OTT and VoLTE Calls

Over and over again I can hear:

Why we need VoLTE, when I can use Skype or Whatsapp (for free)?

Or also:

What is the difference between native (volte) and non-native client?

Of course, everyone likes applications for free. And it’s a great thing we can use our facebook, skype or google accounts practically on any device and from anywhere. Maybe it’s a bit annoying that in order to communicate using such an Over-The-Top (OTT) application, we have to install the same applications as all our buddies.

By OTT we understand an application for a real-time communication using audio, video, and other media over the Internet without the involvement of a mobile operators’ IMS network. But when it comes to mobility we can’t get along without the operators’ infrastructure completely, we still need them to provide us with the internet connectivity.

ott
OTT and LTE

 Skype and other icons are used only for illustration.

There are many aspects and technical details which can’t be covered in this short article. Please, take this post as a brief introduction into the matter.

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VoLTE Policy Control Summary

In IT and particularly in Telco we are obsessed with abbreviations. My wife always loughs and tries to mimic me when she listens to my calls. Today we should be very careful as many of them start on ‘P’ – PCC, PCRF, PCEF, P-CSCF, PGW, PDN, PDG, PDB, PHB. But no worries, there will be abbreviations starting on other letters as well 🙂

In the IMS we have separated signalling and media data. However a full independence of control and user plane is not desirable. We want to control when the media starts and stops, we want to be sure about media routing, we want to ensure Quality of Service (QoS). And, of course, we want to accordingly charge the users.

In order to achieve these requirements we use two techniques in the VoLTE architecture:

  • Policy and Charging Control (PCC)
  • Differentiated Services (DiffServ)

Policy and Charging Control

PCC functionality comprises of Policy Control (e.g. QoS, media gating, ..) and Flow Based Charging. The ETSI TS 29.212, 29.213, 29.214 and 29.203 define Policy and Charging Control Architecture. There are many PCC functions defined. For us the main 3 PCC elements are:

  • Application Function (AF)
  • Policy Charging and Rules Function (PCRF)
  • Policy Control Enforcement Function (PCEF)
Policy and Charging Control (PCC) Architecture
Policy and Charging Control (PCC) Architecture

Application Function

In VoLTE is the AF incorporated within the Proxy-CSCF. The P-CSCF provides the information related to the control plane signaling. The information is taken from SIP/SDP session setup and it is forwarded to the PCRF via the Rx reference point. Each new SIP message that includes an SDP payload or session events (e.g. session termination, modification) can trigger a new request sent towards the PCRF. This ensures that the PCRF gets the proper information in order to perform reliable PCC.

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