7 Paging Systems It is often important to be able to reach certain people very quickly. The conventional telephone network is not always optimal, because a line may be engaged or the person being called might not be available. Mobile telephone systems offer a high degree of reachability of a mobile subscriber, but MSs are not always switched on and are expensive to use. Paging systems fill a particular gap (see Table 7.1). They allow unidi- rectional transmission of information in the form of a tone or a numeric or alphanumeric message to the person being contacted, whose location area is not known (see Table 7.2). A terminal is required that is constantly ready to receive but cannot transmit, and consequently is small, lightweight and inexpensive. A paging message is automatically initiated when a telephone user dials the paging service and, using a telephone keyboard, an Internet terminal or a PC, conveys the pager number of the called person or a short message for it to the computer that responds (see Figure 7.1). One characteristic of the paging system is that the person sending the message can never be certain that it has been received. There can also be a considerable gap between the time a message has been sent and when it is received by the addressee, and during peak load times this can amount to 10 minutes. Table 7.1: Public operation of paging systems in Europe Since 1974 European Paging Service (Eurosignal) Since 1989 Cityruf Since 1990 Euromessage Since 1996 ERMES Table 7.2: Call types of different paging systems Call type Eurosignal Cityruf ERMES Tone only [No. of paging no.] Up to 4 Up to 4 Up to 8 Numeric [No. of num. char.] — Up to 15 20–16 000 Alphanumeric [No. of char.] — Up to 80 400–9000 Transparent data transm. [max. length] — — 4000 bits Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright © 1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 440 7 Paging Systems 0% 7 4 1 2 3 5 8 9 6 * 0% 7 4 1 2 3 5 8 9 6 * Paging Service Switching Centre - Telex - Telephone Networks like: - ISDN - Internet (DTMF Dialing) Call Centre Figure 7.1: Principle of paging Strengths and Weaknesses of Paging Systems Strengths • Inexpensive alerting and information service. • Small receivers. • The user of a paging system can be reached anywhere if the receiver is carried on his/her person. • No additional antenna required (this applies only partially in the case of Eurosignal). • Discrete communication of messages (messages are signalled through vibration, messages can be read on display). Weaknesses • The sender of a paging message receives no confirmation that the mes- sage has been received. • Falsified paging messages and pages by malicious callers can cause the user of a pager to take inappropriate action. Types of Calls Individual calls The paging message is transmitted to the defined paging areas (one or more) or in the dialled paging area. Collective calls A collective call number consists of n individual telephone numbers (receivers with different addresses). The receivers within a paging area are called in consecutive order. 7.1 Paging Service “Cityruf” 441 Group calls Several receivers with the same address are called simultaneously in one or several paging areas. Target calls By dialling additional numbers, the caller specifies the paging area in which the paging message should be transmitted. It is anticipated that the number of users of pagers will increase in the future, despite the availability of services such as the short-message service (SMS) offered by GSM that are regarded as competition. A pager is an ideal extension to the mobile telephone because it can often be reached in places outside the radio coverage range of mobile telephone systems. The miniaturization of terminals, including waterproof wristwatches that incorporate a radio pager receiver, offer the potential for a mass market for paging services that will extend beyond commercial applications to private and leisure use. 7.1 Paging Service “Cityruf” Paging systems that operate in different frequency bands (150–170 MHz or 440–470 MHz) in accordance with POCSAG-Code (the Post Office Code Stan- dardization Advisory Group pager system was developed as long ago as 1981) (CCIR Radio-Paging Code No.1 ) are now being used in many parts of Eu- rope. Subscribers can be reached by practically all communications networks over the POCSAG radio calling service (see Table 7.3). A typical example of the POCSAG paging service is Cityruf, which was introduced in Germany in March 1989. A radio paging system based on POCSAG (see Figure 7.1) consists of: • A paging service switching centre that forms the link between the vari- ous communications services (telephone, data, etc.) and the paging net- work, in which incoming data is prepared for processing in the paging computer. • Paging transmitters with transmitter power up to 100 W. • A paging concentrator, which is used to switch on a paging transmitter at the paging service switching centre and is allocated a paging measure- ment receiver for the automatic propagation control of the modulation paths. • A paging receiver. As the name implies, the Cityruf network is not envisaged to be a wide- area service. It is a regional paging service for cities, and its coverage area is divided into internetworked paging areas. About 50 paging areas with a maximum diameter of 70 km have been implemented in the final configuration. However, subscribers to the Cityruf service are not only booked in locally 442 7 Paging Systems or regionally but in several paging areas and, in fact, throughout Germany. The transmitter systems of Cityruf are designed in such a way that they can guarantee reception within buildings without the need for additional