Method and apparatus for monitoring link activity to prevent system deadlock in a dispatch system

The present invention relates generally to a diapatch system, and more specifically to the implementation of a dispatch system within a cellular system.

In radiotelephone communication systems, many users communicate via radio channels to connect to other radiotelephone and wired telephone systems. Communication over wireless channels can be one of a variety of multi-dimensional access techniques. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA). CDMA techniques have many advantages. An exemplary CDMA system has been transferred to the transferee of the present invention and is referenced herein by K. Gilhousen et al., U.S. Pat. No. 4,901,307 [filed February 13, 1990, "Satellite Repeater or It is described in "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS"].

In the aforementioned patent, a large number of mobile phone system users, each with one transceiver, use satellite repeaters, airborne repeaters, or ground base station transceiver subs that use CDMA spread spectrum communication signals. Multiple access techniques for communicating through the system are disclosed. In using CDMA communication, the frequency spectrum can be reused multiple times, allowing an increase in the capacity of system users.

In a CDMA cellular system, each base station transceiver subsystem provides coverage over a limited geographic area, and remote units within that coverage area are connected to the public switched telephone network (PSTN) via a cellular system switch. Links to.

When the remote device moves to the coverage area of the new base station transceiver subsystem, the routing of the user's call is forwarded to the new base station transceiver subsystem. The signal transmission path from the base station to the remote device is called a forward link, and the signal transmission path from the remote device to the base station is called a reverse link.

In a typical radiotelephone network, the remote device may utilize a vocoding system that encodes voice information in a variable speed format. In variable speed systems, the data speed may be reduced due to pauses in voice activity. Lower data rates reduce the level of interference to other users caused by remote device transmission. In base stations, a bocoding system is used to reconstruct audio information. In addition to audio information, data information alone, or a mixture of the two, may be transmitted by a remote device.

When the remote device is creating its own data for transmission, the built-in vocoder is based on voice activity in 20 milliseconds (ms) frames, for example, about 8,000 bits per second (bps), 4,000. Generates data encoded at four different speeds, such as bps, 2,000 bps and 1,000 bps, from digital samples of audio information. Each frame of vocoder data is formatted with overhead bits such as 9,600bps, 4,800bps, 2,400bps, and 1,200bps data frames. The fastest data frame that corresponds to a 9,600 bps frame is called a "full rate" frame. Data frames at 4,800 bps are called "half rate" frames. Data frames of 2,400 bps are called "quarter rate" frames. And the 1,200 bps data frame is "1/8 rate (eighth) It is called a "rate)" frame. A vocoder suitable for application in this environment is described in US Pat. No. 5,414,796, entitled "VARIABLE RATE VOCODER," issued May 9, 1995 and assigned to the assignee of the invention. Has been done. When the remote device receives data from an external source such as a terminal equipment unit, it continues to process that data in this variable speed frame format.

When the original cellular telephone spectrum license was issued by the government, one of the restrictions on the use of spectrum was that carriers could not provide dispatch services. However, due to the significant advantages of CDMA systems and the inherent costs and issues of deploying and maintaining private dispatch systems, the government is reviewing this issue. The government itself will greatly benefit from such services.

Typical wireless and wired telephone services provide point-to-point services, while dispatch services provide one-to-many services. Common uses of dispatch services are local police radio systems, taxi express systems, Federal Bureau of Investigation and Confidential Investigation Department operations, and general military communications systems.

The basic model of the dispatch system consists of the user's broadcast net. Each broadcast net user monitors one common broadcast forward link signal. When a net user wants to have a conversation, he presses the push-to-talk button and is granted system talker privilege. Typically, the voice of the transmitting user is routed over the broadcast forward link. Ideally, the dispatch system would allow landline and wireless access to the system. The remote device user releases the PTT button when the conversation ends. In response, the remote device creates a push-to-talk-off display that terminates privileges and releases the system for use by other system users.

