Introduction to Computer Networks

Resource Sharing: It refers to making all programs, equipments and especially data available to anyone on the network disregard to the physical location of the resource and the user.

Client-server modelHere, communication generally takes the form of a request message from the client so the server asking for some work to be done. The server then doest the work and send back the reply.

Uses of Computer Network:

  1. resource sharing
  2. high reliability
  3. saving money
  4. client-server model
  5. scalability/ performance
  6. powerful communication medium

 Future merits of computer networks:

  1. access to remote information
  2. person to person communication
  3. interactive entertainment

 Types of transmission technology:

  1. Broadcast networks: they have a single communication channel that is shared by all the machines on the network. Short messages, called packets in certain contexts sent by any machine are received by all others. Broadcast systems generally allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet is transmitted, it is received and processed by every machine on the network.

  1. Point to point networks: it consists of many connections between individual pairs of machines. To go from the source to destination, a packet may have to undergo one or more intermediate points.

LAN: local area networks are privately owned networks within a single building of up to a few kilometers in size. LANs are widely used to connect PCs and workstations in company offices to share resources and exchange information.

 Characteristics of LAN:

  1. SizeLANs are restricted in size and thus if there happens to be any faults, it is found and known in advance. Thus the network management is simplified.
  2. Transmission technology: LAN uses transmission technology consisting of a single cable to which all the machines are attached.

 Subnet: is a collection of routers and communication lines that move packets from the source host to the destination host.

 Ethernet:is a bus-based broadcast network with decentralized control operating at 10, 100, or 1000 Mbps. Computers on an Ethernet can transmit whenever they want to; if two or more packets collide, each computer just waits a random time and tries again later.

Ring: here, each bit propagates around its own; not waiting for the rest of the packet to which it belongs. Typically, each circumnavigates the entire ring in the time it takes to transmit a few bits, often before the complete packet has been transmitted.

Broadcast networks: are classified as follows:

  1. Static:static allocation of channels divides up time into discrete intervals and run a round robin algorithm, allowing each machine to broadcast only when its time comes up. It wastes channel capacity when a machine has nothing to say during its allocated slot.
  2. Dynamic: these are for the common channels either centralized or decentralized. In centralized channel allocation, there is a single entity (a bus arbitration unit) that determines who goes next. It might be done by accepting requests and making a decision according to some internal algorithm. In decentralized channel allocation, there is no central entity; each machine must decide for itself whether or not to transmit.

MAN: Metropolitan Area Network is basically a bigger version of LAN and normally uses similar technology. It might cover a group of nearby corporate offices or a city or public. MAN can support both data and voice; and might even be related to the local cable television network. As the Internet users started growing exponentially, the cable TV network was integrated with the Internet service. In this case, both television signals and the Internet are fed into the centralized head end for subsequent distribution to people’s residence.

WAN: wide area network spans a large geographical area, often a country or continent. It contains a collection of machines called hosts, intended for running user applications (programs). The hosts are connected by a communication subnet which carries messages from host to host. Subnet consists of two distinct components: transmission lines and switching elements.

Transmission lines (circuits, channels, trunks) move bits between machines. The switching elements are specialized computers used to connect two or more transmission lines. When data arrive on an incoming line, the switching element mush chooses an outgoing line to forward them on. Since there is no standard terminology used to name these computers, they are variably called “packet switching nodes, intermediate systems and data switching exchanges”, but a commonly used term is “router”.

When a packet is sent from one router to another via one or more intermediate routers, the packet is received at each intermediate route in its entirety, stored there until the required output line is free and then forwarded. A subnet using this principle is called point-to-point, store-and-forward, or packet-switched subnet. Almost all WAN have store-and-forward subnets. When all the packets are small and of same size, they are known as cells.

Point-to-point subnet is dependent of the router interconnection topology. LANs were designed to have a symmetrical topology while WAN typically have irregular topologies.

 Demerits of wireless LANs:

Though wireless LANs are easy to install, they also have some demerits as they have a capacity 1-2Mbps, which is much slower than wired LANs. The error rates are often much higher too and the transmissions from different computers can interfere with one another.

Gateway:is the machine which connects different and frequently incompatible networks together, providing the necessary translation, both in terms of hardware and software. A collection of interconnected networks is called an inter network or just internet.

A common form of internet is a collection of LANs connected by a WAN. The only real distinction between a subnet and a WAN is whether or not hosts are present. If the system within the cloud curve contains only routers, it is a subnet but if it contains both routers and hosts with their own users, it is a WAN.

Subnet: refers to the collection of routers and communication lines owned by the network operator like AOL.

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