Internet, WWW, Networks & Transmissions

Network Structures

There are many different types of networks and it depends on the size of the network

These include:

• WAN

It is Network connecting computers on different connecting sites which can be 1000's of Km wide.

That is why this is called a wide area network

This is owned by the PSTN ( company ) but can be leased or used by an organisation

The transmission medium is fibre optics

Communications with in the network is done by switches. You don't need a router for this as a switch contains the mac address of the device connected to the switch.

A router is bit more advance but we will talk about it later



• LAN

The lan is a network connecting devices in a single room or building

An example - Office uses

So this is owned by the organisation and it enables communications with in the building or room

The transmission medium used are wireless or twisted pair

There are many advantages in setting up a LAN as this enables communications within computers

For Example you can communicate with your collegues without actually accessing the internet

Also Centralised servers such as application servers and file servers can be connected to a LAN so the people can access files or softwares from these servers rather than installing the software to each individual computer. This makes it more efficient and cheaper

These centralised servers highlight a very important model called the client-server model.



• Internet

Is the largest network linking computers from geographical separate locations. It stands for interconnected network

This definition is very important infact, they usually ask what's the difference between internet and WWW

All we have to know is that internet is the largest or a massive network which connects networks together such as WAN. It also uses the TCP and IP protocols.

The reason why other networks don't use this protocols as the internet is a interconnected network and so it links computers of separate locations. This must be used in order for data to be sent to the correct location

Client server model

You only need to know these points

• One computer acts as a server and it provides a specific service or application.
• One computer acts as a client which sends requests to the server and receives the services

This model is very important as they usually ask this.

Lets take a simple example .You may know the app Google drive

So Google drive helps you backup data or files onto a (file) server. This is the service provided by the server and the client is the browser or you. This is a specific type of service called cloud computing services

The client server model can be broken into two categories

• Thin client

So in thin client it sends request or data to a server and the processing occurs only on the servers . Then the server will send a signal or an output to the client

This is actually PHP coding



• Thick client

So in thick client the server sends the application/software onto the browser and so the proccesing occurs on the browser itself and not on the server.

This is Javascript coding

We use each type for different purposes such as for ecommerce or online stores we have to use thin client as data must be sent to the server. However, if we use an online calculator the browser itself can perform the processing




Peer to peer network

This is fully covered in A2

In this model there is no fixed server or client. Whenever a computer wants to retrieve data it is called a client or a seed. If another computer wants to access data from it, then it acts as a server.

So in this model parts of the document is stored in individual computers and it is not stored in a centralised server.

In other words the services are shared.




Advantages and disadvantage

Indeed each model has its own advantages and you need to remember this as exams ask this

Advantages of Client-Server model



So as all the applications are stored in a central server. This improves the security of data and allows access rights. So users will be able to acces data if they are authorised to

We can also perform centralised backups

Store a virus guard to protect the data

Much more efficient and cheaper as the services doesn't need to be installed on individual computers

Advantages of Peer to peer network

So usually the data is spread over many computers

Part of the documents can be accessed rather than the full file

It avoids congestion of the medium to the main server as not everyone will be accesing the main server simulataneuosly.

And usually there are copies of data from many hosts




The reason why I didn't mention the disadvantages as the advantages of the other options are the disadvantages




Modes of Transmission

When ever they say mode of transmission there are 3 types

• Simplex mode

Data is sent in one direction only

Example is a radio station

• Half-Duplex

When data is sent in both direction but one direction at a time

Example is a walkie talkie

• Duplex

When data is sent in both directions simultaneously

Example is playing online games. The internet is basically duplex




Broadcast and more..

Message or data can be transmitted different ways. If data is sent by a single device to all the devices in the network it is broadcasted.

If data is sent to many destinations but selected it is multicast.

If data is sent only to a single device it is unicast




Topologies

It is the configuration or structure of a network and how devices are connected

There are some main topologies which you must know and each of them have there own advantages and disadvantages.



• Point to point

A single transmission medium between two devices

Think of a leased wire. This is actually a point to point network as you only can communicate with the other device only.



• Bus topology

Endsystems or devices connected to a shared link.

So in this a single shared link is used to connect device to this network



• Mesh topology

It contains direct links with in the devices.

Think about it. If you have 3 devices , one device can communicate directly to the other 2 and its the same for the other 2 devices. In this data can be sent only to a single device or all



• Star topology

Devices connected to a central device

Think of a router. Usually all the devices are connected to this central device

Advantages and disadvantages

• Bus topology

Advantages

The performance of the network or other devices is not affected when a device connected to the network fails

Also fewer cabling is used compare to the other topologies as a single shared wire is used. So more cheaper




Disadvantages

However if the shared link is damaged the whole network fails

Higher chance of congestion as single wire is used so slow transmission

Also the message is broadcasted and so it is less secure

• Mesh Topology

Advantages

This allows direct communication with in the system and so faster and more secure transmission

If a device fails it doesn't affect the network




Disadvantages

Requires alot of cabling so highly expensive and unrealistic

• Star topology

Advantages

Doesn't affect the perfomance of the network if an endsystem fails

More secure as the central device can filter the data




Disadvantages

If central device fails the whole network fails

Requires alot of cabling as devices must be connected to the central device

Depending on the central device there could be data collisions and corruption and so less secure




Hybrid Network

It is a collections of LAN's which contains different supporting technologies or topologies

Example is when a Bus topology is connected to a star topology we then call it a hybrid Network

Usually as a whole the internet is a hybrid network




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