CN100588147C - Low-overhead high-precision time synchronization method of wireless mesh network for industrial monitoring - Google Patents

Abstract

The invention belongs to the field of wireless network communication, and particularly relates to a high-precision time synchronization method for a node and a time reference source in a wireless mesh network. The main technical points are as follows: recording the accurate timestamp of message sending and receiving on the MAC layer; the receiving node needing synchronization collects the time synchronization beacons periodically issued by the time source, and after receiving a plurality of time synchronization beacons, a statistical method is adopted to establish a relation equation of local time and global time at the sending node of the time synchronization beacons, so that the influence of clock crystal oscillator frequency deviation on time synchronization precision is eliminated; and in the network operation process, the parameters of the relational equation are updated on line. The method can eliminate the time uncertainty in the synchronization process to the maximum extent and reduce the synchronization overhead.

Description

The industry monitoring wireless meshed network low-spending high-precision time synchronization process

Technical field

The invention belongs to the wireless communication field, specifically is the high-precision time synchronization method that is used to realize wireless mesh (Mesh) network node and time reference source.

Background technology

Mesh is a kind of novel radio network configuration that occurs in recent years.In this network configuration, each node can be as router, and by constituting the cordless communication network of multi-hop with the collaborative transmission of a plurality of nodes.The low cost of Mesh structural network, robust, advantage such as easy-to-use makes one of its main topology that becomes the wireless communication system of using towards industry monitoring flexibly.

Network time synchronization is the basis of collaborative transmission, is one of key technology that makes up wireless Mesh netword.Face following challenge at industry monitoring with the time synchronized of wireless Mesh netword: one, commercial Application requires cheaply to make that the resource strictness of network node is limited, because the restriction of applied environment, most node relies on powered battery, for the life-span that prolongs node need be reduced the communication overhead that time synchronized causes as far as possible; Two, the communication for coordination between network node as in order to hide the synchronization frequency modulation strategy that disturbs and take, has proposed very high requirement to the precision of time synchronized.

Time synchronization problem is existing big quantity research in distributed system theory, the Internet, real-time control system.Wherein topmost achievement is that the David L.Mills of U.S. Delaware university teaches the NTP (Network Time Protocol) Network Time Protocol that put forward in 1985, and this agreement is used widely at Internet, has formed Internet protocol SNTP.Facts have proved that NTP can satisfy Internet and go up the demand of various application to time synchronized.But Network Time Protocol adopts the time synchronized mechanism of Client/Server mode, and communication overhead is bigger, is not suitable for using in wireless Mesh netword.

In order to reduce communication overhead, people utilize the node in same radio broadcasting territory can receive the characteristics of broadcasting packet simultaneously, have proposed the RBS method for synchronous.RBS adopts the one-way synchronization beacon transfer mode that is periodically sent sync message by time source, if the synchronizing cycle of setting is longer, to greatly reduce communication overhead, but long synchronizing cycle is bigger to the influence of synchronization accuracy, can't satisfies the requirement of industry monitoring with wireless Mesh netword.

Synchronous error in the RBS method mainly is that two aspects cause.On the one hand, because time synchronization protocol is positioned at application layer, need in the sync beacon transmittance process through sending and the processing of recipient MAC layer, network layer, transport layer, the transmission of writing down in the uncertain sync beacon that causes in processing time, time of reception inaccurate is one of major reason that causes synchronous error; On the other hand, to be used for the clock crystal oscillator frequency of timing be incomplete same when dispatching from the factory to each node in the network, as long as the frequency difference when dispatching from the factory just can be thought qualified products in certain scope.The clock crystal oscillator frequency in use generally neither be changeless, can reach the variation of environment in time and change.If crystal oscillator is in the drift in time of running medium frequency, once subsynchronous clock that can synchronous fully two nodes.But there are differences and the drift situation under, as Fig. 2, at t1 constantly, sending node A and the local clock difference that receives between the joint B are D _T1 ^A-＞BAt t2 constantly, the local clock difference of sending node A and reception joint B becomes D _T2 ^A-＞B,

$D_{t2}^{A->B}=D_{t1}^{A->B}+{\mathrm{RD}}_{t1->t2}^{A->B}---\left(1\right)$

In the formula, RD _At1-＞t2 ^A-＞BPart is drifted about to the crystal oscillator frequency of t2 by t1 and is caused.Therefore, the drift of the crystal oscillator frequency of node is another major reason that causes synchronous error.

