4G – IMS

Posted: August 13, 2010 in technical
Tags: , , , , , , , , , , , , ,

I’ve been looking, as one of this blog’s readers asked for, a 3GPP spec to describe the functionality of the PGW (or SGW)..or, for the matter of fact, the 4G network element that deals with the IMS interaction. The common sense advised me the “culprit” in this matter should be the PGW.

[The reader is Jason Miller – RCS Engineer at Verizon Wireless :d ]

And TS 29.061 confirmed my idea 🙂

First let’s take a short look at the 4G – IMS picture.

[nggallery id=44]

As you should know by now:

1. MME, unlike SGSN, only does control-plane traffic, only GTPv2-C signaling

2. SGW does both signaling and user-plane traffic

3. all user-plane traffic that flows from the eNB via the SGW to the PGW is encapsulated in GTPv1-U tunnels

4. there is a “default” bearer structure that represents the connection of the UE to the PGW (PGW being the one that allocates the IP for the UE and it is also the main anchor point for mobility purposes) – this bearer is only used for fallback and the specs don’t recommend sending user-data over it; the QoS and attributes of the default bearer reside in the local 4G HSS database, connected via S6a to the MME

5. in order to be able to better shape the user-plane traffic, there are “dedicated” bearer structures that have a QoS class associated; the PCRF contains the information about these bearers, classifying traffic according to its importance; the PGW is the one responsible to communicate with PCRF, gather these rules and trigger the dedicated bearers creation to sustain this traffic

6. the PGW deals with encapsulation of the downlink data into the correct GTPv1-U tunnels according to the appropriate QoS settings – which results in specific user-plane bearer identifiers being used to identify these tunnels; also, the PGW deals with the decapsulation of the uplink data traffic and routing it to the appropriate destination in the Internet/IMS networks/intranet…etc. As the enforcer of the QoS patterns, the PGW acts as border MPLS router, translating the bearer QoS structures into plain IP QoS structures

In the TS 29.061 – section 13.a, the 3GPP guys shortly define the roles of the PGW:

Interworking with the IP Multimedia Core Network Subsystem (IMS) puts additional requirements on the GGSN/P-GW. When the MS/UE connects to the IP Multimedia Core Network Subsystem (IMS), specific parameters in Session Management messages may be handled. The IMS specific parameters are: IMS signalling flag, P-CSCF address request, returned P-CSCF address(es) and flow identifier(s).

For interworking with the IMS, the Gx interface (see 3GPP TS 29.212 [75]) is used to correlate the session (SIP/SDP) and the bearer (PDP Contexts).

The mechanisms in GGSN/P-GW to support IMS shall be:

–     P-CSCF discovery.

–     Dedicated signalling bearer (e.g. PDP contexts) (with or without enhanced QoS) for IMS signalling; with associated packet filters to permit signalling to/from designated servers.

–     Gx interface for policy and charging control of bearer (e.g. PDP contexts) for IMS media flows.

These mechanisms are however not restricted to the IMS and could be used for other services that could benefit from these mechanisms.

* the details can be read from the TS

  1. Jason says:

    Good effort. Will need to deep dive this a bit, I appreciate the perspective and starting point. Hopefully, I will beat you to putting together a call flow.

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