Transcript COE 308: Computer Architecture (T032) Dr. Marwan Abu

COE 341: Data & Computer Communications (T062)
Dr. Marwan Abu-Amara
Chapter 7:
Data Link Control Protocols
Contents
1.
Error Control
a.
b.
c.
2.
Flow Control
a.
b.
3.
Stop-and-Wait ARQ
Go-Back-N ARQ
Selective-Reject ARQ
Stop-and-Wait flow control
Sliding-Window flow control
High-Level Data Link (HDLC)
COE 341 – Dr. Marwan Abu-Amara
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What is Data Link Control


The logic or procedures used to convert the raw
stream of bits provided by the physical layer into a
“reliable” connection
Requirements and Objectives:






Frame synchronization
Error control
Flow control
Addressing
Multiplexing data and control on connection
Link management
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Error & Flow Control

We now study two main functions of the datalink layer



Error control
Flow control
Why Error & Flow Control together?

We usually lump Error Control and Flow Control
discussions together because the data-link
protocols used for error control are also used for
flow control
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Error Control Definition

Error Control is the second type of methods used to
“handle” errors in frames


Error Control methods are those that use
retransmission in the case an error occurs in a frame


First type is FEC
This process is called Automatic Repeat Request (ARQ)
In general, a retransmission is needed if a:


Frame is erroneous
Frame is lost (i.e. does not arrive or arrives too late)

There are several reasons for this to happen (e.g. existence of
noise, network congestion…)
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Flow Control Definition

We define Flow Control as: “The set of
procedures used to restrict the amount of
data that a TX can send before waiting for an
acknowledgement from the RX” (per
Forouzan)

This is to avoid overwhelming the RX by the flow
of data from TX



RX does not ‘absorb’ the received data instantly
It buffers (temporarily stores) the data it receives to do
some processing before sending it upward to higher
layers
Without flow control, the RX buffer may overflow and
data gets lost
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Flow Control

Assume:



No frames lost
No frames arrive in error
Frames arrive in the same order they were sent,
following a variable propagation delay
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A Model of Frame Transmission
Loss
Only one frame
traveling on the link
at any given time
Error
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Error & Flow Control Protocols

There are three main types of error control
protocols (ARQ-based):


Stop-and-Wait ARQ
Sliding Window ARQ



Go-back-N ARQ
Selective-Reject ARQ
There are two main types of flow control
protocols:


Stop-and-Wait
Sliding Window
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Categories of Error Control
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Categories of Flow Control
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Stop-and-Wait ARQ

Stop-and-Wait ARQ is a simple protocol:




TX keeps a copy of the last frame sent
After receiving the frame, RX sends back an ACK
After receiving this ACK, TX sends another frame
and so on…
Both Data & ACK frames are alternately
numbered with “0” or “1”


Data Frame “0” is acknowledged by ACK “1”
Data Frame “1” is acknowledged by ACK “0”
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Stop-and-Wait ARQ: Normal Operation
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Stop-and-Wait ARQ: Abnormal Operation

Stop-and-Wait ARQ deals with encountered
anomalies as follows:

Lost or Damaged Frames: RX discards them silently
i.e. without sending Negative-ACK (NACK) back to
TX


RX keeps its current value for R (R var. defined on prev.
slide)
Lost or Damaged ACK:



TX discards damaged ACK
TX keeps a timer, after sending a frame, within which ACK
must be received. Otherwise, ACK is considered lost.
In both situations, Lost and Damaged ACK, the TX sends
the frame again
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Stop-and-Wait ARQ: Lost Frame
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Stop-and-Wait ARQ: Lost ACK
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Stop-and-Wait ARQ: Delayed ACK

Another abnormality: TX receiving a late ACK

Timer has already expired


Frame is re-sent again



The ACK was considered lost
This frame will be duplicate at RX and discarded
Also its ACK will be discarded when received back at TX
Then the late ACK arrives

Now TX can send the next frame
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Stop-and-Wait ARQ: Delayed ACK
Lost
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Stop-and-Wait ARQ Piggybacking




Piggybacking is a method
that combines the data
and the ACK in one frame
It is useful in bidirectional
communications
Stations can send their
data along with ACK to
data previously received
Piggybacking is faster
and saves bandwidth
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Stop-and-Wait ARQ Drawback

In Stop-and-Wait ARQ, the line is not efficiently utilized
because only one frame is sent at a time