If the push-to-talk button on the remote device is stuck down, the remote device is granted system caller privileges. System resources are expanded in this way, and other remote devices access the system because the remote device with the push-to-talk button that has stopped and stuck is blocking the system. Is hindered. This type of scenario is called a system deadlock, and it goes without saying that this is a highly undesirable condition. The present invention is a method and device for detecting a system deadlock and truncate its harmful effects on the body and mind.

When the remote device user presses the push-to-talk button, the communication manager can grant the remote device system caller privileges. When the remote device has system caller privileges, his voice signal is broadcast to other remote devices that are members of the dispatch system. When the remote device user releases the push-to-talk button, the communication manager denys the remote device's system caller privilege, which allows the system to have system caller privilege freely with respect to other remote devices. It will be possible. When the remote device's push-to-talk button stops and becomes stuck, the remote device retains system caller privileges, thus preventing other remote devices from obtaining system caller privilege grants. The present invention monitors the voice activity of a signal received from a remote device at a base station. When voice activity falls below a certain level, it is assumed that the remote device no longer requires system caller privileges, and the base station is sent to the communications manager instead of releasing the push-to-talk button. (surrogate) Generate a display. In this way, the communication manager is free to grant system caller privileges to another remote device.

<figref num="1">Block diagram of a typical dispatch system.</figref><figref num="2">A flowchart showing a set of exemplary steps for carrying out the present invention.</figref>

The features, objectives and advantages of the present invention will become even more apparent from the detailed description set forth below when interpreted with the drawings. Figure 1 shows a typical dispatch system. In a preferred embodiment, remote devices 10, 20, 22 and 24 can function as both a dispatch device and a point-to-point telephone. In Figure 1, remote device 10 is currently active, and remote devices 20, 22, and 24 are currently passive listeners. Base station antennas 30, 32, and 34 may provide broadcast forward link channels to remote devices 20, 22, and 24. The base station antenna 30 transmits and receives a dedicated forward traffic channel and a reverse traffic channel to and from the remote device 10. Dedicated traffic channels are, for example, remote device 10 signaling other remote device special signaling information such as power control commands. Similar to a forward link broadcast channel, except that it can receive information). The Mobile Exchange Center (MSC) 38 coordinates signaling to and from a set of base station transceiver subsystems, including base station transceiver subsystems 44, 48, and 50. The system with base station antennas 30, 32, and 34, as well as base station transceiver subsystems 44, 48, 50, and MSC 38 is referred to as base station 28. The communication manager 40 manages the net, such as granting system caller privileges to a remote device whose user presses the "push-to-talk" (PTT) button. In a preferred embodiment, air interface signaling and modulation is the Mobile Station-Base Station Compatibility Standard for, commonly referred to simply as IS-95. Dual-Mode Wideband Spread Spectrum Cellular Systems) Complies with the Code Division Multiple Access (CDMA) systems described in TIA / EIA / IS-95. In IS-95, the remote device is called a mobile station.

It is technically well known that a base station transceiver subsystem may be sectorized into, for example, three sectors. When the term base station or base station transceiver subsystem is used herein, it is implied that the term may refer to the entire base station transceiver subsystem or a single sector of the base station transceiver subsystem.

In FIG. 1, the active remote device 10 has an established bidirectional link with the base station transceiver subsystem 44. To be active, the remote device 10 sends an access channel message requesting a traffic channel to the base station transceiver subsystem 44. Access messages are sent on the access channel. An access channel is a reverse link channel used by a remote device to communicate with a base station. The access channel is a shared slotted random access channel. Only one remote device per base station transceiver subsystem sector per frequency channel can successfully use access channels at one time. Access channels are used for short signaling message exchanges such as calling, answering pages and registering. An access attempt is a series of access probes (access It is sent by a remote device by probe). Each access probe carries the same information, but is transmitted at a higher power level than the previous power level. The access probe continues until a base station acknowledgment is received by the remote device.

