Patent Publication Number: US-2016226530-A1

Title: Multi-Processor Platform for Wireless Communication Terminal Having a Partitioned Protocol Stack

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 13/845,807, filed Mar. 18, 2013, which is a Continuation of U.S. patent application Ser. No. 10/733,861, filed Dec. 11, 2003, now U.S. Pat. No. 8,423,077, which claims benefit to U.S. Provisional Patent Application No. 60/434,448, filed Dec. 18, 2002. U.S. patent application Ser. No. 13/845,807 is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to wireless communication systems and, more particularly, to a multi-processor platform for a wireless communication terminal having a partitioned protocol stack. 
     BACKGROUND OF THE INVENTION 
     It is becoming increasingly apparent that communication systems involving fixed client terminals and server units are no longer the only pervasive means of communication available to large segments of society. In particular, certain current and next-generation client devices are no longer tied to use at a single physical location or limited to a single application. Such portable client terminals are predicted to emerge as ubiquitous communication and computing platforms, capable of enabling the convergence of consumer electronics, computing, and communications. In order for this type of convergence to fulfill its promise, client terminals will need to become capable of accessing a multiplicity of applications and services while seamlessly connecting to a variety of wireless access networks. 
     Such convergence may be evaluated from at least two perspectives. First, the manner in which multiple wireless networks may be configured to facilitate such convergence needs to be considered. This will enable the creation of user scenarios aiding in the development of mobile terminal architectures designed to interoperate with such multiple networks. Secondly, convergence from the perspective of end-users should be understood in order that any proposed system solutions accommodate the needs of such end-users to the greatest extent possible given applicable network constraints. 
     From a network perspective, efforts are being made to achieve such convergence through integration of wireless local area networks (“WLANs”) and third-generation (“3G”) cellular systems developed in accordance with the Universal Mobile Telecommunications System (UMTS). Such 3G cellular systems include, for example, integrated systems based upon Global System Mobile (GSM) and General Packet Radio Service (GPRS) (i.e., GSM/GPRS systems), as well as wideband code division multiple access systems (WCDMA). Varying degrees of integration of a 3G cellular system and a WLAN may be achieved. For example, a certain degree of integration may be obtained merely through sharing of billing and subscriber profile information. On the other hand, a relatively greater degree of integration may be achieved through integration of the core network functionality of the WLAN and the 3G cellular system. Although the latter approach promises to yield a more complete set of network functions, it would constitute an extremely complicated and expensive undertaking. Furthermore, in view of the evolving nature of both the WLAN and UMTS standards, near term prospects of comprehensive integration of WLAN and 3G cellular systems seem rather dim. Accordingly, it is probable that the former type of integration and coordination among systems will likely be the only approach to be implemented within the foreseeable future. 
     Turning now to  FIG. 1 , an illustrative representation is provided of an exemplary wireless communication system  100  within which the former type of integration may be attained by connecting the billing and subscriber profiles for a WLAN  104  and a UMTS network  106 . As may be appreciated from  FIG. 1 , the WLAN  104  and UMTS network  106  share a common authentication system  110  and a common billing system  114 . 
     The UMTS network  106  is comprised of several primary portions including a mobile subscriber terminal  118  and associated Subscriber Identity Module (SIM)  120 , a UMTS radio network  124 , and a UMTS core network  126  containing switching infrastructure and network intelligence. During operation of the system  100 , the subscriber terminal  118  communicates with base stations within the UMTS radio network  124 . Such base stations convert radio signals from the subscriber terminal  118  into digital signals which are provided to the switching infrastructure within the UMTS core network  126 . This switching infrastructure establishes call connections with other subscriber terminals, or routes the digital signal information to the public switched telephone network (PSTN) or other data network (e.g., the public packet data network (PPDN) or the Internet). 
     The SIM  120  is realized as an electronic card and provides subscriber identity information to the subscriber terminal  118 , which transmits this information to the UMTS radio network  124  in order to gain access to the UMTS core network  126 . The UMTS core network  126  then verifies the validity of the subscriber identification information before authorizing access to the subscriber terminal  118 . Within the UMTS network  106 , the SIM  120  is used as the primary subscriber identification and encryption mechanism, although this capability has not been standardized within WLAN environments. However, several approaches have been proposed for development of authentication and encryption solutions for deployment within WLANs using SIM/USIM technology. 
     It is anticipated that SIM/USIM technology will play a key role in enabling the convergence of WLAN and cellular systems at a network level by enabling joint authentication (and by implication also billing). It is further believed that this technology may play a key role in solving many of the security issues that have hindered deployment of WLAN systems. 
     From an end-user perspective, the promise of third generation wireless systems has always been the delivery of a diverse range of services to anyone, anywhere, anytime and at the lowest possible cost. During the early stages of the development of UMTS networks, the vision was that the combination of existing GSM/GPRS networks with the newly developed WCDMA networks would fulfill this promise. However, the development and commercialization of WLAN technologies (specifically 802.11a/b) has been gaining momentum. Among many experts, the current consensus seems to be that both systems will co-exist. In this regard it appears that end users will be less concerned with the availability of a particular technology than with the reliable delivery of multiple different types of advanced services. In order to enable such convergence of service offerings, network operators must ensure the availability of subscriber terminals capable of securely executing a number of different applications. In addition, it will also be desired to deliver such advanced services using the lowest-cost network infrastructure available. Accordingly, the architecture of next-generation mobile terminals will ideally be capable of receiving services or applications via a number of different bearer options (e.g. GSM/GPRS, WCDMA, and 802.11a/b). 
