CN1228894A

CN1228894A - Master-slave synchronisation

Info

Publication number: CN1228894A
Application number: CN 97197625
Authority: CN
Inventors: 尤克·朱哈尼·卡潘恩
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1996-07-19
Filing date: 1997-07-03
Publication date: 1999-09-15
Also published as: FI104592B; FI962910A0; FI962910A; AU3346797A; WO1998004064A1; EP0917779A1

Abstract

The invention relates to synchronization of network elements in a network that uses master-slave synchronization. The use of the known monitoring bit MCB (Master Clock Bit) is expanded in the method so that it is transmitted all the way from the master network element, for example the mobile switching center (MSC). If one of the network elements located between the master network element and a specific slave network element, for example a base station, does not accept the received signal as the synchronization source because of its quality, the network element in question forces the MCB bit located in the signal that the element transmits further to state 1. If the transmission unit of a specific slave network element is locked to the signal which includes the MCB in state 1, or if the element is forced to revert to using the internal clock because of a fault situation of the signal, the transmission unit activates the alert. Before the start of and during the synchronization of the base station clock, the fault status is read from the transmission unit and the synchronization is prevented or interrupted if the alert is on. The alert is given to other units of the base station in question from which the fault monitoring unit further transmits it through a separate operations and maintenance network to the operations center of the network operator.

Description

The principal and subordinate is synchronous

The present invention relates to remain on a phase mutually synchronization between all network element of using the synchronous communication network of principal and subordinate.

A communication network is typically formed by the plurality of network unit with all main lines that these unit are connected with each other.Network can be synchronous, as a synchronous digital classification (SDH) network, or accurate (plesiochronous) synchronously, as a PDH (PDH) network.

The PDH network also comprises digital mobile communication network except other thing, this digital mobile communication network is used as the example that application network of the present invention is described.Please refer to Fig. 1, Fig. 1 represents from the sight of the emission figure of a simplification of GSM network (Groupe Speciale Mobile (special horde)).The subsystem NSS of network is made up of the portable MSC of switching center, and by the system interface at this center, mobile communications network is connected to all other networks, as public switched telephone network PSTN.

The subsystem NSS of network is connected to the subsystem BSS of base station by interface A, the subsystem BSS of base station is made up of all base station controller BSC, and each in all base station controller BSC is all controlled all base transceiver board BTS that are connected to these controllers.Interface between all base stations that are connected at base station controller with it is an Abis interface.In portable switching center, layer 1 physical interface between all radiating portions of base station controller BSC and base station is the line of a 2Mbit/s, promptly 32 of 64kbit/s time slots (=2048kbit/s).On the other hand, all RF parts of all base stations are subjected to the control of base station controller BSC fully, mainly are made up of all transceiver T RX, and all transceiver T RX are implemented to the radio interface of travelling carriage.

The network that is formed by all base stations that are subjected to all base station controller control can be a star, shown in the system on the left side in the drawings, in this case each base station is directly connected to base station controller, it perhaps can be a loop, this loop is made up of all base stations that interlink together, and this loop or terminate in all base stations one as in the system shown in the mid portion of figure like that, perhaps terminate in the base station controller.Network also can be a link, a main line in this case from base station controller to all base stations, from that base station to another base station etc., as shown in the base station system on Fig. 1 the right.Can be with any in above-mentioned all base station network types or their combination when needing in a mobile communications network.

As in other all PDH networks, all network element also should be carried out work in the mode of phase mutually synchronization in a mobile communications network, so that avoid all frame slips.In addition, all base stations need one synchronously accurately, so that for the signal of air interface formation right type and with enough accuracy it is remained in the frequency band that keeps into it.

There are some kinds of diverse ways to can be used for making all network element phase mutually synchronization of a PHD communication network.A clock signal of separating can be input to network element from an energy by the shared accurate clock source of whole network.Each network element can both have an accurate clock source that separates, perhaps they can, for example, receive synchronizing signal by the gps satellite system.Yet, because all above-mentioned methods all are quite expensive, so usual way is synchronous with so-called principal and subordinate.In the principal and subordinate was synchronous, the higher unit of network classification provided synchronizing signal to the lower unit of a network classification, as a part that transmits, only needed an accurate timing source in this case in the highest network element of classification.In the situation of mobile communications network, master clock is in the portable MSC of switching center, provides clock signal to all base station controllers, and all base station controllers further are transmitted to this clock signal all base stations that are subjected to their control.All network element directly from the frequency of the bit rate receive clock of the main line of 2Mbit/s and position mutually.Therefore in the situation of a base station network that forms link, clock signal pass course MSC → BSC → BTS1 → BTS2 → or the like transmission.