The remote device 10 receives the signaling present on the forward broadcast channel on the dedicated forward link traffic channel when it establishes a communication link. In this way, the remote device 10 does not monitor the forward link broadcast channel, but it receives all of the dispatch system information on its own dedicated forward link traffic channel. The remote device 10 communicates back to the base station transceiver subsystem 44 via a dedicated reverse channel. Since the remote device 10 has its own dedicated forward link signal path, remote device special messaging may be included in the signaling. For example, if the remote device 10 can act as both a dispatch system remote device and a point-to-point telephone device, the remote device 10 directs incoming point-to-point calls to the remote device 10 on the forward link traffic channel. You may be notified that you are.

On the other hand, in FIG. 1, passive remote devices 20, 22 and 24 do not have an established reverse link signal to any of the base station transceiver subsystems. Even though the remote devices 20, 22, and 24 are passive, they can still communicate with the base station using access channels. In a preferred embodiment, passive remote devices 20, 22 and 24 use access channels to signal the base station transceiver subsystem whether they require more power from the forward link broadcast channel. ). The base station transceiver subsystem may increase the transmit power level of the forward link broadcast channel in response to a power request access message.

In a standard CDMA system, the process of allocating resources to activate a remote device can take seconds, not just a substantial amount of processing resources. In a preferred embodiment, when a remote device presses a push-to-talk button, a set of resources is allocated to save system resources and avoid associated delays. When the remote device releases the push-to-talk button, the resource remains dedicated to the remote device for a period of time. During the time the user is not pressing the push-to-talk button, the remote device is indicated to be active and is said to be hanging. The hung remote device sends and receives a series of slow idle messages and saves link power control. In this way, when the remote device user subsequently presses the push-to-talk button, the link is fully established and responds immediately. This kind of behavior fits the use of the dispatch system's natural dialog. Resources can be released when the pause between push-to-talk activations exceeds the threshold. After the resource is released, the remote device must reestablish the connection by sending an origination message on the access channel. It is true that only one remote device can talk at any given time, but it is possible that two or more remote devices are active.

The behavior of the system described above can be quite different from standard push-to-talk behavior. A typical push-to-talk system is implemented using one common frequency or a set of two frequencies. Once the remote device user presses the push-to-talk button, he is transmitting on a common frequency, blocking everything else from accessing the channel (block). The user also blocks the channel by pressing his push-to-talk button, even when another user is speaking for the first time. Normally, while the caller is talking, his or her receiver is disabled to avoid feedback. In this way, if a remote device user presses a push-to-talk button and his or her receiver is disabled, that user will not hear his or her own voice. Therefore, if the push-to-talk button on one device stops and does not move, not only will other users not be able to access the system, but the user himself will also be warned if an overriding message is sent. ) You may not be able to hear the message.

In a standard push-to-talk system, there is no requirement for system caller privileges and no corresponding authorization for system caller privileges. Also, there is no way to deny system caller privileges after the remote device presses the push-to-talk button. Also, in a typical system, it is difficult to detect speech activity. The present invention is quite different. In a preferred embodiment, a CDMA multiple access technique is used. (In another embodiment, other multiple access techniques may be used). In a CDMA system, two or more remote devices may transmit at the same frequency at the same time. Even if the remote device transmits continuously, other remote devices in the area should use the same frequency to communicate on the access channel, dedicated traffic channel and forward link broadcast channel, and on others. Remains possible. Also note that while the remote device is busy and generating the reverse link traffic channel signal, it will continue to receive the forward link traffic channel signal. If the remote device user's voice is not included in the forward link traffic channel signal, the speaker on the remote device remains enabled while the remote device is designated as the system caller. In this way, a privileged device can generate a voice message to its remote device even when the person's push-to-talk button is pressed.

When the user first presses the push-to-talk button, the PTT_on display is sent from the remote device to the base station. When the user releases the push-to-talk button, the PTT_off indication is sent from the remote device to the base station. Normally, it is unlikely that other users will be granted push-to-talk access until the PTT_off display is received. One aspect of the invention addresses a situation where the push-to-talk button malfunctions so that the PTT_off display is not transmitted.

