PRESENTATION

ON
OSI REFFERENCE MODAL

NAME:OM KUMAR
ROLL NO.:0809710058
The OSI Reference Model
Data Link Layer: Virtual Vs. Actual
Communication

Virtual Communication Actual Communication

The Data Link Layer
Functions
Concerned with reliable, error-free and efficient communication
between adjacent machines in the network through the following
functions:
functions:
1 Data Framing:
The term “frame” refers to a small block of data used in a specific network.
The data link layer groups raw data bits to/from the physical layer into discrete
frames with error detection/correction code bits added. Framing methods:
– Character count.
– Starting and ending characters, with character stuffing.
– Starting and ending flags with bit stuffing.
– Physical layer coding violations.
2 Error Detection/Correction:
– Error Detection:
• Include enough redundant information in each frame to allow the receiver to deduce that an error
has occurred, but not which error and to request a retransmission.
• Uses error-detecting codes.
– Error Correction:
• Include redundant information in the transmitted frame to enable the receiver not only to deduce
that an error has occurred but also correct the error.
• Uses error-correcting codes.
The Data Link Layer
Functions
3 Services to the network layer:
– Unacknowledged connectionless service:
• Independent frames sent without having the destination acknowledge them.
• Suitable for real-time data such as speech and video where transmission
speed is more important than absolute reliability.
• Utilized in most LANS.
– Acknowledged connectionless service:
• Each frame sent is acknowledged by the receiver.
• Acknowledgment at the layer level is not essential but provides more
efficiency than acknowledgment at higher layers (transport) which is done
only for the whole message.
• A lost acknowledgment may cause a frame to be sent and received several
times.
The Data Link Layer
Functions
– Acknowledged connection-oriented service:
• The sender and receiver establish a connection before any data transmission.
• The message is broken into numbered frames.
• The data link guarantees that each frame sent is received exactly once and in the right
order.

4 Flow control:
Protocols to control the rate the sender transmits frames at a rate
acceptable to the receiver, and the ability to retransmit lost or damaged
frames. This insures that slow receivers are not swamped by fast
senders and further aids error detection/correction.
– Several flow control protocols exist, but all essentially require a form of
feedback to make the sender aware of whether the receiver can keep up.
• Stop-and-wait Protocols:
– A positive acknowledgment frame is send by the receiver to indicate that the frame
has been received and to indicate being ready for the next frame.
– Positive Acknowledgment with Retransmission (PAR); uses timeouts
• Sliding Window Protocols:
– Data frames and acknowledgement frames are mixed in both directions.
– Frames sent contain sequence numbers
– Timeouts used to initiate retransmission of lost frames.
Placement of The Data
Link Protocol
Data Channel

Adjacent routers/hosts shown

 Data Link Layer:
•Framing
The character count method:
– The frame header includes the count of characters in the
frame
– A transmission error can cause an incorrect count causing
the source and destination to get out of synchronization
– Rarely used in actual data link protocols

A character stream with no errors

A character stream with one error

Data Link Layer:
Framing
Using Starting and ending characters, with
character stuffing
• Each frame starts with the ASCII character sequence
DLE (Data Link Escape) and STX (Start of TeXt) and
ends
Network with
Layer DataDLE ETX (End of TeXt)
at the sender

• When binary data is transmitted where (DLE STX or DLE

ETX) can occur in data, character stuffing is used
(additional
Data after DLE byistheinserted
character stuffing in the
Data Link Layer data).
at the sender

• Limited to 8-bit characters and ASCII.

Network Layer Data at the Receiver

Data Link Layer:
Framing
• Bit-Oriented Using Start/End Flags:
– Each frame begins and ends with 01111110
– Bit stuffing: After each five consecutive ones in a data a
zero isData
The Original stuffed
– Stuffed zero bits are removed by the data link layer at
receiving end.
Data appearing on the line after bit stuffing

Data received after destuffing

 Data Link Layer: Error
•Detection/Correction
Simplest error detection : Parity bits and checksum (sum
of 1’s in data).
• Error-detecting and -correcting codes:
– m data bits + r redundant bits added.
– n = m + r transmitted in frame.
– Only 2m code words out of possible 2m+r words are legal.
– The Hamming distance --minimum number of positions any two
legal code words differ-- of a code defines its error
detection/correction ability.
– To detect d errors code Hamming distance = d + 1
– To correct d errors code Hamming distance = 2d + 1
– Some codes are more suitable to correct burst errors rather than
isolated errors.
– Polynomial codes: Cyclic Redundancy Check (CRC) Codes, are
characterized by a generating polynomial G(X)
 Cyclic Redundancy Check
•(CRC)
Based on polynomial arithmetic over finite field.
• View m-bit string a m-1a m-2 . . . a0 as a polynomial of degree m-
1:
M(x) = a m-1 x m-1 + a m-2 x m-2 + …. + a0
• Select a generating polynomial G(x) of degree r.
• Let R(x) be the remainder of xr M(x) / G(x)
• The code word T(x) of length m + r bit generated is then given by:
T(x) = xr M(x) - R(x)
• Assume code word T(x) is transmitted, but T(x) + E(x)
arrives at the receiver:
– If E(x) = 0 then no transmission errors and T(x)/G(x) = 0
– If E(x) 0 then transmission error(s) occurred and:
[T(x) + E(x)] / G(x)  0
Calculation of Polynomial Code
(CRC) Checksum 1. For degree of generating polynomial
G(x) = r , append r zero bits to low-order
of frame. The frame now has m+r bits.
2. Divide the bit string corresponding to
G(X) into the bit string xrM(x) mod(2)
3. Subtract the remainder R(x) from the
bit string xrM(x) mod(2)
Frame: 1 1 0 1 0 1 1 0 1 1
Generator: 1 0 0 1 1
G(X) = X4 + X + 1
Message after appending four 0’s:
1101011011 000 0
Remainder: 1110
Transmitted Frame:
11010110111110
Hardware
Computation of CRC
For G(x) = x16 + x12 + x5 + 1

1 +x5 +x12 +x16

An Example Frame Format with CRC bits

Common CRC Generator
Polynomials

• CRC-32: x32 + x 26 + x 23 + x22 + x16 + x12 + x11 +

x10 + x8 + x7 + x5 + x4 + x2 + x + 1
Used in FDDI, Ethernet.

• CRC-CCITT: x16 + X12 + x5 + 1

 Used in HDLC.

• CRC-8: x 8 + x2 + x + 1
 Used in ATM.
Use of A Hamming Code to
Correct Burst Errors