Design and Implementation of a Multi-hop Zigbee Network Chi-Wen Deng, Li-chun Ko, Yung-chih Liu, Hua-wei Fang Networks and Multimedia Institute Institute for Information Industry, ROC {cwdeng, lcko, ulysses, wallace}@nmi.iii.org.tw Abstract A Multi-hop based sensor network is important for time synchronization and power consumption. The sensor node sends beacons to synchronize accurate time sampling for fine-tuned of duty cycle and could be into the sleep mode periodically for power-saving. Therefore scheduling beacon transmissions is required to prevent the beacon frames of one device from colliding with either the beacon frames or data transmissions of its neighboring devices. In this paper, we develop a multi-hop sensor network based on our implemented beacon-enable network platform. To implement a multi-hop sensor network, there are many time slots must be scheduled for time of each sensor node. Information of relative time slots and absolute time slots could be utilized for scheduling beacon transmission. It is easier to implement a multi-hop network using this approach. Furthermore, Preamble is required to implement time sensitive system due to clock skew and waiting time in the real world. Finally, the experimental result will show performance evaluation in our implementation under different parameters. 1. Introduction IEEE 802.15.4[1] is a standard that describes Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications for Low-Rate Wireless personal Area Networks (LR-WPANs). Under the specification, we implement our beacon-enable sensor network platform in [2]. Based on that, power-saving is necessary for beacon-enable sensor network. To make sensor nodes periodically into the sleep mode and time synchronization, we decide to develop a multi-hop sensor network. Zigbee Specification version 1.1[3] mentions some information how to establish a multi-hop sensor network. But it is not enough to setup a multi-hop sensor network because of information limited. Therefore, we focus on the design and implementation part as well as performance evaluation in a real case. There are lots of studies that discuss some problems in the multi-hop network in detail. The main topics are transmission latency and power consumption. A latency penalty in a multi-hop network is concerned when packets must wait for the next scheduled time for transmission. To reduce latency in multi-hop paths, Many algorithms provide fast data forwarding paths by adding additional wake-up periods on the nodes along paths from source to sinks [4]. An adaptive schedule-based MAC can also increase data transmission rate in a multi-hop network. But it is always more complicated than fixed beacon scheduling network. Furthermore, wireless sensor networks use many small, wireless sensors to sense their environment. Wireless sensors are often battery operated to simplify deployment. It is difficult or impossible to change batteries because many sensor nodes are always placed in their target environment. Thus, sensor nodes must be energy efficient. Due to the property of multi-hop network, a sensor node could periodically be set the sleeping mode in an inactive period. However, nodes closer to the root under the tree topology will perform energy-intensive tasks such as data-aggregation and data forwarding more than others. In [5], the algorithm shows how to maximize lifetime of overall sensor network. This paper is organized as follows. In section II, we will briefly introduce the zigbee specification about multi-hop sensor network part. In section III, we describe how to design our system and platform based on beacon-enable system. There are two kinds of data transmission modes and specific slot information in our system. The Transmitting data flow is also described in this session. In section IV, experimental result in our implemented platform shows that preamble time affects the multi-hop sensor network in a real case. Finally, the conclusion of our implementation result and what we can improve in the future in the last section. 2. Multi-hop networks in Zigbee specification In this session, the information in zigbee specification [3] about multi-hop sensor network will be discussed. We will show that how to implement the multi-hop sensor network based on zigbee specification. 2.1 Scheduling beacon transmissions Figure 1 shows the superframe structure for a beaconing device. Time is divided into beacon interval and superframe duration, and the active portion of the superframe of every device in the network shall avoid the active portion of the others. Oppositely, the inactive portion of the superframe of every device could sleep for power saving.
