An NEC pager, using POCSAG coding branded for the Skyper network

Radio-paging code No. 1 (usually and hereafter called POCSAG) is an asynchronous protocol used to transmit data to pagers. Its usual designation is an acronym of the Post Office Code Standardisation Advisory Group, the name of the group that developed the code under the chairmanship of the British Post Office that used to operate most telecommunications in Britain before privatization.

Before the development and adoption of the POCSAG code, pagers used one of several codes such as GOLAY.

In the 1990s new paging codes were developed that offered higher data transmission rates and other advanced features such as European and Network roaming.

The POCSAG code is generally transmitted at one of three data rates; 512, 1200 or 2400 bits per second.

With Super-POCSAG, 1200 bits per second or 2400 bits per second transmission rates are possible. Super-POCSAG has mostly displaced the POCSAG in the developed world but the transition is still in progress.

The History of POCSAG

In 1976 an international group of engineers began to meet to explore the possibility of developing a new code for wide area paging; paging networks covering regions of entire countries. These meetings were successful and in February 1981 the CCIR (Comité consultatif international pour la radio) the forerunner of the ITU-R accepted the code as Radiopaging Code No.1 (RPC No.1),(Rec, 584). The meetings were chaired by R.H.Tridgell and were attended by representatives of British, European, and Japanese pager manufacturers^[1]

How it works

The modulation used is FSK with a ±4.5 kHz shift on the carrier. The high frequency represents a 0 and the low frequency a 1.^[2]

The ±4.5 kHz frequency shift is used along with a 25 kHz channel spacing, known as "wideband". Some jurisdictions require that all systems move to a "narrowband" configuration, using 12.5 kHz channels and ±2.5 kHz frequency shifts (for example, the U.S. Federal Communications Commission (FCC) has mandated this transition be completed prior to 2013.).^[3]

Often single transmission channels contain blocks of data at more than one of the rates.

Transmission uses 32-bit blocks called codewords. Each codeword carries 21 bits of information (bits 31 through 11), 10 bits of error-correcting code (bits 10 through 1), and an even parity bit (bit 0). Bits 31 through 1 are a binary BCH code (31, 21). The error-correcting code has a 6-bit Hamming distance: each 31-bit codeword differs from every other codeword in at least 6 bits. Consequently, the code can detect and correct up to 2 errors in a codeword.

The generating polynomial g(x) for the BCH (31, 21) code is:^[4]

${\displaystyle {\begin{aligned}g(x)&=x^{10}+x^{9}+x^{8}+x^{6}+x^{5}+x^{3}+1\\&=(x^{5}+x^{2}+1)(x^{5}+x^{4}+x^{3}+x^{2}+1)\end{aligned}}}$

The codewords are either address or data, which is indicated by the first bit transmitted, bit 31. An address codeword contains 18 bits of address (bit 30 through to 13), and 2 function bits (12 & 11). Each data codeword carries 20 bits of data (bits 30 through to 11).

Codewords are transmitted in batches that consist of a sync codeword, defined in the standard as 0x7CD215D8, followed by 16 payload codewords that are either address or data. Any unused codewords are filled with the idle value of 0x7A89C197.

Although the address (also referred to as a RIC - Radio Identity Code or CAP code - Channel Access Protocol code)^[5] is transmitted as 18 bits the actual address is 21-bits long: the remaining three bits are derived from which of the 8 pairs of codewords in the batch the address is sent in. This strategy allows the receiver to turn off for a considerable percentage of the time as it only needs to listen to the pair that applies to it, thus saving a significant amount of battery power.

Before a burst of data there will always be a preamble of at least 576 bits of data containing alternating 1s and 0s, allowing the receiver to synchronize itself to the signal, and is another mechanism that enables the receiver to be turned off for a large percentage of the time.

A message will start with an address codeword followed by a number of data codewords and will continue until another address, a sync, or an idle codeword is sent. When the data bits are extracted they will be in one of two formats.

Message format

There are two message coding formats for the data messages. Numeric messages are sent as 4 bit BCD values, and alphanumeric messages are sent as 7-bit ASCII. The 7-bit ASCII is commonly referred to as 'alpha-paging', and 4-bit BCD is commonly referred to as 'numeric-paging'.

National implementations

Europe

In the UK, most pager transmissions are in five bands at

• 26 MHz (local pagers, mainly hospital systems, POCSAG and voice)
• 49 MHz
• 138 MHz
• 153 to 153.5 MHz
• 454 MHz

The frequency 466.075 MHz was previously used by Hutchison Paging, but the network was shut down in 2000. The frequency is still reserved for paging but is not used.

DAPNET: Decentralized amateur paging network worldwide

In Germany, well known transmissions are at

• 173 MHz range (Fire Departments, Rescue)
• 439.9875 MHz (Amateur-Radio pager-network)
• 144.8625 MHz (Amateur-Radio pager-network VHF in the German-Luxembourgish Border Region)
• 466.075 MHz (nationwide public paging by emessage)
• 465.970 MHz (-same-, old frequency of the POCSAG-Service Skyper)
• 448.425 MHz (nationwide Fire Departments, Rescue by emassage (named e*Bos), old frequency of the POCSAG-Service Telmi)

Licensed paging is possible in any other VHF/UHF bands.

In Spain, nationwide service was provided by Telefónica Mensatel but the network was shut down in 2012.

The Swedish pager network marketed as "Minicall" is encoded as POCSAG and broadcast on these frequencies:

• 169.800 MHz
• 161.4375 MHz

In Switzerland the following frequencies are used:

• Telepage F1: 147,400 MHz (512 bit/s)
• Telepage F2: 147,375 MHz (1200 bit/s)
• Telepage F3: 147,325 MHz (1200 bit/s)
• Telepage F4: 147,300 MHz (1200 bit/s)
• SIKAN: 169,500 MHz

The Belgium POCSAG is used for paging over the A.S.T.R.I.D. network:

• 169.625 MHz: POCSAG 2400 (Fire Departments, Rescue)

In Italy, the 26.225-26.935 MHz band (AM/FM, odd frequency steps) and 40.0125-40.0875 MHz (in 25 kHz steps) may be used for local pagers. These frequencies are often used for on-site hospital paging systems, including voice paging. Use of POCSAG on the 26 MHz and 27 MHz band has been logged by several listeners in Europe, specifically frequencies 26.350 MHz, 26.500 MHz, 26.705 MHz, 26.725 MHz, 26.755 MHz, 27.005 MHz, 27.007 MHz, 27.255 MHz (see note below regarding legal use of 27.255 MHz for paging in the United States). It appears that US-specification paging systems operating on 27.255 MHz have been sold in Italy and other European countries.

The former monopoly operator SIP (which later became TIM) used the following frequencies for their pager service, called Teledrin:

• 161.175 MHz (for tone/voice only and numeric pagers, probably for the alphanumeric pagers);
• 466.075 MHz (for tone/voice only, numeric and alphanumeric pagers, using the narrow frequencies of the TACS phone system)

In France, POCSAG is operated by E*Message over the AlphaPage network on the 466 MHz frequency:

• 466.025 MHz
• 466.050 MHz
• 466.075 MHz
• 466.175 MHz
• 466.20625 MHz
• 466.23125 MHz

References

External links

• MultimonNG: Open Source POCSAG decoder for Windows, Linux and Mac OS X implementing error correction for better reception under weak conditions
• OpenPoc: OpenSource-Application for decoding POCSAG-Transmissions
• PageOne Communications: How a POCSAG paging system operates
• Ham Radio Paging: Putting POCSAG On Packet
• OFCOM: United Kingdom Frequency Allocation Table