     Turning now to  FIG. 2 , a block diagram is provided of the baseband platform of a typical second generation (2G) wireless handset  200 . As shown, handset  200  typically includes a processor  204  (e.g., an ARM7 or the equivalent) and a 16-bit DSP  208 . Firmware of the DSP  208  is typically executed from ROM (not shown), while software executed by the processor  204  is stored in “off-chip” FLASH memory  212 . The handset  200  also typically includes a limited amount of off-chip SRAM  216 , as well as a SIM interface  220  configured to accept an electronic SIM card of the type described above. With slight modification, the platform  200  may also be used to implement dual-mode GSM/GPRS solutions. Typically, a processor  204  of higher speed (e.g., an ARM9 processor) is used in the GSM/GPRS handset, and the clock speed of the 16-bit DSP  208  is also increased. A higher-speed processor  204  such as the ARM9 is not only capable of running the GSM/GPRS protocol stack, but also of concurrently executing applications. 
     Accordingly, from an end user perspective a number of the ingredients necessary to support convergence are present within existing handset technology; namely, sufficient processing and computing capability to underpin a number of different applications and services, and a SIM interface enabling subscriber access to a unified authentication and billing platform. However, existing handsets are generally incapable of supporting multiple radio protocols or “bearers”, thereby limiting the convergence of the different services offered via various bearers. For example, certain existing GSM handsets are capable of accessing and displaying information via Internet web browsing, but are not disposed to seamlessly roam between GSM networks and other types of radio networks such as, for example, WLAN, Bluetooth or 3G WCDMA networks. 
     Accordingly, it would be desirable to provide for seamless mobility between radio networks operative in accordance with different protocols. In order enable such mobility and the consequent convergence in services, it would also be desirable to provide a mobile wireless terminal that inexpensively supports multiple bearers and services, and that further enables service differentiation based upon user identity. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention relates in one aspect to a multi-mode wireless communication device including a first baseband co-processor configured to execute low-level stack operations of a first wireless communications protocol employed within a first wireless communications network. The wireless device also includes a host baseband processor configured to execute (i) a set of protocol stack operations of a second wireless communications protocol employed within a first wireless communications network, and (ii) higher-level stack operations of the first wireless communications protocol. A data communication channel is provided between the host baseband processor and the first baseband co-processor and is capable of carrying data received by the multi-mode wireless communication device from the first wireless communications network or sent by the multi-mode wireless communication device through the first wireless communications network. In a particular implementation the set of protocol stack operations executed by the host baseband processor comprises a complete set of protocol stack operations of the second wireless communications protocol. In other implementations the wireless device further includes a second baseband processor configured to execute low-level stack operations of the second wireless communications protocol, with higher-level protocol stack operations of the second wireless communications protocol being executed by the host baseband processor. 
     The present invention also relates to a method performed in a wireless communication device disposed for communication with first and second wireless communications networks in accordance with first and second wireless communication protocols, respectively. The method includes executing low-level stack operations of the first wireless communications protocol within a first baseband co-processor. A set of protocol stack operations of a second wireless communications protocol and higher-level stack operations of the first wireless communications protocol are also executed within a host baseband processor. A data communication channel capable of carrying data received by the wireless communication device from the first wireless communications network or sent by the wireless communication device through the first wireless communications network is established between the host baseband processor and the first baseband co-processor. In a particular implementation the method further includes executing low-level stack operations of the second wireless communications protocol within a second baseband processor in communication with the host baseband processor via the data communication channel. 
     In another aspect the invention is directed to a multi-mode wireless communication device including a first bearer-specific processor configured to execute low-level stack operations of a first wireless communications protocol employed within a first wireless communications network. The device also includes a second bearer-specific processor configured to execute low-level stack operations of a second wireless communications protocol employed within a second wireless communications network. A primary processor configured to execute higher-level stack operations common to the first and second wireless communications protocols is also provided. The device additionally includes a radio transceiver, and an arrangement for communicating data between the radio transceiver, the primary processor, the first bearer-specific processor and the second bearer-specific processor. In a particular implementation the low-level stack operations of the first wireless communications protocol include physical layer functions and bearer-specific stack functions peculiar to the first wireless communications protocol. Similarly, the low-level stack operations of the second wireless communications protocol may include physical layer functions and bearer-specific stack functions peculiar to the second wireless communications protocol. 
     The present invention also pertains to a multi-mode wireless communication device including a first integrated circuit configured to execute low-level stack operations of a first wireless communications protocol employed within a first wireless communications network. The device also includes a second integrated circuit configured to execute low-level stack operations of a second wireless communications protocol employed within a second wireless communications network. Also included within the device is a third integrated circuit configured to execute higher-level stack operations of the first wireless communications protocol and of the second wireless communications protocol. A first data communications channel is provided between the first integrated circuit and the third integrated circuit, and a second data communications channel is provided between the second integrated circuit and the third integrated circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the nature of the features of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  provides an illustrative representation of an exemplary wireless communication system in which the billing and subscriber profiles for a wireless LAN and a UMTS network are connected. 
