The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between adjacent network nodes in a wide area network (WAN) or between nodes on the same local area network (LAN) segment.
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between adjacent network nodes in a wide area network (WAN) or between nodes on the same local area network (LAN) segment.
RANDOM ACCESS PROTOCOL IN COMMUNICATION AMOGHA A K
In random access ,each station has right to send the data. However , if more than one station tries to send ,collision will occur .To avoid this collision , protocols came into existence.
In random access method , no stations are superior & none is assigned the control over the other .
When a station has a data to send , it uses a procedure defined by a protocol whether to send or not .
Carrier-sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in early Ethernet technology for local area networking.Carrier-sense multiple access with collision detection is a media access control method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting.
RANDOM ACCESS PROTOCOL IN COMMUNICATION AMOGHA A K
In random access ,each station has right to send the data. However , if more than one station tries to send ,collision will occur .To avoid this collision , protocols came into existence.
In random access method , no stations are superior & none is assigned the control over the other .
When a station has a data to send , it uses a procedure defined by a protocol whether to send or not .
Carrier-sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in early Ethernet technology for local area networking.Carrier-sense multiple access with collision detection is a media access control method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting.
Lecture slides from the 3rd-year module on Computer Networks at the University of Birmingham, UK.
This presentation covers the Data-Link layer of the networks stack, primarily Error Control, Flow Control and Framing.
This slide is brief description of DCN of unit-2 .It explains about the different noisy channel and noiseless channel protocols and brief description about HDLC.
Abstract The data can get lost, reordered or duplicated due to the presence of routers and buffer space over the unreliable channel in the conventional networks. The data link layer deals with frame formation, flow control, error control, and addressing and link management. All such functions will be performed only by data link protocols. The sliding window protocol will detect and correct error if the received data have enough redundant bits or repeat a retransmission of data. The paper shows the working of this duplex protocol of data link network. Keywords: ACK, GOBACK, ARQ, NACK.
IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology.
To transmit the data from one node to another, data link layer combines framing, flow control & error control schemes.
We divide the discussion protocols into those that can be used for noiseless(error free) channels and those that can be used for noisy (error creating) channels.
NO1 Uk best vashikaran specialist in delhi vashikaran baba near me online vas...Amil Baba Dawood bangali
Contact with Dawood Bhai Just call on +92322-6382012 and we'll help you. We'll solve all your problems within 12 to 24 hours and with 101% guarantee and with astrology systematic. If you want to take any personal or professional advice then also you can call us on +92322-6382012 , ONLINE LOVE PROBLEM & Other all types of Daily Life Problem's.Then CALL or WHATSAPP us on +92322-6382012 and Get all these problems solutions here by Amil Baba DAWOOD BANGALI
#vashikaranspecialist #astrologer #palmistry #amliyaat #taweez #manpasandshadi #horoscope #spiritual #lovelife #lovespell #marriagespell#aamilbabainpakistan #amilbabainkarachi #powerfullblackmagicspell #kalajadumantarspecialist #realamilbaba #AmilbabainPakistan #astrologerincanada #astrologerindubai #lovespellsmaster #kalajaduspecialist #lovespellsthatwork #aamilbabainlahore#blackmagicformarriage #aamilbaba #kalajadu #kalailam #taweez #wazifaexpert #jadumantar #vashikaranspecialist #astrologer #palmistry #amliyaat #taweez #manpasandshadi #horoscope #spiritual #lovelife #lovespell #marriagespell#aamilbabainpakistan #amilbabainkarachi #powerfullblackmagicspell #kalajadumantarspecialist #realamilbaba #AmilbabainPakistan #astrologerincanada #astrologerindubai #lovespellsmaster #kalajaduspecialist #lovespellsthatwork #aamilbabainlahore #blackmagicforlove #blackmagicformarriage #aamilbaba #kalajadu #kalailam #taweez #wazifaexpert #jadumantar #vashikaranspecialist #astrologer #palmistry #amliyaat #taweez #manpasandshadi #horoscope #spiritual #lovelife #lovespell #marriagespell#aamilbabainpakistan #amilbabainkarachi #powerfullblackmagicspell #kalajadumantarspecialist #realamilbaba #AmilbabainPakistan #astrologerincanada #astrologerindubai #lovespellsmaster #kalajaduspecialist #lovespellsthatwork #aamilbabainlahore #Amilbabainuk #amilbabainspain #amilbabaindubai #Amilbabainnorway #amilbabainkrachi #amilbabainlahore #amilbabaingujranwalan #amilbabainislamabad
Cosmetic shop management system project report.pdfKamal Acharya
Buying new cosmetic products is difficult. It can even be scary for those who have sensitive skin and are prone to skin trouble. The information needed to alleviate this problem is on the back of each product, but it's thought to interpret those ingredient lists unless you have a background in chemistry.
