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Part II: Packet Transmission

Packets on a Network

Packets, Frames, LAN, WAN, Hardware Addresses, Bridges, Switches, Routing and

Protocols

Fall 2005

Qutaibah MalluhiComputer Science and Engineering

Qatar University

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Packets, Frames and Error Detection

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Packet and Circuit Switching

Circuit-switched networks (connection oriented)– Form a dedicated connection (circuit) between two

points

– Guaranteed capacity but high-cost (cost is fixed and is independent of the traffic)

– E.g. telephone system

Computer networks are often called packet switched networks (Connectionless)– Data divided into small pieces (packets)

– Each packet is sent individually

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Motivation for Packet Switching

Coordination - helps transmitter and receiver determine which data have been received correctly and which have not– Better more efficient control on transmission errors

fair Access - each computer can only send one packet at a time

Resource sharing - allows multiple computers to have more effective sharing of the expensive network infrastructure– Most network use shared media which interconnect all

computers – However - only one source

can transmit data at a time

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Dedicated (non packet-switched) Access

5MB file transferred across network with 56Kbps capacity will require 12 minutes:

5x106 bytes * 8 bits/byte ---------------------------------------------- = 11.9 minutes

60 secs/minute * 56x103 bits/second

All other computers will be forced to wait 12 minutes before initiating other transfers

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Packet Switched Access

If a 5 MB file is broken into packets, other computers must only wait until packet (not entire file) has been sent

From previous example, suppose file is broken into 1000 byte packets– Each packet takes less than 0.15 seconds to transmit:

1000 bytes * 8 bits/byte ----------------------------------- = 0.143 seconds 56 * 103 bits/second

Other computer must only wait 0.143 seconds before beginning to transmit

Note: – If both files are both 5MB long, each now takes 24 minutes to

transmit – BUT if second file is only 10KB long, it will be transmitted in

only 2.8 seconds, while 5MB file still takes roughly 12 minutes Packet Switching provides fair and prompt

access to shared network resources Packet Switching provides fair and prompt

access to shared network resources

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Packets Allow Time Division Multiplexing

Dividing data into small packets conceptually permits providing a form of time-division multiplexing

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Packets and frames

Packet is ``generic'' term that refers to a small block of data

Each hardware technology uses different packet format

Frame or hardware frame denotes a packet of a specific format on a specific hardware technology – Need to define a standard format for data to indicate

the beginning and end of the frame » Header and trailer used to ``frame'' the data» Can choose two unused data values for framing. E.g., if

data is limited to printable ASCII, can use ``start of header'' (soh) ``end of text'' (eot)

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Frame Delimiters

Incurs extra overhead – soh and eot take time to transmit, but carry no data

Accommodates transmission problems: – Missing eot indicates sending computer crashed – Missing soh indicates receiving computer missed

beginning of message – Bad frame is discarded

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Data Stuffing

May not afford to reserve two special characters for framing – E.g., transmitting arbitrary binary data – soh and eot can be part of data will be misinterpreted

as framing data Solution: Bit stuffing and byte stuffing

– Inserting extra data to encode reserved bytes– Sender and receiver must agree to encode special

characters for unambiguous transmission Example:

– Byte stuffing translates each reserved byte into two unreserved bytes

– For example, can use esc as prefix, followed by x for soh, y for eot and z for esc:

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Byte Stuffing

Sender scans data and translates each reserved byte into the appropriate encoding pair of bytes

Receiver interprets pairs of bytes and retrieves encoded byte Data still framed by soh and eot

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Bit-Oriented Frames/ Bit Stuffing

Delineate frame with a special bit pattern: 01111110

If bit pattern occurs at the data, use bit-stuffing– sender: insert bit 0 after five consecutive 1’s– receiver: if receive five consecutive 1’s, check next bit

» if it is 0, remove it» if they are 10, mark the end of the frame» if they are 11, error

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Transmission Errors

Transmission errors can occur due to interference from external electromagnetic signals – Data can be changed – Data can be lost – Unwanted data can be generated

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Error detection and correction

Error detection– send additional information so incorrect data can be

detected and rejected– retransmission used to recover from errors– Use for low error rate in data communication

Error correction – send additional information so incorrect data can be

detected, corrected and accepted– higher overhead, used only if retransmission is

impractical, such as simplex transmission, long delay, high error rate

No error control– real time traffic (e.g. real time voice and video)– does not require a 100% error rate