         FIG. 2  is a block diagram of the baseband platform of a typical second generation (2G) wireless handset. 
         FIG. 3  illustratively represents an exemplary layered software architecture of the present invention disposed within a mobile wireless communication terminal. 
         FIG. 4  is a block diagrammatic representation of a mobile terminal incorporating a layered software architecture partitioned among multiple processors in accordance with the invention. 
         FIG. 5  provides a more detailed illustrative representation of a multi-strata software architecture as configured for incorporation within a multi-bearer wireless terminal. 
         FIG. 6  illustratively represents a wireless terminal baseband platform obtained through mapping of the multi-strata software architecture of  FIG. 5  to an existing GSM/GPRS platform architecture. 
         FIG. 7  illustrates a dual-mode wireless terminal baseband platform configured to provide both GSM/GPRS and WCDMA bearer services. 
         FIG. 8  illustrates a dual-mode wireless terminal baseband platform which illustrates the convergence of multiple user applications into a single device. 
         FIG. 9  shows a tri-mode wireless terminal platform configured to provide both GSM/GPRS, WCDMA and wireless local area network (WLAN) bearer services in accordance with the invention. 
         FIG. 10  depicts a block diagram of an exemplary embodiment of a convergent multi-mode wireless terminal platform of the present invention. 
         FIG. 11  illustrates a dual-mode wireless terminal baseband platform with respect to which will be described the provision of both GSM/GPRS and WCDMA bearer services in a time-synchronized manner. 
         FIG. 12  provides an illustrative representation of a counter maintained by a WCDMA master timer of a WCDMA baseband co-processor. 
         FIG. 13  shows a timing diagram which illustratively represents a timing synchronization method predicated upon execution of a direct access read operation. 
         FIG. 14  depicts a timing diagram illustratively representing a timing synchronization method predicated upon execution of an interrupt capture operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  illustratively represents an exemplary layered software architecture  300  of the present invention as configured for inclusion within a mobile wireless communication terminal  310 . The layered software architecture  300  includes an application layer  314  in communication with a common stack functions layer  316 . As is indicated by  FIG. 3 , a set of software routines defining an overall communication protocol for the mobile wireless communication terminal  310  are grouped into a stack of protocol layers; i.e., a protocol stack, comprised of the common stack functions layer  316 , a bearer-specific stack layer  320  and a physical layer  324 . The protocol stack divides the overall communication protocol into hierarchical layers of functionality. 
     As may be appreciated with reference to  FIG. 3 , the “lower” protocol layers comprised of the bearer-specific stack layer  320  and physical layer  324  are specific to a particular communication protocol and radio transceiver design, respectively. In contrast, the “upper” protocol layers comprised of the application layer  314  and common stack functions layer  316  are substantially independent of a particular communications protocol and transceiver design. It follows that in certain implementations it will be convenient to bifurcate the processing of such upper and lower protocol layers among first and second processor modules  330  and  334 , respectively. In this way any second processor module  334  configured to implement a desired radio bearer and transceiver functionality may be inserted within the terminal  310  and communicate with the higher layer protocols executed by the first processor module  330 . 
     It is thus apparent that the functionality of the layered software architecture  300  may be distributed as desired among a plurality of physical processing modules used to realize the communication terminal  310 . Advantageously, the common stack functions layer  316  permits the data streams received from the bearer-specific stack layer  320  to appear the same to the application layer  314  irrespective of the particular communications protocols being implemented by such stack layer  320 . This distribution of functionality enables such additional processing modules  334  to be removed and replaced with other modules configured to implement different communication protocols. 
     Referring to  FIG. 3 , the application layer  314  is comprised of a number of distinct application programs  342  (e.g., voice communication, web browsing, streaming video). Each application program  342  interacts with the common stack functions layer  316 , which provides access to a particular bearer communication channel (e.g. GSM/GPRS, 802.11 or WCDMA). For example, in the case of WCDMA the common stack functions layer  316  would implement the functionality of the Non-Access Stratum (NAS), which performs user authentication based upon the information included within the SIM card  350  inserted into the mobile terminal  310 . Since the NAS is executed by the first processing unit  330  independent of any bearer-specific processing unit  334 , this authentication process is advantageously effected in a bearer-independent manner. That is, in this embodiment the user will always be authenticated using the information within the SIM card  350  irrespective of whether the chosen bearer is WCDMA, 802.11 or GSM/GPRS. 
     Turning now to  FIG. 4 , a block diagrammatic representation is provided of a mobile terminal  400  incorporating a layered software architecture partitioned among multiple processors in accordance with the present invention. As shown, the mobile terminal  400  includes a first processor  410  disposed to execute application layer routines and a set of common stack functions as described above with reference to  FIG. 3 . The mobile terminal  400  further includes a plurality of bearer-specific processors  414 , each of which is configured to implement the bearer-specific and physical layers of the protocol stack for a given radio bearer. A conventional keyboard module  418  is interactively coupled to the processor  410 , which may be implemented as a 16-bit microprocessor having ROM, RAM, a plurality of ports, analog to digital converters and a serial interface. In addition to the on-chip memory capacity, an external ROM  420  and an external RAM  424  may be provided for additional data processing and communication capacity. The terminal  400  further includes a display controller and associated driver circuits  430  configured to drive an LCD screen  434 . 