Instead of buying and hoping for the best, we can use data science to help us predict which products may be good fits for us. It includes various function programs to do the above mentioned tasks.
Data file handling has been effectively used in the program.
The automated cosmetic shop management system should deal with the automation of general workflow and administration process of the shop. The main processes of the system focus on customer's request where the system is able to search the most appropriate products and deliver it to the customers. It should help the employees to quickly identify the list of cosmetic product that have reached the minimum quantity and also keep a track of expired date for each cosmetic product. It should help the employees to find the rack number in which the product is placed.It is also Faster and more efficient way.
Forklift Classes Overview by Intella PartsIntella Parts
Discover the different forklift classes and their specific applications. Learn how to choose the right forklift for your needs to ensure safety, efficiency, and compliance in your operations.
For more technical information, visit our website https://intellaparts.com
COLLEGE BUS MANAGEMENT SYSTEM PROJECT REPORT.pdfKamal Acharya
The College Bus Management system is completely developed by Visual Basic .NET Version. The application is connect with most secured database language MS SQL Server. The application is develop by using best combination of front-end and back-end languages. The application is totally design like flat user interface. This flat user interface is more attractive user interface in 2017. The application is gives more important to the system functionality. The application is to manage the student’s details, driver’s details, bus details, bus route details, bus fees details and more. The application has only one unit for admin. The admin can manage the entire application. The admin can login into the application by using username and password of the admin. The application is develop for big and small colleges. It is more user friendly for non-computer person. Even they can easily learn how to manage the application within hours. The application is more secure by the admin. The system will give an effective output for the VB.Net and SQL Server given as input to the system. The compiled java program given as input to the system, after scanning the program will generate different reports. The application generates the report for users. The admin can view and download the report of the data. The application deliver the excel format reports. Because, excel formatted reports is very easy to understand the income and expense of the college bus. This application is mainly develop for windows operating system users. In 2017, 73% of people enterprises are using windows operating system. So the application will easily install for all the windows operating system users. The application-developed size is very low. The application consumes very low space in disk. Therefore, the user can allocate very minimum local disk space for this application.
Democratizing Fuzzing at Scale by Abhishek Aryaabh.arya
Presented at NUS: Fuzzing and Software Security Summer School 2024
This keynote talks about the democratization of fuzzing at scale, highlighting the collaboration between open source communities, academia, and industry to advance the field of fuzzing. It delves into the history of fuzzing, the development of scalable fuzzing platforms, and the empowerment of community-driven research. The talk will further discuss recent advancements leveraging AI/ML and offer insights into the future evolution of the fuzzing landscape.
Courier management system project report.pdfKamal Acharya
It is now-a-days very important for the people to send or receive articles like imported furniture, electronic items, gifts, business goods and the like. People depend vastly on different transport systems which mostly use the manual way of receiving and delivering the articles. There is no way to track the articles till they are received and there is no way to let the customer know what happened in transit, once he booked some articles. In such a situation, we need a system which completely computerizes the cargo activities including time to time tracking of the articles sent. This need is fulfilled by Courier Management System software which is online software for the cargo management people that enables them to receive the goods from a source and send them to a required destination and track their status from time to time.
Welcome to WIPAC Monthly the magazine brought to you by the LinkedIn Group Water Industry Process Automation & Control.
In this month's edition, along with this month's industry news to celebrate the 13 years since the group was created we have articles including
A case study of the used of Advanced Process Control at the Wastewater Treatment works at Lleida in Spain
A look back on an article on smart wastewater networks in order to see how the industry has measured up in the interim around the adoption of Digital Transformation in the Water Industry.
About
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
• Remote control: Parallel or serial interface.
• Compatible with MAFI CCR system.
• Compatible with IDM8000 CCR.
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
• Easy in configuration using DIP switches.
Technical Specifications
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
Key Features
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
• Remote control: Parallel or serial interface
• Compatible with MAFI CCR system
• Copatiable with IDM8000 CCR
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
Application
• Remote control: Parallel or serial interface.
• Compatible with MAFI CCR system.
• Compatible with IDM8000 CCR.
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
• Easy in configuration using DIP switches.
Quality defects in TMT Bars, Possible causes and Potential Solutions.PrashantGoswami42
Maintaining high-quality standards in the production of TMT bars is crucial for ensuring structural integrity in construction. Addressing common defects through careful monitoring, standardized processes, and advanced technology can significantly improve the quality of TMT bars. Continuous training and adherence to quality control measures will also play a pivotal role in minimizing these defects.
