Computer Networking
网络课件
双语教学
模拟实验
计算机网络教研室
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 after they are
broken is called error control.
 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.
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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.
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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 indicates
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
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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.
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Chapter 7 Data Link Control Protocol
课后习题
Exercises
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
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Answer
Chapter 7 Data Link Control Protocol
课后习题
Exercises
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
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Answer
Chapter 7 Data Link Control Protocol
课后习题
Exercises
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
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Answer
Chapter 7 Data Link Control Protocol
课后习题
Exercises
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
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Answer
Chapter 7 Data Link Control Protocol
课后习题
Exercises
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
课后习题
Exercises
HDLC
6.SDLC is a subset of ________.
Answer
7.BISYNC is defined to only support the 6-bit transcode (SBT),
ASCII .
EBCDIC, and _________
Answer
8.A set of rules that define the exact format of messages
exchanged between computers or between computers and
protocol
people is called a ______________
.
Answer
Chapter 7 Data Link Control Protocol
课后习题
Exercises
9. A technique used to ensure that a fast transmitting node does
not send data faster than a receiving node can receive and
flow control .
process it is called ___________
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
connection to the Internet, is called _____.
PPP
Answer
Chapter 7 Data Link Control Protocol
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