ZyXEL Elite 2864I User Guide - Page 31

BRI BRI, Basic Rate Interface, is the ISDN service provided to a normal home or office. It consists of 2B+D data channels. A D channel offers a speed of 16Kbps for signaling information transmission. Each B channel is for user data transmission at 64Kbps. Each B channel can be used independently for voice or data transmission. A user can place two independent calls through a BRI interface simultaneously. U- or S-Interface ISDN signal transmission is digital from end to end. At every interface point there is a specified signal format. Equipment at every interface point needs to have signal transmission/reception conform to the specified signal format. At the customer site, the interface point for terminating the ISDN line connection from the CO (Central Office) is called the U-interface. A network terminator NT1 terminates the ISDN U-interface and converts it to the S-interface for internal ISDN equipment connection. Non-ISDN capable equipment can use an ISDN TA to connect to the S-interface. In North America, the NT1 U-interface device is the user's responsibility. The user can either install an NT1 to get the S-interface for internal ISDN equipment connections, or have an ISDN TA to directly adapt to the U-interface. In Europe, the telephone company installs the NT1 U-interface device at the customer site. The user only needs to have the ISDN TA to adapt to the S-interface. AT Command Set The AT command set is the industry de facto standard used to control a modem in command state. It got its name from the fact that every command line has a prefix AT. AT commands were first used in the Hayes Smartmodem® and hence adopted by other modem manufacturers and communication software writers. The original set of AT commands such as dialing, answering, etc., is used by everyone and is standardized. The AT command set is extended by each modem manufacturer for control of ever-increasing modem functions and capabilities. This extended or enhanced command set is not standardized whatsoever. The AT command set is used only with an asynchronous serial data interface. To send an AT command to a modem the connected computer must be running a terminal software. ZyXEL has defined a set of extended AT commands to support many of its unique functions and features. 2-5