4.9 - Fundamentals of communication and networking

Streams of bits are transferred over a communication channel at a constant rate The transmitter and the receiver are synchronised using a common clock signal No need for a start or stop bit - more information can be sent per unit of time

There is no clock signal, so additional data (start and stop bits) is used to control the communication Data is transmitted when it is available, rather than at specific intervals - there can be periods of time when the transmission channel is idle

Serial - data bits are sent in a sequence, one after the other, over a single wire Parallel - several bits are sent at the same time over their own dedicated wires or printed circuit tracks

Can operate efficiently at higher bit rates - not affected by skew, and there are fewer wires in close proximity, which minimises crosstalk Specific solutions that are not affected by interference, such as fibre-optic cables, can be used for transmitting over longer distances Also uses fewer wires- cheaper to implement Interfaces have fewer pins, so they are cheaper and easier to produce

Only reliable over short distances and with lower bit rates than serial- mainly used inside a computer and in some early peripherals Can be affected by skew and crosstalk when higher bit rates and longer wires are used

Skew - happens when the bits that are transmitted across parallel links travel at different speeds Crosstalk - occurs when electromagnetic interference between wires that are in proximity (such as parallel links) results in transmitting corrupted data that will need to be re-sent

Easy to connect nodes to network Less cabling needed - cheaper to install

If two (or more) devices try to transmit at the same time, it will cause acollision - signals will interfere with each other Every device attached to the bus can “read” every unencrypted message since signals are transmitted across the whole network - can be a security issue A failure of the main bus cable will bring the whole network to a stop

Physical - the physical layout of the network Logical - the flow of data packets around a network

If one workstation fails, the rest of the network can continue to operate New nodes can be added to the network simply by connecting them to the switch Tends to have better performance - message is passed on to its intended node only

Switch or hub is acentral point of failure - if it fails, none of the connected devices will be able to communicate or access network resources Requires plenty of cable if wired - can be expensive in large networks

Client- a program that typically runs on a device used by an end-user such as a laptop or mobile phone Server- a program on a computer that shares resources with, or provides services to, any authorised client

A client sends arequestto the server The serverprocessesthe request and then sends aresponseback to the client

Servers can be located in secure rooms - easier to monitor a server room than to monitor every device on the network Most important files are stored on servers - easier to make sure all files are backed up

High setup cost - servers will need to be set up and configured If a server fails, many users will be affected

Every computer can be configured to share its resources - owner of each device gives permission to access some or all of the device’s resources Peers can also share hardware such as printers or optical disc drives Peers communicate directly with each other Computers must be switched on to share resources

Low set-up costs - no additional devices needed If one device fails, it has less impact - most users can carry on with their work

Workstations may be at multiple different locations, including at home - difficult to oversee Individual users are likely responsible for backing up their own data

A wireless local area network that is based on international standards Used to enable devices to connect to a network wirelessly

NIC (Network Interface Controller) - connects a device to a network Hub - simply repeats messages to all connected devices Switch - only sends a message to its intended destination device Router - required to connect two separate networks when communication happens Modem - modulates electrical signals so that they can be sent through a cable that uses a different technology Network access device - makes it easier for home devices to connect to the Internet Gateway - allows the transmission of data between dissimilar networks

WEP - uses a symmetric encryption algorithm (same key is used to encrypt and decrypt data) to secure a network WPA - generates new encryption key for each data packeted WPA2 - encrypts each message several times depending on key size

Forwards data packets between computer networks Types: Edge routers -link one network to another Subscriber routers- routers provided to homes and small businesses Enterprise routers- routers that connect large businesses and ISP networks to the internet Core routers- routers that are part of the Internet backbone

Monitors both incoming and outgoing network traffic Will attempt to prevent malicious traffic entering the network

The process of a firewall checking the headers of the packets arriving from the untrusted network against a set of rules orpacket filtersdefined by the network administrator