     As is described hereinafter, in a particular embodiment the inventive software architecture  300  enables new radio bearers to be added to an existing GSM/GPRS platform (see, e.g.,  FIG. 2 ) without modification of the processing modules effecting the core GSM/GPRS functionality. In this way the present invention enables the re-use of existing GSM/GPRS solutions, thereby permitting development of mobile terminal platforms facilitating convergence from both network and user perspectives. As a result, wireless semiconductor and mobile device manufacturers may efficiently and cost effectively migrate their existing single-mode GSM/GPRS platforms to dual-mode (GSM/GPRS &amp; WCDMA) or even multi-mode (GSM/GPRS, WCDMA &amp; 802.11) solutions. This enables the efficient and economical addition of new bearers with minimal redesign of existing mobile terminal platforms. 
     Turning now to  FIG. 5 , a more detailed illustrative representation is provided of the software architecture  500  of the present invention as configured for incorporation within a multi-bearer wireless terminal. As shown, the architecture  500  is organized within a set of four software strata, each of which is defined by different data flow characteristics: an application stratum  504 , communication stratum  506 , protocol stratum  508  and a physical stratum  510 . 
     In the exemplary embodiment the application stratum  504  is comprised of a plurality of user-level application programs  520  (e.g., web browsing, text messaging). As a consequence, the data transfers occurring across the interface  524  between the communication stratum  506  and the application stratum  504  will tend to be “bursty” in nature. 
     The communication stratum  506  implements bearer-independent protocol stack functionality pertinent to maintenance of calls or other connections. In this regard the communication stratum  506  functions to authenticate users on various networks, select an appropriate bearer to use in transport of data packets, and maintain connections at the application level while switching between such bearers. That is, the communication stratum  506  provides application programs  520  access to different bearers, and provides authentication service for all bearers using SIM/USIM mechanisms. The data rates across the interface  530  between the communication stratum  506  and the protocol stratum  508  will tend to be more consistent than across the interface  524 . 
     The protocol stratum  508  implements various bearer-specific protocol stack functions  534 , and is configured to accommodate relatively high peak data rates across the interface  536  with the physical stratum  510 . As shown, the physical stratum  510  is comprised of a number of physical layer modules  550  corresponding to various bearers (e.g., GSM/GPRS, WCDMA and 802.11). It should be noted that  FIG. 5  provides a hierarchical view of the software architecture of  FIG. 5 , which is not constrained to be mapped to a particular hardware configuration. 
     As may be appreciated from  FIG. 5 , the multi-strata software architecture  500  relies upon buffering in order to equalize the data flow among the four defined software strata. Specifically, the application stratum  504  includes a plurality of buffers  556  respectively associated with the plurality of application programs  520 , the protocol stratum includes a plurality of buffers  560  respectively associated with each bearer-specific stack functions  534 , and the physical stratum  510  includes a plurality of buffers  564  respectively associated with each physical layer module  550 . As is described hereinafter, the buffers  556 ,  560  and  564  enables the software architecture  500  to be implemented using a number of different hardware configurations. 
     As an initial example,  FIG. 6  illustratively represents a wireless terminal baseband platform  600  obtained through mapping of the inventive multi-strata software architecture  500  to an existing GSM/GPRS platform architecture. The platform  600  is realized using a single baseband integrated circuit or “chip”  601  comprised of a processor  604  (e.g., an ARM9 processor) and a digital signal processor (DSP)  608 . In this approach, the functions associated with the communication stratum  508 , protocol stratum  506  and SIM/USIM authentication process  602  are executed by the processor  604 . As shown, the processor  604  executes common stack functions  620 , as well as bearer-specific GSM stack functions  622  and GPRS stack functions  624 . Buffers  630  and  632  serve to accommodate the different data rates associated with execution of the common stack functions  620  and the bearer-specific GSM and GPRS stack functions  622  and  624 . Typically, data to be transmitted over the air is stored in on-chip SRAM  616  in order to enable efficient access to such data in connection with the addition or removal of header information and the like. 
     As is indicated by  FIG. 6 , the physical stratum  510  is implemented using the DSP  608 . Although the GPRS physical layer module  650   2  will typically re-use the functionality of the GSM physical layer module  650   1 , from a logical perspective distinct GSM and GPRS functionality may be split among the modules  650   1  and  650   2  as indicated. As shown, the interface between the protocol stratum and the physical stratum is implemented as an on-chip mailbox  610  containing a first physical stratum buffer  660   1  associated with the GSM physical layer module  650   1  and a second physical stratum buffer  660   2  associated with the GPRS physical layer module  650   2 . 
     Turning now to  FIG. 7 , there is illustrated a dual-mode wireless terminal baseband platform  700  configured to provide both GSM/GPRS and WCDMA bearer services. As shown, the baseband platform  700  is architected similarly to the platform  600 , and includes a GSM/GPRS host baseband processor platform  701  comprised of a processor  718  (e.g., an ARM9 processor) and a digital signal processor (DSP)  708 . However, the platform  700  further includes a WCDMA baseband co-processor  704  containing a WCDMA physical layer module  708  and associated buffer  710 . The WCDMA baseband co-processor  704  operates to perform physical layer processing of WCDMA bearer signals, and interfaces with a bearer-specific WCDMA stack functions  716  executed by the host processor  718 . A buffer  722  accommodates the generally different data transfer rates associated with execution of the common stack functions  720  and the WCDMA stack functions  716 . 
     In the embodiment of  FIG. 7 , the WCDMA stack functions  716  implemented using the processor  916  include the bearer-specific functions MAC, RLC, PDCP, BMC and RRC. In like manner the processor  916  is used to implement the common stack functions  720 , which in WCDMA-based configurations would include NAS functions. Finally, the WCDMA baseband co-processor  704  is responsible for all WCDMA-related “Layer 1” or physical layer functions. 
     Again referring to  FIG. 7 , prudent engineering design suggests that the additional processing burden placed upon the processor  718  as a consequence of the addition of a WCDMA bearer should be evaluated. As an initial matter, the processing overhead associated with execution of the bearer-specific WCDMA stack module  716  is considered. For example, assuming that the WCDMA stack function  716 , GSM stack functions  622 , and GPRS stack functions  624  collectively require  30  MIPS of processing power, the processing activity of the processor  718  is profiled below in Table I. 
     
