Why ?

The layer approach for computers network is describe by the  following  two standards :

1. OSI (Open System Interface) Reference model.

2.TCP (Transport Communication Protocol)/IP (Internal Protocol)

A wide area network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries ). This is in contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.

WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization’s LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects to the LAN on one side and a hub within the WAN on the other. Leased lines can be very expensive. Instead of using leased lines, WANs can also be built using less costly circuit switching or packet switching methods. Network protocols including TCP/IP deliver transport and addressing functions. Protocols including Packet over SONET/SDH, MPLS, ATM and Frame relay are often used by service providers to deliver the links that are used in WANs. X.25 was an important early WAN protocol, and is often considered to be the “grandfather” of Frame Relay as many of the underlying protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay.

Transmission rate usually range from 1200 bps to 6 Mbps, although some connections such as ATM and Leased lines can reach speeds greater than 156 Mbps. Typical communication links used in WANs are telephone lines, microwave links & satellite channels.

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• Design to extend over an entire city.
• Single network such as cable, television or a mean of connecting a number of LAN to a larger network.
• Resources shared LAN to LAN and device to device.

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• Usually owned by the same organization that owns the attached devices.
• Internal data transfer rate are much higher than in WAN (wide area network) (more than 10MB to 100MB)
• Usually uses a broadcast network approach.
• At each station there is a transmitter/receiver that communicates over the shared medium.
• Switch LAN and ATM (Asynchronous Transfer Mode ) are being introduced.

Applications of computer networks

1.LAN

- usually multiport  connection

client/server or centralized network structure

Distance :              several meters to several Kilometers 

Broadcast approach :    A second data to E, data is received by B,C,D,E but only E accept it.
                        Others reject it.
                                                  

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Client/Server network

- Client (work station)

client computer make request for services or data on the
network from computer referred to other servers.

- Servers

Server :         + administrate the network (security,maintenance,.......etc )
                 + support large number of clients

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* Provide a dedicated link (connection pathway that transfer data from one device to another) between two devices.
* Entire capacity of the link reserved for transmission between the two devices.

Multipoint :

* More than two specified devices share the same link.

* Capacity of the channel is shared either

- Spatially : several devices use the link simultaneously

- Temporally : User takes turns to use the link

Bandwidth

Definition:

The term bandwidth has a number of technical meanings but since the popularization of the Internet, it has generally referred to the volume of information per unit of time that a transmission medium (like an Internet connection) can handle. Here’s a quick analogy: Data is to available bandwidth as water is to the size of the pipe. In other words, as the bandwidth increases, so does the amount of data that can flow through in a given amount of time, just like as the size of the pipe increases, so does the amount of water hat can flow through during a period of time. An Internet connection with a larger bandwidth can move a set amount of data (say, a video file) much faster than an Internet connection with a lower bandwidth. You can test your bandwidth availability for free using tools available at Internet speed test sites.

Temporally :

 each device take turns to end/recive data through
                       the medium,so each device get a time limit to
                       send/recive data.After the time period the medium
                       will be reserved by another PC for another time
                       period.

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A node can be a :

• Copmuter

• Printer

• or any device capable of sending and/or reciving data generated by nodes on the network.

Connection of two nodes

communication links :

- Twiated pair

- Coaxial cable

- Optical fiber

- Free space (through air)

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