Data Link Control Framing

8/12/2019 Data Link Control Framing

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11.1

Chapter 11Data Link Control

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


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11.2

11-1 FRAMING

The data l ink layer needs to pack bits into frames, so

that each f rame is distinguishable from another.

Fixed-Size Framing

Variable-Size Framing

Topics discussed in this section:


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11.3

Figure 11.1 A frame in a character-oriented protocol


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11.4

Figure 11.2 Byte stuff ing and unstuf f ing


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11.5

Byte stuffing is the process of adding 1extra byte whenever there is a flag or

escape character in the text.

Note


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11.6

Figure 11.3 A frame in a bit-ori ented protocol


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Bit stuffing is the process of adding one

extra 0 whenever five consecutive 1s

follow a 0 in the data, so that the

receiver does not mistake

the pattern 0111110 for a flag.

Note


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Figure 11.4 Bi t stuff ing and unstuff ing


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11-2 FLOW AND ERROR CONTROL

The most important responsibil i ties of the data link

layer are f low controland error control. Collectively,

these functions are known as data l ink control.

Flow Control

Error Control



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Flow control refers to a set of proceduresused to restrict the amount of data

that the sender can send before

waiting for acknowledgment.

Note


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Error control in the data link layer isbased on automatic repeat request,

which is the retransmission of data.

Note


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11-3 PROTOCOLS

Now let us see how the data link layer can combine

framing, flow control, and error control to achieve thedelivery of data from one node to another.


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Figure 11.5 Taxonomy of protocols discussed in this chapter


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11-4 NOISELESS CHANNELS

Let us first assume we have an ideal channel in which

no frames are lost, duplicated, or corrupted. We

introduce two protocols for this type of channel.

Simplest ProtocolStop-and-Wait Protocol



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Figure 11.6 The design of the simplest protocol wi th no flow or error control


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Algorithm 11.1 Sender-site algor ithm for the simplest protocol


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Algorithm 11.2 Receiver-site algori thm for the simplest protocol


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F igure 11.7 shows an example of communication using

this protocol. I t is very simple. The sender sends a

sequence of frames without even thinking about the

receiver.

Example 11.1


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Figure 11.7 F low diagram for Example 11.1


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11.20

Figure 11.8 Design of Stop-and-Wait Protocol


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11.21

F igure 11.9 shows an example of communication using

this protocol. I t is sti l l very simple. The sender sends one

frame and waits for feedback from the receiver. When the

ACK arr ives, the sender sends the next frame. Note that

sending two frames in the protocol involves the sender infour events and the receiver in two events.

Example 11.2


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11.22



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11.23

11-5 NOISY CHANNELS

Although the Stop-and-Wait Protocol gives us an idea

of how to add f low control to i ts predecessor, noiseless

channels are nonexistent. We discuss three protocols

in this section that use error control.

Stop-and-Wait Automatic Repeat RequestGo-Back-N Automatic Repeat Request

Selective Repeat Automatic Repeat Request



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11.24

Error correction in Stop-and-Wait ARQ is

done by keeping a copy of the sentframe and retransmitting of the frame

when the timer expires.

Note


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11.25

In Stop-and-Wait ARQ, we use sequence

numbers to number the frames.The sequence numbers are based on

modulo-2 arithmetic.

Note


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11.26

In Stop-and-Wait ARQ, the

acknowledgment number always

announces in modulo-2 arithmetic the

sequence number of the next frame

expected.

Note


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11.27

Figure 11.10 Design of the Stop-and-Wait ARQ Protocol


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11.28

Algorithm 11.5 Sender-site algor ithm for Stop-and-Wait ARQ

(cont inued)


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Algorithm 11.5 Sender-site algor ithm for Stop-and-Wait ARQ (cont inued)


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11.30

Algorithm 11.6 Receiver-site algor ithm for Stop-and-Wait ARQ Protocol

E l 11 3


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11.31

F igure 11.11 shows an example of Stop-and-Wait ARQ.

F rame 0 is sent and acknowledged. Frame 1 is lost and

resent after the time-out. The resent frame 1 is

acknowledged and the timer stops. Frame 0 is sent and

acknowledged, but the acknowledgment is lost. Thesender has no idea if the frame or the acknowledgment

is lost, so after the time-out, it resends frame 0, which is

acknowledged.

Example 11.3


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11.33

In the Go-Back-N Protocol, the sequence

numbers are modulo 2m,where m is the size of the sequence

number field in bits.

Note


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11.34

Figure 11.12 Send window for Go-Back-N ARQ


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11.35

The send window can slide oneor more slots when a valid

acknowledgment arrives.

Note


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11.36

Figure 11.13 Receive window for Go-Back-N ARQ


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11.37

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.

Note


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11.38

Figure 11.14 Design of Go-Back-N ARQ


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11.39

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.

