A minimum physical distance delivery protocol based on ZigBee in smart grid
© Mu; licensee Springer. 2014
Received: 14 May 2014
Accepted: 2 June 2014
Published: 3 July 2014
ZigBee provides a simple and reliable solution for the advanced measuring infrastructures. However, the current routing algorithms cannot fully satisfy the requirements of the application, and the characteristics of the node deployment and the data flows should be more considered. In this paper, we propose a minimum physical distance (MPD) delivery protocol based on the ZigBee specification in the smart grid to optimize the transmission of the monitoring and command packets which are from or to the ZigBee coordinator (ZC). The physical depth, which is introduced to indicate the least hops to the ZC, and the transmission paths are decided based on the neighbour table information. The simulation results show that the MPD could improve the performance of the monitoring and controlling packet transmission, it provided high reliability and short paths, the bits sent by the devices except the coordinator were reduced and the end-to-end delay was also shortened.
Smart grid is characterized by two-way flow of power in electrical network, and information in communication network to increase energy efficiency, transition to renewable energy sources, reduce greenhouse gas emissions, and build a sustainable economy that ensures prosperity for current and future generations [1–3]. The real-time communication ability of the smart grid will enable utilities to optimize and modernize the power grid in order to realize its full potential . The communication network takes charge of the collection and analysis of real-time data, along with the control of electrical loads for energy reduction and demand response . Advanced metering infrastructure (AMI) is the technology of automatically collecting data from energy metering devices and transferring that data to a central database by communication technology for remote control and analyzing. It is the totality of systems and networks for measuring, collecting, storing, analyzing, and using energy usage data. AMI will link consumers and power utilities together and provide foundation for future distribution automation and other smart grid functionalities . Based on these functions, the nodes in AMI are always irregularly distributed and the communication is low data rate and short range. In the recent report on National Institute of Standard and Technology (NIST) framework and roadmap for smart grid interoperability standards, several wireless communication technologies are identified for smart grid. For examples, ZigBee and the ZigBee Smart Energy Profile (SEP) have been defined as the one of the communication standards for use in the customer premise network domain (including AMI) of the smart grid .
ZigBee technology is characterized by low cost, low power, low data rate, and simplicity . These features, along with its operating over unlicensed spectrum and being a standardized protocol based on IEEE 802.15.4 standards, facilitate easy network deployment and implementation, and make it the most suitable wireless technology for smart grid applications . It has also been selected by a large number of utilities as the communications platform of choice for their smart metering devices as it provides a standardized platform for exchanging data between utilities and smart metering devices and appliances located on customer premises .
ZigBee uses a mixed routing mechanism combined with HRP (hierarchical tree routing) and Z-AODV (ZigBee ad-hoc on demand distance vector) . HRP is based on the address distribution and provides a simple and reliable measure for data transmission, though it is not always efficient and robust. For Z-AODV, each node may initiate routing discovery when necessary; a global shortest path between the source and destination is obtained during the process, and the data frame was sent along the route. However, since the HRP and Z-AODV are designed for different topologies, their benefits are alternative. Moreover, ZigBee devices have limited processing capabilities, storage, power supplies, and communication bandwidth. They may also move about randomly, which results in topology changes of the network. These constraints make it very difficult to find proper routing mechanisms that ensure high network throughput in different environments . For that reason, current network formation and routing protocols described in the ZigBee specification cannot fully address power consumption issues [13, 14]. The deployment in smart grid is newly presented and the characteristics of application need to be more considered [15, 16].
Considering the regular data flows in AMI, they can be classified into two categories. One is the monitoring and controlling frames from or to the sink node, which is the ZigBee coordinator (ZC) in the ZigBee network. The other is the general communication between any other devices except the ZC. For the monitoring and controlling communication, as the ZC is the root of tree structure, these packets are transmitted along the hierarchical paths, where the depths of nodes are strictly monotonic (increasing for command and decreasing for monitoring frames). The depth is defined as the hop counts in the HRP, and it indicates the topological distance to the ZC. Nevertheless, the parameter that implies the spatial distance is needed to find the best route. In our work, we introduce the physical depth (PD), which is the minimum hop counts to the sink, to indicate the physical distance from a local device to the ZC. Since all the one-hop neighbour node information is required to be stored by each device according to the ZigBee specification, the physical distance can be easily updated by diffusion. On that basis, the transmission of the command and monitoring packets could be optimized by the principle of seeking the receiver with the least physical depth in broadcasting transmission. In this way, the monitoring and command frames can be transmitted in the global shortest path with neither the handshake nor the routing discovery.
