmitter Receiver
Information Frame
Control Frame
Timer is set after
each frame
transmission

38
Stop-and-Wait ARQ(Automatic
Repeat reQuest
 Sender keeps a copy of sent frame until
successful delivery is ensured.
 Receiver responds with an ack when it
successfully receives a frame.
 Both data and ack frames must be numbered.
 When sender does not receive an ack within
certain time, it assumes frame is lost, then
retransmits the same frame.

39
Stop-and-Wait ARQ
1. Normal operation
2. The frame is lost
3. The ACK is lost
4. The ACK is delayed

40
Flow Diagram: Normal Operation
Time
Sender Receiver
Time
S = 0
S = 1
R = 0
R = 1
R = 0
Deliver
Deliver

41
Flow Diagram: Lost Frame
Time
Sender Receiver
Time
S = 0 R = 0
R = 1
R = 0
S = 1
Timeout
Deliver
Deliver

42
Flow Diagram: Delayed ACK
Sender Receiver
S = 0 R = 0
R = 1
R = 0
Timeout
S = 1
S = 0
R = 1
Frame 0 expected;
discard
Deliver
Timeout
Deliver

 THINKINGCORNER: Why
Data frames & ACK frames need
to be sequentially numbered?

Need for Sequence Numbers
(a) Frame 1 lost
A
B
Frame
0
Frame
1
ACK
Frame
1
ACK
Time
Time-out
Frame
2
(b) ACK lost
A
B
Frame
0
Frame
1
ACK
Frame
1
ACK
Time
Time-out
Frame
2
ACK

45
Drawbacks of Stop-and-Wait
ARQ
 After each frame sent the host must wait for an ACK.
 inefficient use of bandwidth.
 To improve efficiencyACK should be sent after
multiple frames.
 Alternatives: SlidingWindow protocols
1. Go-back-NARQ
2. Selective Repeat ARQ

46
Pipelining
 One task begins before the other one ends.
 increases efficiency in transmission.
 There is no pipelining in Stop-and-Wait ARQ.
 In sliding window protocol pipelining is
implemented.

Sliding window
 The sliding window is an theoretical concept
that defines the range of sequence numbers
that is the concern of the sender and receiver.

2.GO BACK N ARQ
(Automatic Repeat
reQuest)

49
(sliding window protocol)
Go-Back-N ARQ
 Allows multiple frames to be sent before
waiting for ACK.
 These frames must be numbered differently.
 Frame numbers are called Sequence numbers.
 Frames must be received in the correct order.
 If a frame is lost, the lost frame and all of the
following frames must be retransmitted.

51
The name of Go-back-N: why?
 Re-sending frame
 when the frame is damaged the sender goes back and sends
a set of frames starting from the last one ACKn’d
 the number of retransmitted frames is N
 Example:
 The window size is 4.
 A sender has sent frame 6 and the timer expires for
frame 3 (frame 3 not ACKn’d).The sender goes back
and re-sends the frames 3, 4, 5 and 6.

Sequence numbers
 Frame header contains m bits for sequence
number.
 That allows up to 2m different frame
numbers.
 We can repeat sequence numbers.

53
Sending Window
 Sending more than one frame at once requires
sender to buffer multiple frames.
 Known as "sending window"
 Any of these frames in the window can be lost.

54
"Sliding" Window
 Once the first frames in the window is ACKed
 ACKed frames are removed from the buffer
 More frames are transmitted
 Result:The window slides to the right

55
Receiving Window
 Receiver expects one frame at a time

56
Go-back-N
-Control variables-
 S- holds the sequence number of the recently sent frame
 SF – holds sequence number of the first frame in the window
 SL – holds the sequence number of the last frame
 R – sequence number of the frame expected to be received

57
Go-Back-N: Window Sizes
 For m-bit sequence numbers.
 Send window size: at most 2m-1.
 Up to 2m-1 frames can be sent without ACK.
 Receive window size: 1
 Frames must be received in order.

58
Go-Back-N: Normal Operation

59
Go-Back-N: Lost Frame
ACKs are
cumulative

60
Go-back-N
 Inefficient
 all out of order received packets are discarded
 This is a problem in a noisy link
 many frames must be retransmitted -> bandwidth
consuming
 Solution
 re-send only the damaged frames
 Selective RepeatARQ
 avoid unnecessary retransmissions

61
 Thinking Corner
 Stop-and-Wait is a special case of Go-Back-N.
 What is the send window size in Stop-and-Wait?

63
Selective Repeat ARQ
 Go-Back-N always discards out-of-order
frames
 Losing one frame may result in retransmission of
multiple frames
 Very inefficient in noisy link
 Selective Repeat ARQ allows frames to be
received out of order
 Therefore, receive window > 1

64
Send and Receive Windows
 Sender and receiver share window space
equally
 For m-bit sequence numbers
 Send window: up to 2m-1
 Receive window: up to 2m-1

67
Negative ACK
 Used by receiver to indicate missing frame

68
Selective Repeat: Window
Size

69
PIGGY BACKING
(Bidirectional Transmission)
 Data are transferred both ways
 ACK are "piggybacked" with data frames

links
 https://youtu.be/Ms1XEMZbjgM
 https://youtu.be/Cs8tR8A9jm8
 https://youtu.be/ovEQhPCbSGI

Datalink control(framing,protocols)

More Related Content

What's hot (20)

Viewers also liked (18)

Similar to Datalink control(framing,protocols) (20)

Recently uploaded (20)

Datalink control(framing,protocols)