Note

Example 11 6


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11.40

Example 11.6

F igure 11.16 shows an example of Go-Back-N. This is an

example of a case where the forward channel is rel iable,

but the reverse is not. No data frames are lost, but some

ACKs are delayed and one is lost. The example also

shows how cumulative acknowledgments can help if

acknowledgments are delayed or lost. After ini tialization,there are seven sender events. Request events are

tr iggered by data from the network layer; arr ival events

are triggered by acknowledgments from the physical

layer. There is no time-out event here because alloutstanding frames are acknowledged before the timer

expires. Note that although ACK 2 is lost, ACK 3 serves as

both ACK 2 and ACK 3.


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11.41


Example 11 7


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11.42

F igure 11.17 shows what happens when a frame is lost.

F rames 0, 1, 2, and 3 are sent. However, f rame 1 is lost.The receiver receives frames 2 and 3, but they are

discarded because they are received out of order. The

sender receives no acknowledgment about f rames 1, 2, or

3. I ts timer f inally expires. The sender sends all

outstanding frames (1, 2, and 3) because it does not know

what is wrong. Note that the resending of frames 1, 2, and

3 is the response to one single event. When the sender is

responding to this event, it cannot accept the tr igger ing of

other events. This means that when ACK 2 arr ives, the

sender is sti l l busy with sending frame 3.

Example 11.7

Example 11 7 (continued)


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11.43

The physical layer must wait unti l this event is completed

and the data l ink layer goes back to i ts sleeping state. We

have shown a vertical l ine to indicate the delay. I t is the

same story with ACK 3; but when ACK 3 arr ives, the

sender is busy responding to ACK 2. I t happens againwhen ACK 4 arr ives. Note that before the second timer

expires, all outstanding frames have been sent and the

timer is stopped.

Example 11.7 (continued)


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11.44



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Stop-and-Wait ARQ is a special case of

Go-Back-N ARQ in which the size of the

send window is 1.

Note


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11.46

Figure 11.18 Send window for Selective Repeat ARQ


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Figure 11.19 Receive window for Selective Repeat ARQ

Fi 11 20


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11.48

Figure 11.20 Design of Selective Repeat ARQ


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In Selective Repeat ARQ, the size of the

sender and receiver window

must be at most one-half of 2m.

Note

Algorithm 11 9 Sender site Selective Repeat algorithm


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Algorithm 11.9 Sender-site Selective Repeat algorithm

(cont inued)

Algorithm 11.9 Sender-site Selective Repeat algorithm (cont inued)


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go t .9 Se de s e Se ec e epea a go ( )

(cont inued)

Algorithm 11 9 Sender-site Selective Repeat algorithm (continued)


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Algorithm 11.9 Sender-site Selective Repeat algorithm (continued)

Algorithm 11 10 R i it S l ti R t l i th


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Algorithm 11.10 Receiver-site Selective Repeat algorithm

Algorithm 11.10 Receiver-site Selective Repeat algorithm


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g p g


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Figure 11.22 Delivery of data in Selective Repeat ARQ

Example 11.8


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This example is similar to Example 11.3 in which frame 1

is lost. We show how Selective Repeat behaves in this

case. Figure 11.23 shows the situation. One main

difference is the number of timers. Here, each frame sent

or resent needs a timer, which means that the timers need

to be numbered (0, 1, 2, and 3). The timer for frame 0

starts at the first request, but stops when the ACK for this

frame arr ives. The timer for frame 1 starts at the second

request, restarts when a NAK arr ives, and finally stopswhen the last ACK arr ives. The other two timers start

when the corresponding frames are sent and stop at the

last arr ival event.

p



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11.57

At the receiver site we need to distinguish between the

acceptance of a frame and its delivery to the network

layer. At the second arr ival, frame 2 arr ives and is stored

and marked, but i t cannot be delivered because frame 1 is

missing. At the next arrival, frame 3 arrives and is

marked and stored, but sti l l none of the frames can be

delivered. Only at the last arr ival, when finally a copy of

frame 1 arr ives, can frames 1, 2, and 3 be delivered to the

network layer. There are two conditions for the delivery offrames to the network layer: F irst, a set of consecutive

frames must have arr ived. Second, the set star ts from the

beginning of the window.

p ( )



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11.58

Another important point is that a NAK is sent after the

second arrival, but not after the third, although bothsituations look the same. The reason is that the protocol

does not want to crowd the network with unnecessary

NAKs and unnecessary resent frames. The second NAK

would sti l l be NAK1 to inform the sender to resend frame

1 again; this has already been done. The first NAK sent is

remembered (using the nakSent variable) and is not sent

again unti l the frame slides. A NAK is sent once for eachwindow position and defines the f irst slot in the window.

p ( )



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11.59

The next point is about the ACKs. Notice that only two

ACKs are sent here. The first one acknowledges only thef irst f rame; the second one acknowledges three frames. In

Selective Repeat, ACKs are sent when data are delivered to

the network layer. I f the data belonging to n f rames are

delivered in one shot, only one ACK is sent for all of them.

p ( )


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Figure 11.24 Design of piggybacking in Go-Back-N ARQ


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11-6 HDLC

High-level Data L ink Control (HDLC)is a bit-orientedprotocol for communication over point-to-point and

multipoint l inks. I t implements the ARQ mechanisms

we discussed in this chapter.

