ResultsIntroduction

Increased interest in real-word applications in industrial automation, smart home, smart building and smart-cities will result in significant developments in wireless sensor networks (WSNs) and internet of things (IoT). To increase network lifetime, medium access control (MAC) and physical (PHY) layers need to be carefully designed. To that end, IEEE standards such as IEEE 802.15.4 and IEEE 802.15.4e, as well as wireless highway addressable remote transducer protocol (WirelessHART) consortium, have MAC and PHY layers targeting sensor networks that have limited computation and battery power.

Time synchronized channel hoping (TSCH), which is one of the MAC features in IEEE 802.15.4e standard. There have been no studies that investigate performance of coordinated sampled listening (CSL) and compare it to that of TSCH. CSL is also a MAC feature of IEEE 802.15.4e targeting low energy nodes. The nodes in WSN scenarios under investigation run the IEEE 802.15.4e TSCH/CSL protocols on the Contiki real time operating system.

Materials and methods

In TSCH time is divided into groups of timeslots called slotframe that repeat over time. All the nodes in the network are synchronized to the coordinator. When a node first joins the network, the coordinator/root node assigns a set of timeslots for this node.

Note that during dedicated timeslots, only the nodes assigned to communicate during these timeslots are active, thus, no interference from other nodes.TSCH protocol is a deterministic protocol since a node only wakes up at its own assigned timeslots.

In CSL, a transmitter node sends wakeup frames before a data packet is sent. The receiver wakes up periodically to check for wakeup frames.If the receiver is successful in receiving a data packet from the transmitter, it can send an acknowledgment frame that also contains the periodicity at which the receiver wakes up

Conclusions

Comparison of IEEE 802.15.4e MAC Features602410003 WuBin Wang, 602410128 YuTing Chang

EPS Labs, Texas Instruments, Inc., 12500 TI Blvd, TX 75243, USA

Figure 1. Photograph or drawing of organism, chemical structure, procedure, etc. Don’t use graphics from the web (they look terrible when printed).

We report simulation results obtained for different scenarios. Figure 3 shows a 5 node star topology where leaf nodes are connected directly to the root node. Cooja simulations has been found to be reliable simulations for investigating more complex network behaviors.

AcknowledgmentsThis research is being supported by the FP7-IAPP-SWAP EU contract number 251557, the EU FP7 Calipso Project (FP7-ICT-2011.1.3 PN: 288879) and the Future Internet-Public Private Partnership (Fi-PPP) European project Outsmart (PN 285038) under the 7th Framework Programme FP7-2011-ICT-FI. UAB research is supported by the Spanish Government under project TEC2011-28219 and the Catalan Government under grant 2009 SGR 298. Xavier Vilajosana is funded by the Spanish Ministry of Education under Fullbright-ME grant (INF-2010-0319).

Simulation setup

Figure 1. Time slotted channel hopping illustration.

Figure 2. Coordinated sampled listening operation.

We show simulation results with the current consumption for MSP430 and CC2520 as given in Table I . Note that the software simulation assumes 100% transmission and reception success ratio if there are no collisions.The networks investigated consist of three types of nodes, the root/coordinator node(PAN coordinator), intermediatenode (routing/switching coordinator) and leaf node (end network device).

The root node is responsible for network creation and maintenance. It achieves this via broadcasted beacon frames that convey information about the network parameters; e.g., timing, channel hopping sequence, link information, etc.

The intermediate node is responsible for forwarding traffic received from other neighbor nodes. It can also transmit its own frames that could carry sensor information. In addition, the intermediate node can also transmit beacon frames in beacon slots assigned by the root node. These beacon frames contain exactly the same information as the beacon frames transmitted by the root node. The leaf node does not have any routing/forwarding responsibilities. Rather, its main job is to collect sensor data and send it to the root node.

Figure 3. Star topology.

