Computer Networks - 21CS52 - VTU Notes / Study Materials - Module 2

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Department ofInformation Science Engineering
Computer Networks Module 2
Dr Loganathan D Professor
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21CS32 - Computer Networks - Syllabus
Module 1 Introduction to networks Network
hardware, Network software, Reference models.
Physical Layer Guided transmission media,
Wireless transmission. Module 2 The Data link
layer Design issues of DLL, Error detection and
correction, Elementary data link protocols,
Sliding window protocols. The medium access
control sub layer The channel allocation
problem, Multiple access protocols.
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21CS32 - Computer Networks - Syllabus
Module 3 The Network Layer Network Layer Design
Issues, Routing Algorithms, Congestion Control
Algorithms, QoS. Module 4 The Transport Layer
The Transport Service, Elements of transport
protocols, Congestion control, The internet
transport protocols. Module 5 Application
Layer Principles of Network Applications, The
Web and HTTP, Electronic Mail in the Internet,
DNSThe Internets Directory Service.
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21CS32 - Computer Networks - Syllabus

• Module 2
• The Data Link Layer
• Design issues of DLL,
• Error detection and correction,
• Elementary data link protocols, Sliding window
  protocols.
• The medium access control sub layer
• The channel allocation problem,
• Multiple access protocols.

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Module 2 - The Data link layer
Design issues of DLL

• The Data Link Layer (DLL) uses the services of
  the physical layer to send and receive bits over
  communication channels.
• It has a number of functions, including
• 1. Providing a well-defined service interface to
  the network layer.
• 2. Dealing with transmission errors.
• 3. Regulating the flow of data so that slow
  receivers are not flooded by fast senders.
• To accomplish these goals, the data link layer
  takes the packets it gets from the network layer
  and encapsulates them into frames for
  transmission.
• Each frame contains a frame header, a payload
  field for holding the packet, and a frame
  trailer, as illustrated in Figure.

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Module 2 - The Data link layer
Design issues of DLL

• Frame Header It contains the source and the
  destination addresses of the frame and the
  control bytes. 
• Payload field It contains the message to be
  delivered.
• Trailer It contains the error detection and
  error correction bits.

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Module 2 - The Data link layer
Design issues of DLL

• Although this chapter is explicitly about the
  data link layer and its protocols, many of the
  principles we will study here, such as error
  control and flow control, are found in transport
  and other protocols as well.
• Services Provided to the Network Layer
• The function of the data link layer is to provide
  services to the network layer.
• The principal service is transferring data from
  the network layer on the source machine to the
  network layer on the destination machine.
• The network layer that hands some bits to the
  data link layer for transmission to the
  destination.
• The job of the data link layer is to transmit the
  bits to the destination machine so they can be
  handed over to the network layer there, as shown
  in Figure.

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Module 2 - The Data link layer
Design issues of DLL
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Module 2 - The Data link layer
Design issues of DLL
Placement of the data link protocol.
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Module 2 - The Data link layer
Design issues of DLL

• The data link layer can be designed to offer
  various services. The actual services that are
  offered vary from protocol to protocol. Three
  reasonable possibilities that we will consider in
  turn are
• 1. Unacknowledged connectionless service.
• 2. Acknowledged connectionless service.
• 3. Acknowledged connection-oriented service.
• Getting back to the services, the most
  sophisticated service the data link layer can
  provide to the network layer is
  connection-oriented service.
• With this service, the source and destination
  machines establish a connection before any data
  are transferred.
• Each frame sent over the connection is numbered,
  and the data link layer guarantees that each
  frame sent is indeed received.

Connection-oriented service Connection less
Service
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Module 2 - The Data link layer
Design issues of DLL
Connection-oriented service Connection less
Service

• When connection-oriented service is used,
  transfers go through three distinct phases.
• In the first phase, the connection is established
  by having both sides initialize variables and
  counters needed to keep track of which frames
  have been received and which ones have not.
• In the second phase, one or more frames are
  actually transmitted.
• In the third and final phase, the connection is
  released, freeing up the variables, buffers, and
  other resources used to maintain the connection.

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Module 2 - The Data link layer
Design issues of DLL

• Data Link Layer Design Issues
• Framing
• Error Control
• Flow Control

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Module 2 - The Data link layer
Design issues of DLL

• Framing
• To provide service to the network layer, the data
  link layer must use the service provided to it by
  the physical layer.
• What the physical layer does is accept a raw bit
  stream and attempt to deliver it to the
  destination.
• However, the bit stream received by the data link
  layer is not guaranteed to be error free.
• Some bits may have different values and the
  number of bits received may be less than, equal
  to, or more than the number of bits transmitted.
• It is up to the data link layer to detect and, if
  necessary, correct errors.

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Module 2 - The Data link layer
Design issues of DLL

• Framing
• Breaking up the bit stream into frames is more
  difficult than it at first appears.
• A good design must make it easy for a receiver to
  find the start of new frames while using little
  of the channel bandwidth.
• We will look at four methods to implement
  framing,
• 1. Byte count.
• 2. Flag bytes with byte stuffing.
• 3. Flag bits with bit stuffing.
• 4. Physical layer coding violations.

