|AN INTRODUCTION TO TWO-WAY TECHNOLOGY|
(Source: Motorola, author unknown)
In the United States, the Federal Communications Commission's (FCC) 1994 nationwide and regional spectrum license auctions have been the catalyst for much opportunity in the worldwide wireless communications industry by giving birth to a whole new cadre of applications.
Both the industry and its end users are seeking technologies and applications that will expedite open wireless communications and incorporate the world of advanced messaging. The industry is evolving, rather rapidly, from a diverse collection of converging technologies to systems that will give end users choices to fit their lifestyle, and make it easier for them to get the information they need, when and how they want.
The entrepreneurs in this new wireless capabilities are driving the limits of the frontier into increasingly exciting territory. Narrowband PCS has opened the door for companies willing to take messaging to a new level and move the industry into the future with more robust features.
The birth of the advanced messaging industry occurred in July 1994, with the Narrowband PCS auctions in the US. The availability of nationwide and regional 50-KHz channels introduced the era of two-way advanced messaging and associated applications.
Narrowband PCS enables new technologies, including two-way, voice, and graphical messaging. Infrastructure technology and subscriber devices, along with accessible applications and meaningful content to enable business growth, are just beginning to emerge, and support Narrowband PCS in 901 to 902 MHz bandwidths for 12.5-KHz inbound channels, and 930 to 931 and 940 to 941 MHz bandwidths for 50-KHz outbound channels.
FLEX the worldwide de facto high-speed, one-way paging protocolwas introduced to fulfill new requirements from paging operators that needed to increase the capacities and features on established paging channels.
FLEX operates at 1600, 3200, or 6400 bps; far greater performance levels than POCSAG. And FLEX supports more than one billion addresses, offering the highest channel capacity in the world today. Additionally, FLEX is more robust, offers superior fade protection, and increases battery life by up to five times more than other paging protocols. In fact, FLEX's ability to deliver five times the throughput with a limited number of additional transmitters makes it the most cost-efficient protocol available today.
The ReFLEX protocol has grown from FLEX, and introduces the attributes of two-way messaging. In two-way messaging, the communicator responds back to the system. Simply, the subscriber is carrying a transmitter that operates both automatically and by the subscribers actions. Two-way messaging has many advantages for the service provider, the subscribers, and the callers.
ReFLEX provides two-way communications, but not in the same sense as traditional two-way data systems. ReFLEX is asymmetric, which means that the data transmitted from the communicators back to the system is optimized to a much smaller size. This concentrates the transmitted power into a smaller bandwidth, and thus minimizes the number of receiver sites required to achieve proper coverage.
One of the most critical areas of management for any system is spectrum efficiency. This is because spectrum is valuable, and maximum efficiency in usage is of utmost importance. ReFLEX provides the best spectrum transmitter management for wide-area coverage through the use of automatic registration, locator services, and transmission cell management.
With ReFLEX, transmission usage is maximized because only the transmitter, or localized group of transmitters, in the communicators physical location is used. (In one-way systems, all nationwide transmitters had to broadcast every message because the system never knew where the communicator was located.) All other transmitters or transmitter zones in the system can be devoted to other messages. This gives the entire system peak spectrum efficiency, a key element in maximizing subscriber capacity.
Another benefit of two-way messaging is the communicators ability to confirm the receipt of each message without any subscriber action. This feature has four important advantages.
The first advantage is that when the communicator successfully receives a message, it informs the system of the successful reception. The system now removes the message from the broadcast queue, making room for other messages.
The second advantage occurs if part of the message was not correctly received. In this case, the communicator tells the system which parts of the message (known as the data packets) need to be rebroadcast. This provides maximum efficiency in the amount of broadcast time each message can potentially require while insuring message integrity. (In one-way systems, the message was never rebroadcast because the system never knew if there was an error.)
The third advantage occurs when the subscriber actually reads the message. As an option, the communicator can be configured to inform the system of this action, and the system now knows that the subscriber has reviewed the message. Callers can subsequently confirm that the message was successfully received and (optionally) read, thus eliminating repeat messaging due to caller uncertainty.
The fourth, and possibly the most important, advantage is that all messages are guaranteed to be delivered. If a message cannot be delivered (because, for instance, the communicator is turned off), the system holds the message until the communicator can be located. The system always holds the message, broadcasting it periodically, until it receives the successful confirmation from the communicator.
