Network Data

1. Network Data Organizational Communications and Technologies Prithvi N. Rao Carnegie Mellon University Web: http://www.andrew.cmu.edu/course/90-702/

2. Reading Data Communication Fundamentals (Stallings and van Slyke) Chapter 5 TCP/IP and Other Protocol Architectures (Stallings and van Slyke) Chapter 12

3. Objectives • Be familiar with basic data types • Recognize the difference between analog and digital transmission • Be able to describe how computers handle transmission errors occurring during transmission • Recognize the concept of bandwidth and how it relates to the data transfer capacity of media

4. Introduction: Review of Analog Signals • Amplitude of a wave is measured in decibels • Phase of a wave (0 – 360 degrees) provides information about the position of the wave • Frequency of the wave describes the number of waves present over a given period of time. Measured in cycles per second (hertz).

5. Introduction: Review of Digital Encoding • Sampling of analog signal is the basis for digital encoding • Manchester encoding • Differential Manchester encoding • Non Return to Zero • Non Return to Zero Inverted • Alternate Mark Inversion

6. Digital Versus Analog • Digital technology provides benefits over analog for data transmission • Resilience in terms of immunity to external noise (crosstalk) • Faster and more flexible equipment can be used in digital networksMost ‘networks’ are digital or are moving in that direction

7. Asynchronous Character Structure • Handshaking permits the transmission of data in an orderly fashion • Framing data with special control bits indicating the beginning and end of data • Using common timer or clock to determine when the data starts and stops in a transmission Asynchronous communication uses frames to indicate the beginning and end of each piece of data that is transmitted. Serial communication is an example.

8. Asynchronous Character Structure 7 or 8 Bit Character LSB first Start bit Parity Stop bit

9. Synchronous Data Structure • Handshaking permits the transmission of data in an orderly fashion • Framing data with special control bits indicating the beginning and end of data • Using common timer or clock to determine when data starts and stops in a transmission

10. Synchronous Data Structure • Synchronous communications uses a clock to coordinate the movements of bits through the network • No start and stop bits are required • Can be character (byte) oriented or bit oriented

11. Synchronous Data Structure Frame Check Sequence Cntl Info Variable Length Information Field Flag Flag

12. Communication Strategies • Simplex defines one way communication from sender to receiver • Half Duplex defines bi-directional communication with information traveling in only one way at a time • Full Duplex permits bi-directional communication simultaneously

13. Error Handling • Error detection is an important part consideration of data transmission • Parity checking • Redundancy checking

14. Parity Checking • Involves performing a basic calculation of the number of digital 0’s and 1’s making up a transmission unit • Parity calculated on even or odd number of 1’s • Parity bit is set per frame (byte or character) • Parity checking is found mostly in Asynchronous communication

15. Parity Checking • Both sender and receiver must agree agree on whether to use odd or even parity Example: 1 0 0 0 1 0 1 1 or 1 1 0 0 0 0 0 0 • Even number of 1s (4 or 2) so parity bit set to 1 • Receiving computer checks for even parity seeing parity bit set to 1

16. Cyclic Redundancy Checking • Problem with parity checking is that two different signals could both indicate the same parity • More reliable is CRC or Cyclic Redundancy Check • Check is performed by • Totaling entire transmission • Divide by a constant prime number • Resulting remainder is the CRC validation

17. CRC Example • Consider the following transmission unit0 0 0 0 1 1 1 1 which adds up to 15 binary Divide this number by 17 (constant prime number)Remainder is 15 the CRC validation number • Also called Frame or Block checking because it works on the entire transmission not just the start and end

18. Parity Checking vs CRC • CRC can be used with larger units of data (blocks or frames) • CRC field is made part of the frame;inserted just before the end of the frame delimiter • Parity checking checks one byte at a time • Parity checking can be ambiguous

19. Error Correction • Process of recovery when error is detected • Simple solution is retransmission • Retransmission occurs if receiver does not send and ACK signal • Alternative for retransmission is sending a NACK • Most protocols have some form of acknowledgement

20. Data Transfer Rates • Rate at which signal can move from a 0 to a 1 • Speed of encoding process • Amount of overhead involved in framing • Level of error detection • Amount of flow control or handshaking

21. Flow Control • Required to control the speed of communication • Required when receiver cannot accept rate of delivery of data • Limits the speed of transmission • Receiver not ready tells sender to stop transmitting • Window manipulation can reduce amount of data being transmitted

22. Capacity and Bandwidth

23. Summary • Two basic types of transmission • Analog • Digital • Flow control is based on • Synchronous transmission relying on a clock • Asynchronous transmission indicating start and stop of data • Parity and CRC are two methods for error checking • Bandwidth is the effective capacity of media