In a preferred embodiment, the remote device comprises a multirate vocoder. Multi-speed vocoders transmit at low speeds when voice or data activity is minimized, and at higher speeds when level voice or data activity is high. In a preferred embodiment, the built-in vocabulary is based on audio activity during 20 milliseconds (ms) frames at four different rates, eg, about 8,000 bits per second (bps), 4,000 bps, 2,000 bps, and 1,000 bps. , Generates encoded data from a digital sample of audio information. Each frame of vocoder data is formatted with overhead bits such as 9,600bps, 4,800bps, 2,400bps, and 1,200bps data frames. The fastest data frame that corresponds to a 9,600 bps frame is called a "full rate" frame. Data frames at 4,800 bps are called "half-rate" frames. Data frames at 2,400 bps are called "1/4 rate" frames. And a 1,200 bps data frame is called a "1/8 rate" frame. A vocoder suitable for application in this environment is described in US Pat. No. 5,414,796 [filed May 9, 1995, "VARIALE RATE VOCODER"], which was assigned to the assignee of the present invention. Even when the remote device receives data from an external source such as a terminal device unit, the remote device continues processing the data in variable speed frame format. The present invention can take advantage of the vocoder sending 1/8 rate frames when the smallest link data is present.

There are two different scenarios that the present invention addresses. In the first scenario, the user presses the push-to-talk button. The remote device sends a PTT_on display and receives the channel assignment. However, the user does not speak intentionally or accidentally. In addition, the PTT_off indication is not received at the base station. The base station may not receive the PTT_off display if the user does not release the push-to-talk button. The base station may not receive the PTT_off indication if the button stops and does not move, or if the remote device is otherwise malfunctioning.

In a similar but slightly different second scenario, the user presses a push-to-talk button. The remote device sends a PTT_on display and receives the channel assignment. The user speaks and sends voice traffic to other net members. However, intentionally or accidentally, the user stops speaking for an extended period of time. Again, the PTT_off indication is not received at the base station. The base station may not receive the PTT_off display if the user does not release the push-to-talk button. The base station may not receive the PTT_off indication if the button stops and does not move, or if the remote device is otherwise malfunctioning.

In both scenarios, the system is deadlocked unless one of its members of the net is designated as a higher priority user and the wrong remote device cannot be "interrupted". In such cases, other remote devices on the net cannot be "callers" and therefore disable the network. The present invention prevents such deadlocks by monitoring link (voice or data) activity. The present invention is an assignment of the United States Patent Application No. 08 / 671,132 to this assignee [filed June 24, 1996, "METHOD AND APPRATUS FOR ACCESS". REGULATION AND SYSTEM PROTECTION OF A DISPATCH SYSTEM)], and US Patent Application No. 08 / 671,131 transferred to this assignee [filed June 24, 1996, "Efficient System Access in Dispatch Systems" Methods and equipment for (METHODD AND APPRATUS FOR EFFICIENT SYSTEM ACCESS IN A DISPATCH SYSTEM]] can be used in conjunction with other mechanisms to prevent deadlocks and regulate system access.

There are multiple ways to detect link activity. When a vocoder similar to the one described above is used, the average number of slow frames received over a period of time is calculated. This method is also applicable to data transmission. In this way, the lack of a valid continuous audio signal is still detected when background noise occasionally results in faster frames. Other fixed speed digital vocoders may use different coding methods depending on whether the audio signal is voice or non-voice noise. The base station may monitor the display of the type of coding used to code the signal to detect link activity. Another method can monitor the spectral content of the encoded signal to determine the presence or absence of audio. The absence of data on a data connection can be much easier to detect. For example, a base station may simply look at the average number of bits having the value "1" or the transitions of incoming signals compared to the total number of bits.

