© N. Ganesan, Ph.D., All rights reserved. Chapter Formatting of Data for Transmission

Chapter Objectives Explain the different types of formatting used in information transmission –Character frames, block and packets Show the structure of the data for each of the above formatting procedure Present an example showing the computation of transmission costs for a given amount of data

Chapter Modules Data formatting and character frames Cost of transmission Block mode transmission Data packets and summary

© N. Ganesan, Ph.D., All rights reserved. Module Introduction to Formatting of Information

An Overview of Data Formats used in Transmission Character Frames –Used in character-by-character transmission Block –Blocks of data between two known nodes Packets –Packets of data are transmitted from one node to another node –Most extensively used format in the Internet

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© N. Ganesan, Ph.D., All rights reserved. Module Character Framing

Typical Components of a Character Frame At the center of the frame is the ASCII code of the character Parity bit for error checking Start bit indicating the starting point of the frame Stop bit indicating the ending point of the frame

Character Frame Start BitStop Bit Parity Bit 7-bit ASCII 10-bits Total Frame

Values for the Components of a Character Frame Parity bit –Odd, even or none Start bit –Mostly one bit is used Stop bit –One or two bits depending on the protocol

Practical Implications Of Parameters When using a communication software parameters must be set ahead of starting a session –For example, before calling a remote computer Parameters are also associated with the serial ports of a computer as well

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© N. Ganesan, Ph.D., All rights reserved. Module Calculation of the Cost of Transmission

Character Size Used in Calculations: An Example Problem: –Time required for the transmission of a file An assumption on the character size is often made in computing the cost of transmitting the file

Character Size Assumption Character- by character transmission –10 bits per character –A general assumption Block/Packet transmission –Approximately 8 bits per character as would be explained later –10 bits per character is still used for quick calculations

Case Example: Cost of Transmission Mode –Character by character transmission Speed –1200 bps Stop bits –1 Connect time charge –$20/Hour

Case Example Cont. Page size –5000 characters Compute the cost of sending 3 pages

Cost Computation Transmission speed –1200/10 = 120 Characters per second Time to send one character –1/120 Seconds Time to send 3*5000 characters –3*5000* 1/120 = 125 Seconds

Continuation of Cost Computation Connect time cost for one second –20/3600 Dollars Cost of sending 3 pages –(20/3600)*125 =.69 Dollars –69 Cents

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© N. Ganesan, Ph.D., All rights reserved. Module Packet Mode Transmission and Error Checking

Packet Transmission Preview Packet is the format used in the transmission of information in networks including the Internet

An Overview of Data Packets The information assembled in a packet and sent Components of a packet –Address of sender –Address of receiver –Synch. character, start of text, data, error detection, end of text character

Data Packet Format 1000 Character ETXT (End Of Text) Error Synchronization Character STXT (Start Of Text) Note: Each Character Takes 8 Bits Within The Block Sender’s address Receiver’s address

A Note on the Types of Error Checking Procedures There are two separate error checking procedures are employed Once is performed on each of the characters carried by the block The other is performed on the entire block of data

Error Checking On Data Packet BLOCK PARITY CHECK LRC CHECK Or CRC CHECK

Error Checking Procedures Used Within the Packet On each character –Parity (VRC) On the entire packet of data, one of the following two error checking procedures could be used –Longitudinal Redundancy Checking (LRC) –Cyclic Redundancy Checking (CRC) CRC 16 CRC 32 – More sophisticated

Bit Count on a Block of Data 1000 Characters X 8 bits 8888 The Size of the block= = 8049 ETXT Error Synchronization Character STXT 1000 Characters X 8 bits

Average Number of Bits Used for Carrying One Character Bits per character –8048/1000 = – Approximately 8 bits

Data Packet Format Summary Synchronization Start of text Sender’s address Receiver’s address Block of data Error checking on the block End of text

Salient Points of a Data packet Used extensively in networks Structure of the data packet is determined by the protocol –Ex. The Ethernet protocol An example of a standardized Ethernet protocol –IEEE 802.3

A Sample Ethernet Packet Format

Packets On The Internet Internet X.25 Frame Relay ATM (Cell Technology)

Switches Directs packets within a network Switches do not change the packet format as they only deliver it from one client to another in a single network

A Router Device used for interfacing different networks operating under different protocols Routes the data packets May change the packet format to conform to the destination network protocol

The Emerging Concept of Cells Frame (Packet) concept –Size of the packet is different from one packet to another packet Cell concept –Cells are packets of fixed length –ATM is an example of the cell concept in data formatting

The Advantage of the Cell Concept Fixed cell length –Internetworking devices such as switches and routers can be designed to operate more efficiently This is similar in nature to the containerized transportation of goods Fixed size containers –Helps in the design of the transportation equipment Equipment designed for fixed length cells will inherently operate faster

Other Names for Packets Cell Frame Block Segment

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Important Concepts in Packet Transmission Two methods –Character based (Old) –Packet based (new) Packet –Format determined by protocol –Variable size –E.g. X.25 (analog) and Frame Relay (Digital) are used in WANs

Newer Cell Technology Based on fixed length packets known as cell E.g. ATM (Digital) is used in WANs

© N. Ganesan, Ph.D., All rights reserved. Module Packet Transmission Cost Computation

Case Example: Cost of Transmission Mode –Character by character transmission Speed –1200 bps Stop bits –1 Connect time charge –$20/Hour

Case Example Cont. Page size –5000 characters Compute –Cost of sending 3 pages

Comparison with Character Mode Transmission Character mode transmission –10 bits per character Packet mode transmission –8 bits per character In general, packet mode transmission is more efficient than character mode transmission

Cost of Transmission Under Packet Mode (A Comparison) The cost of sending 3 pages in packet mode can be computed as: –69*(8/10) = 55.2 cents Cost of sending 3 pages in character mode was computed to be: –69 cents A savings of 14 cents therefore is obtained under packet transmission

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