Network Data Organizational Communications and Technologies Prithvi N

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

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

Objectives n n 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

Introduction: Review of Analog Signals n n n 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).

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

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

Asynchronous Character Structure n Handshaking permits the transmission of data in an orderly fashion n n 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.

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

Synchronous Data Structure n Handshaking permits the transmission of data in an orderly fashion n n 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

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

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

Communication Strategies n n n 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

Error Handling n Error detection is an important part consideration of data transmission n Parity checking n Redundancy checking

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

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

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

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

Parity Checking vs CRC n n 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 n Parity checking checks one byte at a time n Parity checking can be ambiguous

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

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

Flow Control n Required to control the speed of communication n n 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

Capacity and Bandwidth Network Bandwidth Source, storage and output Copper Wire Plumbing Pipe Diameter Source, storage and output Small Pipe Coaxial Cable Large Pipe Fiber Optic Cable Larger Pipe

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