Network Topology

Network topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network, especially the physical (real) and logical (virtual) interconnections between nodes

PHYSICAL TOPOLOGY :

The physical layout of devices on a network. or the way that the devices on a network are arranged and how they communicate with each other The way that the workstations are connected to the network through the actual cables that transmit data - the physical structure of the network

LOGICAL TOPOLOGY :

the mapping of the flow of data between the nodes in the network determines the logical topology of the network the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices.

Types/Classification of physical topologies

1.  Linear Bus
2. Star
3. Star-Wired Ring
4. Tree
5.  FDDI
6. Mesh

LINEAR BUS

• A linear bus topology consists of a main run of cable with a terminator at each end.  All nodes (file server, workstations, and peripherals) are connected to the linear cable.
• Ethernet and LocalTalk networks use a linear bus topology.
• The bus cable carries the transmitted message along the cable. As the message arrives at each workstation, the workstation computer checks the destination address contained in the message to see if it matches it's own. If the address does not match, the workstation does nothing more.
• If the workstation address matches that contained in the message, the workstation processes the message. The message is transmitted along the cable and is visible to all computers connected to that cable.

Advantages	Disadvantages
Easy to install	Out-of-date technology
Costs are usually low	If cable breaks, whole network is down
Easy to add systems to network	Can be difficult to troubleshoot
Great for small networks	Unmanageable in a large network

RING

• Each of the systems is connected to its respective neighbor forming a ring.
• The main difference between the bus and ring is that the ring topology does not require termination. Because the systems are connected all together in a loop, there is no beginning and end point as there is with the bus topology.
• This configuration is seen in Fiber Distributed Data Interface (FDDI) networks.

STAR

•  A star topology is designed with each node (fileserver, workstations, and peripherals) connected directly to a central network hub or concentrator
• Data on a star network passes through the hub or concentrator before continuing to its destination.
• The hub or concentrator manages and controls allbfunctions of the network. It also acts as a repeater for the data flow.
• This configuration is common with twisted pair cable; however, it can also be used with  coaxial cable or fiber optic cable.
• The protocols used with star configurations are usually Ethernet or LocalTalk

Advantages

• Easy to install, and wire.
•  Easy to add new workstations
• No disruptions to the network when connecting or removing devices.
• Any non-centralised failure will have very little effect on the network
•  Easy to detect faults and to remove parts.
• Centralized control
• Centralized network/hub monitoring

Disadvantages

• Requires more cable length than a linear topology.
• If the hub or concentrator fails, nodes attached are disabled.
•  More expensive than linear bus topologies because of the cost of the concentrators.

STAR WIRED

•  A star-wired topology may appear (externally) to be the same as a star topology.
•  Internally, the MAU (multistation access unit) of a star-wired ring contains wiring that allows
• information to pass from one device to another in a circle or ring
• The Token Ring protocol uses a star-wired topology.

TREE

•  A tree (hybrid) topology combines characteristics of linear bus and star topologies.
• It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantage

• Point-to-point wiring individual segments

Disadvantages

•  Overall length of each segment is limited by the type of cabling used.
• If the backbone line breaks, the entire segment goes down.
• More difficult to configure and wire than other topologies.

FDDI (Fiber Distributed Data Interface)

• 100mbps
• Normally implemented over fiber optic (fast- Ethernet, UTP)
•  Dual redundancy built in by use of primary and secondary ring
• Automatic bypassing and isolation of fault nodes

Reference : Note (Topology)