5. Computer Networking

• A Network is a way of connecting different computers to allow data and message passing, and resource sharing.

• Early communication techniques included telegraphs (with morris code), telephones and teletypes.

• Advantages

allows shared computers and peripherals

provides communication and cooperation tools for people and computers

increases reliability by having alternate computers available

decreases isolation of many computers

• Applications,

allows remote access to databases and libraries

electronic mail

distributed computational facilities

remote control of applications

• The relationship between various network types is shown below

• The Star Network topology uses many connections to a central server

• When all computers are connected to the same network cable, there are a number of structures which may be used. The bus topology below is simpler, and the tree topology is more flexible and versatile. This approach uses a single cable which is easy to add to

• The ring topology below shows a closed loop of cable which can be simple to implement, but limited in capabilities

• An enhanced ring topology is pictured below

• The figure below shows two ring networks, with a few possible devices hooked to them.


• Advantages

Computers not necessarily from the same manufacturer

Allow computers to communicate information.

Sharing of equipment such as printers, disks, etc.

Programs can run on multiple machines improving performance

Access to machines with better/different resources.

• Several types: Ring, Star, Linear, Point-to-Point

• Local Area Network (LAN), Wide Area Network (WAN)

5.1.1 Why Use A Network?

• Medium for Communication:

Satellite link

Phone line

High Speed (Multiplexed) phone lines

Coax Cable

Fiber Optical Cable

Twisted pair cable

(Theoretically anything can be used)

• General OSI Diagram contains seven layers - Physical Layer

• Physical layer is concerned with transmission of raw bits over a physical circuit.

• Deals with voltages, timing, connections, etc.

• Responsible for bit synchronization and the identification of a signal element as either a 0 or a 1.

• Protocols: RS-232, RS-449, CCITT X.25 and X.21, IEEE 802 - Data Link Layer

• Data link layer breaks input data into “data frames” and processes acknowledgments.

• Object is to provide a error-free transmission line to the network layer.

• Responsible for the reliable delivery of information over a point-to-point or multipoint link.

• Supervises interchange of both link control data and user information

• Protocols: ANSI X3.28, HDLC, X.25, ISDN, IEEE 802 - Network Layer

• Network layer determines the interface between the computer and the intermediate system, how packets are routed.

• Chooses a route from the available data links that form the network.

• Object is to take messages, convert them to packets and send them towards the destination.

• Protocols: CCITT X.25, X.21, IP, CCITT Q.931, ISO 8473 - Transport Layer

• Transport layer takes data from the session layer, splits it up if necessary, and passes this to the network layer.

• Ensures that pieces all arrive correctly at the other end.

• Isolates the user from any concern for the actual movement of the information.

• Protocols: TCP, ISO 8073 - Session Layer

• Session layer defines a connection between users (presentation layer processes).

• Includes specification of the remote machine, authorization, options for the communication, and recovering from errors in broken transport connections.

• The set-up of communications is called binding.

• Protocols: ISO 8327, CCITT X.225, T.62, ECMA 75, NFS, RPC. - Presentation Layer

• Ensures compatible syntax among the communicating processes by adjusting data structures, formats, and codes.

• Presentation layer is generally represented by library routines which the user accesses to perform network operations.

• This layer can also perform transformations such as compression and encryption.

• Protocols: DIS 8823, 8824, 8825, CCITT X.409, T.61 - Application Layer

• Application layer is written by the user, or is a program that performs some function for the user.

• Provides a window by which the user gains access to the communication services provided by the architecture.

• Protocols: DIS 8571, 8832, 9040

5.2 Open Systems


• Open Systems Principles

Entity is a group of functions for a particular task. Accept input, and produce output.

Layer is a group of functions designed to provide a set of services related to the mission of that layer

Entities provide a set of services to the layer above. In doing this they use the services of the underlying layers.

Each layer isolates the layer above from the details of the underlying layers, and thus a lower layers characteristics can change without affecting the rest of the system.

Primitives control the layer’s services and data flow. Primitives fall into 4 groups: Request, Indicate, Response, and Confirm

• We can do a simple comparison of networking models



• A number of basic components are required for networks

• Computer (or dedicated computing equipment)

An obvious must. Without the computer there is no need for networking

Approximate cost $500 to $1,000,000

• Network Interface Hardware

Some computers come with built in network interfaces, If these are for the wrong network types, or there is no network interface, a networking interface must be purchased.

examples are ethernet, latticenet, fibre, etc.

Approximate Cost $200.00 and up

• The Media,

This is the cabling which will connect the computers

10base2 (thin wire) is thin shielded coaxial cable with BNC connectors

10baseF (fiber optic) is costly, but signal transmission and noise properties are very good.

10baseT (twisted pair) is the most popular. It is a pair of twisted copper wires terminated with an RJ-45 connector.

Approximate cost for thick net $40 + $5/station, for thin net $10 + $1/station


• Transceiver

A device which is sometimes included between the network media, and the network interface hardware.

Allows easy changes to media type.

Not used for thin cable computer interfaces.

Approximate costs $100 to $200

• Hub/Concentrator

Connects separate wires and will route local traffic to local wires and remote through external connections.

approximate cost $50/wire

• Bridges, (Data link layer)

Used when hooking one network type to another, or isolating one part of a network from another.

A bridge can be used for hooking an ethernet cable to an optical backbone cable.

Has one input, one output, and will only handle sorting by network address (like ethernet address of machine).

Approximate cost $2,000 to $5,000

Same functionality as router.

• Routers, (Network Layer)

When there are too many networks on the same network, a router can be used to isolate small loops of the network from traffic which does not involve it.

More intelligent than a bridge, multiple inputs/outputs.

Sorts Network packets by IP address.

Approximate cost $5,000 to $10,000.

• Repeaters, (Physical Layer)

If network cables are too long, the repeaters will boost the signal strength so that it may complete it’s journey.

Also allows media to be changed

• Networking Software / Networking Management Software

Handles networking interface control for,

receiving / sending data or files
electronic mail tools
messaging systems

Examples of this are NFS (Network File Server), SNMP (Simple Network Management Protocol)

Approximate costs >$1,000, unless provided with system

• Gateways (Application, Presentation, Session and Transport layers)

• An example of a micro computer with a networked interface