CFD Simulation of By-pass Flow in a HRSG module by R&R Consult.pptxR&R Consult
CFD analysis is incredibly effective at solving mysteries and improving the performance of complex systems!
Here's a great example: At a large natural gas-fired power plant, where they use waste heat to generate steam and energy, they were puzzled that their boiler wasn't producing as much steam as expected.
R&R and Tetra Engineering Group Inc. were asked to solve the issue with reduced steam production.
An inspection had shown that a significant amount of hot flue gas was bypassing the boiler tubes, where the heat was supposed to be transferred.
R&R Consult conducted a CFD analysis, which revealed that 6.3% of the flue gas was bypassing the boiler tubes without transferring heat. The analysis also showed that the flue gas was instead being directed along the sides of the boiler and between the modules that were supposed to capture the heat. This was the cause of the reduced performance.
Based on our results, Tetra Engineering installed covering plates to reduce the bypass flow. This improved the boiler's performance and increased electricity production.
It is always satisfying when we can help solve complex challenges like this. Do your systems also need a check-up or optimization? Give us a call!
Work done in cooperation with James Malloy and David Moelling from Tetra Engineering.
More examples of our work https://www.r-r-consult.dk/en/cases-en/
Automobile Management System Project Report.pdfKamal Acharya
The proposed project is developed to manage the automobile in the automobile dealer company. The main module in this project is login, automobile management, customer management, sales, complaints and reports. The first module is the login. The automobile showroom owner should login to the project for usage. The username and password are verified and if it is correct, next form opens. If the username and password are not correct, it shows the error message.
When a customer search for a automobile, if the automobile is available, they will be taken to a page that shows the details of the automobile including automobile name, automobile ID, quantity, price etc. “Automobile Management System” is useful for maintaining automobiles, customers effectively and hence helps for establishing good relation between customer and automobile organization. It contains various customized modules for effectively maintaining automobiles and stock information accurately and safely.
When the automobile is sold to the customer, stock will be reduced automatically. When a new purchase is made, stock will be increased automatically. While selecting automobiles for sale, the proposed software will automatically check for total number of available stock of that particular item, if the total stock of that particular item is less than 5, software will notify the user to purchase the particular item.
Also when the user tries to sale items which are not in stock, the system will prompt the user that the stock is not enough. Customers of this system can search for a automobile; can purchase a automobile easily by selecting fast. On the other hand the stock of automobiles can be maintained perfectly by the automobile shop manager overcoming the drawbacks of existing system.
Immunizing Image Classifiers Against Localized Adversary Attacksgerogepatton
This paper addresses the vulnerability of deep learning models, particularly convolutional neural networks
(CNN)s, to adversarial attacks and presents a proactive training technique designed to counter them. We
introduce a novel volumization algorithm, which transforms 2D images into 3D volumetric representations.
When combined with 3D convolution and deep curriculum learning optimization (CLO), itsignificantly improves
the immunity of models against localized universal attacks by up to 40%. We evaluate our proposed approach
using contemporary CNN architectures and the modified Canadian Institute for Advanced Research (CIFAR-10
and CIFAR-100) and ImageNet Large Scale Visual Recognition Challenge (ILSVRC12) datasets, showcasing
accuracy improvements over previous techniques. The results indicate that the combination of the volumetric
input and curriculum learning holds significant promise for mitigating adversarial attacks without necessitating
adversary training.
2. Topics to be covered in this unit . . . . . . . . .
Framing
Flow Control
Error Control
Protocols
~ for Noiseless ( ideal ) Channels
~ for Noisy (practical or real ) Channels
HDLC Protocol
3. 2-1 FRAMING
The data link layer needs to pack bits into frames,
so that each frame is distinguishable from another.
Our postal system practices a type of framing.
The simple act of inserting a letter into an envelope
separates one piece of information from another;
the envelope serves as the delimiter.
Topics discussed in this section:
a) Fixed-Size Framing
[ The size itself acts as delimiter or frame boundary ]
b) Variable-Size Framing
[ special character or bit pattern called flag is used as delimiter ]
4. A frame in a character-oriented protocol
A frame in a bit-oriented protocol
5. Byte stuffing and unstuffing
Note
Byte stuffing is the process of adding 1 extra byte
whenever there is a flag or escape character in the text.
6. Bit stuffing and unstuffing
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 01111110 for a flag.
Note
7. 2-2 FLOW AND ERROR CONTROL
The most important responsibilities of the data link
layer are flow control and error control. Collectively,
these functions are known as data link control.