Summary of the invention

Require low expense at powered battery, the synchronization accuracy that can reach low synchronizing cycle is difficult to satisfy the problem of commercial Application demand, and the object of the invention is to propose a kind of industry monitoring wireless Mesh netword low-spending high-precision time synchronization process.

Technical scheme of the present invention is: send and the accurate time stamp of reception at MAC layer recorded message, it is right to make up time stamp, sets up the linear relationship equation of local zone time and length of a game; Be specially:

1) send and receive accurate time stamp at MAC layer recorded message: in network, choose can accurately obtain physical time sending node as time source, with unidirectional delivery mode, gap periods airtime regular time sync beacon; At the MAC layer overall time stamp that writing time, sync beacon was carried;

2) the structure time stamp is right: behind the time synchronized beacon of the receiving node time of receipt (T of R) source issue in the time source broadcast domain, in MAC layer sync beacon writing time time of advent, extract the local time stamp of inserting by sending node in the message, and with the time synchronized beacon in the overall time stamp that carries to form time stamp right;

3) set up the linear relationship equation of local zone time and length of a game: after receiving node is collected several time synchronized beacons, utilize the local zone time of the method match receiving node of adding up and the relation equation of sending node place length of a game, the frequency drift rate of On-line Estimation clock crystal oscillator, and obtain unified time in the network with relation equation, finish with time source synchronously, obtain synchronous with time source synchronization node; After this, the each time of receipt (T of R) sync beacon of described synchronization node all upgrade local zone time with the coefficient of the relation equation of time synchronized beacon transmitting node place length of a game;

The relation equation of described local zone time and time synchronized beacon transmitting node place length of a game is:

T _global＝ρ×T _local+ΔT

Wherein: in the formula, T _GloalBe length of a game, T _LocalBe local zone time, ρ is the drift of the clock of sending node with respect to receiving node place length of a game; Δ T is the time difference of sending node and receiving node two nodes; The T of length of a game _GlobalWith overall time stamp t _GlobalBe occurrence, local zone time T _LocalWith local time stamp t _LocalBe occurrence; Δ t is local time stamp t _LocalWith overall time stamp t _GlobalPoor;

4) the synchronous receiving node that has obtained becomes time source, periodically other receiving node issuing time sync beacon in its broadcast domain;

5) in the Mesh network configuration, along with the diffusion of time synchronized beacon, the node hop-by-hop of time source periphery obtains synchronously in the network, the node up to the edge of network.

Advantage of the present invention is:

1. owing to adopt time synchronized beacon unidirectional delivery mode, compare with the bi-directional mode that prior art is adopted, the present invention has realized low expense.

2. owing to the present invention the drift rate of clock crystal oscillator frequency is estimated, under identical timing tracking accuracy requires, the present invention can adopt long time synchronized beacon send the cycle (as, the present invention adopts about 30 minutes time synchronized beacon to send the cycle, the method of estimating clock crystal oscillator frequency does not adopt about 5 minutes time synchronized beacon to send the cycle), this method has realized low expense.

3. the present invention utilizes the drift rate of linear regression method On-line Estimation clock crystal oscillator frequency, obtains very high timing tracking accuracy.Because synchronizing cycle is longer, the drift of clock crystal oscillator frequency will bring than mistake to timing tracking accuracy.Adopt method of the present invention, in the time that does not obtain before subsynchronous again, the caused error of clock crystal oscillator frequency drift also is taken into account, thereby has improved the precision of time synchronized.

Description of drawings

Fig. 1 is a time synchronized beacon packet sending and receiving timestamp record process.

The time synchronization error schematic diagram that Fig. 2 causes for the crystal oscillator frequency drift.

