There are two basic approaches to building these gateways—complete proto-
col internetworking and protocol encapsulation. The latter approach is known
as SIP Telephony (SIP-T) [1], which is not a separate protocol, but rather the
SIP protocol plus a number of extensions. The gateway appears to the SIP net-
work to be a user agent for many different users, and to the PSTN as a termi-
nating telephone switch, known in North America as either Class 5 or Class 3,
depending on the design.Gateway Location and Routing
Since the gateway has multiple users, it does not REGISTERlike a normal user
agent. A normal registration binds a user’s URI with a number of URIs. A gate-
way instead serves a host of users, either a corporate entity served from a PBX
or Centrex group, a local Internet service provider (ISP) domain, or users asso-
ciated with a particular geographic region, usually identified by a PSTN num-
ber range: country code, Numbering Plan Area (NPA), or area code or
NPA-NXX (area code and local exchange). Instead of modifying SIP registra-
tion, the problem of gateway location and routing has been tackled in the IETF
IP Telephony Working Group (IPTEL WG) with the development of the Tele-
phony Routing over IP (TRIP) [2] protocol. This gateway to the location server
protocol, based on Border Gateway Protocol (BGP)—used to advertise IP
routes between networks—allows a gateway to advertise what PSTN number
range it supports. This information is then available to proxies in routing SIP
URIs containing telephone numbers and telephony URIs.
TRIP is designed for interdomain gateway location—it is not specifically
designed to be used within a domain. However, the need for this same service
within a domain has been identified by the IPTEL WG, which has begun work
on a new version called Telephony Gateway Registration Protocol (TGREP) [3].186 Chapter 11
SIP URIS AND TELEPHONE NUMBERS
As discussed in Chapter 6, “SIP Overview,” SIP URIs can contain telephone
numbers such as sip:[email protected];user=phone. This URI does
not need TRIP or any other protocol for routing, since the SIP request should be
routed to the carrier.comdomain, which then would locate a gateway.
However, when a user dials a telephone number on a SIP device, the resulting
dial stringwill first need to be interpreted based on a dial plan so that it can
be put in global or E.164 format. This could either be represented as a tel URI or
a SIP URI that has the domain of the user agent. In this case, routing informa-
tion is needed, either through gateway routing tables, or automatically deter-
mined using TRIP or TGREP to locate and select an appropriate gateway. In
addition, DNS ENUM queries, as described in Chapter 4, “DNS and ENUM,” may
be performed on this telephone number to resolve it to a URI instead of just
forwarding it to the PSTN.