Main responsibilities of MAC layer:

To effectively share the capacities of multiple access channels for multiple users.

Multiple access channels are broadcast channels, i.e., the same channel is shared by all users on the networks.

Examples of multiple access channels:

• Citizen Band (CB) radio channels;
• Ethernet;
• Cellular networks using CDMA (also called spread spectrum);
• Satellite channels;
• etc.

Examples of point to point channels:

• Telephone (when not in teleconferencing);
• Modem connections;
• Cellular networks using TDMA (believe it or not, most cellular channels are allocated by central control. Once allocated, no other cellular phone can use it until the channel is cleared by the active caller.);
• etc.