1. 交通大學 High-Speed Communication & Computing Laboratory 指導教授:曾煜棋教授
Quick Convergecast in ZigBee/IEEE Tree-Based Wireless Sensor Networks
交通大學
High-Speed Communication & Computing Laboratory
指導教授:曾煜棋教授

2. Convergecast in Zigbee/IEEE802.15.4
Zigbee/IEEE tree-based Wireless Sensor Network contains:
One sink (Zigbee Coordinator)
Some Full function devices (Zigbee Routers)
Some reduced function devices (Zigbee end device)
Each Zigbee router is responsible for collecting sensed data from end devices associated with it and relaying incoming data to the sink.
1.圖: A, B, C:FFD(Zigbee Routers)
2.每個Zigbee Routers下面有一些RFD(Zigbee end device)
3.其中只有Zigbee Routers和是有relay的功能

3. Overview of IEEE802.15.4 and Zigbee Standard
A Zigbee router can announce a beacon to start a superframe
Each superframe consists of an active portion followed by an inactive portion.
Each active portion consists of 16 equal-length slots and can further partitioned into a contention access period (CAP) and a contention free period (CFP)
CAP: Slotted CSMA/CA is used in CAP
CFP: FFDs which require fixed transmission rates can ask for guarantee time slots (GTSs)
1.在Zigbee network中,只有Zigbee router 可以發beacon(在sueperftame的一開始時)
2.CAP:採contention方式來搶slot
CFP:FFD可以向他的coordinator要求GTS,且CPF中可包含multiple GTS, and each GTS may contain multiple slots.

4. An example of Convergecast in a ZigBee/IEEE 802. 15
An example of Convergecast in a ZigBee/IEEE Tree-Based network
On receiving parent router’s beacon, and end device has to wake up for an active portion to sense the environment and communication with its coordinator.
To avoid collision with its neighbor, a router should shift its active portion by a certain amount.
圖例: Convergecast :所有點要將data report到sink,每個點在netwrok上的communication必須follow:
1.Each device (end device or Zigbee router) 必須起來聽他parent的beacon, 且在其parent 的active portion作communication
2.Each Zigbee router 必須和neighbor 錯開active portion以避免碰撞

5. Motivation
Assuming that routers relay packets in contention-free guarantee time slots (GTSs).
The collected sensory data of A in the k-th superframe can be sent to C via the GTS of C in the k-th superframe
The superframe of B in the k-th superframe appears after that of C, the collected data of B in the k-th superframe can only be relayed to C along the GTS of C in the (k+1)-th superframe.
The delay can be eliminated if the active portion of B in the k-th superframe appears before that of C
假設routers relay packets in contention-free guarantee time slots (GTSs):
1.Nodes A在第k個superframe可以將data report給他的parent node C
2.但是node B在第k個superframe的active portion比他的parent node C晚，所以 node B 只能在第k+1個superframe才能將data丟給其parent node C，造成data report時delay變大
由此觀察：若是我們將每個node的active portion 排好schedule，就可以達到降低reporting delay的效果

6. Motivation and problem
The delay in not negligible because of the low duty cycle design of IEEE
Our goal is to solve a minimum delay beacon scheduling problem for quick convergecast in ZigBee/IEEE tree-based wireless sensor networks.

7. Problem definition Minimum delay Beacon scheduling problem:
Given G=(V,E) and k available slots
Find the interference-free slot assignment s(i),
such that the convergecast latency L(G) is minimized.
where L(G) is the maximum of all nodes’s report latencies.