CS-09 – Data Communications and Networks

 

 

AGENDA (Session 6)

 

 

Quiz

 

Data Link Layer

 

Framing

 

Error Control – Flow Control

 

Error Detection

 

Error Correction

 

Elementary Protocols

 

 

 

                                     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Quiz

 

 

1. Basic Transfer rate of a SONET channel is

a) 34 Mbps    b) 51.84Mbps    c) 155 Mbps      d) 100Mbps

 

2. Circuit Switching is preferrable to packet switching for streaming audio/video because

a)    It provides faster connection setup.

b)   It utilises minimum bandwidth.

c)    Congestion can not occur once the call is thru.

d)   It allows interactive communication.

 

 

3. TDM (Time-Division Multiplexing) is implemented by digital switching technology.            TRUE/FALSE

 

4. B-ISDN primary rate is

a) 2B+1D       b) 30B+1D                   c)1A+1C    d) 2B+2D

 

5. B-ISDN network transfers information Synchronously.        TRUE/FALSE

 

6. Making hexagonal cells in a cellular radio technology

a)    Speeds-up scanning of Cellphones.

b)   Allows lower power handsets.

c)    Allows reuse of frequency in a cell.

d)   Allows unambigious mapping of geographical area for usage of a particular frequency.

 

7. ‘GSM’ stands for

a)    Global Satellite based Mobile phone.

b)   Global System for Mobile Communication.

c)    General Service for Mobile Communication.

d)   Global Service for Mobile Communication.

 

8. Geosynchronous Satellite can use ‘C’ band transmission only. TRUE/FALSE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Data Link Layer

 

 

 

 

 

 

 

 

 

 

 

 


·        Takes Data From Physical Layer

·        Passes it to Network Layer in exactly the same format as the Network Layer on other side must have handed it over to the DataLink Layer on the Other side.

·        Responsible for peer-to-peer connection between network layers

·        Provides Services like:

·        Takes care of Errors introduced by Physical medium.

·        Takes care of completeness of Information Transfer.

·        Framing shall be done at Data-Link Layer for implementing the Error Control and recovery mechanism.

 

 

 

 

 

 

 

 

 

 

 

 

 

Framing

 

·        Technology Inside Plant for Information Routing.

 

Character Count Method:

-----------Data Bytes----------------------
 

Count
 
 

 

 


·        Count Might Itself be lost.

·        Destination looses Synchronisation.

 

Start and End of Frame Method:

 

 

 

 


·        Frames start at DLE,STX and end at DLE,ETX

·        Problem with Binary Data solved by stuffing additional DLE being part of data. (Character Stuffing)

·        Assumes 7/8 Bit stream.

 

Start/End Flag and Bit Stuffing:

·        Frame begin and end with 01111110 flag.

·        Five consecutive bits of Data if found ‘1’, stream is stuffed with bit 0, to differentiate it with the flag pattern.

·        Easy Boundry Recognition for any number of Bit Patterns.

 

By Identifying Level Transition:

·        Useful where 1 bit is encoded as 2 bits for the purpose of decoding level transition.

·        High to low Transition – Bit 1, Low to High Transition – Bit 0.

·        Start and End Flags can be defined for No Transition either HIGH-HIGH or LOW-LOW.

 

 

 

 

Correct and Complete Transfer of Information

 

 

Error Control:

·        Positive or Negative response (Control Frame) on frame receipt.

·        Frames can vanish completely, so a timer is started and if acknowledgement doesn’t come in a finite time it is re-transmitted.

·        To avoid multiple frame receipt, due to acknowledgement loss, frames are also numbered.

 

Flow Control:

·        Required to deal with the problem of slow recepient talking to fast sender.

·        Cyclic Buffers are often employed along with flow control to give sufficient opportunity to receipt side processing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Error Detection and Correction

 

·        Errors may come as burst or at indefinite but continous rate.

·        Enough redundancy can be provided with the message so that recepient by itself determines the correct code (ERROR CORRECTING CODE).

