GB2484523A

GB2484523A - Signaling device to improve timing and synchronization in systems dependent upon a remote periodic timing signal

Info

Publication number: GB2484523A
Application number: GB1017394.6A
Authority: GB
Inventors: Roderick Whitlock
Original assignee: Tracker Network UK Ltd
Current assignee: Tracker Network UK Ltd
Priority date: 2010-10-14
Filing date: 2010-10-14
Publication date: 2012-04-18
Anticipated expiration: 2030-10-14
Also published as: WO2012049319A1; GB201017394D0; GB2484523B

Abstract

A signaling device arranged to receive in-bound periodic timing signals and issue out-bound timing signals either in response to receipt of an expected timing signal, within a predetermined time window or automatically if an expected timing signal is not received within the predetermined time window. The device preferably comprises a timer to determine the start time, which may be a predetermined time after a previous trigger or out-bound event, and duration of the window. Preferably the windows duration is less than the expected jitter of the periodic timing signals. Preferably, if the timing signal is received before the start of the window the trigger occurs at the start of the window and if the timing signal is not received before the end of the window the trigger occurs at the end of the window.

Description

Network Timing

Field

The present invention is in the field of timing and synchronisation and relates particularly, but not exclusively, to improvements in timing and synchronisation in systems that depend on a remote periodic timing signal.

Background

The network timing protocol (NTP) is one well-known way of obtaining reliable periodic timing signals from a remote timing source. Such timing signals find application in many systems and applications. A benefit of NTP is that the timing signals originate from timing servers employing extremely accurate atomic clocks, and so, on average and over time, the timing signals can be relied upon to be extremely accurate, as they suffer from exceptionally low drift. Nevertheless, the channel by which the signals are received can introduce a degree of timing variability and thus a timing inaccuracy. For example, the period between received timing signals, which should be constant, can vary according to the volume of other traffic that is travelling over the channel that is used to carry the timing signals. Such a variation in timing will be referred to herein as timing jitter'. In US 2009/0276542, for example, the problem of timing jitter in packet networks (which is referred to therein as packet delay variation (PDV)) is identified and a solution is proposed incorporating a phase-locked loop (PLL) architecture to reduce the effects ofjitter.

An object of the present invention is to provide an alternative solution to reducing the effects of timing jitter.

Summary

According to a first aspect, the present invention provides a signalling device comprising: a receiver to receive in-bound periodic timing signals from a remote timing source; and a transmitter responsive to a trigger event to transmit out-bound signals; wherein a trigger event occurs either on receipt of an expected timing signal within a predetermined time window or automatically if an expected timing signal is not received within the predetermined time window.

Other aspects and embodiments of the invention will become apparent

from the following description and claims.

Brief Description of the Drawings

Various features and advantages of the invention will become apparent from the following description of embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings, of which: FIG. 1 is schematic diagram of a stolen vehicle recovery system; FIGs. 2a -2c are graphs which illustrate respectively, a periodic timing signal, a transmission which is triggered by receipt of a periodic timing signal, and a receiver window, suitable for receiving the triggered transmission, in a jitter-free environment; FIGs. 3a -3c are graphs which illustrate respectively, a periodic timing signal, a transmission which is triggered by receipt of a periodic timing signal, and a receiver window, suitable for receiving the triggered transmission, in an environment in which the periodic timing signal is subject to jitter; FIGs. 4a -4c are graphs which illustrate respectively, a periodic timing signal, a transmission which is triggered by receipt of a periodic timing signal, and a receiver window, suitable for receiving the triggered transmission, in an environment in which the periodic timing signal is subject to jitter, according to an embodiment of the present invention; FIG. 5 is a functional block diagram of a transceiver system suitable for receiving timing signals according to an embodiment of the present invention; FIG. 6 is a flow diagram of a process for operating a transceiver system according to an embodiment of the present invention; and FIG. 7 is a functional block diagram of a stolen vehicle recovery transceiver unit, according to an embodiment of the present invention.

Detailed Description

Various embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. It will be appreciated that the invention is not limited in its application to the details of method and the arrangement of components as set forth in the following description or illustrated in the drawings. It will be apparent to a person skilled in the art that additional embodiments of the present invention not detailed in the description are possible and will fall within the scope of the present claims. Accordingly, the following description should not be interpreted as limiting in any way, and the scope of protection is defined solely by the claims appended hereto.

