Abstract:
A QMIP unit receives and stores data information from a first set of modules. The QMIP unit also receives and stores a flow control indication from each module of a first set of modules. The flow control indication is indicative as to whether each module of the first set of modules is capable of receiving data. The QMIP unit creates a frame which carries the data information and the flow control indication corresponding to one of the first set of modules. The QMIP unit forwards the frame over the common data link. At the far end of the data link, the receiving QMIP unit parses the flow control indication and the data information from the frame and forwards the flow control indication and the data information to a destination module. The destination module processes the data according to normal procedures. In addition, the destination module responds to the flow control indication by ceasing the transmission of data to the sending module if so indicated.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The invention relates to communication systems. More particularly, the invention relates to communication of digital data over a shared link. 
     II. Description of the Related Art 
     FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system  10 . FIG. 1 shows three remote units  12 A,  12 B and  12 C and two base stations  14 . In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit  12 A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows the portable computer remote unit  12 B and the fixed location remote unit  12 C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system (PCS) units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal  18  from the base stations  14  to the remote units  12  and a reverse link signal  20  from the remote units  12  to the base stations  14 . 
     In the discussion that follows, to aid in illustration, the invention is described with reference to a commonly known, wireless link industry standard and the accompanying data standards which have been developed for use in conjunction with that standard. In fact, the generic principles of the invention can be directly applied to many environments. The discussion that follows assumes operation in accordance with the system described in TIA/EIA/IS-95-A published by the Telephone Industry Association entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” commonly referred to as IS-95. 
     In addition, a family of data transmission standards compatible with IS-95 have been adopted to provide data services over a wireless link. Examples of data services are FAX, digital file transfer, network access, standard modem functions and the like. An early standard is described in TIA/EIA/IS-99 entitled “Data Services Option Standard for Wideband Spread Spectrum Digital Cellular System.” Another more recent standard is described in TIA/EIA/IS-707 entitled “Data Service Options for Spread Spectrum Systems.” The IS-99 and IS-707 define radio link protocols which allow the remote unit to emulate standard modem functions. In addition to these two standards, TIA/EIA/IS-657 entitled “Packet Data Services Option for Wideband Spread Spectrum Systems” defines a radio link protocol which allows the remote unit to pass packetized data over the wireless link. 
     FIG. 2 is a block diagram of a standard remote unit  36  which comprises voice and data functionality. A central control unit  26  controls all of the functions needed for wireless voice and data services in addition to personal interface management (PIM), such as voice recording and play back. For example, the central control unit  26  may receive a telephone number entered by a user via a keypad  34  and command a communication unit  24  to establish a voice call to that telephone number. 
     The communication unit  24  provides the actual wireless voice and wireless data access capability in addition to other inherent functions such as voice playback, translation of wireless voice data to digital format for storage by the central control unit  26  and indications of wireless access status. In one embodiment, the communication unit  24  communicates with a base station according to IS-95. The communication unit  24  exchanges audio signals with a earpiece/microphone unit  28 . 
     A display  30  is used to provide visual information to the remote unit user. The central control unit  26  passes information received from both the keypad  34  and the communication unit  24  to the display  30 . For example, the central control unit  26  receives information about the current signal level received from the base station and passes the information to the display  30  where it is displayed for the user—whether or not a wireless channel has been established. In addition, the central control unit  26  passes information concerning the current wireless status to the display  30  during a call. For example, the display  30  may indicate that the channel is connected, disconnected or in the process of being connected. When an incoming call is detected by the communication unit  24 , the central control unit  26  enables a ringer  36  to alert the user. 
     In one embodiment, the central process  26  passes digital data between the communication unit  24  and an external source (not shown) directly. The transfer of data to and from the external source is accomplished by way of a external serial connector  32 . For example, a lap top or personal computer running a data service or a diagnostic monitor may be attached to the remote unit  36  via the external serial connector  32 . In another embodiment, the central control unit  26  passes serial data directly to and from a lap top or personal computer without involvement from the communication unit  24 . 
     In addition to cellular phones, other forms of portable electronics have become prevalent in the business and personal sectors. One device which is becoming increasingly popular is the personal data assistant (PDA). A PDA is like a miniature palm-held computer which allows the user to perform basic computer functions such as word processing, scheduling, spreadsheets and other such functions. 
