30.4 PAGING PROTOCOLS

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30.4 PAGING PROTOCOLS

Though a number of paging protocols are available, the most widely used protocol is the one standardized by POCSAG (Post Office Code Standardization Advisory group). This POCSAG protocol was standardized by the British post office and used worldwide in both commercial paging systems and amateur paging systems.

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Post Office Code Standardization Advisory Group (POCSAG) formulated a paging protocol, known as the POCSAG protocol, which is used extensively in most paging systems.

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POCSAG supports three data rates: 512bps, 1200bps, and 2400bps. FSK modulation is used with ±4.5kHz frequency shift. Data is NRZ (non-return to zero) coded with higher frequency representing a 1 and lower frequency representing a 0. While transmitting the data, the most significant bit is sent first.

The basic unit of data is called a codeword. Each codeword is 32 bits long. The first bit of the codeword (bit 31) indicates whether the codeword is pager address or message-if it is 0, the codeword is the address; if it is 1, the codeword is the message.

Message codewords use bits 30-11 (20 bits) as message data, bits 10 to 1 are used for error correction, and bit 0 is the even parity bit. Alphanumeric text is coded in ASCII format in the message.

Address codewords use bits 30-13 (18 bits) as address, bits 12, 11 indicate the type and format of the pager, and bits 10 to 1 are used for error correction. Bit 0 is the even parity bit.

For both types of codewords, 1- or 2-bit errors can be corrected. Codewords are sent in batches of 16, each batch preceded by a special seventeenth codeword that contains the fixed frame synchronization pattern. Batches of codewords are preceded by a preamble (101010), which must be at least 576 bits long.

When the paging controller sends the codewords continuously in the above pattern, each pager has to synchronize with the received data, using the preamble and synchronization pattern, and then decode the address codeword. If it matches its own address, the corresponding message is taken.



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Principles of Digital Communication Systems and Computer Networks
Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)
ISBN: 1584503297
EAN: 2147483647
Year: 2003
Pages: 313
Authors: K V Prasad

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