A generally known stolen vehicle recovery (SVR) system, in which embodiments of the present invention find application, is illustrated in FIG. 1.

One kind of commercially available SVR product is TRACKER LocateTM, produced and sold by TRACKER Network (UK) Limited. In this and similar known SVR systems, a vehicle-mounted transceiver unit (VMTU) is installed into a vehicle, and can be activated, for example by remote VHF activation signals, when the vehicle is reported as having been stolen. Once active, the VMTU emits a short-range radio signal, which can be detected by police forces and used to track and recover stolen vehicles.

The present applicant has appreciated the benefits of using battery powered VMTUs in vehicles. Such benefits include easier installation, as there is no need to tap into a vehicle's power lines. Nevertheless, a downside of using battery powered VMTUs is that a battery has a limited lifespan, after which the battery needs to be replaced. One challenge in using battery powered VMTUs is to extend the battery life by reducing power consumption. One operation of certain VMTUs is to receive periodic transmissions from fixed base stations.

Hence, there is an opportunity to conserve battery power by energising the receiver circuitry of a VMTU only during a period when the base station transmissions are expected. The longer the period in which the receiver circuitry is switched off, the more battery power can be conserved.

With reference to FIG. 1, an accurate time source 100, such as an NTP time source, is connected to a plurality of NTP time servers 105. Three time servers are illustrated in FIG. 1, but there may be fewer or more. The NTP time servers 105 provide timing signals to remote entities in response to time requests, in a known way. Timing signals can be communicated via local area networks, wide area networks, such as the Internet 110, via wireless radio channels, for example employing GPRS, or in any other convenient way. One of the time servers 105 is shown as being connected to a wireless transmitter 115, for example a GPRS transmitter, which can transmit timing signals. The present applicant has found that timing signals delivered by GPRS tend to be less accurate than those delivered via the fixed Internet. This is believed to be due to the variable volumes of traffic that have to be handled by the GPRS transmitter systems. Thus, timing signals that are delivered via GPRS are seen to suffer from timing jitter, whereby the period between periodic timing signals varies.

As shown in FIG. 1, a fixed base station transceiver (BSS) 120 (of which two are shown for illustration purposes only, and there may be more or fewer) is arranged to request and receive the timing signals from the time server 105. The BSS could instead, in other scenarios, be any kind of signalling device or apparatus that has the capability to receive an inbound timing signal and transmit an outbound signal at least occasionally using the inbound signal as a timing reference.

The BSS 120 is arranged to synchronise its operation using the received timing signals and use the timing signals to trigger short transmissions to the battery powered VMTUs (not shown in FIG. 1), which are mounted in vehicles 125. The short transmissions include, for example, status and control information and may, of course, include a signal to activate the short-range stolen vehicle signals. As already indicated, the VMTUs are arranged so that the radio receiving capability is switched on periodically, only during a pre-determined time window, for the purposes of receiving the BSS transmissions.

The received BSS transmissions are used by each VMTUs to align an on-board timer, which controls the timing of the time window.

The present applicant has measured timing jitter of NTP timing signals, which are received via a GPRS broadcast, of up to 400ms or more. In other words, it has been shown that the arrival time of any given NTP timing signal may experience an error of up to or beyond ±200ms from the expected time of arrival. The consequence of this can be significant, in terms of VMTU battery life, bearing in mind that the pre-determined time window has to be sufficiently wide to accommodate the maximum likely degree of timing jitter. For example, in the event a BSS transmission has a duration of 0.Ss, accommodating an error of +200ms nearly doubles the time a VMTU receiver circuit needs to be activated for to ensure that BSS transmissions are received. An effective doubling of the time window could reduce VMTU battery life by 50%; for example reducing the battery life from ten years to five years.

The impact of timing signal jitter will now be illustrated with reference to the graphs in FIGs. 2 and 3.

The graph in FIG. 2a represents a periodic NTP timing signal 200 (only three relatively short timing signals are shown), which is delivered periodically by a time server 105. This graph shows the timing signal in a perfect scenario, in which there is no timing jitter. This means that the time between timing signals is constant, that is time period wTl equals time period wT2. Let it be assumed for this example that the period wTl is lOOs.

The graph in FIG. 2b represents a BSS transmission 210, which is transmitted by a BSS 120 in response to receipt of timing signal 200. The BSS transmission has a slot period having a duration wT3, for example of SOOms. It will be apparent that none of the graphs are drawn to scale. For reasons of simplicity of explanation only, the graph does not show any time delay between the transmission of the timing signal by the time server 105 and the triggering of the BSS transmission by the BSS 120. As there is no jitter in the NTP timing signal 200, the period between the BSS transmissions is constant and also equal to wT 1.

