So, let’s talk about 4G and IMS. This will describe the registration to the IMS core, when the IMS equipment is located in a Visited Network, in roaming.

The normal lines (and arrows) represent the main protocol:

- while in the 4G environment (UE, eNodeB, MME, SGW, PGW): it is eGTP (GTPv2-C) protocol

- while in the IMS environment(P-CSCF, I-CSCF, S-CSCF, HSS): it is plain IP or Diameter protocol

The dotted lines (and arrows) represent the inner IP protocol messages, which are encapsulated in GTPv1-U header.

The specs impacted by this are:

TS 23.401 : for the 4G UE equipment Initial Attach to the network

TS 23.203 : description of the IP-CAN Session Establishment procedures

TS 23.228 : description of the P-CSCF Discovery procedures

TS 21.905 : vocabulary for 3GPP Specifications

TS 29.061 : interworking of 4G and IMS

TS 29.212 : PCC (Policy and Charging Control) over Gx interface

TS 29.213 : PPC signaling flows and QoS parameters

TS 24.229 : IP multimedia call control over SIP and SDP

RFC 3261 : SIP – Session Initiation Protocol

The IMS protocols are just too many and diverse to list here all the TSs and RFCs related to them.

Let’s describe – shortly, very shortly – the messages exchanged here:

[Some of the message descriptions are simply and shamelessly copy-pasted from the spec :p  ]

1. Attach Request [TS 23.401]

2. Attach Request [TS 23.401]

The eNodeB derives the MME from the RRC parameters carrying the old GUMMEI and the indicated Selected Network. If that MME is not associated with the eNodeB or the old GUMMEI is not available, the eNodeB selects an MME as described in clause 4.3.8.3 on “MME selection function”. The eNodeB forwards the Attach Request message to the new MME contained in a S1-MME control message (Initial UE message) together with the Selected Network and TAI+ECGI of the cell from where it received the message to the new MME.

3. Create Session Request [TS 23.401]

If a subscribed PDN address is allocated for the UE for this APN, the PDN subscription context contains the UE’s IPv4 address and/or the IPv6 prefix and optionally the PDN GW identity. In case the PDN subscription context contains a subscribed IPv4 address and/or IPv6 prefix, the MME indicates it in the PDN address. For Attach Type indicating “Initial Attach”, if the UE does not provide an APN, the MME shall use the PDN GW corresponding to the default APN for default bearer activation. If the UE provides an APN, this APN shall be employed for default bearer activation. For Attach type indicating “Handover”, if the UE provides an APN, the MME shall use the PDN GW corresponding to the provided APN for default bearer activation, If the UE does not provide an APN, and the subscription context from HSS contains a PDN GW identity corresponding to the default APN, the MME shall use the PDN GW corresponding to the default APN for default bearer activation. The case where the Attach type indicates “Handover” and the UE does not provide an APN, and the subscription context from HSS does not contain a PDN GW identity corresponding to the default APN constitutes an error case. If the attach type indicates “Initial Attach” and the selected PDN subscription context contains no PDN GW identity the new MME selects a PDN GW as described in clause 4.3.8.1 on PDN GW selection function (3GPP accesses). If the PDN subscription context contains a dynamically allocated PDN GW identity and the Attach Type does not indicate “Handover” the MME may select a new PDN GW as described in clause PDN GW selection function, e.g. to allocate a PDN GW that allows for more efficient routing. The new MME selects a Serving GW as described in clause 4.3.8.2 on Serving GW selection function and allocates an EPS Bearer Identity for the Default Bearer associated with the UE. Then it sends a Create Session Request (IMSI, MSISDN, MME TEID for control plane, PDN GW address, PDN Address, APN, RAT type, Default EPS Bearer QoS, PDN Type, APN-AMBR, EPS Bearer Identity, Protocol Configuration Options, Handover Indication, ME Identity, User Location Information (ECGI), MS Info Change Reporting support indication, Selection Mode, Charging Characteristics, Trace Reference, Trace Type, Trigger Id, OMC Identity, Maximum APN Restriction, Dual Address Bearer Flag, the Protocol Type over S5/S8) message to the selected Serving GW.

