[A special welcome to those who
read "Why Fax over IP
Fails" at Webtorials.com.]
by William A. Flanagan, Publisher
The basic Session Initiation Protocol
(SIP) doesn't anticipate the presence of Network Address
Translation (NAT) between IP phones. Some day, when IPv6
completely displaces v4, that may work. Today almost all
sites are behind NAT as part of firewall protection at each
site--which creates problems for inbound connection requests.
To "punch a hole in the firewall"
requires other protocols and a server or two on the public
Internet. We'll get to STUN, TURN, and ICE in future issues (my
and Unified Communications has a complete
description). ICE (Interactive Connectivity Establishment)
may use both STUN and TURN servers on the Internet to provide the
information needed by endpoints to set up connections through NAT
devices. Done well, they deal with the issues of VoIP
configuration raised in earlier issues (VL-84,
project intends to overcome the connectivity problems with:
Google put the source code for webRTC out for use with very liberal
free licensing, including certain Google patents required by the
capable of real-time communications. There are several use
Isn't that third item what the phone companies do for voice?
Almost. Telephones have a globally unique address space,
defined by Recommendation E.164 of the International
Telecommunications Union (ITU). The telephone number includes
country and area codes. Every phone installed on the PSTN
automatically has an address known to the network, the E.164
address. Control of how the number are allocated--only by
local exchange carriers--means the PSTN can find any called
number. The average web server lacks complete knowledge of all
computers on the Internet, but it can ask for help.
A user clicks on a button in a web
page to set up multimedia communications with the host server.
This is similar to click-to-talk or -chat or -video conference
with a sales agent.
A special-interest web site (cars,
construction, golf, etc.) helps its members contact each
Any visitor to the web site can
request a connection to any browser on the Web.
The Domain Name Service (DNS) traditionally maps URLs and email
addresses to IP addresses. End users must ensure their domain
names are entered in DNS, usually by their ISPs. Users of VoIP
can register a voice instrument (for example a SIP phone) in an
additional DNS record--but it's not inherent or automatic as for
To convert an E.164 phone number to an IP address requires two
It's possible, then, for such a web server to connect any two hosts.
ENUM (Electronic NUmber Mapping),
an extension of DNS, converts the E.164 address to a URL.
Looking up that URL in a
specialized DNS server finds the IP; for a PSTN phone
the IP address would be on a gateway to perform media and
Five years ago a program called Rich
Communication Suite started to define multimedia communications as
a way to add features and value to cellular voice calls. The
3rd Generation Partnership Project (3GPP), a group of telecom
associations, adopted the work and continues it, now marketed as
Joyn. GPP published a series of Joyn specifications; ver. 5.1 is
Each carrier that deploys Joyn operates an Internet-protocol
Multimedia Server (IMS). It controls the signaling that sets up each
medium: voice, video, file transfer, presence, etc. Other servers
can deliver information streams or they can come from subscriber
equipment. Extending all services over IP anticipates the all-IP
transport of 4G/LTE networks.
Joyn operates within (and among) cellular carriers. Joyn, developed
for cellular phones, deals with known E.164 addresses. Each
carrier's home registry tracks the location of every mobile device.
The seems to be no need for NAT.
Under RCS it is the IMS server that coordinates signaling and call
setup--the same functions performed by the web server in webRTC and
by a softswitch or IP PBX.
Both Joyn and webRTC suffer when compared to the PSTN because of
directory services. IP-based devices may register with a local
server, a web site, or an IP PBX--but there is no universal VoIP
directory until ENUM is completed. All E.164 telephone numbers are
known to the network--that is, every phone is registered by default
in a global arrangement. A SIP address (SIP:UserName@domain.TLD)
also relies on a user to populate a record in DNS and faces those
problems with NAT. That's why many IP to IP calls today route
through the PSTN.
Conclusion: Closing down the circuit-switched PSTN would create
major problems until a modified DNS can enable webRTC, Joyn, and
SIP/ to find everyone, anywhere.
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