Its ACK must be awaited for
During this waiting period no frames are sent
Inefficiency gets even worse when

TX speed is high


Propagation distance is high


TX quickly sends frame then sits idle
It takes longer for the frame and its ACK to reach destination
Both cases leave the TX waiting idle for longer times
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Stop-and-Wait Efficiency

Let:





: Time to transmit a frame = Lf / R
: Time to transmit an ACK = LACK / R
: Propagation time = (link distance) / V
: Processing time
We define the frame total time as (see next
slide):


Tf
TACK
TProp
TProc
Ttotal = Tf + 2 TProp + TProc + TACK
The utilization of the link is defined as:

U = (Tf)/(Ttotal)
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Stop-and-Wait Efficiency

It is common to ignore TProc and TACK
because they are negligible compared
to other times
In which case the utilization becomes:

U = (Tf)/(Tf +2Tprop) = 1/(1 + 2a)
Tprop
 Where: a = Tprop/Tf
 a is called the “length of the link in bits”: length
of medium in bits compared to frame length (Lf) Tproc
Notice that for very small a (i.e. when 1st

TX
RX
Tf



transmitted bit reaches RX, source will still
be transmitting), U 100%
Notice that for very big a (i.e. frame
transmission is completed before 1st bit
reaches destination),
U 0%
Tprop
TACK
Efficient for links where a << 1 (long
frames on a short link, i.e. fills link)
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Stop and Wait Efficiency: Example

Compare the efficiency of stop-and-wait error control for two
links using the parameter ‘a’:

Fame size, L = 1000 characters of 8 bits each, = 8000 bits

Satellite link between 2 ground stations


d = 2 x 36,000 km, Data rate, R = 1 Mbps


Typical wave velocity, V = 3 x 108 m/s

a





T prop
Tf
d
 V 
L
R
2  36 106
3 10  30
8000
1106
8
200-m optical fiber link
Data rate, R = 1 Gbps
Typical wave velocity, V = 2 x 108 m/s
a
T prop
Tf
200
V
2 108  0.125


L
8000
R
1109
d
Frame TX time, tf = L/R = 8000/(1x106)

Frame TX time, tf = L/R = 8000/(1x109)
= 8 ms
= 8 ms
Propagation time, tprop = d/V

Propagation time, tprop = d/V
= 2x36x106/(3x108) = 240 ms
= 200/(2x108) = 1 ms
End of first frame reaches RX after

End of first frame reaches RX after
8+240 = 248 ms from start
8+1 = 9 ms from start
ACK takes 240 ms more to reach TX,

ACK takes 1 ms more to reach TX,
which sends 2nd frame after 488 ms
which sends 2nd frame after 10 ms
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COE 341 – Dr. Marwan Abu-Amara
Utilization = 8/488 = 1.6% (=1/(2a+1))

Utilization = 8/10 = 80% (=1/(2a+1))
Sliding Window Protocol
Stop-and-Wait can be very inefficient when a > 1
 Protocol:




Assumes full duplex line
Source A and Destination B have buffers each of size W frames
For k-bit sequence numbers:







Frames are numbered: 0, 1, 2, …, 2k-1, 0, 1, … (modulo
2 k)
ACKs (RRs) are numbered: 0, 1, 2, …, 2k-1, 0, 1, …
(modulo 2k)
A is allowed to transmit up to W frames without waiting for an ACK
B can receive up to W consecutive frames
ACK J (or RR J), where 0  J  2k-1, sent by B, means B has
received frames up to frame J-1 and is ready to receive frame J
B can also send RNR J: B has received all frames up to J-1 and is
not ready to receive any more
Window size W, can be less or equal to 2k-1
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Sliding Window Protocol

Example of Sliding-Window-Protocol: k = 3 bits, W = 7
Observations:
• A may Tx W = 7 frames
(F0, F1, …, F6)
• After F0, F1, & F2 are Txed, window is shrunk (i.e.
can not transmit except
F3, F4, …, F6)
• When B sends RR3, A
knows F0, F1 & F2 have
been received and B is
ready to receive F3
• Window is advanced to
cover 7 frames (starting
with F3 up to F1)
• A sends F3, F4, F5, & F6
• B responds with RR4
when F3 is received – A
advances the window by
one position to include F2
W
W
W
W
COE 341 – Dr. Marwan Abu-Amara
W = distance between first unacknowledged
frame and last frame that can be sent
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Go-back-N ARQ

Go-back-N ARQ improves the efficiency of the line by
sending up to W frames before worrying about ACK



