F451 - 3.1.5 Data Transmission

a. Describe the characteristics of LAN and WAN

LOCAL AREA NETWORK

A collection of computers and peripherals over a small geographical area, usually one building or site

LAN can be hard-wired using cable or connected via a short-range wireless system

LAN is far more secure

LAN requires no extra communication device

Allows sharing of devices

E.g. school computer room

WIDE AREA NETWORK

A collectio nof computers over a large geographical area

Machines in a WAN tend to be spread more remotely

WAN tends to use third party communication links

Data on a WAN is subject to interception/attack

WAN requires a modem

E.g. internet

b. Show an understanding of the ahrdware and software needed for a LAN and for accessing a WAN

LOCAL AREA NETWORK

SOFTWARE

- Network Operating System (NOS)

- Communicatiosn software

- Drivers for hardware

- Network versions of applications software

HARDWARE

- Network Interface Card (NIC)

- Cabling/wireless access points

- Server

b. Show an understanding of the ahrdware and software needed for a LAN and for accessing a WAN

LOCAL AREA NETWORK

SOFTWARE

- Network Operating System (NOS)

- Communicatiosn software

- Drivers for hardware

- Network versions of applications software

HARDWARE

- Network Interface Card (NIC)

- Cabling/wireless access points

- Server

WIDE AREA NETWORK

SOFTWARE

- Same as LAN

HARDWARE

- Fibre-optic cable

- Telephone cable

- Infrared/microwave satellite

- Need for a modem/router - to link the LAN to a WAN

WIDE AREA NETWORK

SOFTWARE

- Same as LAN

HARDWARE

- Fibre-optic cable

- Telephone cable

- Infrared/microwave satellite

- Need for a modem/router - to link the LAN to a WAN

NETWORKING

Why network computers?

- Allow each computer to access up-to-date information

- Shared access to database/system

- E.g. ticketing system - avoid double-bookings

Advantages of networking

- Sharing of hardware, software and data files and can use ANY machine

Disadvantages of networking

- Fault in hardware may cause problems throughout the network

- Security of files is worse

- Viruses spread more easily

c. Describe the different types of data transmission

SERIAL DATA TRANSMISSION

A byte is sent, one bit at a time, through A SINGLE WIRE

Slower, but simpler and more reliable

PARALLEL DATA TRANSMISSION

A byte is sent through multiple wires, one wire for each bit in the byte

Therefore, multiple bits are transmitted at a time

Faster but less reliable because the bits can become muddled up as bits will naturally be transmitted at slightly different rates

SIMPLEX DATA TRANSMISSION

Communication in one direction only

HALF-DUPLEX DATA TRANSMISSION

Communication in both directions but one way at a time

DUPLEX DATA TRANSMISSION

Communication in both directions simultaneously

d. Explain the relationship between bit rates and the time sensitivity of the information

BIT RATE

The rate at which data is transferred

How many bits that can be sent in a specific time period

Measured in bits per second/baud

E.g. - Streaming of a video to a machine

- Requires large volumeof data

- Video file is large and is used immediately

- Time sensitive because delay will interrupt video output

- If delay, video will freeze/lag

- Therefore, requires high bit rate otherwise data becomes useless

Bit rates continued...

E.g. - Simple Text files

- Use a small volume of data

- Tend to have less data to transmit than a video file

- Not needed immediately

- Therefore, bit rate is unimportant because there is time for data to download even at a slow rate

Consequently, number of bits per second is improtant and dependent on the data being sent

e. Recognise that errors can occur in data transmission, and explain methods of detecting and correcting these errors

PARITY CHECKS

Number of ones in a byte is either always odd or always even

System has been intialised to always expect either odd or even number of ones in a byte

Each byte has one bit called the parity bit which does not contain data

It is set to 1 or 0 in order to maintain the agreed off or even parity

When transmitted, the number of ones should match agreed parity

If not, the byte contains an error

Continued...

USE OF PARITY IN DATA BLOCKS TO BECOME SELF-CORRECTING

Each byte has a parity bit

Each bit place has also got a parity bit

All parity is checked

If there is an error in the parity for a byte and the parity for a place value

Then where they intersect will be the faulty bit

If it is 0, change it to 1

if it is 1, change it to 0

If more than one error in the block has occured then data is re-transmitted

CHECK SUMS

Blocks of bytes are added together before transmission

This total is transmitted with the block

The same calculation is done on the data blocks at the destination

The result is compared with the transmitted value

If different, a transmission error has occured

ECHOING

When a set of data is transmitted from one device to another...

The received data is retransmitted back to the sender...

It is them compared to the original data

Any differences will signify a transmission error

And the original data is retransmitted

f. Describe packet switching and circuit switching

PACKET SWITCHING

File of data split into packets which are sent onto the network

Each packet is sent to the destination via different (individual) routes

Pakcets arrive at the destination at different times and in a wrong order

Packets must be reordered at destination

CIRCUIT SWITCHING

Route is deserved before transmission

For the duration of the transmission

All packets follow the same route

Packets arrive at the destination in the correct order

g. Explain the difference in use of packet switching and circuit switching

h. Define the term protocol and explain the importance of a protocol to the transmission of data

PROTOCOL

A set of rules to govern communication between devices

i. Describe the need for communication between devices and between computers, and explain the need for protocols to establish links

A handshake signal is a signal sent between two devices to ensure that they are both ready to communicate

A handshake signal is sent from one device and acknowledged by the other device

This states that both devices are now ready for communication

(Establishes medium for communicaton)

Why necessary?- Unless both devices follow the same rules, there can be no communication

j. Explain the need for both physical and logical protocols and the need for layering in an interface

PHYSICAL PROTOCOLS

Wireless/hard-wired?

What frequencies?

Serial or parallel?

Radio or microwaves?

Copper cable or fibre-optic?

LOGICAL PROTOCOLS

- Baud rate

- Error correction technique

- Routing, Flow control, Packet size, Compression type and File type

continued...

LAYERING

Protocols are arranged in layers because it allows different rules within a protocol to be set up independently

Layers are ordered which simplifies the creation of the protocol

Allows manufacturers to design for particular layers

Changes can be made by altering a single layer and the links to the other layers in contact with it