The rest of this paper is organized as follows: The related works are reviewed in section 2. Section 3 briefly introduces the ZigBee specification, and the minimum physical distance broadcasting algorithm is proposed in section 4. In section 5, simulation results are presented. Finally, the conclusion is shown in section 6.
2 Related works
Recently, the ZigBee network has been proposed in different research areas in power systems. The IEEE 802.15.4 was used to construct a wireless, non-intrusive, intelligent, and low-cost energy management system [22, 23]. The motor terminal voltages and current information are sent through a WSN for energy evaluation and condition monitoring. Some progress in system design, network deployment, and data compression have been made by integrating ZigBee networks into Advanced Metering Infrastructure (AMI), fault monitoring, and the like [24, 25]. Typically, a Power Monitoring Module (PMM) is proposed in , which integrates ZigBee and digital signal processing techniques for wireless communication and real-time power parameters computation. The proposed system is stand-alone and communicates wirelessly with outside systems hence can be used in monitoring different points in the power system.
The MPD proposed in this paper is based on the hierarchical structure, in which the HRP is commonly used. For the HRP, paper  gives a modified tree routing mechanism with the introduction of neighbour table. The transmission cost (e.g. hops) via each neighbour device is estimated and compared to improve the routing path. It has a better performance with less power consumption per packet transfer and a long life cycle. But this algorithm is based on the two-hop neighbour information and it may lead to severe energy and memory overhead in ZigBee networks. Studies [28–30] propose several similar routing algorithms for hierarchical topology. The information in neighbour tables was used to get the shorter paths. Based on the Distributed Address Allocation Mechanism (DAAM), if the addresses of source and destination nodes are given, the hierarchical route can be decided without any other information. The local device may calculate the hierarchical hop counts for every device in the neighbourhood and choose the one with the least hops as the next hop node. This mechanism can only grant the best hierarchical path, while our work may find the route with the shortest physical distance. Moreover, the MPD utilizes the broadcasting transmission for better performance, the receiving node but not the sending one may decide whether it should retransmit the packet. The most relevant work is in , a tiny and efficient protocol is proposed based on broadcasting. The algorithm may provide less frame bits and shorter delay. Meanwhile, the protocol is also proved reliable. The simulation results show that the method has a high-enough packet delivery fraction, in the static network, this ratio was no less than 95%. However, the protocol is proposed in common wireless networks and the improvement need further evaluation due to the undetermined MAC layer standards. Our scheme fits for the ZigBee specification, the neighbour information could be used for further optimization. And we compare our method with other popular routing algorithms in the ZigBee network to evaluate the performance objectively.
3 ZigBee specification and routing methods
3.1 Link quality indication
ZigBee devices support the function of testing the link quality indication (LQI) measurement every time they receive a frame. The LQI measurement is a characterization of the strength and/or quality of a received packet. The measurement may be implemented using receiver energy detection (ED), a signal-to-noise ratio estimation, or a combination of these methods. The use of the LQI result by the network or application layers is not specified in IEEE 802.15.4 standard.
The LQI measurement shall be performed for each received packet, and the result shall be reported to the MAC sublayer. The minimum and maximum LQI values (0 × 00 and 0 × ff) should be associated with the lowest and highest quality IEEE 802.15.4 signals detectable by the receiver, and link quality (LQ) values in between is uniformly distributed between these two limits. The LQI information of every single received packet can be simply acquired according to the standard with no more extra calculation and communication .
3.2 Address allocation and HRP
where Aparent represents the address of the parent and 1 ≤ n ≤ Rm.
The hierarchical topology in the ZigBee network is based on the DAAM. In this tree-shaped structure, if the destination address is in the address space that a node is managing, the node forwards the packet to one of its child nodes. Otherwise, it forwards the packet to its parent.
3.3 Neighbour table
Each ZigBee device maintains a neighbour table which has all its neighbours' information in the one-hop transmission range. The contents for a neighbour entry are the network's personal area network (PAN) identifier; node's extended address, network address, device type, relationship, LQI, etc. Optionally, additional information such as the depth can be included. Entries in the table are created when the node joins an existing network. Conversely, the neighbour entry is removed when the neighbour node leaves the network. Since the information on the neighbour table is updated every time a device receives any frame from the some neighbour node, the information of the neighbour table can be said to be up-to-date all the time.
