Topology

The physical topology of a network describes the layout of the cables and workstations and the location of all network components.

Common topologies:

• Bus
• Ring
• Star
• Mesh

BUS TOPOLOGY


In a bus topology, all computers are attached to a single continuous cable that is terminated at both ends, which is the simplest way to create a physical network.

RING TOPOLOGY

In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another.

The good news about laying out cable in a ring is that the cable design is simple. The bad news is that, as with bus topology, any break, such as adding or removing a computer, disrupts the entire network. Also, because you have to “break” the ring in order to add another station, it is very difficult to reconfigure without bringing down the whole network. For this reason, the physical ring topology is seldom used.

STAR TOPOLOGY

Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable. The central point is a device known as a hub.

Although this setup uses more cable than a bus, a star topology is much more fault tolerant than a bus topology. This means that if a failure occurs along one of the cables connecting to the hub, only that portion of the network is affected, not the entire network. It also means that you can add new stations just by running a single new cable.

MESH TOPOLOGY

In a mesh topology, a path exists from each station to every other station in the network. While not usually seen in LANs, a variation on this type of topology—the hybrid mesh—is used on the Internet and other WANs in a limited fashion.

Hybrid mesh topology networks can have multiple connections between some locations, but this is done only for redundancy.

Also, it is not a true mesh because there is not a connection between each and every node, just a few for backup purposes.

Physical Media 1

Straight Through Cable

In a UTP implementation of a straight-through cable, the wires on both cable ends are in the same order.

The diagram shows the pinouts of the straight-through cable.

You can determine that the wiring is a straight-through cable by holding both ends of the UTP cable side by side and seeing that the order of the wires on both ends is identical.

You can use a straight-through cable for the following tasks:

· Connecting a router to a hub or switch

· Connecting a server to a hub or switch

· Connecting workstations to a hub or switch

Cross Over cable

In the implementation of a crossover, the wires on each end of the cable are crossed.

Transmit to Receive and Receive to Transmit on each side, for both tip and ring.

Diagram shows the UTP crossover implementation.

Notice that pin 1 on one side connects to pin 3 on the oth

er side, and pin 2 connects to pin 6 on the opposite end.

You can use a crossover cable for the following tasks:

· Connecting uplinks between switches

· Connecting hubs to switches

· Connecting a hub to another hub

Fiber Optics

SPEED: Fiber optic networks operate at high speeds - up into the gigabits

BANDWIDTH: large carrying capacity

DISTANCE: Signals can be transmitted further without needing to be "refreshed" or strengthened.

RESISTANCE: Greater resistance to electromagnetic noise such as radios, motors or other nearby cables.

MAINTENANCE: Fiber optic cables costs much less to maintain.

A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. Looking at the components in a fiber-optic chain will give a better understanding of how the system works in conjunction with wire based systems. At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are funneled into the fiber-optic medium where they transmit themselves down the line. Think of a fiber cable in terms of very long cardboard roll (from the inside roll of paper towel) that is coated with a mirror.

Physical Media

The common types of physical media used for networking are:

• Coaxial
• Twisted-pair
• Fiber-optic - Bandwidth Upto 100s of Gbps

Coaxial cable

Coaxial cable consists of a central copper core surrounded by an insulator, a braided metal shielding, called braiding, and an outer cover, called the sheath or jacket. EG: Cable Tv network use the coaxial cable.

Thinnet (10Base2)

Thinnet, also known as thin Ethernet, was the most popular medium for Ethernet LANs in the 1980s. Like Thicknet, Thinnet is rarely used on modern networks, although you may encounter it on networks installed in the 1980s or on newer small office or home office LANs.

IEEE has designated Thinnet as 10Base2 Ethernet, with the “10” representing its data transmission rate of 10 Mbps, the “Base” representing the fact that it uses baseband transmission, and the “2” representing its maximum segment length of 185 (or roughly 200) m.

