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2009-03-03 Network topologies and data flow

posted Feb 27, 2009, 2:14 AM by Eddie Woo   [ updated Jun 12, 2009, 5:40 PM ]

Class notes - 'network topologies'

The word topology is derived from topos, meaning place or location, and 'ology' from logos, meaning study of, or logic. A network is a group of entities that can communicate with each other.

In the textbook on page 108, a question is asked: 'how are the computers connected to one another in a network?' The answer is topology; however, this has nothing to do with the physical locations of the computer hardware. There are three major network topologies.

1. Point-to-point

Two points are connected with each other, with only one link. There are two 'sub-types' of this topology: permanent, and switched. In a switched sub-type, a network switch or switchboard is used:

2. Mesh


The mesh topology is a natural extension of a permanent point-to-point network. There is an obvious problem - it is too complex. However, it has the advantages of multiple ways to access a node ('redundancy'). and hence the network is independent of any central computer.

The partial mesh removes some superfluous connections to simplify the topology. This form is more flexible and more suited to certain situations and setups.

3. Tree (hierarchical)



This topology models the way people interact in a structured organisation. Nodes at the bottom belong to people of a lower status.

4. Ring

Every computer (or 'node') in a ring network is connected to its two neighbors. This is the most effective setup as far as the quantity of connections is concerned.

Class notes - 'controlling data flow'

Token ring

A protocol is needed to establish control in a ring network. When communications are established, special 'tokens' start flowing in the network, like trains on railway tracks.

When your computer wants to send data, it waits for the next token and puts its own data in it. At this stage, the token becomes a 'frame'. The frame will eventually reach the destination, which will detect the frame as something that it needs to process. The data is copied onto the recieving computer and sent back to the sender so that the sender can confirm that the data was received.

CSMA/CD

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. In a bus network, traffic will often overwhelm the connection and there will be a collision. All nodes in the network will sense the collision. A jamming signal is sent and only the sender of the jamming signal may send data.

Network topologies

For further details, please refer to IPT: Network Topologies

Point to point, mesh, and tree topologies

   Advantages  Disadvantages
Point to point
  • Simple
  • No special networking devices e.g. hubs required
  • Less wiring in wired networks required if the overall layout of the network is simple
  • Easy to secure
  • No need to share connections .'. less traffic through connections
  • Only one node can be connected to at any given time (switches can provide access to several different nodes, but not simultaneously)
  • Could require more wiring in some setups
  • Prone to data collisions
Mesh
  • Direct access to all other nodes
  • Data always takes the quickest route; does not need to pass through other nodes to get to destination
  • No need to share connections .'. less traffic through connections
  • If the direct connection between two nodes fails, alternative routes can be used
  • Requires the most wiring in wired networks
    • As a consequence, usually difficult to install and repair
Tree
  • Optimised for hierarchical organisations e.g. most corporations and institutions
  • Less wiring required in wired networks than mesh topology
  • Less-frequently used connections are omitted in this topology, hence saving resources e.g. money
  • Less redundant connections results in less "backup"/"fallback" routes if the main connection fails
  • Connections between hierarchical layers are indirect, with more middle nodes for data pass through as the layers in between increase in quantity; this results in more traffic

Wireless topologies

In wireless networks, topologies are much more flexible and dynamic as new connections can be created on demand. As a result, most wireless networks use what could be described as "hybrid" topologies that combine the features of various mainstream topologies, making the topology optimal for the task at hand.

Devices that are capable of wireless networking usually make use of many different technologies. For example, Infrared is always limited to point-to-point; however, an device equipped with Infrared such as a mobile phone may also be equipped with the ability to operate using other networking protocols such as Bluetooth, WAP, and Cellular telephony.

Data flow

For further details, please refer to IPT: Controlling Data Flow

CSMA/CD protocol

Carrier Sense Multiple Access with Collision Detection refers to a protocol commonly found on bus networks, by which the many nodes (multiple access) in a network are able to resolve the problem of data collision. All the nodes in the network are aware (carrier sense) if other nodes are transmitting data, and check for this before transmitting data themselves.

If two nodes transmit at the same time and the inevitable data collision occurs, a jamming signal is sent to make all nodes aware of the collision (collision detection). All nodes stop transmitting, and wait a random amount of time before re-attempting transmission.

Token ring protocol

Empty tokens and frames - see the 'class notes' above.

CSMA/CA

CSMA/CA is Carrier Sense Multiple Access with collision avoidance instead of collision detection - collision detection is not required as collisions are avoided in the first place. Collisions are avoided by requiring nodes to 'listen' for existing signals before transmission.

CSMA/CARP

CSMA/CARP is Carrier Sense Multiple Access with Collision Avoidance and Resolutions using Priorities. Different levels of priority for the nodes instead of simply forcing nodes to detect collisions or listen for existing signals.

MACAW (wireless)

MACAW stands for Multiple Access with Collision Avoidance for Wireless. It is essentially CSMA/CA for wireless networks instead of wired networks.

PCF (wireless)

PCF stands for Point Coordination Function. It is used on wireless networks - the wireless access point (in simple English, the 'router thing with the antenna on it') is required to communicate with 'listener' nodes to check if the airwaves are free of interference as a result of nodes communicating.


N.B. This post was originally titled, "Questions for Focus Group".
Ċ
Eddie Woo,
Feb 27, 2009, 2:16 AM
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mesh.png
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Eddie Woo,
Jun 11, 2009, 3:09 AM
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