ISPCS 2008 – International IEEE Symposium on Precision Clock Synchronization for Measurement, Control and Communication Ann Arbor, Michigan, September 22–26, 2008
Non invasive Time Synchronization for ZigBee Wireless Sensor Networks P. Ferrari1, A. Flammini1, D. Marioli1, E. Sisinni1, A. Taroni2 1
University of Brescia – DEA – Brescia – Via Branze,38 – 25123 – Italy Carlo Cattaneo University, Castellanza (VA) – C.so Matteotti,22 – 21053 – Italy Email:
[email protected], Tel.+390303715445 Fax. +39030380014
2
Abstract –A precise time synchronization is essential in many Wireless Sensor Networking applications, e.g. in order to correlate observations of the same event from different sites or to efficiently share the wireless channel. Generally, wireless sensors are designed having in mind low cost and low power. In the past, several protocols have been suggested, each one with its pros and cons, aiming at the maximization of the accuracy vs. consumption ratio. However, their performance have been generally tested on particular hardware setup and with proprietary communication protocols. In this paper we present a non invasive implementation of a synchronization protocol over a full ZigBee-compliant platform. Experimental evaluation on real prototypes has been performed in order to ensure feasibility and verify performances. Our approach preserves compatibility with this emerging standard still offering synchronization error on the order of few μs if synchronization procedure is executed every 4 s. Keywords – Wireless Sensor Neworking, Time synchronization, Distributed system
I.
INTRODUCTION
ZigBee (ZB) [1] is an emerging standard for Wireless Sensor Networks (WSNs). It targets low distance, low data rate, low power consumption and low cost applications; according to standard nomenclature, it implements a Low Rate–Wireless Personal Area Network (LR-WPAN). However, there is a lot of confusion around what the term ZB really describes and its name has been often misused (for instance in [2] the term is misused, since it only refers to an IEEE802.15.4-compliant hardware platform). ZB defines upper layers (network and application) of the ISO protocol reference model. On the contrary, in regards to the physical and data link ones, it relies over another standard, the well accepted IEEE802.15.4 [3], which offers a gross transfer rate of 250 kbps in the 2.4 GHz ISM unlicensed band. Although ZB is designed for event-based applications, it can take advantage from sensors clock synchronization, in order to give an unique time reference to all measurements and events. However, synchronization procedure should be simple, in order to not waste power, and it should preserve compatibility with the available ZB stacks, including the socalled ZB-processors [4]. In the past several protocols have been proposed and used for time synchronization in wired and wireless networks. Mill’s Network Time Protocol (NTP) [5] has been widely used in the Internet for decades. However, traditional synchronization schemes are not suitable for use in WSNs
due to the specific requirements of those networks in terms of precision (that must be high) and cost (nodes have typically limited power, computational and storage resources). In fact, most of the protocols designed for wired environments exchange several huge messages and also need to store them (for statistical processing). For this reason, solutions like the IEEE1588v2 or even the IEEE802.1AS [6], that has been also proposed for wireless audio video broadcasting, must be discarded. On the other hand, their basic principle (i.e. pairwise message exchanges within a hierarchical master/slave structure) and the concept of transparent clock can be borrowed and usefully adopted in multi hop WSNs. Recently, a significant amount of research on time synchronization for wireless sensor has been published [7,8]. Taking advantage from the well-known TPSN (Timing-sync Protocol for Sensor Networks) protocol [9], we propose a non invasive modification that preserves all the features of standard ZB stack and allows to reach time synchronization on the order of few μs without decreasing power efficiency. The rest of this paper is structured as follows. In Section II we present an overview of ZB architecture and the time synchronization from the ZB point of view. After describing our implementation in Section III, we present some experimental results in Section IV. Finally, in section V some concluding remarks are reported. II. ZIGBEE AND TIME SYNCHRONIZATION Key feature of ZB is the capability of handling both single (star topology) and multi hop (cluster tree and mesh topologies) networks. The cluster-tree is a hierarchical topology, probably the most interesting solution of this standard. In this case the network consists of clusters, each having a “router” as a cluster head and multiple devices as leaf nodes. A “PAN coordinator” initiates the network and serves as the root. The network is formed by parent-child relationships, where new nodes associate as children with the existing coordinators. This well-defined structure simplifies multi-hop routing and allows energy saving; each node maintains synchronization of data exchanges with its parent coordinator only. As a matter of fact, two different kinds of devices can participate in ZB-WSNs: Full Function Devices (FFDs) and Reduced Function Devices (RFDs). They differentiate on the basis of capabilities and, thus, on the resources needed to operate. An RFD does not have routing
