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 ūüôā

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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IMS from 10.000 feet

Over and over I need to go through the basics. One would think that everything was already said but sometimes a refresher is a good thing. Especially when we rely a bit on what we guess than on what we really know. I’m not any exception ūüôā

There are many¬†changes we can witness these days in operators’ networks.

The core network has become much more complex. We still have the 2G/3G mix in place, we added new LTE network, WiFi network, some operators are even introducing WebRTC GW. I’ll leave aside the IoT/M2M traffic – however very soon it will be the most important guy in town (now 2nd – 3rd biggest source of revenue for mobile operators).

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Headers in User-centric networks

One of the key reasons why we need the IMS is the support of multiple devices for the same public identity. Simply put we don’t care what physical device our counterpart has, we just call and it is up to the network to select the particular device or more devices. For example my buddy can be connected via 3G or 4G and wifi from his smartphone, and he can be also connected from a web browser using the WebRTC (and well there are already some operators testing this).

As the logic says the key headers which are being used for routing will remain the same.

The examples here are taken from real networks (AS/CSCF). The exact ids and IPs were modified. You can enjoy the diversity of headers we can find all around the world. There could be¬†much more of them, but I don’t want to have the post too long and boring.

 

Request-URI
INVITE sip:0123456789;
phone-context=ims.mnc000.mcc000.3gppnetwork.org@ims.mnc000.mcc000.3gppnetwork.org;
user=phone SIP/2.0

or

INVITE sip:+12345678912@ims.operator.com;
user=phone SIP/2.0

or just

INVITE sip:12345678912@server.networks.operator.com SIP/2.0

 

P-Prefered-Identity
P-Preferred-Identity: sip:+12345678912@ims.operator.com

P-Asserted-Identity
P-Asserted-Identity: sip:+12345678912@ims.operator.com
P-Served-User
P-Served-User: <sip:+12345678912@ims.operator.com>;
sescase=orig;regstate=reg

 

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Much Ado about Registration

Registration is something what was missing in the times of wires. Ok, not completely but it was done more or less once when an MSISDN was assigned to some line.

IMS Registration

IMS Registration

In contrast to the previous types of the networks the 4G network is ‘user-centric’. It means the user can use multiple devices and identities and we have to deal with it. The main purpose of the registration is to create a binding between user (her public identity) and IP address of the device, so we know where we can send the data to. That’s why there is a Contact header in the SIP REGISTER message. The Contact header contains an address which identifies the current location of the user (Point-of-Presence – e.g. IP/FQDN of a particular client). During the registration is the Contact Address¬†is linked to¬†a Public Identity (IMPU, AOR in SIP terminology). The IMPU is an equivalent to MSISDN in GSM and has to be present in the To header of the SIP REGISTER. One identity can by used by more terminals and as each terminal can have different capabilities this has to be also taken into account. This information is part of the Contact header.

Contact: <sip:119560022547@152.67.235.234:49635;transport=tcp>;
   expires=3200;
   +g.oma.sip-im;
   language="en,fr";
   +u.asmc.apn="6a6044869e2aba3d23d4cfc0ef1384d00da28854c2408ddbbf";
   +sip.instance="<urn:uuid:D4196919-ED8A-4E00-9436-B9CDD1E76813>"

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