FIG. 2 is a flowchart illustrating the basic operation of the present invention. In a preferred embodiment, the system is run by base station 28 (FIG. 1). This system is very likely to be located within the MSC38 (Figure 1), while some of those operations can occur within the base station transceiver subsystem. In a very common embodiment, the system may be located at any part of the communication system. The system shown in Figure 2 runs once for each remote device that has system caller privileges.

From start block 100, the action begins when a system caller privilege authorization is received for the remote device, block 102. Such authorization typically responds to PTT_on indications received from remote devices. The two counts are also set to their initial values in block 102. When the first data is transferred from the remote device to the base station, the base station determines if there is sufficient link activity to indicate the reception of active voice or data communication, block 106. If not, the T1 count is incremented to reflect the passage of time, block 108. Block 110 asks if a denial of system caller privileges has been received. Such a denial may be received if the remote device releases the push-to-talk button, or if the remote device is interrupted by another remote device. If a denial is received, the flow ends at block 114. If no denial is received, the flow continues to block 112. If the T1 count does not exceed the threshold of threshold 1 in block 112, the flow returns to block 106 and continues. When the T1 count exceeds the threshold of threshold 1 in block 112, the base station creates an alternative PTT_off display and sends it to the communications manager in block 134, thereby using it by other remote devices. Release the system for.

When link activity is detected in block 106, it enters the second phase of the flowchart dealing with the second scenario above. As data continues to be transferred from the remote device to the base station, block 116 monitors for link activity. At block 118, the T2 count is incremented to reflect the passage of time if there is not enough link activity to indicate the reception of active voice or data communication. Block 120 asks if a denial of system caller privileges has been received. If a denial is received, the flow ends at block 128. If no denial is received, the flow continues to block 122. If the T2 count does not exceed the threshold of threshold 2 in block 122, the flow returns to block 116 and continues. When the T2 count exceeds the threshold of threshold 2 in block 122, the base station raises an alternative PTT_off indication and sends it to the communications manager in block 134, thereby for use by other remote devices. Release the system and the flow ends at block 138. If link activity is detected in block 116, the T2 count is reset, block 126. Block 130 asks if a denial of system caller privileges has been received. If a denial is received, the flow ends at block 136. If no denial is received, the flow continues to block 116.

The mechanisms deployed in blocks 106 and 116 for detecting link activity can be the same or different. One example of a link detection mechanism is to count the average number of slow frames received over a fixed period of time. In this way, the lack of a valid continuous audio signal is still detected when background noise occasionally results in higher speed frames. In a preferred embodiment, the vocoder frame is transferred over the air at a rate of 1 per 20 msec. The mechanism of blocks 106 and 116 may monitor 16 consecutive frames. If 15 of 16 of the frames have 1/8 rate data, the link activity is not enough to indicate active use.

When the base station sends an alternative PTT_off display to the communication manager, the communication manager may or may not be able to distinguish the alternative display from the PTT_off display received directly from the remote device. When the communication manager receives an alternative PTT_off indication, it may respond by sending a denial of system caller privileges to the remote device. If the communication manager cannot distinguish between an alternative PTT_off display and a PTT_off display created by a remote device, the communication manager may send a denial in response to any PTT_off display it receives. If the transmit manager can distinguish between the two, it should only send a denial when an alternative PTT_off is received. In a preferred embodiment, the denial is sent to the remote device on the reverse link traffic channel. In response to the denial, the remote device either stops transmitting the active audio signal and becomes inactive or goes into a hung state. The remote device may respond by alerting the user or taking corrective action.

Note what the flowchart shown in Figure 2 works for. The top loop, which usually consists of blocks 106, 108, 110 and 112, relates to the first scenario. The top loop monitors link activity until active voice or data communication is received. When the upper loop ends, the lower loop, which generally consists of blocks 116, 118, 120 and 122, is involved in the second scenario. The lower loop continues to monitor the link activity of the incoming signal as the call progresses. Note that the values for threshold 1 and threshold 2 do not have to be the same. If these values are selected as the same value, the loop collapses into a single loop. In the most common situation, threshold 1 has a value less than threshold 2. When a user first presses the push-to-talk button, it is assumed that the user wants to communicate with something. If the user does not start communicating messages immediately, it is probably due to an error or a deliberate attempt to deadlock the system. The threshold 2 is long because it is assumed that when a user starts talking, the user can pause during the conversation. Threshold 1 may have a value such as 5-10 seconds. Threshold 2 may have a value such as 10-20 seconds.