The graph in FIG. 2c represents the time window 220 during which a receiver circuit of a VMTU is activated to receive the BSS transmission 210.

Again, no time delay is shown. As can be seen, the time window 220 has exactly the same duration as the BSS transmission 200, that being SOOms. Such a matching of the BSS transmission period and the receiver circuit activation period is permissible as the time of arrival of the BSS transmission is completely predictable, as being exactly wTl seconds after receipt of a previous BSS transmission.

Obviously, the graphs in FIGs. 2a-2c represent a perfect system, which is unattainable in reality.

In contrast, the graphs in FIG. 3 are illustrative of a system in which NTP timing signals suffer from jitter. As before, the graphs do not indicate any time delay.

The graph in FIG. 3a, represents a timing signal 300 which suffers from jitter, such that time period xTl no longer equals time period xT2. Indeed, the expected degree of jitter is indicated by bounds 302 around the timing signal 300, having a time period denoted by xT3. Let is be assumed that the jitter, xT3, for this example is 400ms. An NTP timing signal 300 may appear at any point within the bounds 302 specified by time period xT3. Of the three timing signals shown, it can be seen that, due to jitter, the first occurs significantly S earlier than expected, the second occurs significantly later than expected, and the third occurs only slightly earlier than expected. The expected times are indicated by dummy timing pulses shown at the mid-point of each bounded time period 302.

The graph in FIG. 3b represents a BSS transmission 310, which is transmitted by a BSS in response to receipt of the respective timing signal 300.

Again, the BSS transmission has a slot period of wT3 = SOOms. Clearly, the period between BSS transmissions is not constant and a bounded time period 312, having a period of at least xT4 = xT3 = SOOms, indicates the time during which each BSS transmission 310 may be triggered, due to the jitter exhibited by the timing signal 300.

The graph in FIG. 3c represents the time window 320 during which a receiver circuit of a VMTU must be activated in order to receive the BSS transmission 310. As can be seen, the time window 320 has to accommodate the possible receipt of the BSS transmission 310 at any time within a further time period denoted xTS. It will be appreciated that the time period xTS equates to a period of at least xT4 + wT3 (since it must accommodate BSS signal triggering at any point from the start to the end of the bounded timeframe 312).

Applying the exemplary values, in which xT3 = 400ms and wT3 = SOOms, it is clear that xTS has a period of at least 900ms. By comparison to the graph in FIG. 2c, it can be seen that the receiver circuit associated with the graph in FIG. 4c must be active for nearly twice as long.

An embodiment of the present invention will now be exemplified with reference to the graphs in FIGs. 4a-4c.

The graph in FIG. 4a is the same as in FIG. 3a, and shows a sequence of timing signals 400 that are subject to jitter. As before, of the three timing signals shown, it can be seen that the first occurs significantly earlier than expected, the second occurs significantly later than expected, and the third occurs only slightly earlier than expected. Time periods xTl-xT3 are the same as in FIG. 3a.

The graph in FIG. 4b illustrates a triggering of three BSS transmissions, according to the present embodiment. In FIG. 4b, a time window 412, which is defined to have a period yT4, is determined by the BSS to be the time period during which BSS transmissions must be triggered. In this example, the period yT4 may be, for example, lOms, in contrast to which xT3 may be 400ms.

The BSS 120 then employs the following logic for triggering a BSS transmission: -If a timing signal 400 is received in advance of the time window 412, then the BSS transmission 410 is triggered at the start of the time window 412 (see, for example, BSS transmission 410a); -If a timing signal 400 is received within the time window 412, then the BSS transmission 410 is triggered by the arrival of the timing signal (see, for example, BSS transmission 410c); or -If a timing signal 400 has not been received by the end of the time window 412, then the BSS transmission 410 is triggered automatically at the end of the time window 412 (see, for example, BSS transmission 4 lOb).

In this way, the BSS transmissions are triggered to take place within a time window 412, which is relatively short compared with the maximum jitter exhibited by the timing signals 400.

The BSS causes each subsequent time window 412 to start at a predetermined period, yT6, from the start of the last BSS transmission. The predetermined period is defined by the BSS by reference to the expected perfect period, wTl, between timing signals and the period of the time window, yT4, as follows: yT6wTl -1⁄2(yT4) In effect, yT6 is calculated by the BSS so that the next timing signal, in a perfect world, would occur at the mid-point of the next timing window, 412.