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Module 2 - The Data link layer
Design issues of DLL

• Framing involves,
• Detecting start of the frame
• How does the station detect a frame?
• Detecting end of frame
• Handling errors
• Framing overhead (bandwidth)
• Framing synchronization
• Two types of Framing -
• 1. Fixed-size and 2. Variable size - frame counts

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Module 2 - The Data link layer
Design issues of DLL
Framing - 1. Byte count method
Byte counting is a technique used in framing at
the data link layer. In this method, the length
of the frame (number of bytes) is explicitly
specified in the frame itself.
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Module 2 - The Data link layer
Design issues of DLL
Framing 2. Flag bytes with byte stuffing Method

• Byte stuffing is a method used to avoid confusion
  between data and control characters in a data
  stream. The second framing method gets around the
  problem of resynchronization after an error by
  having each frame start and end with special
  bytes.
• Often the same byte, called a flag byte, is used
  as both the starting and ending delimiter. This
  byte is shown in Figure as FLAG.
• However, there is a still a problem we have to
  solve. It may happen that the flag byte occurs in
  the data, especially when binary data such as
  photographs or songs are being transmitted.

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Module 2 - The Data link layer
Design issues of DLL
Framing 2. Flag bytes with byte stuffing Method

• One way to solve this problem is to have the
  senders data link layer insert a special escape
  byte (ESC) just before each accidental flag
  byte in the data. Thus, a framing flag byte can
  be distinguished from one in the data by the
  absence or presence of an escape byte before it.
• The data link layer on the receiving end removes
  the escape bytes before giving the data to the
  network layer. This technique is called byte
  stuffing.

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Module 2 - The Data link layer
Design issues of DLL
Framing 2. Flag bytes with byte stuffing Method
insert a special escape byte (ESC) just before
each accidental flag byte in the data.
Used in PPP (Point-to-Point Protocol), which is
used to carry packets over communications
links. The second framing method gets around the
problem of synchronization after an error by
having each frame start and end with special
bytes. Often the same byte, called a flag byte,
is used as both the starting and ending delimiter.
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Module 2 - The Data link layer
Design issues of DLL
Framing 3. Flag bits with bit stuffing.

• It was developed for the very popular HDLC (High
  level Data Link Control) protocol.
• Each frame begins and ends with a special bit
  pattern, 01111110 or 0x7E in hexadecimal. This
  pattern is a flag byte. When the receiver sees
  five consecutive incoming 1 bits, followed by a 0
  bit, it automatically deletes the 0 bit. Just as
  byte stuffing is completely transparent to the
  network layer in both computers, so is bit
  stuffing. If the user data contain the flag
  pattern, 01111110, this flag is transmitted as
  011111010 but stored in the receivers memory as
  01111110.

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Module 2 - The Data link layer
Design issues of DLL
Framing 3. Flag bits with bit stuffing.
0 added every after five consecutive ones -111110
111110
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Module 2 - The Data link layer
Design issues of DLL
Framing 4. Physical layer coding violations

• In networking, especially in the context of
  Ethernet, physical layer coding violations refer
  to situations where the normal coding rules are
  intentionally violated to create a unique signal
  pattern.
• In 100BASE-TX Ethernet, the normal Manchester
  encoding might be violated for certain control or
  special-purpose signals.
• The encoding of bits as signals often includes
  redundancy to help the receiver.
• This redundancy means that some signals will not
  occur in regular data.
• This can be detected by devices interpreting the
  signal.
• Used for special signaling or synchronization
  purposes in the physical layer.

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Module 2 - The Data link layer
Connection oriented or Connectionless Oriented
Services

• Connection oriented Services
• Requires the establishment of a connection before
  data transfer.
• TCP (Transmission Control Protocol) is a
  prominent example of a connection-oriented
  protocol.
• Implements flow control mechanisms to manage the
  rate of data transmission.
• Offers a reliable communication channel with
  error checking and correction mechanisms.
• Connectionless Oriented Services
• Does not require a pre-established connection
  before data transmission.
• UDP (User Datagram Protocol) is a common
  connectionless protocol.
• Provides no guarantee of reliable data delivery.
• Error checking may be minimal or absent.

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Module 2 - The Data link layer
Modem, Multiplexing, and error detection,

• Data Communication
• Understanding the fundamentals of data
  communication, including concepts like modem
  (Modulation and Demodulation), multiplexing, and
  error detection, provides a solid foundation for
  networking.
• Modulation is the process of encoding information
  in a transmitted signal, while demodulation is
  the process of extracting information from the
  transmitted signal. 
• Multiplexing is a technique used to combine and
  send the multiple data streams over a single
  medium. The process of combining the data streams
  is known as multiplexing and hardware used for
  multiplexing is known as a multiplexer.
• Error is a condition when the receivers
  information does not match the senders
  information.

