Tuesday, 6 August 2013


An access method is a set of rules governing the network nodes that share the transmission medium. There are three methods of of media access methods:
1.     CONTENATION- It means that the PCs are contending (competing) for the use of transmission medium. Any PC can transmit data at any time on first come first serve basis. But the system breaks down when 2 PC try to transmit data at the same time. This usually happens when the network is too busy & tries to transmit data which results in collisions of data packets. Mechanisms like Carrier Sensing Multiple Access (CSMA) are usually used to minimize the collisions. Each computer “listens” to the network before attempting to transmit data. If the network is busy then the computer avoids transmitting data until the network settles down. Another mechanism is Carrier Detection (CD) in which computers continue to “listen” to network as they transmit data. If the computer detects another signal that is interfering with the signal it is sending then it stops transmitting. Both computers then wait for random interval time & try to retransmit.  
2.   POLLING- One device is responsible for polling the other devices to see whether they are ready for transmission or reception of data. Polling-based systems use a device called master device or controller. This access method is not widely used on networks because polling itself can cause some network traffic.
3.  TOKEN PASSING- The computer takes turns to use the transmission medium. This system utilizes a frame called token, which circulates around the network. A computer that needs to transmit waits until it receives a token, at which time the computer is permitted to transmit. The computer then passes the token frame to the next station on the network. 
Topology is the map of network. The 2 basic categories are as follows:
        1.  PHYSICAL TOPOLOGY- It describes actual layout of the network.
        2.  LOGICAL TOPOLOGY- it describes the logical pathway followed by a signal as it passes through the network nodes.
The 4 basic types of network topology is as follows:

      1.    BUS- The bus topology is ideally suited for networks that use contention-based access methods. All PCs are connected to the same central cable called bus or backbone. The cable is terminated at each end. A special connector called terminator is placed at the end to prevent signals from reflecting back on the cable and causing interference. Wiring is normally done point to point. A faulty cable or workstation will take the entire LAN down.
      2.  RING- All PCs are connected to one another in shape of ring, so that each device is connected directly to two other devices, one on either side of it. Each device has a receiver and a transmitter and acts as a repeater that passes the signal to the next device in ring. As the signal is regenerated at each device, the signal degeneration is low. Faulty workstations can be bypassed. More cabling required than bus topology.
    3STAR- All PCs are connected to a central part, a server or hub. The hub receives signals from other network devices & routes them to destinations. Star hubs can be interconnected to form a tree or hierarchical topology. It has the greatest cable lengths of any topology.
     4.  MESH- A mesh topology is really a hybrid model representing an all-channel sort of physical topology. Every device is directly connected to every other device in network. Any new device must be connected to all existing device. If there is any break in the transmission medium, the data transfer takes alternative routes. Cabling is more extensive and complicated.