The RAT type is provided in this message for the later PCC decision. The subscribed APN AMBR for the APN is also provided in this message. The MSISDN is included if provided in the subscription data from the HSS. Handover Indication is included if the Attach type indicates handover. Selection Mode indicates whether a subscribed APN was selected, or a non-subscribed APN sent by the UE was selected.. Charging Characteristics indicates which kind of charging the bearer context is liable for. The MME may change the requested PDN type according to the subscription data for this APN as described in clause 5.3.1.1. The MME shall set the Dual Address Bearer Flag when the PDN type is set to IPv4v6 and all SGSNs which the UE may be handed over to are Release 8 or above supporting dual addressing, which is determined based on node pre-configuration by the operator. The Protocol Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface.

The charging characteristics for the PS subscription and individually subscribed APNs as well as the way of handling Charging Characteristics and whether to send them or not to the P GW is defined in TS 32.251 [44]. The MME shall include Trace Reference, Trace Type, Trigger Id, and OMC Identity if S GW and/or P GW trace is activated. The MME shall copy Trace Reference, Trace Type, and OMC Identity from the trace information received from the HLR or OMC.

The Maximum APN Restriction denotes the most stringent restriction as required by any already active bearer context. If there are no already active bearer contexts, this value is set to the least restrictive type (see clause 15.4 of TS 23.060 [7]). If the P GW receives the Maximum APN Restriction, then the P GW shall check if the Maximum APN Restriction value does not conflict with the APN Restriction value associated with this bearer context request. If there is no conflict the request shall be allowed, otherwise the request shall be rejected with sending an appropriate error cause to the UE.

NOTE 7: The Dual Address Bearer Flag is not used when the Protocol Type over S5/S8 is PMIP.

Tags: 4G, Diameter, eNodeB, EPC, HSS, I-CSCF, IMS, LTE, MME, P-CSCF, passion, PCRF, PGW, Register, S-CSCF, SGW, techie, UE

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.

[Show as slideshow]

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

Tags: 4G, HSS, I-CSCF, IMS, LTE, P-CSCF, passion, PCRF, PGW, S-CSCF, SAE, SGW, techie, UE

The 3GPP project consists of major telecommunications organizations worldwide and one of the latest architectures described by this organization is 4G – SAE – System Architecture Evolution. SAE consists of a radio network, named LTE – Long Term Evolution and a core network, named EPC – Evolved Packet Core, a network architecture based only on IP addressing. The radio network is

out of the scope of this paper, the focus being on the components located in EPC. Another architectural design created by 3GPP is IMS – IP Multimedia Subsystem, dealing initially with VoIP signaling, and later on with multiple types of services and applications, like push-to-talk, presence and MBMS.

The figure below describes the components most commonly found in EPC, along with their IMS interaction.

EPC – IMS Architecture

As described in Figure 1, one of the main components of the EPC is the eNodeB, which has the functionality of the antenna and the controller. This component has the role of UE – User Equipment radio management, it is the first point of contact for the UE when connecting to a 4G network, it deals with the signaling for the UE management and also with the forwarding of the user-plane traffic to and from the UE. The signaling protocol specific to 4G is called GTPv2 or eGTP – evolved GTP. This protocol is present on the following EPC interfaces: S1-MME, S11 and S5/S8 and S4 (not presented in the picture, it appears between the SGSN and SGW). The user-plane protocol present in EPC is GTPv1 – user plane, and it appears on the following interfaces: S1-U and S5/S8. One or more eNodeBs (a pool of eNBs) is managed by an entity called MME – Mobility Management Entity. This device has the role of authenticating the UE to the HSS, it manages the UE sessions and controls its mobility over the network, and, unlike its homologous SGSN (Serving GPRS Support Node) device from 3G, only has role in the signaling path of the EPC, no user-plane traffic flowing through it. The MME is the entity responsible with the selection of the following entity, SGW – Serving Gateway (it’s homologous in 3G being the GGSN – Gateway GPRS Support Node). The SGW is the local mobility anchor for the UE: it manages the UE sessions and mobility, whether the UE is in active or in idle state, does QoS enforcement and forwards the control-plane and user-plane messages to the next entity, the PGW – PDN (Packet Data Network) Gateway. This entity has the role of allocating IP addresses to the UE, to route the traffic between the EPC and the PDN, to filter the traffic and assign it to different QoS classes, as well as to enforce this QoS for certain traffic. It is as well the mobility anchor for inter-working with non-3GPP technologies.

