Network Topologies

Topologies

The first thing to consider about a network is its physical shape, or the design layout, which will be extremely important when you select a wiring scheme and design the wiring for a new installation.

Network really has two shapes, or two types of topology; one is physical and the other is logical. The physical topology is the shape you can see, and the logical topology is the shape that the data travels in.

Physical Topologies

Physical topology is further divided in two section

·         Point-to-point connections

·         Multipoint connections

Point-to-point connections

Only two devices are involved in a point-to-point connection, with one wire (or air, in the case of wireless) sitting between them.'

Ethernet Networks

In late 1978, the first experimental network system was created to interconnect the Xerox Altos PCs to one another and to servers and laser printers. This first experimental network was called the Alto Aloha Network.

In 1979 the name was changed to Ethernet, to make it clear that the system could support any computer not just Altos and to point out that the new network mechanisms had evolved well beyond the Aloha system.

The base word ether was chosen as a way of describing an essential feature of the system; the physical medium (a cable) carries bits to all stations

A star configuration is simple: Each of several devices has its own cable that connects to a central hub, or sometimes a switch, multipoint repeater, or even a Multistation Access Unit (MAU). Data passes through the hub to reach other devices on the network. Ethernet over unshielded twisted pair (UTP), whether it is 10BaseT, 100BaseT, or Gigabit, all use a star topology.

Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer which acts as a router to transmit messages. If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.

You will find that a star topology is most common in networks. This is mainly because of the ease of configuring and troubleshooting it. If a wire or a single port on the hub or switch goes bad, only one network node goes down, which prevents a huge impact on productivity overall (unless the entire hub or switch fails-in which case, the whole LAN goes down). However, because a star topology involves a central hub or switch as well as a lot more cabling, it costs more to implement.

Disadvantages of a Star Network

·         Twisted pair cables typically used in star topologies are not as immune to interferences as coxiale cable

·         Expensive because of additional cabling and central hub require

·         If the centralize device fails the entire system is affected.

Advantages of Star Network

·         Easy to Install: Each device on network simply requires a cable run between it and the concentrator device.

·         Flexible: Devices can be added or removed without affecting the other devices on the network.

·         A single device or cable failure will not bring down the network

·         Easy to set up and to expand.as each device on the network simply requires a cable run between it and the concentrator device

·         Any non-centralised failure will have very little effect on the network, whereas on a ring network it would all fail with one fault.

·         Data Packets are sent quickly as they do not have to travel through any unnecessary nodes.

·         Performance is greater with speeds capable of 10mbps to 100mbps or more

·         The ability to isolate individual devices in troubleshooting An intelligent central hub or switch that can help diagnose and manage the network Adjusting traffic levels so that computers that place heavy loads on the network are moved to separate hubs

Hierarchical Topology (also known as Tree)

The type of network topology in which a central 'root' node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy - the hierarchy of the tree is symmetrical, each node in the network having a specific fixed number, f, of nodes connected to it at the next lower level in the hierarchy, the number, f, being referred to as the 'branching factor' of the hierarchical tree.

Bus Topology

In bus topologies, all computers are connected to a single cable or "trunk or backbone", by a transceiver either directly or by using a short drop cable. All ends of the cable must be terminated, that is plugged into a device such as a computer or terminator. Most bus topologies use coax cables.

Disadvantages

·         Entire network shuts down if there is a break in the main cable.

·         Difficult to identify the problem if the entire network shuts down.

·         Performance: Coax technology is usually limited to a maximum of 10mbs.

·         Not intended for use as a standalone solution in a large building.

·         Coax technology is usually limited to a maximum of 10mbs.

·         Limited cable length and number of stations.

·         Not intended for use as a standalone solution in a large building.

·         If there is a problem with the cable, the entire network goes down.

·         Performance degrades as additional computers are added or on heavy traffic.

·         Low security (all computers on the bus can see all data transmissions).

·         If one node fails, the whole network will shut down.

·         You are limited with the number of devices that you can have on a single segment.

Advantages

·         Inexpensive: Does not require additional hardware to interconnect the attached devices.

·         Easy to Install: Coax cable is durable and performs well in harsh environments.

·         Flexible: New devices can be added by simply installing a new ‘T’ connector.

·         Well suited for temporary or small networks not requiring high speeds(quick setup)

·         Initially less expensive than other topologies.

·         Requires less cable length than a star topology

Ring

In a ring topology network computers are connected by a single loop of cable, the data signals travel around the loop in one direction, passing through each computer. Ring topology is an active topology because each computer repeats (boosts) the signal before passing it on to the next computer. One method of transmitting data around a ring is called token passing. The token is passed from computer to computer until it gets to a computer that has data to send.

Advantages

·         Data is quickly transferred without a 'bottle neck'

·         The transmission of data is relatively simple as packets travel in one direction only.

·         Adding additional nodes has very little impact on bandwidth

·         It prevents network collisions because of the media access method or architecture required.

·         All devices have equal access.

Disadvantages

·         Because all stations are wired together, to add a station you must shut down the network temporarily.

·         It is difficult to troubleshoot the ring.

·         Data packets must pass through every computer between the sender and recipient Therefore this makes it slower.

·         If any of the nodes fail then the ring is broken and data cannot be transmitted successfully.