We should be noted that a network element necessarily can not receive a synchronizing signal from several directions simultaneously.Even base station network is a loop, but with regard to master clock signal, it is not a loop, and loop is interrupted at a point, so that all base stations only receive their clock signal from a direction.Yet with regard to speech and data passes, loop is not interrupted, although lost in this case with master clock synchronously, a fault in all links can not exert an influence to voice delivery.

Shortcoming during principal and subordinate in a mobile communications network is synchronous is in all network failure situations, for example when fault occurs in the connection of MSC BSC, for example the isolated part from the downward network of BSC makes oneself synchronously according to the clock that is formed by certain network element, and therefore all base stations float to outside their frequency band.Yet, base station even can keep its synchronous and all frequencies by all long interruptions in synchronization link.Yet, pass through the imagination synchronizing signal from (rated frequency) high-quality clock source when a base station, its frequency is carried out timing produce a harmful situation, and in fact be not this situation.

The frequency drift of a base station is to frequency band outside, except other thing, may cause that also the operator's of operator's a network and a competition network enters a state some hrs that can not use.The state that can not use is paid the compensation of other network operator possibly and is lost new all users because lost reputation by the expense of all losses, and the operator is caused all economic losses.

In known master-slave method, if there is communicating to connect of and base station controller the base station, then the base station draws and allows synchronous conclusion.In all 20 minutes intervals, repeatedly take place synchronously, make the position of base station clock mutually with (position of the clock of MSC → BSC → BTS) receive is carried out n time mutually and compared, and to these phasic difference calculating mean values from network.If according to last mean value, phasic difference need is large compensation too, then this result is abandoned.Therefore, keep communicating to connect in mutually at whole sync bit exactly for activating synchronous event and its successional needs.

The weakness of this known method is in some network failure situation or in a combined situation of several simultaneous mistakes, all signals are switched on, work is carried out in communicating to connect in the BSC BTS of interval, and still some in all base stations have lost their synchronism.When if two unidirectional faults take place in fact simultaneously and do not allow to comprise signal to the alarm signal of far-end as a clock source, above-mentioned situation may, for example, in a loop network, take place.When all network corrections operation and particularly these operations afterwards in all intervention nodes may stay on its internal clocking and nobody also similar situation can take place when noticing it.

Can improve the synchronous reliability of principal and subordinate by in clock signal, adding a special control bit MCB (MasterClockBit (master clock position)).We are described this problem with reference to Fig. 2.Control bit MCB is added on the clock signal of the base station BTS 0 of loop network, and base station BTS 0 is connected to base station controller BSC, and therefore loop from base station BTS 0, in base station BTS 0 end.This control bit is in the logic state of agreeing, for example, and in the state 0.In all slave stations in loop network one, for example, base station BTS 1 receives its value when being 0 MCB control bit, and the base station knows that clock signal is from real master clock.The emitter TRU of base station makes it own synchronous with master clock, and further the internal bus B by the base station is transmitted into all clock oscillators with clock signal as a reference signal, and these clock oscillators are to connect by all other PLL of funtion part that use is arranged in the base station to realize.In addition, the base station is transmitted into next base station BTS 2 with clock signal and its MCB position that is arranged on " 0 ".The emitter TRU of next base station detects reception from the MCB position clock signal is real signal, and further clock signal and MBC position is transmitted into next base station BTS 3 etc.Base station BTS 2 can not be synchronous with the clock signal that arrives from direction 2 (from the direction of BASE STATION3 (BTS3)), because this signal forms loop in BASE STATION3, and by with position LCB=1 this point being indicated to BTS2.

In each base station, all clock inputs of emitter all input directions have been distributed to, in the drawings with

numeral

1 and 2 two of representing in them.If more direction is arranged, then they are showed continuously with numeral.Give priority to all directions, make that direction 1 is such direction, clock signal is from this direction, and system mainly is synchronous with this direction, and direction 2 is such directions, and clock signal is transmitted into this direction, and system supplymentary ground and this direction are synchronous.One in all clock inputs in priority list is the internal clocking of emitter.The all clock input in loop of all devices by them coupled together in mode shown in Figure 2.

