System information

really fundamental differences in order to throw light on the next explanation, which
covers the settings we might need to change on devices external to Asterisk, such as IP
phones.
Have you ever considered the fact that an analog phone is a totally dumb device (we
know that a basic model is very, very cheap) that needs to connect to an intelligent
network (the PSTN), whereas an IP phone (e.g., SIP or IAX2) is a very intelligent device
that connects to a dumb network (the Internet, or any regular IP network)? Figures
9-1 and 9-2 illustrate the difference.
Figure 9-1. The old days: dumb devices connect to a smart network
Figure 9-2. The situation today: smart devices connect through a dumb network
Could we take two analog phones, connect them directly to each other and have the
functionality we would normally associate with a regular phone? No, of course not,
because the network supplies everything: the actual power to the phone, the dialtone
(from the local exchange or CO), the caller ID information, the ringing tone (from the
remote [closest to the destination phone] exchange or CO), all the signaling required,
and so on.
Conversely, could we take two IP phones, connect them directly to each other, and get
some sensible functionality? Sure we could, because all the intelligence is inside the IP
phones themselves—they provide the tones we hear (dialtone, ringing, busy) and run
the protocol that does all the required signaling (usually SIP). In fact, you can try this
for yourself—most mid-price IP phones have a built-in Ethernet switch, so you can
actually connect the two IP phones directly to each other with a regular (straight-
through) Ethernet patch cable, or just connect them through a regular switch. They
will need to have fixed IP addresses in the absence of a DHCP server, and you can
Devices External to the Asterisk Server | 183