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Friday, October 2, 2009

Logical Topologies

The Logical topology defines how the systems communicate across the physical topologies. In CISSP terms, you may hear logical topology referred to as the LAN media access method or network access method. There are two main types of logical topologies:

shared media topology
token-based topology

Shared Media

In a shared media topology, all the systems have the ability to access the physical layout whenever they need it. The main advantage in a shared media topology is that the systems have unrestricted access to the physical media. Of course, the main disadvantage to this topology is collisions. If two systems send information out on the wire at the same time, the packets collide and kill both packets. Ethernet is an example of a shared media topology.


To help avoid the collision problem, Ethernet uses a protocol called Carrier Sense Multiple Access/Collision Detection (CSMA/CD). In this protocol, each system monitors the wire, listening for traffic. If traffic is detected, the system waits until it hears no traffic before it sends packets out. If a situation occurs where two systems send out packets at the same time and a collision occurs, each system waits for a period of time before it retries. This time period is different for each system, so that the collision does not occur again.

For small networks, the shared media topology works fine; however, as you begin to add more systems to the network, there is a greater opportunity for collisions. To help reduce the number of collisions, many networks are broken up into several smaller networks with the use of switches or hubs, and each network is then referred to as its own collision domain.

Shared media networks are typically deployed in a bus, star, or hybrid physical topology.

Token Based

The token-based topology works by using a token to provide access to the physical media. In a token-based network, there is a token that travels around the network. When a system needs to send out packets, it grabs the token off of the wire, attaches it to the packets that are sent, and sends it back out on the wire. As the token travels around the network, each system examines the token. When the packets arrive at the destination systems, those systems copy the information off of the wire and the token continues its journey until it gets back to the sender. When the sender receives the token back, it pulls the token off of the wire and sends out a new empty token to be used by the next machine.

Token-based networks do not have the same collision problems that Ethernet-based networks do because of the need to have possession of the token to communicate. However, one problem that does occur with token-based networks is latency. Because each machine has to wait until it can use the token, there is often a delay in when communications actually occur.

Token-based network are typically configured in physical ring topology because the token needs to be delivered back to the originating machine for it to release. The ring topology best facilitates this requirement.

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4 comments:

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