Computer Networks

Basic Definition

A computer network can be defined as a network of data processing nodes that are interconnected for the purpose of data communication, or alternatively as a communications network in which the end instruments are computers.

The nodes that one may find on a network can include:

Servers: computers used to store the shared information and have all the other computers reference that information over a network.

Clients: computers on a network that use, but do not provide, network resources.

Peers: computers on a network that that both use and provide network resources.

Clasification

Networks are often broadly classified in terms of the typical communication patterns that one may find on them. Three common types of networks are:

1. Server-based (client/server) – contain clients and the servers that support them. Typical communication in a client/server system involves the client sending a request for data, the server waiting for requests, processing received requests and sending responses, and the clients waiting for, and using, the response.
2. Peer (peer-to-peer) – contain only clients, no servers, and use network to share resources among individual peers.
3. Hybrid – client/server that also contains peers sharing resources (most common for corporations).

Client/Server networks offer a single strong central security point, with central file storage, which provides multi-user capability and easy backup. It also gives the ability to pool the available hardware and software, lowering overall costs. Optimized dedicated servers can make networks run faster. Dedicated server hardware is usually expensive, and the server must run often-expensive network operating system software. A dedicated network administrator is usually required.

DATA TRANSMISSION MODES

1. SIMPEX

The simplest signal flow technique is the Simplex configuration. Simplex allows transmission in only one direction and is a unidirectional channel. Note the difference between simplex and half-duplex. Half duplex refers to two-way communications where only one party can transmit at a time. Simplex refers to one-way communications where one party is the transmitter and the other is the receiver. An example of simplex communications is a simple radio, which you can receive data from stations but can’t transmit data.
Advantages of Simplex
• Cheapest Communication method
Disadvantages of Simplex
• Only allows for communication in one direction

2. HALF DUPLEX

Half Duplex refers to the transmission of data in just one direction at a time. For example, a walkie-talkie is a half-duplex device because only one party can talk at a time. In contrast, a telephone is a full-duplex device because both parties can talk simultaneously.

Most modems contain a switch that lets you select between half-duplex and full-duplex modes. The correct choice depends on which program you are using to transmit data through the modem.
Advantages of Half Duplex
• Costs less than full duplex
• Enables for two-way Communications.
Disadvantages of Half Duplex
• Only one device can transmit at a time
• Costs more than simplex

3. FULL DUPLEX

Full Duplex refers to the transmission of data in two directions simultaneously. For example, a telephone is a full-duplex device because both parties can talk at once. In contrast, a walkie-talkie is a half-duplex device because only one party can transmit at a time.

Most modems have a switch that lets you choose between full-duplex and half-duplex modes. The choice depends on which communications program you are running.
Advantages of Full Duplex
• Enables for two-way Communications simultaneously.
Disadvantages of Full Duplex
• The most expensive method in terms of equipment because two bandwidth channels are needed.

Topology

A network configuration is also called a network topology. A network topology is the shape or physical connectivity of the network.

1. Bus

In a bus topology each node (computer, server, peripheral etc.) attaches directly to a common cable. This topology most often serves as the backbone for a network. In some instances, such as in classrooms or labs, a bus will connect small workgroups. Since a hub is not required in a bus topology, the set-up cost is relatively low. However, this topology’s wiring scheme is unstructured (without a central point of concentration) making it difficult to troubleshoot. Often if one PC goes down, the whole network can shut down.

Usually the bus must be terminated. Termination is the process of stopping signals sent through a network. Without termination, signals bounce back and forth, causing a logjam over a network.

Bus networks are simple, easy to use, and reliable. They require the least amount of cable and are easy to extend. Repeaters can be used to boost signal and extend bus.

Heavy network traffic can slow a bus considerably. Each connection weakens the signal, causing distortion among too many connections.

2. Star

A star topology, on the other hand, is relatively easy to troubleshoot due to its structured wiring scheme. With this topology, each node has a dedicated set of wires connecting it to a central network hub. The failure of one connection will not usually affect the others. And, since all traffic passes through the hub, the hub becomes a central point for isolating network problems and gathering network statistics.

The star topology can have a number of different transmission mechanisms, depending on the nature of the central hub.
• Broadcast Star Network: The hub receives and resends the signal to all of the nodes on a network.
• Switched Star Network: The hub sends the message to only the destination node.
• Active Hub (Multi-port Repeater): Regenerates the electric signal and sends it to all the nodes connected to the hub.
• Passive Hub: Does not regenerate the signal; simply passes it along to all the nodes connected to the hub.
• Hybrid Star Network: Placing another star hub where a client node might otherwise go.