Configurations and Transfer ModesFrames

Control Field



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High-Level Data Link Control (HDLC)

HDLC was defined by ISO for use on both point-to-point and multipoint data links.

It supports full-duplex communication

Other similar protocols are Synchronous Data Link Control (SDLC) by IBM

Advanced Data Communication Control Procedure(ADCCP) by ANSI


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HDLC Overview

Broadly HDLC features are as follows: Reliable protocol

selective repeat or go-back-N Full-duplex communication

receive and transmit at the same time Bit-oriented protocol

use bits to stuff flags occurring in data Flow control

adjust window size based on receiver capability Uses physical layer clocking and synchronization to send

and receive frames


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HDLC Overview

Defines three types of stations Primary

Secondary

Combined

Defines three types of data transfer mode Normal Response mode

Asynchronous Response mode

Asynchronous Balanced mode

Three types of frames Unnumbered information

Supervisory

HDLC


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HDLC

The three stations are : Primary station

Has the responsibility of controlling the operation of dataflow in the link.

Handles error recovery Frames issued by the primary station are called commands.

Secondary station, Operates under the control of the primary station. Frames issued by a secondary station are called responses.

Combined station, Acts as both as primary and secondary station.


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HDLC


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HDLC

The three modes of data transfer operations are Normal Response Mode(NRM)

Mainly used in terminal-mainframe networks. In this case, Secondaries (terminals) can only transmit when specifically

instructed by the primary station in response to a polling Unbalanced configuration, good for multi-point links

Asynchronous Response Mode(ARM) Same as NRM except that the secondaries can initiate transmissions

without direct polling from the primary station Transmission proceeds when channel is detected idle , used mostly

in point-to-point-links

Asynchronous Balanced Mode(ABM) Mainly used in point-to-point links, for communication between

combined stations

Fi 11 25 N l d


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Figure 11.25 Normal response mode

Figure 11 26 A h b l d d


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Figure 11.26 Asynchronous balanced mode


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Non-operational Modes

Normal Disconnected Mode

Asynchronous Disconnected Mode

Both the above modes mean that the secondary node islogically disconnected from the primary node

Figure 11 27 HDLC frames


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Figure 11.27 HDLC frames

HDLC frame format


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HDLC frame format

Flag: 01111110- start and ending delimiter. Bits are stuffed for flagsin data frames

FCS: 16-bit CRC using generating polynomialG(x) = x16+ x12+ x5+ 1

Address field: In unbalanced configuration, every secondary is assigned a unique

address. Contains address of secondary station in both command andresponse frames

In balanced mode, command frame has destination address andresponse frame has sending nodesaddress

Group addresses are also possible. E.g., One command sent to all thesecondaries.

Figure 11 28 Control f ield format for the different f rame types


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Figure 11.28 Control f ield format for the different f rame types

In I-frames, N(s)is the sequence number of the frame being sent, andR(s)is the sequence number of the frame being expected.

The P/F bit, known as the poll/final bit, is used with different meaningin different contexts.

It is used to indicate pollingand to indicate the final of I-frame, etc

HDLC


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HDLC

There are three different classes of frames usedin HDLC

Unnumbered frames, used in link setup anddisconnection, and hence do not contain ACK.

Information frames, which carry actual information.

Supervisory frames, which are used for error and flowcontrol purposes and hence contain send and receivesequence numbers

HDLC


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HDLC

There are four different supervisory frames SS=00, Receiver Ready(RR), and N(R) ACKs all

frames received up to and including the one withsequence number N(R) - 1

SS=10, Receiver Not Ready(RNR).

SS=01, Reject; all frames with sequence numberN(R) or higher are rejected, which in turns ACKsframes with sequence number N(R) -1.

SS=11, Selective Reject; the receive rejects the framewith sequence number N(R)


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Table 11.1 U-f rame control command and response

Example 11.9


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Figure 11.29 shows how U-frames can be used for connection

establishment and connection release.

Node A asks for a connection with a set asynchronous balanced mode

(SABM) frame; node B gives a positive response with an

unnumbered acknowledgment (UA) frame.

After these two exchanges, data can be transferred between the two

nodes (not shown in the figure).

After data transfer, node A sends a DISC (disconnect) frame torelease the connection; it is confirmed by node B responding with a

UA (unnumbered acknowledgment).

Figure 11 29 Example of connection and disconnection


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Figure 11.29 Example of connection and disconnection

Data Link Control Framing

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