CN101466142A - Synchronization method for layered time comparative clock in wireless sensor network - Google Patents
Synchronization method for layered time comparative clock in wireless sensor network Download PDFInfo
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Abstract
Disclosed is a layered time comparative clock synchronized method in the wireless sensor network, which belongs to the wireless network technology field. The method comprises the following steps: step one, the network initialization is started, and the network is layered from a basic station/a coordinator according to the signal intensity and the device category; step two, the whole network is divided into outer clock source synchronization and inner clock source synchronization, the nodes are synchronized by adopting the timestamp comparative method, and the upper-level nodes and the lower-level nodes respectively take the outer clock source synchronization and the inner clock source synchronization; step three, the synchronizing cycle and the clock drift parameter are regulated according to previous synchronization effects; step four, the old timestamp is automatically discarded. Compared with prior art, the method can better correspond with the accuracy of the wireless sensor network time synchronization method, and has the advantages of low cost and solidity.
Description
Technical field
The present invention relates to the method in a kind of radio network technique field, specifically is synchronization method for layered time comparative clock in a kind of wireless sensor network.
Background technology
In recent years, the wireless sensor network of forming by power consumption wireless sensor (wireless sensornetworks, WSNs) use extensive further.Time synchronized is the important component part that WSNs uses, and aspects such as control messages conflict, data fusion, node locating all require to keep between node clock synchronization.
Domestic and international many universities and research institution have all carried out further investigation and have proposed multiple method for synchronizing time the simultaneous techniques of wireless sensor network, satisfy the application need of sensor network from different aspects.Up to the present, proposed tens kinds of implementation methods, RBS, TPSN, LTS, TS/MS, TSync, DMTS, FTSP, AD, TSS and TDP have typically been arranged.Though these methods have satisfied application need from different aspects, corresponding they self have many defectives, and are better such as the RBS precision, but can't use in the procotol that has.TS/MS, Tsync precision are better, but the method complexity is somewhat excessive for operational capability limited in the wireless network and network overhead.
Find by prior art documents, Rui Fan, Indraneel Chakraborty, and NancyLynch, at the Lecture Notes on August 23rd, 2005 Computer Science, Principlesof Distributed Systems (computer science lecture notes, the distributed system principle) article of delivering in " Clock Synchronization for Wireless Networks " (clock synchronization of wireless network), this article has proposed a kind of method of synchronization, take the mode that external clock is synchronous and internal clocking is run simultaneously, by continuous comparison time coordinate size, get its maximum (may be immediate with the outside reference time when also being synchronous in fact) and be its logical time, adopt the fixed cycle synchronizing cycle.The computing cost and the communication overhead of this method are less, avoided the estimation of the processing data packets time-delay of randomness maximum, avoided some algorithms may be simultaneously with the possibility of clock clawback, but this method is only better to stable single-hop networks effect, synchronization accuracy is relatively poor in larger dynamic network, and this is not enough for some practical application obviously.
Summary of the invention
The objective of the invention is at above-mentioned the deficiencies in the prior art, synchronization method for layered time comparative clock in a kind of wireless sensor network is provided, compare by layering and timestamp, reduced the expense of synchronous communication, adopt and estimate to approach clock drift, adjust the method for parameter ρ, further reduce synchronous expense, simultaneously, abandoned the timestamp that lost efficacy, reduced the expense to the node storage, the present invention can be applied in the dynamic multihop network and go.
The present invention is achieved through the following technical solutions, and the present invention includes following steps:
Whether step 4, node i check current effective array coordinate greater than array length, if set up, the timestamp of inefficacy is abandoned in current effective array coordinate zero clearing, if be false, current effective array coordinate continues counting, repeating step two.
