Many factors, associated with both the TDMA and the VSATPlus II, make the PolarSat product an excellent choice for telephony trunking applications. In addition to the inherent technological advantages of TDMA, such as full-mesh connectivity, hubless operation and automatic uplink power control, a key VSATPlus II feature is the E1 card. The E1 card provides a direct digital connection to private automated branch exchanges (PABXs) and switches. A variety of signaling protocols are presently supported and new protocols can be easily implemented because of the card's flexible software architecture. The following sections describe how these features work with a variety of signaling protocols.
When there is intense traffic concentration at the terrestrial interface point and when bandwidth requirements are fairly constant, preassigned circuits are often used for high-volume trunks between major switching centers. However, many times the number of interswitch trunks needed to support this traffic is less than a full El span (30 circuits). For such cases, the VSATPlus II's E1 card offers split channel mode and fractional El operation, which saves in hardware costs and conserves satellite bandwidth.
The El interface card splits the El data stream into 30 independent channels. Any one, several, or all of these channels may be transmitted to any other node, in any order, where they are reframed and transmitted to the user. Because the channels are independent from each other, only the active channels of the El data stream are transmitted over the satellite by the VSATPlus II terminal.
VSATPlus II supports demand assigned (switched) service with standard line and register signaling protocols for voice service. It also operates as local exchange or toll-class switch (for example, class 4/5 switch) and allows private automated branch exchanges (PABXs) to connect without modification. DAMA operations reduce the number of required terrestrial circuits and total satellite channels. DAMA operation is decentralized. The failure of any station will not affect the operation of the rest of the network.
The E1 card can operate in transparent mode. In a 32-channel transparent mode of operation, the E1 card transparently passes 2.048 Mbps data. The card retains the terrestrial interfacing characteristics of the E1 card, but it does not require multiframe alignment (MFA) signals and does not support split-channel operations.
When the VSATPlus II network is configured with the StreamView NMS, including the optional DAMA service management feature, the StreamView can provide DAMA network traffic information as one of the variety of advanced management features.
The E1 card supports a variety of voice compression algorithms. Clear channel 64 kbps PCM is supported as well as 16, 24, and 32 kbps adaptive differential pulse code modulation (ADPCM) algorithms. Furthermore, emerging ITU standards for lower rate encoding schemes, such as G.729 (8 kbps CELP) and G.728 (16 kbps LD-CELP), are being developed for the E1 card as well as the associated subscriber analog voice card. Added features of the voice compression algorithms are fax and in-band data bypass support. Group III fax signals are supported up to 14.4 kbps. This means that in a 16 kbps compressed voice channel, 14.4 kbps fax transmissions can be sent. Without such a feature, fax signals would be severely distorted by the compression algorithm.
A major challenge of satellite-based telephony networks is the need to minimize the call setup delay. PolarSat has elegantly overcome this problem through protocol spoofing. The overall effect reduces the call setup delay caused by the satellite link to approximately three seconds!
The line and register signaling protocol is optimized for satellite and TDMA operation. It creates the engine that implements universal protocol conversion, reformatting the line and register signals into a generic form. Using protocol conversion, VSATPlus II technology allows different switches to operate within the same network.
Voice Card Interoperability
The voice card is interoperable with the E1 card. The voice card provides users with four ports of analog voice. The interface may be a 2- or 4-wire E&M (Type I through V) trunk circuit or a 2-wire subscriber circuit (FXO or FXS).