The graph in FIG. 4c illustrates one benefit of employing a BSS that triggers BSS transmissions, according to the present embodiment, based on a combination of remote timing signals 400 and an internally generated time window, yT4. As can be seen, by comparison to the graph in FIG. 3c, the time, yTS, for which a receiver circuit in a VMTU is required to be activated, can be significantly reduced to approaching wT3 + yT4, which equates to 5 lOms. In a practical scenario, the receiver circuit would typically be switched on slightly earlier than required and switched off slightly later than required, for example by an additional Sms at the start and end, in order to ensure BSS transmission capture.

In practice, the receiver circuit is timed to switch on and off by a timer in the \MTU, which is reset on arrival of each BSS transmission. As such, the timing of the VMTU receiver circuit operation is substantially independent of actual NTP timing signals, and is, instead dependent upon the time of receipt of each BSS transmission.

If the effect of jitter on the timing signals is thought of as high frequency noise, the effect of defining a time window 412 is analogous to introducing a low pass filter on the noise. For example, as consecutive timing signals 400 occur at any time within the associated bounded timeframes 402, the maximum consequential movement of the respective BSS transmissions 410 is bounded by the relatively, far shorter time window 412.

It will be appreciated that, irrespective of the magnitude of the jitter experienced by the NTP timing signals, the maximum consequential movement in BSS transmission, according to the present embodiment, is +(1⁄2 x (yT4)) = +Sms.

It will also be appreciate that, over time, the there is insignificant long term drift in the NTP timing signals, as they rely on atomic clocks that have negligible long term drift or instability.

A functional block diagram of the BSS sub-system 500 that drive the BSS transmissions is illustrated in FIG. 5. As shown, timing signals 400 are received by a timing signal receiver 505. The received timing signals are communicated to a comparator 510. The comparator 510 also receives time window 412 timing information from a time window generator 515. The comparator 510 operates to establish if the timing signals are received in advance of, during, or after the respective time window, by applying the logic presented above for triggering a BSS transmission. The comparator 510 communicates a signal in response to applying the logic to a trigger 520, which in tum communicates a trigger signal to a message output transmitter 525. The output message transmitter 525 also receives message payload information from a message payload generator 530. In response to receipt of a trigger signal, the output message transmitter transmits a BSS transmission 410 carrying the message payload. In addition, the output message transmitter 525 communicates a signal to the time window generator 515, in order to reset a timer thereof for calculating the start time of the next time window 412.

The operation of the BSS sub-system described above is illustrated in more detail, according to an embodiment of the invention, by the flow diagram in FIG. 6.

According to FIG. 6, to begin with [step 600] an initial NTP timing signal is received by the BSS 120, a BSS transmission 410 is triggered immediately 605 [step 605] and the time window generator 515 is reset [step 610]. The process then enters a timing loop, which operates until a calculated end time of the next time window 410. The loop repeatedly checks for receipt of the next NTP timing signal 400 [step 615]. If the timing signal has been received (which, by implication in this embodiment, must be before the end of the next time window), the process checks to see whether the next time window has started [step 620]. If the window has already started, then the loop exits and triggers a next BSS transmission [step 605]. If the window has not started, then a delay is applied [step 625] until the start of the window, at which point the next BSS transmission is triggered [step 605].

Altematively, if [step 615] a timing signal has not been received, the process checks to see whether the present time window has ended [step 630]. If the time window has not ended, then the process loops and checks again to see whether the timing signal has been received [step 615]. If the time window has ended [step 630] then the process is set to discard, or ignore, the late timing signal, and then the next BSS transmission is triggered immediately. This process continues indefinitely.

The foregoing process can be implemented by a standard computing platform, for example running a MicrosoftTM WindowsTM operating system and appropriate application software. The receiver 505 and transmitter 525 components in FIG. 5 can be standard components, for example employed in existing SVR systems, for example using VHF communications of the kind employed in standard cellular mobile communications.