Topics discussed in this section:
a) Flow Control
b) Error Control
8. Flow control refers to a set of procedures
used to restrict the amount of data
that the sender can send before
waiting for acknowledgment.
Error control in the data link layer is
based on automatic repeat request (ARQ),
which is the retransmission of data.
9. 2-3 PROTOCOLS
The protocols are normally implemented in software by using one
of the common programming languages.
To make our discussions language-free, we have written in
pseudo code a version of each protocol that concentrates mostly on
the procedure instead of delving into the details of language rules.
10. 2-3 a) NOISELESS CHANNELS
Let us first assume we have an ideal channel in
which no frames are lost, duplicated, or corrupted.
In other words, the channel is error free and as a
result we don’t require any error control
mechanism in the corresponding protocols.
We introduce two protocols for this type of channel.
Topics discussed in this section:
a) Simplest Protocol
b) Stop-and-Wait Protocol
11. 1. Simplest Protocol
We assume that the receiver can immediately handle any frame
it receives with a processing time that is small enough to be
negligible.
In other words, the receiver can never be overwhelmed with
incoming frames.
Hence there is no need for flow control in this scheme.
15. 2. Stop-and-Wait Protocol
If the data frames arrives at the receiver site faster than they can
processed, the frames must be stored until their use.
Normally, the receiver does not have enough storage space, especially
if it is receiving data from many sources.
Hence to prevent the receiver from becoming overwhelmed with
incoming frames, we somehow need to tell the sender to slow down. As
such there must be feedback from the receiver to the sender. In other
words we need to employ a flow control mechanism in the protocol.
Acknowledgement (ACK) frames that are auxiliary frames help in this
regard.
The protocol is called Stop-and-Wait protocol because the sender
sends one frame, stops until it receives confirmation from the receiver
(okay to go ahead ) and then sends the next frame.
16. a) Design
Note :The regions highlighted with red shade indicate additions to the previous protocol.
20. 2-3 b) NOISY CHANNELS
Although the Stop-and-Wait Protocol gives us an idea
of how to add flow control to its predecessor, noiseless
channels are nonexistent. We discuss three protocols
in this section that use error control.
Topics discussed in this section:
a) Stop-and-Wait Automatic Repeat Request
b) Go-Back-N Automatic Repeat Request
c) Selective Repeat Automatic Repeat Request
21. 3. Stop-and-Wait ARQ Protocol
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.
In Stop-and-Wait ARQ, the acknowledgment number
always announces in modulo-2 arithmetic the sequence
number of the next frame expected.
26. c) Flow Diagram
Data Frame lost in the
middle of transmission.
Acknowledgement Frame lost
in the middle of transmission.
27. Efficiency & Pipelining
The Stop & Wait ARQ discussed in the previous section is
very inefficient if our channel is thick and long. By thick it is
meant that the channel has a large bandwidth; by long, we
mean the round-trip delay is long.
The product of these two is called the bandwidth-delay
product. The bandwidth-delay product is a measure of the
number of bits we can send out of our system while waiting
for news from the receiver.
Pipelining is a technique in which a task is often begun before
the previous task has ended. This is known as pipelining.
If we employ pipelining concept then several frames can be
sent before we receive news about the previous frames.
Pipelining improves efficiency of the transmission if the
number of bits in transition is large with respect to the
bandwidth-delay product.
28. Efficiency : 1st Numerical
1) Assume that, in a Stop-and-Wait ARQ system, the bandwidth of
the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What
is the bandwidth-delay product? If the system data frames are 1000
bits in length, what is the utilization percentage of the link?
Solution
The bandwidth-delay product is
The system can send 20,000 bits during the time it takes for the
data to go from the sender to the receiver and then back again.
However, the system sends only 1000 bits. We can say that the link
utilization is only 1000/20,000, or 5 percent. For this reason, for a
link with a high bandwidth or long delay, the use of Stop-and-Wait
ARQ wastes the capacity of the link.
29. Efficiency : 2nd Numerical
What is the utilization percentage of the link in
previous numerical example, if we have a protocol that
can send up to 15 frames before stopping and
worrying about the acknowledgments?
Solution
The bandwidth-delay product is still 20,000 bits. The
system can send up to 15 frames or 15,000 bits during a
round trip. This means the utilization is 15,000/20,000, or
75 percent. Of course, if there are damaged frames, the
utilization percentage is much less because frames have
to be resent.
30. 4. Go-Back-N ARQ Protocol
To improve efficiency of transmission, we should have a
protocol in which multiple frames must be in transition
while waiting for acknowledgement.
