4.9 Fundamentals of communication and networking

add security

Serial transmission

data bits are sent one bit at a time over a single wire

Where is serial transmission used?

networks, phones, keyboard and mouse

Parallel transmission

data bits are sent over multiple wires at the same time

Where is parallel transmission used?

distances no more than 2m so to connect components inside the CPU

Disadvantages of serial transmission

slower than parallel given the same frequency

Disadvantages of parallel transmission

cannot transmit data over very high frequencies and distances

Errors that occur with parallel transmission: Skew

due to slightly different properties of each wire resulting in bits travelling at different speeds and therefore arriving at different times so the bits are out of order

Errors that can occur with parallel transmission: Crosstalk

occurs when electromagnetic interference between adjacent parallel . Gets more pronounced as frequency increases and means in data is corrupt it has to be retransmitted

Synchronous transmission

streams of bits are transferred over a communication channel at a constant rate. Transmitter and receiver are synchronised using a clock signal

Asynchronous transmission

data is transmitted when it is available rather than at specific intervals. No clock signal so start and stop bits are used to control the communication

Why are start and stop bits used?

signal the start and stop of communication

Advantages of synchronous transmission

more information can be sent per unit of time as there is no need for a start and stop bit

What is synchronous transmission commonly used for?

parallel transmission for example clock signal is used to synchronise the signals of the control bus and of the data transmitted over the address and data buses

Bitrate

the number of bits transferred between devices in a second, measured in bps

What does bps stand for?

bits per second

Baud rate

rate at which the signal changes state, measured in bps

Bitrate equation

=baud rate*number of bits per signal change

Bandwidth

the maximum rate of data transfer of a communication channel

Relationship between bandwidth and bitrate

directly proportional: the greater the amount of data that can be transmitted over a channel, the more bits can be transmitted per second

Latency

the time delay between initiating a transmission and it being received

Protocol

a set of rules that determine the communication between devices

What does a protocol need to define?

transmission speed (bitrate/baud rate), transmission method (a/synchronous), error checking method, format of data being transmitted

Bus topology

one cable that connects every device on the network, called backbone

Physical components of bus topology

each end of backbone there is a terminator which absorbs the signal when they reach the end to prevent interference

Operations of bus topology

network signals are sent through backbone to reach all connected devices. Each message contains a destination address that identifies its intended recipient. MAC address encoded into NIC of each device used for this purpose Receiving device picks up communication and all other devices ignore it

bus topology: collisions

if two or more devices try to transmit at the same time it will cause a collision

bus topology: security

security risk as every device attached to the bus can read every message unless message is encrypted

bus topology: reliability

if backbone breaks whole network will stop however if one client breaks or wire to client then rest of the network is fine

Star topology

each device on the network is connected to a central switch or hub

Operations of star topology: hub

network signals reach hub are sent out to all connected devices. Hub does not process signals that pass through it but just repeats them on to all of its connections

Operations of star topology: switch

uses a table that maps each device to the port it is connected to. So switch is able to direct messages to the correct recipients

star topology: security and speed

improves security as messages do not pass though every cable in the netork

star topology: reliability

if switch fails none of clients can communicate. But failure of client rest of network can still operate.

PAN

personal area network, connected devices that are located within a few meters of each other

LAN

local area network, a single network that is located in a small geographical area such as someones house

WAN

wide area network, a network that connects two or more networks over a wider geographical area

What is a server?

a program on a computer that shares resources or provides services to any authorised client P

Peer-to-peer

no central servers and every computer shares resources

Client server properties

used in small, medium and large organisations; no access to other users files; backup is centralised and usually automated; can be expensive to set up and manage; user IDS, passwords centrally controlled

Peer-to-peer properties

files and programs stored on individual computers; suitable for a home computer network; cheap to set up and maintain; can be used for sharing of music and streaming of live events; each computer on the network can act as both client and server

Napstar

created in 1999 it enabled peer to peer sharing of music but was actually the theft of copyright music

What do wireless local area networks use to connect?

radio waves

Purpose of WiFI

enables devices to connect to a network via a WAP so they can access or share network resources

Components needed for wireless networking

device needs NIC, all stations in the WAP share radio frequency and each station is constantly tuned in on this frequency to pick up transmissions, WAP requires connection to router

Network interface card (NIC)

a card that enables device to connect to a server. Its a printed circuit board inside the computer and specifically designed to allow the computer to connect via cable or wirelessly. Also detects speed of data transmission available between device and network.

Service set identifier (SSID)

a locally unique 32 bit character code that identifies a device on a wireless network. Each code is unique to WLAN and therefore acts as an identifier allowing frame of data to be transmitted around WLAN, NIC must also be programmed with the same 32 bit code

CSMA/CA

carrier sense multiple access/collision avoidance is a protocol used to check whether the channel is idle so the data is sent if not it will wait a random amount of time before checking again

Hidden node problem

when two or more nodes

RTS/CTS

request to send/clear to send protocol helps overcome hidden node problem. RTS sends message to access point

WPA

wifi protected access

WPA2

wifi protected access 2

What are WPA and WPA II?

security protocols and certification programs used to secure wireless networks. WPA2 is built into wireless NIC and provide encryption of data transmission for each data packet sent.