t model) have the po- retain or discard a generated waveform. tential to increase capabilities in a variety of ways. For example, a lone CR is limited in its ability to access spectrum, but a pair of CRs can sense the spectrum, Software communications architecture identify an unused band, rendezvous there, and com- The primary motivations for SDR technology are lower municate. A network of CRs enables other significant life cycle costs and increased interoperability. The increases in capabilities. Software for Mobile Ad hoc basic hardware for SDR is more expensive than for Networking (MANET), although maturing slowly, is single-point radio system, but a single piece of hardware a key enabling technology. performs as many radios. The single piece of hardware The medium access control (MAC) layer is critical in requires only one logistics tail for service, training, CR networks. If the CR is employing advanced spectrum replacement parts, and SO on. One of the driving costs in access techniques, a robust MAC that mitigates the SDR development is that of software development. The hidden node problem is needed. In a "static spectrum JTRS acquisitions are controlling these costs by ensuring access" environment, a more traditional MAC is possible. software reuse. The approach for reuse is based on the A carrier sense collision detection (802.11 MAC) mode Software Communications Architecture (SCA), which is not possible because a radio cannot receive in the exact is a set of standards that describes the software environ- same band in which it is transmitting, SO a carrier sense ment. It is currently in release 2.2.1. Software written to collision avoidance approach is frequently used. Request- be SCA compliant is more easily ported from one JTRS to-send (RTS) and clear-to-send (CTS) messaging are radio to another. The waveforms are maintained in a JTRS popular wireless MACs amenable to MANETs. Other Joint Program Office (JPO) library. approaches include TDMA or CDMA MACs. A CR can be implemented under the SCA standards. 