antennas. The maximum number of subscribers that the system is capable of addressing is two million. Fifteen paging messages can be sent in one second. Users of Cityruf have a choice between different calling classes based on different monthly charges: Calling class 0 (tone only) Input is over the telephone. Tone-only devices issue up to four optically and acoustically different signals. Calling class 1 (numeric) A maximum of 15 digits or special characters can be entered directly over the telephone using a supplementary device or with a dual-tone multiple-frequency (DTMF) signal generator, and then appear in the display of the terminal. Calling class 2 (alphanumeric) Text messages up to 80 characters long can be entered, for example over the Internet or using an acoustic coupler device over the PSTN. Several messages can be transmitted in succes- sion. The following types of calls are available for the three calling classes: Individual calls A radio call is broadcast in the registered paging areas or to the selected paging area. Group calls A number of receivers are addressed over a group number at the same time. Collective calls Up to 20 individual paging numbers are assembled in a list and then dialled up automatically in succession. Target calls The receiver of a call is assigned a special number. To reach this person, a caller first dials the paging number and then the suffix number of the radio paging area in which the call is to be transmitted. In addition to terminals and accessories for Cityruf, radio paging network operators offer the supplementary service Inforuf, which allows the user to receive information such as stock market updates, business news and weather reports. To access this information, users require a special Inforuf receiver that receives the Inforuf signals along with the Cityruf signals and is capable of storing 80 000 characters and reading them out on its 80-character dis- play. The current information providers include Reuters, Telerate and pooled information services. Cityruf is transmitted on the following frequencies: • 465.970 MHz • 466.075 MHz • 466.230 MHz 7.1 Paging Service “Cityruf” 443 S C 17 Code Words = SC + 8 Frames 1 20/1 32 2-19 Address Bits 22-31 Coding Bits Code Word Frame of Two Code Words Message Flag Function Even Bits Parity0 = Address Word 1 = Message Word Synchronization Preamble First Batch Second Batch Subsequent Batches 1.0625 s1.125 s 1.0625 s Figure 7.2: Message coding and block format with POCSAG The transmission rate is 512 bit/s or 1200 bit/s. The digital signals are NRZ (Non-Return to Zero) coded and modulated through the use of differential frequency-shift keying (DFSK). The transmitters emit bursts of data blocks with code words (see Figure 7.2) [2]. Each burst begins with a 1.125 s long preamble, which is followed by a num- ber of data blocks of 1.0625 s duration. The preamble enables synchronization to take place on the signal pulse on the receiver side, and is a prerequisite for error-free message coding. The data blocks consist of 17 code words, the first of which is used in the synchronization. The remaining 16 code words, each with 32 bits, contain an initial bit that indicates whether the code word is an address or a message, 20 address or message bits, 10 bits for error detection and correction, and a parity bit. The 16 code words form eight frames, each of which contains two code words. Each terminal is only addressed in a specific frame. The message for a receiver can be of any length, and is sent in the form of message words based on the address of the person receiving the message. Paging areas can be divided into several radio coverage areas, with all transmitters within an area transmitting in a common frequency. The three frequencies are not used simultaneously in a transmitting zone, but are stag- gered and cyclical. Transmission never takes place simultaneously on the same frequency in the adjacent transmitting zones, so that a three-site cluster oc- curs. The time (time slot) during which transmission can take place on a frequency can be adapted to traffic volume. The advantages of this procedure are that adjacent zones can be decoupled through radio engineering and a receiver only needs to be operational when its frequency is being sent. If no messages are being sent on the frequency of the receiver or if it recognizes that a radio call is not directed at it, the receiver remains in battery-saving idle state. Cityruf receivers are small in size, include storage options for characters which are received and use minimal power only. Table 7.3 lists the service codes required for network access. 444 7 Paging Systems Table 7.3: Cityruf network access codes Access over Code/paging number/paging area PSTN/ISDN Tone only 0164/XXXXXX/YY Numeric 0168/XXXXXX/YY Alphanumeric 01691/XXXXXX/YY Bureau service 016951 IDN Telex network 1691/XXXXXX/YY Datex-L 1692/XXXXXX/YY T-Online *1691#/XXXXXX/YY Development of Service 1989 Start of service to the public. 1990 Internetworking of Cityruf with the systems ALPHAPAGE in France, TELEDRIN in Italy, EUROPAGE in Great Britain. 1991 Introduction of the Inforuf service for closed user groups, introduc- tion of alphanumeric messages with multifrequency dialling using the access code 0168, internetworking of Cityruf with radio paging system in Switzerland. 1992 Automatic dialling through telephone answering machines, alphanu- meric access through access code 01691 at higher transmission rates (up to 24 kbit/s). 1994 Remote control and telemonitoring with Cityruf, input of numeric mes- sages using voice. 