A network interface card (NIC), which is also commonly referred to as a network adapter card, is normally installed in the system unit of a personal computer and provides an interface between the computer on the media in the form of a LAN cable.

In addition to providing a physical interface to the LAN, the adapter card contains instructions, usually in the form of read only memory, which perform network access control functions, as well as the framing of data for transmission onto the network and the removal of framing from data received from the network.

To illustrate an example of access control performed by a network adapter, let us assume that the LAN is contention-based. Then, the adapter card will ‘listen’ to the LAN prior to attempting to transmit data.

If no activity is heard on the LAN the adapter card will transmit data onto the media, whereas the presence of activity will result in the adapter deferring transmission and returning to a listening state.

One of the problems associated with an access methodology based upon contention is the fact that two or more workstations that listen and have data to transmit will do so.

As you might expect, this will result in the collision of data, and a network adapter which supports a contention access scheme will contain collision detection circuitry.

Then, once a collision has been detected each adapter card will employ circuitry which results in the generation of a random time interval to be used prior to attempting to retransmit.

This method of LAN access is referred to as Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and represents the access method used on Ethernet LANs. 

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