Slot time

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Slot time is a concept in computer networking. It is twice the time it takes for an electronic pulse (OSI Layer 1 - Physical) to travel the length of the maximum theoretical distance between two nodes. In CSMA/CD networks such as ethernet, Network Interface Controllers (NICs) wait a minimum of the slot time (which should be a constant, NOT dependent on the individual network -i.e., it is a standard across all CSMA/CD networks that use a common NIC) before transmitting, allowing time (the maximum theoretical time - slot time) for the pulse to reach the NIC that intends to send.

Since a pulse will never exceed slot time (the maximum theoretical time for a frame to travel a network), the NIC waits a minimum of slot time before transmitting, in order to allow any pulse that was initiated at the time that the waiting NIC was requested to send, to reach the waiting NIC. By allowing the pulse to reach the waiting NIC, a local collision occurs rather than a late collision occurring. By having the collision occur at the NIC (local) and not on the wire (late) CSMA/CD implementation can take more control over the situation.

Some times for slot time include:

Speed Slot time* Time Interval
10 Mbit/s 512 bit times 51.2 microseconds
100 Mbit/s 512 bit times 5.12 microseconds
1 Gbit/s 4096 bit times 4.096 microseconds
10 Gbit/s Not applicable Not applicable
  • Slot time is only applicable to half-duplex transmissions. Since slot time is the time required to wait for the medium to be free from transmissions, there is no time required to wait for full-duplex transmissions. 10 Gbit/s is a full duplex technology, so slot time is not applicable here.
See also: bit time