If the link between two base stations of loop, for example, the link between all base station BTS 1 and BTS2 has been interrupted, and first base station BTS 2 that then is positioned at point of interruption back is not accepted clock signal.In this case, the emitter TRU of base station begins with its internal clocking and further it is launched as clock signal, but the MCB position is changed over " 1 ".BTS2 can not be synchronous with the clock signal that arrives from direction 2 (from the direction of BTS3), because BTS2 forms loop by BTS 3, this is by showing in feedback with position LCB=1.From this value of MCB position, it is not original clock signal that next base station detects used clock signal, although base station and this clock signal are synchronous.As a summary, we can notice that the MCB position is to launch from the main platform of network, the MCB position is that in all base stations from the link that is formed by all base stations that are in 0 state one launches in a mobile communications network, when Network Transmission is passed through in this position, whether the clock that it points out to be included in the signal of being discussed is to take place in the main platform (MCB=0) of network, whether point out perhaps that a failure condition has forced its internal clocking of usefulness (MCB=1) in all network element, be positioned at network element and this clock synchronization after the fault in this case.

If base station network links, it is enough then in mobile communications network the MCB position being added on the clock signal.In the situation of a fault, network automatically is divided into two parts in all links, and all network element that are positioned at the fault back are locked on the internal clocking that is positioned at fault base station the most nearby.

In the network of a formation loop, in the situation of a fault, from other direction, in other words, from direction 2, it is possible taking master clock signal to all base station, sees also Fig. 2.In this case, the clock signal that must will arrive from this direction in one or another kind of mode is that all base stations must be informed synchronously with it in master clock signal and all base stations after all.This can be added on the clock signal of leaving as the base station BTS 0 of main platform work by monitor bits LCB except the MCB position that another is specific (Loop Clock Bit (loop clock position)) and realize, sees also Fig. 2.When signal left main platform, two monitor bits all were in 0 state.0 of LCB position is worth to all reception base station BTS 1 ..., BTS3 points out that clock signal is not the clock signal of a rotating formation loop.In normal state, the further transmit clock signal in each base station of loop and with MCB position and LCB position the two all remains in 0 state.In addition, also go back the clock signal emission in the direction that clock signal arrives in the base station, and the MCB position remained in the state 0 as one the base station is locked in signal on the original master clock signal.On the other hand, the LCB position is changed to 1, it points out that clock signal is a rotating master clock signal, and the base station of front is not locked to this signal in this case.If the value of LCB position be 0 and the value of MCB position be 0, this means that then the base station must make its this clock signal own and that arrive from direction 2 synchronous, as it arrives from master clock.

Can not solve when base station network is one how to keep synchronous problem when forming the network of loop and two faults taking place simultaneously in loop according to the device of above-mentioned former technology.In Fig. 3, first fault between BTS0 and BTS1 prevent clock signal from direction 1 to base station BTS 1 emitter, and cause in the direction at BTS3 from BTS0 emission remote-end alarm position FEA at the second failure between BTS3 and the BTS0, therefore do not allow BTS3 to be locked on the signal that arrives from the direction of being discussed.Difficulty is by such fact, and promptly with regard to voice, loop is a real loop, and in the situation of an interruption, is formed into automatically by other branch road that the connection of base station controller causes.For synchronously, loop is not a real loop, but it is by link BTS0 ..., BTS3 forms.So, can interruptedly not proceed communication at an intercourse, but all base station BTS 1 to BTS3 do not receive master clock signal.

At first, base station BTS 1 is returned to and uses internal clocking and will the two is transmitted into direction 2 in the MCB position in the state 1 and LCB position, in other words, is transmitted into base station BTS 2.BTS2 also returns to and uses internal clocking and further ladies and gentlemen above-mentioned are not had all variations ground to be transmitted into base station BTS 3 in direction 2.BTS3 is returned to and uses internal clocking and will be included in MCB position in the state 1 (=signal is not a master clock signal) and the clock signal emission of the LCB position in state 0 (=signal is not rotating clock signal) is gone back.This signal arrives base station BTS 2 from be in deputy direction priority list, so this base station oneself is locked in it on this clock signal and further base station BTS 1 is got back in this signal emission.All values of all monitor bits equal 1/0, so BTS1 also is locked on this clock signal.Last result and Fig. 3 meet, and in Fig. 3, make the internal clock signal of all base station BTS 1 to BTS3 and base station BTS 3 synchronous.Also can see above-mentioned problem like this, make network as a result as first fault as a link by synchronously.When second failure takes place, the device of the link of close fault, BTS3 is returned to all other the devices that use internal clocking and will be transmitted into link in the MCB position of state 1, and these chain circuit devices keep the synchronous of them in the direction of being discussed.