Additionally, ReFLEX two-way communicators maintain all the traditional advantages of today's one-way pagers, such as a small and convenient physical design, long battery life, low cost, and reliability.
A key feature of ReFLEX is the ability of the subscriber to reply to a message. This function creates the closed-loop performance that truly makes advanced messaging a quantum leap in wireless communications.
The ReFLEX system offers the subscriber two ways to respond to a received message, as well as the ability to originate a message.
In the first method for responding to a received message, the subscriber can select a response from a list of up to 120 pre-established, subscriber- specific messages stored in the communicator and the Terminal. To optimize airtime, only the selected messages index identifier is sent back to the Terminal.
In the second method, the message sender can insert up to 128 possible multiple-choice replies into the message. From this imbedded list, the subscriber can select one and return it to the system, where the sender can access and review it.
Note: The system provides three ways for the caller to review the response. In the first method, known as CallQuery, the caller can call back to the system at a later time and, through the use of a transaction ID, retrieve the response. In the second method, known as AutoMESSAGE, if the caller is also a subscriber in the system, the system can be instructed to send the response directly to the to the callers communicator. In the third method, known as AutoNOTE, the caller can have the system send the response in the form of an electronic message, or E-mail.
Additionally, the subscriber has the ability to send a message at any time to another subscriber (one-way or two-way), a fax machine, an Internet address, etc. The subscriber selects the individual or group from a pre-established list of up to 32 addresses stored in the communicator. Then, from a 64-message subset list of the subscribers 120-message master list, the subscriber selects a message, and instructs the system to send it to the selected address.
The 2-WAY Terminal uses the Mailbox to store messages in the system for later retrieval by the subscriber. These messages are not sent to the subscribers PMU directly at the time of entry. They can be in the form of numeric, text, or voice.
After a caller leaves a message, the system stores the message, and sends a special message to the subscribers PMU that says a new message is in the Mailbox. After retrieving the message, the subscriber can have it forwarded to another destination.
Subscribers can have all their messages stored in the 2-WAY Terminal by using the Mailbox. If no system-defined purge time has been set, a subscriber can save messages indefinitely. However, there is a finite amount of message spaces allowed for each subscriber. When the storage space for the subscribers messages is nearly full (number of messages or length), the subscriber is informed of this when accessing the system or by a special message.
When accessing the Mailbox, voice messages are replayed from the recording; text and numeric messages are optionally enunciated. The subscriber has the option of retransmitting, retrieving, transferring, saving, or deleting the message. The subscriber makes the selection as each message is announced.
A subscriber can transfer Mailbox messages to another subscriber. The system makes a copy of the message, transmits the copy, and retains the original for the sender. The Mailbox message being transferred can be marked as private so that the receiving party cannot divert or transfer the message. Also, the transferred message can be marked as urgent so that the receiving party receives a priority reminder message.
A subscriber can modify assigned features, and activate and deactivate them during normal entry call. (Entry to the administrative mode is password protected.) Subscriber password, legitimization codes used by callers, and custom voice prompts are some of the features that can be managed through this feature.
The 2-WAY Terminal system offers many new features for callers, all designed to make the use of two-way messaging a simple and pleasant experience.
At any time during interaction with the system, a caller can request help through the use of the help menu. This feature helps guide the caller through the process or feature he is currently in at the time help was requested.
A caller can deposit messages in numeric, alphanumeric, voice, fax, electronic, and voice mail formats.
An authorized caller can request automatic message repetition. Messages can be repeated up to a preconfigured maximum number of times and at preset time intervals. The subscriber in the legitimization codes controls message repetition.
A caller has a choice of four priority levels for message delivery, namely Emergency, and Priorities One, Two, and Three. Emergency messages have the highest priority while Priority 2 is the default level. Also, the system automatically raises the priority level of reminder pages for urgent Mailbox messages.
An authorized caller can have a message set at a specific time and day. This is known as a wake-up call. The caller enters the time and date, a periodic rebroadcast (if desired), and the message.
Automatic features improve the messaging process by adding useful information to the callers message and by supporting useful capabilities, such as message storage and group calling.
A message number from 1 to 99 is assigned to each message. This number is used during message retrieval and transfer. The message number is added to the text of a message so that the subscriber can identify missing messages.