Described each node comprises the steps: according to the logical time that relatively obtains this node of interior time source and external clock source synchronized result
The first step equals the reservation synchronization times of node i and the product of node synchronizing cycle if node i is found the logical timer of node i, and node i begins synchronously, and check whether external clock reference exists, if exist, then send the external sync service request, entered for second step; Whether external clock reference exists that all to begin internal clocking synchronous, promptly carries out for the 3rd step, and the reservation synchronization times of node i increases by one simultaneously, but that the 0th node layer does not carry out internal clocking all the time is synchronous;
Second step, it is synchronous that node i is carried out external clock reference: if node i is received the GPS synchronizing signal, current effective array coordinate i.current of node i adds one, gps time stabbed deposits array i.local[i.current in] and i.max_gps in, i.local[i.current] upgrade synchronously with phy clock, i.local[] represent the best estimate of node i to this node time coordinate of living in, i.max_gps represents that nearest gps time stabs;
The 3rd step, it is synchronous that node i is carried out internal clocking: node i to directly under superior node j send synchronization service request, if node i receive the synchronization frame sync_message that sends from superior node j (j.local[j.current], j.max_gps), node i checks whether the same new with the i.max_gps of node i the gps time of node j stabs, check j.local[j.current subsequently] whether than i.global[i.current] big, i.global[] the expression node i is to other node time best estimates of this node belonging network, if both are all satisfied, representation node i is to the estimation i.global[i.current of other nodes] error arranged, then upgrade i.global[i.current] for j.local[j.current], j.global[j.current] doubly upgrade with (1-ρ)/(the 1+ ρ) of phy clock;
In the 4th step, node i is array i.local[relatively] each element, getting the maximum is i.mlocal, relatively array i.global[] each element, getting the maximum is i.mglobal;
In the 5th step, node i compares the size of i.mlocal and i.mglobal, and getting the greater is the logical timer that i.logical represents node i, and it is synchronous to finish nodal clock, and i.logical upgrades synchronously with physical time.
Compared with prior art, the present invention includes following beneficial effect:
1, the present invention relatively avoids estimation to data packet transmission time-delay between node by timestamp; Can know internodal transmission delay because channel energy detection and channel confliction detect from the building of wireless network of reality with the packet capturing analysis, the influence of the task scheduling of node, randomness is very large, the employing timestamp has relatively got around the estimation to these factors, thereby under the condition that guarantees certain precision, reduce overhead;
2, the present invention has reduced the internodal synchronous error limit by the method for layering; In other words, the synchronization accuracy of the internodal synchronous error limit and node all depends on the maximum delay of this node belonging network, so with a big network hierarchy, be divided into several little nets and carry out helping to reduce error synchronously;
3, the present invention has reduced the expense of synchronous communication by the method for variable period; In actual engineering, the employed crystal oscillator of each node has than big difference, and the precision that has is good, the low precision that has.If obviously be to increase no small expense to limited network channel flow the unified synchronizing cycle of imposing uniformity without examining individual cases, increase whole network energy consumption can be avoided unnecessary energy consumption and flow expense and allow node add up voluntarily to change with the cycle;
4, the present invention is by estimating to approach clock drift, adjust the method for parameter ρ, further reduce synchronous expense, in front in the node synchronizing process, can see that accurate if ρ estimates, obviously this node is estimated more accurate to the clock of other nodes, it is longer that the corresponding synchronous cycle just can establish, and natural expense is just little;
5, the present invention has reduced the expense to the node storage by the i.Current zero clearing being abandoned the timestamp of inefficacy; Though along with the continuous development of semiconductor technology, resource is more and more abundanter on the sheet,, the situation that resource is subjected to bigger restriction for the employed single-chip microcomputer of node remains in a period of time in the future and can't change can seeming.Two best estimate arrays are not account for how many resources in the short period of time in this synchronization mechanism, but the time grown, obviously not all right, old excessively simultaneously timestamp also is nonsensical.So abandon the timestamp of inefficacy.
To sum up, the present invention can better meet the moderate accuracy of unlimited sensor network time synchronized algorithm, low expense, healthy and strong requirement.
Description of drawings
Fig. 1 is the workflow diagram of the inventive method;
Fig. 2 is embodiments of the invention simulation result figure;
Among the figure, (a), (b) be logical frequency variation diagram after the process parameter adjustment, (c) for comparing block diagram the synchronizing cycle before and after handling for the unprocessed crystal oscillator frequency variation diagram of simulation.
Embodiment
Below in conjunction with accompanying drawing embodiments of the invention are elaborated: present embodiment is being to implement under the prerequisite with the technical solution of the present invention, provided detailed execution mode and concrete operating process, but protection scope of the present invention is not limited to following embodiment.
