© 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

End of Module

© 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

End of Module

© 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

End of Module

© 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

SynchronizationCharacter

STXT(Start Of Text)

Note: Each Character Takes8 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

10111011

10101010110 110100100010001001011

BLOCK

PARITY CHECK

LRC CHECKOr 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 bits8 8 88

The Size of the block= 8+8+8000+8+8+8+8= 8049

ETXT

Error

SynchronizationCharacter

STXT

1000 Characters X 8 bits

Average Number of Bits Used for Carrying One Character

• Bits per character– 8048/1000 = 8.048– 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

End of Module

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

End of Module

End of Module

END OF CHAPTER