       
         
           
               
               
             
               
                 TABLE I 
               
               
                   
               
             
            
               
                 MIPS available(@ 104 MHz) 
                 104  
               
               
                 Number of wait states for external memory access 
                 10 
               
               
                 Cache hit ratio 
                 83% 
               
               
                 Stack MIPS requirement 
                 30 
               
               
                 MIPS Remaining 
                 104 − 30*0.83 − 
               
               
                   
                 (30*0.17*10) = 28.1 
               
               
                   
               
            
           
         
       
     
     As may be apparent from Table I, the processor  718  possesses sufficient processing resources to implement both the bearer-specific WCDMA stack function  716  and GSM/GPRS stack functions  622  and  624 . That is, the present invention enables the mapping of the WCDMA stack function  716  onto a processor of the type employed in realizing existing GSM/GPRS solutions, while providing a WCDMA baseband co-processor  704  to effect the WCDMA physical layer functions. Since the WCDMA physical layer is anticipated to be of substantially greater complexity than the GSM/GPRS physical layers, it may often be appropriate to realize the WCDMA baseband co-processor  704  as an application specific integrated circuit (ASIC) rather than using a general purpose digital signal processor (DSP). It is also of course possible to integrate all of the required physical, protocol and communications stratum GSM/GPRS arid WCDMA functionality within a single baseband integrated circuit, but this nullifies the advantages associated with the modular approach described above. 
     As indicated above, when a pair of integrated circuits (i.e., host baseband processor platform  701  and WCDMA baseband co-processor  704 ) are used to implement the dual-mode platform  700 , memory mapping is used to define the interface between the protocol stratum and the physical stratum. Since this interface has the benefit of being standardized, the augmentation of existing 2.5 G platforms to include WCDMA functionality in accordance with the invention is simplified. The memory mapping defining this interface will typically be effected by establishing a shared area within the memory of the host baseband processor platform  701 . This shared memory space may be logically configured as a dual-port RAM segmented into a number of areas, each containing a different type of data. These data types may comprise, for example, control information transferred between the protocol stacks and physical layers and uplink/downlink data. During operation of the platform  700 , this shared memory space facilitates the exchange of data between the host baseband processor platform  701  and WCDMA baseband co-processor  704  at regular intervals. Typically, such an interval will correspond to the duration of a frame (e.g., 10 ms in the case of WCDMA). At the end of each frame, the WCDMA baseband co-processor  704  will interrupt the host baseband processor platform  701  and signal that new information is available for reading. When the host baseband processor platform  701  reads such new information, it also writes new information into the shared memory space for reading by the WCDMA baseband co-processor  704 . In the exemplary embodiment the host baseband processor platform  701  may interrupt the WCDMA baseband processor  704  at any time should it desire to write new data into the shared memory space. 
       FIG. 8  illustrates a dual-mode wireless terminal baseband platform  800  which illustrates the manner in which the present invention facilitates convergence of user applications into a single device. As was demonstrated above with reference to Table I, the present invention enables existing 2.5 G platforms to be augmented to accommodate new high-speed bearer services (e.g., WCDMA) while retaining sufficient significant processing resources to permit execution of user applications. For example, if the subject device is a feature phone, the remaining processing resources could be used to execute an application enabling decoding of a multi-media message or the like. Should more advanced application execution capabilities be required, the architecture depicted in FIG.  8  may be employed. As shown, in the embodiment of  FIG. 8  the application stratum  504  has been mapped to an application processor  804  external to the host baseband processor platform  701 . The application processor  804  is configured to run an operating system capable of executing complex applications such as, for example, MPEG-4 encoding or the equivalent. As is illustrated by  FIG. 8 , the application processor  804  may be connected to the host baseband processor platform  701  using a relatively fast serial connection  810 . In general, the buffering of data between the application stratum  504  and the communication stratum  506  may be handled by the application processor  804 . 
     Referring now to  FIG. 9 , there is shown a tri-mode wireless terminal platform  900  configured to provide both GSM/GPRS, WCDMA and wireless local area network (WLAN) bearer services. As a consequence of the high peak data rates characterizing various WLAN protocols (e.g., IEEE 802.11), in the embodiment of  FIG. 9  the protocol stratum  508  is seen to be implemented across a host baseband processor platform  901  and a WLAN baseband co-processor  904 . As shown, the protocol stratum  508  for the WLAN bearer is comprised of a WLAN upper medium access control (MAC) layer  908  executed by a processor  916 , and a WLAN lower MAC &amp; physical layer  910  executed by the WLAN baseband co-processor  904 . The WLAN upper MAC layer  908  will generally be executed by the host baseband processor platform  901 . This bifurcation of the processing of the WLAN MAC layer will generally be desirable in view of the lower processing requirements associated with execution of the WLAN upper MAC layer  908  relative to execution of the WLAN lower MAC &amp; physical layer  910 ; that is, execution of the WLAN lower MAC &amp; physical layer  910  requires relatively more processing power and such execution will thus often be effected using a separate chip. Again, the different data flow characteristics of the WLAN upper MAC layer  908  and the WLAN lower MAC &amp; physical layer  910  are accommodated using buffers  920  and  924 , respectively. 
     When considering the addition of a new bearer to the inventive wireless terminal platform, at least two parameters will generally warrant consideration; namely, the peak and average data rates. While the peak data rate of the new bearer may be relatively high, average data rates may be significantly lower. For example, in the case of both 802.11b and WCDMA bearers the average data rates will typically be in the range of approximately only 200-384 kbps, while peak data rates may be significantly higher. This phenomenon tends to arise for at least two reasons. First, the 11 Mbps communication bandwidth offered by 802.11b systems is shared by all users within the applicable coverage area or “hotspot”. Secondly, data and video compression enable better utilization of bandwidth and thus require a lower average data rate. In accordance with the invention, splitting of the MAC layer in the manner described above may prevent bottlenecks from developing across the memory interfaces associated with the host baseband processor platform during the processing of such peak data rates. By such splitting of the MAC layer, the peak data rate associated with processing of the lower MAC portions by a separate WLAN baseband chip may be on the order of 11 Mbps, while the average data rate associated with processing of the upper MAC portions via the host baseband processor platform may be much lower (e.g., 300-400 Kbps). 
     From a logical perspective, each physical stratum buffer (i.e., the buffers  660 ,  710  and  924 ) is implemented as a dual-port RAM in the embodiment of  FIG. 9 . In the case of the physical stratum buffer  710 , a first port is read and written to by the host baseband processor platform  901  while a second port of the buffer  710  is asynchronously accessed by the WCDMA physical layer  708 . It will generally be preferred to implement the buffer  710  such that the WCDMA baseband co-processor  704  does not serve as a master on the bus connected thereto. This results in all accesses of the first port being initiated by the host baseband processor platform  901 , which permits the bus to be easily shared by program and data memory. 
     Each protocol stratum buffer (i.e., the buffers  630 ,  632 ,  722  and  920 ) generally constitutes a block of locations within the memory of the host baseband processor platform  901 . This memory space may be allocated statically or dynamically, and is used primarily as a repository for data to be potentially re-transmitted to the extent required by the applicable Layer 2 protocols. For example, in the case of TCP the protocol stratum  508  may transmit a packet out and then wait for an acknowledgement (i.e., an ACK) to be received from the TCP peer to which the packet was transmitted. If an ACK is not received, the subject data is retransmitted from the protocol stratum  508 . In this case the communication stratum  506  is not involved in the retransmission, which is consistent with an architecture in which such retransmission is implemented as a bearer specific function. 
     Similar to the protocol stratum buffers, an application stratum buffer (not shown) generally constitutes a block of locations within the memory of the host baseband processor platform  901 . This buffer functions to store data generated by applications until such data is ready for transmission. In this way the application stratum buffer supports the switching of the communication stratum between bearers of different speeds. 
     Referring again to  FIGS. 7-9 , the common stack functions  720  generally comprise various stack functions applicable to the bearers supported by the platform  700 . One such common stack function  720  which will generally be implemented is the Session Management function. As an example of such implementation, consider the case when the wireless terminal platform of the present invention is incorporated within a wireless terminal used to browse the Web. In this case the wireless terminal would initiate a TCP/IP session, during which the IP packets could be transported via any supported bearer (e.g. WCDMA or 802.11). That is, when a user of the wireless terminal “opens” its browser program, a connection (C1) is created through which a particular bearer (e.g., WCDMA) is used to transport the IP packets. Assume next that the user enters a hotspot area in which a faster 802.11 air interface is available. This situation is detected by the communication stratum  506 , which will now invoke the 802.11 air interface to carry the IP packets. However, the connection is still C1 from a session perspective, and the user of the wireless terminal will be unaware that a different physical layer is being used to actually transport the IP packets. 
     The common stack functions  720  may also implement various authentication operations. To this end the common stack functions  720  will often contain all the software necessary to, for example, read a SIM card and generate the secure keys and the like necessary to encrypt data in connection with a desired authentication operation. 
     Again directing attention to  FIGS. 7-9 , in the exemplary embodiment the bearer-specific WCDMA stack functions  716  are comprised of the following: MAC (Medium Access Control), RLC (Radio Link Control), PDCP (Packet Data Converge Protocol) and RRC (Radio Resource Control). The MAC, RLC and PDCP functions are involved in regulating functionality within the data domain, while the RRC is responsible for control functionality. In contrast, bearer-specific WLAN protocol stacks generally consist only of a MAC layer. In both cases, at least three primary constraints applicable to the bearer specific protocol stack functions should be considered in determining the manner in which the terminal architecture of the present invention may be configured to support convergence. Specifically, these constraints relate to the code space, data space and MIPS required to integrate multiple bearers using the approaches described above. 
     Required Code Space 
     The total code space required to implement a GSM/GPRS stack will be somewhat dependent upon the details of various implementations, but is generally expected to require an average of approximately 1.1 MB of program memory. As is indicated by Table II, moving to a dual-mode GSM/GPRS &amp; WCDMA solution will tend to increase this code space requirement to approximately 3 MB. However, the addition of an 802.11b bearer is expected to have only negligible impact upon program memory requirements. This is because the complexity of the WCDMA protocol stack is such that its size will typically be largely determinant of overall program memory requirements. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Technology 
                 Code Space Required 
               
