Exercises Chapter 7 Data Link Control Protocol 课后

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Transcript Exercises Chapter 7 Data Link Control Protocol 课后

Computer Networking

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CHAPTER 7 DATA LINK CONTROL PROTOCOL

1 The first section

2

Exercises

3

Online lecture Department of Computer Networking Application

Chapter 7 Data Link Control Protocol

7.1 INTRODUCTION

A set of rules that define the

exact format of

messages

exchanged

between computers or between computers and people is called a

protocol

.

Data link protocols

are necessary so that all of the devices on the network are operating by the same set of rules and are able to communicate with each other.

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Chapter 7 Data Link Control Protocol 7.2 DATA LINK PROTOCOLS FUNCTIONS

Communication startup

Character identification and framing

Message identification The DTE must separate

the characters on

the communications line into messages

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Chapter 7 Data Link Control Protocol 7.2 DATA LINK PROTOCOLS FUNCTIONS

Line Control

The part of a protocol consisting of the rules that specify the way the receiving terminal signals the sending terminal if it has received data correctly, how and under what

circumstances

the line will be turned around, and whether the receiving terminal can accept more data is called line control.

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Chapter 7 Data Link Control Protocol 7.2 DATA LINK PROTOCOLS FUNCTIONS

Error Control

The part of the protocol consisting of rules specifying what happens when an error is detected, what to do if communications suddenly and

unexplainably cease , and the way communications are reestablished

broken is called error control .

after they are

Termination

The part of a protocol consisting of rules for ending the communications under normal and abnormal

circumstances

is called termination .

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Chapter 7 Data Link Control Protocol

7.3 DESIRABLE ATTRIBUTES OF DATA LINK PROTOCOLS

Transparency

The ability of data terminal equipment

(DTE) to be able to transmit and receive any bit pattern as data is called transparency .

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Chapter 7 Data Link Control Protocol

7.3 DESIRABLE ATTRIBUTES OF DATA LINK PROTOCOLS

Code independence

The protocol allow the transmission of data from any data coding system.

Efficiency

The protocol should use as few characters as possible to control the data transmission so that most of

the line capacity

can be used for actual data transmission.

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Chapter 7 Data Link Control Protocol

7.4 PROTOCOL IMPLEMENTION A data link protocol is implemented

by transmitting

certain bit patterns or characters on the communications

circuit.

7.5 PREDETERMINED COMMUNICATIONS PARAMETERS

Predetermined communication parameters are those

that are set manually by switches or specified as parameters in software.

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Chapter 7 Data Link Control Protocol

7.7 LINE ACCESS

When two stations on a circuit transmit at the same

time, a(n) collision

occurs.

There are three primary ways that circuit access can

be obtained

:

contention,polling,and token passing .

7.7.1 Contention

When any

station

that has traffic looks to see if the circuit is free, and if it is, begins sending its traffic the line control technique is called contention . www.gxmu.edu.cn

Chapter 7 Data Link Control Protocol Contention systems work best _____.

a. on circuits that have only two DTEs b. when the speed of the circuit is relatively fast c. on circuits that have

not too heavy message traffic

7.7.2

Polling

When a

central or control station

asks each station on the circuit or network if it has traffic to send it is using a line control technique called polling . www.gxmu.edu.cn

Chapter 7 Data Link Control Protocol 7.7.2

Polling

Roll call polling Roll call polling is the most common implementation of a polling system.

In order to know what stations to poll and in what sequence, the master station uses a

polling list

.

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Chapter 7 Data Link Control Protocol 7.7.2

Polling

Fast select polling

Fast select polling speeds up the polling

process because a slave station

that doesn’t have traffic isn’t required to return a character to the

master station

.

Hub polling

P128

7.7.3 Token passing

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Chapter 7 Data Link Control Protocol

7.8 MESSAGE FORMAT

Data messages normally consist of three

parts: the header, text, and trailer

. The header of a message contains and conveys information about the message, such as

the destination node’s address , a sequence number , and perhaps a date and time.

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Chapter 7 Data Link Control Protocol Figure 7-1 Message format showing examples of the kinds of control characters the protocol might employ.

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Chapter 7 Data Link Control Protocol

7.9 FLOW CONTROL

Techniques to ensure that a fast transmitting node does not send data faster than the receiving node can receive and process it are called flow control .

7.9.1 Stop-and-wait Flow Control

Stop-and-wait Flow Control is the simplest form control.

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Chapter 7 Data Link Control Protocol Figure 7-2 The data flow of stop-and-wait flow control.

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Chapter 7 Data Link Control Protocol

7.9.2

Sliding Window Flow Control

The flow control technique that allows

multiple frames to be in transit on the line at

one time is called sliding window .