7.2 Euromessage In addition to Cityruf, which is a national service, there is also a European- wide radio paging service based on POCSAG called Euromessage (European Messaging). This is an extension of Cityruf that was set up in March 1990 to include an international service through the internetworking of the na- tional radio paging services in Germany (Cityruf), France (Alphapage), Italy (Teledrin) and Great Britain (Europage). Subscribers to this service wishing to be reached whilst abroad must notify the service operator with details about the period of time they will be away and in which paging area they can be located. All messages that arrive for a subscriber during this period are then rerouted to the appropriate paging area. Euromessage is considered to be an intermediate solution until the avail- ability of the Pan-European standardized radio paging service ERMES. 7.3 RDS Paging System 445 16 bit Block 2Block 1 Block 3 Block 4 87.7 ms Control 10 bit Information Control 10 bit Information 16 bit Group 104 bit Figure 7.3: The RDS block format 7.3 RDS Paging System RDS (Radio Data System), which was specified by the European Broadcast Union (EBU) and passed in 1984, is used for transmitting supplementary information over VHF radio broadcast transmitters such as: • Transmitter recognition • Alternative transmitter frequencies • Programme information • Traffic information • Paging The supplementary information consists of digital data, which is combined into groups of 104 bits each (see Figure 7.3). Each of these groups has a code to indicate which type of supplementary information it contains. Groups that contain tuning and switching information are transmitted more frequently than other groups. The countries that use RDS for transmitting radio paging services are Swe- den since 1978, France since 1987 and Ireland. Other countries, including Spain and Norway, are planning to introduce the service. Germany intro- duced RDS in 1988. The RDS paging service offers the advantages that investment costs are low and subscribers can be reached anywhere because of countrywide VHF radio broadcast coverage with the existing transmitter network and shared use of VHF frequencies. Alphanumeric messages cannot be transmitted in the RDS system. 7.4 ERMES ERMES (European Radio Messaging System) is a European-wide paging ser- vice that was developed as the result of a resolution adopted by the CEPT countries in 1986. The standardization work begun under the responsibility of CEPT was continued by ETSI in 1989 and completed in 1992. 446 7 Paging Systems By late 1990, 27 European PTTs and radio paging network operators had signed a memorandum of understanding (MoU) that guaranteed the imple- mentation of the ERMES radio paging service. The reason why European countries are so keen on ERMES is that it offers significant advantages over existing paging services. Compared with POCSAG-based paging systems, ERMES has a higher channel capacity because of a high transmission rate; furthermore, because of the availability of transparent data transmission, it is possible to transmit any data with up to 64 kbit/s. Added to this is the pos- sibility of international roaming and, in the case of several operators, national roaming. ERMES represents the first standardized paging system in Europe that operates in the same frequency band throughout Europe and guarantees the accessibility of its subscribers throughout Europe. Because of its 6250 bit/s bit rate, the capacity of an ERMES channel is four times higher than a 1200 bit/s POCSAG channel. Between 300 000 and 400 000 subscribers can be served per channel, which equates to a system ca- pacity of around six million subscribers. ERMES pagers are being designed to require extremely little battery power and are smaller than equivalent POC- SAG models. 7.4.1 The Services of the ERMES Paging System ERMES offers its subscribers a number of basic services as well as supple- mentary services. The basic services that each operator is required to provide include [3]: • Tone-only calling in which ERMES supports up to eight different tone signals per Radio Identity Code (RIC). This means that a tone receiver with a RIC can produce eight different alarm signals. • Numeric radio paging in which the receiver has a display with at least 20 digits and also supports the tone-only function. • Alphanumeric paging with a minimum of 400 characters and a receiver that is also suitable for tone-only calls and numeric pages. • Transparent data transmission at 64 kbit/s, which can also be used for process control, telemetry and alarm activation. • Roaming: the network recognizes the paging area in which a subscriber is located. ERMES is planning the following supplementary services that can be of- fered by an operator as an option: • Standard text allows an alphanumeric page to be sent through the input of DTMF codes over the telephone. Each code is linked to a standard text, such as “Meet in one hour”. 7.4 ERMES 447 • Group call (see Section 7.1). • Collective call (see Section 7.1). • Call forwarding to another receiver. • Storage of incoming messages, which the subscriber can then request to be sent at a later time. • Numbering of messages and automatic retransmission of the number of the last message. The receiver will recognize from the sequence number whether certain messages have been received. • Repetition of call when requested. • Temporary call barring. • Transmission of calls with different categories of priority. • Target call in which the caller determines the paging area in which a call should be broadcast. • Closed user group. • Display of the category of urgency of a message and acceptance of a message based on its urgency. • The person calling can indicate the time when a message should be sent. • Encryption of messages. 