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Module 2 - The Data link layer
RJ 45 8 pin Connector - Ethernet Cable Color
Coding Diagram 
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Module 2 - The Data link layer
Modes of Transmission Medium

• Modes of Transmission Medium
• Simplex mode In this mode, out of two devices,
  only one device can transmit the data, and the
  other device can only receive the data. Example-
  Input from keyboards, monitors, TV broadcasting,
  Radio broadcasting, etc.
• Half Duplex mode In this mode, out of two
  devices, both devices can send and receive the
  data but only one at a time not simultaneously.
  Examples- Walkie-Talkie, Railway Track, etc.
• Full-Duplex mode In this mode, both devices can
  send and receive the data simultaneously.
  Examples- Telephone Systems, Chatting
  applications, etc.

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Module 2 - The Data link layer

• Virtual Communication
• Virtual communication relies on digital
  platforms, such as email, video calls, instant
  messaging, and social media using digital medium.
• Participants may face more distractions in a
  virtual environment, affecting focus and
  attention.
• Face-to-face communication
• Face-to-face communication involves direct,
  in-person interaction between individuals
• Face-to-face conversations are typically not
  documented, relying on participants' memory for
  recall.

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Module 2 - The Data link layer
Design issues of DLL

• Frame how support in DLL?
• Error Control?
• Flow Control?
• Error Detection and Correction?
• Error Correcting Codes?

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Module 2 - The Data link layer
Design issues of DLL

• Error Control
• 1. If Sender receives a negative acknowledgement
  or positive acknowledgement about a frame
• 2. In case of malfunctioning hardware or a
  faulty communication channel. Timer will be used
  and to assign sequence numbers for outgoing
  frames
• The whole issue of managing the timers and
  sequence numbers so as to ensure that each frame
  is ultimately passed to the network layer at the
  destination exactly once, no more and no less, is
  an important part of the duties of the data link
  layer

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Module 2 - The Data link layer
Design issues of DLL

• Error Control
• Marking the start and end of each frame, we come
  to the next problem
• How to make sure all frames are eventually
  delivered to the network layer at the destination
  and in the proper order.
• Not bothering about unacknowledged connectionless
  service it might be fine if the sender just kept
  outputting frames without regard to whether they
  were arriving properly.
• But for reliable, connection-oriented service it
  would not be fine at all.

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Module 2 - The Data link layer
Design issues of DLL

• Error Control
• Typically, the protocol calls for the receiver to
  send back special control frames bearing Positive
  Acknowledgement or negative acknowledgement.
• If the sender receives a positive acknowledgement
  about a frame, it knows the frame has arrived
  safely.
• On the other hand, a negative acknowledgement
  means that something has gone wrong and the frame
  must be transmitted again.

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Module 2 - The Data link layer
Design issues of DLL

• Error Control
• In case of malfunctioning hardware or a faulty
  communication channel.
• Timer will be used and to assign sequence numbers
  for outgoing frames.
• This possibility is dealt with by introducing
  timers into the data link layer. When the sender
  transmits a frame, it generally also starts a
  timer.
• The timer is set to expire after an interval long
  enough for the frame to reach the destination, be
  processed there, and have the acknowledgement
  propagate back to the sender.
• Even if repeat to re transmit To prevent this
  from happening, it is generally necessary to
  assign sequence numbers to outgoing frames, so
  that the receiver can distinguish retransmissions
  from originals.

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Module 2 - The Data link layer
Design issues of DLL

• Flow Control
• Another important design issue that occurs in the
  data link layer is what to do with a sender that
  systematically wants to transmit frames faster
  than the receiver can accept them.
• Clearly, something has to be done to prevent this
  situation.
• NICs (Network Interface Cards) are sometimes
• said to run at wire speed, meaning that
  they can
• handle frames as fast as they can arrive on
  the link.

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Module 2 - The Data link layer
Design issues of DLL

• Flow Control
• Two approaches are commonly used to prevent the
  overflow control.
• Feedback-based flow control
• Rate-based flow control
• In the first one, feedback-based flow control,
  the receiver sends back information to the sender
  giving it permission to send more data, or at
  least telling the sender how the receiver is
  doing.
• In the second one, rate-based flow control, the
  protocol has a built-in mechanism that limits the
  rate at which senders may transmit data, without
  using feedback from the receiver.

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Module 2 - The Data link layer
Design issues of DLL
Error Detection and Correction
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Module 2 - The Data link layer
Design issues of DLL

• Error Detection and Correction
• Parity is commonly used in the design of
  error-checking codes.
• There are two main types of parity Odd Parity
  and Even Parity
• Although this chapter is explicitly about the
  data link layer and its protocols, many of the
  principles we will study here, such as error
  control and flow control, are found in transport
  and other protocols as well.
• The former strategy uses error-correcting codes
  and the latter uses error-detecting codes
• The use of error-correcting codes is often
  referred to as FEC (Forward Error Correction).

