Definition of pen register
A pen register is an electronic device that records all numbers called from a particular telephone line. The term has come to include any device or program that performs similar functions to an original pen register, including programs monitoring Internet communications.
The United States statutes governing pen registers are codified under 18 U.S.C., Chapter 206.
The term pen register originally referred to a device for recording telegraph signals on a strip of paper. Samuel F. B. Morse's 1840 telegraph patent described such a register as consisting of a lever holding an armature on one end, opposite an electromagnet, with a fountain pen, pencil or other marking instrument on the other end, and a clockwork mechanism to advance a paper recording tape under the marker.
The term telegraph register came to be a generic term for such a recording device in the later 19th century. See for example, Frank Wood's Telegraph Register. Where the record was made in ink with a pen, the term pen register emerged. By the end of the 19th century, pen registers were widely used to record pulsed electrical signals in many contexts. For example, one fire-alarm system used a "double pen-register", and another used a "single or multiple pen register".
As pulse dialing came into use for telephone exchanges, pen registers had obvious applications as diagnostic instruments for recording sequences of telephone dial pulses. With the passage of time, any instrument that could be used for this purpose came to be defined as a pen register. Title 18 of the United States Code defines a pen register as:
a device or process which records or decodes routing, addressing, or signalling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided, however, that such information shall not include the contents of any communication, but such term does not include any device or process used by a provider or customer of a wire or electronic communication service for billing, or recording as an incident to billing, for communications services provided by such provider or any device or process used by a provider or customer of a wire communication service for cost accounting or other like purposes in the ordinary course of its business.
This is the current definition of a Pen Register, as amended by passage of the 2001 USA PATRIOT Act. The original statutory definition of a pen register was created in 1984 as part of the Electronic Communications Privacy Act, which defined a "Pen Register" as:
A device which records or decodes electronic or other impulses which identify the numbers called or otherwise transmitted on the telephone line to which such device is dedicated.
A pen register is similar to a trap and trace device. A trap and trace device would show what numbers had called a specific telephone, i.e. all incoming phone numbers. A pen register rather would show what numbers a phone had called, i.e. all outgoing phone numbers. The two terms are often used in concert, especially in the context of Internet communications. They are often jointly referred to as "Pen Register or Trap and Trace devices," to reflect the fact that the same program will probably do both functions in the modern era, and the distinction is not that important. The term 'pen register' is often used to describe both pen registers and trap and trace devices.
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