RING topology
The RING network consists of a closed loop of networked devices. Each device is directly connected to another device in the loop. The RING network can cover greater distances than some other types of networks (e.g. BUS networks) due to each host’s ability to regenerate the message as it passes through it and the fact that there is no central hub requiring each device to connect to.
The most common implementation of the RING topology is the Token Ring network. These networks use a 'Token' that is passed along the RING to prevent data collision. Data can only be sent by the computer (host) with the Token, and the Token can only be held for a set period of time before being passed on to the next computer, if when the Token is received no data is required to be sent, then the Token is passed on immediately.
Original RING networks were implemented using coaxial cable limiting the maximum bandwidth achievable by the network to either 4Mbps or 16Mbps.
RING networks can be expensive to implement and costly to run – any work on the structure of the ring, or fault resulting in an effective break in the ring, results in a period of downtime (all ends of the RING network must be terminated in order for the RING to function). Also, if there is a break in any of the cables, the whole network is down. On the upside, they have the potential to deliver fantastic bandwidth over large distances.
FDDI topology
The Fiber Distributed Data Interface (FDDI) network builds on the basis of the single RING network, but consists of two RING networks running side by side. This architecture is know as 'dual-ring', and the data on each RING flows in an opposite direction (counter-rotating).
The two rings are known as ‘Primary’ and ‘Secondary’. The Secondary ring is generally used as a back-up RING, with the Primary delivering up to 100Mbps capacity. In some circumstances where the Secondary ring is not being used for backup, the capacity can be doubled and therefore effectively deliver 200Mbps of bandwidth.
An advantage of FDDI over a standard RING network is the fact that FDDI uses fibre optics, and so is capable of far higher bandwidth and further distances between hosts (and so overall distance).
Also, by using two rings instead of one, FDDI allows for an element of redundancy that a RING network cannot. If a computer is taken out of the network, the secondary ring forms a loop using the remaining computers and thus maintains the network between the remaining computers.
STAR topology
The STAR network is built using a central 'connection' known as the hub, off which all other network devices are connected. This 'hub' can be a hub, switch or router, and depending on the device, ranges from a simple data link to actively processing which messages are sent to which machine.
The basic hub design consists of messages being sent from a single computer to the hub, and the hub then forward the message onto all other computers connected to the hub.
STAR networks have a typical bandwidth of 100Mbps, though this does vary, and use Ethernet cabling (CAT 5).
Data collision can be high on a STAR network, and this increases as the network grows. Whenever there is a data collision, the whole network is 'frozen' for an instance and then effectively 'reset'. As data collision increases as more and more devices are added to the network, this can have a major impact on performance.
Devices such as switches and routers reduce data collision, as these actively manage data direction, effectively reducing the traffic on the network and therefore reducing the possibility of data collision.
Comparison of STAR and RING topologies
Below is a table showing comparison of RING and STAR networks:
|
| RING | STAR |
|---|---|---|
| Reliability | The RING network is limited by design, as the whole network is down whenever there is a fault in the line | Good, the network remains up, even if there is a failure taking out a computer. |
| Cost | RING networks are reasonably easy to implement, as there is no central hub. | A central hub is require and cabling needs to be routed from all attached devices to this hub. |
| Speed | The RING network can offer high speeds on reasonable size networks as data only goes in one direction. Though there on larger networks speed becomes an issue do to data having to pass through every computer between the sender and receiver. |
Can offer excellent speed, even on larger networks if combined will switches and routers. Data is only sent from host to receiver, with few other devices involved. |
| Maintainability | Network has to be down to add or remove a computer from the network. Though only cabling is required between to a computer on the network to the new computer | Very good as new computers can be added to the network without any down time. Though if new cabling is required this can prove to be an issue, as it has to be routed to the nearest hub. |
| Expandability | There is a limit to the number of computers that can be included on a RING network before there is a drop in performance. | There is good potential for expansion on a STAR network, as with the use of switches and routers, additional network activity is minimal. |
| Data Collision | Data Collision is not an issue on a RING network, as only the computer that currently holds the token can transmit data. | Can become an issue when there is a large amount of network activity either on large networks or those that the functionality of switches and routers. |
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