# <center><i class="fa fa-edit"></i> ALOHA Basics: Utilization, Fairness, Channel Sharing Protocols, Abstraction for Shared Medium </center>
###### tags: `Internship`
:::info
**Goal:**
- [x] Learn ALOHA System Model

**Resources:**
[ALOHA Random Access Protocol (Tutorials Point)](https://www.youtube.com/watch?v=g32GA0YR26U)
[Lecture PDF](http://web.mit.edu/6.02/www/currentsemester/handouts/L18_slides.pdf)
:::

### Utilization
0 <= U <= 1
Utilization < 1 when:
* Nodes are backlogged (have packets in transmit queues)
* Perfect utilization: let one node transmit all the time, but this would be unfair
* Insufficient offered load (not enough packets to transmit to use full channel capacity)

### Fairness
1/N <= F <= 1
* x = throughput of node
* Distribution with lower standard deviation = fairer
* Note that the denominator of F is N*variance
* F = 1/N : implies a single node gets all the throughput
* F = 1 : perfect fairness
* Often a tradeoff between fairness and utilization

### Channel Sharing Protocols
Protocol: MAC; Rules of engagement
Time division
* Share time “slots”
* Prearranged: TDMA
* Not prearranged: contention protocols (Alohanet)
Frequency division
* Give each transmitter its own frequency, receivers choose “station”
Code division
* Unique orthogonal pseudorandom code for each transmitter
* Channel adds transmissions to create combined signal
* Receiver listens to one “dimension” of combined signal using dot product of code with combined signal

### Abstraction for Shared Medium
* Time divided into slots of equal length
* Each node can start transmission only at beginning of time slot
* All packets are the same size and take the same amount of time to transmit (equal to some integral multiple of time slots)
* Collision if transmission of two or more nodes overlap
* None of packets are received correctly
* Even if collision only involves part of the pacekt, the entire packet is assumed to be lost
* Transmitting nodes can detect collisions
* Can retransmit later
* Each node has a queue of packets waiting for transmission (backlogged)
* Different context:
* Nodes can hear each other perfectly (Ethernet) or not at all (satellite ground stations) or partially (WiFi or cell phone)
* Assume all nodes want to send packets to a fixed master (e.g. base station)