1.6.1 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.
1.6.2 OSI Network Model
• 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.
• General OSI Diagram contains seven layers
1.6.2.1 - 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
1.6.2.2 - 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
1.6.2.3 - 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
1.6.2.4 - 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.
• Protocols: TCP, ISO 8073
1.6.2.5 - 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.
1.6.2.6 - 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
1.6.2.7 - 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
1.6.2.8 - 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
1.6.2.9 - Networking Hardware
• 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 $30.00 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
- 10base2
• 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
- etc
- 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
• Basic network hardware can be described with the 7-layer ISO model,