Fig. 3 is the realization flow of Mesh method for synchronizing network time.

Embodiment

The invention will be further described below in conjunction with embodiment and accompanying drawing.

The present invention is operation as follows specifically:

1) sends and receive accurate time stamp at MAC layer recorded message: in network, choose sending node A and (that is: can accurately obtain the node of physical time, as gateway node) as time source, with unidirectional delivery mode, regular time (as: present embodiment is 30 minutes), periodic broadcast time synchronized beacon (time synchronization beacon TSB) at interval; In order to improve synchronization accuracy, at the MAC layer overall time stamp that writing time, sync beacon was carried;

2) the structure time stamp is right: behind the time synchronized beacon of receiving node B (that is: the node in the time source broadcast domain) time of receipt (T of R) source issue, in MAC layer sync beacon writing time time of advent, extracting the overall time stamp that carries in the local time stamp inserted by sending node B in the message and the time synchronized beacon, to form time stamp right.

Recorded message sends and receives precise time, obtains the right method of the used time stamp of linear regression specifically referring to Fig. 1.Sending node A writes down the time at that time after the start of frame bits or sync bit that send MAC layer message, and with this time value (having overall time stamp) insertion message tail subsequently.Receiving node B is same record time value at that time behind start of frame bits of receiving message or sync bit.After treating that whole message receives, extract the local time stamp of inserting by sending node A in the message.It is right that the overall time stamp that this this locality time stamp and receiving node B write down is previously formed time stamp jointly.

3) set up the linear relationship equation of local zone time and length of a game: after receiving node B collects several time synchronized beacons, utilize the local zone time at the method match receiving node B place that adds up and the relation equation of sending node A place length of a game, the frequency drift rate of On-line Estimation clock crystal oscillator, and obtain unified time in the network with relation equation, finish with time source synchronously, obtaining becomes synchronization node with the synchronous node of time source; After this, the each time of receipt (T of R) sync beacon of described synchronization node is all upgraded the coefficient of the relation equation of local zone time and time synchronized beacon transmitting node A place length of a game.

Specify linear regression method of the present invention below:

If: two nodes that are in the same broadcast domain are respectively sending node, receiving node, sending node

A transmitting time sync beacon is filled in overall time stamp t before the actual transmission of message _GlobalWhen receiving node begins to collect the time synchronized beacon, at first write down the local time stamp t of this moment _Local, form time stamp to (t _Local, Δ t), wherein Δ t is local time stamp t _LocalWith overall time stamp t _GlobalPoor.When sending node collected a plurality of time stamps to after, sending node is set up the relation equation of following local zone time and sending node A place length of a game:

T _global＝ρ×T _local+ΔT (2)

In the formula, T _GlobalBe length of a game's (time synchronously), T _LocalBe local zone time (not carrying out the synchronous time) that ρ is that the clock (promptly being used to obtain length of a game) of sending node is with respect to the drift of (promptly being used to obtain local zone time) of receiving node clock; Δ t is the time difference at sending node and receiving node two places.These two parameters can be asked for by the least square method of standard, that is:

$\mathrm{\ρ}=\frac{\mathrm{Lxy}}{\mathrm{Lxx}}---\left(3\right)$

ΔT＝T _global-ρT _local (4)

In the formula (3),

$\mathrm{Lxx}=\underset{i=1}{\overset{n}{\mathrm{\Σ}}}{(T_{\mathrm{local}}-{\stackrel{\‾}{T}}_{\mathrm{local}})}^2---\left(5\right)$

$\mathrm{Lyy}=\underset{i=1}{\overset{n}{\mathrm{\Σ}}}(T_{\mathrm{local}}-{\stackrel{\‾}{T}}_{\mathrm{local}})(T_{\mathrm{global}}-{\stackrel{\‾}{T}}_{\mathrm{global}})---\left(6\right)$

In the formula (4),

${\stackrel{\‾}{T}}_{\mathrm{local}}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\Σ}}}{T}_{\mathrm{local}}^i---\left(7\right)$