The part that the network management personnel does not receive network no longer is in the advise of same of the such fact of synchronous regime.This situation may continue a few weeks longer or even the several months, up to the part of the network of being discussed till seriously its frequency band has been left in drift.

The purpose of this invention is to provide a method, being returned to soon in all different failure situations synchronously of base station network that forms link and formation loop with one of this method used master clock, and this method guarantees that base station receives the authentic communication about the synchronizing signal quality in using.The failure situations that we have guaranteed above-mentioned principal and subordinate's synchronising base station network in this mode can not cause that the base station drift leaves its frequency band.

In order to realize this purpose, we make the synchronizing signal group selection clock signal of undernet unit from the candidate, these candidates' synchronizing signal comprises master sync signal, all synchronizing signals of the internal clock signal of undernet unit and all other network element of arriving from all different directions in network.

All undernets unit has been coupled together the network that forms loop as, formed in the network of loop, will be input to a undernet unit by the master sync signal that the master network unit provides at this.All there is a main direction each undernet unit, and each undernet unit is primary and the master sync signal that arrives from this main direction is synchronous.Also all there is an auxiliary direction each undernet unit, the undernet unit has been selected this unit to be transmitted into this auxiliary direction as the signal of synchronizing signal, and each undernet unit is first and foremost non-but on the back burnerlyly and the signal Synchronization that arrives from this auxiliary direction.

System will about signal be master sync signal or by other a slave unit for the information of the internal clock signal that uses to select with about whether allowing to select clock signal to be added on each candidate's who arrives from the direction of network the synchronizing signal as the additional information of synchronizing signal.If select certain signal that is different from master sync signal as synchronizing signal, then the undernet unit produces an alarm signal.

The method according to this invention is used the network monitoring position MCB (master clock position) of known formation loop.

First, the use of expansion MCB position, make from the master network unit, for example, the emission of MCB position is gone from portable switching center (MSC), if and in all network element between portable switching center and specific undernet unit one, for example, a base station is not because the quality problems of received signal are accepted this received signal as synchronisation source, then when launched the MCB position base station again, described network element forced the MCB position in signal to change to state 1.

Second, if the emitter of a specific base stations is locked on the signal that is included in the MCB in the state 1, if perhaps because a failure condition in received signal forces the base station to be returned to the use internal clocking, then emitter activates report to the police (FEr, frequency error).Give all other devices of in question base station with this alarm transmission, further alarm transmission is given network operation personnel's operation center from these plant failure monitoring arrangements by an operation and maintenance network that separates.

The 3rd, base station clock begin preceding synchronously and when base station clock was synchronous, if exist to report to the police, then the base station was read all error conditions of emitter and is prevented or interrupt synchronously.

We combine with reference to appended all figure and with a preferential embodiment now, and present invention is described.

Fig. 1 represents a mobile communications network,

Fig. 2 represents from the base station network of the viewpoint of known method for synchronous,

Fig. 3 is illustrated in two synchronous situation in the simultaneous situation of fault,

Fig. 4 represents immediately the situation in the fault back,

Fig. 5 represents the situation when having arrived last base station about the notice of interrupting,

Fig. 6 represent when network by the situation synchronously time the once more,

Fig. 7 be illustrated in the network that forms link fault and

Fig. 8 represent when network by the situation synchronously time the once more.

The network that is used in the example is a mobile communications network.When moment t=t0 fault takes place between dominant base BTS0 and first base station BTS 1, we are described in detail the everything that takes place in the network at the formation loop of Fig. 2 behind the failure condition.In normal state, as shown in Figure 2, the base station receives and further is transmitted in all MCB and the LCB position in the state 0, but MCB position in state 0 and the LCB position in state 1 are returned.

Fig. 4 is illustrated in the situation of fault back.Fault one takes place, and base station BTS 1 has just lost the master clock signal that arrives in the input 1 of emitter, and follows it together, has lost all monitor bits.When the base station detects when losing, it is returned to uses internal clocking and internal clock signal is transmitted into next base station, and it is not an indication of this fact of master clock signal as clock signal that the MCB signal is transformed to state 1 from state 0.In addition, the emitter TRU of base station BTS 1 sends alarm signal FEr to the monitoring arrangement of base station.All these will spend the time of about 200ms.For the sake of clarity and normal condition (Fig. 2) add straight line below more all values of altered all monitor bits.Can see that as us all base station BTS 2 and BTS3 also do not react to fault, but they are transmitted to information " OK " their all monitoring arrangements.