     In order to increase the utility of a PDA, wireless functionality has been introduced to the PDA. When PDA and wireless functions are combined into a single unit having a common palm-sized casing, the resulting unit is referred to as a smartphone. A smartphone may send and retrieve e-mail, access the Internet, act as a pager and cellular telephone and provide many other wireless functions. 
     When the functionality of a remote unit is combined with the functionality of a PDA, typically a single display is used to provide information concerning the wireless link and concerning the operations of the PDA. For example, the same screen which is used to display the dialed digits and the received signal strength is also used to display e-mail messages and soft key functions. Therefore, the central processor in the remote unit must communicate with a central processor of the PDA to provide such information for display. In addition to wireless status, other functionality in the smartphone may require the transfer of information between the two portions of the smartphone. For example, the smartphone may incorporate an answering machine or voice-memo function. The digital voice samples may be stored in memory associated with the PDA portion of the smartphone. The samples are passed to the remote unit portion of the smartphone when accessed by the user so that the vocoder and speaker portion of the remote unit may be used to replay the message. In a like manner, when an incoming message is recorded, digital voice samples must be passed from the remote unit portion of the smartphone to the PDA portion. 
     When building a remote unit module for integration into a smartphone, it is advantageous to avoid extensive modification of the existing remote unit design. In this way, development costs as well as on-going production costs may be reduced due to economies of scale. 
     When an electronic device is designed such that a first plurality of modules communicate with a second plurality of modules over a common data link, some means of controlling the flow of data and regulating access to the common data link must be used. In some environments, it is advantageous if the modules themselves operate transparent to the use of the common data link. For example, the modules may be designed as part of an application specific integrated circuit (ASIC) which is configured to be directly connected to a set of modules rather than connected over a common data link. In order to avoid re-designing the ASIC to operate over a common data link, it is advantageous if the modules themselves operate in the same manner as if a direct connection existed. 
     However, a problem is encountered when an attempt is made to use the available digital connector on a remote unit to transfer more than one type of information. For example, referring again to FIG. 2, when IS-99 FAX data is being received and passed over the external serial connector  32 , the protocol used to transfer the data assumes a dedicated link over the external serial connector  32 . In particular, the protocol used assumes that all information is passed to a common destination. If during the transfer of an IS-99 FAX, the remote unit has updated wireless status information for the display, no means exists to transfer the information over the external serial connector  32  during the IS-99 FAX transfer because the IS-99 transfer preempts the transfer of any other type of data. 
     Therefore, there is a need in the art for a means of transferring data between multiple entities over a single serial link. The invention fulfills this need in an efficient manner. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention comprises a method of communicating between multiple entities over a common data link using a QUALCOMM multiplex interface protocol (QMIP) unit. The QMIP unit receives and stores data information from a first set of modules. For example, the first set of modules can be functional modules within a wireless access unit or they can be application modules within a smartphone. In one embodiment, the QMIP unit comprises a memory configured to be a queue which stores an indication of the module and the information. The QMIP unit also receives and stores a flow control indication from each module of a first set of modules. The flow control indication is indicative as to whether each module of the first set of modules is capable of receiving data. For example, if the entity communicating with the module from the far side of the common data link has filled the data input queue of the module, the module sets the flow control indication to signal the far-side entity to cease the transmission of data to the module. The QMIP unit creates a frame which carries the data information and the flow control indication corresponding to one of the first set of modules. The QMIP unit forwards the frame over the common data link. 
     At the far end of the data link, the receiving QMIP unit receives the frame and parses the flow control indication and the data information from the frame. The receiving QMIP forwards the flow control indication and the data information to a destination module. The destination module processes the data according to normal procedures. In addition, the destination module responds to the flow control indication by ceasing the transmission of data to the sending module if so indicated. 
     In a general embodiment of the invention, a digital apparatus has a first set of modules. Each module has a data input, a data output and a flow control output. Each module is configured to provide a flow control indication at the flow control output if unable to accept additional data at the data input. 
     A first connection module, such as the QMIP unit, has a transmission input coupled to the data output of each module of the first set of modules. The first connection module also has a flow control input coupled to the flow control output of each module of the first set of modules. The first connection unit also has a transmission output. The first connection module is configured to receive information from the transmission input and to create a frame comprising the information and the flow control indication corresponding to one of the first set of modules. The first connection unit produces the frame on the transmission output. 
     A common data link connects the first connection module to a second connection module. The common data link has a first input coupled to the transmission output of the first connection module and has a first output. The common data link is configured to pass the frame from the first input to the first output. 