In other words, we need to let more than one frame be
outstanding to keep the channel busy while the sender is
waiting for acknowledgement.
Go-Back-N is one such protocol. In this protocol
we can send several frames before receiving
acknowledgements; we keep a copy of these frames
until the acknowledgements arrive.
31. i) Sequence Numbers
In the Go-Back-N Protocol, the sequence
numbers are modulo 2m, where m is the
size of the sequence number field in bits.
For example,
~ if m=2 then the sequence numbers will range from 0 to 3.
( 0, 1, 2, 3 )
~ If m=4 then the sequence numbers will range from 0 to 15.
( 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 )
33. The send window is an abstract concept
defining an imaginary box of size 2m − 1
with three variables: Sf, Sn, and Ssize.
The send window can slide one
or more slots when a valid
acknowledgment arrives.
35. 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.
37. Window size for Go-Back-N ARQ
Note
Will work correctly
Will work erroneously
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.
40. Receiver Site
No significant changes here compared to previous protocol
41. c) Flow Diagram [ for Example 1 ]
Acknowledgement Frame-2 lost.
Note : ACK3 acts as cumulative acknowledgement for both Frames 1 & 2.
42. Flow Diagram [ for Example 2 ]
Data Frame lost in the
middle of transmission.
Retransmission of
Frames 1, 2 and 3.
43. Stop-and-Wait ARQ is a special case of
Go-Back-N ARQ in which the size of the
send window is 1.
Note
44. 5. Selective Repeat ARQ Protocol
Go-Back-N ARQ simplifies the process at the receiver
site. The receiver keeps track of only one variable, and
there is no need to buffer out-of-order frames; they are
simply discarded.
However, this protocol is very inefficient for a noisy link.
In a noisy link a frame has higher probability of damage,
which means the resending of multiple frames. This
resending uses up the bandwidth and slows down the
transmission.
For noisy links, there is another mechanism that does
not resend N frames when just one frame is damaged;
only the damaged frame is resent. This mechanism is
called Selective Repeat ARQ. It is more efficient for noisy
links, but processing at the receiver is more complex.
45. Windows
a) Send window for Selective Repeat ARQ
b) Receive window for Selective Repeat ARQ
47. Selective Repeat ARQ, window size
Will work erroneously
Will work correctly
Note : In Selective Repeat ARQ, the size of the sender and
receiver window must be at most one-half of 2m.
53. c) Flow diagram
Data Frame lost in the
middle of transmission.
Frame 2 is received and
NAK is sent for Frame 1
Note : ACK4 acts as cumulative acknowledgement for Frames 1, 2 and 3.
55. 11-6 HDLC
High-level Data Link Control (HDLC) is a bit-oriented
protocol for communication over point-to-point and
multipoint links. It implements the ARQ mechanisms
we discussed in this chapter.
Topics discussed in this section:
a) Configurations and Transfer Modes
~ Normal Response Mode [NRM]
~ Asynchronous Balanced Mode [ABM]
b) Frames
c) Control Field
61. Numerical 1 on connection & disconnection in HDLC
Figure in the next slide 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 to
release the connection; it is confirmed by node B
responding with a UA (unnumbered acknowledgment).
63. Numerical 2 on piggybacking without error in HDLC
Figure in the next slide shows an exchange using piggybacking. Node
A begins the exchange of information with an I-frame numbered 0
followed by another I-frame numbered 1. Node B piggybacks its
acknowledgment of both frames onto an I-frame of its own. Node 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. Node B transmits its
second and third I-frames (numbered 1 and 2) before accepting
further frames from node A. Its N(R) information, therefore, has not
changed: B frames 1 and 2 indicate that node B is still expecting A’s
frame 2 to arrive next. Node A has sent all its data. Therefore, it
cannot piggyback an acknowledgment onto an I-frame and sends an
S-frame instead. The RR code indicates that A is still ready to receive.
The number 3 in the N(R) field tells B that frames 0, 1, and 2 have all
been accepted and that A is now expecting frame number 3.
65. Numerical 3 on piggybacking with error in HDLC
Figure in the next slide shows an exchange in which a
frame is lost. Node B sends three data frames (0, 1, and
2), but frame 1 is lost. When node A receives frame 2, it
discards it and sends a REJ frame for frame 1. Note that
the protocol being used is Go-Back-N with the special use
of an REJ frame as a NAK frame. The NAK frame does
two things here: It confirms the receipt of frame 0 and
declares that frame 1 and any following frames must be
resent. Node B, after receiving the REJ frame, resends
frames 1 and 2. Node A acknowledges the receipt by
sending an RR frame (ACK) with acknowledgment
number 3.