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Module 2 - The Data link layer
Design issues of DLL
Single-bit error
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Module 2 - The Data link layer
Design issues of DLL

• Erasure channel Other types of errors also
  exist. Sometimes, the location of an error will
  be known, perhaps because the physical layer
  received an analog signal that was far from the
  expected value for a 0 or 1 and declared the bit
  to be lost. This situation is called an erasure
  channel.
• We will examine both error-correcting codes and
  error-detecting codes next.
• Error-correcting codes are also seen in the
  physical layer, particularly for noisy channels,
  and in higher layers, particularly for real-time
  media and content distribution.
• Error-detecting codes are commonly used in data
  link, network, and transport layers.

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Module 2 - The Data link layer
Error-Correcting Codes
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Module 2 - The Data link layer
Error-Correcting Codes

• Errors and Error Correcting Codes (EEC)
• We will examine four different error-correcting
  codes/models
• 1. Hamming codes.
• 2. Binary convolutional codes.
• 3. Reed-Solomon codes.
• 4. Low-Density Parity Check (LDPC) codes.
• All of these codes add redundancy to the
  information that is sent.
• A frame consists of m data (i.e., message) bits
  and r redundant (i.e. check bits) bits.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes add redundancy to
• Block code
• Systematic code
• linear code
• Convolutional codes

Types 1. Hamming codes. 2. Binary convolutional
codes. 3. Reed-Solomon codes. 4. Low-Density
Parity Check (LDPC) codes.
In general, block codes, systematic codes, linear
codes and binary conventional codes are all types
of error-correcting codes used in various types
of error correcting models in digital
communication systems.
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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes add redundancy in
• In a block code, the r check bits are computed
  solely as a function of the m data bits with
  which they are associated, as though the m bits
  were looked up in a large table to find their
  corresponding r check bits.
• In a systematic code, the m data bits are sent
  directly, along with the check bits, rather than
  being encoded themselves before they are sent.
• In a linear code, the r check bits are computed
  as a linear function of the m data bits.
• Convolutional codes use shift registers and
  modulo-2 addition (XOR) to encode input data.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Hamming code
• The Hamming distance is a metric used to measure
  the difference between two strings of equal
  length.
• For example, consider the two strings 101010
  and 111000. The Hamming distance between these
  two strings is 3, since there are three positions
  where the corresponding symbols are different
  the first, fourth, and sixth positions
• Hamming code is a type of linear block code that
  is used to detect and correct errors in digital
  communication systems.
• It is also a systematic code that adds redundant
  bits to the message to detect and correct errors.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Hamming code
• In Hamming codes the bits of the codeword are
  numbered consecutively, starting with bit 1 at
  the left end, bit 2 to its immediate right, and
  so on.
• Basic Hamming code above corrects 1-bit errors
  only.
• Trick to use it to correct burst errors (A burst
  error is a type of error when two or more bits in
  the data unit have changed from 0 to 1 or vice
  versa )
• The bits that are powers of 2 (1, 2, 4, 8, 16,
  etc.) are check bits.
• The rest (3, 5, 6, 7, 9, etc.) are filled up with
  the m data bits. This pattern is shown for an
  (11,7) Hamming code with 7 data bits and 4 check
  bits in figure.
• Let the total length of a block be n (i.e., n m
  r). We will describe this as an (n,m) code. An
  n-bit unit containing data and check bits is
  referred to as an nbit codeword.

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Module 2 - The Data link layer
Error-Correcting Codes
Error-Correcting Codes - Hamming code
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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Hamming code
• How it works?
• Example 1
• 21 (as sum of powers of 2) 1 4 16
• Bit 21 is checked by check bits 1, 4 and 16.
• No other bit is checked by exactly these 3 check
  bits.
• Assume one-bit error
• If any data bit bad, then multiple check bits
  will be bad.
• Example 2
• 23 (as sum of powers of 2) 1 2 4 16
• Bit 21 is checked by check bits 1,2, 4 and 16.
• If nbit codeword will check and compare, all data
  bit are goodit might a good frame.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Binary Convolutional
  code
• The second code we will look at is a
  convolutional code. This code is the only one we
  will cover that is not a block code.
• In a convolutional code, an encoder processes a
  sequence of input bits and generates a sequence
  of output bits.
• The output depends on the current and previous
  input bits.
• That is, the encoder has memory.
• Convolutional codes are specified in terms of
  their Code rate and constraint length.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Binary Convolutional
  code
• Basically, there are mainly two parameters that
  define the convolutional coding which are as
  follows
• Constraint length
• The number of previous bits on which the output
  depends is called the constraint length of the
  code
• Code rate
• It is the ratio of a number of bits shifted at
  once within the shift register (denoted by k) to
  the total number of bits in an encoded
  (generated) bit stream (denoted by n).

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Binary Convolutional
  code
• A convolutional code can be represented as (n,k,
  K) where
• k is the number of bits shifted into the encoder
  at one time.
• Generally, k 1.
• n is the number of encoder output bits
  corresponding to k information bits.
• The code-rate, Rc  k/n .
• The encoder memory, a shift register of size k,
  is the constraint length.
• n is a function of the present input bits and the
  contents of K.
• The state of the encoder is given by the value of
  (K - 1) bits.