     The second connection module has a reception input coupled to the first output of the common data link and has a flow control output and a data output. The second connection module is configured to parse the frame and to pass the flow control indication and the information to an intended module from a second set of modules from the flow control output and the data output, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objectives, and advantages of the invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawing: 
     FIG. 1 is an exemplary embodiment of a terrestrial wireless communication system. 
     FIG. 2 is a block diagram of a standard remote unit which comprises voice and data functionality. 
     FIG. 3 is a block diagram showing a standard configuration for a smartphone. 
     FIG. 4 is a block diagram representatively illustrating the functional and applicational modules within a smartphone. 
     FIG. 5 is a flowchart showing the operation of the QMIP unit. 
     FIG. 6 is a representative drawing showing the elements of an exemplifying frame created by the QMIP. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described herein with reference to a wireless telecommunication environment in order to illustrate the basic principles of the invention. However, the invention has broad applicability to a variety of situations in which a data link is used to transfer data between a plurality of entities. 
     FIG. 3 is a block diagram showing a standard configuration for a smartphone  50 . The smartphone  50  is comprised of a wireless access unit  76  which performs wireless functions and a number of other elements which support the functions of a personal data assistant. In one embodiment, the wireless access unit  76  comprises an antenna, a central control unit and an earpiece/microphone unit. Among other functions, the wireless access unit  76  performs the over-the-air operations, the processing of requests for origination and detection of incoming voice, data and test mode calls, wireless status monitoring, short message handling (such as IS-637 paging messages), IS-99, IS-657 and IS-707 data services functions, diagnostic functions and audio control for the earpiece and microphone. Therefore, if a voice signal is received over the wireless link, the wireless access unit  76  processes the signal and outputs an audible signal for the smartphone user. If a digital data signal is received over the wireless link, the wireless access unit  76  passes the received data over a data link such as a serial link  74 . In another embodiment, the data link may be a parallel link or other type of data bus. 
     In addition to the digital data information, the wireless access unit  76  provides wireless link status information and diagnostic information to a system controller  66  over the serial link  74 . The system controller  66  acts as an interface between the wireless access unit  76  and the remainder of the smartphone  50 . For example, when an incoming call is detected by the wireless access unit  76 , it passes a ring indication over the serial link  74  to the system controller  66 . The system controller  66  passes the ring indication to a ringer  64 . The ringer  64  attempts to alert the smartphone user such as by producing an audible sound, or a vibration. If the user answers the incoming call, the system controller  66  passes an off-hook indication to the wireless access unit  76  over the serial link  74 . 
     If a digital data call is received, the wireless access unit  76  passes the digital information over the serial link  74 . If the digital information is FAX data or other serial data stream such as IS-707.4 compliant data, the information is routed to a serial data handling entity such as FAX  60 . If the information is an e-mail message or other packetized data stream such as IS-707.5 compliant data, the information is routed to a packet data handling entity such as e-mail module  54 . 
     When the smartphone user wishes to place an outgoing voice call, he may enter a number of digits using a user interface  62 . The user interface  62  may be a standard key board, a soft-key system, a mouse, a touch screen system which works in conjunction with a display  52  or any other means by which the user may pass information to the smartphone  50 . The system controller  66  passes the dialed digits to the wireless access unit  76  within a command to initiate a telephone call. 
     The wireless access unit  76  passes information concerning wireless status to the system controller  66 . The system controller  66  passes some of the information to the display  52  where it is displayed for the user. For example, smartphones typically display an indication of the current received signal strength being received from the base station by the wireless access unit  76 . This information is passed repeatedly from the wireless access unit  76  to the system controller  66  and to the display  52  even during an active call. 
     In one embodiment, the smartphone  50  also comprises a voice mail function  56 . If a voice call is received through the wireless access unit  76  and the user is not available to answer the call, the smartphone  50  may record a message for later retrieval by the user. The wireless access unit  76  receives digital voice samples over the wireless link. Instead of decoding the digital voice signals and outputting audio signals over an earpiece, the wireless access unit  76  passes the digital voice samples over the serial link  74  to the system unit controller  66 . The system controller  66  forwards the digital voice samples to a memory storage location within the voice mail function  56 . When the user retrieves the voice mail, the system unit controller  66  passes the digital voice samples from the voice mail function  56  to the wireless access unit  76  over the serial link  74 . The wireless access unit  76  decodes the stored voice samples and outputs the resultant signal over the earpiece. 