4 Minimum physical distance broadcasting algorithm
In the smart grid application, the monitoring and command packets include the electricity consumption information gathering from the appliances and the instructions controlling the equipment. These packets are either collected or sent by the smart meter. In other words, the communication has a definite terminal, the ZC. The depth is defined as the transmission hops in the HRP. It indicates the topology distance between the local device and the ZC. This information is not helpful to find the global shortest path since the hierarchical topology limits the transmission to parent–child links. We introduce the concept of the physical depth (PD), which is defined as the minimum hop counts to the ZC. So the PD is able to indicate the spatial distance to some extents.
Thus, the ZC is the only device with the PD 0 in the network. For a device in the neighbourhood of the ZC, its PD may be set to 1 since it may find the entry of the ZC in its neighbour table. If the PD information is required in the neighbour table, all the nodes within the transmission range of the PD 1 devices may have PD 2; the PD of any device can be recursively decided. The principle can be simply stated as follows: The PD value of a certain node is one plus the minimum PD value in its neighbour table.
where Q d is introduced to determine whether node X is a router node or an end device at depth d. One can see that the depth information is redundant in this case. Therefore, it can be replaced by the PD. On that basis, the delivery protocol is investigated.
In the MPD, we focus on the following parameters in the transmission. The address of the other terminal except the ZC, S/D add (this device is also called S/D node in the following for convenience); the frame type, M/C (1 for monitoring frame and 0 for command); the sequence number, SN; the retired times, RT; the PD of the S/D node, PDs; the remaining hops to the destination, PD r; a flag, f Opt, indicates whether this frame is allowed to be rebroadcasted by other nodes which cannot find the routing table entries corresponding to the S/D add (1 for True and 0 for false). All the variables above are included in the frame header. With the array of [S/D, M/C, SN, RT], which we called broadcasting frame identity array (BFIA), the frame can be uniquely recognized. And the other parameters, PDs, PD r, and f Opt are used to control the transmission.
Our algorithm is based on an assumption that all the transmission links are half-duplex and symmetric. On the first attempt to communicate with the ZC for a certain node, suppose it is a monitoring frame (the principle is the same for the controlling one), the S/D add is set the address of this device, M/C is 1, SN is one plus the SN value in the last monitoring packets transmission, RT and PD s depends on the real situation, PD r is equal to the PDs minus 1, and f Opt has to be 1. And the packet is broadcasted to its neighbourhood by the S/D device.
Each node that receives this packet may compare the PD r value with its own PD, if the PD r is smaller, the frame is abandoned. Otherwise, the device may rebroadcast the packet. Meanwhile, the PD r value is decremented by 1 and an entry containing the S/D add and the PD r is built in the routing table. The sending node is asked to monitor the channel, if some packet with the same BFIA and the PD r is received, it may regard the packets as the transmission acknowledgement and reserve the corresponding routing entry, otherwise, the entry is deleted after a preset expiration time. Based on this mechanism, the frame is forwarded to the ZC and the path must be the global shortest.
When a packet is successfully delivered to the ZC, all the nodes in the transmission path may update their routing tables. The entry should at least includes the S/D add, PD s and PD r. For the later communication between this device and the ZC, the f Opt could be set to 0, and the nodes only have to check the existence of the corresponding routing table entry to decide whether it should rebroadcast this packet or abandon it. For the monitoring packets, the PD r is the PD value of the local device, while it is equal to (PD s-PD) for the controlling ones.
The assumption of symmetric links can be roughly satisfied in most cases for the deployed nodes except the ZC. Owing to the sufficient power supply and capability of using more complicated peripherals, its transmitting range can be larger than the receiving one. This may lead to the routing errors and link failures. In our algorithm, all the nodes that could hear from the ZC are required to send a simple inquiry with its network address. The particular frame can be only responded by the ZC. If a node receives the response within a preset expire time, its PD is decided as 1. Otherwise, it may ignored the neighbour table entry corresponding to the ZC and decide its depth based on the least PD value of the other neighbours. Note that for the controlling packets transmission, it is possible to have one or more less hops comparing with the monitoring ones since the ZC has the capability of sending message directly to the high PD devices. On that case, the ZC cannot receive the rebroadcasting to confirm the transmission. While the PD 1 node in the monitoring link may find the PD r value is less than expected. It is required to announce the situation. So the ZC is aware of the asymmetric link, and it may repeat the packets for bits error control. This mechanism not only solves the asymmetric link problem. The nodes close to the ZC tend to take part in the communication more frequently, thus they may consume the batteries faster. This framework can also reduce the energy consumption for low PD devices.
