The Aloha Protocol

1
The Aloha Protocol

• Slides taken from Liam Kilmartin Dept. of Elec.
  Eng., NUI, Galway

2
ALOHA Protocols

• Carrier sense type medium access control
  protocols are very inefficient for satellite
  networks due to the long propagation delay
• ALOHA protocols are very basic transmit at will
  protocols
• Consider two variants
• Pure ALOHA protocol
• Slotted ALOHA protocol

3
Pure ALOHA Protocol

• Any earth station with a packet of data to
  transmit can transmit it onto the shared
  satellite channel at any time
• Collision will occur if that earth stations
  transmission arrives at the satellite at the same
  time as another earth stations transmission
• Therefore, a collision can occur if
• Another earth station is already transmitting
  when this earth station starts
• Another earth station starts to transmit while
  this earth station is transmitting

4
Analysis of Pure ALOHA protocol

• Assume that there are a large number (N) of earth
  stations in the network
• All earth stations transmit frames with a fixed
  (average) length of T seconds
• Each earth station transmits with a fixed
  probability (p) in the time period (T)
• Thus, the average number of frames transmitted in
  the system in the time period T will be Np

5
Analysis of Pure ALOHA protocol

• Danger period for an earth stations
  transmission starts T seconds before it initiates
  its frame transmission and ends T seconds after
  it completes its frame
• During this time period of 2T, the average number
  of frames transmitted will be E2Np2G
• A Poisson probability distribution indicates the
  probability of k events occurring in a unit
  time

6
Analysis of Pure ALOHA protocol

• For transmission to be successful, no other earth
  station should transmit during the unit time of
  interest (2T). Thus the probability of a
  successful transmission will be
• p(k0)e-Ee-2G
• Therefore, the system throughput for the time
  period T will be SNumber of transmission
  attempts in time period T x probability of
  successful transmission, or
• SGe-2G

7
Pure Aloha Throughput
8
Operation of Protocol

• Optimum Throughput occurs at G0.5 or when
• Average number of attempts to ensure successful
  transmission is

9
Slotted ALOHA protocol

• Enhancement of pure ALOHA in that stations can
  only start to transmit frame so that it arrives
  at the satellite at the beginning of defined time
  slots of duration T
• Danger period for this system is only the T
  seconds prior to the start of stations frame and
  thus ENp and SGe-G
• For this system, optimum throughput occurs if G1

10
Slotted Aloha Throughput
11
Efficiency of Slotted ALOHA

• Successful throughput S read from graph (e.g.
  Soptimum0.368 or 36.8 of timeslot contain
  successful transmissions)
• Number of frames with no transmissions can be
  found from Poisson distribution p(k0)0.368 or
  36.8 slots
• Remaining time slots must contain collisions