Page 69 - M. Abrahim Thesis

Computer Network 2021

In older forms of Ethernet, such a permanent collision, preclude the system from
working. Yet in 1000BASE-TX, sophisticated circuitry can accommodate this
permanent collision, resulting from an attempt to get as much data as possible over
UTP.

https://www.youtube.com/watch?v=_NX99ad2FUA

3.7.4 Media Access Control

Media Access Control (MAC) refers to protocols that determine which computer on a
shared-medium environment (collision domain) is allowed to transmit the data. MAC, with LLC,
comprises the IEEE version of Layer 2. MAC and LLC are both sublayers of Layer 2.

Two broad categories of MAC exist:

• Deterministic (taking turns)

• Nondeterministic (first come, first served) Token Ring and FDDI are deterministic,
and Ethernet/802.3 is nondeterministic (also called probabilistic).

CSMA/CD is an example of a nondeterministic MAC protocol.

To use this shared-medium technology, Ethernet allows the networking devices to
arbitrate for the right to transmit. Stations on a CSMA/CD network listen for quiet, at which time
it’s okay to transmit. However, if two stations transmit at the same time, a collision occurs and
neither station’s transmission succeeds. All other stations on the network also hear the collision
and wait for silence. The transmitting stations, in turn, each wait a random period of time (a
backoff period) before retransmitting, thus minimizing the probability of a second collision, as
shown in Figure 3-16.

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