${\stackrel{\‾}{T}}_{\mathrm{global}}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\Σ}}}{T}_{\mathrm{global}}^i---\left(8\right)$

In periodic sync beacon broadcasting process, the parameter ρ of relation equation and Δ T bring in constant renewal in.When being obtained the T of length of a game by synchronous node _GlobalThe time, it can be with local zone time T _LocalThe temporal information through type (2) of record is converted to the T of length of a game _Global

Wherein overall time stamp t _GlobalBe the T of length of a game _GlobalAn occurrence of variable, local time stamp t _LocalBe local zone time T _LocalAn occurrence of variable.

4) having obtained synchronous receiving node B becomes time source, periodically other receiving node B issuing time sync beacon in its broadcast domain.

5) in the Mesh network configuration, along with the diffusion of time synchronized beacon, the node hop-by-hop of time source periphery obtains synchronously in the network, the node up to the edge of network.

Fig. 3 has illustrated the realization flow of the time synchronized of Mesh network.Label is that the receiving node B that 1 time source one is jumped in the communication range obtains earlier synchronously, outwards diffusion time sync beacon, label is that 2 the receiving node B apart from the time source double bounce communication range could obtain synchronously then.

Claims (3)

1. industry monitoring wireless Mesh netword low-spending high-precision time synchronization process is characterized in that: send and receive accurate time stamp at MAC layer recorded message, it is right to make up time stamp, sets up the linear relationship equation of local zone time and length of a game; Be specially:

1) send and receive accurate time stamp at MAC layer recorded message: in network, choose can accurately obtain physical time sending node (A) as time source, with unidirectional delivery mode, gap periods airtime regular time sync beacon; At the MAC layer overall time stamp that writing time, sync beacon was carried;

2) the structure time stamp is right: behind the time synchronized beacon of the receiving node time of receipt (T of R) source issue in the time source broadcast domain, in MAC layer sync beacon writing time time of advent, extract the local time stamp of inserting by sending node (B) in the message, and with the time synchronized beacon in the overall time stamp that carries to form time stamp right;

3) set up the linear relationship equation of local zone time and length of a game: after receiving node (B) is collected several time synchronized beacons, utilize the local zone time of the method match receiving node of adding up (B) and the relation equation that sending node (A) is located length of a game, the frequency drift rate of On-line Estimation clock crystal oscillator, and obtain unified time in the network with relation equation, finish with time source synchronously, obtain synchronous with time source synchronization node; After this, the each time of receipt (T of R) sync beacon of described synchronization node is all upgraded the parameter that local zone time and time synchronized beacon transmitting node (A) are located the relation equation of length of a game;

The relation equation that described local zone time and time synchronized beacon transmitting node (A) are located length of a game is:

T _global＝ρ×T _local+ΔT

Wherein: in the formula, T _GlobalBe length of a game, T _LocalBe local zone time, ρ is the drift of the clock of sending node with respect to the receiving node clock; Δ T is the timing differential of sending node and receiving node two nodes; The T of length of a game _GlobalWith overall time stamp t _GlobalBe occurrence, local zone time T _LocalWith local time stamp t _LocalBe occurrence; Δ t is local time stamp t _LocalWith overall time stamp t _GlobalPoor;

4) the synchronous receiving node (B) that has obtained becomes time source, periodically other receiving node (B) issuing time sync beacon in its broadcast domain;

5) in the Mesh network configuration, along with the diffusion of time synchronized beacon, the node hop-by-hop of time source periphery obtains synchronously in the network, the node up to the edge of network.

2. by the described industry monitoring of claim 1 wireless Mesh netword low-spending high-precision time synchronization process, it is characterized in that: with the local zone time of the method match receiving node of adding up (B) and the relation equation that sending node (A) is located length of a game, the method for described statistics adopts linear regression method.

3. by the described industry monitoring of claim 1 wireless Mesh netword low-spending high-precision time synchronization process, it is characterized in that: the described T of length of a game _GlobalBe the time after synchronous; Local zone time T _LocalFor not carrying out the synchronous time.

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