Fig. 5 represents a situation, has arrived base station BTS 3 about the information that takes place to interrupt in this case in monitor bits MCB.BTS2 has received by the clock signal of BTS1 emission with about such fact, and promptly signal is launched the information of going back with this clock signal and the MCB position in state 1 from internal clocking and it.The clock synchronization of base station BTS 2 and base station BTS 1, and emitter sends alarm signal FEr to the monitoring arrangement of base station.Last base station BTS 3 of link has received the MCB position state 1 from direction 1 emission by BTS2.The priority of all clocks input of base station BTS 3 is such: if not from direction 1 but receive master clock signals from direction 2, then can reciever to 2 as input.In this mode, the base station with it synchronously and the master clock signal that receives from direction 2 couple together, and it will be in the LCB position in the state 1 to this direction emission.Simultaneously it will be transmitted into the direction 1 feedback branch from the master clock signal that direction 2 receives, and and it together will be in the MCB position in the state 0 (signal is the indication of this fact of master clock signal) and the LCB position in state 0 does not form loop as clock signal and therefore allow BTS2 to be locked in an indication of this fact on this clock signal to 1 emission of the direction in feedback branch.

Interim when these incidents take place, the emitter of base station BTS 3 sends maintained synchronously sign of " OK " signal conduct and master clock signal to monitoring arrangement in the whole time.The internal clocking of all base station BTS 1 and BTS2 and BTS1 is synchronous, and all emitters provide fault-signal FEr.

Everything continues to go on from the situation of Fig. 5, makes base station BTS 2 be connected with master clock signal by the BTS3 emission, and emitter to monitoring arrangement send " OK " signal as base station BTS 2 by an indication of synchronous this fact.This base station also will the master clock signal in feedback branch be transmitted into base station BTS 1 and the MCB position that is included in the state 0 is that indication of real this fact of master clock signal and the LCB position in state 0 are as allowing the indication of this fact synchronously of base station and this clock signal as in question signal in this signal.In this mode and master clock synchronously from base station BTS 3, arrive base station BTS 2, now near base station BTS 1.

Fig. 6 represents one as last base station, the situation after base station BTS 1 has made oneself and master clock signal synchronously.Master clock signal in all feedback branches transmission and arrived first base station from all monitor bits state 0 that BTS3 begins.The emitter of this base station also and from the master clock signal that direction 2 receives sends " OK " signal synchronously and to monitoring arrangement.This base station is with master clock signal, and the MCB position in state 0 (clock signal of being discussed is an indication of this fact of master clock signal) and the LCB position in state 0 (clock signal is not an indication of this fact of clock signal of returning immediately) is transmitted into dominant base BTS0.

In above-mentioned mode, synchronously from last base station BTS 3 beginning and when each base station backward court during all monitor bits of base station of close fault from a base station transmits to another base station.

All still there is such fact in now all base stations regardless of interruption synchronously with master clock signal.In the situation of this example, changed the about 200ms of expense for all variations in each base station, so, amount to the time of having changed the about 5 * 200ms=1sec of expense more synchronously.

When fault is excluded, all base stations of loop all and the clock signal that arrives from direction 1 rather than from direction 2 synchronous.The synchronous base station below fault, in above-mentioned example from base station BTS 1, and when all monitor bits MCB and LCB during from a base station transmits to another base station to last base station transmits.This mechanism is identical with above-mentioned mechanism, does not need it is described in more detail.In this mode, all base stations keep they synchronously and no matter changed the input direction of master clock signal.

Fig. 7 is illustrated in a base station network, and all base stations form link in this base station network.Fig. 7 represents a situation, and all synchronous variations have taken place to interrupt and taken place between all base station BTS 1 and BTS2 in this case.Because in question network is not a network that forms loop, so without the LCB position.In other words, before fault takes place, in normal situation, as mentioned previously master clock signal from a base station transmits to another base station.Each base station is also all launched unaltered clock signal and its MCB position backward.Do not allow and to be used for synchronously at the clock in the direction 2 of base station.

When a fault has taken place, be located immediately at base station BTS after the fault and be returned to and use internal clocking and emitter to send alarm signal FEr to monitoring arrangement.Also be not transferred to BTS3 about the information of fault constantly at this, so send " OK " signal to monitoring arrangement at its emitter of this point.In case its internal clock signal and the MCB position in state 1 have been transmitted into the clock input of the direction 1 of BTS3 as BTS2, when not being one of this fact of master clock indication as the clock of being discussed, BTS3 just sends alarm signal FEr and returns MCB position at state 1.In the situation of this point as shown in Figure 7, the part of network in Fig. 7, promptly all base station BTS 2 are synchronous with one of the other parts of coming automatic network different clock signal with BTS3.