Present embodiment is applied to wireless sensor network and builds in the application of clock synchronization research of the Experimental Network in the project, mainly adopted based on the ASAP1810/20 development board of purple core company and the CC2420 wireless senser development board of TI, and developed the networking experiment thereon.
Wireless sensor network is made up of 126 slave units and a main equipment in the present embodiment, and wherein main equipment is provided with the gps clock source, and the Network Layering number of plies is divided according to its physical location and function voluntarily by each node.
As shown in Figure 1, present embodiment comprises the steps:
Whether step 4, node i check current effective array coordinate greater than array length, if set up, the timestamp of inefficacy is abandoned in current effective array coordinate zero clearing, if be false, current effective array coordinate continues counting, repeating step two.
Described each node comprises the steps: according to the logical time that relatively obtains this node of interior time source and external clock source synchronized result
The first step, set up two array: i.local[p in the node i], i.global[p], eight variable: i.current, i.next_sync, i.mlocal, i.mglobal, i.max_gps, ρ, i.logical, τ, wherein, i.current represents current effective array coordinate of node i; I.local[] the expression node i is to the best estimate of this node time coordinate of living in; I.global[] the expression node i is to other node time best estimates of this node belonging network; I.mlocal represents i.local[] middle maximum; I.mglobal represents i.global[] middle maximum; I.max_gps represents that nearest gps time stabs; ρ represents clock drift, the ppm of unit (per 1,000,000/); τ represents node synchronizing cycle; P represents array length; I.logical represents the logical timer of node i; I.next_sync represents the predetermined synchronization times of node i;
In second step, if node i is found i.logical=i.next_sync* τ, node i begins synchronously, and checks whether external clock reference exists, if exist, then sends the external sync service request, enters step 3; Whether external clock reference exists that all to begin internal clocking synchronous, and promptly execution in step four, and variable i .next_sync adds one simultaneously, but that the 0th node layer does not carry out internal clocking all the time is synchronous;
The 3rd step, node i is carried out the external clock reference synchronizing process: if node i is received the GPS synchronizing signal, i.current adds one, gps time is stabbed deposit i.local[i.current in] and i.max_gps in, i.local[i.current] upgrade synchronously with phy clock;
The 4th step, node i is carried out the internal clocking synchronizing process: node i to directly under superior node j send synchronization service request, if node i receive the synchronization frame sync_message that sends from superior node j (j.local[j.current], j.max_gps), node i checks whether the same new with the i.max_gps of node i the gps time of node j stabs, check j.local[j.current subsequently] whether than i.global[i.current] big, if both are all satisfied, representation node i is to the estimation i.global[i.current of other nodes] error arranged, then upgrade i.global[i.current] for j.local[j.current], j.global[j.current] doubly upgrade with (1-ρ)/(the 1+ ρ) of phy clock;
In the 5th step, node i is array i.local[relatively] each element, getting the maximum is i.mlocal, relatively array i.global[] each element, getting the maximum is i.mglobal;
In the 6th step, node i is the size of i.mlocal and i.mglobal relatively, gets the greater and be i.logical and be that logical timer has been finished nodal clock is synchronous, and i.logical upgrades synchronously with physical time.
In the present embodiment, internodal synchronous error such as following formula: | i.log ical (t)-j.log ical (t) |≤2 (D+ ρ (T+D)), internodal synchronous error approximately is controlled between node in 2 times of the signal delays, network configuration is the downward recursion of binary tree structure, the purpose of present embodiment is to use this method, under try one's best little network overhead and node computing cost, satisfy the synchronization accuracy requirement of network.
The form of present embodiment method with software write in each development board, is the wireless sensor network node building network with this development board again, observes the network service situation, mainly is that the affirmation network is set up, and top mode inserts external clock reference.In an embodiment, crystal oscillator drift ρ gets 10
-4Second, synchronizing cycle, τ got 1 second.The Network Layering number of plies is decided according to the message capacity and the signal cover of node.
