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☻Error control☻ ☻ Network architecture ☻

☻ Protocols ☻ ☻ Transmission Efficiency and Throughput ☻

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Causes of errors

• Errors are caused by: – various kinds of surrounding noise which

disturbs the signal going through a medium like copper, coaxial cable, etc.

– properties of the medium • attenuation distortion (high frequencies lose

power more rapidly than low frequencies)• delay distortion (different frequencies travel

through the medium at different speeds)

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Error Prevention

• Shielding

• Relocating cables

• Conditioning (carriers guarantee the maximum number of errors that can occur)– C-type conditioning compensates for attenuation and

delay distortions.

– D-type conditioning improves signal to noise ratio.

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ERROR DETECTION AND CONTROLPARITY CHECKING

Single Parity bit: Total number of 1 bits must always be even.V = 0110101 (7-bit ASCII code). Since, the number of 1’s is even, add a 0 as the eighth bit.Therefore, 8-bit representation of V is 01101010.

Similarly, W = 0001101 (7-bit ASCII code)Since, the number of 1’s is odd, add a 1 as the eighth bit.Therefore, 8-bit representation of W is 00011011.

(In odd parity system, total number of 1 bits is always odd.)What is the drawback with the single parity method?

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CYCLICAL PARITY CHECKING

This method requires two parity bits per character.Assuming six bits of code (bits 1 through 6) add twoparity bits (bits 7 and 8) such that bit 7 is the parity forbits 1, 3 and 5, while bit 8 is the parity for bits 2, 4, and 6.Again, total number of ones is even in both cases.

0 1 1 0 0 1 1 0

Parity 1

Parity 2

How is this method better than having a single parity bit?

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M-of-N CodesThe code is designed in such a way that there will always

be M 1’s and N-M 0’s in each valid character of the code.

Example

4-of-8 Code (from IBM)

In this 8-bit code there must be exactly 4 ones and 4 zeros.

Valid characters: Invalid characters:

00001111 00000111

01011010 11100000

00011110

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Parity Bit Yes Yes No Yes No No No Yes No No No

Bit place 1 2 3 4 5 6 7 8 9 10 11

Data 1 1 0 0 1 1 0

Parity 1 0 1 0

Transmit 1 0 1 1 1 0 0 0 1 1 0

The table above shows how the character 1100110 is converted into its hamming code equivalent. Even parity is used in this case.

Bit 1 checks: 1, 3, 5, 7, 9, 11

Bit 2 checks: 2, 3, 6, 7, 10, 11

Bit 4 checks: 4, 5, 6, 7

Bit 8 checks: 8, 9, 10, 11

Hamming codes

Hamming codes are Forward error correcting codes

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Cyclical Redundancy Check (CRC)

• This is applied to an entire block of data in synchronous communication.

• A 16-bit (or more commonly 32-bit) number is calculated from the entire block, and

attached to the end of the block by the sender.

• The receiver performs a similar calculation and compares the 16-bit value to see if it is the same. If they are not the same, it indicates an error in the transmission.

• This is a highly reliable scheme with almost 100% error detection capability.

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Transmission Efficiency and throughput

Transmission efficiency is defined as:

In asynchronous transmission, efficiency = 70%

In synchronous transmission, efficiency is much

higher

sent bits ofnumber Total

sent bits ofnumber Total ninformatio

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Throughput

Efficiency = 80%Error rate = 1%Modem speed = 9600 bits per second

Throughput = 9600 x 0.80 x (1 – 0.01) = 7603.2 bits per second

This is also called transmission rate of information bits. Note: Throughput is less than efficiency.

Throughput: number of information bits received per second after also accounting

for retransmissions due to errors.

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Network Architecture

Defines:

1) The way communications functions are divided into layers.

2) Protocols, standards and messages at each layer.

Objective of the layered approach:

1) Each layer performs one set of functions.

2) Each layer isolates the layers above it from the complexities below

Protocols in each layer are the set of rules agreed to and followed by both parties for successful communication.

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Layered Network Architecture

• Several layers are involved in data communications (7 in OSI, 4 in TCP/IP)

• The most important layers (and the ones in TCP/IP) are:– Application layer: handles the details of particular

applications (e.g., Telnet, Ftp, SMTP, SNMP). – Transport layer:provides reliable flow of data between end

system hosts for the application layer.– Network layer: performs addressing and routing.– Link Layer: responsible for error control, flow control,

message delineation, link management (media access control). Also called network interface layer.

General principle: division of work across layers.

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OSI Layered Protocol

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

Network layer

Data Link layer

Physical layer

Network layer

Data Link layer

Physical layer

Host Computer Host Computer

(Intermediate node)

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Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

ApplicationProgramming

Interface

What if your Web-browser used an Implementation of the OSI model?