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capabilities and can act as end node only. It can only communicate with its parent, i.e. the node that allowed it to join the network. On the contrary, an FFD has routing capabilities and can be configured as the WSN coordinator. According to this distinction, ZB implements three types of logical devices: coordinator (must be an FFD), router (must be an FFD) and end device (may be an FFD or an RFD). In order to reduce latency and packet losses, the current ZB standard requires FFDs to be always on, which in practice means that if they are battery-powered their lifetime is on the order of a few days. On the contrary, RFD are optimized for low power consumption and are allowed to enter in the deep sleep modality offered by the hardware, as better explained in the superframe description that follows. It is evident that mesh topology allows only FFDs and it is not a power efficient solution As we previously said, ZB has been optimized for single sporadic event notification, as suggested by the adoption of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) as medium access protocol. In regard to time synchronization, it is based on the exchange of particular packets, called Beacon, that FFDs may emit on a regular basis. However, only star and tree networks may use beacon-oriented communications, but many commercially available stacks do not fully support this feature. In addition, Beacons do not use CSMA/CA and a lot of burden must be done in order to avoid collisions between them or with data frames. As previously stated, each cluster is managed by one coordinator (i.e. Zigbee Router - FFD), which generates periodic Beacon frames to synchronize its child nodes (belonging to its cluster). Since many nodes of different clusters may occupy the same operating space, their transmissions must be accurately scheduled to not overlap. Obviously, there is a tradeoff with respect to the network scalability, since this approach obliges each node to collect information about all its neighbors and their parents. This in turn means that each node must compile such a database, greatly affecting memory resource requirements. The ZB standard suggests to divide the time into different non overlapping timeslots each one corresponding to the superframe active portion of that node (see Fig.1). WPAN Coordinator (FFD)
Superframe (Level k)
Active portion of the superframe
Cluster Head (FFD) Level k End Device (RFD) Level k+1
Coordinator Beacon
Superframe (Level k+1)
Cluster Head Beacon
Fig. 1. Beacon scheduling under ZB.
In fact, the activity of a beacon-enabled node is organized into regular repetitive patterns (superframes) that initiate with the Beacon transmission. The time between two successive Beacons is divided into an active and a sleep portion of variable lengths. In the latter, nodes are not allowed to
communicate. A node that wants to join the network must choose the time collocation of the active portion of its superframe structure with any suitable algorithm able to avoid overlapping. Nevertheless, the standard do not specifically address this aspect. If there are no free nonoverlapping time slots, the device shall not transmit Beacons and shall operate on the network as an end device (i.e. it cannot route packets). III. THE PROPOSED APPROACH Authors’ basic idea is to completely avoid the use of Beacons still ensuring that all nodes share the same sense of time in order to improve power efficiency; also the traditional “listen-before-talk” strategy of the CSMA/CA protocol should be preserved. In particular, we want to permit node synchronization without using a complex and rigid structure as the superframe time organization. This approach lowers the node protocol complexity since no particular scheduling algorithm must be employed and it is not needed to acquire information about neighbors. It must be also remembered that beacons are handled at the MAC level of the protocol and we do not want to operate at intermediate levels of the stack to be implementation independent. As better explained in the following, we only access the application layer of the protocol stack (no matter who made it) and exploit a minimal software integration to furnish low level timestamping, when this feature in not already provided. A. The TPSN protocol What we suggest is a modified implementation of the well-known TPSN [9] protocol in order to be fully compatible with ZB without increasing consumption. TPSN is a traditional sender-receiver based synchronization protocol that uses a tree to organize the clock hierarchy. In the original proposal, there is a level discovery phase and an actual synchronization phase. The first step of the algorithm is to create a hierarchical topology in the network. Every node is assigned a level in this hierarchical structure. The basic hypothesis underlying this phase is that a node belonging to level k+1 can communicate with at least one node belonging to level k. Only one node is assigned to level “0”, which it is called the “root node”. The level discovery phase is initiated by the root sending a level discovery packet which contains the identity and the level of the sender. The immediate neighbors (i.e. MAC neighbors) of the root node receive this packet and assign themselves a level “1”. After establishing their own level, they broadcast a new level discovery packet. This process is continued and eventually every node in the network is assigned a level. After a node obtains his level, it discards any such future packets. This makes sure that no flooding congestion takes place in this phase. Once the hierarchical structure has been established, the root node initiates the second stage of the algorithm, the so
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Level k+2
ZDO
APS
T4,k+1
Synchronization “retrofitting”
T1,k+2
NWK IEEE802.15.4
T2,k T3,k
Acknowledgement
Cluster Head (FFD) Level k+1
Synchro End Point
Level k
T1,k+1
Synchronization Pulse
Cluster Head (FFD) Level k
Level k+1
Security
WPAN Coordinator (FFD)
With regard to peer-to-peer (e.g. router toward router) data transfer, the devices wishing to communicate must be receiving constantly or must be synchronized (for instance using Beacon) and can use direct transmission. The timestamping feature is one of the most critical aspect in synchronization methods. If we work at the highest level, as an application tool, we preserve a full compatibility with the selected stack, but the results are poor in terms of jitter. Otherwise, if we work at the lower levels in order to achieve the best performance, we often are too invasive. In our implementation we worked at the lowest and highest levels of the stack, thus being completely non invasive with respect to the stack (see Fig.4).
