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Pager Protocols

One-way Pager Protocols
A pager is designed to receive the paging signals that are transmitted under control of the paging terminal. These paging signals are designed to carry pager-specific address information in order to alert an individual pager. In addition, message data to display as well as an indication if a special beep pattern should be heard is forwarded to the pager. Because the traditional (one-way) pager is not capable of requesting retransmission in the event of an error, this information must be transmitted in some reliable form to the receiver.

Different manufacturers have developed an array of techniques to forward the required information to remote pagers. These techniques, known as encoding formats, define the techniques employed to represent the information carrying data elements of the protocol as well as how to interpret the overall data content. For example, in one encoding format, data is sent as a stream of binary digits. Each digit is represented by a square wave. A binary zero is represented by a waveform with a period of 2.0 milliseconds, while a binary one is represented by a square wave with a period of 4.0 milliseconds. In another digital paging technique, a sine wave is transmitted continually; if the phase of this signal is not changed from one cycle to the next then this represents a sequence of 11 or 00. If the phase of the next sine wave is inverted, then this represents the sequence 10 or 01.

In many cases, encoding formats send additional data, known as Error Detection and Correction Codes, that are capable of detecting and recovering incorrectly received data. With error correction, pager receipt reliability is improved dramatically. Both analog and digital transmission techniques are used to transfer information to pagers.

Most paging formats are manufacturer-specific and often proprietary. There are a few paging protocols that have been developed and put into the public domain so that many different manufacturers may produce compatible pagers. Among these public domain protocols are POCSAG, Swedish Format (MBS), the Radio Data System (RDS) format and the European Radio Message System (ERMES) format. Each of these formats were developed in Europe.

The different paging formats have certain advantages and disadvantages when compared with each other. In some paging networks that are designed to support pagers that are not only purchased from the carrier but may also be purchased from electronic and department stores, the paging terminal must support multiple paging formats. In cases where the carrier has complete control over the pagers used by its subscribers, the network can be limited to very few different formats. Manufacturers have designed their formats in such a way as to attempt to avoid "false" alerting of other manufacturers pagers when a different format is being transmitted over the same frequency. When there is a mix of paging formats on the same frequency, it is the function of the paging terminal to optimize the mix of paging formats to provide the most efficient method of transmitting all of the outstanding pages as quickly as possible.

Most pagers are always "powered on" so that they may receive a page at any time. Conserving the power drain in a pager will increase the interval between the charging or replacing of the battery. In an attempt to maximize battery life, each pager and paging format normally employs some type of "battery saver" technique that allows the pager to enter a low power mode for short periods of time. The following sections describe how paging information is formatted under the various encoding techniques.

Two-Tone Formats
One of the earliest radio paging formats used a technique known as Two-Tone to alert pagers. The two-tone analog paging technique transmits two sequential audio tones,usually of a different tone duration, to represent a unique pager address. The number of tones that are used determines the maximum number of pagers that can be supported. In a system that supports 30 different tones (because each tone must be different), 30 x29 or 870 different pagers could be supported. Tones in the range of 500 Hz to 4,000 Hz are typical. For example, the frequency combination of 3,636 Hz followed by 880 Hz would alert one pager, while the sequence of 880 Hz followed by 3,636 Hz would alert a different pager. Many times, pagers require a short time period between the two tones,known as "the Gap," for proper operation. In addition, another gap normally is required after the second tone before another two-tone sequence is transmitted on behalf of another pager. The timing diagram for this paging format appears as follows:

t 1 t 2 t 3 t 4
Tone A Gap A Tone B Gap B

In some two-tone pagers, the length of the transmitted tones and the time period of the gap control the type of beep that is heard. In some implementations, the "B" tone is repeated with a short interval in order to cause a double beep tone to be heard. The t1 and t2 tone time periods typically are in the order of 240 milliseconds. Gap A could be 0 to 10 milliseconds, and Gap B could be 500 ms. Therefore, a typical page easily could take more than 1 second.

The two tone encoding format is used for tone only as well as tone and voice paging. If this is a voice page, after transmitting the B tone and waiting the Gap B period, the speaker of the pager is connected directly to the analog signal so that transmitted analog voice signals are heard on the pager. In many cases, it is up to the user to press a button to turn off the speaker after the voice message is heard. If this is not done, the pager acts just like a radio tuned to the paging frequency, and it will allow the user to listen to all paging signals and voice messages being sent.

The two-tone format has several drawbacks due to the limited number of pagers that can be supported on one radio frequency, and the excessively long tone and gap periods. By today's standards, this encoding format is far too slow. Many systems still support this format because of very old pagers that still are on the network. However, in the time it takes to transmit a single tone only page, other paging formats could send as many as 60 tone pages!

Five/Six Tone Format
The Five/Six tone analog paging format was an improvement on the two-tone format. The format uses 11 different tones; ten of these tones represent the digits 0, 1, 2, up to 9. The eleventh tone is known as the Repeat, or "R" tone. Up to 100,000 pagers can be supported with these 11 tones. Every pager is assigned a number from 00,000 to 99,999. To alert the pager, the five tones that define its number are transmitted sequentially; to alert pager 15243, the five tones representing these five digits are sent in a row.

The specifications only define the minimum time period of a tone and not necessarily the maximum period. If a pager number has repeated digits, it is not possible to determine if this is a repeated digit or just a long tone. Therefore, the "R" tone is introduced. When this tone is heard, it is assumed that the prior digit is being transmitted again. If the digit is repeated a third time, the original tone is transmitted. To alert pager 11333, the tone sequence 1R3R3 is transmitted. An optional twelfth frequency can be transmitted after the five-digit number is sent. This sixth digit indicates that a different beep pattern should sound.