For this, the frames must be sequentially numbered
Sequence number must be included in the header of
the frame


This is called Pipelining (several tasks are started before the
1st is finished)
If m bits are reserved for sequence number then the
sequence numbers range from 0 to 2m – 1
Example:

m = 3 bits, then seq. numbers range is 0-7 and could be used
as:

0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1,…
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Go-back-N ARQ Sliding Window

At the TX side, to hold the outstanding frames until
they are individually ACK, we use a window



Each time a proper ACK is received for a frame the
window slides past this frame, hence the name
Sliding Window


The window size W cannot exceed 2m – 1
W is fixed in this protocol but may be variable in others (e.g.
TCP)
The acknowledged frames can be purged out of the TX
memory
As an example, next slide shows that frame 0 and 1
have been acknowledged

So the sliding window slides just past them in (b)
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TX Sliding Window
In (b), frame 0 and frame 1 were properly acknowledged
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RX Sliding Window




At the RX side, the size of the window is
always 1
The RX window is centered on the next
expected frame number
If any other frame arrives (i.e. out of sequence
arrival), it is immediately discarded
If the right frame arrives, the window slides
past it to zoom on the next expected frame
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RX Sliding Window
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Go-back-N ARQ Control Variables

TX keeps track of three variables




The frame size W is related to these variables by:


S: Sequence number of the recently sent frame
SF: Sequence number of the first frame in the window
SL: Sequence number of the last frame in the window
W = SL– SF + 1
On the other hand the RX has only one variable:

R: Expected frame sequence number
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Go-back-N ARQ: TX & RX Control Variables
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Timers & Acknowledgments





TX sets a timer for each frame sent
RX has no timers
RX sends an ACK if a frame arrives with no errors and in order
If RX receives damaged or out of sequence frame, it silently
discards them until it receives the expected frame
 The silence of the RX causes the TX timer to expire
 This in turn causes the TX to go back and send all frames starting
from the non-acknowledged frame
 This is why it is called Go-back-N ARQ
Example:
 If TX has already sent frame-6 but the timer for frame-3 expires
without receiving an ACK for it, the TX goes back and send frames
3, 4, 5, 6
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Go-back-N ARQ Normal Operation
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Go-back-N ARQ Lost Frame Operation
Here, ACK2 acknowledges
frames 0 & 1 at the same time
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Quiz: Window Size Condition for Go-back-N
ARQ


Earlier, we mentioned
that the size of the
window must be W  2m
– 1 i.e. W < 2m where
“m” is the number of bits
reserved (in the frame
overhead) for the
sequence number
By comparing the
figures across explain
the need for this
condition
COE 341 – Dr. Marwan Abu-Amara
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Sliding Window Protocol

Animation for Sliding Window protocol

Sliding Window Protocol Simulation
(http://www.cs.stir.ac.uk/~kjt/software/comms/jasper/S
WP3.html)
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Example: Problem 7-9
a)
b)
c)
Two neighboring nodes (A and B) use a slidingwindow protocol with a 3-bit sequence number. As
the ARQ mechanism, go-back-N is used with a
window size of 4. Assuming A is transmitting and B is
receiving, show the window positions for the
following succession of events:
Before A sends any frames
After A sends frame 0, 1, 2 and B acknowledges 0, 1
and the ACKs are received by A
After A sends frames 3, 4, and 5 and B
acknowledges 4 and the ACK is received by A
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Example: Problem 7-9 - Solution
W
a)
•••
0
1
2
3
b)
4
5
6
7
0
•••
4
5
6
7
0
•••
7
0
•••
W
•••
0
1
2
3
c)
W
•••
0
1
2
3
4
5
COE 341 – Dr. Marwan Abu-Amara
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Sliding Window Protocol - Piggybacking
 When
using sliding window protocol in full
duplex connections:
Node A maintains its own transmit window
 Node B maintains its own transmit window
 A frame contains: data field + ACK field
 There is a sequence number for the data field, and a
sequence number for the ACK field

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Selective-Repeat ARQ

Go-back-N ARQ simplifies the job of the RX



However, this protocol is inefficient for noisy links
where the probability of damaged frames is high


RX keeps track of one control variable R only
Out-of-sequence frames are simply dropped, not buffered
This means many retransmissions will occur which uses up
the bandwidth of the link
For noisy links, a more efficient mechanism is used in
which only the damaged frame is retransmitted (not
all N frames)

