Analog Voice and Digital Data
Most scientists agree that what makes human beings radically different from all other mammals is that we talk with each other. There has not been enough time for evolution to make human brains that much different from apes, and DNA research has reinforced this belief. Human speech, however, learned rapidly after birth, appears to drive many of the differences in brain organization that allow people to carry on a conversation with their parents but not with their dogs. This came as a surprise to...
Quantizing
In reality, the analog signals that produce 64-kb s analog voice are digitized twice. The PAM samples are first converted to a string of 0s and 1s through the quantizing process, and then they are converted once again into a form suited for long-distance transmission over a digital telecommunications link. It is also possible to quantize and then use a modem to send digitized voice over an analog link, but this possibility is not considered here. Not only is this rarely done today, but 64-kb s...
Digital Computer Networks
The growth of the analog voice network in the United States was rapid by the standards of the day. By 1941, even after the hard times of the global Depression in the 1930s, more than 40 percent of homes in the United States had telephones. Almost everyone who wanted a telephone could get a telephone, although even local service could be expensive in some cases. Perhaps surprisingly, use of the telephone for routine business purposes was slow to catch on. Businesses generally had an office...
Postdivestiture PSTN
Since 1984, the architecture of the PSTN is taken on a structure of LEC, LATA, POP, and IXC. Instead of a hierarchy of switches, SS7 has allowed for a system of IXC backbone switches linked in a flatter structure. There is often still a hierarchy of access switches and backbone switches, but nothing as elaborate as the class switching structure that prevailed before 1984. The architecture of the postdivestiture PSTN is shown in Figure 3-11. All of the blocks are voice switches, distinguished...
Network Requirements
While H.323 and SIP both provide common and as yet incompatible languages by which the end points in a VoIP conversation may signal each other, interoperate with gatekeepers, and code and decode audio signals, they are comparatively vague when it comes to the network facilities over which telephony services should run. For example, H.323 states that Novell's IPX protocol is as valid a network layer as IP for voice, and even the IETF mentions this as a possibility. These network layer protocols...
Session Initialization Protocol
The ITU-T is not the only standards organization to weigh in with a proposal for establishing IP telephony connections and packetizing audio for them. The Internet Engineering Task Force IETF , which more than any other body in the loose world of Internet standards manages or at least legitimizes the development of the Internet Protocol suite, has its own proposal for VoIP systems. This is called the Session Initialization Protocol SIP . Proponents of SIP claim that H.323, arising as it does...
Connectionless Networking
What other choices might there be for a philosophy around which to design a shared network Isn't a connection of some type, either physical or virtual, always necessary to get information to the correct place In fact, many data networks use no defined circuits to identify the end points of information transfer, leaving that job to a unique identifier or address that is assigned to each potential recipient. Such a network is commonly referred to as a connectionless network, for obvious reasons....