To preserve battery life, five-tone pagers are grouped into one of ten battery-saver groups. It is up to the paging terminal equipment to "batch together" all page alerts by battery-saver group. Prior to alerting each of the pagers in a group, a single long tone known as "the Preamble," is transmitted first. This preamble is one of the ten audio frequencies that represent the ten digits. The preamble "wakes up" all of the pagers in this battery-saver group to look for its specific five-tone sequence. If the battery-saver tone is not heard, the pager can "go back to sleep" for a while, occasionally looking to see if its particular long tone is being transmitted.

With the addition of the battery-saver tone, one million pagers can be supported by the format. The actual time to transmit a single page within a batch of pagers typically is in the order of 225 milliseconds (not amortizing the time taken to transmit the common preamble code). Therefore, more than four times the number of tone-only pagers can be supported on a single channel utilizing the 5/6 tone format as can be achieved under the two-tone paging format. The 5/6 tone format can support tone only as well as tone and voice paging.

Golay Format
The Golay paging technique is a proprietary encoding mechanism that was developed by Motorola Inc. This is a digital encoding mechanism, meaning that the paging information is represented by signals that can be interpreted as a stream of zeros and ones. It is up to the pager to receive this data stream and to extract pager-specific information from this transmission.

The Golay format is capable of transmitting tone only, numeric, alphanumeric and voice pages. Much thought was put into the development of the format so that the information is transmitted in such a way as maximize the probability that the data will be received intact. Error-correcting codes are transmitted along with the data so that even if a number of data bits are received incorrectly, the pager can detect and ignore this incorrect data and replace it with the proper information. To further improve reception probability, data bits are transmitted in a particular order to reduce the probability that if a short radio burst corrupts sequential data, the entire data sequence can be recovered correctly.

For purposes of improving battery life and avoiding false alerts due to other encoding formats sharing the same frequency, pagers are divided into groups. A preamble code,as in 5/6 tone paging, is transmitted prior to page alerts. Only pagers that fall within the group number specified by the preamble code transmitted need look for their particular pager address within the stream of paging data that follows.

To improve page alert reliability while making pager message transmission move quickly, the pager address information is transmitted at 300 bits per second while any numeric or alphanumeric data transmitted to the pager is sent at 600 bits per second. The function code of the pager that determines one of four beep patterns associated with the particular pager address is encoded as part of the pager address information that sent. Many page alerts consisting of address and message data can be transmitted sequentially directly following the common preamble code for a group of pagers. This"batching" technique amortizes the time it takes to transmit the preamble code over a number of individual page alerts.

The combination of information encoding techniques, variable transmission speed, sending of pages back to back, batching, the order in that data bits are transmitted, and the use of the Error Correcting Codes to detect and correct corrupted data makes Golay a reliable, efficient protocol that performs well during short bursts of radio interference. To compare the transmission speed of Golay relative to analog 5/6 tone encoding, in the Golay encoding technique an individual tone-only page can be transmitted in approximately 202 milliseconds, including its error detection and correction data. Therefore, in less time than it takes to send a 5/6 tone, encoded, tone only page, a more reliable transmission of a Golay tone-only page can be sent.

NEC-D3 Format
The NEC-D3 encoding format was developed by NEC America for use in its R3-D3 pagers. This digital encoding mechanism is capable of transmitting tone only or numeric pages. Information is transmitted to the pager at the rate of 200 bits per second. For purposes of conserving battery life, two techniques are used. First, prior to transmitting any page alerts under this encoding mechanism, a special preamble is sent, signaling all pagers that page alerts are coming. Until this preamble is detected, the pagers remain in a reduced power mode. The second technique used is to divide the pagers into four groups based on the address of the pager. A fixed amount of transmission time and, therefore, a fixed amount of bits to be transmitted, is allocated to each of the four groups. As many encoded pages and their associated messages as can fit within the number of bits allocated to each group may be transmitted at one time. If there are not enough pages to fill to group, "idle codes" are filled in. Idle codes are ignored by all pagers.

The four fixed time periods are allocated back to back following the preamble. Therefore, if a pager's address indicates that it falls within group four, the pager can almost completely power down during the transmission time for Group One, Group Two and Group Three. Likewise, a pager within Group One will remain powered up looking for its unique pager address, but after the fixed time period for Group One has passed,the pager can power down during the time allocated to Groups Two, Three and Four. Error-correcting-code information is transmitted with each pager address and each block of message information. An extra data bit also is transmitted with address and message data so that the total number of "one" bits in each transmission block always is even. This extra data bit is known as the "even parity" bit. The parity checking adds yet another manner of detecting transmission errors.

Address and message data is transmitted in blocks of 32 bits. Each address consists of 20 data bits, 10 check bits and 1 parity bit. Each message data block also consists of 20 data bits, 10 check bits and 1 parity bit. Each 20 bit data block can contain 5 digits of a numeric page. There are 20 blocks of 32 bits that are transmitted for each group. Therefore, each group is allocated 3.275 seconds to transmit its pages.

Each individual tone-only page in NEC-D3 is transmitted in 160 milliseconds. This does not consider the amortization of the preamble code or the delays that may been countered to transmit the prior group if this was the only page to send. In a system with a large paging volume consisting of pagers that are evenly spread across the four groups, NEC-D3 can be a fast transmission protocol.

