Wednesday, July 6, 2011

How Caller ID Works

Back in May I wrote a couple of posts using a book I published about ten years ago titled Introduction to Telecommunications Networks. As a follow-up to those basic telephony posts, here's something on Caller ID.

Caller ID, also referred to as Caller Identification Technology was introduced in New Jersey by Bell Atlantic in 1987 and became widely available in the United States with the implementation of a switching technology called Common Channel Signaling System 7 (SS7). I'll write about SS7 in a later post, for let's just say SS7 is necessary to provide Custom Local Area Signaling Services (CLASS). CLASS services include Caller ID, Call Return, Repeat Dialing, Priority Ringing, Select Call Forwarding, Call Trace and Call Blocking.

To transmit Caller ID information the SS7 system sends the telephone numbers of the caller and the recipient in the form of a signal to a transfer point, before the call arrives at the receiving end. The caller ID information, also referred to as the Calling Party Number (CPN) field is placed .5 seconds after the first ring between the first and second rings of a telephone call by the SS7 system in the transmitting central office (CO).

Caller ID or Calling Party Number (CPN) field

As the call travels over the caller to receiver route it may pass through network elements which have not yet been upgraded to SS7. If this happens the CPN information will disappear and the receiver will not be able to identify who is calling.

CPN information is delivered in frequency modulated digital data format. Simply put it’s a series of bits, or 1’s and 0’s, that are transmitted sequentially. The bits are organized into groups of 8, with each 8 bit group referred to as a Byte. Each 8 bit combination, or Byte, represents a letter of the alphabet, number or special character. In the CPN field a 1 is represented by a 1200 Hz tone and a 0 is represented by a 2200 Hz tone.

Let’s look at the layout of the CPN field in more detail.

Calling Party Number (CPN) field Detail

The message info fields include the type of information is included in the CPN field and how long (how many Bytes) the CPN field is. The variable length number field can include name and/or address information. Caller ID requires the use of telephone capable of displaying caller ID information or a display box attached to a non caller ID display phone. The receiver must wait for the time between the first and second ring to see caller ID information. If the receiver picks up the phone before the CPN information is received, there will be no caller ID information. This easy accessibility to caller numbers and other information concerns many people and has created a privacy debate.

The Constitutionality of caller ID has been repeatedly challenged in court. People have three major concerns: the right to be left alone, the right to be free from unreasonable searches or seizures and the right to not be subjected to unreasonable government intrusions. Call blocking has originated as a solution to this privacy problem. Call Blocking allows the caller to prevent their number from being displayed on the receiver’s caller ID unit. Caller ID can be blocked two ways. Per-Line Blocking blocks the callers ID from all connected telephones. Per-Call Blocking requires the caller to dial *67 prior to dialing any call for which the caller wishes to be anonymous. When a CPN is blocked, the telephone company replaces the CPN information with “Private Name Private Number” or something similar.

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