In the most common implementation, the invention is a method and apparatus for monitoring the absence of audio or data information in a push-to-talk system to prevent system deadlock. Needless to say, the general principles shown in Figure 2 could be applied to a large number of different embodiments. For example, the lowest speed frame does not have to be a 1/8 rate frame in another embodiment. The present invention also prevents deadlock if a microphone or other voice path component fails in a remote device and therefore no voice signal is transmitted from that remote device. There are many modifications and practices within the technical scope of the invention. The practice may include all the elements of the invention, but does not necessarily follow the flowchart of FIG. For example, the fact that the entire flowchart process is aborted when a base station receives a PTT_off indication from a remote device at any given time is evident in the figure. Obviously, the same effect could be achieved by using interruptions rather than regular inquiry. Also, the blocks can be rearranged in the flow without affecting the operation of the system. It should also be noted that even if the body of this specification refers to "remote" devices, some of the devices may be wired devices.

The description of a preferred embodiment is provided to allow one of ordinary skill in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles set forth herein can be applied to other embodiments without the use of the capabilities of the present invention. Therefore, the invention is not limited to the examples presented herein, but must be given the broadest scope consistent with the principles and novel features disclosed herein. The inventions described in the claims of the original application of the present application are described below. [1] A method for preventing deadlocks in a dispatch system in which a set of remote devices communicate with each other by broadcasting one at a time, with the following steps: Receive a request to become a system caller from a remote device at a base station; At the base station, the communication manager receives authorization to become the system caller for the remote device; The base station monitors a series of data from the remote device to detect voice activity; The base station sends the communication manager an alternative request to quit as the system caller of the remote device when the voice activity falls below a threshold. [2] How a single remote device at any given time can prevent deadlock in a push-to-talk system that may be designated as a system caller, with the following steps: Monitor link activity from remote devices currently designated as the system caller; If the lack of link activity is detected, a denial of system caller privileges is sent to the remote device. [3] The method according to [2] above, wherein the monitoring step comprises the following steps: Examine the spectral content of the signal received from the remote device; Compare the spectral content to a first predetermined threshold; The lack of link activity is determined when the spectral content is below the first predetermined threshold. [4] The method according to [2] above, wherein the signal received from the remote device comprises digital data, wherein the examination step comprises the following steps: Examine the transition between the "1" and "0" logical states of the digital data; If the average number of the transitions is below the first predetermined threshold, it is determined that the lack of link activity is present. [5] The method according to [2] above, wherein the signal received from the remote device comprises digital data, wherein the monitoring step further comprises the following steps: The average number of occurrences of the logical state "1" in the digital data is calculated by comparing with the total number of received digital data; If the average number of occurrences is below the first threshold or above the second threshold, it is determined that the lack of link activity is present. [6] The signal from the remote device comprises variable velocity data organized in frames, with each frame having one of a preselected set of data velocities determining the lack of link activity. The method according to [2] above, further comprising the step of evaluating the frame in order to do so. [7] The signal from the remote device comprises variable speed data organized in frames, wherein each frame has one of a set of preselected data speeds, wherein the monitored step is The method according to [2] above, further comprising the following steps: Find the average number of frames received from the remote device at the lowest of the set of preselected data velocities. If the average number of the frames exceeds the first threshold during the first length of time, it is determined that the lack of link activity is present. [8] The method according to [7] above, wherein the monitored step further comprises the following steps: Detects the initial presence of link activity when a large number of frames are received at a speed other than the lowest of the set of preselected data rates; If the average number of the frames exceeds the second threshold during the second length of time, after detecting the initial presence of the link activity, it is determined that the lack of the link activity is present. [9] The method according to [8] above, wherein the length of the second time is larger than the length of the first time. [10] How to prevent deadlocks in a