19 Discuss LAN Properties.
The characteristics of a networking system are specified in the IEEE standards. These characteristics are speed, topologies, access methods and media. Dealing with them one by one:
- Speed: Many factors are responsible for controlling the speed of a network. The maximum speed of a network system is specified in the IEEE standards. The speed is generally measured in megabits per second (Mbps) with some faster systems using gigabits per second (Gbps). 1Gbps = 1000Mbps.
- Topology: Topologies are the layouts of the network both in terms of physical as well as logical. There are various patterns of layout that can be followed. The various layouts that can be adopted are bus, star, ring, mesh (partial or full) and wireless.
- Access Methods: Access methods are the mechanism followed by the systems to access the network media and send across data. These are mandatory to ensure that communication between two systems on a network can take place. It provides an adequate opportunity to every system on the network to use the same. CSMA/CD, CSMA/CD, and Token Passing are the access methods used in a network. Discussing them in detail:
- CSMA/CD: The full form of the acronym is Carrier-Sense Multiple Access with Collision Detection. It is the most popular media access method and is defined in the IEEE 802.3 standard. It is associated with 802.3 Ethernet networking which again is the most popular networking system. On a network that is using CSMA/CD, a system desiring to send data to another, first conducts a check to find if the network media is free. This check is essential as every piece of network media used in a LAN carries the signal one at a time. Once it is detected that the network media is free, the sending system transmits the data to the destination.
As long as the transmission happens in this particular mode, there would be no problems and the CD part of the access method would not be required. The problem occurs when two systems try transmitting data simultaneously. It is likely to happen as the frequency of communications is many a times in a single second. When two systems communicate at the same time it is known as collision.
Collision needs to be detected and the same is done by detecting fragments of the transmission. Both the systems wait for some time before they actually start to transmit again. This time is randomly calculated and is only a fraction of a second. This is known as backoff. The system transmits again once the backoff period is over. Incase the system succeeds in transmitting, well and good, otherwise it keeps trying until it finally succeeds.
The advantage of this system is that it does not take much to work efficiently, but the shortcoming is that as more systems are added, collisions that are more frequent occur. This results in the network becoming slower. The advantages far outweigh the disadvantage and so a solution has been found in the form of using switches for creating multiple collisions domains.
- CSMA/CA: the full form of the acronym is Carrier-Sense Multiple Access with Collision Avoidance. This method is based on avoidance of signal rather than detection. It is associated with 802.11 wireless standards and is in use in Apple's LocalTalk network.
On these networks, before the data is actually sent a signal expressing the intent to transmit is sent. When a potential collision is detected in a network by a system, it waits before sending out the data transmission. This helps to avoid transmission collisions. Just as in the case of CSMA/CD, the backoff time is observed in this method also. Once a backoff time expires, the system again listens for a clear channel. If a clear channel is not found, another backoff time is allowed to expire. The same process is repeated till the wait time does not reach zero.
The disadvantage of this system is its choice of notifying its intention of transmitting data through the broadcast method. The broadcast method generates a lot of traffic, which causes congestion.
- Token Passing: Token passing is defined in the IEEE 802.5 standard. Until Ethernet networking came to be preferred it dominated the scene of choosing access methods. In this, a special data frame known as a token is passed among the systems that are on the network. A system is enabled to transmit data when it possesses the token that is only one in number. On arrival of the data, the computer at the receiving end sends out a verification message to the computer that sent the data. The sender computer creates a new data and the process is ready to be repeated. The duration for which a token can be possessed is governed by the standards.
The advantage of this method is lack of collisions, as transmission can only take place when a token is in possession and only one token existing at one time, there is no dispute. The speed of this system does not falter even with heavy traffic. This system works well in facilities like videoconferencing. The disadvantage of this system is the requirement of specialized software and hardware, which further impacts the costs.
- Media: The media that are available fro transporting the signals in a network are mentioned in the IEEE specifications. The common network media types are the different types of cables (twisted pair, coaxial, fiber optics) infrared, radio frequency.