ion with possible fragmentation/ After down-conversion, filtering, and equalization, the defragmentation structures. From this stage, the output is symbols are converted to bits by a symbol detector/de- bits, which are input to the network-processing layer. The modulator combination, which may include a matched network layer is designed for end-to-end connectivity filter or some other detection mechanism as well as support. The output of the network layer is passed to the a structure for mapping symbols to bits. A symbol is se- application layer, which performs some sort of user lected that most closely matches the received signal. At functions and interface (speaker/microphone, graphical this stage, timing recovery is also necessary, but for user interface, keypad, or some other sort of human- symbols rather than the carrier. Then the output from computer interface). the demodulator is in bits. The bits that are represented by that symbol are then passed to the forward error correcting function to correct User application occasional bit errors. Finally, the received and error- The user's application may range from voice telephony, corrected bits are parsed into the various fields of mes- to data networking, to text messaging, to graphic display, sage, header, address, traffic, etc. The message fields are to live video. Each application has its own unique set of then examined by the protocol layers eventually de- requirements, which, in turn, translate into different livering messages to an application (e.g., Web browser or implications on the performance requirements of the voice coder (VoCoder)), thus delivering the function expected by the user. For voice telephony today, the dominant mode is to SDRs must provide a wide variety of computational code the voice to a moderate data rate. Data rates from resources to be able to gracefully anticipate the wide 4800 bps to 13,000 bps are popular in that they provide The software defined radio as a platform for cognitive radio CHAPTER 2.4a excellent voice quality and l