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
Trang 1Paging 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
Table 7.2: Call types of different paging systems
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)
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1 2 3
5
8 96
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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
Trang 37.1 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
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 wide-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
Trang 4or 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
Trang 57.1 Paging Service “Cityruf” 443
S C
17 Code Words
= SC + 8 Frames
1 2-19 Address Bits 20/1 22-31 Coding Bits 32
Code Word
Frame of Two Code Words
0 = Address Word
1 = Message Word
Synchronization
Preamble First Batch Second Batch Subsequent
Batches 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
Trang 6Table 7.3: Cityruf network access codes
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
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
Trang 77.3 RDS Paging System 445
16 bit
Block 2 Block 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
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
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
Trang 8By 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
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”
Trang 97.4 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
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
Trang 10Other 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
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