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By Root 479 0

term. POCSAG (also known as Post Office Code Standardization Advisory Group) was born from British telecommunications and was the forefather of numerous other paging protocols including Super POCSAG, Flex, Mobi, and several other proprietary ones. POCSAG is a fairly simple Asynchronous Protocol using a Frequency Modulation (FM) known as Frequency Shift Keying (FSK) for transmitting data. Data is transmitted in 32-bit blocks using a frequency shift of +/- 4.5 kHz on the carrier frequency. The frequency shift represents a 0 or a 1 depending on the shift up or down. Originally, this enabled data to be sent at 512 bits per second. 512 bits per second is slow by any standard, but viable when sending plain text. Subsequent versions and predecessors of POCSAG provided significantly more bandwidth. Most notably among these is the FLEX protocol. FLEX is a proprietary protocol developed by Motorola and is still used on many pager systems today. Similarly to POCSAG, FLEX uses Frequency Shift Keying (FSK) to transmit data. The FLEX pager protocol is able to achieve much higher speeds including 1600, 3200, and 6400 bits per second by using a four level modulation of the carrier frequency.

The transmit frequencies used for pager services spans the gamut of the VHF and UHF frequency bands. Pager services started in the 35 MHz range and go all the way on through the 900 MHz space. Now that pagers are not as widely used by consumers and are more utilized in certain industries and special use groups, the frequencies seem to be weighted in a couple of areas. 152 MHz to 158 MHz is a hotspot for many medical and hospital paging systems. 420 MHz through 540 MHz is a collage of corporate, industrial, and privately owned paging systems. And 920 MHz to 940 MHz seems to be the prevailing frequency for the remainder of consumer pagers. There is no doubt that someone who takes the time to carefully scan through all of the VHF and UHF frequencies would find additional spots where POCSAG or its predecessors are being transmitted.

A common trait amongst all of the pager protocols is their inherent lack of security. As with many communication protocols, those used for paging systems were not designed with security in mind; a topic that has been detailed before within the pages of 2600. POCSAG and FLEX broadcast data completely unencrypted and often over a significantly large geographical area. While this may be fine for simple communications of non-sensitive information, it is completely unacceptable for personally identifiable information such as names, Social Security numbers, date of births, addresses, or the specifics of medical treatments being given to a person. The telecommunication companies rely on the fact that transmitted pager data is obfuscated using FSK modulation as a means of security. They also hide behind laws such as Counterfeit Access Device Law, 18 USC 1029, that that make it illegal to use a radio scanner to knowingly or with intent, eavesdrop on a wire or electronic communication. And let's not forget the Electronic Communications Privacy Act, 18 USC 2510, that prohibits anyone from intercepting messages sent to display pagers both numeric and or alphanumeric. And, while these laws are in place, there is absolutely no technological means that is stopping a person from accidentally or intentionally intercepting these transmissions and using them for personal gain. Knowing that this threat exists, it would be deplorable for companies or any organization to send sensitive information across these systems, yet that is exactly what is happening!

The System Setup

Because such tasks would be illegal as defined above, I'll state what a person "could do" and the type of information they "could see," should they be so inclined to intercept POCSAG and FLEX transmissions with a radio scanner and a data slicer. This information is intended to be for educational purposes only and to provide awareness to the issues. The equipment needed for intercepting, collecting, and decoding pager transmissions involves three key components. These are: