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Chapter

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

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  1. Chapter Formatting of Data for Transmission

  2. 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

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

  4. Module Introduction to Formatting of Information

  5. 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

  6. End of Module

  7. Module Character Framing

  8. 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

  9. Character Frame Parity Bit Stop Bit Start Bit 7-bit ASCII 10-bits Total Frame

  10. 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

  11. 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

  12. End of Module

  13. Module Calculation of the Cost of Transmission

  14. 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

  15. 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

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

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

  18. 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

  19. 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

  20. End of Module

  21. Module Packet Mode Transmission and Error Checking

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

  23. 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

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

  25. 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

  26. Error Checking On Data Packet LRC CHECK Or CRC CHECK 10101010110 1 0 1 1 1 0 1 1 110100100010001001011 BLOCK PARITY CHECK

  27. 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

  28. 1000 Characters X 8 bits Bit Count on a Block of Data STXT Error 1000 Characters X 8 bits 8 8 8 8 ETXT Synchronization Character The Size of the block= 8+8+8000+8+8+8+8= 8049

  29. Average Number of Bits Used for Carrying One Character • Bits per character • 8048/1000 = 8.048 • Approximately 8 bits

  30. 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

  31. 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

  32. A Sample Ethernet Packet Format

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

  34. 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

  35. 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

  36. 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

  37. 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

  38. Other Names for Packets • Cell • Frame • Block • Segment

  39. End of Module

  40. 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

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

  42. Module Packet Transmission Cost Computation

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

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

  45. 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

  46. 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

  47. End of Module

  48. End of Module END OF CHAPTER

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