Another important aspect of the EPC is the QoS and the way it is enforced by the EPC elements. The traffic is assigned to different QoS classes based on the rules present in the PCRF. The HSS – Home Subscriber Server is a database system used to keep the SAE – related information about a certain UE, like the authentication information or the APN – Access Point Name it can connect to. Unlike HSS, the PCRF – Policy Control and Rules Function contains the charging information about a certain user subscription, information based on which the PCEF – Policy Control Enforcement Function component of the PGW provides QoS authorization (class identifiers and bitrates) and enforces this on the traffic routed through this device. In order to signal the creation of a QoS pattern for the traffic, the EPC uses the concept of bearers. A bearer is a data structure reserved on all the EPC components, it refers to the connection between a certain UE and an APN, for a certain traffic (identified by a TFT – Traffic Flow Template – a set of TCP/IP port numbers and a QoS: QCI – QoS Class Identifier and a set of uplink and downlink bit rates). The role of the bearer is to have independent classes of traffic granularly identified on the EPC components, situation useful when doing traffic shaping and for charging. One other important situation where these bearers are very useful is the handover process; the handover process is the process when an UE moves from the coverage area of an eNB to another eNB’s area. The target antenna and the connected EPC components may or may not support the bitrates and bandwidth supported before the mobility took place. In the case where the target components no longer support the services the UE had before mobility, the EPC drops some of the bearers; the decision of what bearers to drop, meaning what services will no longer be available for that user is taken based on the bearers classification, created from a field called ARP – Allocation and Retention Priority: the bearers with the poorest ARP will be the ones dropped in a situation like the one described.

The PGW connects the UE to an APN via the Gi interface. This interface is a simple IP addressing network, but the APN can be an intranet, the Internet or an IMS – IP Multimedia Subsystem network. In case this is an IMS network, the PGW will most probably be connected to the P-CSCF entity of IMS. The center of the IMS is the CSCF – Call Session Control Function, functionality divided into three components: a Proxy – P-CSCF, an Interrogating unit – I-CSCF and a Serving unit – S-CSCF. The P is the first point of contact in the IMS network, whether the user is in the home network or in roaming; it is also the entity sitting in the signaling path, being able to do message inspection, can do compression of the SIP header (SigComp) and it is the one establishing IPSec sessions to the UE. If it includes a PDF – Policy Decision Function component, it can also do media-plane QoS enforcement and bandwidth management. The S is the central SIP server of the architecture, doing registrations, inspection of the messages (as it sits in the message path) and it decides the SIP AS – Application Server which serves a certain service request. In its turn, the S is assigned to the UE by the HSS. Being in the path of the messages, the S is also responsible for charging records generation. The I is another component located at the edge of the administration domain, where the other servers locate it by doing DNS interrogations (as it uses NAPTR, DNS and SRV records). It has the role of interrogating the HSS and finding out which S that HSS is allocating for that specific user. Just as the EPC, the IMS also relies on the existence of an HSS database, as well as a PCRF system. In case there are multiple IMS networks working together, and therefore multiple HSS databases present, a new element appears – SLF – Subscriber Location Function, which has the purpose of delivering a view from all the databases in order to find information about a user. The protocol dominant in IMS is SIP – Session Initiation Protocol, standardized by IETF, having multiple extensions and improvements added to it. The protocol used to access the HSS is called Diameter, the more secure and more flexible follower of RADIUS protocol. The interfaces to HSS are all running Diameter, as well as the P interface to PCRF, Rx.

Tags: 4G, eNB, EPC, HSS, I-CSCF, IMS, LTE, MME, P-CSCF, PCRF, PGW, QoS, S-CSCF, SAE, SGW, SLF, techie