How to avoid collision?

COLLISION AVOIDANCE

                    A collision occurs on your network when something happens to the data sent from the physical network medium that prevents it from reaching its destination. Mainly, it encounters another signal from another host on the network that yields a resulting useless signal on the network when the signals combine. The collision occurs when the sending device does not receive a clear response back within the allotted time. Collision avoidance is the maintenance of systems and practices designed to prevent vehicles such as cars, trains, ships, and airplanes from colliding with other vehicles. Examples include: Collision avoidance (spacecraft) Collision avoidance system in automobiles. Airborne collision avoidance systems for aircraft.

Wireless technologies, unlike wired IEEE 802.3 technologies, are half duplex. That means a wireless device (AP or endpoint) can listen or talk, but not both at the same time. In addition, in any given radio frequency (RF) region that contains multiple wireless devices, only one device can talk at a time. This creates difficulties in using RF as a shared medium. For instance, because only one device can be transmitting at a time, a single slow device has the potential to slow down all the wireless traffic in that region.

Wired technologies have techniques for collision detection and collision avoidance, such as CSMA/CD (Carrier Sense Multiple Access/Collision Detection) on 802.3 networks. On a wired network, if a collision is detected, packets can be resent.

802.11 WLANs have mechanisms in place to make sure that happens, including CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) and RTS/CTS (Request To Send/Clear To Send).

A collision can only occur at the physical layer in the OSI model. When multiple devices share a common media at the physical layer, which happens when you have multiple devices connected with a hub, there is a possibility that you will have a collision. The network area where a collision may occur is called a collision domain.

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