Week1: Study Slotted ALOHA

home : https://hackmd.io/@bariqsufif/mynotes

tag: NTUST-RESEARCH

Random Access

• Multiple access techniques are for continuous-time applications like voice and video using dedicated channel
• Data don't require continuous transmission, but generated at random time instances, therefore dedicated channel assignment will be inefficient
• In packet radio user data is collected into packets of N bits, and once a packet is formed it is transmitted over the channel

Pure ALOHA

• Vulnerable period Vp is the time interval during which the packets are susceptible to collisions with transmissions from other users

• The packet A will suffer a colission if other terminals transmit packets during the period t1 to t1+2τ
• It is assumed that packet transmissions occur with a Poisson distribution having mean arrival rate of λ packets per second
• if τ is the packet duration in seconds, then the traffic load L is given by L=λτ
• if L>1 then the packets generated by the users exceed the maximum transmission rate of the channel, therefore the rate of new generated packets must lie within 0<L<1
• Performance of random access techniques is characterized by the throughput T, given as the total offered load times the probability of succesful transmission, T=L Pr(no collision) where Pr(no collision) is the probability of a user making a successful packet transmission
• The probability that n packets are generated by the user population during a given packet duration interval is assumed to be Poisson distributed and is given as
• For the pure ALOHA, the vulnerable period is double the packet duration
• Thus, the probability of no collision during the interval 2τ is found by evaluating at k=0
  
• Corresponding throughput

Slotted ALOHA

• In slotted ALOHA, time is assumed to be slotted in timeslots of duration τ, and users can only start their packet transmissions at the beginning of the next timeslot after the packet has formed
• NO partial overlap of transmitted packets, which increases throughput
• The vulnerable period is the packet duration τ
• Thus, the probability of no collision during the interval τ is found by evaluating at k=0
  
• Corresponding throughput

Analytical Result of Slotted ALOHA

% Analytical Throughput of slotted ALOHA
L=0:0.1:5;
T=L.*exp(-L);
plot(L,T);
title('Slotted ALOHA Protocol');
xlabel('traffic load');
ylabel('corresponding throughput');

• The maximum throughput occurs at L=1

Comparison of Analytical vs Simulation

The flowchart of my simulation is presented below

Plot of my simulation vs analytical result

the traffic vs throughput plot is very near to the analytical result.
I vary the backoff time so that the traffic also vary. more backoff time> less traffic

the matlab code and flowchart file can be found here https://github.com/bariqfirmansyah/slottedaloha