     The smartphone  50  may connect to an external data source such as if the smartphone is placed in a dock and connected to a computer. In this case, information may be passed over an external connector  82 . For example, a wireless connection may communicate serial data or packetized data between an externally connected component and a base station. Digital data is received from the external connector  82  and passed to the system controller  66 . The system controller  66  passes the information over the serial link  74  to the wireless access unit  76  for transmission over the wireless link. An analogous reverse path is also established. 
     FIG. 4 is a block diagram functionally illustrating the operation of the smartphone  50 . The wireless access unit  76  is shown to have an antenna  100 , a transceiver  102  and various functional modules  104 A through  104 N. For example, the diagnostic functional module  104 A provides diagnostic functionality such as monitoring current signal strengths. The voice functional module  104 B provides vocoding and other functions associated with the transmission and reception of audio signals. The data services functional module  104 C provides data services functions as described above. The wireless access unit can comprise other functional modules as represented by functional module  104 N. 
     When integrating the wireless access unit  76  into a smartphone  50 , it is advantageous that the functional modules  104 A- 104 N operate in the same manner as if the wireless access unit  76  were the main component of a standard telephone unit. In this way, the wireless access unit.  76  is truly modular and can be used to implement wireless functions for a variety of different equipment. In other words, it is advantageous if the coupling of the remainder of the smartphone  50  is transparent to the functional modules  104 A- 104 N. 
     An issue arises when one or more of the functional modules  104 A- 104 N are operating contemporaneously. In general, several of the functional modules  104 A- 104 N operate contemporaneously in standard operation. For example, typically the diagnostic functional module  104 A operates continually whether or not one or more of the other functional modules is operating. In addition, it is common for the voice functional module  104 B to operate at the same time as one or more of the data functional modules so that a user may use the smartphone  50  as a telephone while continuing to transmit fax information or to receive an e-mail message, for example. In a simple telephone unit, each of the functional modules  104 A- 104 N is directly coupled to a corresponding application module  108 A- 108 N. However, as noted above, in the case of the wireless access unit  76  within the smartphone  50 , the functional modules  104 A- 104 N interface with the other components of the smartphone  50  through the shared serial interface  74 , also shown in FIG.  4 . 
     In addition to the wireless access unit  76 , the smartphone  50  contains the application modules  108 A- 108 N. For example, the user interface application module  108 A receives input from the human user of the smartphone  50 . The TCP/IP stack application module  108 B operates the web browser or e-mail applications on the smartphone  50 . The fax application module  108 C transmits and receives fax information. Likewise, the smartphone  50  may contain other application modules as represented by the application module  108 N. Some means of multiplexing communication over the serial interface  74  is needed to facilitate communication between the application modules and functional modules. In FIG. 4, the functional modules  104 A- 104 N and the application modules  108 A- 108 N can be thought of as logical or virtual circuits. In one embodiment, each of these modules represents a portion of software, hardware including general purpose circuits as well as application-specific circuits, and firmware. 
     Typically, the wireless link is not capable of transmitting data as quickly as it can be transferred over the serial interface  74 . For example, if the TCP/IP stack application module  108 B is forwarding an e-mail message for transmission over the wireless link, the TCP/IP stack application module  108 B can transmit data over the serial interface  74  to the functional module  104 C at a faster rate than the data services functional module  104 C can forward the data over the wireless link via the transceiver  102  and the antenna  100 . For this reason, some means of flow control over the serial interface  74  is necessary in order to avoid overflowing the memory storage capability of the data service functional module  104 C. The flow control process must take place on a module-by-module basis. 
     Therefore, on each side of the serial interface  74 , a QUALCOMM multiplex interface protocol (QMIP) unit  110  is used to regulate access to the serial interface  74  and to provide flow control. The QMIP units  110 A and  110 B on either side of the serial interface  74  operate in the same manner. In the discussions that follow, the operation of passing information from the QMIP unit  110 B to the QMIP  110 A is described. It is to be understood that the reverse process operates in the same manner. The QMIP  110  can be implemented as a microprocessor and associated memory, as one or more software modules or a combination of these. 
     In one embodiment, the QMIP unit  110 B is designed to use most standard data terminal equipment/data communication equipment (DTE/DCE) interfaces such as EIA-232, EIA-422, EIA-423, universal serial bus (USB) as well as a shared memory or inter-process message queuing. Only receive and transmit data signals are passed over the serial interface  74 . Other signals such as data terminal ready (DTR) and data set ready (DSR), data carrier detect (DCD), request to send (RTS) and clear to send (CTS) can be supported. Hardware flow control can also be supported if available. The software flow control is provided on a virtual circuit basis as discussed below with respect to FIG. 5, but hardware flow control shall suspend traffic on the serial interface  74  if asserted for all virtual circuits. 