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Module 2 - The Data link layer
Error-Correcting Codes
Error-Correcting Codes - Binary Convolutional
code
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Module 2 - The Data link layer
Error-Correcting Codes
For example, if 111 is input and the initial
state is all zeros, the internal state, written
left to right, will become 100000, 110000, and
111000 after the first, second, and third bits
have been input. The output bits will be 11,
followed by 10, and then 01. It takes seven
shifts to flush an input completely so that it
does not affect the output. The constraint length
of this code is thus k 7.
Error-Correcting Codes - Binary Convolutional
code
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Module 2 - The Data link layer
Error-Correcting Codes

• Convolutional codes have been popular in practice
  because it is easy to factor
• the uncertainty of a bit being a 0 or a 1
  into the decoding.
• For example, suppose -1V is the logical 0 level
  and 1V is the logical 1 level, we might receive
  0.9V and -0.1V for 2 bits.
• Instead of mapping these signals to 1 and 0 right
  away, we would like to treat 0.9V as very
  likely a 1 and -0.1V as maybe a 0 and
  correct the sequence as a whole.
• This approach of working with the uncertainty of
  a bit is called soft-decision decoding.
  Conversely, deciding whether each bit is a 0 or a
  1 before subsequent error correction is called
  hard-decision decoding.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Reed-Solomon code.
• Reed-Solomon codes are the code that was
  introduced by Irving S. Reed and Gustave Solomon.
  
• The encoder of Reed-Solomon codes differs from a
  binary encoder in that it operates on multiple
  bits rather than individual bits.
• So basically, Reed-Solomon codes help in
  recovering corrupted messages that are being
  transferred over a network.
• In Reed-Solomon codes, we have
• Binary Encoder and 
• Binary Decoder

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Reed-Solomon code.
• Parameters of Reed-Solomon code
• (n, k) code is used to encode m-bit symbols.
• Block length(n) is given by 2m-1 symbols.
• In Reed-Solomon codes, message size is given by
  (n-2t) where t number of errors corrected.
• Parity check size is given by (n-k) or 2t
  symbols.
• Minimum distance(a) is given by (2t1).
• Message size is of k bits.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - Reed-Solomon code.
• Decoding
• At the receiver end we perform the following
  methods
• The receiver receives r(x) at the receiver end.
• If s(x) r(x) then r(x)/g(x) has no remainder.
• If it has remainder, then r(x) p(x) g(x)
  e(x) where e(x) is an error polynomial

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - LDPC (Low-Density Parity
  Check) code
• The final error-correcting code we will cover is
  the LDPC (Low-Density Parity Check) code. This
  corrects errors.
• LDPC codes are linear block codes that were
  invented by Robert Gallagher. In an LDPC code,
  each output bit is formed from only a fraction of
  the input bits.
• This leads to a matrix representation of the code
  that has a low density of 1s, hence the name for
  the code. The received codewords are decoded with
  an approximation algorithm that iteratively
  improves on a best fit of the received data to a
  legal codeword.

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Module 2 - The Data link layer
Error-Correcting Codes

• Error-Correcting Codes - LDPC (Low-Density Parity
  Check) code
• Representations for LDPC codes Basically there
  are two different possibilities to represent LDPC
  codes.
• Like all linear block codes they can be described
  via matrices.
• The second possibility is a graphical
  representation.
• A LDPC code is represented by , where is the
  block length, is the number of 1s in each column
  and is the number of 1s in each row, holding the
  following properties -
• j is the small fixed number of 1s in each
  column, where j gt 3
• k is the small fixed number of 1s in each row,
  where k gt j.

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Module 2 - The Data link layer
Error-Correcting Codes
Low - Density Parity Check Matrix This examples
illustrates an (12, 3, 4) LDPC matrix, i.e. n
12, j 3 and k 4. This implies that each
equation operates on 4 code symbols and each code
symbol appears in 3 equations. Unlike parity
check matrix of the Hamming code, this code does
not have any diagonal 1s in parity bits.
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Module 2 - The Data link layer
Error-Detecting Codes
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Module 2 - The Data link layer
Error-Detecting Codes

• Data can be corrupted during transmission. Some
  applications require that errors be detected and
  corrected.

In a single-bit error, only 1 bit in the data
unit has changed.
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Module 2 - The Data link layer
Error-Detecting Codes
A burst error means that 2 or more bits in the
data unit have changed.
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Module 2 - The Data link layer
Error-Detecting Codes

• To detect or correct errors, we need to send
  extra (redundant) bits with data.
• we can divide our message into blocks(frames),
  each of k bits, called datawords (message). We
  can add r redundant bits to each block to make
  the length n k r.
• The resulting n-bit blocks are called codewords
• An error-detecting code can detect only the types
  of errors for which it is designed other types
  of errors may remain undetected.

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Module 2 - The Data link layer
Error-Detecting Codes

• We will examine three different error-detecting
  codes.
• They are all linear, systematic block codes
• 1. Parity.
• 2. Checksums.
• 3. Cyclic Redundancy Checks (CRCs).

