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31.6 NETWORKS
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.
31.6.1 Introduction
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
Significant issues in networks are,
Network power - does the network distribute power, or does each station need to be separately powered.
Network tees - to tap into the network cable simple connections can be used, or the connection can have circuitry.
Transmission type - switched voltage levels (baseband), single band FSK (carrierband), multiple frequency FSK (broadband). The FSK methods allow much higher network speeds.
Bus access - When network nodes share the same wire they may try to talk at the same time (this is called a network collision). Different schemes for dealing with collisions include,
CSMA/CD (Collision Sense Multiple Access/Collision Detection) - if two nodes start talking and detect a collision then they will stop, wait a random time, and then start again.
CSMA/BA (Collision Sense Multiple Access/Bitwise Arbitration) - if two nodes start talking at the same time the will stop and use their node addresses to determine which one goes first.
Master-Slave - one device one the network is the master and is the only one that may start communication. slave devices will only respond to requests from the master.
Token Passing - A token, or permission to talk, is passed sequentially around a network so that only one station may talk at a time.
31.6.2 OSI Network Model
General OSI Diagram contains seven layers
31.6.2.1 - 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
31.6.2.2 - 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
31.6.2.3 - 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
31.6.2.4 - 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.
31.6.2.5 - 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.
31.6.2.6 - 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
31.6.2.7 - 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
31.6.2.8 - Open Systems
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
31.6.2.9 - Networking Hardware
A number of basic components are required for networks
Computer (or dedicated computing equipment)
- 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.
- 10baseT (twisted pair) is the most popular. It is a pair of twisted copper wires terminated with an RJ-45 connector.
- A device which is sometimes included between the network media, and the network interface hardware.
- Connects separate wires and will route local traffic to local wires and remote through external connections.
- Has one input, one output, and will only handle sorting by network address (like ethernet address of machine).
- 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.
- If network cables are too long, the repeaters will boost the signal strength so that it may complete it's journey.
Networking Software / Networking Management Software
Gateways (Application, Presentation, Session and Transport layers)
An example of a micro computer with a networked interface
Basic network hardware can be described with the 7-layer ISO model,
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