dispatch system with the following steps: Establish communication resources for use as a system caller by the first remote device; Receive a request for system caller privileges from the first remote device; Grant the system caller privilege to the first remote device; Receive a traffic signal from the first remote device; Monitor link activity for said traffic signals; If the link becomes inactive, the system caller privilege to the first remote device is denied. [11] The method according to [10] above, wherein the link is defined as inactive when the link activity is reduced below a predetermined level. [12] The communication resource for use by the first remote device, subject to subsequent requests to become the system caller, without the need to repeat the steps of establishing the communication resource. The method according to [10] above, further comprising a step of receiving. [13] The method according to [10] above, wherein the traffic signal is a code division multiple access signal. [14] The method according to [10] above, wherein the traffic signal is a time division multiple access signal. [15] The method according to [10] above, wherein the step of denying the system caller privilege to the first remote device further comprises the following steps: The base station sends an alternative request to the communications manager to terminate the system caller privilege to the first remote unit; Respond to the alternative request by sending a denial of system caller privileges from the communication manager to the base station; The base station sends the denial of system caller privileges to the first remote device. [16] The method according to [15] above, further comprising the following steps: An off display requesting release from the system caller privilege is sent from the remote device to the base station; A request for terminating the system caller privilege of the first remote device is sent from the base station to the communication manager in response to the off display; The communication manager sends the denial indication to the base station. [17] The traffic signal from the first remote device comprises variable speed data organized in frames, where each frame has one of a set of preselected data speeds. The method according to [10] above, wherein the step to be monitored further comprises the following steps: The base station finds the average number of frames in the traffic signal at the lowest data rate of the set of preselected data rates; It is determined that the link activity is below the predetermined level when the average number of the frames exceeds the first threshold during the length of the first time. [18] The method according to [17] above, wherein the monitoring step further comprises the following steps: The base station detects the initial presence of the link activity; When the average number of the frames exceeds the second threshold during the second length of time, it is determined that the link activity has fallen below the predetermined level after detecting the initial presence of the link activity. .. [19] The step of detecting the initial presence of the link activity comprises detecting the number of frames in the traffic channel at a data rate other than the lowest data rate of the data rates. The method described in [18] above. [20] A system to prevent deadlocks within a dispatch system configured to serve remote devices, including: With a remote device designated as a system caller, sending variable speed data for broadcast transmission to one set of other remote devices; A base station that receives the variable speed data, monitors the variable speed data to detect link activity, and broadcasts the variable speed data to other remote devices in the set. Transmission to the remote device so that when the link activity falls below a predetermined level, an alternative remote device open display signal is received from the base station and the remote device can be instructed to stop transmitting the variable speed data. A communication manager that issues an open display signal to the base station for the purpose. [21] A single remote device is a device that prevents deadlock in a push-to-talk system designated as a system caller at any point in time. A means for monitoring link activity from the first remote device currently designated as the system caller, and When a lack of link activity is detected, a means for sending a denial of system caller privilege to the first remote device, and The device further comprising. [22] The signal from the first remote device comprises variable velocity data organized in frames, wherein each frame has one of a set of preselected data velocities, wherein the monitor. The means for: The device according to [21] above, further comprising: With means for detecting the average number of frames received from the first remote device at the lowest data rate of the set of preselected data rates; A means for detecting the average number of said frames above the first threshold during the first length of time as the lack of linking activity. [23] The device according to [21] above, wherein the monitoring means further comprises: A means for detecting the initial presence of link activity when the number of frames is received at a speed other than the lowest data speed in the preselected set of data speeds. A means for determining the lack of link activity after detecting the initial presence of the link activity when the average number of frames exceeds the second threshold during the second length of time.