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Module 2 - The Data link layer
Error-Detecting Codes

• Parity Error detecting Code
• Parity Bit Method  
• A parity bit is an extra bit included in binary
  message to make total number of 1s either odd or
  even.
• Parity word denotes number of 1s in a binary
  string.
• Always transmit according to the rule
• On receipt, if rule is violated, word is invalid
• There are two parity system even and odd parity
  checks.

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Module 2 - The Data link layer
Error-Detecting Codes

• Parity Error detecting Code
• Almost all block codes used today belong to a
  subset called linear block codes. A linear block
  code is a code in which the XOR (addition
  modulo-2) of two valid codewords creates another
  valid codeword.
• Even Parity Bit
• Doing this is equivalent to computing the (even)
  parity bit as the modulo 2 sum or XOR of the data
  bits.

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Module 2 - The Data link layer
Error-Detecting Codes
Parity Error detecting Code Even Parity Bit
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Module 2 - The Data link layer
Error-Detecting Codes

• Parity Error detecting Code
• Disadvantage
• One difficulty with this scheme is that a single
  parity bit (either even or odd parity check) can
  only reliably detect a single-bit error in the
  block.
• If the block is badly garbled by a long burst
  error, the probability that the error will be
  detected is only 0.5, which is hardly acceptable.
• However, there is something else we can do that
  provides better protection against burst errors
  we can compute the parity bits over the data in a
  different order than the order in which the data
  bits are transmitted.
• Doing so is called interleaving.

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Module 2 - The Data link layer
Error-Detecting Codes
Parity Error detecting Code Interleaving
procedure Suppose that a certain error detection
code has generated the four codewords 10011101,
00101100, 10100101 and 11110000. The next
diagrams illustrates an interleaving procedure
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Module 2 - The Data link layer
Error-Detecting Codes
Parity Error detecting Code
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Module 2 - The Data link layer
Parity Error detecting Code
Periodic interleaving The sequential blocks of
code may be written to a matrix in a row-wise
manner and then read out from the matrix in a
column-wise manner
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Module 2 - The Data link layer
Error-Detecting Codes

• Checksums Error detecting Code
• The second kind of error-detecting code, the
  checksum, is closely related to groups of parity
  bits.
• The word checksum is often used to mean a
  group of check bits associated with a message,
  regardless of how are calculated.
• A group of parity bits is one example of a
  checksum.
• However, there are other, stronger checksums
  based on a running sum of the data bits of the
  message.
• The checksum is usually placed at the end of the
  message, as the complement of the sum function.
• This way, errors may be detected by summing the
  entire received codeword, both data bits and
  checksum. If the result comes out to be zero, no
  error has been detected.

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Module 2 - The Data link layer
Error-Detecting Codes

• Checksums Error detecting Code
• One example of a checksum is the 16-bit Internet
  checksum used on all Internet packets as part of
  the IP protocol

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Module 2 - The Data link layer
Error-Detecting Codes

• Checksums Error detecting Code
• The Receiver after receiving data checksum
  passes it to checksum checker.
• Checksum checker divides this data unit into
  various subunits of equal length and adds all
  these subunits.
• These subunits also contain checksum as one of
  the subunits.
• The resultant bit is then complemented.
• If the complemented result is zero, it means the
  data is error-free.
• If the result is non-zero it means the data
  contains an error and Receiver rejects it. 

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Module 2 - The Data link layer
Error-Detecting Codes

• Checksums Error detecting Code
• Ones or Twos complement arithmetic process
• Modern computers run twos complement arithmetic,
  in which a negative number is the ones
  complement plus one.
• On a twos complement computer, the ones
  complement sum is equivalent to taking the sum
  modulo and adding any overflow of the high
  order bits back into the low-order bits.
• This algorithm gives a more uniform coverage of
  the data by the checksum bits.
• There is another benefit, too. Ones complement
  has two representations of zero, all 0s and all
  1s. This allows one value (e.g., all 0s) to
  indicate that there is no checksum, without the
  need for another field.

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Module 2 - The Data link layer
Error-Detecting Codes

• Checksums Error detecting Code
• The Internet checksum in particular is efficient
  and simple but provides weak protection in some
  cases precisely because it is a simple sum.
• It does not detect the deletion or addition of
  zero data, nor swapping parts of the message, and
  it provides weak protection against message
  splices in which parts of two packets are put
  together.
• These errors may seem very unlikely to occur by
  random processes, but they are just the sort of
  errors that can occur with buggy hardware.
• A better choice is Fletchers checksum (Fletcher,
  1982). It includes a positional component, adding
  the product of the data and its position to the
  running sum.
• This provides stronger detection of changes in
  the position of data.