POCSAG Format
The POCSAG encoding format was developed during the period from 1975 to 1978 by a group of international engineers who were looking to create a mutually agreeable code for wide-area paging. Because the meeting was chaired by the British Post Office, and the group was known as the Post Office Code Standardisation Advisory Group, the acronym POCSAG was adopted. Official international recognition by the international standards organization known as the CCIR in February 1981 adopted the POCSAG standard as Recommendation Number 584, Radiopaging Code 1. This encoding format has been implemented by a large number of manufacturers around the world.

POCSAG is a digital encoding format that is specified to operate at 512 bits per second. The paging format has also been implemented, without modification to the encoding mechanism, to operate at 1200 bits per second. During 1991, successful paging operations have been reported at 2400 bits per second (refer to a later section of this chapter regarding the Telocator High Speed Paging Committee). The higher speeds allow more pages per second to be transmitted and can therefore support a larger customer base over a single channel than lower speeds. A mix of speeds can be supported over one frequency. Pagers are fixed to operate at a single speed; even at the slowest speed and under high traffic volumes, POCSAG can support approximately 15 tone-only pages per second. For comparison purposes, a single tone-only page at the slowest rate (not considering preamble and batching overhead) requires 62 milliseconds. At 1200 bits per second, only 27 milliseconds are required to send a page;at 2400 bits per second, only 13 milliseconds are required. With the introduction of high speed POCSAG, paging systems have increased the number of page alerts that can be delivered in any period of time almost 100-fold from the early days of only two-tone paging.

The POCSAG coding format can support up to 2 million individual pagers. Tone-only,numeric and alphanumeric paging are supported, and up to four beep patterns can be associated with each pager address.

POCSAG can be very efficient when transmitting large volumes of traffic. In systems with a mixture of encoding formats and light loads of POCSAG pagers, the format can be inefficient and can waste air time.

Every POCSAG pager falls into one of eight pager groups based on pager address. A POCSAG transmission consists of a 576-bit preamble code that is used to "wake up" pagers that are in a battery-saver mode. A batch of pages consist of a synchronization code followed by eight transmission "frames." A frame is a fixed number of bits that begins at a fixed time duration after the transmission of the sync code. Each frame is transmitted back to back following the sync code. After the eighth frame is transmitted,the next batch consisting of a sync code and eight more frames can be sent.

A pager that falls into a particular group may only be paged by placing its pager address within the particular frame number fixed for that pager (thus creating the eight groups). If there is no room to put the pager address within a frame, then the pager can not be alerted until that frame number comes around in the next batch, and there is room to put that pager address within that frame. No more than two pagers can be alerted within one frame but, in numeric or alphanumeric paging-only systems, only one pager can be alerted in a frame. Any message data for a pager is encoded and transmitted in fixed blocks, known as "codewords," starting directly after the codeword that contains the pager address. Most times the encoded message information is transmitted across many frames, thus blocking other pages from being placed within these frames. It is up to the paging terminal to pack tone-only, numeric and alphanumeric information for different pagers within frames in the most efficient manner possible to minimize the total number of frames required to output a given number of page requests. Inefficient packing mechanisms can increase the airtime required to send a particular number of pages.

All pager address and message information have an error detection and correction code associated with it to detect small error bursts and to correct single bit errors that may occur within a single codeword. The error detection and correction mechanisms of POCSAG do not perform as well as those of Golay over long streams of bits because of the better error codes used within Golay and the order in that Golay transmits its address and message information.

Because of its speed, efficiency, the number of pager manufacturers with compatible pagers available and its international acceptance, POCSAG has become a popular paging format in high-volume paging applications.

Mark IV/V/VI Formats
The Multitone Electronics Mark IV, V and VI encoding formats are digital formats, and can accommodate tone only, numeric display and voice paging. Unlike other digital formats that transmit a fixed number of binary ones and zeros in a fixed period of time, these formats require 2 milliseconds to transmit a binary 0 and 4 milliseconds to transmit a binary 1; the data transmission rate varies between 250 and 500 bits per second. Up to 100,000 pagers can be supported over a single frequency, and up to eight different beep patterns can be specified for each pager.

The Mark IV™ format was designed to transmit tone-only pages, to forward a single digit to a pager or to forward up to four digits to a pager depending on the way information is encoded. Multitone pagers using the Mark IV™ format can display one digit and sequentially display four digits. Pagers utilizing the Mark V™ pager provided a five-digit display and could display up to ten digits. These pagers have been used extensively within hospital private paging systems.

The Multitone pagers using the Mark V™I format are capable of displaying a ten-digit message; this format also provides a means for the paging terminal to periodically transmit the time of day. When not displaying a message, the pager will display the time continually; this time remains accurate for short periods and is adjusted when the paging terminal sends its next update. The time-of-day transmissions also are used to determine if the pager has moved out of range of the transmitter. If the periodic page is not detected, the pager will alert the user that it is "out of range." Prior to transmitting page alerts, a preamble is transmitted that takes the pagers out of a battery-saver mode; page alerts then are transmitted back to back. Because of the time to transmit a binary "1" is double the time for a "0," the duration of a tone-only page depends on its address. A single page alert can be transmitted in 224 to 276 milliseconds.

A single page alert is created from a stream of four-bit fields. Each digit of the pager number and each digit of the message is encoded within a single four-bit field. The pager function information that determines the beep pattern (indicating if one, four, five or ten digits are being sent; if this is time-of-day data; and if speech will follow) is encoded within other four bit fields in the Mark paging formats.

