You don't need to know most of what's in this chapter to use IP networks for audio. Just as a rock-and-roll roadie can plug XLRs together without knowing anything about op-amps and  printed circuit board (PCB) ground planes, you can connect and use IP audio gear without knowing much about packets and queues.
But you are reading this book, so either you are the curious sort or you have a need to know what is going on behind the RJ-45. You will be rewarded. You know that fixing tricky problems in the analog world calls for an understanding of the underlying technology; and just the same, an awareness of how data networks function will help you in the AoIP world.
Building large and complex systems will require you to have knowledge of how the components interact. All of IT, telephony, and media are going to eventually be built on IP, so now is a great time to get comfortable and proficient with it. Anyway, discovery is fun, right?
We're not going to go so deep that you are overwhelmed with unnecessary details. But there will be enough for you to get a feel for how networks operate, so that you will have a foundation for parsing the lingo you hear from IT folks, and you'll be ready to design AoIP systems, as well as configure them after they are installed. While the topic at hand is general IP/Ethernet data networking, we'll explain a lot of AoIP-specific points along the way that would not be covered in a general networking text.
AoIP systems are built upon standard components, so if you understand data networking generally, you'll be ready for AoIP and the specifics of Livewire. Network engineering is a rich topic, abounding with information and nuance, and it's in constant flux. Fortunately, AoIP uses only a small subset that is easy to learn and use. That is mainly because most of the complexity comes with the IP routing that is the foundation of the Internet. We use only a small piece of that within the local networks that host most AoIP.
Should you want to embark on a journey leading to the top of the IP mountain, bookstores have shelves heaving with networking advice and information. We think that this is one of the important advantages of IP for audio. Because of today's data network ubiquity, there are oceans of books and plenty of other learning resources on IP networking in general (although not much that we've found for AoIP specifically, which is why we are moved to write this book).
See the References and Resources chapter for some starting points on these deeper networking references. You can probably even find some educational programs in your hometown. If you become really serious, you might explore courses that lead to certified credentials. Cisco has defined a broad range of these, for which testing is conducted by a third party. The Society of Broadcast Engineers (SBE) also offers a certificate in data networking for broadcasting.
2.1 TDM VERSUS IP
Time-division multiplexing (TDM) is a term invented by telephone engineers to describe a system design where a common resource—cable, backplane, radio-frequency spectrum—is divided into channels that are separated by time. This is in contrast to space-division multiplexing (SDM), which dedicates individual circuits, and frequency-division multiplexing (FDM), which separates channels by modulating them into different frequency bands. An example of SDM would be POTS (plainold telephone service) telephone lines, and an example of FDM would be the analog microwave radios that carried telephone calls in the 1950s and 1960s. Radio broadcasting is, of course, another example of FDM.
TDM was a natural companion to first-generation digitization. Once audio is made into bits, it is quite easy to offset these into timeslots. Simple logic functions comprised of counters and muxes are up to the task. In the early 1960s, engineers had to use the building blocks at their disposal.
The first application of TDM was the T1 line, which was used as a "pair gain" scheme to obtain more channels from existing copper pairs. It was invented in 1962 and remains widely used to this day. It multiplexes 24 channels of 8-bit audio onto two copper pairs, taking advantage of the connection's ability to pass frequencies much higher than the usual 3.4-kHz speech audio.
Switching caught-up 20 years later with the introduction of the AT&T 5ESS central office switch in 1982. The line interface part of the 5ESS was comprised of many racks full of card cages holding "circuit packs" that adapted analog POTS lines to a digital backplane where the voice channels were divvied-up into timeslots.
Like all TDM equipment, the switching subsection read a full cycle of timeslots into a memory and then wrote them out in a different order. This pattern was followed by other vendors of central office (CO) switches, such as Northern Telecom, Ericsson, Alcatel, and Siemens. Smaller versions were made for PBX applications, as exemplified by the popular Nortel Meridian family.
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