Routing methods

Until the 1980s, telephony service providers were difficult to tell apart. After all, telephony traffic was all carried the same way, using copper cables, so there was no real need for any niche operators. Therefore, the telephony world was populated with a number of monoliths, such as AT&T in the USA, BT in the UK, and many others around the globe. In many countries, these providers were nationalized and thus were pure monopolies. In countries where this was not the case, competition was not strong and markets tended to be carved up so that monopolies or cosy cartels were the order of the day. In the US, this was evidenced by the antitrust case "United States versus AT&T", which in 1982 led to the breakup of AT&T into a smaller core company and seven regional Bell companies.

Mobile telephony, introduced in the 1980s but not widespread until the following decade, shook things up slightly. However, the massive investment required to provide an effective service worked against independent start-ups gaining significant market share, and the net result was that the same monoliths simply diversified into another route to market.

IP telephony is now changing the market significantly. Suddenly it is possible to make a phone call without touching the traditional telecoms network, although it is true to say that the old telephone companies are still at the forefront of Internet circuit provision services. However, newcomers do not require quite the same level of investment to gain a foothold, so the customer is offered more choice and market forces drive prices down and the standard of services up. There is still some way to go, particularly as the majority of calls are required to break out of the Internet and on to the PSTN or mobile infrastructure at some point. It should be pointed out, though, that this is not necessarily a bad thing. The PSTN, in particular, is still a low-cost, high-availability network.

Out of the box, so to speak, Asterisk is designed to work with IP-telephony protocols such as SIP and IAX2. The only routing requirement once it is installed is to have a connection to an IP network, often the Internet but not necessarily so. It is perfectly feasible to have your Asterisk box hooked up to a private WAN with no external routes. Your call destination options would be limited to the extent of the network, but you'd have great functionality and call quality!

In the real world, though, people like to make phone calls outside their LAN/WAN. One option is to set up an account with an Internet Telephony Service Provider (ITSP), pass all outbound calls through that trunk, and let the ITSP worry about how to route your calls. This has advantages in that it is a very simple setup (in all systems simple is a good thing) and it is usually the cheapest means of making international or long-distance calls. The ITSP will route the call over the Internet to a point near the call destination, where it will break-out onto the PSTN or mobile infrastructure. In this chapter, we want to look at adding a bit more complexity, controlled using some of the techniques discussed in Chapter 1, so that we increase our routing options with a view to reducing call costs, increasing resilience and ensuring quality. This means adding the ability to route calls from our Asterisk system through to different service providers, typically PSTN and mobile networks.