MAC
T2,k+1
End Device (RFD) Level k+2
ZIGBEE
called synchronization phase. In this phase, a node belonging to level k+1 synchronizes to a node belonging to level k. Eventually every node is synchronized to the root node and we achieve network-wide time synchronization. The main improvement suggested by the original TPSN protocol is the adoption of a MAC level timestamping, that allows to precisely mark the begin of a transmission and its reception. In our implementation the first phase is superfluous, since the root is always the coordinator and hierarchical clock structure coincides with the cluster tree-topology [10]. Synchronization among nodes of level k (closer to the root) and level k+1 is achieved by means of a pair wise packets exchange, called “Synchronization Pulse” and “Acknowledgement” packet, respectively, as shown in Fig.2.
T3,k+1
PHY
T4,k+2
Timestamping Time
Fig. 2. Two-way message exchanges between nodes of different hierarchical levels.
B. Implementation within the ZigBee protocol Referring to ZB, in non Beacon-enabled networks, communications are always initiated by the end device (see Fig.3). When an RFD wakes up from its deep sleep, it first sends a “Data Request” command to check for data pending at its parent (coordinator or router). FFD - Level k+1 (T2,k)
(T3,k)
RFD - Level k+2 Data Request ACK
DATA
(T1,k+1)
(T4,k+1)
ACK
Fig. 3. Indirect transmission in nonbeacon-enabled ZigBee network.
After the reception of this packet, the parent must always reply sending its data. In our implementation, we used this couple of packets (“Data Request” and “Data”) to retrieve clocks offset and drift. Referring to Fig.3, “Data” packet contains both information (T2,k) and (T3,k), while both information (T1,k+1) and (T4,k+1) are known by the end device, as better explained in the following. In addition, the acknowledge frames defined in the standard allow for increased protocol robustness (ACK, different from the Acknowledgement packet of the synchronization protocol).
Transceiver
Fig. 4.The proposed “retro fitted” ZigBee protocol stack (ZDO: Zigbee Device Object, APS: Application Support, NWK: Network, MAC: Medium Access Control, PHY: Physical).
In fact, we added an accurate timestamping block at the physical level while offset and drift compensation of the local clock are performed at the application level. The former statement is not a strong requirement, especially at the receiving stage, since every transceiver compliant with the IEEE802.15.4-2003 standard offers such a feature. In fact, timestamping is used to track Beacons with a time resolution of “one symbol” (i.e. 16 μs when operating at 2.4 GHz). However, no information is provided on how it must be obtained or which point of the frame must be used as a reference. In addition, it must be noticed that the latest public available version of the ZB specifications [1] is only based on the IEEE802.15.4-2003 [3] and not on the more recent IEEE802.15.4-2006 [11]. In particular, according to what stated in [11], also transmitted or received data frames can be optionally timestamped at the MAC level with a resolution of one symbol and a minimum precision (according to the standard nomenclature) of 16 symbols. For this reason, within the ZB standard timestamping is optionally performed at the network level and disabled by default. C. Practical realization The transceiver we used in this work is the MC13192 from Freescale, which has an input-capture feature fired by the reception of the Frame Length Identifier –FLI– of the
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incoming packet (see Fig.5). As another example, the CC2420 from TI offers a digital line which is raised when the Start Frame Delimiter –SFD– is decoded. As stated in the previous section, the standard do not indicate the reference point and only in [11] has been added the attribute “macSyncSymbolOffset”, i.e. the offset, measured in symbols, between the symbol boundary at which the MAC layer captures the timestamp of each transmitted or received frame, and the onset of the first symbol past the SFD. 32μs
160μs 4 bytes
1 byte
1 byte
Max 125 bytes
ΔμC, i = ((T4 i – T3 i) + (T2 i – T1 i))/2 OμC, i = T4 i – T3 i – ΔμC, i
With regard to the delay, it can be estimated sporadically since it can be considered almost constant (i.e. ΔμC,i= ΔμC). The aim of every synchronization algorithm is to minimize the offset among all clocks and some forms of drift compensation must be employed. The average drift among R successive synchronization pulses can be estimated as (usually 4 ≤ R ≤ 64):
2 bytes
ρ= PREAMBLE
SFD
FLI
PAYLOAD DATA
FCS
Timestamp transceiver MC1319x
Fig. 5. IEEE802.15.4 physical layer packet (inbound timestamping).
