Data link control involves framing data, flow and error control, and common protocols like HDLC and PPP. Framing involves adding source/destination addresses to frames for transmission. Flow control restricts how much data the sender sends before waiting for acknowledgment. Error control uses techniques like automatic repeat request to retransmit lost data frames. HDLC and PPP are protocols that define frame formats and control procedures for point-to-point links.

1. Data Link Control
1. Framing
2. Flow and Error Control
3. Protocols
4. Noiseless Channels
5. Noisy Channels
6. HDLC
7. Point-to-Point Protocol

2. Framing
• Data link layer needs to pack bits into frames, so that each
frame is distinguishable from another
• Separate a message from one source to a destination, or
from other messages to other destinations, by adding a
sender address and a destination address

3. Fixed-size framing: there is no need for defining the
boundaries of the frames.
Variable-size framing
Need a way to define the end of the frame and the
beginning of the next
Used in LANs
Two Approaches
Character-oriented approach and bit-oriented approach

4. Character-Oriented Protocols
• Frame structure
Header carries the source and destination address and some control
information.
Trailer carries error detection and correction bits
To separate one from the next, an 8 bit flag is added at the beginnin
g and end of each frame

5. • Any pattern used for flag can also be part of the information, i
n that case the receiver can detect end of frame in middle, thin
k it has reached at the end. To fix this problem byte stuffing is
used.
• In this a special byte called ESC (Escape character) is added b
efore to the flag byte. Whenever the receiver encounters ESC
byte, it is removed and the next byte is treated as data not as
delimeter.
• Again problem if there is ESC in data, then one ESC before
data ESC.

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

7. Bit-Oriented Protocols
• Frame structure
• Bit stuffing: process of adding one extra 0 whenever five consecutive
1s follow a 0 in the data

8. Flow and Error Control
• Data link control = flow control + 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 acknowledgement
• Error control in the data link layer is based on automatic repeat request
(ARQ), which is the retransmission of data
• All the protocols are unidirectional i.e. the data frames travel from source
to destination node. Only ACK, NAK(Negative ACK) frames can flow in
opposite direction.

9. Noiseless Channels: Simplest Protocol
• Simplest protocol with no flow or error control. The receiver
immediately handle the frame.

10. Simplest Protocol
• Sender-site algorithm
• Receiver-site algorithm

11. Stop-and-Wait Protocol
• Simple tokens of ACK and flow control added

12. Stop-and-Wait Protocol
• Sender-site algorithm
• Receiver-site algorithm

13. Stop-and-Wait Protocol: Example

14. Noisy Channels: Stop-and-Wait ARQ
• Stop-and-wait Automatic Repeat Request (ARQ)
• Error correction in Stop-and-Wait ARQ is done by keeping
a copy of the sent frame and retransmitting of the frame
when the timer expires
• In Stop-and-Wait ARQ, we use sequence numbers to
number the frames. The sequence numbers are based on
modulo-2 arithmetic
• Acknowledgment number always announces in modulo-2
arithmetic the sequence number of the next frame expected.

15. Stop-and-Wait ARQ

16. Stop-and-Wait ARQ
• Sender-site algorithm

17. Stop-and-Wait ARQ
• Receiver-site algorithm

18. Stop-and-Wait ARQ: Example

19. Go-Back-N ARQ
• Pipelining : A task is often begun before the previous task
has ended. To improve the efficiency of transmission
multiple frames must be in trasition while waiting for
acknowledgement
• In the Go-Back-N Protocol, the sequence numbers are in
sequence using modulo 2m, where m is the size of the
sequence number field in bits. If m= 4, then sequence
numbers will be from 0 to 15. after that the sequence will
be repeated.
• In this protocol the abstract concept of sliding window is
used, that defines the range of sequence numbers that is the
concern of sender and the receiver.

20. The range which is the concern of the sender is called the send
sliding window.
The range which is the concern of the sender is called the receive
sliding window
The send window is an abstract concept defining an imaginary
box of size 2m − 1 with three variables:
Sf, Sn, and Ssize
The send window can slide one or more slots when a valid
acknowledgment arrives.
The send window divides the possible sequence numbers in four
regions.
The send window can slide one or more slots when a valid ackno
wledgement arrives.

22. • The receive window is an abstract concept defining an
imaginary box of size 1 with one single variable Rn. The
window slides when a correct frame has arrived; sliding
occurs one slot at a time.

