nonvoice services may be assigned a number of time slots appropriate to the service. For example, as more efficient speech CODECs are perfected, increased capacity may be achieved by the assignment of a reduced number of time slots for voice traffic. TDMA also facilitates the introduction of digital data and signaling services as well as the possible later introduction of such further capacity improvements as digital speech interpolation (DSI). 18.6.4 Code Division Multiple Access (CDMA) CDMA means code division multiple access, which is a form of spread spectrum using direct sequence spreading. There is a second class of spread spectrum called frequency hop, which is used in the GSM system, but is not an access technique. Using spread spectrum techniques accomplishes just the opposite of what we were trying to accomplish in Section 9.2.3.5. There bit packing was used to conserve bandwidth by packing as many bits as possible in 1 Hz of bandwidth. With spread spectrum we do the reverse by spreading the information signal over a very wide bandwidth. Delay spread is a variance in delay due to dispersion; emitted signals on delayed paths due to reflection, diffraction/refraction. Lee reports a typical urban delay spread of about 3 usec. 18.6 NETWORK ACCESS TECHNIQUES Conventional AM requires about twice the bandwidth of the audio information signal with its two sidebands of information (i.e., approximately +4 kHz). 8 On the other hand, depending on its modulation index, frequency modulation could be considered a type of spread spectrum in that it produces a much wider bandwidth than its transmitted informa- tion requires. As with all other spread spectrum systems, a signal-to-noise advantage is gained with FM, depending on its modulation index. For example, with AMPS, a typical FM system, 30 kHz is required to transmit the nominal 4-kHz voice channel. If we are spreading a voice channel over a very wide frequency band, it would seem that we are defeating the purpose of frequency conservation. With spread spectrum, wi