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Data Link Layer:
UNIT-III
Data Link Control

2
Outline
 Framing
 Byte vs. bit oriented protocols
 Data Link Control
 Flow control
 Error control
 Protocols

3
Framing
 Process of wrapping data with certain info before
sending out
 A frame typically consists of
 Flag: indication for start and end of a frame
 Header: source/destination addresses, as well as other
control information
 Data from the upper layer
 Trailer: error detection/correction code

4
Byte vs. Bit Oriented
 Framing in byte-oriented protocols
 Framing in bit-oriented protocols

5
Byte Stuffing
 Process of adding extra byte whenever there is
an escape or a flag character in the data

6
Bit Stuffing
 Process of adding extra bit to ensure flag
sequence does not appear in the data
01111110

7
Flow Control and Error Control
 Flow control
 A set of procedures that tells the sender how
much data can be sent before waiting for
acknowledgment
 Error control
 Includes both error detection and correction
 Allows receiver to inform sender of lost or
duplicate frames
 Mostly based on Automatic Repeat Request
(ARQ)

9
Protocols for Noiseless Channel
 Assuming channel is error free
 Not realistic…
 No need for error control

10
"Simplest" Mechanism
 Assuming
 Noiseless channel
 Unlimited buffer and speed for the receiver

11
"Simplest" : Pseudo Code
 Sender
 Receiver

13
Stop-and-Wait Mechanism
 Still noiseless channel
 Receiver has limited buffer
 Requires flow control
 Sender sends one frame at a time and
wait for an acknowledgment

15
Stop-and-Wait: Pseudo Code
 Sender side

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Stop-and-Wait: Pseudo Code
 Receiver side

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Stop-and-Wait: Flow Diagram

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Noisy Channel
 Realistic
 Error can and will happen
 Require error control
 Mechanisms:
 Stop-and-Wait ARQ
 Go-Back-N ARQ
 Selective Repeat ARQ

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Stop-and-Wait ARQ
 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.

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

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 Thinking Corner
 Why data frames need to be numbered?

23
Flow Diagram: Lost Frame
Time
Sender Receiver
Time
S = 0 R = 0
R = 1
R = 0
S = 1
Timeout
Deliver
Deliver

24
Flow Diagram: Lost ACK
Time
Sender Receiver
Time
S = 0 R = 0
R = 1
R = 0
S = 1
Timeout S = 1
S = 0
R = 0
Frame 0 expected;
discard
Deliver
Deliver

25
 Why ACK frames need to be numbered?

26
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

27
Bidirectional Transmission
 Data are transferred both ways
 ACK are "piggybacked" with data frames

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Example
 Assuming a communication system where:
 Stop-and-Wait ARQ is used
 Bandwidth of the link is 1 Mbps
 Propagation delay is 10 ms
 One-way data flow
 Questions
 What is the bandwidth-roundtrip-delay product?
 If the system data frames are 1000 bits in length,
what is the utilization percentage of the link?

29
Improving Link Utilization
 Previous example demonstrates major
disadvantage of Stop-and-Wait ARQ
 Prefer to send more frames before waiting
for ACK
 Example:
 Recalculate the link utilization if we allow up
to 15 frames to be sent before waiting for an
ACK

30
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

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Sequence Numbers
 Frame header contains m bits for
sequence number
 That allows up to 2m different frame
numbers
 How big should m be?

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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

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"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

34
Receiving Window
 Receiver expects one frame at a time

36
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

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Go-Back-N: Normal Operation

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Go-Back-N: Lost Frame
ACKs are
cumulative

40
 What is a problem if send window is
greater than 2m-1?

42
 Stop-and-Wait is a special case of Go-
Back-N.
 What is the send window size in Stop-and-
Wait?

43
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

44
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

47
Negative ACK
 Used by receiver to indicate missing frame

48
Selective Repeat: Window Size

Data Link Control Protocols:
HDLC and PPP

50
HDLC
 High-level Data Link Control
 Bit-oriented protocol
 Support both
 Point-to-point links
 Multipoint links

51
Normal Response Mode
 "NRM" mode
 Used in both point-to-point and multi-point link

52
Asynchronous Balance Mode
 "ABM"
 Supports only point-to-point links
 Each station is both primary and
secondary

53
HDLC Frames
 Information frame (I-frame)
 Supervisory frame (S-frame)
 Unnumbered frame (U-frame)
Frame Check Sequence
(error detection code)

54
Control Field Format
 N(S) – Frame sequence number
 N(R) – Ack sequence number
 P/F
 Poll (primary  secondary)
 Final (secondary  primary)

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Connection and Disconnection

59
PPP
 Point-to-Point Protocol
 Byte-oriented protocol
 Most common protocol for point-to-point
access
 Dial-up access
 ADSL
 GPRS/EDGE/3G

60
PPP Frame Format
 Escape byte: 01111101
 Control field uses HDLC's U-frame format
 No flow or error control

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PPP Stack
 Link Control Protocol (LCP)
 Authentication Protocol (AP)
 Network Control Protocol (NCP)

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LCP: Link Control Protocol
 Responsible for establishing, maintaining,
configuring, and terminating links

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PPP Authentication
 Two protocols are supported:
 Password Authentication Protocol (PAP)
 Challenge Handshake Authentication Protocol
(CHAP)

68
NCP: Network Control Protocol
 A set of control protocols to allow data from the
network layer to be encapsulated into a PPP
frame
 One common protocol: IPCP
(Internetwork Protocol Control Protocol)
 Allow negotiation at the network layer

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Example: PPP Session (cont'd)

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