Parity information is transmitted as a means of determining if a bit has been corrupted during transmission. Odd parity bits are bits that are set to "0" or "1" so that the total number of "1" bits within segments of the format always will add up to an odd number. For further error detection, the pager address is transmitted twice; if the same address is not received both times, data corruption is assumed, and the pager is not alerted. In the Mark IV, V and VI paging formats, error detection is limited to parity bits and the repeat of information. The more sophisticated error detection and correction codes transmitted as part of many other digital transmission formats are not used.

Swedish MBS Format
The Swedish MBS paging format supports paging over large geographical areas without the necessity of having to allocate the same frequency throughout the network. The format is transmitted along with the main signals of a radio station within a segment of the transmission known as the subcarrier. These subcarrier signals are part of every FM radio and television transmission and normally are removed and ignored by receivers. The original MBS format was designed to support tone-only paging and numeric display paging of up to 12 digits. Later modifications to the format have extended it to support longer numeric messages as well as alphanumeric display.

The Swedish MBS digital encoding format initially was developed for use in the public radio paging system operated by the Swedish Telecommunications Administration. This paging format is designed to be transmitted over FM radio station subcarriers. Pagers utilizing this format are capable of scanning the FM radio band and will lock into special signals that are continually transmitted to indicate that paging data is being sent over a particular radio frequency. When a pager first is turned on, it can take up to 30 seconds to locate an FM station that is transmitting the paging signals.

The MBS format is transmitted at 1187.5 bits per second. The six-digit pager address allows up to one million pagers to be alerted within one paging network. An additional two-digit code allows multiple networks to exist at one time. Each network must be given a unique two-digit identification number that is transmitted continually along with page alert signals. When there are no page alerts to transmit, this network identification code continues to transmit. When a pager is first turned on or if it moves out of range of a particular FM radio station, the pager will scan at a 57,000 cycles per second (kilohertz) signal that is continually transmitted and will look for the network identification code that is encoded within that transmission. When the correct network code is detected, the pager will lock on to that radio station and look for its unique pager address.

The paging format is oriented around sequential transmissions of 26 bits. These transmissions contain 16 bits of information and 10 bits of error detection and correction coding. The six-digit pager address is expressed as six four-bit binary digits, thereby requiring a 52-bit transmission; a tone-only page can be transmitted in 44 milliseconds. For numeric or alphanumeric pagers, the paging data immediately follows the pager address in 26-bit blocks. Four numeric digits can be transmitted within the 16 data bits contained in each transmission block. Blocks containing message data do not contain the network identification code that is part of the address block. The original paging format specified that no more than 12 digits (three data blocks) could follow a pager address because the pagers themselves expect the network code to appear within four data blocks. Remember, pagers scanning for the correct frequency are looking for this code in order to lock into the channel. In the expanded MBS that supports longer messages, mechanisms have been provided to have this network code appear during the transmission of long messages.

In order to provide for battery conservation, all MBS pagers are grouped into one of 100 groups based on the first two digits of the pager address. When a pager is fully powered, it looks for its group number to be transmitted. Once found, it looks for its unique four-digit address within that group. The paging terminal must batch all pages according to its group number. When there are no more pages for a specific group, the pager powers down for 32.707 seconds. During this battery-conservation period, the receiver will not detect pages. For maximum battery life, the paging system must be able to send new pages to this group as soon as possible after this 32.7-second period. If there are no pages for a group, a dummy number must be sent so that the pagers will return to their low-power mode. This paging format currently is used by a Cue Nationwide Paging in the United States.

Radio Data System (RDS) Format
The Radio Data System (RDS) format also was developed by the Swedish Telecommunications Administration. Like the Swedish MBS format, it is transmitted via FM subcarrier. However, the RDS system not only provides tone only, numeric and alphanumeric paging services, but provides many other non-paging radio services. The RDS format is a general radio receiver format that allows for a variety of different services including the ability for traffic information to be continually updated on displays within automobiles or causing an in-car cassette tape player to stop temporarily and tune the car radio to a traffic or weather report.

With RDS, if you like a particular type of radio program format (jazz, country, talk or classical, for example), a button can be pressed that scans only for strong stations that currently are playing your favorite format. This is accomplished by forwarding RDS information within the subcarrier of the FM radio station that contains information regarding the current type of programming ongoing within the voice portion of the radio channel. During a talk show, for example, a coded signal is sent on the subcarrier indicating what type of topic is being discussed (politics, entertainment, news, weather)but during a music program, other codes are sent indicating what type of music is playing (country, new age, blues, rock).

The goal of the designers of the RDS system was to have the format adopted throughout Europe. The format is being used for nationwide paging within France, in the Caribbean and in many other locations around the world.

Paging within the RDS format is designed to handle a ten-digit numeric display, 18 digit numeric display and 80-character alphanumeric display messages. Like MBS, the RDS system operates at 1187.5 bits per second and is based on a 26-bit word consisting of 16 data bits and 10 error detection and correction coding bits. The error code allows correct information to be received even if an error burst of five bits occurs within the 26 bit block. Four blocks of 26 bits are interpreted together as a 104-bit signal referred to as a "group." Depending on the type of information contained within the group, a different "group type" code is defined and transmitted within that group. Because different types of services can reside simultaneously within a RDS network, different group-type codes will be transmitted sequentially. If more than 104 bits are required to completely send the page alert and message data when a page alert is transmitted to a pager, there is no requirement that the next segment of the transmission be sent in the next group.

A different group type used to send information for another data service may appear during the transmission of paging data. In other words, in the RDS format, information for different services are multiplexed into a single transmission stream; for example, paging is known as Group Type 7A.

