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When many nodes share one broadcast channel, such as a radio frequency or an old shared Ethernet cable, two frames sent at once collide and are both lost. A multiple access protocol is the set of rules that decides who may transmit when. These protocols fall into three families: channel partitioning (divide the channel by time, frequency or code), random access (transmit freely and recover from collisions, as in ALOHA and CSMA), and taking turns (polling or passing a token). Each family trades efficiency under heavy load against delay and fairness under light load.
Simulate the channel in class: students "send" by raising a hand in time slots, and two raised hands mean a collision. Run the same activity under fixed time-slot rules, then random rules, then a passed token, and record throughput. Follow with the Kurose and Ross derivation of slotted ALOHA's maximum efficiency, and finish by explaining why Wi-Fi avoids collisions instead of detecting them.
On a point-to-point link, one sender talks to one receiver and there is nothing to coordinate. Many links are different: a Wi-Fi channel, a cellular frequency or a classic shared Ethernet cable is a broadcast channel that every attached node hears. If two nodes transmit at once, their signals interfere, a collision, and both frames are lost. A multiple access protocol decides who may send when, using only the channel itself to coordinate.
The first family is channel partitioning. Time division (TDMA) gives each node a fixed, repeating time slot. Frequency division (FDMA) gives each node its own frequency band. Code division (CDMA) lets all nodes send at once using different codes. Partitioning is fair and collision-free, but a node may use only its share, even when everyone else is idle.
The second family is random access: send when you have data, and recover from collisions. It began with ALOHAnet, a packet radio network built at the University of Hawaii around 1970 under Norman Abramson. In slotted ALOHA, time is divided into slots. A node sends at the start of a slot, and after a collision it retries in each later slot with some probability p. With many active nodes, the best possible efficiency is 1/e, about 37% of slots.
CSMA (carrier sense multiple access) adds a simple rule: listen before talking, and wait if the channel is busy. Collisions still occur, because a signal takes time to travel and a distant node may sense an idle channel after another node has already started. CSMA/CD (collision detection), used by classic shared Ethernet, also listens while sending and stops at once when it hears a collision. It then waits a random time chosen by binary exponential backoff, doubling the range of possible waits after each collision. Wireless networks use CSMA/CA (collision avoidance) instead, because a radio cannot easily hear others while transmitting. An 802.11 node waits a random backoff before sending, and the receiver confirms each frame with an acknowledgment.
The third family is taking turns. In polling, a controller invites each node to send in order. In token passing, a special frame circulates and only its holder may transmit. These schemes avoid collisions and use the channel well under load, but the controller or token is a single point of failure.
Today's switched, full-duplex Ethernet has no shared medium, so CSMA/CD is mostly history there. The same ideas remain central in Wi-Fi, cellular systems and other shared radio links.