Because one purpose of multi-hop network is to allow nodes periodically to sleep in order to conserve power, the Beacon order shall be set larger than the superframe order. The inactive portion of the superframe could be divided into lots of non-overlapping time slots. By the way, the neighbor node can schedule its active portion of the superframe to one of these non-overlapping time slots. An example of scheduling beacon transmission and parent-child superframe positioning relationship shows in figure 2. Beacon Interval Superframe Duration Beacon Inactive Period Figure 1. The superframe structure for a beaconing device For any node except coordinator, the transmitting time shall be relative to the beacon transmitting time of the parent. To avoid the beacon and data transmission collision, a node has to record the transmitting time about its parent, neighbor and the parent of the neighbor. The purpose of having a device know when the parent of its neighbor is active is to maintain the integrity of the parent-child communication link by alleviating the hidden node problems. In other words, a device will never transmit at the same time as the parent of its neighbor. R4 R2 R1 R5 R3 R6 R7 Figure 2. An example of scheduling beacon transmission and parent-child superframe positioning relationship 2.1 TxOffset information During the passive scan procedure, the new device shall build its neighbor table after collecting the information from the beacon notification indication. The beacon frame of a device shall include the transmitting time relative to the beacon transmitting time of the parent devices. Therefore, a device that receives the beacon frame shall store both the local timestamp of the beacon frame and the offset included in the beacon payload in its neighbor table. According to the record, the device can evaluate the appropriate non-overlapping time slot. Figure 3 shows that format of the MAC sub-layer beacon payload. The value TxOffset indicates the difference in time, measured in symbols, between the beacon transmission time of the device and the beacon transmission time of its parent. Bits:0-7 Protocol ID 8-11 Stack profile 12-15 16-17 nwkcproto Reserved col- Version 18 Router capacity 19-22 Device depth 23 End device capacity 24-47 Tx Offset Figure 3. Format of the MAC sub-layer beacon payload 3. Multi-hop network design In this section, we ll discuss how to design our multi-hop network system based on our beacon-enabled network device [2]. The beacon-enabled system comprises a robust RTOS called MOS (Mini Operating System) and beacon Tx and Rx subsystem. Based on that, there are several important things that we must consider in the process of designing a Multi-hop network. After constructing a multi-hop network, we first talk about two data transmission models used in our sensor network. And then we utilize the information absolute slots and relative slots to reduce complicated Txoffset calculation. Finally, the difference between general beacon-enable network and multi-hop network will be discussed in Data Transmission mechanism. 3.1 Data Transmission models Data transmission in a tree multi-hop network shall be accomplished using the parent-child links to route along the tree. Every child must track its parent beacon. During the of the parent, data transmission from a parent to its child must be completed by indirect transmission as well as transmission from child to its parent. We have two available data transmission models under the multi-hop network: source-driven and query driven [6]. 3.1.1 source-driven Figure 4(a) illustrates the source-driven data transmission model. A leaf node periodically transmits data to its parent using the parent-child links. The leaf s parent node relays the data to its parent until the data is received by the coordinator. The data from a leaf node may be transmitted periodically or trigger by some events. 3.1.2 query-driven Figure 4(b) illustrates the query-driven data transmission model. A coordinator may broadcast the query command to the specific node. The query command will be relayed using parent-child links in the tree network. All children of the node will be received the query command until all node of the tree networks is received. The specific node received the query command will send the response to its parent immediately. Finally, the response will be transfer to the coordinator to perform that the query process completed.
R1!! Where? Data? Start Initial variable no CSMA-CA Calculate transmitting time parent calculate RX beacon time Determine destination broadcast Calculate RX and TX beacon time children calculate TX beacon time or next R1 Figure 4. (a)source-driven data transmission. (b)query-driven data transmission, coordinator broadcasts the query request for R1 and then R1 send the specific data back immediately. CSMA-CA RX Random Delay PLME-CCA.request Pending for confirm retry 3.2 Absolute slot and relative slot In a static scheduling beacon method, all nodes in a multi-hop sensor network shall have the same BO and SO, unless the multi-hop network uses an adaptive scheduling beacon method to power saving or transmission latency reduction. Therefore, we could evaluate how many non-overlaying beacon time slots could be utilized under the condition of known SO and BO. The non-overlapping time slots could be classified into relative slots and absolute slots. Relative time slots