Group Type 1A is transmitted at least once per second. This group contains special information that is used to keep receivers synchronized and locked into the channel, and provides encoded timing signals that are used to provide pager battery-life conservation. These timing signals break each minute up into 10 intervals. The last digit of the pager address determines in which interval the pager alert signal will be transmitted. A pager can interpret the timing signals that are transmitted each second and determine how long to stay in a battery-saving mode before it needs to increase power to detect a possible page alert. Because of encoding format rules, a pager never needs to power up for more than 18 seconds of each minute and, in most cases, is powered off much more quickly because it "knows" that no pagers are being alerted within its timing interval.

European Radio Message Standard (ERMES) Format
The European Radio Message Standard (ERMES) is a standard that was developed by a subcommittee of the European Telecommunications Standards Institute (ETSI) responsible for all communication standards throughout that continent. The committee was charged with developing a European-wide radio paging network. Although each part of the network will be operated by carriers within each country, subscribers can be alerted on their pagers regardless of where they are located within the network. Likewise, callers may call from anywhere in the network and use the same input protocol to alert any subscriber anywhere in the network.

When fully implemented, ERMES is expected to operate in more than 16 European countries with a combined population exceeding 320 million. In January 1990, 26 operators from 16 countries signed a Memorandum of Understanding (MOU), indicating their agreement to create a service based on this standard. In support, all signers of the MOU have agreed to allocate the frequency range 169.4 - 169.8 MHz to this new service.

Because ERMES would involve system operators, pager manufacturers, paging terminal manufacturers and transmitter equipment manufacturers together, it was decided that an entirely new standard would be created. This would avoid giving any individual entity a bigger head start than any other in developing ERMES-compatible equipment. The standard is comprehensive in that it encompasses various analog and digital telephone input protocols; data network input protocols; protocols for moving tone-only, numeric, alphanumeric and data messages to paging terminals, protocols for encoding information to the pager; protocols for moving information to the transmitter equipment. This section will be limited to a discussion of the ERMES pager encoding format.

The ERMES digital encoding format supports tone-only, numeric and alphanumeric paging in addition to data transfer capabilities. The format operates at 6250 bits per second. Like the MBS and RDS formats, pagers operate on multiple frequencies, scanning for the best frequency for optimum reception. Scanning pagers allow for operations at different frequencies at different points in the network. The specification denotes 16 frequencies over which pagers are to operate.

The paging format uses a modulation mechanism known as "Four Level Pulse Amplitude Modulated FM." In this mechanism, two binary bits of information are transmitted simultaneously through the transmission of one of four signaling frequencies. One set of frequencies is interpreted as the two binary bits "00," another as "01," another as "10," and the final frequency as "11." Therefore, with frequency transitions at the rate of 3,125 per second, 6,250 bits of information may be transferred.

Under the ERMES protocol, every hour is broken up into 60 cycles, each one minute in duration (cycles 0 - 59). Each cycle is divided into five equal sub-sequences of 12 seconds each (sub-sequences 0 - 5). Finally, each 12-second period is divided into 16 separate batches (batches A - P). The batch number, sub-sequence number and cycle number of each transmission is encoded into the system information partition of each batch. Over the 16 different frequencies that support the ERMES format, the first batch to be sent in each sub-sequence is a different batch number. Batch A is the first batch sent on Channel 1, the sixteenth batch on Channel 2, the second batch on Channel 3, the fifteenth batch on Channel 4, etc. This methodology allows a pager to step through paging frequency channels without losing any messages. For battery-conservation purposes, pagers are designed to be activated starting at one of the 16 batches. Furthermore, a pager can be programmed to be paged only in particular sub-sequences or even in particular cycles.

Each pager is specified by a 35-bit address known as the Radio Identity Code (RIC). This unique address consists of 13 bits that are specific to the "home system," where the subscriber database information is maintained, and a 22-bit local address for the specific pager. This large address field will accommodate a global address scheme to support hundreds of millions of pagers.

A batch contains separation partitions of information known as the synchronization partition, system information partition, the address partition and the message partition. Within each batch, the address partition contains the first 18 bits (the initial address) of the unique pager number ordered in descending order. This technique allows a pager quickly to determine if its unique address is not part of this batch so that it may return to battery-saving mode. All pagers whose addresses are larger than the initial address can return immediately to battery-saver mode. Messages are transmitted directly after the address partition.

Each message is preceded by a 36-bit message header. This header contains the 22 bit local pager address, a message number and additional information about the message. The message number is an important feature of ERMES pagers; if the pager finds that the next message number received is not the next number expected, then the pager will alert the subscriber that a message has been missed. The ERMES system provides a feature where a subscriber may call in to retrieve any lost messages. Note that if this pager is part of a group of pagers being alerted at one time, a special indicator is set in the header to indicate that the sequence number is not being sent and should not be checked.

Some of the additional information that is contained in the message header indicates if this is a tone-only, numeric, alphanumeric or data page; indicates which of eight different beep patterns should be used; indicates if this is a priority page; defines which of many different character sets should be used to display the message; and several other functions. The format also allows for the remote programming of pager parameters over the air as well as the temporary creation of pager groups.

The system partition, address partition and message partition of each batch consists of bit sequences that are some multiple of 18 bits. Associated with each 18 bits of data is a 12-bit error detection and correction code; together they create a 30-bit "codeword." With the 12-bit code added, any two errors in the 30-bit codeword can be corrected, and any three errors may be detected by the pager.