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Module 2 - The Data link layer
Error-Detecting Codes

• CRC (Cyclic Redundancy Check) Error detecting
  Code
• Although the two Parity check and Checksum
  schemes may sometimes be adequate at higher
  layers, in practice, a third and stronger kind of
  error-detecting code is in widespread use at the
  link layer is the CRC (Cyclic Redundancy Check),
  also known as a polynomial code.
• Data
• CRC Generator
• CRC Bits reminder bits not Quotient bits
• CRC Generator -gt n bits
• CRC Bits -gt (n-1) bits

Data (n-1) 0s CRC Generator -gt n bits
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Module 2 - The Data link layer
Error-Detecting Codes

• CRC enables the transmit end to append an R-bit
  check code to the K-bit data to be sent, generate
  a new frame, and send the frame to the receive
  end.
• When receiving the new frame, the receive end
  checks whether the received data is correct based
  on the received data and check code.

CRC Code
Data
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Module 2 - The Data link layer
Error-Detecting Codes

• CRC (Cyclic Redundancy Check) Error detecting
  Code
• CRC Cyclic Redundancy Check or polynomial code
• Consider bits of a message to be coefficients of
  a polynomial
• M(x) 11000 1x4 1x3 0x2
  0x1 0x0
• Of course real messages will be
  much longer and hence of higher degree
• Agree on a small-degree generator polynomial G(x)
  of degree r
• Note G(x) has r digits to the right of
  the leading 1, hence r1 total
• Divide xrM(x) by G(x) using modulo 2 division
  (algebraic field theory does not have carries for
  addition or borrows for subtraction. Both
  addition and subtraction are identical to
  exclusive OR.) getting the remainder polynomial
  R(x)
• Transmit T(x) xrM(x) - R(x) note that this has
  remainder 0 when divided by G(x)
• Receiver rejects frame if the remainder it
  computers is not 0

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Module 2 - The Data link layer
Error-Detecting Codes
CRC (Cyclic Redundancy Check) Error detecting
Code The algorithm for computing the CRC is as
follows 1. Let r be the degree of G(x). Append r
zero bits to the low-order end of the frame so it
now contains m r bits and corresponds to the
polynomial x rM(x). 2. Divide the bit string
corresponding to G(x) into the bit string
corresponding to x rM(x), using modulo 2
division. 3. Subtract the remainder (which is
always r or fewer bits) from the bit string
corresponding to x rM(x) using modulo 2
subtraction. The result is the check summed frame
to be transmitted. Call its polynomial T(x).
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Module 2 - The Data link layer
Error-Detecting Codes
An R-bit check code is appended to the K-bit
data, and the entire code length becomes N bits.
This type of code is also referred to as (N,K)
code. For a given (N,K) code, it can be proved
that there is a polynomial g(x) whose highest
power is NKR, and an R-bit check code can be
generated based on g(x). The algorithm is based
on the GF(2) polynomial arithmetic, as shown in
the figure.
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Figure illustrates the calculation for a frame
1101011111 using the generator G(x) x 4 x 1.
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Module 2 - The Data link layer
Error-Detecting Codes

• Certain polynomials have become international
  standards.
• The one used in IEEE 802 followed the example of
  Ethernet and is
• Although the calculation required to compute the
  CRC may seem complicated, it is easy to compute
  and verify CRCs in hardware with simple shift
  register Circuits.

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Module 2 - The Data link layer
Error-Detecting Codes

• Easily computed with feedback shift register
  hardware
• Detects any single-bit error
• Proper choice of G(x) gives detection of any two
  bit errors
• Proper choice of G(x) gives detection of any odd
  number of bit errors
• Detects any burst error of length lt r
• Received message is T(x)E(x) for some error
  polynomial E(x)
• If E(x) represents a burst of length lt r then
  it can be written as xi(F(x)) where the degree of
  F(x) is lt r
• If G(x) has an x0 term it cant divide xi and no
  degree r polynomial divides a polynomial of
  degree lt r

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• To start with, we assume that the physical layer,
  data link layer, and network layer are
  independent processes that communicate by passing
  messages back and forth.
• A common implementation is shown in Figure.
• The physical layer process and some of the data
  link layer process run on dedicate hardware
  called a NIC (Network Interface Card).
• The link layer process often taking the form of a
  device driver (OS).
• Three processes (physical layer, data link layer,
  and network layer ) offloaded to dedicated
  hardware called a network accelerator, or three
  processes running on the main CPU.
• In any event, treating the three layers as
  separate processes makes the discussion
  conceptually cleaner and also serves to emphasize
  the independence of the layers.

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Another key assumption is that machine A wants to
  send a long stream of data to machine B, using a
  reliable, connection-oriented service.
• Later, we will consider the case where B also
  wants to send data to A simultaneously.
• A is assumed to have an infinite supply of data
  ready to send and never has to wait for data to
  be produced.
• Instead, when As data link layer asks for data,
  the network layer is always able to comply
  immediately.
• We also assume that machines do not crash.
• That is, these protocols deal with communication
  errors, but not the problems caused by computers
  crashing and rebooting

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Utopia
• Data are transmitted in one direction only.
• Both the transmitting and receiving network
  layers are always ready.
• Processing time can be ignored.
• Infinite buffer space is available.
• And best of all, the communication channel
  between the data link layers never damages or
  loses frames.
• This thoroughly unrealistic protocol, which we
  will nickname Utopia, is simply to show the
  basic structure on which we will build.

