Page 212 - Asterisk™: The Future of Telephony

Primary Rate ISDN is very popular, due to its technical benefits and generally compet-
itive pricing at higher densities. If you believe you will require more than a dozen or so
PSTN lines, you should look into Primary Rate ISDN pricing.
From a technical perspective, ISDN-PRI is always preferable to CAS.

Signaling System 7
Signaling System 7 (SS7) is the signaling system used by carriers. It is conceptually
similar to ISDN, and it is instrumental in providing a mechanism for the carriers to
transmit the additional information ISDN endpoints typically need to pass. However,
the technology of SS7 is different from that of ISDN; one big difference is that SS7 runs
on a completely separate network from the actual trunks that carry the calls.

SS7 support in Asterisk is on the horizon, as there is much interest in making Asterisk
compatible with the carrier networks. An open source version of SS7 (http://
www.openss7.org) exists, but work is still needed for full SS7 compliance, and as of this
writing it is not known whether this version will be integrated with Asterisk. Another
promising source of SS7 support comes from Sangoma Technologies, which offers SS7
functionality in many of its products.
It should be noted that adding support for SS7 in Asterisk is not going to be as simple
as writing a proper driver. Connecting equipment to an SS7 network will not be possible
without that equipment having passed extremely rigorous certification processes. Even
then, it seems doubtful that any traditional carrier is going to be in a hurry to allow
such a thing to happen, mostly for strategic and political reasons.

Packet-Switched Networks

In the mid-1990s, network performance improved to the point where it became possible
to send a stream of media information in real time across a network connection. Because
the media stream is chopped up into segments, which are then wrapped in an address-
ing envelope, such connections are referred to as packet-based. The challenge, of course,
is to send a flood of these packets between two endpoints, ensuring that the packets
arrive in the same order in which they were sent, in less than 150 milliseconds, with
none lost. This is the essence of Voice over IP.
Conclusion

This chapter has explored the technologies currently in use in the PSTN. In the next
chapter, we will discuss protocols for VoIP: the carrying of telephone connections
across IP-based networks. These protocols define different mechanisms for carrying
telephone conversations, but their significance is far greater than just that. Bringing the
telephone network into the data network will finally erase the line between telephones
and computers, which holds the promise of a revolutionary evolution in the way we
communicate.

184 | Chapter 7: Understanding Telephony