Tone Testing

The AMPS system uses two types of tones in its signaling sequence. These tones serve multiple purposes, and consistent testing of the applications involving these tones in both the handset and the base station ensures proper interaction during call setups and handoffs in the field.SATS A supervis ory audio tone (SAT) is one of three frequencies (0: 5,970Hz, 1: 6, 000Hz, 2:6,030Hz) used throughout a conversation on the forward and reverse voice channels as a means of identifying that the base station is talking to the intended phone and as a means of signaling between the phone and the base station. The base station assigns one of these SAT frequencies to each of its voice radios. When a user accepts or originates a call, the base station tells the mobile which forward voice channel (FVC) to use as well as which SAT to expect. This is known as the SAT color code in the mobile station control message. The base station then begins transmitting the specific SAT on that FVC. The handset moves to that channel, listening for the correct SAT. If all goes well, the handset will detect, filter and modulate with the same tone on the reverse voice channel (RVC). This completes the loop, and the network is assured that the user is on the correct radio. Not only does the ha! ndset have to move to the correct frequency channel, but it also must find and retransmit the correct SAT.

SATs also are used after a call is established as a means of signaling the occurrence of events such as confirming orders, flash requests and ensuring the integrity of the link between the base station and the handset. This is done in cooperation with the signaling tone. The SAT also is important during handoffs, when the handset and base station can use the closing of the SAT loop to ensure that the handoff took place correctly.

In frequency-domain, multiple-access systems such as AMPS, users are separated by frequencies. A frequency reuse system usually requires significant planning to optimize increased spectrum efficiency. Even with an optimally designed reuse pattern, some co-channel interference probably will occur. This then plays into the first reason for the SAT, which is to ensure that the user is using the right frequency on the right base station. A phone that is on the same frequency but transmitting on a different base station would have a different SAT frequency assigned, and the base station then would be able to reject the signal from the interfering call. The SAT frequency of the base station must be within +/-1Hz for the mobile. Because the SAT must be detected and modulated within fairly exacting specifications, this process ensures that both the base station's voice radio and the handset are operating correctly and will be able to provide high-quality service to the customer durin! g the call.

In the event that an SAT is not detected or the SAT frequency does not match what is expected, the "fade timing status" is enabled. This clock counts five seconds. If no SAT has been detected, it turns off the transmitter, assuming that the call was lost to a fade.

THE ST The signaling tone (ST) is the second type of tone used in AMPS and is generated by the handset. It is a 10kHz tone with a +/-8kHz deviation of the carrier. The ST is used with the SAT throughout the phone call. The on or off states of the SAT and ST, as well as the lengths of the ST, indicate particular occurrences. For instance, with the SAT and ST on for 1.8 seconds, a call release is indicated to the network.

The mobile also can confirm various orders with the SAT/ST using the same method. For instance, the handset can confirm an order to perform a handoff by sending the ST and the SAT for 50ms. A variety of other sequences indicates particular alerting, acknowledgment and conversation states using the SAT and the ST. The most common is the hook-flash, which is the SAT and ST sent for 400ms, followed by a transition to SAT only.

TESTING Testing the operation of these tones is essential to ensuring proper operation of the network and applies equally to base-station and mobile-station testing.

Handset tone testing requires a specialized handset test instrument capable of performing call-processing tasks. The test instrument initiates various actions that simulate a base station. Then the ST and SAT can be monitored. For instance, a call is originated by the handset. As the handset moves to the FVC, the test instrument can generate a known good SAT, which the handset should detect, filter and modulate back on the RVC. This SAT then can be tested for the correct frequency and deviation. Generally, the test should be performed three times to ensure the handset can identify and retransmit all of the SAT frequencies correctly.

Testing the ST requires the test instrument to command the mobile to perform certain tasks or to instruct the technician to initiate handset action. For instance, during an autotest, the instrument might prompt a technician to press the SEND key for a hookflash or press the END key to perform a mobile release. The instrument then can measure the ST to ensure it is at the correct frequency (10kHz), with the correct deviation (+/-8kHz), transmitted for the correct amount of time (in the case of a hookflash, 400ms; in the case of a mobile release, 1,800ms).

Testing the operation of base-station tones also is important. If an SAT is not being modulated correctly on a radio, that channel will drop any attempted call setup. The actual testing on the base station is not that different from the mobile. Most base stations can be configured to generate a specific SAT on a particular voice channel. A typical service monitor then can be used to measure the deviation of the carrier (which should be +/-2kHz) as well as the frequency of the tone. This test is performed on each channel.

After power and frequency, SAT and ST tests are among the most important. Proper testing of these parameters with the necessary equipment needs to be an essential part of the carrier's overall quality plan.