When fault was repaired, base station BTS 2 was received in the master clock signal in the input of direction 1 once more.The input of its emitter and direction 1 is connected and sends " OK " signal to monitoring arrangement.This spends about 200ms.In this point, base station BTS 2 and master clock are synchronous, but base station BTS 3 discord master clocks are synchronous, in fact in its inside warning arranged still.Base station BTS 2 is given in MCB position in the state 0 as feedback data.In the situation of this point as shown in Figure 8.

Just after base station BTS 2 had changed to master clock signal and its numerical value 0 MCB position and is transmitted into base station BTS 3 from 1, the master clock signal of a back base station and reception was synchronous, and by sending " OK " signal warning is removed.BTS 3 still gets back to the MCB position in state 0 as feedback data, has turned back to master clock signal synchronously fully in this point.

The method according to this invention also can be used for improving the synchronous readability in all network failure situations.

Claims

1. method that makes all network element synchronization in a communication network, this network comprises

-one master network unit, it provides master sync signal to network,

The network element of-several subordinates, each in them all produce an internal clock signal and they and comprise and be used for receiving all synchronizing signal candidates that arrive from several different directions of network and the equipment that is used for selecting from all candidates synchronizing signal.

Wherein

The network element of all subordinates has been coupled together the network that forms loop as, in this network, will be input to the network element of a subordinate by the master sync signal that the master network unit provides,

The network element that the network element of each subordinate all has a main direction and each subordinate master sync signal all primary and that arrive from this main direction synchronous and

The network element of each subordinate all has an auxiliary direction, the signal that the network element of subordinate has selected the unit as synchronizing signal is transmitted into this auxiliary direction, and the network element of each subordinate is all first and foremost non-but on the back burnerlyly and the signal Synchronization that arrives from this auxiliary direction.

Basis the process of claim 1 wherein when network in a failure condition, prevent synchronously the time once more subordinate network element reality synchronizing process generation and

A) be positioned at the fault back all subordinates network element and to be positioned at the internal clocking of network element of first subordinate of fault back synchronous,

B) network element of last subordinate receives the master sync signal from auxiliary direction, makes it own synchronous with this synchronizing signal, and further this synchronizing signal is transmitted into the network element of the subordinate of the front in main direction,

C) network element of the subordinate of front makes it own and synchronous from the master sync signal of auxiliary direction reception, and further this synchronizing signal is transmitted into the network element in the subordinate formerly of main direction,

D) repeat operation in step c, up to the network element of first subordinate that is positioned at the fault back and the master clock signal that receives from auxiliary direction synchronously till.

3. a basis the process of claim 1 wherein

Will about signal be master sync signal or the information of the internal clock signal of selecting for use by the unit of another subordinate and about whether allow to select this signal to be added to as the additional information of synchronizing signal on each the synchronizing signal candidate that arrives from network and

If certain signal of selecting to be different from master sync signal is as synchronizing signal, then the network element of subordinate produces an alarm signal that is launched into network operation personnel's operation center.

4. method according to claim 3, wherein each the synchronizing signal candidate that arrives from the direction of network all comprises monitor bits (MCB), when monitor bits is in state 0, monitor bits points out that candidate is a master sync signal, with when monitor bits is in state 1, monitor bits points out that candidate is the internal clock signal of the network element of another subordinate.

5. method according to claim 3, wherein the monitor bits (MCB) in the master sync signal that will be provided by the master network unit is set in state 0.

6. method according to claim 3, second information that wherein is added to each the synchronizing signal candidate that arrives from the direction of network comprises a monitor bits (LCB), when monitor bits is in state 1, monitor bits is pointed out by the network element of subordinate the synchronizing signal candidate to be returned, and necessarily can not select in question candidate as synchronizing signal in this case.

7. basis the process of claim 1 wherein that communication network is that the network element of a mobile communications network and subordinate is a base station (BTS).

8. method according to claim 7, wherein the master network unit is a portable switching center.

9. the method according to claim 7 wherein is added to all monitor bits on the master sync signal in portable switching center.

10. the method according to claim 7 wherein is added to all monitor bits on the master sync signal in base station controller.