It is called Selective-Repeat ARQ
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Selective-Repeat ARQ TX & RX Windows

Window configuration for the TX is the same as for
Go-back-N ARQ
 However, the window size should be W  (½  2m) i.e. W 
2(m-1)

RX window must have the same size as the TX



This a major difference with Go-back-N ARQ where window
W = 1 for the RX
RX window specifies a range of a acceptable frame
sequence numbers
Also, in selective-repeat ARQ the RX uses negative
ACK (NACK) to report the sequence of a damaged
frame to the TX
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Selective-Repeat ARQ: TX & RX Control Variables
Here, S = SF
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Selective-Repeat ARQ Operation: Lost Frame
Example
Here, ACK2 acknowledges
frames 0 & 1 at the same time
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Quiz: Window Size Condition for SelectiveRepeat ARQ


Earlier, we mentioned
that the size of the
window must be W 
2m – 1 where “m” is the
number of bits reserved
(in the frame overhead)
for the sequence
number
By comparing the
figures across explain
the need for this
condition
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Sliding Window Protocol - Efficiency
Refer to Appendix A
 When window size is W (for error free), link utilization,
U, is given by

W  (2a  1)
 1

U  W
W  (2a  1)

 2a  1
where a = Tprop/Tf (i.e. length of link in bits)
 Sliding window protocol can achieve 100% utilization if
W  (2a + 1)
 The smaller the W needed the better! (Why?)

To get high value for U, small value for a is needed as well !!
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Sliding Window Efficiency: Example

Compare the efficiency of Sliding Window flow control for two
links using the parameter ‘a’:

Fame size, L = 1000 characters of 8 bits each, = 8000 bits

Satellite link between 2 ground stations


d = 2 x 36,000 km, Data rate, R = 1 Mbps


Typical wave velocity, V = 3 x 108 m/s

Frame TX time, tf = L/R = 8 ms

Propagation time, tprop = d/V = 240 ms

a = tprop / tf = 30

100 % link utilization is achieved with

window size W:
W  (2 a+1)  (2 x 30 +1)  61
W = 61, k = 6 bit
(Large window and buffer sizes)
For k = 3 bits, W = 7:
Utilization U = W/(2a+1) = 7/(61)
COE 341 – Dr. Marwan Abu-Amara
= 11.5% > 1.6% for Stop and wait.





200-m optical fiber link
Data rate, R = 1 Gbps
Typical wave velocity, V = 2 x 108 m/s
Frame TX time, tf = L/R = 8 ms
Propagation time, tprop = d/V = 1 ms
a = tprop / tf = 0.125
100 % link utilization is achieved with
window size W:
W  (2 a+1)  (2 x 0.125 +1)  1.25
i.e. W = 2 (A window of just 2 frames)
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Stop-and-Wait & Sliding Window as Flow
Control Protocols

Problem:


Qualitatively explain how the Stop-and-Wait and
the Sliding Window protocols could be used for
flow control
Hint:

Think about the use of ACK and when the RX could
send it back to the TX
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High-Level Data Link Control Protocol (HDLC)




One of the most important data link control protocols
and it is the basis for many data link control protocols.
The job of the HDLC layer (Link Layer, Layer 2) is to
ensure that data passed up to the next layer has
been received exactly as transmitted (i.e error free,
without loss and in the correct order)
Another important job is flow control, which ensures
that data is transmitted only as fast as the receiver
can receive it.
There are two distinct HDLC implementations



HDLC NRM (see (SDLC), and
HDLC Link Access Procedure Balanced (LAPB)
Usually when referring to HDLC people mean LAPB
or some variation
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High-Level Data Link Control Protocol (HDLC)


To satisfy a variety of applications, HDLC defines
three types of stations, two link configurations, and
three types of data transfer modes of operation.
Station types:




Primary Station (PS):
 Responsible for controlling the operation of the link
 Frames issued by the PS are called commands
Secondary Station (SS): Frames issued by the SS are called
responses – operates under the control of a primary station
Combined Station: issues commands and responses
Link configurations:


Unbalanced: one primary plus one or more secondary
Balanced: two combined stations
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High-Level Data Link Control Protocol (HDLC)
 Transfer modes:




Normal Response Mode (NRM) – used in
unbalanced config.; secondary may only tx data in
response to a command from primary
Asynchronous Response Mode (ARM) – used in
unbalanced config.; Secondary may initiate data tx
without explicit permission; primary still retains line
control (initialization, error recovery, …)
Asynchronous Balanced Mode (ABM) – used in
balanced config.; either combined station may tx
data without receiving permission from other station
Animation for HDLC
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HDLC – Applications