For higher reception probability, a method known as "codeword interleaving" is used within the message partition of the batch. Every nine codewords are grouped together and are referred to as a "codeblock." Rather than transmit the bits in codeword order - Codeword 1 to Codeword 9 - the bits are transmitted starting from bit 29 (the most significant bit) of Codeword 1, to bit 29 of Codeword 2, to bit 29 of Codeword 3 and soon until bit 29 of Codeword 9. Then transmission continues from bit 28 of Codeword 1, to bit 28 of Codeword 2 and so on. Treating the nine codewords as nine rows of 30 columns and transmitting the information by column rather than by row reduces the probability that a radio-burst error will affect the error-correcting capabilities of the protocol. If a number of bits in a row are corrupted, the error is spread across many different codewords, each of which has the ability to correct any two-bit error.

Hexadecimal Sequential Code (HSC) Format
The Hexadecimal Sequential Code (HSC) format, introduced in 1979, is the only analog paging format that was designed to provide numeric display paging services. The HSC format is a way of providing display paging to carriers who have analog transmitter equipment not capable of transmitting the signals associated with digital paging formats. HSC supports tone-only, numeric display, voice paging and a combination of numeric display with voice.

HSC is a variation of the 5/6 tone format. It continues to use the ten tones that represent digits 0 - 9, the "R" tone to repeat the last digit and the "X" tone used to signal a second beep pattern. Four additional tones (one being NO TONE, a frequency of zero) are defined, resulting in 16 different tones; these tones are referred to as the 0 - 9 and A - F tones. (The word Hexadecimal in HSC comes from the 16 tones that are used.)

Careful design of the HSC encoding format ensures that 5/6 tone pagers operating on the same frequency are not paged accidentally when HSC signals are transmitted. Three of the tones in HSC, "A," "B," and "D," appear as "tone gaps" in normal 5/6 tone paging and cause those pagers to reset their decoders to ignoring the tone sequence received thus far. By partitioning a transmitted sequence with these tones, normal 5/6 tone pagers will not be alerted accidentally. If an A, B or D tone appears after any four tones, 5/6 tone pagers will not recognize a five-tone sequence that might match its pager address. If a telephone number is being transmitted, the tones 0 - 9 cause these digits to be displayed; the digit "C" causes a hyphen to be displayed; and the codes A, Band D are not displayed. After four sequential digits of a transmitted telephone number,the D tone is sent to avoid 5/6 tone pager false alerts. The pager display will not be affected.

Five combinations of HSC tones are used to activate special features of HSC pagers. These codes -- BD, CB, CD, dB and DC -- are ignored by 5/6 tone pagers because they appear as tone gaps. Each of these codes is followed by a numeric digit to further define a feature code; in this way, many different pager capabilities may be activated. Some of these codes are used to:

A combination of codes allow a display message to be received as well as a voice to be heard, a feature unique to HSC receivers.

Morse Code
In the United States, every carrier who is allocated the use of a frequency is assigned a unique station identification code. The Federal Communications Commission (FCC) requires that this code be transmitted at least once an hour. The format of the transmission is not specified; it could be a voice message giving the call sign of the station, but more often, the information is transmitted as a Morse Code signal. This code, originally developed to send messages over telegraph wires, consists of short (dot) and long (dash) tones that, in combination, represent letters and numbers.

The coded signal is created in a variety of ways. In analog systems, a tone of any frequency can be selected and used to send the dot and dash signals. Many all digital systems do not have the ability to transmit analog tones, therefore, short and long bursts of binary "1s" and "0s" are sent. Regardless of the mechanism used to generate the sound of a dot and a dash, the station identification is transmitted. Typically, in the United States, a radio station code is specified as a three-letter and three-number code, such as KQX143. Because of FCC requirements to transmit station information even during periods of heavy paging transmission, Morse-Coded station identification often is treated within a paging terminal as a high-priority page that is sent once per hour.

Letters and numbers are encoded in Morse Code as follows:

1  • — — — —  A  • —  K  — • —  U  • • — 
2  • • — — —  B  — • • •  L  • — • •  V  • • • — 
3  • • • — —  C  — • — •  M  — —  W  • — — 
4  • • • • —  D  — • •  N  — •  X  — • • — 
5  • • • • •  E  •  O  — — —  Y  — — — 
6  — • • • •  F  • • — •  P  • — — •  Z  — — • • 
7  — — • • •  G  — — •  Q  — — • —     
8  — — — • •  H  • • • •  R  • — •     
9  — — — — •  I  • •  S  • • •     
0  — — — — —  J  • — — —  T  —     

FLEX™ Format
Motorola has developed the FLEX™ family of high-speed transport protocols which is being positioned as a standard for the wireless communications industry. While Motorola owns and controls this family of protocols, Motorola is licensing the protocols to enable their acceptance globally. The first family member is the FLEX™ high speed one-way paging protocol. It is capable of operating at data speeds of 1600, 3200 or 6400 bits per second. The format supports the delivery of tone only, numeric, alphanumeric and binary data to remote receivers. It sends paging data in fixed size batches which utilize a data interleaving scheme similar to that of the Motorola Golay format (described briefly as part of the ERMES discussion). This interleaving provides for a high degree of burst error protection. Error protection is further enhanced by imbedded checksum information within the transmitted data. A number of consecutive interleaved blocks are grouped together into frames. Sequences of frames are repeated cyclically each hour.

The protocol is time synchronized, transmitting frames at very specific time intervals. The first frame typically starts on the hour. Pagers may be set up to normally expect its pages to arrive during certain transmission frames, yet, under system control operate to receive pages in frames which occur more often. This may be used when a channel is lightly loaded in order to send several pages at one time rather than spreading them over a longer period of time. It also allows FLEX™ to be introduced into an existing paging channel minimizing the amount of airtime used by FLEX™ until traffic volumes dictate additional airtime. When pagers are operating to only expect transmissions to occur in their normal frames, battery life may be extended at the expense of a slightly longer message latency.

