Star Topology ring Topology bus Topology Logical Physical mesh Topology

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. A point-to-point link is typified by two devices monopolizing the media-similar to two teenagers talking on the telephone with one another, not allowing anyone else to use the phone on either side.

Multipoint connections

In a multipoint connection, multiple machines share the cabling. Multipoint connections might be a group of computers strung together in a long line on an old-fashioned ThinNet (10Base2) cable, or it could be a party line of telephones, all sharing a common phone connection. In fact, even your local cable TV provider uses a multipoint system to get every person in the neighborhood hooked up. In every multipoint connection, each device must be able to identify itself. This is where addressing at the hardware level starts. The device's address must be unique on the channel that it shares with those other devices, or else confusion reigns. Just ask any network administrator who has accidentally assigned the same logical address to two computers. It's not fun dealing with any type of addressing conflict.

logical topology

A logical topology describes how components communicate across the physical topology. The physical and logical topologies are independent of each other. For example, any variety of Ethernet uses a logical bus topology when components communicate, regardless of the physical layout of the cabling. This means that in Ethernet, you might be using 10BaseT with a physical star topology to connect components together; however, these components are using a logical bus topology to communicate.

Media Type	Physical Topology	Logical Topology
Ethernet	Bus, star, or point-to-point	Bus
FDDI	Ring	Ring
Token Ring	Star	Ring

Token Ring is another good example of a communication protocol that has a different physical topology from its logical one. Physically, Token Ring uses a star topology, similar to 10BaseT Ethernet. Logically, however, Token Ring components use a ring topology to communicate between devices. This can create confusion when you are trying to determine how components are connected together and how they communicate. FDDI, on the other hand, is straightforward. FDDI’s physical and logical topologies are the same: a ring.

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

In the diagram you can see two ethernet configurations. On the left the computers are connected together with a single cable coming from the router/switch, this is called a bus or thin ethernet configuration. On the right side of the diagram each computer connects directly to the router/switch. this is how most ethernets are configured today. In this topology management of the network is made much easier (such as adding and removing devices), because of the central point. If computers are connected in a row, along a single cable this is called a bus topology, if they branch out from a single junction or hub this is known as a star topology. When computers are connected to a cable that forms a continuous loop this is called a ring topology. We will go trough all of these topologies in coming section.

Star Topology

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.

Notes:

·         A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central 'root' node and only one hierarchical level below it would exhibit the physical topology of a star.

·         A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology.

·         The branching factor, f, is independent of the total number of nodes in the network and, therefore, if the nodes in the network require ports for connection to other nodes the total number of ports per node may be kept low even though the total number of nodes is large - this makes the effect of the cost of adding ports to each node totally dependent upon the branching factor and may therefore be kept as low as required without any effect upon the total number of nodes that are possible.

·         The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less that the total number of nodes in the network.

·         If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy.

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.

The number of computers on a bus network will affect network performance, since only one computer at a time can send data, the more computers you have on the network the more computers there will be waiting send data. A line break at any point along the trunk cable will result in total network failure. Computers on a bus only listen for data being sent they do not move data from one computer to the next, this is called passive topology. 
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

MeshTopology

A Mesh topology Provides each device with a point-to-point connection to every other device in the network. These are most commonly used in WAN's, which connect networks over telecommunication links. Mesh topologies use routers to determine the best path. Mesh networks provide redundancy, in the event of a link failure, meshed networks enable data to be routed through any other site connected to the network. Because each device has a point-to-point connection to every other device, mesh topologies are the most expensive and difficult to maintain.

Mesh networks differ from other networks in that the component parts can all connect to each other via multiple hops, and they generally are not mobile. Mobile ad-hoc networking (MANET), featured in many consumer devices, is a subsection of mesh networking. Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed.

This concept is applicable to wireless networks, wired networks, and software interaction. There are three distinct generations of wireless mesh architectures. In the first generation one radio provides both backhaul (packet relaying) and client services (access to a laptop). In the second generation, one radio relayed packets over multiple hops while another provided client access. This significantly improved backhaul bandwidth and latency. Third generation wireless mesh products use two or more radios for the backhaul for higher bandwidth and low latency. Third generation mesh products are replacing previous generation products as more demanding applications like voice and video need to be relayed wirelessly over many hops of the mesh network.

Advantages of Mesh topology

·         Extremely reliable. Data has access to fastest paths and can load balance.

·         Provides redundancy and fault tolerance between devices and ensures the best possibility that the network is always available.

Diadvantage of Mesh

·         Uses the most cabling to implement.

·         Has a high administrative overhead.

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.

If there is a line break, or if you are adding or removing a device anywhere in the ring this will bring down the network. In an effort to provide a solution to this problem, some network implementations (such as FDDI) support the use of a double-ring. If the primary ring breaks, or a device fails, the secondary ring can be used as a backup.

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.

Wireless

A wireless network consists of wireless NICs and access points. NICs come in different models including PC Card, ISA, PCI, etc. Access points act as wireless hubs to link multiple wireless NICs into a single subnet. Access points also have at least one fixed Ethernet port to allow the wireless network to be bridged to a traditional wired Ethernet network, such as the organization's network infrastructure. Wireless and wired devices can coexist on the same network.

Wireless topologies seem odd at first because there are no physical wires to guide you to the actual topology shapes that they use. In fact, wireless topologies are implemented in a star, a mesh, or a cellular configuration.

BSS wireless topology

In the star configuration, the wireless topology is called a Basic Service Set (BSS). It consists of a wireless access point connected to a wired network, and it enables each wireless device to connect to the access point and through it to all other devices.

Independent Basic Service Set (IBSS)

In the case of the mesh configuration, the wireless network, the Independent Basic Service Set (IBSS), enables each wireless device to connect to any other wireless device within range.

Extended Service Set (ESS)

In the cellular topology, the wireless network, referred to as an Extended Service Set (ESS),

consists of a series of overlapping wireless cells, each with its own WAP. Devices can actually move among cells and continue working seamlessly, regardless of which cell they happen to be in. It's easiest to think of this as a radio station. Imagine you're driving down a long road and you have your radio tuned to 95.5 FM. As you go along, you eventually fade out of 95.5 FM for one area, but you fade into 95.5 FM for the next area. If these two stations were playing the exact same program, you wouldn't even know that you had changed from one to another.

The ESS cascades wireless access points, enabling seamless access to data as a mobile wireless device moves along the network.