The synchronization phase needs also an accurate timestamping of the transmission start. We studied two different approaches with our prototypes; the first one is based on the “streaming” mode of data transmission towards the radio chip. We are able to intercept the interrupt request associated with the transmission of the first two bytes of the outgoing stream and subsequently read the transceiver timer (the same used in the reception phase). On the coordinator side only, we also encapsulate this value within the outgoing data packet. Again, this is not a strong restriction, and a similar approach has been already used for instance in [2] with the previously cited CC2420. The second possibility is available when the MC13192 is used, and takes advantage of the “time triggered” feature of this device. It is possible to postpone packet transmission later in time exploiting a sort of output-compare behavior of the transceiver timer; this further improve timestamp accuracy. It should be highlighted that timestamping is related to the transceiver oscillator, that is normally a high quality one. Thanks to isochronous reading of other oscillators, timestamps can be related to different oscillators. In addition, as the synchronization parent is always on, the end-device can take advantage from the knowledge of (T2,k) of consecutive synchronizing “Data Request” actions. Concerning offset and drift compensation, we perform a simple calculation at the application level. The one-way delay ΔμC and the offset between parent and child node clocks OμC can be computed according to well known equations [9,12]. In particular, if we suppose a symmetrical channel and consider the single hop portion of the network near the root (i.e. k=0), equations are: T2i = T1 i – OμC,i + ΔμC,i T4 i = T3 i + OμC,i + ΔμC,i
(1) (2)
where the nomenclature is as in Fig.2 and i is the cycle number; remember that both timers have the same resolution. They can be rearranged as in:
(3) (4)
1 R (O μC,i − O μC,i −1 ) ∑ R i=1 (T3i − T3i−1 )
(5).
The adjustment of the slave clock is performed according to the following algorithm, derived from [12]: Step 1: the “Acknowledgement” arrival timestamp in the slave clock time reference in the cycle i T3i is obtained by means of the equation:
T4 i = T4*i−1 +
(T3i − T3i−1 ) 1+ ρ
(6)
where the corrected timestamp T4*i-1 of the previous cycle is available after Step 3. Step 2: the residual offset from the master is evaluated considering also the transmission delay: O μC,i = T4 i − T3i − Δ μC
(7).
Offset is verified against the average value of the last 4 offsets; a simple non-linear filter suppresses any out-of-range value. Step 3: now the corrected timestamp can be calculated:
(8)
T4 *i = T4 i − O μC,i
In order to update the slave clock time reference we simply force T4i = T4*i, introducing a step variation to slave clock. However, amortization algorithms can be employed to eliminate such step variations (e.g. introducing a linear amortization). The previous algorithm is executed every synchronization cycles. Between them, the timestamp (in the slave clock time reference) of a generic event T*gen,S can be calculated with the following equation: * * Tgen, S = T4 i +
(T
gen,S
− T4 i )
1+ ρ
(9)
where T4*i and T4i are referred to the last synchronization cycle and Tgen,S is the value of the slave clock at the considered event.
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IV. EXPERIMENTAL RESULTS Preliminary tests have been conducted with two nodes implementing the BeeKit [13] stack from Freescale. One of them was configured as the PAN coordinator, the other as an end device. As previously stated, all timestamps were based on the timer of the radio device, clocked at 62.5kHz. This choice is dictated by low power considerations and, obviously, it doesn’t offer a very high resolution. (Currently, attempts to use the smallest available time grain of 0.5μs are being carried on). A passive sniffer showing the packet duration in air has been implemented using the CC2420 transceiver (a digital line shows the actual message timestamp point, i.e. the SFD). It has been used to estimate the so called outbound and inbound latency, with the first one being the propagation time between the clock timestamp point and the communication medium for outbound messages and the second one the propagation time between the communication medium and the clock timestamp point for inbound messages. In regards to outgoing messages, a digital line (PTA5_ZC for coordinator and PTA5_ZED for end device in Fig.6 and Fig.7) toggles when timestamping occurs and the delay is measured with respect to the sniffer signal (SFD in Fig.6 and Fig.7).
of messages. On the contrary, for incoming messages it has been supposed that latency is ΔIN=32μs, since the hardware timestamping occurs after the reception of the FLI (see Fig.5). In order to verify algorithm implementation, the node itself has been used to compute the one-way delay ΔμC. It has been obtained ΔμC=21 clock ticks, i.e. (ΔμC)·TCK≤ΔOUT+ΔIN
