ow distortion to the un- Web browsing places a different type of restriction on trained listener. The digital modem, in turn, is generally an SDR. The typical browser needs to be able to store the more robust to degraded link conditions than analog images associated with each Web page in order to make the voice would be under identical link conditions. browsing process more efficient, by eliminating the re- Another criterion for voice communications is low dundant transmission of pages recently seen. This implies latency. Much real experience with voice communica- some large data cache function, normally implemented by tions makes it clear that if the one-way delay for each link a local hard drive disk. Recently, such memories are exceeds 50 milliseconds, then users have difficulty in implemented by high-speed flash memory as a substitute that they expect a response from the far speaker and, for rotating electromechanical components. hearing none, they begin to talk just as the response ar- rives, creating frequent speech collisions. In radio net- works involving ad hoc networking, due to the delay Design choices introduced by each node as it receives and retransmits Several aspects of the receiver shown in Figure 2.4a-1 the voice signaling, it can be quite difficult to achieve are of interest. One of the salient features is that the uniformly low delay. Since the ad hoc network in- effect of all processing between the LNA and the FEC troduces jitter in packet delivery, the receiver must add stage can be largely modeled linearly. This means that the a jitter buffer to accommodate a practical upper bound in signal processing chain does not have to be implemented latency of late packets. All of this conspires to add con- in the way shown in Figure 2.4a-1(a) The carrier recover siderable voice latency. In response, voice networks have loop does not have to be implemented at the mixer stage. established packet protocols that allocate traffic time It can just as easily be implemented immediately before slots to the vo