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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MESH TOPOLOGY

The mesh topology is the last topology we discuss. In this layout, every system is connected to every other system. The main advantage of this topology is high availability. The main disadvantage of this topology is cost, both administrative and physical. Because each system is connected to each other, the amount of cabling and maintenance necessary can be prohibitive, especially in larger networks. The formula for determining the amount of cable needed in a mesh network is:


(N x (N - 1))/2, where N is the number of systems to be interconnected

In our example in Figure 5, we have six systems that require 15 cables to create a mesh network. This topology is mainly used in Wide Area Network environments or in environments where high availability outweighs the costs associated with this amount of interconnection.

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TREE TOPOLOGY

The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the headend, where one or more cables start, and each of these may have branches. The branches in turn may have additional branches to allow quite complex layouts.

Again, a transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement. First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended. Second, a mechanism is needed to regulate transmission.

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BUS TOPOLOGY

For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus.

A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.

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ring topology

In the ring topology, the network consists of a set of repeaters joined by point-topoint links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater. The links are unidirectional; that is, data are transmitted in one direction only and all are oriented in the same way. Thus, data circulate around the ring in one direction (clockwise or counterclockwise).


Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. As with the bus and tree, data are transmitted in frames. As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.

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STAR NETWORK

In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion. A transmission of a frame from one station to the node is retransmitted on all of the outgoing links.

In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.

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LAN topology

LAN topology

There are four basic types of LAN topology.




* STAR
* RING
* BUS
* TREE

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