Networks

About routers, switches, hubs, servers, clients and NIC's

OSI Model

OSI Model

The OSI Model, or the Open Systems Interconnection, is a model to standardise the functions of communication devices. This is done with abstract layers, which characterise each type of device. These layers' functions are grouped logically. Each layer is served by the layer below and serves the layer above. The layers are like so:


7) Application layer

6) Presentation layer

5) Session layer

4) Transport layer

3) Network layer

2) Data link layer

1) Physical layer


I'm going to delegate to the wikipedia page: http://en.wikipedia.org/wiki/OSI_model, so you can understand what each layer individually does.

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NICs

NIC

NIC stands for network interface card/controller. This card allows a client computer to interface with networks, such as the internet. This card includes all the physical circuitry needed to interpret data received over the network. A NIC is a hardware component that connects to one of the client's buses.


Example of use:

Use in PCs to connect to the internet.

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Hub

Hub

This in not a sophisticated as a switch, but an Ethernet Hub is still useful. A hub has no sense of collision detection, barely aware of frames and normally operates on bits. Because of no collision detection, this device isn't as efficient as a switch. Also, hubs have chaining limitations, which increase as the speed of the network increases. The hub operates on the physical level of the OSI model, meaning it doesn't read any incoming data, and just replicates the corresponding bit, or symbol, on every port.


Example of use:

Used in a cheap token network.

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Switch

Switch

A switch is more sophisticated than a hub, because it includes things such as collision detecting and echo removal. Collision detecting is when a device that can only serve one packet at a time, has two packets received at the same time. Normally, the computer listens to the network, assuring that there are no current packets before sending their packet. Sometimes, this fails and there is a collision. The switch can prevent this, sending one packet and then the next packet. In a hub, where this doesn't happen, the computers will have to resend their individual packets later. Some switches route data at the OSI data link layer. But some, called multi-layer switches, operate on the OSI layer 3 and above.


Example of use:

Used in a more expensive token network.

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Router

Router

A router reads the address information and determines its final destination. Then, it forwards the data to the appropriate internetwork, or client. A router typically functions to "traffic forwarding", which simply forwards data received from the ISP to the destination node. This forwarding is affected by the router's routing policy or routing table.


Example of use:

Connecting to your ISP with a modem.

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Server

Server

This is an essential component in the star topology, all other topologies operate without servers. This is a single computer that is dedicated to running one program as a host. This then serves multiple clients with the service it's running. Clients then connect over networking or the client runs on the same computer. In the case of the Star topology, the server redirects appropriately and receives data to be forwarded.


Example of use:

Used in a star topology.

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Client

Client

A client is a peripheral or computer connected to the network. There are 3 types of clients:


Fat client - this uses local storage and local CPU power

Hybrid client - this uses local CPU power but relies on server storage

Thin client - this relies on the CPU and the storage of data on the server


An example of a Fat client, or a rich client, is a computer running a CAD program, which then shares its work over the network.


An example of a Hybrid client is a computer running a game. This relies on the storage from the server, but does local processing.


An example of a Thin client is a web based email client. This client sends a request to the server, which then responds with the needed data. This also stores emails in this case, and the client only does minimal processing, like displaying graphics and sending data.


Example of use:

Used in all topologies.

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Star Topology

Star topology

A star topology operates with a central server, which sends and receives data. Clients on a star topology can send and receive data simultaneously to other clients, which increases efficiency a lot.


Pros:

Efficient

Can safely remove individual computers without the network crashing


Cons:

If the main server fails, the whole network fails



Example of use:

Organisations, schools etc.


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Token ring topology

Token ring topology

A token ring topology is a topology where the clients are connected in a ring, with frames (carriers for data) circle anti-clockwise. There's no server in this topology. The client sets the frame to contain data, then adds the message and the address. The clients then read the address information, deciding whether or not they are the specified client. If they are, they read the message and change the symbol, or bit, to read. Then, the sending client changes the symbol back to empty and empties the frame, so it can again carry data.


Pros:

If one computer fails and the switch has an auto-short mechanism; the network can continue without it.

Less expensive.


Cons:

The data may not take the most direct route to the destination node, meaning that efficiency and speed is decreased.


Example of use:

Local home networks (not as widely used).

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Bus topology

Bus topology

The bus topology implements a single, central bus, which is shared between all clients. The data travels down the bus until it reaches it's destination node.


Pros:

Easily remove clients.


Cons:

Whole network breaks if the central bus breaks.


Example of use:

Not very widely used now (outdated).

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