Chapter 11 Data Link Control

Data Communications, Kwangwoon University

11-1

Chapter 11 Data Link Control

1. Framing 2. Flow and Error Control3. Protocols4. Noiseless Channels5. Noisy Channels6. HDLC7. Point-to-Point Protocol


11-2

Framing

• Data link layer needs to pack bits into frames, so that each Data link layer needs to pack bits into frames, so that each frame is distinguishable from anotherframe 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

• Fixed-size framing: ATM (Chapter 18)

• Variable-size framing

– Need a way to define the end of the frame and the beginning of the next

– Character-oriented approach and bit-oriented approach


11-3

Character-Oriented Protocols

• Frame structureFrame structure

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


11-4

Bit-Oriented Protocols

• Frame structureFrame structure

• Bit stuffing: process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data


11-5

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), whi

ch is the retransmission of data• ACK, NAK(Negative ACK), Piggybacking (ACKs and NAKs in data frames)


11-6

Noiseless Channels: Simplest Protocol

• Simplest protocol with no flow or error control


11-7

Simplest Protocol

• Sender-site algorithm

• Receiver-site algorithm


11-8

Stop-and-Wait Protocol

• Simple tokens of ACK and flow control added


11-9

Stop-and-Wait Protocol• Sender-site algorithm

• Receiver-site algorithm


11-10

Stop-and-Wait Protocol: Example


11-11

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.


11-12

Stop-and-Wait ARQ


11-13

Stop-and-Wait ARQ• Sender-site algorithm


11-14

Stop-and-Wait ARQ• Receiver-site algorithm


11-15

Stop-and-Wait ARQ: Example


11-16

Go-Back-N ARQ• Pipelining improves the efficiency of the transmission• In the Go-Back-N Protocol, the sequence numbers are modulo 2m, where m is the size

of the sequence number field in bits• The send window is an abstract concept defining an imaginary box of size 2m − 1 wit

h three variables: Sf, Sn, and Ssize

• The send window can slide one or more slots when a valid acknowledgment arrives.


11-17

Go-Back-N ARQ• Receive window for Go-Back-N ARQ• 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.


11-18

Go-Back-N ARQ• Sliding windows, Timers, ACK, Resending a frame


11-19

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


11-20

Go-Back-N ARQ: Sender Algorithm


11-21

Go-Back-N ARQ: Receiver Algorithm


11-22

Go-Back-N ARQ: Example 1


11-23

Go-Back-N ARQ: Example 2


11-24

Selective Repeat ARQ• Sender window size

• Receive window size


11-25

Selective Repeat ARQ


11-26

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

of 2m


11-27

Selective Repeat ARQ: Sender-Site Algorithm


11-28

Selective Repeat ARQ: Receiver-Site Algorithm


11-29

Selective Repeat ARQ: Example


11-30

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


11-31

HDLC• High-level Data Link Control• Two common transfer mode: normal response mode (NRM) and

asynchronous balanced mode (ABM)


11-32

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 synchroniz

ation pattern for receiver• FCS field: 2- or 4-byte ITU-T CRC for error detection


11-33

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)


11-34

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

reject (SREJ)


11-35

HDLC: Frames• Control field for U-frame


11-36

HDLC: Example 1• Connection and disconnection


11-37

HDLC: Example 2• Piggybacking without error


11-38

HDLC: Example 3• Piggybacking with error


11-39

HDLC: Bit Stuffing and Unstuffing


11-40

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


11-41

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


11-42

PPP: Transition States


11-43

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


11-44

LCP: Encapsulated in a Frame


11-45

LCP: Common Options

• Options are inserted in the information field of the configuration packets


11-46

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)


11-47

Password Authentication Protocol (PAP)


11-48

Challenge Handshake Authentication Protocol (CHAP)

• Three-way hand-shaking authentication protocol with greater security than PAP


11-49

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


11-50

IPCP Packet

IP Datagram in a PPP frame


11-51

Multiple PPP


11-52

Example (1)


11-53

Example (2)

Documents

Chapter 11 Data Link Control