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Transcript of N. Ganesan, Ph.D., All rights reserved. Chapter Formatting of Data for Transmission.
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© N. Ganesan, Ph.D. , All rights reserved.
Chapter
Formatting of Data for Transmission
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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
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Chapter Modules
• Data formatting and character frames
• Cost of transmission• Block mode transmission• Data packets and summary
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© N. Ganesan, Ph.D. , All rights reserved.
Module
Introduction to Formatting of Information
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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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End of Module
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© N. Ganesan, Ph.D. , All rights reserved.
Module
Character Framing
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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
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Character Frame
Start BitStop Bit
Parity Bit
7-bit ASCII
10-bits Total Frame
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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
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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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End of Module
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© N. Ganesan, Ph.D. , All rights reserved.
Module
Calculation of the Cost of Transmission
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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
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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
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Case Example: Cost of Transmission
• Mode– Character by character transmission
• Speed– 1200 bps
• Stop bits– 1
• Connect time charge– $20/Hour
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Case Example Cont.
• Page size– 5000 characters
• Compute the cost of sending 3 pages
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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
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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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End of Module
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© N. Ganesan, Ph.D. , All rights reserved.
Module
Packet Mode Transmission and Error Checking
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Packet Transmission Preview
• Packet is the format used in the transmission of information in networks including the Internet
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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
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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
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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
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Error Checking On Data Packet
10111011
10101010110 110100100010001001011
BLOCK
PARITY CHECK
LRC CHECKOr CRC CHECK
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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
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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
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Average Number of Bits Used for Carrying One Character
• Bits per character– 8048/1000 = 8.048– Approximately 8 bits
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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
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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
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A Sample Ethernet Packet Format
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Packets On The Internet
Internet
X.25
Frame Relay
ATM(Cell Technology)
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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
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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
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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
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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
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Other Names for Packets
• Cell• Frame• Block• Segment
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End of Module
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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
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Newer Cell Technology
• Based on fixed length packets known as cell
• E.g. ATM (Digital) is used in WANs
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© N. Ganesan, Ph.D. , All rights reserved.
Module
Packet Transmission Cost Computation
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Case Example: Cost of Transmission
• Mode– Character by character transmission
• Speed– 1200 bps
• Stop bits– 1
• Connect time charge– $20/Hour
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Case Example Cont.
• Page size– 5000 characters
• Compute– Cost of sending 3 pages
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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
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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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End of Module
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End of Module
END OF CHAPTER