Thicknet (10Base5)

Thicknet cabling, also called thickwire Ethernet, is a rigid coaxial cable approximately 1-cm thick used for the original Ethernet networks. Because it is often covered with a yellow sheath, Thicknet is sometimes called “yellow Ethernet” or “yellow garden hose.”

IEEE designates Thicknet as 10Base5 Ethernet. The “10” represents its throughput of 10 Mbps, the “Base” stands for baseband transmission, and the “5” represents the maximum segment length of a Thicknet cable, which is 500 m. You will almost never find Thicknet on new networks, but you may find it on older networks.

Twisted-Pair Cable

Twisted-pair (TP) cable is similar to telephone wiring and consists of color-coded pairs of insulated copper wires. The more twists per inch in a pair of wires, the more resistant the pair will be to all forms of noise. Higher-quality, more expensive twisted-pair cable contains more twists per foot. The number of twists per meter or foot is known as the twist ratio.

Twisted-pair cable is the most common form of cabling found on LANs today. It is relatively inexpensive, flexible, and easy to install, and it can span a significant distance before requiring a repeater (though not as far as coax).Twisted-pair cable easily accommodates several different topologies, although it is most often implemented in star or star-hybrid topologies.

One drawback to twisted-pair is that, because of its flexibility, it is more prone to physical damage than coaxial cable. This problem is a minor factor given its many advantages over coax. All twisted-pair cable falls into one of two categories: shielded twisted-pair (STP) or unshielded twisted-pair (UTP).

Shielded Twisted-Pair (STP)

As the name implies, shielded twisted-pair (STP) cable consists of twisted wire pairs that are not only individually insulated, but also surrounded by a shielding made of a metallic substance such as foil. Some STP use a braided metal shielding. The shielding acts as a barrier to external electromagnetic forces, thus preventing them from affecting the signals traveling over the wire inside the shielding. The shielding may be grounded to enhance its Protective effect

Unshielded Twisted-Pair (UTP)

Unshielded twisted-pair (UTP) cabling consists of one or more insulated wire pairs encased in a plastic sheath. As its name implies, UTP does not contain additional shielding for the twisted pairs. As a result, UTP is both less expensive and less resistant to noise than STP.

OSI Layer

OSI Reference Model

The OSI reference model illustrates the networking process as being divided into seven layers. This theoretical construct makes it easier to learn and understand the concepts involved. At the top of the model is the application that requires access to a resource on the network, and at the bottom is the network medium itself. As data moves down through the layers of the model, the various protocols operating there prepare and package it for transmission over the network. Once the data arrives at its destination, it moves up through the layers on the receiving system, where the same protocols perform the same process in reverse.

Networking

A network is a group of two or more personal computers linked together. It is a collection of hosts that are able to communicate with each other often by relying on the service of a number of dedicated hosts that relay data between participants. Host can be computers, X - terminals or intelligent printers.

All networks offer advantages relative to using a standalone computer (a personal computer that uses programs and data only from its local disks). Most importantly, networks enable multiple users to share devices and data that, collectively, are referred to as the networks’ resources.

For any organization, sharing devices saves money. For example, rather than buying 20 printers for 20 staff members, you can buy one printer and have those 20 staff members share it over a network. Sharing devices also saves time.

Data sharing before the advent of Networks

Another advantage to networks is that they allow you to manage, or administer, hardware and software on multiple computers from one central location. Imagine you work in the Information Technology (IT) department of a multinational insurance company and must verify that each of 5000 insurance agents across the world uses the same version of Word Perfect. Without a network you could never keep up! Networks, along with network management software, allow you to manage computers in your office or around the world from one computer.

The computer on which you are actually working is referred to as the local computer. The computer that you are controlling or working via the network is referred to as the remote computer. Because they allow you to share devices and administer computers centrally, networks increase productivity. It’s not surprising, then, that most businesses depend on their networks to stay competitive.