Each message gets a time, or a time and date, stamp. As an option, time and date stamp can be added to a subscribers message.
Messages can be automatically stored for subscribers for a predetermined period of time. All messages are stored on the file server, and can be compressed for storage and movement efficiency. Limits on the number and size of stored messages are defined for each type of message (data, voice, etc.).
Messages are automatically deleted after a configurable time, or purge time, have expired. Purge times can be unique for each feature on a system wide basis.
Subscribers can be placed in groups, whereby a single message can be automatically sent to multiple subscribers. When one message is received for a group, it is distributed to all members of that group. Each group is individually defined, with its own subscriber number, and has most of the regular subscriber features. Groups also offer these features:
Automatic system management includes applying limits. The total message traffic for each subscriber can be limited. This can be done by limiting the total size (for example, characters) of all combined messages and the number of messages. Normally, after the limit is reached, messages are refused. The length of each message for each subscriber can be limited. This limit can be imposed during a specific time period (hour, day, week or month). During this time period, if a message exceeds a limit, the system can either defer the message or truncate the message at the limit.
The infrastructure subsystems include:
The addition of the return communications channel to paging applications creates a cornucopia of new messaging applications. These applications require the paging Terminal to provide increased message processing, file storage, system availability, and networking capabilities.
The 2-WAY Terminal is designed to meet the needs of current applications, the 2-WAY Terminal uses advanced RISC processors. Its modular design provides the processing power, network interfaces, and capacity also to meet the needs of future messaging applications. The 2-WAY Terminal accepts the voice and data input paging messages through the Public Switched Telephone Network (PSTN) and the Internet (future) wirelines, compresses voice pages for transmission, maintains a subscriber registration and location database, communicates with other paging Terminals or directly to its Controller (depending on the location of the subscriber) and returns the message-received status and any subscriber reply message back to the caller.
Features of the 2-WAY Terminal include:
The Controller batches and schedules outbound messages, coordinates inbound responses and messages with the outbound messages, performs retransmissions as required, and returns message status and replies to the 2-WAY Terminal. For maximum network efficiency, the Controller batches and queues multiple simultaneous InFLEXion sub channels into a region. The Controller can also batch InFLEXion and ReFLEX messages for multiple regions on a common link. The Controller is available in configurations for small, medium, and large transmitter and receiver site counts. Like the 2-WAY Terminal, the Controller is available in redundant hardware configurations for higher service reliability. (Note: InFLEXion equipment is no longer in production.)
As the centerpiece of the new two-way messaging system, the Controller interfaces with every component of the messaging system: The 2-WAY Terminal (the Terminal), the 2-Way Transmitter (the transmitters), the 2-Way Receiver (the receivers), and the multiplexer (the remote concentrators for stand-alone receiver networking). As a result, the Controller is also responsible for managing transmitter and receiver alarm reporting.
The Controller hardware and software architecture is intentionally designed to accommodate easy growth and functional enhancements. In fact, most of the functional enhancements are software based. The system software is modular and based on a multitasking platform, which makes it easy to upgrade and ensures high reliability.
The 2-Way Transmitter maximizes the efficiency of the radio frequency spectrum. With linear modulation as specified for InFLEXion messaging, as well as the 2- and 4-level capabilities required for ReFLEX messaging, the 2-Way Transmitter provides all the functionality required for higher messaging speeds, new messaging signaling schemes, advanced two-way messaging, and other value-added messaging services.
The 2-Way Transmitter uses linear technology to gather the greatest spectrum efficiency in the new 50-KHz channels, yet meets all modulation requirements, size, weight, costs, heat dissipation, and performance criteria. Using automatic cutback circuitry and front-to-rear air flow, the 2-Way Transmitter's power amplifier also incorporates dual-device modules that permit two RF devices to work together to produce extremely high gain. These modules use graphite heat sinks to remove heat much more quickly than aluminum castings. Front-to-back forced airflow provides more consistent circulation while minimizing dust intake.
The 2-Way Transmitter exciter module uses highly sophisticated modulation techniques and a state-of-the-art custom integrated circuit with a built-in IQ (In-phase and Quadrature) digital modulator. This accurately shapes waves for broadcasting ReFLEX and InFLEXion messages with data streams in excess of 10,000 bps. The bottom line: The 2-Way Transmitter technology provides unequaled modulation accuracy up to a single hertz, as well as spectral purity on a 50-KHz bandwidth that is second to none, and an exciter module that offers smart power control for maintaining maximum power output under adverse conditions.
