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In computer networking, slot time is the time that should elapse between a first electronic pulse being sent and a second one following it. It is equal to twice the time it takes for an electronic pulse to travel the theoretical maximum distance between two devices on the network. Slot time need only be taken into account during a half-duplex transmission.
A computer network is a group of computers and other devices joined by wired or wireless connections. This allows communication and resource sharing between the various devices known as nodes. A network interface controller such as a network interface card, network adapter, or LAN adapter allows physical access to a node through the network.
When information is sent from one node to the other, the network interface controller must take slot time into account. Since slot time is twice the time it takes for a pulse to travel through the network, a pulse can never take longer than the slot time to reach its destination. After transmitting a first pulse, the network interface controller waits at least the length of the slot time before it transmits a second pulse.
This is important because it creates a local collision instead of a late collision. A local collision is one that occurs between two pulses at the network interface controller. This type of collision can be detected by the network interface controller, which then deals with the problem. A late collision happens in the wire and may not be detectable by the network interface controller.
Slot time is important only in a half-duplex communication system. Any system composed of devices that can communicate with one another in both directions is known as a duplex system. In a half-duplex system, the communications can only move in one direction at a time. This means that if one node sends a signal, the other cannot reply until the first finishes sending. The walkie-talkie is a familiar example of this kind of system.
The slot time of a half-duplex system is important because it lets the nodes know how long to wait before attempting to respond to a signal. In a full-duplex communication system, information can move in both directions at once without interference. In this case, slot time is irrelevant because the nodes do not have to wait to respond to each other. Full-duplex systems tend to be faster than half-duplex systems for this reason.
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