chap11.ppt

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Data Communications, Kwangwoon Univers

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Chapter 11 Data Link Control

1.

Framing

2.

Flow and Error Control

3.

Protocols

4.

Noiseless Channels

5.

Noisy Channels

6.

HDLC

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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

• 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

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Character-Oriented Protocols

•

Frame structure

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Bit-Oriented Protocols

•

Frame structure

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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 sen

der can send before waiting for acknowledgement

•

Error control

in the data link layer is based on

automatic repeat request

(ARQ), which i

s the retransmission of data

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Noiseless Channels: Simplest Protocol

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Simplest Protocol

• Sender-site algorithm

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Stop-and-Wait Protocol

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Stop-and-Wait Protocol

• Sender-site algorithm

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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

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Stop-and-Wait ARQ

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Stop-and-Wait ARQ

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Go-Back-N ARQ

• Pipelining improves the efficiency of the transmission

• In the Go-Back-N Protocol, the sequence numbers are modulo 2

m

, where m is the siz

e of the sequence number field in bits

• The send window is an abstract concept defining an imaginary box of size 2

m

− 1 wit

h three variables: S

_f

, S

_n

, and S

_size

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Go-Back-N ARQ

• Receive window for Go-Back-N ARQ

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Go-Back-N ARQ

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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}

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Selective Repeat ARQ

• Sender window size

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Selective Repeat ARQ: Window Size

• The size of the sender and receiver window must be at most one-half of

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Piggybacking

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HDLC

• High-level Data Link Control

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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 synchronizati on pattern for receiver

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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

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HDLC: Frames

• Control field for S-frame

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HDLC: Frames

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HDLC: Example 1

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HDLC: Example 2

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HDLC: Example 3

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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

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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

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PPP: Multiplexing

• PPP uses another set of other protocols to establish the link, authenticate

the parties, and carry the network layer data

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LCP: Common Options

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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)

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Challenge Handshake Authentication

Protocol (CHAP)

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Network Control Protocol:

NCP

• PPP is a multiple-network layer protocol.

• It can carry a network data packet from protocols defined by the Intern

et, OSI, Xerox, DECnet, AppleTalk, Novel

• IPCP (IP Control Protocol)

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IPCP Packet

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