NRM:

Point-multipoint (multi-drop line): one computer
(primary) polls multiple terminals (secondary
stations)



number of terminals are connected to a host computer
ARM: rarely used
ABM: most widely used (no polling involved)

Full duplex point-to-point
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HDLC NRM
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HDLC ABM
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HDLC – Frame Structure
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HDLC – Frame Format

Flag:




Address:






Size: 1 or 2 Bytes
Used for error & flow control
Information:



Size: 1 Byte (or more for larger networks)
If primary station created the frame, the address is that of the destination
secondary station
If secondary station created the frame, the address is that of the source
secondary station
Networks not using “primary/secondary” (e.g. Ethernet) use 2-Byte address
(source/destination)
Control:


Size: 1 Byte
Special pattern 0 1 1 1 1 1 1 0 used as frame begin/end and synch.
Used in Header and trailer
Size: Varies from network to network. Always fixed within a network
Contains user data from Network layer or Network Management information
FCS:


Size: 2 or 4 Bytes
Implements ITU-T CRC for error detection
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HDLC – Frame Structure – Flag Field

Flag Field: unique pattern 01111110
 Used for synchronization
 To prevent this pattern form occurring in data  bit stuffing
procedure is used


Tx-er inserts a 0 after each 5 1s
Rx-er, after detecting flag, monitors incoming bits – when a pattern of
5 1s appears; the 6th/7th bit are checked:



•
If 0, it is deleted
If 10, this is a flag
If 11, this is an ABORT
Pitfalls of bit stuffing: one bit errors can split one frame into two or
merge two frames into one
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HDLC Data Transparency

If the data field of an HDLC frame contains a
pattern identical to the flag pattern (01111110),
the RX will interpret it as end-of-frame flag



Next bits will be considered part of the next frame
This is called lack of Transparency
Bit Stuffing is the process of adding one extra
0 whenever there are five consecutive 1s in the
data so that the receiver does not mistake the
data for a flag
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HDLC Bit Stuffing & Removal
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HDLC Bit Stuffing

Problems: With bit stuffing, Single-bit errors could
split a frame into 2 or merge two frames into 1.
Frame
Splitting
01111100
(Bit stuffed)
01111110
01111110
Frame
Merging
(Bit stuffed)
01111100
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HDLC Frame Structure – Address Field
Extended Address Field
Address field identifies the secondary station that
transmitted or is to receive frame
 Not used (but included for uniformity) for point-to-point
links
 Extendable – by prior arrangement
 Address = 11111111 (single octet) used by the primary
to broadcast to all secondary stations

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HDLC Frame Structure – Control Field

Three types of frames: I, S, and U





Information frame (I): carry user data (upper layers) – flow and error control info
is piggybacked on these frames as well
Supervisory frame (S): carry flow and error control info when piggybacking is not
used
Unnumbered frame (U): provide supplementary link control
First 2 bits of the control field determine the type of frame, the remaining are
organized as shown in figure 7.7.c and d (next slide)
Poll/Final (P/F) bit:


In command frames (P): used to solicit response from peer entity
In response frames (F): indicate response is the result of soliciting command
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HDLC Frame Structure – Control Field
 Extension

of sequence number
7-bit sequence numbers rather than 3-bit ones
 “Set-mode”
command  extends control field to 16 bit
for S and I frames
 Unnumbered frames always use 8-bit
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HDLC Frame Structure – Information/FCS Fields

Information field:
Present ONLY in I-frames and some U-frames
 Contains integer number of octets
 Length is variable – up to some system defined maximum

 FCS
field:
CRC error detecting code
 Calculated from ALL remaining bits in frame (excluding flag
field)
 Normally 16 bits (CRC-CCITT polynomial = X16+X12+X5+1), or
 32-bit optional FCS using CRC-32

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HDLC Frames


HDLC uses Synchronous transmission
HDLC defines three types of frames

Information frames (I-frames)


Supervisory frames (S-frames)


Used to transport user data and user control information
relating to the user data (e.g. piggybacking)
Used to transport control information only
Unnumbered frames (U-frames)

Used in the link management
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HDLC Generic Frame Format
Indicates that the first
transmitted field is the Header
flag, then the address, then the
control…
Trailer
Header
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HDLC I, S & U Frames Format
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HDLC I-Frames