·        Sufficient redundancy can be provided in the data to determine whether error is there (ERROR DETECTION)

 

Error Correcting Code

·        Hamming Distance = How many bits differ in two codes

 

10001001

    xxx

10110001                d=3

 

3 bit change will change 1 code to other code.

 

·        To detect ‘d’ errors d+1 distance is sufficient.

·        To correct ‘d’ errors 2d+1 distance is required.

·        So= 1 bit error can be corrected by 3 bit distance between codes.

·        Requirement of Reduntant bits can be worked out to

 

2r >= m+r+1

 

where m = Message Bits, r = additional redundant bits

 

·        For 1 Bit error correction Hamming code is a successful implementation of above.

·        1,2,4,16 are check bits. Other bits are expressed as summation of one of the above check bits.

·        Bits can be inverted if corresponding check-bit is wrong.

 

 

Error Detecting Codes

 

·       Error Correcting Codes are preferred over a simplex channel.

·       Detection of Error forms a basis for retransmission request in a duplex channel.

·       Efficient transmission is possible by error detection and re-transmission approach.

·       Block Level or Matrix Level parity Methods are sometimes insufficient for error detection.

·       Polynomial (also called CRC –Cyclic Redundancy) codes are extensively used.

·       Bit stream is treated as coefficients of polynomials of (k-1) degree, where k is no. of bits in frame.

Xk-1 + Xk-2 +………..+ X0

·        Sender and Receiver agree on same Generator Polynomial G(x).

·       Checksum appended as per m message bits and is checked by dividing received frame with G(x). No Remainder = No Error.

·       R checkbits shall detect all burst errors of length <=r.

·       Standards: CRC-12, CRC-16, CRC-CCITT.

·       Shift Registers can be used to implement calculation of CRC value.

·       Real data may still give problem with polynomials as they are designed for random errors.

 

 

 

 

 

 

Elementary Protocol Models – Unrestricted Simplex

 

The Protocol:

·       Sender Collects Message from Network layer.

·       Prepares a frame buffer.

·       Transfers to Physical layer.

·       Receiver keeps waiting for frame arrival event.

·       On Event it collects frame from physical layer.

·       Transfers message to network Layer.

 

Analysis:

·       Flow Management is absent.

·       Works on a perfect channel only.

·       Unrealistic, thus little pratical use.

·       Given here for explanation of wait for and event at receiver side.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Elementary Protocol Models –  Simplex Stop & Wait

 

The Protocol:

·       Sender Collects Message from Network layer.

·       Prepares a frame buffer.

·       Transfers to Physical layer.

·       Sender Waits for Go-Ahead.

·       Receiver keeps waiting for frame arrival event.

·       On Event it collects frame from physical layer.

·       Transfers message to network Layer.

·       Receiver Sends Go-Ahead (Acknowledgement) to Sender.

 

Analysis:

·       Flow Management is possible.

·       Loss of ACK frame can repeat frame earlier sent by sender.

·       Receiver can not detect repeated frame.

·       Channel on Other Side is required for returning the control frame.

·       Half-Duplex Physical channel can suffice as traffic at a time is only in one direction.

 

 

 

 

 

 

 

 

 

 

 

 

 

Elementary Protocol Models –  Simplex Stop & Wait

The Protocol:

·       Sender Collects Message from Network layer.

·       Prepares a frame buffer with Info and Frame Sequence no.

·       Transfers to Physical layer.

·       Start a timer to time-out if ACK is not received.

·       Sender Waits for Next-Frame Event .

·       Receiver keeps waiting for frame arrival event.

·       On Event it collects frame from physical layer.

·       Check whether FrameSequence is as expected

·       Transfers message to network Layer.

·       Receiver Sends Go-Ahead (Acknowledgement) to Sender.

 

Analysis:

·       Only Positive ACK is sent and Retransmission is controlled by timer. (PAR – positive ack with retransmission, ARQ – Automatic Repeat Request)

·       For a One to One ACK model 1 bit Frame Sequence number is sufficient.

·       May not be very efficient as acknowledgement timer has to reset on every frame transaction.

·       Still a Half-Duplex Model.