Paging technology: POCSAG, Golay paging codes Part 2–Although POCSAG paging code remains widely used, Golay sequential code is no longer used in most new system designs. Here are some of the advantages and disadvantages of these two early paging protocol
Table 1 at the left compares some main characteristics of four digital paging formats: POCSAG, Golay sequential code, Flex protocol, and ERMES.* Probably the most important reasons for paging system operators to use digital instead of analog formats and for the creation of the newer digital formats, Flex protocol and ERMES, is the ability to send more information, faster, to more people. The POCSAG digital paging scheme was developed by British Telecom as a standard signaling format in the United Kingdom. It is now a widely accepted format worldwide. Official international recognition of this code was given in February 1981 when it was accepted by the International Radio Consultative Committee (CCIR, according to the French abbreviation) as the recommended Radiopaging Code No. 1 (RPC1). The CCIR is a body appointed by the United Nations to recommend worldwide standards in radio matters. POCSAG is a synchronous paging scheme. Messages are always sent in batches, and each pager address is located in a specific frame within a batch. Figure 1 at the left shows the POCSAG paging coding format. Battery energy consumption is reduced (“battery-saving”) because each pager, after detecting the preamble and synchronizing to the frame sync word, turns off its receiver circuits until the proper frame appears. When it does look for its address in the appropriate frame, it will find it unless: there is no page, so an idle codeword is found. a message codeword from another subscriber’s page is using the frame. In the latter case, if a message is intended for your pager, its page will be delayed until a future batch. The number of characters that can be sent in a message codeword are 5.0 numeric or 2.8 alpha. That limit means that for a numeric pager to receive a caller’s 10-digit phone number, two message codewords must be sent following the pager’s address codeword. POCSAG pagers support tone, numeric and alphanumeric messages, but not voice. Golay sequential coding (GSC) is an asynchronous paging scheme. Messages are sent individually or in batches. A pager can receive its address individually or anywhere in a batch. Figure 2 at the left shows the Golay paging coding format. For Golay, battery-saving is achieved through use of the preamble. There are 10 preambles, and the pagers are divided into groups based on the preamble. The pager samples every 1.38 seconds for 250ms looking for the preamble, which lasts 1.42 seconds. If its assigned preamble is found, its receiver circuitry stays on; otherwise, it returns to battery- saving mode. The polarity of the preamble signifies whether an individual or a batch call is due. Following the preamble is the start code, which marks the end of the preamble and supplies timing information to the pager. Within the address codeword, word 1 has 50 possible values, and word 2 has about 2,000 possible values, so the number of possible address codes is 100,000. Because the 10 preambles are also used to divide the pagers into groups, the maximum code capacity of 1 million is reached. Golay pagers support tone, voice, numeric, and alphanumeric messages. In some areas, there is a growing demand for increased capacity because existing systems are reaching their maximum capacity. Also there is a demand for more sophisticated messaging capability.
Next month: How Motorola’s flexible wide-area synchronous protocol (Flex) paging format and the European Radio Message System (ERMES) paging format respond to capacity and advanced messaging capability demands.
*POCSAG stands for Post Office Code Standardisation Advisory Group. Golay sequential code is named after Marcel Jules Edouard Golay (b. 1902), a physicist who constructed the code about 1955. Flex is short for flexible wide-area synchronous protocol. ERMES stands for European radio message system. Paging technology series Part 1: “Systems and Services,” February 1996. Part 2: “POCSAG, Golay Paging Codes,” March 1996.
Back issues printed within the past two years can be ordered for $5 each, postpaid. Call Intertec Publishing customer service at 800-441-0294. Issues printed more than two years ago and individual article photocopies are unavailable from the publisher. Individual photocopies of articles printed since January 1983 can be ordered from the UMI Information Store, 800-248-0360.