     FIG. 5 is a flowchart showing the operation of the QMIP unit  110 B as it creates a QMIP frame. The functions described in FIG. 5 are used to develop a frame for transmission over the serial interface  74 . FIG. 6 is a representative drawing showing the elements of an exemplifying frame format used by the QMIP  110 B. FIG. 6 is used below to aid in illustration of the functions shown in FIG. 5. A frame  172  shown in FIG. 6 is comprised of four portions: a control/address portion  174 , an additional optional control portion  176 , an information portion  178 , and an end flag portion  180 . The exemplifying frame format shown in FIG. 6 is only one of many frame formats and a plurality of frame formats can be developed, such as, for example, by the simple rearrangement of the portions within the frame, the simple re-arrangement of the bits within the portions or by the addition or removal of bits or portions. 
     The QMIP  110 B is configure to receive information from each of the functional modules  104 A- 104 N. Typically, the QMIP  110 B comprises one or more memory queues which are used to store the information from the various functional modules as it arrives. The basic functions disclosed in FIG. 5 assume that the information has already been received by the QMIP  110 B and placed within a queue. Either the queue itself uniquely identifies the functional module from which the information is received, or, alternatively, an entry in the queue indicates from which of the functional modules the information is received. As noted above, the functional modules operate transparently to the operation of the QMIP  110 B and, therefore, in some cases may not self-identify themselves within the information. 
     In block  120 , the QMIP  110 B retrieves information from the queue corresponding to one of the functional modules  104 . In block  122 , the QMIP  110  determines whether the functional module  104  is able to accept data. As noted above, the QMIP  110 B facilitates bi-directional communication over the serial link  74 . Typically, one or more of the functional modules  104  utilizes flow control for data reception. For example, if the data services functional module  104 C is receiving data from the TCP/IP stack application module  108 B at a rate faster than the data services functional module  104 C can queue the data for transmission over the wireless link, periodically the data services functional module  104 C exerts a flow control indication intended for the TCP/IP stack to cease the flow of data to the data service functional module  104 C. As the stored data within the data service functional module  104 C is transmitted over the wireless link, the data services functional module  104 C exerts an indication intended for the TCP/IP stack application module  108 B indicating that it is once again able to accept data. According to the invention, the flow control information corresponding to data received by the data services functional module  104 C can be included in a QMIP frame  172  which also carries information to the corresponding application module  108 B. In addition, in some cases, a frame is created which carries control information but does not carry payload data information for transmission to the application module. 
     Within the frame  172 , a flow control bit  188  within the control/address portion is used to implement software flow control on a logical circuit basis. Thus, in one embodiment, when the functional modules  104  send flow control information, the QMIP  110  intercepts the flow control information and queues it for transmission according to the flowchart shown in FIG.  5 . In block  122 , the QMIP  110 B determines whether the corresponding functional module  104  has indicated that it is unable to accept more data by reference to the flow control queue. If the functional module  104  can no longer accept data, the flow control bit  188  is set to 1 as indicated in block  124 . If the functional module  104  is able to accept data, the flow control bit  188  is set to 0 as indicated in block  126 . In either case, flow continues to block  128 . 
     In block  128 , the QMIP  110 B determines whether the persistence timer associated with the functional module has expired. If the persistence timer has expired, a poll bit  182  within the control/address portion  174  is set to 1 in block  130 . If the persistence timer has not expired, the poll bit  182  is set to 0 in block  132 . 
     The use of the poll bit  182  is designed to prevent deadlock in the following situation. Assume that a functional module  104  notifies the QMIP  110 B that it can no longer accept data. The QMIP  110 B creates a frame  172  with the flow control bit  188  set to 1. When the corresponding indication is received at the corresponding application module  108 , the application module  108  ceases to transmit data to the specified functional module  104  and begins to wait for a frame with a flow control bit set to 0. If the functional module  104  sends an indication to the QMIP  110 B that it is able to accept data and the QMIP  110 B creates a corresponding frame  172  with the flow control bit  188  set to 0, a deadlock occurs if the frame is lost or corrupted in transmission so that the corresponding application module  108  does not receive the indication. In such a case, the functional module  104  is waiting for data from the application module  108  and the application module  108  is waiting for the indication from the functional module  104  that it is ready for additional data. 