23. • Sliding windows, Timers, ACK, Resending a frame

24. Go-Back-N ARQ: Send Window Size
• In Go-Back-N ARQ, the size of the send window must be less than 2m;
the size of the receiver window is always 1
• Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the
size of the send window is 1

25. Go-Back-N ARQ: Sender Algorithm

27. Go-Back-N ARQ: Receiver Algorithm

28. Go-Back-N ARQ: Example 1

29. Go-Back-N ARQ: Example 2

30. Selective Repeat ARQ
Go back N ARQ simplifies the process at the receiver site. The
receiver keeps track of only one variable and there is no need to
buffer out of order frames, they are simply discarded.
However this protocol is very inefficient for a noisy link, where a
frame has higher probability of damage, which means resending of
the multiple frames. This resending uses up the bandwidth and slows
down the transmission.
For noisy links there is another mechanism that does not resend N
frames when just one frame is damaged, only the damaged frame is
resent. This mechanism is called Selective ARQ. It is more efficient
for noisy links, but the processing at the receiver side is more
complex.

31. Selective Repeat ARQ
• Sender window size
The size of the send window is much smaller i.e 2m-1
If m =4, the sequence numbers will be from 0 to 15, but the size of
window is just 8.

32. • Receive window size
The size of the receive window is the same as the send
window. The colored frames are the frames received out of
order and are waiting for their neighbors.

33. Selective Repeat ARQ

34. Selective Repeat ARQ: Window Size
• The size of the sender and receiver window must be at most one-half
of 2m

35. Selective Repeat ARQ: Sender-Site Algorithm

37. Selective Repeat ARQ: Receiver-Site Algorithm

39. Selective Repeat ARQ: Example

40. Piggybacking
• To improve the efficiency of the bidirectional protocols
• Piggybacking in Go-Back-N ARQ

41. HDLC
• High-level Data Link Control
• Two common transfer mode: normal response mode (NRM) and
asynchronous balanced mode (ABM)

42. HDLC: Frames
• I(information)-frames, S(supervisory)-frames, U(unnumbered frame)-
frames
• Flag field: 01111110 to identify both the beginning and the end of a
frame and serve as synchronization pattern for receiver
• FCS field: 2- or 4-byte ITU-T CRC for error detection

43. HDLC: Frames
• Control Field: 1- or 2-byte segment of the frame used for flow and
error control
• Determine the type of frame and define its functionality
• Control field for I-frame: P/F (poll/final bit for primary/secondary)

44. HDLC: Frames
• Control field for S-frame
• Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective
reject (SREJ)

45. HDLC: Frames
• Control field for U-frame

46. HDLC: Example 1
• Connection and disconnection

47. HDLC: Example 2
• Piggybacking without error

48. HDLC: Example 3
• Piggybacking with error

49. HDLC: Bit Stuffing and Unstuffing

50. Point-to-Point Protocol: PPP
• One of the most common protocols for point-to-point access
• Many Internet users who need to connect their home computer to the
server of an Internet service provider use PPP
• A point-to-point link protocol is required to control and manage the
transfer of data
• PPP defines/provides
– the format of the frame to be exchanged between devices
– how two devices negotiate the establishment of the link and the exchange of data
– how network layer data are encapsulated in the data link frame
– how two devices can authenticate each other
– multiple network layer services
– connection over multiple links
– Network address configuration
• But, several services are missing for simplicity
– no flow control, simple error control (detection and discard), no sophisticate
addressing for multipoint configuration

51. PPP Frame
• Flag: 01111110 the same as HDLC, but it treated as a byte because of
PPP is a byte-oriented protocol
• Address: 11111111 (broadcast address)
• Control: No need because PPP has no flow control and limited error
control
• PPP is a byte-oriented protocol using byte stuffing with the escape byte
01111101

52. PPP: Transition States

53. PPP: Multiplexing
• PPP uses another set of other protocols to establish the link, authenticate
the parties, and carry the network layer data
• Three sets of protocols defined for powerful PPP: LCP, two APs, several
NCPs

54. LCP: Encapsulated in a Frame

55. LCP: Common Options
• Options are inserted in the information field of the
configuration packets

56. Authentication
• Authentication means validating the identity of a user who
needs to access
• PPP is designed for use over dial-up links
 User authentication is necessary
• PPP has two protocols for authentication
– Password Authentication Protocol (PAP)
– Challenge Handshake Authentication Protocol
(CHAP)

57. Password Authentication Protocol (PAP)

58. Challenge Handshake Authentication
Protocol (CHAP)
• Three-way hand-shaking authentication protocol with greater security
than PAP

59. Network Control Protocol: NCP
• PPP is a multiple-network layer protocol.
• It can carry a network data packet from protocols defined by the
Internet, OSI, Xerox, DECnet, AppleTalk, Novel
• IPCP (IP Control Protocol)
– Configures the link used to carry IP packets in the Internet

60. IPCP Packet
IP Datagram in a PPP frame

61. Multiple PPP

62. Example (1)

63. Example (2)