Figure 4. Average power and latency in star topology.Figure 4 shows the overall power and latency for TSCH and

CSL. In terms of power, CSL consumes 6 times more power than TSCH, while in terms of latency, TSCH has greater than 5 times worse latency. The reason for higher latency in TSCH can be explained with the fact that TSCH uses dedicated slots and if a packet is not transmitted in a slotframe, then it has to wait one slotframe before it is transmitted. The difference in power consumption between TSCH and CSL can be attributed to their operation. In TSCH a node is awake for beacon receptions and transmission/reception of data during dedicated timeslots. At other times, the node is asleep. In CSL, on the other hand, if a node is a transmitter, then a train of wakeup frames is transmitted before the data frame is transmitted. If the node is a receiver, it wakes up periodically (at a rate higher than slotframe size in TSCH) to sense the medium. Hence, in CSL a node is less power efficient during transmission or reception of data frames. Thus, the 6x difference between the power consumptions.

Figure 5. Four Hops Linear Topology with Several Leaf Nodes, 4 leaf nodes are used.

Figure 6. Two Hops Linear Topology with Several Leaf Nodes, 8 leaf nodes are used

While the scenarios considered here are for a limited number of nodes, given the fundamental design difference between TSCH and CSL, we believe that for larger number of nodes, CSL will perform worse. This is not just due to the fact that there is overhead with transmission of data frames, but also due to the fact that transmissions are uncoordinated and can lead to a lot of collisions. Multi-channel operation can be enabled with CSL to improve its performance and reduce collisions; however, we believe that even in that scenario, the lack of centralized controller that sees all the links in the network will still lead to interference between nodes.

We believe that TSCH is good for applications with low packet error rate requirement and CSL is good for realtime applications with low latency requirement.

We compared via simulations power and latency performance of TSCH with that of CSL in multiple practical deployment settings. To that end, we showed that TSCH achieves better power efficiency than CSL, while having higher latency.

Figure 7 shows the overall performance of the network depicted in Figure 5. As one would expect, the power consumption and latency impact is higher for TSCH, even though, it does not change the trends with earlier observations when compared to CSL performance.

Figure 8 shows the overall performance of the network depicted in Figure 6. As one would expect, the power consumption and latency impact is higher for TSCH, even though, it does not change the trends with earlier observations when compared to CSL performance. Compared to the performance of the previous scenario, the average power consumption is lower since there are more leaf nodes and the latency is lower because there are less hops.

Figure 7. Overall average power and latency.

Figure 8. Overall average power and latency.

AcknowledgmentsThis research is being supported by the FP7-IAPP-SWAP EU contract number 251557, the EU FP7 Calipso Project (FP7-ICT-2011.1.3 PN: 288879) and the Future Internet-Public Private Partnership (Fi-PPP) European project Outsmart (PN 285038) under the 7th Framework Programme FP7-2011-ICT-FI. UAB research is supported by the Spanish Government under project TEC2011-28219 and the Catalan Government under grant 2009 SGR 298. Xavier Vilajosana is funded by the Spanish Ministry of Education under Fullbright-ME grant (INF-2010-0319).

For further informationPlease contact {j-zhou2, axhafa, r-vedantham, ryan.nuzzaci, arvindkr, xlu}@ti.com. More information on this and related projects can be obtained at http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6803159.

Literature citedJ. Yick et. al, ”Wireless Sensor Network Survey”, Computer Networks,Elsevier, 2008, pp.

2292-2330.M. R. Palattella, et al., ”Standardized Protocol Stack for the Internet of (Important) Things”, IEEE Communications Surveys and Tutorials, vol.15, no. 3, 2013, pp. 1389-1406.F. Chen, R. German, and F. Dressler, ”Towards IEEE 802.15.4e: A Study of Performance

Aspects”, IEEE , 2010, pp. 68-73,W-C. Jeong and J. Lee, ”Performance Evaluation of IEEE 802.154e DSME MAC Protocol for Wireless Sensor Networks”, IEEE 2012 The First Workshop on Enabling Technologies for Smartphone and Internet of Things (ETSIoT), pp. 7-12.

J. Lee and W-C. Jeong, ”Performance Evaluation of IEEE 802.154e DSME MAC Protocol under WLAN Interference”, IEEE ICTC 2012, pp. 741-746.

B. X. Yen, D. T. Hop, and M. Yoo, ”Redundant transmission in wireless networked control system over IEEE 802.15.4e”, IEEE ICOIN 2013, pp-628-631.

A. E. Xhafa, et. al, ”Towards a perpetual wireless sensor node”, IEEE Sensors 2013, accepted.IEEE Standard for Information technology - Telecommunications and information

exchanges between systems - Local and Metropolitan Area Networks - Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs) , IEEE, 2006.