On the basis of this actual networking experiment, the relation of adjusting parameter ρ and adjust between τ synchronizing cycle has been carried out analog simulation, simulation result as shown in Figure 2, figure (a) is the unprocessed crystal oscillator frequency variation diagram of simulation, (b) be logical frequency variation diagram after the process parameter adjustment, the ordinate among this two width of cloth figure is represented the poor of original frequency and reference frequency, logical frequency and reference frequency respectively.And scheme (c) is a block diagram, and compare the synchronizing cycle before and after expression will be handled.Wherein, the synchronizing cycle before No. 1 post representative is handled, the synchronizing cycle after No. 2 post representatives are handled.As known in the figure, treated logical frequency will be got well much than original frequency stability, can satisfy under the condition of certain timing accuracy thus, prolong synchronizing cycle as much as possible, this wireless sensor network for the energy consumption sensitivity has very big meaning, is especially launching power consumption under the situation of computing power consumption.
Claims (2)
1, synchronization method for layered time comparative clock in a kind of wireless sensor network is characterized in that, comprises the steps:
Step 1, network begins initialization: with total base station/telegon is top layer, send the membership acknowledgement frame according to signal strength signal intensity and device category, after corresponding equipment receives acknowledgement frame, and echo reply frame, by that analogy, finish, confirm that simultaneously network reaches stable state whole Network Layering setting;
Step 2, from top layer, successively begin synchronously, be divided into two parts synchronously: external clock source is synchronous and the internal clock source is synchronous, in the external clock reference synchronizing process, if node i is received the GPS synchronizing signal, current effective array coordinate i.current of node i adds one, external clock reference provides time reference synchronously for internal clock source synchronously, and each node is according to the logical time that relatively obtains this node of interior time source and external clock source synchronized result;
Step 3, each node is unified sets synchronizing cycle, and each node is adjusted clock drift parameter ρ, synchronizing cycle τ according to initial several times synchronized result and synchronization accuracy subsequently, satisfies synchronization accuracy if node is subsynchronous, then increase τ synchronizing cycle, reduce overhead; If do not satisfy, then adjust clock drift parameter ρ, and will reduce τ synchronizing cycle;
Whether step 4, node i check current effective array coordinate greater than array length, if set up, the timestamp of inefficacy is abandoned in current effective array coordinate zero clearing, if be false, current effective array coordinate continues counting, repeating step two.
2, synchronization method for layered time comparative clock in the wireless sensor network according to claim 1 is characterized in that, described each node comprises the steps: according to the logical time that relatively obtains this node of interior time source and external clock source synchronized result
The first step equals the reservation synchronization times of node i and the product of node synchronizing cycle if node i is found the logical timer of node i, and node i begins synchronously, and check whether external clock reference exists, if exist, then send the external sync service request, entered for second step; Whether external clock reference exists that all to begin internal clocking synchronous, promptly carries out for the 3rd step, and the reservation synchronization times of node i increases by one simultaneously, but that the 0th node layer does not carry out internal clocking all the time is synchronous;
Second step, it is synchronous that node i is carried out external clock reference: if node i is received the GPS synchronizing signal, current effective array coordinate i.current of node i adds one, gps time stabbed deposits array i.local[i.current in] and i.max_gps in, i.local[i.current] upgrade synchronously with phy clock, i.local[] represent the best estimate of node i to this node time coordinate of living in, i.max_gps represents that nearest gps time stabs;
The 3rd step, it is synchronous that node i is carried out internal clocking: node i to directly under superior node j send synchronization service request, if node i receive the synchronization frame sync_message that sends from superior node j (j.local[j.current], j.max_gps), node i checks whether the same new with the i.max_gps of node i the gps time of node j stabs, check j.local[j.current subsequently] whether than i.global[i.current] big, i.global[] the expression node i is to other node time best estimates of this node belonging network, if both are all satisfied, representation node i is to the estimation i.global[i.current of other nodes] error arranged, then upgrade i.global[i.current] for j.local[j.current], j.global[j.current] doubly upgrade with (1-ρ)/(the 1+ ρ) of phy clock;
In the 4th step, node i is array i.local[relatively] each element, getting the maximum is i.mlocal, relatively array i.global[] each element, getting the maximum is i.mglobal;
In the 5th step, node i compares the size of i.mlocal and i.mglobal, and getting the greater is the logical timer that i.logical represents node i, and it is synchronous to finish nodal clock, and i.logical upgrades synchronously with physical time.
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