Network Data

Organizational Communications and TechnologiesPrithvi N. Rao

Carnegie Mellon UniversityWeb: 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 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 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 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

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 networks

Most ‘networks’ are digital or are moving in that direction

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.

Asynchronous Character Structure

Start bit Parity

Stop bit7 or 8 Bit CharacterLSB first

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

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

Synchronous Data Structure

FlagCntlInfo

Variable Length Information Field

Frame CheckSequence

Flag

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

Error Handling Error detection is an important part

consideration of data transmission

Parity checking

Redundancy checking

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

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

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

CRC Example Consider the following transmission unit

0 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

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

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

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

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

Capacity and Bandwidth

Network Plumbing

Bandwidth Pipe Diameter

Source, storage and output

Source, storage and output

Copper Wire Small Pipe

Coaxial Cable

Large Pipe

Fiber Optic Cable

Larger Pipe

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