At its highest operating speed, the protocol simultaneously delivers 4 time multiplexed data streams at a time, allowing each pager to continue operation at a low speed (1600 BPS) while the channel carries 6400 BPS. The paging format is capable of addressing more than 1 billion device addresses. Frames are created in such a way as to minimize the time it takes a pager to determine if the frame contains a message for its address. This further improves the battery life of FLEX™ based pagers.

FLEX™ provides a range of operating modes which allow the paging terminal a great degree of flexibility in optimally transmitting pages on queue based upon traffic loading conditions. Long messages may be segmented into shorter pieces and reassembled by the receiver. This allows shorter messages to be transmitted between the segments of larger messages. Other features allow certain page requests to be replaced by shorter transmission sequences thereby allowing more pages to be delivered in a given period of time.

APOC Format
The Advanced Paging Operations Code (APOC) is a proprietary pager protocol of Philips Paging. The format has been designed to allow POCSAG paging systems to gracefully introduce new paging features and capabilities in an existing paging network. These enhancements include extra channel capacity, extended battery economy and additional network features. It has been designed to coexist with standard POCSAG transmissions.

Battery life extensions are accomplished by grouping APOC pager transmissions into cycles which traverse many individual POCSAG batches. More than a tenfold increase in battery life may be accomplished by allowing the pager to remain in a low power mode more often than that achieved in standard POCSAG. Of course, battery life is extended at the cost of a small increase in the latency of a page transmission.

The APOC format is particularly suitable for alphanumeric paging. It incorporates a built in dictionary held in each pager and the transmission of short data sequences which reference this dictionary, in order to provide a considerable degree of message compression. This not only results in increased reliability in sending long textual messages, but effectively increases channel capacity by reducing the number of data bits required to send a message. APOC also addresses the efficiency of numeric page transmission by providing a mode of operation which allows numeric pages which do not contain any special characters such as spaces, hyphens and parenthesis, to be sent with, typically, a 20% increase in channel capacity. A 50% increase in capacity can be achieved for 7-digit numeric messages.

Under the APOC enhancements, mixed data transmission speeds may operate on a single channel. Standard APOC operates as a 1200 bit per second transmission, but, data codewords may be sent at 2400, 3200, 4800 or 6400 bits per second. This is accomplished by sending specialized POCSAG synchronization codewords at the 1200 bit per second rate, which indicate that in the period of a standard 1200 bit per second POCSAG batch, data will be transmitted at a specific data rate. In order to improve page reception reliability beyond that of standard POCSAG, APOC incorporates a codeword interleaving technique similar to ERMES to increase the error detection and correction capabilities of certain types of page transmissions, well above that of normal POCSAG pages.

APOC introduces the concept of an extended address field which increases pager addressing beyond the 2 million pager numbers of standard POCSAG. It also introduces a control field which provides for new paging capabilities. These new fields are used to provide such features as:

The synchronization technique employed in APOC allows APOC pagers to recognize POCSAG transmissions allowing these receivers to roam between APOC and unmodified POCSAG networks.

Two-way Paging
The most recent pager technology advancement at the time of this writing, is in the introduction of two way paging technologies. Paging receivers equipped with battery efficient transmitters, are now capable of returning responses which are eventually returned to the paging control terminal. The control terminal in turn, can then forward a response to the originator of the page request through any means of communication supported by the paging terminal. This could include the sending of another page, a voice mail response, telephone dial out, fax delivery, forwarding of E-mail, etc.

Various responses from receivers are possible, based upon the features of the pager and paging protocols utilized by the portable transceiver. Some possible pager responses could be:

Different devices could offer some or all of these responses. Some of these responses could cause a status update to be requested at the paging control terminal for caller pickup or could initiate the forwarding of a response to the caller.

Some of the two way protocols support autonomous registration. This is the sending of a special control packet from the pager when it is powered on or enters the coverage region of a paging network. In this way, the paging control system can determine if there are any waiting messages to send to the pager when it is first becomes known to the network. This can be further extended in a multi-city network to provide for automatic roaming capabilities.

For proper operation of a two way paging system, information sent to the pager must be tagged in such a way that the eventual response or responses sent from the receiver,can be associated with a particular message sent. The responses could come a considerable period of time after the message is originally transmitted. Pager protocols(encoding formats) either had to be expanded upon to convey additional information in support of two way paging or entirely new formats had to be developed. There are several competing two way paging technologies in existence today. Some of these two way paging protocols will be reviewed in this section.

Two way paging requires a network of radio receivers to interpret the transmissions from the pagers. It is possible for more than one receiver in such a network to detect a transmission from a single pager. A number of different components exist in a two way paging system to properly process paging responses and move them back to the paging control system which is to ultimately alert the message originator. It is the function of some of these components to address the duplicate response situation.

New protocols have been developed between each of the network components to properly convey response information from the pager to the central control terminal. The discussions in this section will be limited to some of the pager protocols in place and not the protocols used within the network components.

ReFLEX™
ReFLEX™ is a Motorola designed two way advanced messaging FM protocol. The protocol defines the manner in which message and control data is sent to the paging and messaging receiver as well as the manner used to return the various types of responses from the receiver. Protocol flexibility allows receivers to transmit their responses on a different frequency than that over which the message was received. Further features permit ReFLEX™ operations over 25 kHz or 50 kHz wide channels, providing for higher data rates and customer capacity at the higher bandwidth. Proper system operation in each of these operating modes dictate that ReFLEX™ operate as a time synchronized protocol. The paging receivers utilize the same time base as well as information conveyed within the transmitted data stream to precisely time when a pager is to send its responses. The response time periods may be dynamically changed or assigned based upon the current loading conditions of the radio channel.

