1. Fast Join and Synchronization Schema in the IEEE 802.15.4e MAC
Speaker: Liang-Lin Yan
Advisor: Dr. Ho-Ting Wu
2015/10/29

2. Outline Introduction Overview on the IEEE 802.15.4e TSCH
Fast synchronization algorithms
Analytical models
Experimental evaluation
Conclusion
Reference

3. Introduction
The Internet of Things (IoT) represents, in the context of networking, one of the most relevant innovations of the third millennium .
In enabling IoT the low power and short range wireless communication technologies play a key role .

4. Introduction(cont.)
One of the leading standard in this field is the IEEE MAC, and the new IEEE e amendment extends its features making it more suitable for the industrial market.
It introduces the Time Slotted Channel Hopping (TSCH) mode to increase the reliability and the energy efficiency of short range wireless communications in noisy environments.

5. Introduction(cont.)
In a IEEE e network, all nodes share a common time slotted baseline, organized as a periodic sequence of slotframes, and they can wake up only in those timeslots according to the needs of higher layer protocols.
In this manner, the duty cycle could be minimized, extending the network lifetime due to a smaller energy consumption.

6. Introduction(cont.)
A critical aspect of TSCH, which is worth to be investigated, is related to the set up of the global synchronization at the network bootstrap.
Electing at least one node to broadcast the Absolute Slot Number (ASN). To this aim, specific Enhanced Beacon (EB) frames are used.

7. Introduction(cont.)
The adoption of TSCH for the transmission of EBs may lead to a longer time needed for the global synchronization of all the nodes, because it is necessary that the synchronizer and the new joining nodes are aligned on the same frequency.
TSCH may impairs the overall energy efficiency of the network because nodes are forced to remain awake for a long time till they are able to gain the synchronization.

8. IEEE e TSCH
The IEEE e TSCH MAC adds channel hopping to time slotted access. Channel hopping mitigates the effects of interference and multipath fading
Moreover, the use of several frequencies increases the network capacity, because more nodes can transmit their frames at the same time, using different channel offsets.

9. IEEE e TSCH(cont.)
In TSCH, as stated before, motes synchronize on a slotframe structure. Each mote follows a schedule which tells it what to do in each slot: transmit, receive, or sleep.
A TSCH PAN is formed when a device, usually the PAN coordinator, advertises network presence by sending EBs. The device wishing to join the network begins “passively” (using a preferred channel) or “actively” (scanning for the network).

10. IEEE e TSCH(cont.)
Once new node received a valid EB, it joins the network by sending a Join Request command frame to the advertising device.
When the new mote is accepted into the network, the advertiser activates it by setting up slotframes and links between it and other existing motes.
一旦想加入的節點收到一個有效的EB，他會傳一個加入的要求給發送者
當新節點加入網路後會重新設定slotframes及與網路內其他節點之間的link，當然這些slotframes及link都可以在之後做修改或刪除

11. IEEE e TSCH(cont.)
It is important to highlight that in TSCH, device-to-device synchronization is necessary to maintain connectivity with neighbors.
To this aim, Acknowledgement-Based and Frame- Based methods have been defined; they allow the receiver to calculate the difference between expected and actual (ACK or frame) arrival times and to tune its clock accordingly to stay synchronized with the sender.

12. IEEE e TSCH(cont.)
To maintain synchronization in a network with a very low duty- cycle, there is the need for keep-alive packets. An adaptive synchronization technique where the nodes measure the clock drifts of the neighbors and adjust the period of keep-alive packets accordingly.

13. Fast synchronization algorithms
For sake of clarity, in Fig. 1 it is shown an example
scheduling structure, which contains a multi-slotframe lasting
five slotframes. A node is allowed to send EBs only in the first
timeslot of each slotframe, i.e., the advertisement slot. Unless
otherwise specified, a node can schedule the transmission of
EBs in only one of the available advertisement slots of the
multi-slotframe. Possible collisions might arise when two or
more nodes schedule the transmission of their respective EBs
in the same timeslot-channel offset pair.

14. Fast synchronization algorithms (cont.)
Random Vertical filling (RV) -The coordinator transmits EBs in the first advertisement slot of the multi-superframe structure using chof = 0.
Any new synchronized node, instead, has to transmit in the same advertisement slot with a randomly chosen channel offset.
如圖1藍色部分

15. Fast synchronization algorithms (cont.)
Random Horizontal filling (RH) - The coordinator transmits EBs in the first advertisement slot of the multislotframe structure using a chof = 0
Other synchronized node will chose randomly one of the available advertisement slots of the multi -slotframe structure using again a chof = 0.
如圖1黃色

16. Analytical models
Based on the following assumptions (unless otherwise specified):
There are N nodes, already synchronized, in radio visibility that send EBs. They will be referred to as “synchronizer” nodes.
The EBs are sent with a frequency 1/TM where TM is the multi- slotframe duration.
The probability that each synchronizer node transmits at a certain frequency in a given timeslot is uniformly distributed and it is equal to 1/C, where C is the number of channels in use.