mean that any node uses the time slot relative to its parent beacon transmitting time slot. Absolute slot mean that any node uses the time slot relative to the PAN coordinator beacon transmitting time slot. Obviously, any node could select one suitable absolute time slot that avoids the absolute time slot of its neighbors and the parent of its neighbors. Following this rule, a node can easily attend beacon scheduling. In our implementation system, we utilize the information of absolute slots and relative slots instead of beacon transmission time offset. First, we append the field of absolute time slots information and relative time slot information in the beacon payload instead of TxOffset. Next, any node received beacon notification indication will record the information about absolute time slots and relative time slots to its neighbor tables. According to relative and absolute time slots, the node that wants to join a multi-hop network could select a suitable absolute time slot. This method makes the microprocessor reduce complicated computation and the error due to resolution. 3.3 Tx and Rx mechanism Time to Rx and Tx is accurate for beacon-enable network. This characteristic causes the multi-hop sensor network must be considered that timing to transmit and receive packet. 3.3.1 Rx mechanism Reference to [3], Rx mechanism of multi-hop system is almost the same as Rx mechanism of beacon-enable system. The data transmission must be completed in time of its parent or its time. CHANNEL_ACCESS_FAIL Data Transmission TX RX NO_ACK failed failed TX ON PD-DATA.request Pending for confirm RX ON Pending for ACK SUCCESS SUCCESS Figure 6. The transmission mechanism of a device in the multi-hop network 3.3.2 Tx mechanism In a multi-hop network, any device must follow the rule of scheduling beacon before joining the network. Except the coordinator and the enddevice, there are two active time for any device in the multi-hop network. One active period is used to communicate with the parent and another is used for the children. Figure 6 shows the flowchart of Tx mechanism of the multi-hop network. The time to transmit data using CSMA-CA algorithm has to be evaluated under several conditions. First, if the system wants to transmit data to parents, the time to transmit must be within the time of its parent according to Rx beacon time. Second, if the system wants to transmit data not to the parent, the time to transmit must be within its time according to Tx beacon time. Finally, the condition for broadcast is also considered. The system must evaluate the moment is closed to Rx beacon time or Tx beacon time. If the moment in the Rx beacon time or Tx beacon time, the system must calculate the remaining time is enough to transmit the packet. If it isn t, transmission is started until next time. Thus, Recording Rx time, Tx time and self-status are necessary to implement data transmission in a multi-hop network. 4. Performance evaluation We consider the influence of inaccuracy due to hardware limited and evaluate the performance under different parameters in a real case. 4.1 Implementation platform Chipcon CC2420 DBK Chipcon [7] is used as our implementation platform. It has an 8Mhz Atmel Atmega128 micro controller with embedded 4k SRAM
and 128K flash memory. The transceiver is Chipcon s CC2420 is a true single-chip 2.4GHz IEEE 802.15.4 compliant RF transceiver designed for low-power and low-voltage wireless application. 4.2 Performance Evaluation We consider the delay of system operation in transmission progress. The delay in packet transmission could be summarized as: a. RTOS check and run a task in the schedule queue. b. CSMA/CA backoff time (It is a random value and then we select an average value). c. Turn on RX and CCA. d. aturnaroundtime in TX procedure. e. Preamble, SFD and LEN header in the PHY layer. f. The header and CRC check in the PHY, MAC, NWK and APS layer. g. Packet payload h. Time to wait for ACK. i. IFS Time. j. Check returned ACK. In our implementation, we reserve 54 symbols to wait for ACK and 40 symbols long inter-frame spacing (LIFS) after transmitting each packet. By the way, the system delay per packet could be evaluated. The packet size (bytes) and transmission rate has the following relationship under different BE (the back-off exponent, which is related to how many back-off periods a device shall wait before attempting to assess a channel): Figure 7. The packet transmission rate is evaluated under the different packet size. Because of the delay and the backoff periods that wait before attempting to assess a channel, transmission rate increases depending on the packet size. And the transmission rate per packet in a read case is more less then 250 kbps in an ideal case. According to 802.15.4 specification, SD (super frame duration) and BI (beacon interval) are controlled by two parameters: SO (superframe order) and BO (beacon order). From function (1) and (2) where abasesuperframeduration is constant 960 in symbols. From function (1) and the overall evaluated delay, the packet number in superframe duration could be evaluated in function (3). SD = abasesuperframeduration*2 SO (1) BI = abasesuperframeduration*2 BO (2) SO N = 960* 2 max BeaconSize * 2 94 BE P *2 + 178 + 20* rand (0,2 ) With different packet size, we plot the maximum number of transmitting packets N (under BE=3) is related to packet size P bytes (includes MAC header) in figure 8. The experimental result can show that when SO is less than 1, we could transmit a few packets. Figure 8. The maximum number of transmitting packets N when SO between 0 and 6 and packet size P=30/60 /90 under BE=3. 