I-frames are designed to carry user data from
Network-layer
They can include flow & error control information
(piggybacking)
The bits of the I-frame Control field are:


First bit is always “0”. It means it is an I-frame
Next 3-bits, called N(S), define the sequence of the frame


Next bit is called P/F (Poll or Final)



So possible sequences are 0,1,2,3,4,5,6,7
When a primary station polls other stations, it sets this bit to 1
When a secondary station responds to a poll, it sets this bit to 1
Next 3-bits, called N(R), define the value of the ACK when
piggybacking is used
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HDLC I-Frames
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HDLC S-Frames
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HDLC U-Frames
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U-frame control command and response
Command/response
Meaning
SNRM
Set normal response mode
SNRME
Set normal response mode (extended)
SABM
Set asynchronous balanced mode
SABME
Set asynchronous balanced mode (extended)
UP
Unnumbered poll
UI
Unnumbered information
UA
Unnumbered acknowledgment
RD
Request disconnect
DISC
Disconnect
DM
Disconnect mode
RIM
Request information mode
SIM
Set initialization mode
RSET
Reset
XID
Exchange ID
FRMR
Frame reject
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HDLC – Operation



HDLC operation consists of the exchange of Iframes, S-frames, and U-frames between two
stations.
Table 7.1 list types of Control/Response
functions for various frame types
The operations of HDLC involve three phases:

Initialization (by either side): U-Frames


Data Transfer (by the two sides): I- and S-Frames


Both agree on various options
Exchange of user data and control info for flow and error
control
Disconnect (by either side): U-Frames

Signaling termination of operation
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HDLC – Operation
Initialization:
(Link Setup & Disconnect)
Issue SABME command and starts timer
 B responds with UA (or DM if request is
rejected)
 A receives UA and initializes its variables
 To disconnect: issue DISC command
 Followed by UA

SABME: Set Asynchronous balanced/extended mode;7-bit sequence
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HDLC – Operation

Data Transfer:





Once initialization is complete a logical path is
established.
Both sides start to send I-frames (Full-duplex
exchange) starting with seq. number 0
N(S), N(R) both are seq. number to support
flow and error control
N(R) is the ACK for the I-frame received; it
enables the HDLC module to indicate which
number I-frame it expect to receive next.
RR is used (S-frame) when there is no
reverse user data (I-frame) traffic
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HDLC – Operation

Disconnect:




Either side can issue a DISC frame
to request disconnect
The remote entity MUST accept
the request by sending UA.
Any outstanding I-frame will be
lost.
Busy condition: Notice the use of
P/F bit.




When A is unable to keep up with
the speed of the transmitter “B” or
buffer is full.
A sends RNR, to halt the
transmission of B
To check the readiness of A,
periodically B sends RR frame with
P set.
Once the condition of being busy is
cleared A responds with F=1
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HDLC – Operation

Reject Recovery:




I-frame 4 was lost
B receives I-frame 5 (out of
order) – responds with REJ 4
A resend I-frame 4 and all
subsequent frames (Go-back-N)
Timeout Recovery:




A sends I-frame 3 – but it is lost
Timer expires before
acknowledgement arrives
A polls Node B
B responds indicating it is still
waiting for frame 3 – B set the F
bit because this a response to A’s
solicitation
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HDLC Example: Piggybacking w/o Errors





Next slide shows an exchange using piggybacking when
there is no error
Station A begins the exchange of information with an Iframe numbered 0 followed by another I-frame numbered 1
Station B piggybacks its acknowledgment of both frames
onto an I-frame of its own
Station B’s first I-frame is also numbered 0 [N(S) field] and
contains a 2 in its N(R) field, acknowledging the receipt of
A’s frames 1 and 0 and indicating that it expects frame 2 to
arrive next
Station B transmits its second and third I-frames
(numbered 1 and 2) before accepting further frames from
station A

Its N(R) information, therefore, has not changed: B frames 1 and 2
indicate that station B is still expecting A’s frame 2 to arrive next
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HDLC Example: Piggybacking w/o Errors
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HDLC Example: Piggybacking with Errors




Suppose frame 1 sent from station B to station
A has an error
Station A informs station B to resend frames 1
and 2 (the system is using the Go-Back-N
mechanism)
Station A sends a reject supervisory frame to
announce the error in frame 1
Next slide shows the exchange
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HDLC Example: Piggybacking with Errors
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