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.