     In order to prevent this deadlock, according to the invention, when the QMIP  110 A receives a frame with a flow control bit set to 1, it sets a corresponding persistence timer associated with either the functional module  104  or the application module  108 , alternatively a timer associated with both the functional module  104  and the application module  108 . If the timer expires before a frame is received with a flow control bit set equal to 1, the QMIP  110 A creates a frame with the poll bit set equal to 1and indicating the address of the corresponding functional module  104 . When the QMIP  110 B receives the message, if the corresponding functional module  104  is currently capable of receiving data, the QMIP  110 B creates a frame  172  with a flow control bit  188  set to 0 indicating that the corresponding functional module is able to accept data. 
     In block  134 , the QMIP  110 B determines whether the information or the functional module  104  has a high priority indication. If so, an optional control bit  184  is set to 1 within the block  136 . If not, the optional control bit  184  is set to 0 in the block  138 . In one embodiment, the additional optional control portion  176  is used to specify a class of service. The high priority frame can receive preferential processing within the QMIP  110 A, QMIP  110 B, within the destination module or a combination of these. 
     In block  140 , the QMIP  110 B determines whether the information is in-band control data. If the application module for which this frame is intended has implemented a special procedure for in-band control, an in-band control bit  186  is set to 1 in block  142 . If not, in block  144  the in-band control bit  186  is set to 0 within block  144 . 
     In block  146 , the QMIP  110  sets an address field  190  to identify the functional module  104 . When the QMIP  110 A receives the frame, it uses the address field to determine the proper destination application module  108 . For example, the QMIP  110 A may determine the proper application module  108  by reference to a look-up table. In another embodiment, the QMIP  110 B refers to a look-up table which maps the functional module  104  address to the corresponding application module  108  address and the QMIP  110 B sets the address field  190  to the corresponding application module  108  address. In either case, the look-up table can be predetermined by design, determined by a higher level protocol or determined on a virtual circuit basis. 
     In block  148 , the information received from the application module  108 , if any, is placed within the information portion  178  of the frame  172 . The information portion  178  contains the data that is being communicated from the functional module  104  to the application module  108 . In one embodiment, the maximum size of the information field is 36 bits. In one embodiment, any information which corresponds to an ending flag character or escape character is replaced with a 2-byte sequence. 
     In block  152 , the QMIP  110 B places an end flag indication in the end flag portion  180 . The ending flag delineates the end of the frame. In one embodiment, it consists of an 8-bit binary sequence such as  8 E (HEX). The end flag is used by the QMIP  110 A to identify the end of a frame. 
     In one embodiment, an escape character is used to provide data transparency. In one embodiment, it consists of the binary sequence  8 D (HEX). The escape character can be used to pass data values which equal the ending flag designation value. 
     The process of receiving and processing the frames executed by the QMIP  110 A basically follows the analogous reverse-process to that shown in FIG.  5 . The QMIP  110 A receives the frames, parses them to determine the destination, flow control status and other information as described above. In one embodiment, frames with unrecognized control/address portion  174  values are discarded by the receiving QMIP  110 A. 
     By the processes and mechanisms just described, multiple entities can communicate over a common data link while flow control is individually performed according to the abilities and demands of the various entities. 
     One advantage of the invention is its relative simplicity. The execution of the invention requires minimal processing power. In addition, the code space required to store the processes of the invention is relatively small. These two advantages are important in the smartphone environment where the processing power and memory capacity are limited to reduce cost. In addition, the invention allows for the efficient transfer of data bytes due to the minimal overhead associated with the frame format. The frame is not cluttered with the unnecessary and burdensome overhead content associated with most standard multiplexing protocols. As noted above, one type of data which may be transferred using the QMIP is voice data. Therefore, it is important to limit the amount of delay, especially random delay, caused by the multiplex protocol in order to preserve voice quality. Therefore, by limiting the overhead and, thereby, increasing the data carrying capacity of the band-limited serial link, the invention preserves voice quality. In addition to the rapid transfer of the data over the band-limited serial link, the relatively simple multiplex protocol also creates the frames quite quickly, thereby further decreasing the latency and random delay associated with the multiplex protocol. Each of these considerations allows the invention to be implemented in an inexpensive, time-sensitive, memory-conservative and processing-conservative manner. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiment can be considered in all respects only as illustrative and not as restrictive, and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.