Performance Evaluation of IEEE 802.15.4 MAC With Different Backoff Ranges In WSNs

Reference :

Performance Evaluation of IEEE 802.15.4 MAC With Different Backoff Ranges In Wireless Sensor Networks
Jeong-Gil Ko, Yong-Hyun Cho and Hyogon Kim
Department of Computer Science and Engineering
Korea University

The IEEE 802.15.4 MAC layer can not support different throughput performance for individual nodes with the current specifications. If certain nodes are sending data more frequently compared to the other, in standard MAC it is hard to achieve network efficiency. The authors modified the IEEE 802.15.4 standard and additionally proposed a new state transition scheme.
By adjusting the minBE value of some nodes to a smaller value and by dynamically changing the value depending on the transmission conditions, the authors shortened the backoff delay of nodes with frequent transmission.

Standard IEEE 802.15.4 limits the default value of the minBE(minimum Backoff Exponent) as 3, and aMaxBE (Maximum BE) as 5, giving only the range [0::2^3] to [0::2^5] for randomly selecting the actual backoff value. They additionally adapt the State Transition Scheme to pursue efficient use of the differentiation. This modification will increase the network efficiency because nodes send data frequently will have more opportunity to transmit compared to the nodes that remain idle.

Standard MAC of IEEE 802.154.4 :
the initial value of BE (backoff exponent) is given as macMinBE and the system selects abackoff time from a random number between [0::2^BE – 1]. while the backoff time is set, the system starts backoff and the slots in the CAP of the superframe are skipped away. If a selected backoff is longer than the number of CAP that are left in the current superframe, the backoff is paused until the next superframe and continues the backoff on the CAP of the next superframe.

After the backoff time is over, the system performs CCA (Clear Channel Assessment) to check if the channel is busy or not. If the channel is sensed busy, the value of BE is increased by one and a new random number is selected from the new range for another backoff period. If the channel is sensed idle for two consecutive CCA (for example when CW=0) the channel is considered idle and the system can start transmission.

MAC Modification :
a. Backoff Exponential Range Modifications
in their MAC modification,The minBE which indicates the minimum value of the BE can be set to a numberbetween 0 and 3. the default standard value is given as 3. To give differentiation to individual nodes, they suggest that the minimum value of IEEE 702.15.4 MAC be flexible to changes. If the BE value is decremented to a value less than 3, the lower boundary of the possible backoff value will decrease also. It will shorten the waiting time when CCA detects the channel busy or when a packet collides with a different packet in the channel. This give a higher chance of selecting a shorter backoff time. This makes the node try the CCA more frequently, leading it to a higher possibility of making a successful trasmission. Because of this the throughput will increase significantly compared to other nodes with longer waiting.

b. State Transmission Scheme
In the MAC modifications, we let the value of the minBE be a number between 1 and 3. The value of the minBE change flexibly as the condition of the node changes. Each node has three states, noData, postData,sendData, with each state having minBE value of 3,2,1 respectively.
Figure :

f6feb_1.png

Simulation Result :
it can be seen that the modified node with a lower minBE values has higher throughput than a node with no modifications.

Reference :

Performance Evaluation of IEEE 802.15.4 MAC With Different Backoff Ranges In Wireless Sensor Networks
Jeong-Gil Ko, Yong-Hyun Cho and Hyogon Kim
Department of Computer Science and Engineering
Korea University

Note : This resume is created for self-learning only. Author and Publisher hold copyrights

I will read all of whatever you have been read to waste boring vacation

February 06 2008,
Taipei City
High Speed Network Lab

Udin Harun

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