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Assumptions
• Each layer (physical, data link and network) are
  independent of each other.
• Machine A wants to send a long data stream to
  machine B using a reliable connection-oriented
  service.
• The data link layer encapsulates the packet in a
  frame by adding a data link header and trailer.
• The software (and hardware) is dedicated full
  time to handling just one channel.
• Checksums are computed and used to manage
  erroneous frames.
• A frame header and tail will never be given to
  the network layer.

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Five data structures that describe a frame
  boolean, integer, packet, frame kind and frame.
• A frame is composed of four fields (frame,
  header) kind, seq, ack, and info
• Frame sequence numbers are always in the range of
  0 and a predefined maximum constant and are
  incremented in a circular manner. Timers are used
  to manage lost or missing frames or
  acknowledgements Network layer can be disabled to
  control the number of packets to prevent the
  network layerfrom swamping it with packets for
  which it has no buffer space

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• An Unrestricted Simplex Protocol UTOPIA
• Data transmitted in one direction only
• Transmitting and receiving layers are always
  ready
• Processing time is ignored
• Infinite buffer space
• Ideal communication channel - no loss, no errors

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• A Simplex Stop-and-Wait Protocol
• Consider a slow receiver, No buffer space and
  Slow Processor but channel is still error free
  and traffic is still simplex
• Possible solutions
• Introduction of delays
• Send information very slowly
• Stop and wait
• Sender only sends out the succeeding packet
  afterreceiving an acknowledgement,
• Data traffic is simplex,
• Communication channel needs to be bidirectional
• Communication channel can be half-duplex

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• A Simplex Protocol for a Noisy Channel
• Lost or damaged frames
• Damage must be detectable through checksums
• Possible solution
• Receiver only acknowledges when there is an
  undamaged frame received.
• What happens if the acknowledgement frame gets
  lost? The protocol would keep passing wrong
  information up to the network protocol.
• The proper solution requires the receiver to
  distinguish a frame that it is seeing for
  thefirst time from a retransmission.
• Use sequence numbers

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• A Simplex Protocol for a Noisy Channel
• What is the minimum number of bits needed for the
  sequence number?
• To ensure that the frame header is also small
  1-bit sequence number is sufficient
• Protocols in which the sender waits for a
  positive acknowledgement before advancing to
  thenext data item
• PAR (Positive Acknowledgement with
  Retransmission)
• ARQ (Automatic Repeat request)
• Timeout interval must be long enough to prevent
  premature timeouts
• It cannot be set too short allow time for the
  frame to reach the receiver, to get processed and
  to receive an acknowledgement

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• A Simplex Protocol for a Noisy Channel
• Sender can wait for three things
• an acknowledgement frame arrives undamaged
• an acknowledgement frame arrives damaged timer
  goes off
• Valid frames are passed on to the network layer
• Duplicate frames and damaged frames are not
  passed on to the network layer.

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
DLL Protocols

• Stop and Wait

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
DLL Protocols

• Stop and Wait Rules

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
DLL Protocols Flow Control
ARQ (Automatic Repeat request) Or PAR
(Positive Acknowledgement with Retransmission).
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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
A utopian simplex Protocol Sender Side
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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
A utopian simplex Protocol Receiver Side
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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
A Simplex Stop-and-Wait Protocol for an
Error-Free Channel Sender Side
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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS
A Simplex Stop-and-Wait Protocol for an
Error-Free Channel Receiver Side
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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Sendand-Wait Vs Sliding Window Protocols
• Sendand-Wait - Only one frame can send at a
  time
• Where as Sliding Window Protocols allows to send
  multiple frames at a time
• The sliding window is a technique for sending
  multiple frames at a time.
• It controls the data packets between the two
  devices where reliable and gradual delivery of
  data frames is needed.
• It is also used in TCP (Transmission Control
  Protocol).

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Sliding Window Protocols
• There are three protocols, which are
  bidirectional protocols that belong to a class
  called sliding window protocols.
• A One-Bit Sliding Window Protocol
• A Protocol Using Go-Back-N ARQ
• A Protocol Using Selective Repeat
• The three differ among themselves in terms of
  efficiency, complexity, and buffer requirement.
• In these, as in all sliding window protocols,
  each outbound frame contains a sequence number,
  ranging from 0 up to some maximum.

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Module 2 - The Data link layer
ELEMENTARY DATA LINK PROTOCOLS

• Sliding Window Protocols
• The maximum is usually 2n - 1 so the sequence
  number fits exactly in an n-bit field.
• The stop-and-wait sliding window protocol uses n
  1, restricting the sequence numbers to 0 and 1,
  but more sophisticated versions can use an
  arbitrary n.
• The essence of all sliding window protocols is
  that at any instant of time, the sender maintains
  a set of sequence numbers corresponding to frames
  it is permitted to send.
• These frames are said to fall within the s