Telos VX - Addressing

To Next Page

To Previous Page

NOTES, RESOURCES, ADDITIONAL INFORMATION | 71

server components listed therein – or they might be included, but not labeled according to the

standard names bulleted above. Instead these functions would just be provided as part of the

system “black box.” Cisco and Microsoft VoIP products fall into this category, for example.

Addressing

SIP addresses, also called SIP URIs (Uniform Resource Identiﬁers), are in the form sip:user@

host. The user portion of the address can be a text name or a telephone number, and the host

portion can be a domain name or an IP address. The address resolution process normally

begins with a URI and ends with a username at an IP address. Just as with email, the sender

needs no information about the physical location or IP address of the receiver. This is one of

the powerful features of SIP – it automatically implements portability and mobility.

Examples of valid SIP addresses:

The usual form is an email address preﬁxed by ‘sip:’:

sip:joesmith@company.com

You can call a PBX telephone at a business this way:

sip:123@telos-systems.com (extension phone 123 at Telos Systems)

If you don’t have a name or extension, you might want to contact the receptionist:

sip:receptionist@telos-systems.com

Here’s an internal machine-to-machine message, such as from an on-air phone system to a

PBX or gateway to initiate a PSTN call:

sip:12162417225@168.123.23.1

Note that in this case, an IP number is provided to identify the concrete machine that is to

receive the message. You usually don’t want to use DNS for this because it takes time for the

look-up step, and because there may well not be a DNS name associated with a machine being

used as a telephone server.

To assist readability, SIP lets you use ‘+’, ‘-‘, and ‘.’ separators. It removes them prior to pro-

cessing:

sip:+1-216-241-7225@telos-systems.com

As you can see, SIP bridges the telephone and Internet worlds. Both Web-type and PSTN

telephone number addresses are possible, and users on either network can reach those on the

other.

Often, address resolution involves multiple steps and SIP-message hops. A DNS (Domain

Name Service) server, a SIP proxy server and a SIP redirect server might all be involved in a

single name resolution, for example.

A few other points of interest regarding SIP: Some servers associated with SIP systems can

accept unformatted text names, but this is not part of the standard.

URIs are not URLs. URIs are independent of the location of the named object. Email addresses

are an example of URIs. In SIP, a Request-URI is deﬁned to indicate the name of the destination

for the SIP Request (INVITE, REGISTER, etc.). URLs (Universal Resource Locator) describes the

location of a resource available on the Internet. For example, http://www.telos-systems.com is

the URL for a Web home page. It is resolved by DNS to a concrete IP address.

PSTN telephone numbers are sometimes called ‘E.164’ numbers, a designation applied in an

ITU-T standard that describes the format of telephone numbers to be used worldwide. ENUM