17. Analytical models(cont.)
The joining node is tuned on one of the available channels listening for an EB.
The nodes willing to join the network have initially a duty-cycle equal to 100 %, that is, their radios are always on, till they gain the synchronization.

18. Analytical models(cont.)
表這是一些符號的解釋

19. Analytical models-RV(cont.)
In the RV scheme, the EB transmission is allowed in only one timeslot every multi-slotframe and the corresponding channel is chosen randomly.
The joining node A acquires the synchronization need to satisfied two conditions:
the channel, fB, used by a node sending an EB is the same channel, fA,used by the node A to listen EBs (i.e., fB = fA);
no collisions nor errors happen.

20. Analytical models-RV(cont.)
Probability of a channel is selected by only one synchronizer:
P1表示某個channel只被一個節點選擇為傳送EB的機率
C:表示TSCH 排程中使用的channel數
平均有多少個channel，只被一個節點選擇來傳送EB
Mean number of channel where only one EB is transmitted:

21. Analytical models-RV(cont.)
Mean number of TM periods need for synchronization:
N =1, packet deliver ratio=1
假設在沒有錯誤的情況下
C: 為可用的channel數
MS:平均加入網路需要幾個multi-slotframe的時間
TM:為multi-slotframe的時間
因為同步者有C個可用的channel，每個channel選到的機率都是 uniform discrete的隨機變數，所以平均值為1+C/2個multi-slotframes(每個multi-slotframes只能傳送一個 EB)
當packet delivery ratio 小於1時的情況
When packet delivery ratio < 1:

22. Analytical models-RV(cont.)
When packet delivery ratio < 1 and N >1:
在考慮N>1時，只有一個EB傳送的channel數目會改變，會影響到MS
TS是平均同步時間 = nulti-slotframe 的長度乘上 個數
Average synchronization time:

23. Analytical models-RV(cont.)
The optimal number of node(N):
The optimal synchronization:

24. Analytical models-RH (cont.)
As for the RV model, there are N nodes (including the PAN coordinator) already synchronized that send periodically EB frames with a period equal to TM = Sf *Tf .
Each of the N nodes chooses randomly an advertisement slot in the first Sf slotframes (after it joins the network) and it uses always this advertisement slot for transmitting EBs.
According to TSCH operations, the physical channel used in the selected slot will be different at every consecutive sloframe.
SF: 兩連續EB之間的slotframe數
TF: slotframe的週期(幾個slot)
TM:multi-slotframe的長度(時間)
Multi-slotframe的週期會是 SF*TF
每個節點就會隨機選擇一個slotframe的第一個 slot作為ad slot
根據TSCH的機制因為ASN會變，所以一樣會有跳頻的效果

25. Analytical models-RH(cont.)
Probability of a slotframe is selected by only one synchronizer :
SF: 兩連續EB之間的slotframe數
UF:平均有幾個slotframe的AD slot 只有一個EB傳送
Mean number of ad. slot where only one EB is transmitted:

26. Analytical models-RH(cont.)
Average period for EB transmission:
SF: 兩連續EB之間的slotframe數
UF:平均有幾個slotframe的AD slot 只有一個EB傳送
TF: slotframe的長度
MT:平均EB傳送的週期

27. Analytical models-RH(cont.)
Average synchronization time:
平均同步時間 =平均EB傳送的週期 * multi-slotframe
MS:平均加入網路需要幾個multi-slotframe的時間
MT:平均EB的週期(multi-slotframe的時間)

28. Analytical models(cont.)

29. Experimental evaluation
Implemented in the OpenWSN stack.
Using TelosB motes.
Multi-superframe of 15 slotframes & each slotframe 101 timeslots.
Topology:
考慮了100個樣本 信心水準95%

30. Experimental evaluation (cont.)
TS: 平均同步時間 TM: multi-slotframe的時間 縱軸單位是multi-slotframe 橫軸傳送EB的節點個數

31. Experimental evaluation (cont.)
TS: 平均同步時間 TM: multi-slotframe的時間 縱軸單位是multi-slotframe 橫軸傳送EB的節點個數

32. Experimental evaluation (cont.)

33. Conclusion
They provide the closed form expression. The theoretical and experimental result are similarly the same. So we can use these expression to calculate the optimal situation.
The joining operations required by a node to become part of a TSCH network can be effectively boosted by increasing the node density or reduce the average inter arrival time between consecutive EBs.
They don’t consider collision scenario , when N > number of available channel.
由於他們 計算出來 的攻勢跟實驗結果相當接近，我們可以藉由他們的公式算出最理想的情況是由幾個節點來發送EB，且所需要的最佳同步時間是多少
可藉由增加節點的密度或降低連續兩個EB的inter arrival time，來增加網路建立的速度，但也是有個上限
他們 沒有模擬會發生碰撞的情形，當 N >可以使用的channel時

34. Reference
Vogli, E.; Ribezzo, G.; Grieco, L.A.; Boggia, G, “Fast join and synchronization schema in the IEEE e MAC,” IEEE Wireless Communications and Networking Conference Workshops (WCNCW), 9-12 March, 2015, pp.85-90