# CAS API protocols

To communicate across a trunk line, parties must signal one another. The scheme used to signal across a telephone line is called a protocol. Many different protocol standards are in use throughout the world.

The following table describes the CAS API protocols:

Protocol | Description
Analog loop start (LPS) | Includes protocols that use loop start signaling, where the presence or absence of current flow is interpreted by a switch as protocol signaling events.
Australian P2 (AP2) | Used in Australia for connections between the PBX and some PSTN carriers. It uses two-bit line signaling (derived from CCITT recommendation Q.421), and in-band DTMF register signaling.
EL7 | Implements the application computer variant of the EL7 protocol found on an Ericsson MD110 PBX.
European digital channel associated signaling (EUC) | Includes channel associated signaling protocols used in certain European countries. The protocols use two-bit line signaling specified by national standards for use over E1 trunks. The register signaling is either carried by in-band DTMF tones (not compelled) or by out-of-band decadic pulses.
Feature group D (FGD) | Implements the specifications of the feature group D (FGD) switched access service. This service provides interconnection to the BOC (Bell Operating Companies) network for the provision of message telecommunications service/wide area telecommunications service (MTS/WATS) and MTS/WATS-type services. FGD service, which provides access to the trunk side of suitably equipped BOC switching systems, is available for termination and originating access.
Ground start (GDS) | Cover digital interfaces (T1 trunks) connected to a private branch exchange (PBX) or PSTN switches. The protocol can handle two signaling variations: FX (foreign exchange) and SA (special access).
Mercury exchange line (MELCAS) | Implements the Mercury exchange line CAS protocol.
MF-Socotel (MFS) | C onforms to the Spanish National Specifications for channel associated signaling over E1 trunks. It uses single bit steady-state line signaling and MF-Socotel MF compelled in-band register signaling.
Multi-frequency compelled protocols based on the R2 standard (MFC) | Includes the CCITT Signaling System R2 (Recommendations Q.421 to Q.442, CCITT Blue Book, 1988) implementation and numerous national variations. These protocols run on E1 trunks and use two-bit line signaling on the signaling channel associated with each voice channel and in-band MF compelled register signaling.
NEC PBX (NEC) | A digital line interface protocol that implements the specifications of the 30 DLI using the PA-30 DTS package (Annex 303-15-B 2/2 from NEC, NEC Australia PTY, LTD).
Off-premises station (OPS) | Covers only digital interfaces. The protocol can handle T1 or E1 digital trunks, of signaling types FX (foreign exchange) or SA (special access).
Operator workstation (STA) | Other side of the analog loop start protocol used to implement stations that connect to analog phones.
Pulsed E and M (EAM) | Includes country-specific protocols that use one-bit line signaling in a pulsed form, and variations on the compelled in-band MF register signaling specified by the CCITT in the 1988 Blue Book. These protocols run on E1 trunks and are specified in different countries by national regulatory agencies.
Signaling system 5 (SS5) | Specified by the CCITT Recommendations Q.140 to Q.154 (CCITT Red Book, Volume VI Fascicle VI.2, Geneva 1985). The protocol uses in-band compelled signal frequency tones to perform line signaling and in-band MF tones for register signaling. Since no signaling bits are used, this protocol works the same way on T1 and E1 trunks.
System R1.5 (R15) | Includes channel associated signaling protocols used for E1 lines in Russia (based on CCITT recommendations Q.511 and Q.544). The protocols use two-bit steady-state line signaling. Register signaling is either carried by in-band MF tones (MF acknowledged pulses) or by out-of-band decadic pulses. Two R15 protocol software modules are included with the CAS API. One is used (r150) for controlling inbound calls and the other (r151) is used for controlling outbound calls.
Digital and analog wink start (WNK) | Includes protocols used on T1 in the USA, Hong Kong, and Taiwan. The protocol uses one-bit signaling, two-bit signaling, or presence or absence of current, and owes its name to the wink (brief presence of current or variation of the signaling bit) that the inbound side uses to acknowledge line seizure. Register signaling is performed by in-band DTMF or MF tones, or by out-of-band decadic pulses.

Most of the protocols in these families have country-specific variations. Dialogic provides parameter files that determine how protocols interact with telephone networks in different countries. The package for each country contains software modules you need to download to enable telephony boards to communicate with telephone networks in that country.