               
                   
                   
               
             
            
               
                   
                 GSM/GPRS 
                 1.1 MBytes 
               
               
                   
                 WCDMA 
                 1.9 MBytes 
               
               
                   
                   
               
            
           
         
       
     
     Required Data Space 
     As in the case of code space requirements, overall data space requirements will be dependent upon the specifics of various implementation approaches. However, it is generally anticipated that a single mode GSM/GPRS solution will require approximately 512 KB of data memory. As is indicated by Table III, extending this solution to a dual-mode GSM/GPRS &amp; WCDMA implementation will tend to increase the memory requirements to 1 MB. Similarly, the addition of an 802.11b bearer will generally require an additional 128 KB of data memory. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                 Technology 
                 Data Space Required 
               
               
                   
                   
               
             
            
               
                   
                 GSM/GPRS 
                 512 KBytes 
               
               
                   
                 WCDMA 
                 512 KBytes 
               
               
                   
                 WLAN 
                 128 KBytes 
               
               
                   
                   
               
            
           
         
       
     
     MIPS Required 
     The third parameter which should be considered in the design of the converged terminal architecture of the present invention relates to the processing resources required by the various bearer services which may be supported. In the specific cases of WCDMA and 802.11, different factors will be determinative of the required processing resources. In the case of 802.11a/b, the primary factor is the maximum data rate supported. In contrast, the required control overhead associated with a WCDMA bearer will typically primarily account for its consumption of processing resources. As indicated by Table IV, it is expected that execution of an exemplary implementation of an 802.11a/b WLAN MAC will require approximately 10 MIPS (assuming zero wait state access to all memories), while execution of a WCDMA service at 384 kbps will require approximately 30 MIPS. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 Technology 
                 MIPS Required 
               
               
                   
                   
               
             
            
               
                   
                 GSM/GPRS 
                 &lt;10 MIPS  
               
               
                   
                 WCDMA 
                 30 MIPS 
               
               
                   
                 WLAN 
                 10 MIPS 
               
               
                   
                   
               
            
           
         
       
