VL-87                    Feb 2013

[A special welcome to those who read "Why Fax over IP Fails" at Webtorials.com.]

Joyn/RCS vs. webRTC

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 book VoIP 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, VL-73).


The webRTC 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 webRTC javascript software.   The goal is to make browsers capable of real-time communications.  There are several use cases.
  1. 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.

  2. A special-interest web site (cars, construction, golf, etc.) helps its members contact each other.

  3. Any visitor to the web site can request a connection to any browser on the Web.

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.

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 PSTN phones.

To convert an E.164 phone number to an IP address requires two steps:
  1. ENUM (Electronic NUmber Mapping), an extension of DNS, converts the E.164 address to a URL.

  2. 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 signaling conversions.

It's possible, then, for such a web server to connect any two hosts.


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 current.

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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