Star networks are easy to modify and one can add new nodes without disturbing the rest of the network. Intelligent hubs provide for central monitoring and managing. Often there are facilities to use several different cable types with hubs.

Central hub failure will lead to total network failure. They are also costly to cable since all network cables must be pulled to one central point.

3. Ring

A ring topology features a logically closed loop of cable – a ring. Data packets travel in a single direction around the ring from one network device to the next. Each network device acts as a repeater, meaning it regenerates the signal. If one device fails, the entire network goes down.

4. Tree (Hierarchy)

The hierarchical topology is one of the more common topologies found today. The software to control the network is relatively simple and the topology provides a concentration point for control and error resolution. The node at the highest point in the hierarchy usually controls the network.

Whilst this type of network is attractive for its simplicity it does present a potential significant bottleneck problem. In some instances the uppermost node will control all the traffic. Not only can this cause a bottleneck, but it can also present reliability problems if this node fails.

5. Mesh

The mesh topology has been used more frequently in recent years. Its primary attraction is its relative immunity to bottlenecks and channel/node failures. Due to the multiplicity of paths between nodes, traffic can easily be routed around failed or busy nodes. Given that this approach is very expensive in comparison to other topologies, some users will still prefer the reliability of the mesh network to that of others (especially for networks that only have a few nodes that need to be connected together).

CABLE TYPE

Cable is what physically connects network devices together, serving as the conduit for information traveling from one computing device to another. The type of cable you choose for your network will be dictated in part by the network’s topology, size and media access method. Small networks may employ only a single cable type, whereas large networks tend to use a combination.

In Project 802, the IEEE established specifications for cables carrying Ethernet signals. 10Base5, 10Base2, 10Base-T and 10Base-F refer to thick coaxial, thin coaxial, unshielded twisted-pair and fiber-optic cables respectively.

The “10” refers to the Ethernet transmission speed – 10 Mbps. The “Base” refers to base band (single communications channel on each cable). Originally, the last character referred to the maximum cable distance in hundreds of meters. This naming convention changed, however, with the introduction of 10Base-T and 10Base-F. In these instances, the T and F refer to the cable types (twisted-pair and fiber-optic).

1. Coaxial Cable

Coaxial cable includes a copper wire surrounded by insulation, a secondary conductor that acts as a ground, and a plastic outside covering. Because of coaxial cable’s two layers of shielding, it is relatively immune to electronic noise, such as motors, and can thus transmit data packets long distances. Coaxial cable is a good choice for running the lengths of buildings (in a bus topology) as a network backbone.

Local area networks (LANs) primarily use two sizes of coaxial cable, commonly referred to as thick and thin. Thick coaxial cable can extend longer distances than thin and was a popular backbone (bus) cable in the 1970s and 1980s. However, thick is more expensive than thin and difficult to install. Today, thin (which looks similar to a cable television connection) is used more frequently than thick.

2. Twisted-Pair Cable

Twisted-pair cable consists of two insulated wires that are twisted around each other and covered with a plastic casing. It is available in two varieties, unshielded and shielded. Unshielded twisted-pair (UTP) is similar in appearance to the wire used for telephone systems. UTP cabling wire is grouped into categories, numbered 1-5. The higher the category rating, the more tightly the wires are twisted, allowing faster data transmission without cross talk. Since many buildings are pre-wired with extra UTP cables, and because UTP is inexpensive and easy to install, it has become a very popular network media over the last few years.

Shielded twisted-pair cable (STP) adds a layer of shielding to UTP. Although STP is less affected by noise interference than UTP and can transmit data further, it is more expensive and more difficult to install

3. Fiber-Optic Cable

Fiber-optic cable is constructed of flexible glass and plastic. It transmits information via photons, or light. More resistant to electronic interference than the other media types, fiber-optic is ideal for environments with a considerable amount of noise (electrical interference). Furthermore, since fiber-optic cable can transmit signals further than coaxial and twisted-pair, more and more educational institutions are installing it as a backbone in large facilities and between buildings. The cost of installing and maintaining fiber-optic cable remains too high, however, for it to be a viable network media connection for classroom computers.

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