     
     Attention is now directed to  FIG. 10 , which depicts a block diagram of a particular physical implementation of a multi-mode wireless terminal platform  1000  consistent with the present invention. The multi-mode terminal platform  1000  is configured for implementation within a wireless terminal (not shown) possessing GSM/GPRS, WCDMA and 802.11a/802.11b bearer capabilities. The inventive platform  1000  is implemented using a three distinct integrated circuits; however, the WCDMA and WLAN functionalities could easily be integrated into a single chip if desired. As shown in  FIG. 10 , the platform  1000  includes a GSM/GPRS chip  1002  as modified to implement WCDMA and 802.11 upper level MAC protocol stack functionality in the manner described above. The GSM/GPRS chip  1002  is connected to a WCDMA chip  1004  operative to effect physical layer processing of the WCDMA bearer. Similarly, the GSM/GPRS chip  1002  is connected to a 802.11 chip  1010  configured to execute the lower MAC &amp; physical layers of an 802.11 bearer. As shown, the GSM/GPRS chip  1002  also interfaces with SRAM  1014  and flash memory  1020 . The platform  1000  advantageously affords a significant degree of flexibility as various types of terminals may be developed using a common set of chip designs. In this way a given GSM/GPRS chip design may be used to produce wireless terminals having at least the following types of capabilities: single-mode GSM/GPRS; dual-mode GSM/GPRS &amp; WCDMA; dual-mode GSM/GPRS &amp; 802.11; and multi mode GSM/GPRS, WCDMA &amp; 802.11. 
     In a particular implementation of the wireless terminal platform  1000 , the WCDMA chip  1004  may be realized using, for example, a SPINNERcore chip available from Zyray Wireless of San Diego, Calif. Similarly, the 802.11 chip  1010  may be implemented using an HFA 3860 or an HFA 3724 from Intersil Corporation of Irvine, Calif. 
     Turning now to  FIG. 11 , there is illustrated a dual-mode wireless terminal baseband platform  1100  with respect to which will be described the provision of both GSM/GPRS and WCDMA bearer services in a time-synchronized manner. As shown, the baseband platform  1100  includes a “host” GSM/GPRS baseband processor  1101  comprised of a Layer 2 processor  1108  and a GSM/GPRS modem  1110 . In the exemplary embodiment the Layer 2 processor  1108  comprises an ARM9 processor available from ARM, Inc. As shown, the host baseband processor  1101  further includes a master timer  1112  adapted to maintain counter values utilized by the GSM/GPRS modem  1110 . The platform  1100  further includes a WCDMA baseband co-processor  1104 , which contains a WCDMA modem  1116 . The WCDMA baseband co-processor  1104  further includes a master timer  1118  configured to maintain counter values utilized by the WCDMA modem  1116 . The WCDMA baseband co-processor  1104  operates to perform physical layer processing of WCDMA bearer signals, and interfaces with the host baseband processor  1101  through a baseband interface  1122 . Various bearer-specific WCDMA stack functions are executed by the Layer 2 processor  1108  with respect to WCDMA bearer signals communicated to and from the WCDMA modem  1116  via the baseband interface  1122 . In the embodiment of  FIG. 11 , the baseband interface  1122  comprises a shared area within the memory of the WCDMA baseband co-processor  1104 . This shared memory space may be logically configured as a dual-port RAM segmented into a number of areas, each containing a different type of data. These data types may comprise, for example, control information transferred between the protocol stacks and physical layers and uplink/downlink data. As shown, communication between the baseband interface  1122  and the WCDMA master timer  1118  may be effected via a direct access read operation  1150  or over an Advanced High Speed (AHB) bus  1160 . A description of an exemplary set of specifications for the AHB bus  1160  are set forth in, for example, the AMBA Specification, Revision 2.0 available from ARM, Inc. (www.arm.com). 
     During operation of the dual-mode wireless terminal baseband platform  1100 , the Layer 2 processor  1108  executes various WCDMA-specific functions (e.g., MAC, RLC, PDCP, BMC and RRC), GSM/GPRS stack functions, as well as various common stack functions. In WCDMA-based configurations such as  FIG. 11 , these common stack functions would include NAS functions. Finally, the WCDMA baseband co-processor  1104  is responsible for all WCDMA-related “Layer 1” or physical layer functions. 
     In the embodiment of  FIG. 11 , the host GSM/GPRS baseband processor  1101  acts as a master device with respect to the WCDMA baseband co-processor  1104 . The host GSM/GPRS processor  1101  runs a protocol stack interface that reads and writes to the baseband interface  1122  as well as to various registers of the WCDMA baseband co-processor  1104 . During operation of the platform  1100 , the shared memory space comprising the baseband interface  1122  facilitates the exchange of data between the host baseband processor  1101  and the WCDMA baseband co-processor  1104  at regular intervals. When the host baseband processor  1101  reads new information stored within this shared memory space, it also writes new information for reading by the WCDMA baseband co-processor  1104 . In the exemplary embodiment the host baseband processor  1101  may interrupt the WCDMA baseband co-processor  1104  at any time should it desire to write new data into the shared memory space of the baseband interface  1122 . This interaction between the host GSM/GPRS baseband processor  1101  and the WCDMA baseband co-processor  1104  facilitates operation of the platform  1100  within a dual-mode system. 
     During operation of the dual-mode wireless terminal baseband platform  1100 , both the GSM master timer  1112  and the WCDMA master timer  1118  update various counters consistent with the GSM and WCDMA protocols, respectively. These counters are relevant to control of for example, processing of the respective incoming (Rx) and outgoing (Tx) data streams processed by the GSM/GPRS modem  1110  and the WCDMA modem  1116 . 