7.4.2 ERMES Network Architecture The system structure for ERMES is presented in Figure 7.4 [4]. The Paging Network Controller (PNC) processes the input received over the telephone, data network and from other networks, with consideration of the services agreed with a subscriber and stored in the network controller. With ERMES, messages for a tone or numeric pager can be conveyed over DTMF (Dual- Tone Multiple Frequency) signals. Paging text messages can be input over any conventional data network (ISDN, PSPDN, CSPDN, etc.) using standardized UPC protocols. Furthermore, the broadcast of a radio page can be initiated through arrangement with the operator. The network controller is therefore responsible for the following tasks: • Provide a user with access to ERMES over the fixed network. • Control and administer the database containing subscriber data. • Provide the possibility of roaming through its link to other ERMES networks over an interface in accordance with CCITT Rec. X.200. 448 7 Paging Systems Other ERMES Networks Database PNC Control Network Paging PAC Control Area Paging Service Broker Base Stations Figure 7.4: ERMES system structure • Control radio transmission in the service areas. Up to 64 Paging Area Controllers (PAC) can be connected to a network controller (see Figure 7.4). An area controller organizes the broadcast of a call and, in accordance with the agreement with the subscriber, activates one or more base stations in order to ensure that coverage is provided to the paging area concerned. Depending on the priority of incoming pages, the PAC forwards them to the base stations. The base stations (BS) transmit with up to 100 W and can reach receivers within a radius of up to 15 km. 7.4.3 Technical Parameters of the ERMES Paging System Sixteen channels with a bandwidth of 25 kHz in the 169.4125–169.8125 MHz frequency band have been allocated to ERMES in Europe. With the use of the 4-PAM/FM modulation procedure the data transmission rate is 6.25 kbit/s or 3.125 kBaud/s. An abbreviated cyclical code (30,18) derived from BCH (31,21) is used as an error-correction code. This code is supported by interleaving with a depth of nine. Network operators and users have been assigned identities to ensure that messages in the ERMES system are routed correctly [1]. An operator identity OPID is 13 bits long (see Figure 7.5). The zone and country codes are derived from CCITT Rec. E.212. The 3- bit long operator code supports eight different operators per country. Should further operator codes be necessary, additional country codes will be granted to differentiate between them. [...]... Chichester, 1993 References 451 ¨ [3] A Ortqvist ERMES’s Role in Europe In 5th Nordic Seminar on Digital Mobile Radio Communications (DMR V), p 119, Helsinki, Finnland, Dec 1992 [4] B Walke Mobile data communications in Germany—A survey In Proceedings of 6th International Symposium on Personal and Indoors Mobile Radio Communications, pp 799–802, The Hague, Netherlands, Sept 1994 ... initial addresses are transmitted in descending order If the initial address received by a receiver is smaller than its own then it cannot receive any further radio pages in this batch Message Partition This part contains the local address of a receiver, the message number, the type of message and the message itself References [1] G Edbom The Concept for World Wide Radio Paging In 41st IEEE Vehicular Technology... structure of the user identity (Radio Identity Code, RIC) consists of 35 bits and is shown in Figure 7.6 The RIC contains the 13-bit long OPID and a further 22 bits for the initial address and the batch type, which together form a so-called local address A receiver is able to have several user identities stored Depending on its description, an ERMES network can be set up as a: • Time Division Network (TDN):... in neighbouring or overlapping paging areas is at di erent time intervals • Frequency Division Network (FDN): Transmission in neighbouring or overlapping paging areas is on di erent frequencies A combination of the two types of operation is also possible The channel and block structures of the transmission protocol are presented in Figure 7.7 The periodic sequence forms the primary structure of 60-minute... minute and is used in the coordination between di erent networks Because the receivers are designed to save on battery power, they can only listen in on one or a few cycles Each cycle contains five subsequences, each 12 s in duration In TDN the transmission for a paging area depends on the traffic volume and takes place in at least one of these subsequences Each subsequence is divided into 16 batches, with... Initial Address APT Address Partition Terminator Figure 7.7: Message coding and block format in ERMES • Two system information words (SI1, SI2), of which SI1 notifies the receiver of the operator identity (ID) and the paging area (PA) number SI2 gives the position of the batch in the sequence and the channel of the transmitter • An additional system information word that contains either the area code,... 16th having a length of 190 words Through the last 4 bits in the RIC, the receiver is allocated one of the 16 possible types of batches and can be addressed in the batch type specified for it A batch is divided into four parts: Synchronization Partition The preamble word (PW) and the synchronization word (SW) enable the receiver to carry out the symbol or code word synchronization for the batch System . di erent tone signals per Radio Identity Code (RIC). This means that a tone receiver with a RIC can produce eight di erent alarm signals. • Numeric radio. European PTTs and radio paging network operators had signed a memorandum of understanding (MoU) that guaranteed the imple- mentation of the ERMES radio paging