These compact receivers are used to receive the transmissions from the communicators. Upon receiving the signal, the 2-Way Receiver relays the message to the Controller. With a diversity receiver and antenna scheme, the receiver has greater signal sensitivity.
The 2-Way Receiver can be co-located with the 2-Way Transmitter. Stand-alone units also can be installed as required to provide effective coverage for inbound messaging.
Infrastructure suppliers are working in tandem to ensure that various infrastructure components can be mixed and matched based on customer requirements. To date, specific interface and protocol specifications have been defined between the infrastructure subsystems to enable the interoperability of many infrastructure suppliers Terminals, controllers, transmitters, and receivers on the same network.
The forward and reverse
Two-way messaging fits well with the current widespread use of voice mail, e-mail, and fax services, in addition to more traditional telephone and one-way paging services.
The delivery of advanced messaging is augmented by a family of over-the-air (OTA) message protocols the FLEX family. Within this family, FLEX provides one-way text paging, ReFLEX provides two-way text messaging, and InFLEXion provides two-way voice and data messaging (no longer available). The FLEX protocols provide higher transmission speeds than former OTA protocols, with a high resistance to fading. Two-way messaging, using the ReFLEX and InFLEXion protocols, provides guaranteed message delivery because the receipt of messages is acknowledged by the subscriber unit; if a receipt is not received after a transmission, the system can retransmit it intermittently until it is received.
In order to obtain the maximum benefit from advanced messaging services, a means is needed to combine all available services using a new technology.
The transmitter system
Accepted pages from Messaging Center are encoded and batched at the Controller. The encoded information is then sent out via the RF network to, depending on the protocol, the Nucleus or 2-Way Transmitter transmitter.
The transmitters are linked to the Controller via the distribution network. This includes all network elements from the Controller out to the Nucleus or Transmitter module transmitters. It includes any Terminal servers, routers, modems, DSU/CSU, etc., that are required to network the Controller and the transmitters.
2-Way Transmitter feature
With linear modulation as specified for InFLEXion messaging, as well as the 2- and 4-level capabilities required for ReFLEX messaging, the 2-Way Transmitter (Transmitter module) two-way linear transmitter provides all the functionality required for higher messaging speeds, new messaging signaling schemes, advanced two-way messaging, and other value-added messaging services.
The Transmitter module uses linear technology to gather the greatest spectrum efficiency in the new 50-KHz channels, yet meets all modulation requirements, size, weight, costs, heat dissipation, and performance criteria. Using automatic cutback circuitry and front-to-rear air flow, the Transmitter module's power amplifier also incorporates dual-device modules that permit two RF devices to work together to produce extremely high gain.
The Transmitter module exciter module uses highly sophisticated modulation techniques and a state-of-the-art custom integrated circuit with a built-in IQ (In-phase and Quadrature) digital modulator. This accurately shapes waves for broadcasting ReFLEX and InFLEXion messages with data streams in excess of 10,000 bps. The bottom line: The Transmitter module technology provides unequaled modulation accuracy, as well as spectral purity on a 50-KHz bandwidth that is second to none, and an exciter module that offers smart power control for maintaining maximum power output under adverse conditions.
Base station configuration
Each transmitter site consists of a Transmitter module and a collocated 2-Way Receiver. The output from the transmitter is connected to a main transmit antenna.
The Transmitter module is designed to maximize the radio frequency spectrum. It is capable of 2/4 level modulation, which makes it capable of FLEX and ReFLEX messaging.
An external local controller, integrated with the transmitter, decodes the incoming protocol sent from the Controller. The local controller is a 19-inch rack mount module that is two rack units (3.5 inches) high. It also provides the ability to remotely accept software downloads from the Controller to the Transmitter module. For local access, a laptop interface is available for programming transmitter parameters.
The decoded information is sent from the local controller via a cabling interface on the backplane to the Transmitter module. The Transmitter module is built based on a modular concept. The following components make up a Transmitter module for a single sub channel:
The power supplies are switching power supply units that supply the DC voltages required by the other modules in the transmitter.