     Turning now to  FIG. 12 , an illustrative representation is provided of a counter  1200  maintained by the WCDMA master timer  1118  of the WCDMA baseband co processor  1104 . The counter  1200  includes two fields; namely, a sample counter  1204  and slot counter  1208 . In the exemplary embodiment both of the counters  1204  and  1208  are free-running at every rising edge of the 15.36 MHz system clock (not shown) of the WCDMA baseband co-processor  1104 . The sample counter  1204  is incremented at the 15.36 MHz clock rate and rolls over to 0 upon reaching a count of 10239. The slot counter  1208  increments (when its count is less than 14) or rolls over (when its count is equal to 14) when the sample counter  1204  rolls over from 10239 to 0. 
     As is known to those skilled in the art, the structure of counters will vary among communication systems adhering to different protocols. For example, the structure of counters maintained by the host GSM/GPRS baseband processor  1101  differs from that depicted in  FIG. 12 . 
     During operation of the platform  1100 , the host GSM/GPRS baseband processor  1101  is disposed to synchronize its counters to the counters maintained by the WCDMA baseband co-processor  1104 . In general, the host GSM/GPRS baseband processor  1101  initiates this synchronization process by either directly or indirectly determining the values of the counters maintained by the WCDMA master timer  1118 . Once the values of the counters maintained by the WCDMA master timer  1118  have been captured, the host GSM baseband processor  1101  compares the values of the WCDMA counter values to those maintained by the GSM master timer  1112  and determines the timing relationship between the processors  1101 ,  1104 . The determination of this timing relationship effectively synchronizes, within the wireless device incorporating the dual-mode wireless terminal baseband platform  1100 , the timing of the applicable WCDMA and GSM/GPRS networks. Establishing such timing synchronization permits the wireless device incorporating the dual-mode wireless terminal baseband platform  1100  to operate contemporaneously in WCDMA and GSM/GPRS networks, and/or to be “handed off” between such networks. 
     There exist at least two potential methods for synchronizing or determining the relationship between the GSM/GPRS and WCDMA counters. Specifically, the host GSM/GPRS baseband processor  1101  may determine the values of the counters maintained by the WCDMA master timer  1118  through execution of either a “direct access read” or an “interrupt capture” method. These methods are described with reference to  FIGS. 13 and 14 , respectively. 
     Referring now to  FIG. 13 , there is shown a timing diagram  1300  which illustratively represents a timing synchronization method predicated upon execution of a direct access read operation. Pursuant to this synchronization method, the GSM/GPRS baseband processor  1101  performs a direct access read operation upon the “live” counter values generated by the WCDMA master timer  1118 . Consistent with this direct access approach, the fields of a given counter value generated by the WCDMA master timer  1118  are each read  1150  ( FIG. 11 ) by the GSM/GPRS baseband processor  1101  during a different deterministic WCDMA clock cycle. In this regard the term “deterministic” indicates that the instantaneous value of at least one counter maintained by the GSM master timer  1112  is known at the time of executing this direct access read operation; that is, the GSM/GPRS baseband processor  1101  will generally be configured to perform this direct access read operation when a particular GSM counter reaches a predetermined value. In  FIG. 13 , bsel is representative of a re-synchronized read pulse received from the GSM/GPRS baseband processor  1101 . In addition, baddr represents an address bus capable of addressing registers of the WCDMA master timer  1118 , and brdata corresponds to the data bus through which a register of the WCDMA master timer  1118  is read in connection with read operation  1150 . 
     Attention is now directed to  FIG. 14 , which depicts a timing diagram  1400  illustratively representing a timing synchronization method predicated upon execution of an interrupt capture operation. As mentioned above, the direct access approach illustrated by  FIG. 13  generally requires that each field of a given counter value maintained by the WCDMA master timer  1118  be read during a different deterministic WCDMA clock cycle. In the approach of  FIG. 14 , all fields of a WCDMA counter may be captured during the same deterministic clock cycle (i.e., during the WCDMA clock cycle which occurs upon a given counter maintained by the GSM master timer  1112  reaching a predetermined value). In particular, when a particular GSM counter reaches a predetermined value the GSM/GPRS baseband processor  1101  sends an interrupt pulse  1410  to a resynchronization pulse generator  1420  ( FIG. 11 ) of the WCDMA baseband processor  1104 . In response, the resynchronization pulse generator  1420  generates a resynchronization pulse  1430  which is provided to the WCDMA master timer  1118 . Upon receipt of this interrupt pulse by the WCDMA master timer  1118 , the WCDMA modern  1116  is instructed to capture a value  1440  of its sample counter  1204  and a value  1450  of its slot counter  1208  and store them within its sample_cnt_cap and slot_cnt_cap registers, respectively. This advantageously permits the GSM/GPRS baseband processor  1001  to access these stored values pursuant to a direct access read operation. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well-known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following Claims and their equivalents define the scope of the invention.