Networks (Part 2) Use of a Modem The different modems Baud Rate Bandwidth Server and Client machines Network Topologies ( Bus, Ring and Star)

Recap What is a Network? What are the two main types of Networks? What is the difference between LAN and WLAN? What are the advantages of a Network? What is the WWW?

What is a Modem A modem is on of the main requirements of a network A modem could be an Analogue to Digital and a Digital to Analogue device The work modem comes from two words Modulate and Demodulate which basically refer to what the task of a modem

Use of a Modem The modem allows the computer to be connected to a telephone line so that data can be sent and received Without a modem there would be no communication available The modem is responsible from changing analogue data to digital data for the computer to understand and vice versa

Different Modems Due to technology advances many different modems have been developed These are some of them; 1. External PSTN Modem 2. Internal PSTN Modem 3. ADSL Modem 4. Cable Modem

1. External PSTN Modem PSTN: Public Switched Telephone Network This modem is most common modem used in our homes This modem is simply plugged into the computer via the serial port, with its own power cable The telephone cable is plugged into the modem

2. Internal PSTN Modem This modem is also a common modem found in homes It is the same as the external PSTN modem, but it is directly connected to the motherboard Due to being connected to the motherboard it is found inside the tower hence it shares power with the computer

3. ADSL Modem ADSL stands fro Asymmetric Digital Subscriber Line The ADSL modem is an external modem and taking over the other modems It plugs to the PC using either USB The ADSL modem still uses the PSTN line but has a two way connection this makes the service much faster than normal dial-up

4. Cable Modem The cable modem does not use a telephone line Instead of a telephone line it uses the cables that are used for Televisions The cables are used to to connect to the ISP (Internet Service Probider)

How Modems Work The telephone-lines use analogue technology which is not compatible with computers, the modem is used to convert analogue signals into digital signals 1. When information is being sent, the digital data sent by the computer is delivered to the modem 2. The modem converts it to analogue data which in turn is transferred to the telephone line 3. When information is being received by the PC, information from the telephone line is converted from analogue to digital by the modem

…. Digital data is passed to the modem. The modem converts it into analogue, since the telephone line is analogue. Modem receives analogue data. Data is converted back into digital format.

Baud Rate The baud rate of modem determines how fast information is sent or received by the modem Most dial-up modems have a speed of 56kbps Most modern modems such as ADSL can reach speeds of 4096Kbps

Client - Server Machines Client-server relationship is a method of network organization In this relationship work stations make use of resources available in the server One computer has the role of central resource manager for the network.

The Server The server is a computer offering a service Services could be a set of resources such as files, printers, databases, CD-ROM The server handles the requests by a client which asks to make use of these resources Some types of servers are …

1. File Server The file server is the central disk storage for any work stations on the network The file server software identifies each user’s files separately so that other users cannot use them Users can access their own files from any client station on the network using a username and passwork

2. Printer Server This server allows all the work stations to use a printer 3. CD – ROM Server This allows all the work stations to obtain data from a CD-ROM disk

Database Server This server manages a large database. Work stations can access data in the database and, if authorized, can edit the database. The database processing is usually carried out by the server, with the query being sent by a work station of the server and the results assembled by the server and returned to the client station

Bandwidth One of the main problems with client-server technologies is bandwidth. Bandwidth determines the amount of data that can travel through the network at a given moment. For example in a LAN the bandwidth’s capacity is 100Mbits/sec, that means that at any time the maximum amount of data that can pass through the network is 100Mbits. This may lead to some problems if the data that needs to be passed through the number is greater than this limit. In that case the communication between the computers will be corrupted.

Network Topologies 1. Bus Topology 2. Ring Topology 3. Star Topology

What is a Topology A topology is the layout of the network The topology of a network is basically how the computers are connected to each other and the network The topology used depends on the need of the network and this changes depending on the building

1. Bus Topology A bus network allows work stations to be connected to the network in a line fashion Work stations will be set up one after each other (just like bus stops) COMPUTER

2. Ring Topology A ring network has each of its computers connected to a network ring Signals are sent in a round about fashion COMPUTER

In both bus and ring network setups, it is usually preferable to connect the computers to the network using spurs. This is preferred because if a computer breaks down, the rest of the network will not be affected. In the case of the network setups without the spurs, precautions must be taken to prevent a breakdown of the whole network, if a single computer breaks down.

3. Star Topology In a star network all devices are connected to one central computer The central computer (called hub) is a computer which has separate connections to each computer. COMPUTER SERVER COMPUTER (HUB)