ves. Those who use ISDN-based systems may want the option to select transmission rates (often running between 128 and 384 Kbps) depending upon the conference. The ability to deliver this flexibility will come from the codec itself (through HO bonding) or from an external device (an inverse multiplexer). Some high-end group systems include a built-in inverse multiplexer, used to bond multiple B channels (or switched 56 circuits) together into higher bandwidth aggregates. Low-end roll-abouts do not bond beyond two channels. If speeds above 128 Kbps are required, bonding must be performed by a separate I-MUX. All else being equal, group systems that are data-rate agile (that function as a DTE device and can thereby follow a network clock) are preferable to those that are not. The ability to clock off the network allows the codec to detect changes and adjust its data rate accordingly. During procurement, a customer should ask about the network (physical layer) interfaces that are offered. These might include Bell System interfaces (T-1) (e.g., RS- 449, RS-422, RS-423, RS-366-A, RS-530) and ITU-T (V.35, X.21 and ISDN). Typically ITU-T V.35 connections are used in the United States for circuit-switched digital connections (excluding ISDN). The V.35 specification defines a high-speed serial communications interface (an electrical connection) between a network access device (e.g., a multiplexer or a codec) and network switching equipment. V.35 makes its Group Systems connection using a large rectangular connector with large hold-down screws. Although the ITU-T has formally stated that the V.35 specification is out of date, it is still widely used and provides a solid connection that is almost impossible to dislodge accidentally. X.21 connections are used for circuit-switched digital connections in Europe and the rest of the world. Like V.35 X.21 is a digital signaling interface, defined by the CCITT/ITU-T, and used to connect DCE to DTE. X.21 describes the names and functions of eight wires. The physical connector has 15