The CSMA/CD
access rules
1) Carrier sense—Each station/transmitter continuously listens for traffic
on the medium to determine when gaps between frame transmissions occur.
2) Multiple-access—Stations/transmitter may begin transmitting any time they
detect that the network is quiet/idle (there is no traffic).
3) Collision detect—If two or more stations/transmitters in the same CSMA/CD
network (collision domain) begin transmitting at approximately the same time,
the bit streams from the transmitting stations will interfere (collide) with
each other, and both transmissions will be unreadable. If that happens, each
transmitting station must be capable of detecting that a collision has occurred
before it has finished sending its frame.
. All devices on the network have
equal-priority access to the medium.
. Multiple nodes may simultaneously receive data from the medium but
only one node can transmit at a time.
. The technique used to arbitrate is called Carrier Sense Multiple
Access with collision detection (CSMA/CD).
. A node wishing to send data, first “listens” the medium.
. If some activity is going on then the node will differ its transmission
until the activity ceases and a predetermined period of silence passes.
. This period of inactivity is known as IPG (inter packet gap)
. The IPG delineates each packet and allows all stations to detect
carrier sense as inactive (IPG values is one round trip delay)
. If two or more nodes simultaneously starts transmitting then a
Collision occurs.
. Each transmitting node monitors for “collision” and if detects one,
stops immediately and sends a 32 bit jamming sequence.
. Jam period guarantees that stations at the extremes of the network
are able to detect collision condition.
. If the collision is detected during preamble, Preamble/SFD sequence
is completed and then Jam sequence is sent.
. After collision the MAC retries
until either it is successful or a maximum number of attempts have been made
and all have terminated due to collisions.
. The scheduling of the retransmissions is determined by a controlled
randomization process called “truncated binary exponential backoff”.
. The delay is integer multiple of slot time.
. The number of slot times to delay before the nth retransmission
attempt is chosen as a uniformly distributed random integer r in the range:
0 <= r < 2k
Where k = min (n, 10)
. If all attempts fail, this
event is reported as an error.
. Algorithms used to generate the integer r should be designed to
minimize the correlation between the numbers generated by any two stations at
any given time.
. A round-trip delay called Slot time determines how long it takes to
detect a collision.
. Slot time is fixed as 512 bit times for Ethernet.
. If a collision is detected after slot time then its called as Late
collision and transmission is aborted immediately.