The performance of the proposed minimum physical depth delivery protocol is discussed in this section. The simulation was implemented by the MATLAB. Some parameters in the simulation were set as follows: the time duration was 300 s; the simulation area was 300 m × 300 m; the network consists of 50 nodes except the ZC, and all of them were the ZigBee routers; the nodes were randomly deployed following the uniform distribution; Cm, Rm, and Lm were set to 4, 4, and 5, separately; the data packet size was 100 bits; the packet interval time was 1 s; the first packet arrival time followed a uniform distribution from 10 to 11 for each node; the maximum retries was four times; the transmitter power of the ZC is five times the power of other devices; for each scenario, the simulation was carried out 500 times to calculate the average. To make a comprehensive comparison, the two modes of the MPD in which the f Opt was 0 and 1 were both tested. Our algorithm was compared with the two specified routing algorithms, the HRP and the Z-AODV. As a classic method in the hierarchical topology, the EHRP in  was also included.
The node mobility should be considered in the wireless networks. In our simulations, each node was stationary for a random time that followed a uniform distribution form ts − 50 to ts + 50 s. Then, the node moved to a new position which is randomly chosen, and the moving speed was uniformly distributed from 1 to 10 m/s. The devices in the smart grid application did not move a lot, so the ts value was ranged from 100 to 350 s. As the duration was 300 s, the ts = 350 meant all the nodes may not move during the scenario.
We also made a mapping from the LQI in the received data to the signal power. In our simulation, the channel followed Rayleigh fading with aδ2of 5. We mapped the best LQI (−3-dB loss) to 0 × ff (255) and the lowest quality compliant signals detectable by the receiver (−20-dB loss) to 0 × 00 (0); the values in between were uniformly distributed.
Although the routing method is mainly a NWK layer protocol, its performance is not only based on the NWK behaviours but also the MAC schemes. The MPD is quite different from the existing algorithms, even if it is designed for the ZigBee specification, some functions in the IEEE 802.15.4 (e.g. the beacon enable mode, GTS allocation, the superframe structure and etc.) may be imcompatible with our algorithm. It has two meanings; the MPD cannot support such modes. Consequently, the MAC and NWK headers are redundant if we just consider the transmission. To make a meaningful comparison, we did not include all the frame headers in our simulations, only the information related to the routing and the used transmission mechanism was focused on.
Similar as the analysis of the network load, the simulation on the end-to-end delay is also a complicated task related to the MAC scheme. Moreover, most simulation software, such as the OPNET, NS2, and NS3, do not provide the broadcasting-based transmission strategy used in the MPD. We can only estimate the delays according to the IEEE 802.15.4 protocol. Since the carrier transmission time is so short that can be ignored, the delay is mainly caused by the device processing time and the CSMA/CA mechanism. The former factor was tested in the OPNET as reference, we made a simple network with two nodes (not including the ZC) and watched the processing time of the intermediate node with the fixed packet size (100 bit). When trying to send the message, a node may listen to the channel and decide to transmit or wait for a random time based on the CSMA/CA scheme. The simulation in our work is a rough comparison, as a comprehensive work, the accurate test data may be obtained by designing compatible models in our further work.
The overall comparison in monitoring and controlling packets transmission
Packet delivery fraction
Average hop counts
MPD (f Opt =1)
MPD (f Opt =0)
The ZigBee is a fitting communication protocol for the AMI of the smart grid. However, the routing algorithms specified in the ZigBee specification could not fully satisfy the requirements on the energy consumption and the characteristics of the application should be more considered. In this paper, we proposed a minimum physical depth delivery protocol to improve the monitoring and controlling data transmission in the AMI. By introducing the concept of the physical depth, the packet was forwarded along the trajectory of the devices with the least PD in the neighbourhood. Considering the low payload of the frames, the broadcasting mechanism was used to save the bandwidth and the receiving acknowledgement was removed. Our algorithm was compared with the ones specified in the ZigBee and some improved classic methods. The simulation results showed that the MPD could improve the routing performance for the monitoring and controlling packets. It could reduce the network throughput and the hop counts effectively, and the end-to-end delay was slightly decreased. And it had a good-enough packet delivery fraction to meet the requirement of reliability.
Our further work will focus on two issues. One is to design a fully compatible MAC scheme for the minimum physical depth delivery protocol to test the performance more accurately and apply the algorithm in the real application. The other one is to make the protocol aware of the channel quality based on the LQI to avoid the high bit error rate.
This work is funded by the National Science Foundation of China, (NSFC: 61271411, 61372097), the Project of Tianjin University Technology Development Fund (20130714), and the Doctoral Scientific Foundation of Tianjin Normal University (52XB1106).
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