The 2-Way Transmitter exciter module uses highly sophisticated modulation techniques and a state-of-the-art custom linearization IC with a built-in Inphase and Quadrature (I/Q) digital modulator that accurately shapes waves for broadcasting FLEX, ReFLEX , and InFLEXion messages that contain data streams in excess of ten thousand bits per second.
The Transmitter module uses linear technology to gather the greatest spectrum efficiency in the 50-KHz channel while maintaining all required modulation techniques, size and weight limitations, heat dissipation specification, and performing goals.
As it uses automatic cutback circuitry and front-to-back airflow, the 2-Way Transmitter power amplifier also incorporates dual device modules that cause two RF devices to work together to produce extremely high gain. These modules use graphite heat sinks to remove heat much more quickly than aluminum castings. Front-to-back airflow provides more consistent circulation while minimizing dust intake.
The Control module is equipped with a 32-bit microprocessor and Digital Signal Processor that manage high speed messaging format. The Digital Signal Processor wave shapes all data going to the exciter module to maintain a data rise time within two microseconds. The Control module is the second controller board in the Transmitter module. It talks to the local controller that performs the data compression for InFLEXion. At the same time, it controls the data interface to the exciter. For example, to command the transmitter to key up in FLEX, ReFLEX , or InFLEXion.
The 2-Way Transmitter can be expanded by the addition of more power supplies and linear amplifiers, and replacement of the internal combiner, to support up to four sub channels. This is due to the fact that as subscriber capacity is added, sub channels are added at the site with the same InFLEXion power output per sub channel, so as to maintain transmitter site separation. The number of sub channels that the transmitter can handle is also limited by the power output of the transmitter. If a transmitter is upgraded to handle more simultaneous sub channel transmissions, its power output capability must be increased accordingly.
As in FLEX systems, the transmitter network uses the GPS for synchronization. For sites with collocated transmitters and receivers, all equipment shares the same GPS antenna. For sites with stand-alone receivers, a separate GPS antenna is required.
The transmitters can report alarms back to the Controller through various means:
Polled Diagnostics from Controller
The Controller can be set up to poll the transmitters for alarm at preset time intervals. The system operator has the ability to prioritize messaging over polled diagnostics.
Real-Time Alarm Reporting
The transmitters are equipped with dial-out modems that dial back immediately if alarms programmed for real-time alarm reporting have been triggered. Real-time alarm reporting can also be via leased lines.
The system operator can dial in from the Messaging Center to each of the transmitters to interrogate the station for alarms. The dial-in process is password protected.
The receiver system
The 2-Way Receiver are receivers that send back the ACK's, NACK's, and other scheduled and nonscheduled transmissions to the Controller.
2-Way Receiver feature
The 2-Way Receiver (2-Way Receiver) receiver is used to receive the transmissions from the communicators. Upon receiving the signal, the 2-Way Receiver relays the message to the Controller. With a diversity receiver and antenna scheme, the receiver has greater signal sensitivity.
The 2-Way Receiver can be collocated with the 2-Way Transmitter. Stand-alone units also can be installed as required to provide effective coverage for inbound messaging.
The multiplexer component of the 2-Way Receiver is a receiver concentrator that receives transmissions from multiple 2-Way Receiver sites in a particular geographic area, and sends this information directly to the Controller or to the 2-WAY Terminal for delivery to the Controller. The multiplexer reduces recurring costs associated with the terrestrial network return paths.
Base station configuration
Each 2-Way Receiver is a 19-inch rack-mount module, which is either one or two rack units high. Each receiver is capable of receiving signals on a single channel only. It utilizes advanced DSP demodulation techniques for inbound FSK message and is speed agile from 800 to 9600 bps.
The 2-Way Receiver uses two separate antenna input for micro diversity purposes. Micro-diversity is used to gain base receiver sensitivity by mitigating the effects of fast fading, also known as Raleigh fading. Signals from two separate antennas are combined and bit decisions are made in the receiver on the composite signal.
Collocated 2-Way Receivers can support remote a software download from the Controller while stand-alone receivers require software download from the Controller via leased lines or dial-up lines.
Two-way subscriber products
A full line of subscriber devices has been developed (also called Personal Messaging Units, or PMU's) that give subscribers a wide selection. In addition, other manufacturers will make chip sets available that make it even easier for other vendors to supply subscriber units.