1. Application sends message down to Network API and application layer communicates with application

CLIENT *openserver (char 180.232.23.10,char 80)

Get (http:180.232.23.10:80)

2. Presentation layer encodes message into commonly used data storage format (for example Ascii)

3. Session layer notifies end-system host of a pending data transmission.

Get (http:180.232.23.10:80)

4. Transport layer splits data into packets and controls flow

Get (http:180.232.23.10:80)

5. Network layer encapsulates message with sender and destination address

Get (http:180.232.23.10:80)TO

FRM

6. Data Link layer breaks up message into DL size packets

(http:180.23201111110 8 bits 8 or 16 bits 01111110

01111110 8 bits 8 or 16 bits 01111110.23.10:80)

GetTO

FRM

01111110

Begin Flag Address Control

8 bits 8 or 16 bits

Information

01111110

Ending Flag

7. Physical layer transforms bits into physical representation (voltages, sign-waves, pulses of light, etc.) and sends it onto the “wire” between two network devices.

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Network layer

Data Link layer

Physical layer

Network layer

Data Link layer

Physical layer

(Intermediate node)

1. Physical layer receives physical representation of bits turns them back DL packets

2. Data Link layer re-assembles NL packet

(http:180.23201111110 8 bits 8 or 16 bits 01111110

01111110 8 bits 8 or 16 bits 01111110.23.10:80)

GetTO

FRM

01111110

Begin Flag Address Control

8 bits 8 or 16 bits

Information

01111110

Ending Flag

3 Network layer receives packet and calculates best port to get to end-system – then sends packet back down to DLL

Get (http:180.232.23.10:80)TO

FRM

4. Data Link layer breaks up message into DL size packets

(http:180.23201111110 8 bits 8 or 16 bits 01111110

01111110 8 bits 8 or 16 bits 01111110.23.10:80)

GetTO

FRM

01111110

Begin Flag Address Control

8 bits 8 or 16 bits

Information

01111110

Ending Flag

5. Physical layer transforms bits into physical representation (voltages, sign-waves, pulses of light, etc.) and sends it onto the “wire” between two network devices.

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Get (http:180.232.23.10:80)

2. Presentation layer encodes message back into format used by application layer

3. Session layer notifies sending system that message has been received, and terminates connection

Get (http:180.232.23.10:80)

4. Transport layer re-assembles message (if needed)

Get (http:180.232.23.10:80)

5. Network layer removes packet/addressing information and sends information up to TL

Get (http:180.232.23.10:80)TO

FRM

6. Data Link layer re-assembles packet and sends it up to NL

(http:180.23201111110 8 bits 8 or 16 bits 01111110

01111110 8 bits 8 or 16 bits 01111110.23.10:80)

GetTO

FRM

01111110

Begin Flag Address Control

8 bits 8 or 16 bits

Information

01111110

Ending Flag

7. Physical layer receives physical representation of bits from the wire, and sends bit-patterns up to DLL

SERVER

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

ApplicationProgramming

Interface

1. Application layer sends message through Network API up to the server application

CLIENT *openserver (char 180.232.23.10,char 80)

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OSI Layered Protocol

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

Network layer

Data Link layer

Physical layer

Network layer

Data Link layer

Physical layer

Host Computer Host Computer

(Intermediate node)

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The Level 3 ApproachNetwork Layer Primary Attribute

Addressing

QoS

MultiplexingLow Error RateFault Tolerance

High Capacity

Physical Medium

1

2

3

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Layers in protocols

Source: Kurose and Ross (2001), “Computer Networking: A Top-Down Approach Featuring the Internet”

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Implementing an Architecture

• Each layer appends its own header to the application data.• At the receiving end, each layer strips off the corresponding

header.

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Data Link protocols

BSC (Binary Synchronous Communications)SDLC (Synchronous Data Link Control)HDLC (High Level Data Link Control)

Protocol Features and Issues•Communications line control (polling/selecting)•Framing•Addressing•Synchronization•Data transparency•Error control•Flow control•Fragmentation and reassembly

Need a reliable way of exchanging information at data link layer

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Multipoint SDLC network

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SDLC Frame

01111110

Begin Flag Address Control

8 bits 8 or 16 bits

Information

Variable length 16 bits

FrameCheckSequence

01111110

Ending Flag

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How Data Link Protocol Works(Automatic Repeat Request - ARQ - method)

• Stop and wait ARQ:– Sender stops and waits for response from

receiver after each packet– Receiver sends ACK if no errors in message– Receiver sends NACK if errors in message.

This is a half-duplex method used in BSC protocol.

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A B

frame 0

Ack1

frame 1

Ack0

Stop and wait ARQ

In this case, there are only two frames numbered 0 and 1

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Protocol for error correction(Automatic Repeat Request)

• Continuous ARQ: – sender does not wait for response from receiver

after each packet– receiver asks for retransmission of erroneous

packets.

This is a full-duplex method.

It is also called sliding window protocol.

It is used in SDLC protocol.

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A B

frame 0

1

234

REJ(5)

6

7567

Continuous ARQ

Frame 0 received okay

Frame 1 received okay

Frame 2 received okay

Frame 3 received okay

Frame 4 received okay

Frame 5 not received

Frame 6 received okay

Frame 7 received okay

Frame 5 received okay

Frame 6 received okay

Frame 7 received okay

5

Note: Received Receipts will go back from B to A (not animated)