Computer networking

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9 Computer networking

Asynchronous Transfer Mode (ATM)	A fast and expensive method of setting up a computer network; tends to be used to provide the backbone to a system
Backbone	A physical connection between hubs that carries all data back to the server
Bandwidth	The amount of information in bits per second (bps), kilobits per second (kbps), or megabits per second (Mbps) that can be sent via a communication channel or a network connection in a set period of time, i.e. the speed of the system
Bus	• A set of wires between computers where two signals can travel side by side • The signals can start and finish at any point in the system • The bus has no control of what is sent and when it is sent
Collision	When two or more computers try to send data at the same time down the same network
Deterministic	The method of calculating how long it will take data to travel round a network and how quickly the system can be accessed
Ethernet	A method of setting up a computer network developed by Xerox, using cabling to connect the parts of the system together
Global Area Network (GAN)	Worldwide connecting systems, e.g. • Internet • Telecommunications
Hub	• Equipment that receives information from a network interface card • Re-times the information • Sends the information to its destination
IP Address	• Each node on a system is given its own unique number • Formed by four larger numbers separated by dots, e.g. 12.345.678.9 • Allows data to be sent to the correct destination
ISDN Connection	A digital telephone line allowing computer connections
Local Area Network (LAN)	A method of linking computers, printers, etc. within a building to enable the sharing of data between computers
Metropolitan Area Network (MAN)	Linking of local area networks over a local district
Modem	Device connecting computers to the telephone system Consists of: • Modulator – converts computer signal to an audio signal • Demodulator – converts incoming audio signal to a digital signal
Network	A method of linking computers, printers, etc. to enable the sharing of data between them
Network Interface Cards (NICs)	• Cards that control the flow of data between a computer and the hub • Have collision detection circuits • Therefore if two sets of data are sent at identical times from two different machines One card will automatically delay transmission Therefore prevent a collision
Node	Equipment that communicates in a network, e.g. • Workstation • Printer • Ultrasound equipment
Router	• Equipment that connects: Workstations Networks • Increases the capacity of the network
Segment	Section of a network where several nodes are linked together
Server	A method of enabling computers to communicate with each other either by using another computer or software on a computer
Switches	Send information directly to any linked computer in a segment without using the hub
Token Ring	A method developed by IBM, of setting up a computer network using cabling that is joined in a ring
Wide Area Network (WAN)	A method of linking computers to external users via a modem
Wireless System	Used in place of a modem where direct connections are expensive or difficult Uses: • Radio transmitters • Microwaves • Lasers

Basic local area network (simplified)

Equipment to be Linked (Nodes)	• Server (Main computer) • Computers • Workstations • Imaging systems • Printers • Scanners • Modems
Preparation	• All nodes to be networked are identified • Each piece of node is given a unique, identifying, address • Each piece of equipment has a Network Interface Card
Connections	• The equipment is connected to the network via: The network interface card The hub The backbone • Using a networking standard Ethernet Token Ring Asynchronous transfer mode
Data Journey	Example • Information is sent from a computer • The network interface card Checks if there is information being transmitted on the system Either transmits data or holds the data back until there is space on the system • The hub Receives the data Either transmits data or holds the data back until there is space on the system Transmits the data to the backbone • The backbone Links together several hubs Either transmits data or holds the data back until there is space on the system Transmits the data to the server • The server Stores the information Sends information to routers if required • The router Connects local area networks Transmits information between the networks

Network standards

Ethernet	• Developed by Xerox • Linkage is via a bus Connections between computers where two signals can travel side by side The signals can start and finish at any point in the system The bus has no control of what is sent and when it is sent • Depends on network interface cards controlling the flow of data • Only one single set of data can access the system at a time • Although several sets of data can be in the system at any time • Simple to install • Relatively inexpensive system • Not possible to calculate the time taken to send data Fig. 9.1 Ethernet connection.
	Developments 10Base5 • Original system • Using thick cable • Maximum length 500 m 10Base2 (Cheapernet or Thinnet) • Thin coaxial cable used • Maximum length 185 m 10BaseT • Unshielded twisted pair cables • Maximum length 100 m • Each computer connected directly to the hub System speed • 10 Mbits per second Used for e-mails Access to hospital databases Sending reports • 100 Mbits per second – for fast Ethernet system Used for sending images Note • Both standard and fast systems can be used on the same system • The system is advancing; speeds of 100 Gigabits per second are now possible
Token Ring	• Developed by IBM • Network is joined via a ring of cables • Token Small package of data circulates round the system If a computer wishes to send data – The token goes to the computer – Allows full access to the system • Data can only be sent if the computer concerned has the token • The system is expensive to install • The system is described as being deterministic • Speeds 16 Mbits per second Fig. 9.2 Token ring connections.
Asynchronous Transfer Mode (ATM)	• Expensive to install • Used for the backbones of Ethernet systems • Transmits data in packets of fixed length 53 byte cells • Allows fast switching times due to the small packet size • Speeds 155 Mbits per second internally 5 Gbits per second between sites Fig. 9.3 ATM backbone.