Random access protocols are a category of multiple access (MAC sublayer) protocols in which no station has a scheduled turn or fixed priority — any station with data ready may attempt to transmit at any time, without needing permission or a pre-assigned time slot. Because multiple stations can attempt to transmit simultaneously, collisions are possible, and the protocol must define rules for how stations detect, handle, and recover from them (e.g. random backoff before retrying). Examples include ALOHA, CSMA, CSMA/CD, and CSMA/CA.

Pure ALOHA vs Slotted ALOHA

Aspect	Pure ALOHA	Slotted ALOHA
Transmission timing	A station transmits immediately whenever it has data ready, with no synchronization	Time is divided into fixed-length slots (= 1 frame transmission time); a station may only begin transmitting at the start of a slot
Vulnerable time	2 x frame transmission time (a collision can occur if any other station starts within one frame-time before or after)	1 x frame transmission time (since all stations are synchronized to slot boundaries, overlaps can only happen if two stations pick the same slot)
Maximum throughput	~18.4% (S = G x e^(-2G), max at G=0.5)	~36.8% (S = G x e^(-G), max at G=1), exactly double pure ALOHA
Synchronization needed	No	Yes — requires a global clock/synchronization mechanism among all stations
Delay at low load	Lower (no waiting for slot boundary)	Slightly higher (must wait for next slot start)

By halving the vulnerable time, slotted ALOHA significantly reduces the collision probability at any given load, roughly doubling the maximum achievable throughput compared to pure ALOHA — at the cost of requiring all stations to be time-synchronized.