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Error Detection Techniques

Will See three techniques– Parity– Checksum– Cyclic redundancy check (CRC)

Checksum and CRC are two widely used techniques

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Parity Error Detection

A parity bit is an extra bit transmitted with a data item Parity refers to the number of bits set to 1 in the data item

– Even parity - an even number of bits are 1» data: 10011101, parity bit 1» use XOR to compute

– Odd parity - an odd number of bits are 1» data: 10011101, parity bit 0» use XNOR to compute

Procedure:– Transmitter and receiver agree on which parity to use– Sender computes parity bit and sends it with data– If a transmission error changes one of the bits in the data from a

1 to a 0 or from a 0 to a 1, parity of resulting bits will be wrong – Receiver detects error in data because of incorrect parity

Parity can only detect odd or practically single-bit errors (not even bit errors)

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Checksum

Treat data as a sequence of integers– Can be 8-, 16- or 32-bit integers

Compute and send arithmetic sum – Easy to compute– Checksum computed over data– Checksum appended to frame– Typically use 1s-complement arithmetic – Add carry to result

Example - 16-bit checksum with 1s complement arithmetic

Handles multiple bit errors Cannot handle all errors

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Checksum May Fail to Detect Errors

Second bit reversed in each item Checksum is the same

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Cyclic Redundancy Check (CRC)

Idea: given a k-bit frame, the transmitter generates an n-bit sequence known as the Cyclic Redundancy Check (CRC), so that the resulting (k+n)-bit frame is exactly divisible (modulo-2) by some predetermined number called a generator (specified by a generator polynomial).

Consider data in message as coefficients of a polynomial

Divide that coefficient set by a known polynomial Transmit remainder as CRC

– Good error detection properties – In practice can detect 99.98% of errors– Easy to implement in hardware

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Cyclic Redundancy Check (CRC) Commonly used polynomial generators:

– CRC-12: – CRC-16:– CRC-CCITT:

Assume the degree of the generator is n n zero bits appended to the end of the frame Modulo-2 operations used to determine the remainder CRC Sender sends the data plus CRC (notice that data plus CRC

should be divisible by the generator polynomial) Receiver performs the modulo-2 operations on (k+n)-bit frame

(original data plus CRC) using the same generator If the remainder is zeros, assume NO error. Otherwise,

discard the frame.

1231112 xxxxx121516 xxx151216 xxx

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Insight on CRC Computation Data bits are 10011 Generator polynomial (x3+ 1) has degree 3

– I.e., 3 bit CRC Therefore, append three check bits to the data bits, producing

the code word 10011000 (x7+ x4+ x3). If we divide the data codeword by the generator polynomial,

the result has a remainder of 1. – The remainder constitute the CRC

Subtracting (same as adding/XORing) to the original data codeword word produces 10011001 (x7+ x4+ x3+ 1), which, is what get sent by the sender.– Notice that this should be divisible by the generator

If the receiver divides what he receives by the generator polynomial, the result should be zero if there are no errors.

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CRC Computation Example (1)

GeneratorPolynomial

Data

Quotient

1001 10011

001

Sender Receiver

10011001Transmit

Remainder (CRC)

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Building Blocks For CRC

Exclusive or

Shift register– a shows status before shift– b shows status after shift– output same as top bit

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Example Of CRC Hardware

Computes 16-bit CRC Registers initialized to

zero Bits of message shifted in CRC found in registers

151216 xxxUses the generator polynomial

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CRC Computation Example (1)

GeneratorPolynomial

Data

Quotient

1001 10011

001

Sender Receiver

10011001Transmit

Remainder (CRC)

000001010100000001010100001

10011000

ShiftReg._____

InputBit_____

+

input

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CRC Computation Example (2)

0000000100110111111001111111010110111101000100100100

111001010000

ShiftReg._____

InputBit_____

+

Input11100101000

+ +

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Illustration Of Frame Using CRC

Error detection typically done for each frame CRC cover data only Error in frame typically causes receiver to discard

frame

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Summary

Packet technology– Invented to provide fair access in shared network– Sender divides data into small packets that travel independently

Packets specific to a particular hardware network technology are called frames

Each frame has a specific format that identifies the beginning and end of the frame

Data (bit or byte)-stuffing are needed when special characters appear in data

Error detection and correction is used to identify and isolate transmission errors

To detect transmission errors– Sender adds information to packet– Receiver checks