Two-way subscriber devices maintain all the traditional advantages of today's one-way subscriber devices. These include a small and convenient physical design, long battery life, low cost, and reliability.
The ReFLEX PMU
The ReFLEX PMU is an alphanumeric (text) belt-worn communicator with a four-line (three for text, one for icons), 20-character display, 95 Kbytes of memory, pleasing tone or vibration alerts, and it operates on an AA battery. A flip-top cover houses the transmitter antenna and, when closed, protects the display. The ReFLEX PMU has six keys, with a user-friendly soft key design. The subscriber sees the function of the keys as each display screen is scrolled through.
The ReFLEX PMU supports up to 120 preprogrammed (canned) messages.
Anybody needing a quick answer to a question from a mobile associate or family member can benefit from the ReFLEX PMU. Some examples are:
Couriers and delivery services
Home message centers
Office information centers
The ReFLEX PMU provides a total of 95 Kbytes of message storage. The memory for storing each message is dynamically allocated. That is, there could be many short messages, a few long messages, or a mixture. A maximum of 760 messages can be stored. (The maximum message size is 500 characters for alphanumeric messages and 40 characters for numeric messages on the forward channel.) Personal and information services messages are stored in the same mailbox. When the memory in the ReFLEX PMU becomes full, new incoming messages automatically overwrite the memory used by the oldest messages.
The subscriber can lock messages by moving them to the message storage folder. This allows messages to be saved by preventing them from being overwritten by new incoming messages. Half of message memory can be utilized for the message storage folder.
The subscriber can delete all messages or one message at a time. From the options menu, all messages are deleted (except the ones stored in the message storage folder). Message deletion is performed when a message is being previewed in either the subscribers system Mailbox or the PMU folder. No messages can be deleted when they are being read from the Mailbox or folder.
If no multiple choice responses are embedded in the message, the ReFLEX PMU allows the subscriber to select one of several preprogrammed responses and transmit a code representing one of these messages back to the system. Here, the code is read, the response is selected, and is then transmitted to the message originator.
There are two areas where canned messages are stored. The first area may hold up to 16 canned messages that can be up to 15 characters in length. The second area can contain a maximum of 104 messages with a maximum length of 255 characters each. All of these messages are available on the forward channel. Some or all of these are valid on the reverse channel. There is a field in the EEPROM that is programmed to represent the dividing line between the forward- and reverse-channel canned messages.
Address Book Usage
The ReFLEX PMU allows the user to select a person from a preprogrammed Address Book and select a canned message to send to that person.
Serial Port Operation
The ReFLEX PMU has a serial port, which supports XMODEM for message downloading and uploading. In addition, there are commands that are offered through the serial port that manage the message memory and some of the subscriber options. The protocol for this port is eight-bit data, one-stop bit, and no parity.
To preview a message, the subscriber presses the Mailbox button on the main menu. The first three lines of the display contain the first three messages in the Mailbox. The first twelve characters of each message are displayed. Messages are read, deleted, or moved to a folder from this menu.
To preview a message in a folder, the user presses the folder button from the main menu. The first three lines of the display contain the first three messages in the folder. The first twelve characters of each message are displayed. Messages can be read or deleted from this menu.
If the read button is pressed from the preview screen, then the first three lines of the specified message are shown. The ReFLEX PMU supports forward and backward scrolling of messages. This is accomplished by pressing the down and up arrow keys, respectively. Repetitive scrolling is also supported by pressing and holding down the down or up arrow keys. The ReFLEX PMU also allows the subscriber to scroll to the previous (left arrow) or next (right arrow) message.
Messages can be moved to a folder from this menu. Each message is time stamped.
Message Reception during
The ReFLEX PMU alerts the user as soon as a new message arrives. In the case when the user happens to be reading a previously received message at the time when a new message arrives, the ReFLEX PMU does not notify the user that a new message has arrived until the user has finished reading the current message.
The power-up screen can be programmed to support up to 20 characters.
Reminder alert is an option in the code plug. Both the interval between the reminder alerts and the time at which the reminder alert ceases are programmable.
RF Programming (OTA)
The ReFLEX PMU can be programmed via over-the-air programming. A security feature prevents programming from unauthorized sources.
Hard Contact Programming
PMU options can be programmed via the RS-232 port. Passwords can be enabled on an individual basis. If an invalid password is entered more than eight times, the PMU becomes disabled.
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