There are two common implementations

of

sliding window flow control: go-back-n and selective repeat .

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Chapter 7 Data Link Control Protocol Figure 7-3 The data flow in a simple sliding window flow control.

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Chapter 7 Data Link Control Protocol

7.10 ASYNCHRONOUS DATA LINK PROTOCOLS Asynchronous data link protocols deal with individual characters rather than blocks of data .

7.10.1 Point-to-Point Protocol(PPP)

A protocol that is primarily used by PC workstations to

send IP over dial-up lines , typically when dialing in to an ISP

for connection to the Internet, is called PPP.

PPP has largely replaced the

SLIP

protocol because it

has better error detection .

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Chapter 7 Data Link Control Protocol Figure 7-4 PPP’s frame format.

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Chapter 7 Data Link Control Protocol

7.10.2 XMODEM Protocol

The XMODEM protocol was developed for use between PCs, especially for transfers of data files between them.

7.10.3 Other Asynchronous Protocols A sophisticated check character at the end of each block of data is called a cyclic redundancy check(CRC) .

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Chapter 7 Data Link Control Protocol

7.11

SYNCHRONOUS DATA LINK PROTOCOLS

Synchronous data link protocols typically deal with blocks of data,not individual characters.

The three types are

character-oriented protocol

,

byte-count-oriented protocol

,and

bit-oriented protocols

.

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Chapter 7 Data Link Control Protocol 7.11.1 Character-Oriented Protocols A character-oriented protocol uses special

characters to indicate

the beginnings and ends of messages.

The best known character-oriented protocol

is the Binary Synchronous Communications Protocol(BSC) ,also known as BISYNC.

BISYNC is defined to only support the 6-bit transcode (SBT) , EBCDIC, and ASCII .

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Chapter 7 Data Link Control Protocol 7.11.2 Byte-Count-Oriented Protocols Byte-count-oriented protocols have a special characters to mark the beginning of the

header,followed by a count field that

indicate

s

how many characters are in the

data portion of the messages

.

The best known byte-count-oriented

protocol is the

Digital Data Communications Message Protocol(DDCMP)

.

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Chapter 7 Data Link Control Protocol 7.11.3 Bit-Oriented Protocols A bit-oriented protocol uses only one special

character,called the flag character

,which marks the beginning and end of parts of the message. The best known bit-oriented protocols are the

ISO’s High-Level Data Link Control (HDLC)

protocol. SDLC is a subset of HDLC. 7.12 OTHER DATA LINK PROTOCOLS P143 www.gxmu.edu.cn

Chapter 7 Data Link Control Protocol

7.13

PROTOCOL CONVERSION

A protocol converter changes one protocol to another, when, for example, a message is sent between two networks that use different protocols. www.gxmu.edu.cn

Chapter 7 Data Link Control Protocol

1.When an HDLC node receives a flag character, it knows that (

C

) .

× A 、 an error has occurred × B 、 it should signal the sender to stop sending traffic

C 、 a frame is beginning or ending × D 、 it should switch to transparent mode Redo Next Answer

Chapter 7 Data Link Control Protocol

2.When two stations on a circuit transmit at the same time, a(n) (

D

) occurs.

× A 、 division × B 、 polling × C 、 bit stuffing

D 、 collision Redo Next Answer

Chapter 7 Data Link Control Protocol

3.In order to know what stations to poll and in what sequence, the master station uses a (

B

) .

× A 、 message header

B 、 polling list × C 、 start of header character × D 、 cyclic redundancy check Redo Next Answer

Chapter 7 Data Link Control Protocol

4. Go-back-n and selective repeat are two common implementations of (

D

) .

× A 、 bit oriented protocols × B 、 byte protocols × C 、 byte-count oriented protocols

D 、 sliding window flow control Redo Next Answer

Chapter 7 Data Link Control Protocol √

5. When a receiver must acknowledge every block of data before the next block is sent, the (

A

) type of flow control is being used.

A 、 stop-and-wait × B 、 stop-and-check × C 、 sliding frame × D 、 sliding window Redo Next Answer

Chapter 7 Data Link Control Protocol

Answer 7.BISYNC is defined to only support the 6-bit transcode (SBT), Answer 8.A set of rules that define the exact format of messages exchanged between computers or between computers and Answer

Chapter 7 Data Link Control Protocol

9. A technique used to ensure that a fast transmitting node does not send data faster than a receiving node can receive and Answer 10.A protocol that is primarily used by PC workstations to send IP over dial-up lines, typically when dialing in to an ISP for Answer

Chapter 7 Data Link Control Protocol www.gxmu.edu.cn