A requirement that often arises in modern communication systems is the need for several devices to communicate time related messages between themselves. A block diagram illustrating an example prior art multi-device system incorporating a separate timer for each device is shown in FIG. 1. The example system, generally referenced 10, comprises three devices 12, labeled device A, B and C. Each device comprises a control unit 14 and timer 16, labeled Control Unit A, B and C and Timer A, B and C, respectively.
In many cases, each of the devices maintains its own time base which (1) may or may not be driven by a hardware clock at a frequency suitable for that device; (2) may or may not be adjusted by the device itself (such as to compensate for drifts due to temperature, etc. or to simplify control of the device); and (3) may suffer from inherent drifts when compared to the timers in other devices. One example of a multi-device system is a multi-mode cellular handset comprising two or more different radio access technology (RAT) PHYs.
In recent years, the world has witnessed explosive growth in the demand for wireless communications and it is predicted that this demand will increase in the future. There are already over 1.5 billion users subscribing to cellular telephone services and the number is continually increasing. The number of GSM users around the world alone has already crossed the 1.2 billion mark, as an example of the increased use of cellular services. One in five people around the world now have a mobile phone and in some developed markets mobile penetration has already approached 100%. It is predicted that by 2010 there will be over 2.3 billion individual wireless subscribers worldwide.
In some countries, the number of cellular subscribers already exceeds the number of fixed line telephone installations. In many cases, the revenues from mobile services exceeds that for fixed line services even though the amount of traffic generated through mobile phones is less than in fixed networks.
Other related wireless technologies have experienced growth similar to that of cellular. For example, cordless telephony, two way radio trunking systems, paging (one way and two way), messaging, wireless local area networks (WLANs), wireless local loops (WLLs), WiMAX and Ultra Wideband (UWB) based MANs.
Currently, the majority of users subscribe to digital cellular networks. Almost all new cellular handsets sold to customers are based on digital technology, typically third generation digital technology. Currently, fourth generation digital networks are being designed and tested which will be able to support much higher data rates. The first generation analog systems comprise the well known protocols AMPS, TACS, etc. The digital systems comprise the second and third generation services GSM/GPRS/EGPRS, TDMA (IS-136), CDMA (IS-95), UMTS (WCDMA), etc. Future fourth generation cellular services are intended to provide mobile data at rates of 100 Mbps or more.
A problem arises in wireless communication systems, however, where different RAT PHY devices within a cellular handset need to communicate time related messages between each other. Since each device operates using its own time base, a mechanism of synchronizing all the different time bases to a common time base shared by all the devices is needed.