The ReFLEX™ protocol shares many attributes with the FLEX™ one way FLEX™ paging protocol and is able to be mixed on the same channel with the other FLEX™ type protocols. The outbound paging channels may operate at 1600, 3200 or 6400 bits per second although ReFLEX™ may also operate over multiple 6400 bit per second data streams if sufficient bandwidth is available. The same paging capabilities and page transmission features of FLEX™, including segmentation of long messages and the ability to utilize alternate mechanisms to reduce the transmission time of certain messages, exist within ReFLEX™. But the ReFLEX™ protocol introduces an array of specialized over-the-air commands to address the control of messaging unit responses.

Messaging units transmitted responses fall into two categories, solicited and unsolicited. The protocol dictates when unsolicited response are allowed to arrive. There are several types of unsolicited responses which may be sent. Some unsolicited responses could result in a subsequent solicited response. The outbound transmission equipment has complete control, through the ReFLEX™ protocol defined commands, to precisely schedule the transmission of certain responses which are pending in the messaging unit. The ReFLEX™ protocol provides a choice of multiple signaling speeds for response messages sent from the messaging unit, to allow the system structure to operate with the fewest number of receiver sites for a given traffic level. The ability to schedule location specific transmissions provides a reuse feature for the operating system allowing maximum utilization of the RF spectrum.

InFLEXion™
Another member in the family of FLEX™ protocols is InFLEXion™, Motorola's advanced voice and data messaging protocol utilizing both FM and Linear modulation signaling methods. The FCC action to release 50 kHz wide Narrow Band PCS channels has allowed the InFLEXion™ technology to become available. The InFLEXion™ protocol allows for the creation of a voice and data messaging network with frequency reuse capabilities similar to that of cellular telephony. The protocol is based upon the ReFLEX™ protocol already discussed. It is also possible to operate completely in a simulcast mode. Other FLEX™ type protocols may be mixed on an InFLEXion™ based channel. InFLEXion™ based systems make specific use of the ReFLEX™ capability to request a messaging unit to respond to a location request, combined with directed transmission of pages within specific geographic areas. The messaging unit response enables the network control equipment to determine the specific geographic area in which the messaging unit is located. Using this response, a voice message may be sent to the nearest transmitter(s) to which the pager is currently located. The voice messaging over the 50 kHz channels features variable compression techniques allowing equal to or greater than 24 times the subscriber capacity of today's 25 kHz channels. Simultaneously, other voice messages may be delivered on the same frequency to messaging units which are geographically separated from each other so that there will be no transmission interference. With InFLEXion™ Data signaling the throughput on the 50 kHz channel allows signaling of up to 112 Kbps As in ReFLEX™ systems, the InFLEXion™ protocol provides a choice of multiple inbound signaling speeds to allow the system structure to operate with the fewest number of receiver sites for a given traffic level.

NexNet
NexNet is a proprietary two way paging system designed by Nexus Telecommunications Ltd. of Israel. It utilizes spread spectrum technology integrated into their paging receivers to transmit response messages. In its initial implementation, messages are forwarded to pagers through the utilization of the POCSAG protocol. The additional messaging information required for time synchronization of the transmitted messages with the two way pager responses sent at a later time, as well as the transmission of two way paging control data, is imbedded within standard POCSAG output data. NexNet outbound data in support of two way paging is sent in such a way as to be totally transparent to normal POCSAG transmissions so that two way paging may coexist in any preexisting one way paging system.

The Nexus two way paging device, known as the Twager™, sends responses formatted according to the NexNet™ "uplink" protocol specification. In the Spread Spectrum technique, the data is transmitted over a range of different frequencies and is detected by receiving equipment located throughout the coverage region. Each Twager™ utilizes different sets of frequency combinations (frequency hops)when transmitting their data. Complete response messages may be recovered at receivers even though some portions of the response message may have been corrupted. This is because a message segment which is corrupted because of interference from other transmissions or by a simultaneous transmission from another Twager™ at one or more frequencies, has a high probability of being properly received at other frequencies over which this segment is transmitted.

In order to simplify the reception of the spread spectrum frequency hopped signals, the transmissions from the Twagers™ are required to synchronize to a precise clock. This precise time synchronization is achieved through the transmission of precision timing data which is imbedded in the POCSAG output stream. But, this is accomplished in a manner which permits this data to be overlaid into existing paging networks without the need to add any specialized transmission equipment.

New Code Formats From the PCIA High Speed Paging Committee
Since paging began, new paging formats have been developed to incorporate new transmission technologies and to bring new features to the subscribers. New transmission techniques may result in higher transmission speeds and/or higher reliability in the receipt of paging data or a lower cost to transmit information over larger geographical areas. New transmission formats may result in sending more information with fewer data bits, better battery saving capabilities to prolong battery life, or to provide services and features that go beyond that of traditional paging. For example, a variation of the POCSAG format fully compatible with normal POCSAG pages, allows for the forwarding of spreadsheets, computer data and even computer programs over paging channels for reception by laptop or palmtop computer devices. This technique, described by the TRT protocol specification as part of the TDP protocol, extended the POCSAG format for the transmission of data, a capability which was not part of the original specification.

The PCIA High Speed Paging Committee is the paging association-sponsored team of carriers, paging terminal manufacturers, transmitter manufacturers and pager manufacturers who together have defined some of the desired capabilities for paging into the 21st century and together review technologies which address these requirements.

END OF SECTION

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