Analog Legacy Protocol Enhancements

18th, Oct 2019

Welcome to another issue of GL Communications' Newsletter providing information and insight into our T1 E1 Protocol Analyzer software supporting legacy protocols (2-Wire call, 4-wire SS1/SS4, voice call signaling, single frequency signaling) analysis and emulation- applicable to air traffic management and communication.


Signaling that occurs between switches (such as private branch exchanges (PBX), local exchange carrier and interexchange carrier) in telephone networks is sent either within the voice traffic (in-band signaling) or on a separate dedicated signaling channel (out of band signaling). The Channel Associated Signaling (CAS) is an in band method to set up and tear down calls. This supervisory signaling is carried as "on-hook" and "off-hook". The addressing is carried as DTMF (Dual Tone Multi Frequency) or as MF (Multi Frequency). This form of signaling is still used to route calls on the public switched telephone network including subscriber lines like 2-wire local loops, on trunks between central offices, and on access trunks such as PBXs.

GL’s CAS and SS1 legacy protocol test solution supports simulation and analysis of signaling and voice communications interface between the National Airspace System (NAS) Voice System (NVS) and the legacy NAS voice switches, utilizing Ground to Ground (G/G) trunks that support analog interphone call processing.

GL's T1/E1 Cards and USB units provide T1 and VF I/O jacks through which one can monitor or inject audio signals from or onto a designated T1 timeslot, or VF line. The T1/E1 hardware test platform along with analyzer and emulator supporting legacy protocols can generate selective SS-1/ SS-4 signaling, voice call signaling, 2-wire call, single frequency signaling, MFC-R2 analog, E and M signaling types of signaling and voice being carried over communication channel and can analyze these signals in real time.

Legacy Protocols Simulation

SS1 (Selective Signaling 1) Protocol Analyzer

Selective Signaling "SS-1" is a 2 and/or 3-digit dial system is used by the Federal Aviation Administration (FAA) within their NAS ground-to-ground air traffic control network. It is used on leased and FAA lines between various control centers, Lockheed Martin Flight Service Hub's "FS-21", Military, National Weather Service and non-governmental aviation facilities for the control and management of aircraft flights.

GL’s T1 E1 test platform supports selective signaling in accordance with FAA-STD-054 for the analog G/G interface. SS1 Emulator (Dialer) application provides the ability to setup and dial tone sequences that make up SS1 dial digits. SS1 Analyzer detects and analyzes tone sequences that make up the SS1 dial digits. Sequences of pulse and guard tones are detected, decoded, and assembled into their corresponding dial digits. The tone sequences are also verified for compliance against a specification parameter which can correspond to published standards or user criteria.

SS1 Analyzer Detecting and Analyzing Dial Code

SS1 Dial Code in a Waveform Viewer  

MAPS™ CAS Single Frequency Signaling Emulator

In analog circuits, channel bandwidth is divided into a 300 – 3400 Hz “voiceband” used for speech and/or data, and a “supervisory band” centered at 2400/2600/2800 Hz used for the signaling tone. Because both the supervisory signal and line traffic are carried in the same channel, filters are normally required at each end of the connection to separate the line signal from the voiceband traffic.

MAPS™ CAS Single Frequency (SF) simulator can generate signaling tone on Idle and Active Modes with configurable operational frequency (2400/2600/2800 Hz) bands for analog G/G interface. It also has efficient filters to extract voice from line signals and filters to inform the state of the remote-end efficiently.
The ready scripts allow to control call terminate, answer, reject calls, transmit voice file, record the extracted voice, Playback/Listen to the recorded file, and call event tracking are few other related features supported during SF signaling on Idle and Active mode operation.

The following are the examples of SF Signaling in Active Mode and Idle Mode:

SF Call Simulation (Idle Mode

SF Call Simulation (Active Mode)

Voice Call Signaling Emulator

Voice call lines are typically used for four-wire. A Voice detect (VOX) circuit is used on the receive pair to detect incoming audio and signal the voice switch. No signaling is used on the transmit pair.

MAPS™ CAS supports voice call signaling by providing voice activated circuitry (VOX) to control voice connectivity and ensure immediate activation of the circuit upon receipt of incoming voice. During out-going calls, it seizes (opens) any circuit designated by the government as a voice call line and transmits the voice into the circuit, whenever the circuit is activated.

The following depicts an incoming voice call signaling script configured in MAPS™ CAS to detect an incoming voice call by the presence of voice signals on the line and ensures immediate activation of the circuit upon receipt of the incoming voice.

MAPS CAS Call Generation

2-Wire FXO FXS Call Simulation

FXS Port on tProbe™ mimics a two-wire FXS device, such as a telephone wall jack. This feature allows interfacing with an FXO device such as a telephone. Similarly, the FXO Port on tProbe™ unit can mimic a two-wire FXO device such as a telephone. This feature allows one to capture and analyze data from a two-wire telephone line, as well as to generate and transmit analog data onto that two-wire line connected to switch.

The scripts provide standard FXO FXS features such as monitoring hook status, sending ring signals, “ON hook” and “OFF hook” indications (loop closure) and transmitting DTMF digits (i.e. dialing the telephone number). tProbe™ FXO /FXS ports can also capture the incoming traffic into a file (i.e. A-law or Mu-law formats) and playback/listen to the file and transmit traffic such as tones, noise, voice files (i.e. A-law or Mu-law formats), and fax (at varying Min and Max Data rates).

The following depicts simulation of ‘RINGING 20Hz Frequency’ and ‘Loop Start’ call scenarios, when the FXO port of the tProbe™ is connected to the Central Office (Switch).

MAPS™ FXO Answer Incoming Call from Central Office

MAPS™ FXO Placing Call to Central Office

MAPS™ CAS Emulator
Channel Associated Signaling (CAS) Simulator Brochure
Channel Associated Signaling (CAS) Analyzer Brochure
SS1 Signaling Analysis and Emulation Brochure
MAPS™ Brochure
Analog Phone Simulator (MAPS™ APS) Brochure
MAPS™ APIs for Complete Automation Brochure
GL Product Lists

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