4.3 The influence of preamble on multi-hop sensor network When every micro processor works, the oscillator of each device will cause the time computation error. It is hard to avoid a small amount of difference in frequency. It is always a big trouble of implementing beacon-enable network because of time synchronize is sensitive. To synchronize all devices in the multi-hop network, we always early turn on the receiver to track its parent beacon and turn on the transmitter to send its beacon because of clock skew. This redundant waiting time and reserved time is called preamble. If a device is late to receive the beacon of its parent because of clock skew, it will cause sync loss. Relatively, the device must have some preparing time in order to transmitting on time. Both of that redundant time causes all devices to affect its active and inactive time. Table 1. Preamble setting under different BO in tracking beacon progress BO 8 9 10 11 12 13 14 Preamble 1 2 3 6 12 24 48 Length of the preambles because of tracking beacon packet depends on beacon order. In our platform, the clock error rate is within 30PPM. We choice 320 us (which equals the transmission time of 20 symbols of IEEE 802.15.4 at 2.4GHz) as our basic preamble unit, which is the same as basic random back-off unit of the original CSMA/CA algorithm. Table 1 concludes the different values of preamble setting under different BO in our implementation. (3)
However, there are the other preambles in a multi-hop system except for receiving beacon. To transmit beacon in time, the device must turn on the transmitter early. This preamble cost 4 back-offs and the time to encode and decode beacon packet cost 28 back-offs. The reserved time is mainly used to transmit beacon packet in time and check the received beacon. According to all the preambles in the multi-hop system, we can compute the ratio of preambles to the active period under different SO from the function (1). In figure 9, the experimental result can show that the preamble causes almost no impact on it as long as SO is larger than or equal to 5. (The affection is less than 2%). Data transmission latency is another important thing in a multi-hop system. Under the condition that BO is equal to 14 and the number of hop is equal to 5, the transmission latency is about 20 minutes in the worst path. Thus, we will focus on how to implement an adaptive beacon scheduling method in the multi-hop system in the future. 6. References [1] IEEE 802.15 WPAN Task Group 4, IEEE 802.15.4-2003 Standard, IEEE, 2003. [2] Li-chun Ko, Yung-chih Liu, Hua-wei Fang, Desgin and Implementation of IEEE 802.15.4 Beacon-enabled Network Devices, In Fourth Annual IEEE International Conference on Pervasive Computing and Communications Workshops (PERCOMW'06), 2006. [3] Zigbee Standard Organization, Zigbee Specification Version 1.0, Zigbee Alliance, June 2005. Figure 9. The ratio of preambles to active period is evaluated under the different SO. 5. Conclusions and future work In this paper, an implemented multi-hop system is introduced. The design principle is to track and transmit beacons accurately. And then an approach is shown that how to schedule beacon time slot by using specific time slot information. It is easier to implement a multi-hop system by using this approach. Furthermore, our system supports two kinds of data transmission models and the transmission mechanism of a multi-hop enabled device is shown. Based on beacon-enable system the multi-hop system is more time sensitive to track beacon and transmit beacon in time. In a real world, the delay due to hardware limited is must be considered. We show that the transmission rate in our implemented platform. And then the final experimental result shows that the influence of the preambles on the multi-hop system. It causes a great impact on active period of a multi-hop enabled device under some SO parameters. Further work will include a more detailed analysis of preambles due to sleep in an inactive period and the time to wake the device up. The device that sleep and wake up periodically will cause inaccurate time to transmit beacon packet and it makes the next hop must reserved some redundant time to receive beacon. However, the preamble is variable depending on which beacon time slot a device of the parent selects. In our system, the child could easily know the absolute time slot that the parent select from the beacon payload and then adjust the preamble. [4] Yuan Li, Wei Ye, John Heidemann, Energy and Latency Control in Low Duty Cycle MAC Protocols In USC/ISI Technical Report ISI-TR-595. August 2004 [5] Diba Mirza, Maryam Owrang, Curt Schurgers, "Energy-efficient Wakeup Scheduling for Maximizing Lifetime of IEEE 802.15.4 Networks," International Conference on Wireless Internet (WICON 05), Budapest, Hungary, pp. 130-137, July 2005. [6] Ossama Younis and Sonia Fahmy, "A Scalable Framework for Distributed Time Synchronization in Multi-hop Sensor Networks," In Proceedings of the Second IEEE International Conference on Sensor and Ad Hoc Communications and Networks (IEEE SECON 05), Santa Clara, September 2005. [7] Data sheet for CC2420 DBK is available at http://ww w.chipcon.com/