Adaptive Backoff Exponent

Goal :

1. Propose and efficient management of BEs based on the decision criterion

2. Present a brief study of the CSMA-CA mechanism used in IEEE 802.15.4, with emphasis on the improper BE distribution which result in frequent packet collisions and a loss in systems performance.
3. propose algorithm called the Adaptive Backoff Exponent (ABE) which reduce the probability of devices choosing identical number of backoff periods


NOTE :

1. The BE determines the number of backoff periods the device shall wait before accessing the channel
2. The power consumption requirements make CSMA-CA use fewer Bes which increase the probability of devices choosing identical Bes and as a result wait for the same number of backoff periodes in same cases
3. this inefficiency degrades system performance at congestion scenarios by bringing in more collisions
4. CSMA-CA is implemented in units of time called ‘backoff period’ where one backoff period is equal to aUnitBackoffPeriod (=20 Symbols).
5. A study of the system performance at collision scenarios reveals an exponential increase in the number of packet drops, for higher datarate operation. The drop in system performance can be attributed to the numerous poor link quality packets. Link quality is an attribute to characterize the quality of each incoming packet. This measurement is implemented as a Signal-to-Noise Ratio (SNR) estimation in most cases. Often, poor link quality is a direct consequence of the hidden node problem
6. because of the power consumption constrains the BE of devices is never allowed to vary out the range 3-5. As a result, devices often choose identical number of backoff periods and detect an idle channel. Thus, the devices on choosing an identical backoff periods transmit data/MAC-command packets without being aware that another node has also detected an idle channel. This result in frequent confrontations among nodes which result in collisions, packet drops and as a consequence retransmission
7. in CSMA-CA, each device shall maintain three variables for each transmission attempt : NB, CW and BE
8. the higher the value of BE, the longer the device will spend trying to access the channel in some cases. The longer wait adds up to the power consumption of the device. Therefore a lower BE range will ensure the devices will never spend too much time waiting for channel access. This makes two or more devices using the same number of backoff periods. As a result they detect an idle channel simultaneously and proceed with their transmissions which results in frequent collisions
9. the choice of the correct value of backoff periods if out of scope of the IEEE 802.15.4 standard. It could possibly be determined by the application developer
10. Beacon Modification : ABE algorithm utilizes additional space withn the beacon payload to convey the coordinator’s decision on the macMinBE adjustment. Hence additional space is required to indicate the number of incmenting/decrementing nodes and their respective node addresses

ABE Algorithm :
1. The minimum backoff Exponent (macMinBE) shall be variable
2. All devices that contributing more to the network traffic are slapped with higher macMinBE, and devices which contribute less to the network congestion will use lower minimum backoff exponents. Therefore devices with longer macMinBE are likely to wait longer than devices with lower macMinBE
3. the coordinator keep track of each node’s contribution to the network traffic during of the analyzing cycle to decide the macMinBE
4. the devices are now allowed to use a higher backoff exponent (aMaxBE = 7). At the beginning of the transmission the devices shall start off with a macMinBE of 3. However as time progress and the node in question contributes unevenly to the network traffic, its macMinBE can be either decremented or incremented. Hence on the next transmisson it shall use the new macMinBE
5. the algorithm is implemented in three different phases : (a). The analyzing phase (at coordinator), (b) the decision phase (at coordinator), (c), the implementation phase (at end nodes)

 
Three Important Principles of ABE :
1. The idea is providing a higher range of backoff exponents to the devices, to reduce the probability of devices choosing the same number of backoff periods to sense the channel.
2. The backoff exponent shall be variable, hence devices are not likely to start off with the same backoff exponent when they wish to start a data transmission.
3. The way the minimum backoff exponent (macMinBE) is maintained.

Performance Analysis :
1) Simulation : Network throughput, End-to-End Delay, Delivery Ratio, Energy Consumption vs Traffic load (pkts/sec)