IEEE 802.15.4 standard defined medium access control (MAC) protocol to achieve low-power transmissions in low-rate and short-distance WPANs. CSMA/CA does not protect the hidden node problem (HNP) to minimize power consumption. In this paper, the authors propose grouping strategy to solve the IEEE 802.15.4 HNP.
There are two kinds of WPAN topologies: the star (infrastructure) and the peer-to-peer (ad-hoc) topologies. The coordinator and nodes (devices) have two kinds of transmission manners. One is to contend channel in the contention access period (CAP), and the other is to access channel in a collision-free manner during a contention-free period (CFP).
Devices try to access channel using the modified CSMA/CA mechanism. When a node has frame(s) in the transmission buffer, it first selects a random backoff value from the initial contention window (CW) and the enter backoff mode. Different from the standard CSMA/CA, this node starts sensing channel only when the backoff countdown process is complete. During the sensing period, if node perceives the channel idle for two consecutive unit backoff periods (UBPs), it will transmit the frame. UBP is the basic time unit in the backoff process. In the other hand, if node detects that the channel is busy during carrier sensing period, it stop listening and selects another random backoff value from the doubled CW for the next attempt.Then the node turns of its transceiver and starts to coundown the new chosen backoff time. Because of the blinding countdown, the channel utilization could be very low and the access delay could be very long.
The active superframe portion consists of CAP and CFP. The interval of active portion and the interval of beacon frame (interval of superframe) are decided by Superframe Order (SO) and Beacon Order (BO) respectively. An active and a superframe interval both consist of 16 equal size time slots. One slot of the active period equals to 3X2^SO UBPs and the superframe interval equals to 3X2^BO. BO and SO can be set from 0 to 14. Thus, the interval of inactive portion is equal to 16X3X(2^BO-2^SO) UBPs.
The authors called two possible collision cases in WPAN : Contention Collision (CC) and Hidden Node Collision (HNP).
Grouping Strategy Phases :
- Hidden node situation discovery.
- Hidden relationship collection
- Nodes grouping
- Bandwidth allocation
Mathematical Analysis :
The authors also use trigonometric function to prove that the area covered by the transmission ranges of five selected nodes is able to cover the whole network.
The authors simulate the grouping strategy, the hidden node environment and non-hidden node environment.
1. The derived goodputs measured in percentage of the total channel bandwidth/ traffic load.
2. The battery lifetime vs. The traffic load
Grouping Strategy for Solving Hidden Node Problem in IEEE 802.15.4 LR-WPAN
by Lain-Jinn Hwang, Shiann-Tsong Sheu‡, Yun-Yen Shih‡ and Yen-Chieh Cheng.
Proceedings of the First International Conference on Wireless Internet (WICON’05) IEEE
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July 05, 2008
High Speed Network Lab