Performance Evaluation of the IEEE 802.15.4 MAC for Low-Rate Low-Power Wireless Networks

Source :
Performance Evaluation of the IEEE 802.15.4 MAC for Low-Rate Low-Power Wireless Networks
Gang Lu, Bhaskar Krisnamachari, Cauligi S.
Univ of Southern California

The authors provide this paper one of the first simulation-based performance eveluations of the new medium access protocol in IEEE 802.15.4, focusing on its beacon-enabled mode for a star topology network. The authors provide an analysis comparing the energy costs of beacon tracking and non-tracking modes for synchronization.

Compared to wireless local area networks (WLAN) which aim to provide high-throughput, low-latency for traditional transfer and multimedia applications. the required data rate for LR-WPAN applications is expected to be only on the order of tens of kbps. The goal of IEEE 802.15.4 is to provide a physical-layer and MAC-layer standard with ultra-low complexity, cost , and power for low-data-rate wireless connectivity among cheap fixed devices.

Data Transfer :
A node wishing to send data to the PAN coordinator needs to receive a beacon to understand the current superframe structure. If it has been allocated a GTS, it sends its data during the CFP, otherwise, it sends its data using CSMA-CA in the CAP.
Coordinator needs to indicate in its beacon when messages are pending for devices. Devices themselves need to poll the coordinator to determine whether they have any messages pending.

Low Duty Cycle :
The coordinator announces the superframe structure to devices in the PAN periodically through beacon frame. By changing the active and inactive portion via the parameters SO and BO, the WPAN can operate under low duty cycle to conserve energy.

Contention Access Period (CAP)
Three variables are maintained at each device for a channe; access : NB, CW and BE. NB is the number of times the CSMA-CA backoffs while attempting the current transmission, and is rest to 0 for each new data transmission. CW is the contention window length, which is reset to 2 either for a new data transmission or when the channel is found to be busy. BE is the backoff exponent, which is related to the backoff periods a device should wait before attempting carrier sensing again.

in CSMA-CA, alot of energy is generally consumed by the long backoff period which is required during high traffic periods to avoid collision. However, in IEEE 802.15.4 supports a Battery Life Extension(BLE) mode, in which the CSMA-CA backoff exponent is limited to the range 0-2. This reduces the period of idle listening in low offered traffic applications. A network device can put its radio to sleep to conserve energy immediately after the reception of acknowledgment packet if there is no more data to be sent or received.

Collision Free Period (CFP)

a device requiring dedicated bandwidth or low-latency transmission can be assigned a guaranteed time slot(GTS) in CFP by the PAN coordinator. when a device wishes to transmit aframe using GTS, it first checks a list on the beacon frame to see whether it has been allocated a valid GTS. If a valid GTS is found, the device enables its receiver at a time prior to the start of the GTS and transmits the data during the GTS period. The MAC of the PAN coordinator ensures that its receiver is enabled for all allocated guaranteed time slots.

Synchronization
The PAN coordinator transmits beacon frames periodically to announce the superframe structure in PAN.Devices need to synchronize with a coordinator.There are two methods of synchronization : tracking and non-tracking.

With tracking, the device receives the first beacon, gets currentsuperframe structure, knows when to active its receiver for the next beacon and keep track of it.
with non-tracking. the device attempts to acquire the beacon only once.
let see original paper for detail🙂.

CSMA-CA in CAP

The CSMA-CA algorithm in IEEE 802.15.4 employs a small backoff period,i.e. Battery Life Extention (BLE), to reduce the idle listening energy, to reduce the idle listening energy consumption.

Duty Cycle
In IEEE 802.15.4, the coordinator decides the duty cycle and beacon interval that the PAN operates on. This is done by setting the SO and BO parameters. The duty cycle is given the expression 2^SO / 2^BO. Low duty cycle conserves energy by putting devices to sleep. However, a low duty cycle reduces the bandwidth and increase latency.

Guaranteed Time Slots (GTS)
GTS can be assigned in the CFP to applications which require dedicated bandwidth or low latency regardless of the traffic load in the network. However, as in TDA, the time slot assigned to an application could be wasted if there is no data transmission request.

Syncronization

With beacon tracking, a device enables its radio periodically to receive beacon frame which consume energy. However, because the device knows the time for next beacon frame, it can keep sleeping until that time, which may conserve energy. On the contrary, without beacon tracking, a device does not spend energy on periodical beacon reception. However, since it does not know where the beacon appear, it has to enable its radio immediately to search a beacon when it need to communicate with the coordinator, which may incur high energy cost due to long idle period.

Performance Evaluation
The authors implemented prototype in the NS-2 network simulator with the CMU wireless extention. also analyze of tracking versus Non-Tracking modes.

Reference :
Performance Evaluation of the IEEE 802.15.4 MAC for Low-Rate Low-Power Wireless Networks
Gang Lu, Bhaskar Krisnamachari, Cauligi S.
Univ of Southern California

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

Waiting for you from formosa island, may Allah SWT always love you all
February 08 2008,
Taipei City
High Speed Network Lab

Udin Harun

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