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ICND1 break down -- Network Topologies

Network topology refers to the way in which the connections within the network are made.

There are two types of topology: Physical topology and logical topology.

A physical topology defines the way in which the computers, printers, network devices, and other devices are connected. There are three main physical topology: bus, star and ring.

line bus
line bus

Pic 1. bus physical topology

A bus topology consists of a main run of cable with a terminator at each end (See pic. 1). All nodes (file server, workstations, and peripherals) are connected to the cable in a line. Ethernet and LocalTalk networks use a bus topology.


Pic 2. star physical topology

A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or concentrator (See pic. 2). This category includes both star and extended-star topologies. Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. A physical star topology costs more to implement than the physical bus topology, but physical star topology have an important advantage -- if one cable connecting end device to the central device fails, the rest of the network remains operational. This is why star topology is the most common physical topology in Ethernet LANs.

In a ring topology, computers and other network devices are cabled together with the last device connected to the first to form a circle, or ring. This category includes both single-ring and dual-ring topologies. In a single -ring topology, all the devices on the network share a single cable, and the data travels in one direction only. While signe ring is suceptible to a single failure, the dual-ring topology introduces fault tolerance by creating two rings with two cables, which allow data to sent in both diretions. If one ring fails, data can be transmitted on the other ring.

Besides the three physical topologies mentioned above, there are other physical topologies. One example is tree topology.


Pic 3. tree physical topology

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See pic. 3). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs

The logical topology, in contrast,  describes the paths that the signals travel from one point on a network to another without regard to the physical interconnection of the devices.

Logical topologies are bound to the network protocols that direct how the data moves across a network. The Ethernet protocol (layer 1) is a common logical bus topology protocol. LocalTalk (layer 1) is a common logical bus or star topology protocol. IBM's Token Ring (layer 2) is a common logical ring topology protocol.

Besides bus, star and ring topology, there are other logical topologies such as mesh and tree topologies. The following pictures and tables listed the most common logical topologies.

logical topology
logical topology

Mesh Topology Devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network. In a partial mesh, at least one device maintains multiple connections to other devices.
Star Topology All devices are connected to a central hub. Nodes communicate across the network by passing data through the hub. When a star network is expanded to include an additional network device that is connected to the main network devices, the topology is referred to as an extended-star topology.
Bus Topology All devices are connected to a central cable, called the bus or backbone. The sending node broadcasts the data to the entire network. The various nodes hear it and look to see if the data is for them. If so, they keep the data. If not they ignore the data. An example of a logical bus topology is an Ethernet hub.
Ring Topology All devices are connected to one another in the shape of a closed loop. In the Ring Topology, the sending node passes a token or small message around the ring. The computer wish to transmit catches the token, attaches a message to it, and then lets it continue to travel around the network. After one full circle, the sending node will destroy the token. Since only the computer with the token at a particular time may transmit a message so that collision will never occur.The Token Ring and Fiber Distributed Data Interface (FDDI) are examples of a Ring Logical Network
Tree Topology A hybrid topology. Groups of star-configured networks are connected to a linear bus backbone.This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

A network's logical topology is not necessarily the same as its physical topology. For example, IBM's Token Ring is a logical ring topology, it is physically set up in a star topology.

ICND1 and ICND2 break down

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