A collision domain is a physical network segment where data packets can "collide" with one another for being sent on a shared medium, in particular in the Ethernet networking protocol. A network collision is a scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. Meanwhile, another device does the same, and the two competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network. [1]
If a group of Ethernet or Fast Ethernet devices in a CSMA LAN are connected by repeaters they will compete for access on the network. This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions. Collisions decrease network efficiency; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.
The basic strategy goes like this: [2]
- A computer listens on the cable to see if another computer is transmitting, which is indicated by a voltage change on the cable. If busy, the computer waits and listens.
- When the cable is not busy, a computer attempts to transmit.
- Another computer may attempt to transmit at the same time, which causes a collision.
- Both computers that attempted to transmit must back off, wait a random period of time, which is generated independently by each computer, (see Exponential backoff), otherwise they would immediately collide again, and then each computer attempts to transmit again.
Computers on the network detect collisions by looking for abnormally changing voltages. Signals from multiple systems overlap and distort one another. Overlapping signals will push the voltage above the allowable limit. This is detected by attached computers, which reject the corrupted frames (called runts).
To relieve the network of collision domains, it is recommended to use a network switch which increases the number of collision domains, but decreases each collision domain's size. This is because each port on a switch is its own collision domain.
See also
Broadcast domain A broadcast domain is logical division of computer network.
Exponential backoff A method used to rapidly decrease the possibility of two systems colliding with one another by generating a random number (in the case of Ethernet, the number of milli-seconds to delay), multiplied by an exponetially increasing (per iteration) multiplier.
References
- ^ CCNA Study Guide Lammle, Todd (2004). CCNA Study Guide (Fourth edition ed.). Sybex Inc.. ISBN 0-7821-4311-3.
- ^ Professor of MIT University Tanenbaum, Andrew S. (2003). Computernetwerken (Computer Networks) (Fourth edition ed.). Pearson Prentice Hill. ISBN 90-430-0698-X.
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