Editing ALOHAnet (section)

=== Slotted ALOHA ===
[[file:Slotted ALOHA.svg|thumb|alt=Graph of frames being sent from 8 different stations according to the slotted ALOHA protocol with respect to time, with frames in the same slots shaded to denote collision.|Slotted ALOHA protocol. Boxes indicate frames. Shaded boxes indicate frames which are in the same slots.]]

An improvement to the original ALOHA protocol was Slotted ALOHA, which introduced discrete time slots and increased the maximum throughput.<ref>{{Cite journal |last=Roberts |first=Lawrence G. |author-link=Lawrence Roberts (scientist) |date=April 1975 |title=ALOHA Packet System With and Without Slots and Capture |journal=Computer Communications Review |volume=5 |issue=2 |pages=28–42 |doi=10.1145/1024916.1024920|s2cid=3062884 }}</ref> A station can start a transmission only at the beginning of a time slot, and thus collisions are reduced. In this case, only transmission-attempts within 1 frame-time and not 2 consecutive frame-times need to be considered, since collisions can only occur during each time slot. Thus, the probability of there being zero transmission attempts by other stations in a single time slot is:

<math>Prob_{slotted} = e^{-G}</math>

the probability of a transmission requiring exactly k attempts is (k-1 collisions and 1 success):<ref name="tann" />

<math>Prob_{slotted} k =  e^{-G} ( 1 - e^{-G} )^{k-1}</math>

The throughput is:

<math>S_{slotted}=Ge^{-G}</math>

The maximum throughput is ''1/e'' frames per frame-time (reached when ''G'' = 1), which is approximately 0.368 frames per frame-time, or 36.8%.

Slotted ALOHA is used in low-data-rate tactical [[communications satellite|satellite communications networks]] by military forces, in subscriber-based satellite communications networks, mobile telephony call setup, [[Switched video|set-top box communications]] and in the contactless [[RFID]] technologies.