Data Transmission

Question 1

Serial and Parallel Transmission

Answer 1

Serial - Bits are sent one after the other over a single wire from source to destination.

Uses of serial transmission
- transmission to another computer or external devices
- Medium to long distances
- USB (Keyboard, mouse etc)

• Serial transmission is cheaper
• less complex and smaller cables (lower cost)
• little interference with high frequency than parallel transmission so results in higher net transfer.

Parallel Transmission – Multiple bits are sent simultaneously over parallel wires from source to destination.
- As each individual wire has slightly different properties, there is a possibility that
bits could travel at slightly different speeds over each of the wires. This produces a
problem known as data skew. parallel transmission is only used over short distances.

Uses of parallel transmission
- Fast transmission within a computer system
- short distances (skew)
- Integrated Circuits and Buses

Parallel wires can suffer from cross talk, the interference between wires. This can result in data corruption. Stronger the signal the worse the problem

Question 2

Simplex, Half-Duplex and Full-Duplex

Answer 2

Simplex - data transmission can only take place in one direction

Half-Duplex - communication that can happen in both directions but not at the same time

Full-Duplex - communication that takes place in both directions at the same time.

Question 3

Multiplexing

Answer 3

Multiplexing
- several independent data sources are combined to be sent along a single route to a specific destination.
- multiple devices need to send a signal along the same medium
- Multiplexor organises these signals in a line

Question 4

Switching

Answer 4

Switching
- process of examining packets and routing them to the correct destination
- knows location of nearby devices

Packets Are Made out of:
- Length (bits in packet)
- Packet ID, rearrange packets in correct order at destination
- Source Address, IP address of the source
- Destination address, IP address of the device that should receive the data
- Protocol, which protocol has been used to form the packet
- Data
- Checksum, verifies the data that was sent matches the data that was received

Question 5

Circuit Switching

Answer 5

Circuit Switching
- channel is created between the two nodes, which remains constantly in place until it is no longer needed.

Question 6

Packer Switching

Answer 6

Packet Switching
- Data is broken into packets
- constant connection is not maintained.
- packets arrive at a node and are forwarded on towards their destination. Multiple nodes may be transmitting at the same time, so packets going to a single destination do not necessarily travel together, but as long as they are forwarded to the correct destination, they can be re-assembled when they have all been received.

• Better security as it is very difficult to intercept packets on different routes
• Makes more efficient use of data lines as there Is no waiting during gaps.
• Less likely to be affected by network failures

Question 7

Collision Detection

Answer 7

Collision Detection
- A collision occurs when two or more devices attempt to transmit data using the same connection medium at the same time.
- A protocol called carrier sense multiple access/collision detection (CSMA/CD) determines how collisions are detected and handled. Nodes ‘listen’ to the network and can only transmit when it is quiet.
- if two nodes begin to transmit at exactly the same moment, a collision will occur and a jamming signal is sent to prevent further transmissions.
- Both nodes will wait a random amount of time and then attempt retransmission. As the number of collisions increases, network performance of the network is degraded. As the number of nodes on a LAN increases, so do the number of collisions.

Question 8

Routing on a network

Answer 8

static routing - each router has access to a table which tells it which path to use to forward data based on destination address
- simple to implement
- not suitable for networks that are constantly changing like adding devices and removing devices

dynamic routing - routers learn the location of other devices based on monitoring address of data that is forwarded. Changes to the network can be automatically detected (not instant) by the router