According to the present embodiment, the timing and duration of time windows 412 are calculated using the standard clocks and counters that are employed in such standard computing platforms. The present inventors have appreciated that, while standard clocks and counters are nowhere near as accurate as an atomic clocks, for example in terms of long term drift and thermal stability, such timers and counters are typically accurate enough to provide satisfactory time windows over relatively short periods. For example, in order to provide an accurate 1 Oms window after 1 OOs, a clock would need to be accurate to about one part in 1 o4, which is well within the accuracy of a standard crystal clock in a PC or the like. The degree of long term drift that would be presented by operation of a standard crystal based clock over months or years would, however, be unsatisfactory and, in time, a PC clock would fall out of synchronisation with the accurate timing source 100. Nevertheless, the effects of such long term drift are avoided, because the system still depends on the accurate timing source 100, on average, over time. In other words, the long term drift of the system depends entirely on the accuracy of the accurate timing source.

While embodiments of the present invention may be implemented entirely in software, certain elements in other embodiments may be implemented in hardware. For example, the receipt of a timing signal may be detected and cause a hardware interrupt, which triggers the operation of a processor to send a BSS transmission. There are many ways, in hardware, software and in a combination of both, by which embodiments of the present invention may be implemented.

FIG. 7 is a functional diagram of an exemplary VMTU 700. The VMTU comprises a receiver circuit 705 for receiving BSS transmissions. The receiver circuit 705 delivers received BSS transmissions to a microprocessor 710, which is arranged to process the received transmissions. The microprocessor 710 is typically an embedded processor which operates under the control of firmware instructions, which are stored in an area of EEPROM (not shown). The microprocessor is arranged to identify instructions to emit short-range radio signals 715, via an SYR signal transmitter 720, which can be detected by police forces and used to track and recover stolen vehicles. In addition, the microprocessor communicates the arrival of a BSS transmission 410 to a receiver window generator circuit 725. In response, the receiver window generator circuit 725 instructs a receiver switch circuit 730 to switch the receiver 705 off (or it could wait until the calculated end of the present window to switch off), and then calculates when to switch the receiver circuit back on S again in time to receive the next BSS transmission. After the calculate time period, the receiver switch circuit switches the receiver circuit back on again.

As with the BSS, the receiver switch circuit 730 employs a standard crystal clock, or even an embedded clock of the microprocessor 710.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged and will be apparent to the skilled person. For example, the ability to reduce the effects of jitter, as has been described herein, may find application in many devices or systems that rely on some kind of remotely generated timing signal, and embodiments of the present invention are in no way limited to NTP timing sources, fixed networks, wireless networks or SVR systems. Indeed, the principles taught herein may find application in all manner of communications systems, network based databases, wireless communications systems or the like, any of which may require an accurate timing reference. It is to be understood that any feature described in relation to any one embodiment may be used alone, or, if the context permits, in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

Claims 1. A signalling device comprising: -a receiver to receive in-bound periodic timing signals from a remote timing source; and -a transmitter responsive to a trigger event to transmit out-bound signals; wherein a trigger event occurs either on receipt of an expected timing signal within a predetermined time window or automatically if an expected timing signal is not received within the predetermined time window.
2. A signalling device according to claim 1, further comprising a timer to determine the start time and duration of the time window.
3. A signalling device according to either preceding claim, wherein the time window is timed to start at a predetermined time after a previous trigger event and/or outbound transmission.
4. A signalling device according to claim 2, wherein the accuracy of the timer exceeds a maximum expected timing jitter of the periodic timing signal.
5. A signalling device according to any one of the preceding claims, wherein the time window has a duration that is less than the maximum expected timing jitter of the periodic timing signals.
6. A signalling device according to any one of the preceding claims, wherein the duration of the timing window is less than half of the expected and/or measured jitter.
7. A signalling device according to any one of the preceding claims, wherein the duration of the timing window is less than one tenth of the expected and/or measured jitter.
8. A signalling device according to any one of the preceding claims, in which a trigger event occurs automatically at the start of the time window, if an expected timing signal is received before the start of the time window.
9. A signalling device according to any one of the preceding claims, in which a trigger event occurs automatically at the end of the timing window, if an expected timing signal has not been received by the end of the timing window.
10. A wireless base station, for communicating wirelessly with mobile transceivers, the wireless base station comprising a signalling device according to any one of the preceding claims.
11. A stolen vehicle recovery system, comprising at least one wireless base station according to claim 10.
12. A system comprising: -a wireless base station according to claim 10; and -at least one mobile transceiver, which is operable to receive out-bound signals from the base station, the mobile transceiver having a receiver, which is periodically active only during a time window, and which is controlled by the wireless base station to coincide with the time window of the signalling device.