As I was saying in part 3: let’s get down to business
The 4G portion of the 4G – IMS registration has been described in Part 1;
The IP-CAN Session Establishment has been described in Part 2;
The generic IMS specifications (at least part of them) have been described in Part 3;

Now, let’s analyze the messages from the IMS – SIP signaling. As I was saying, these messages are tunneled between the eNodeB and PGW via GTPv1-U protocol. Then they reach the P-CSCF and are forwarded in the IMS core. This P-CSCF entity, often called simply P, can be located – and usually it is, specially in the roaming scenarios, … located in the visited network.

Before continuing to the description of each of the messages in the IMS registration, let’s take another look at the 4G – IMS architecture, as well as to the registration flow that we describe:

4G-IMS Architecture

4G-IMS RegistrationFlow

16. P-CSCF Discovery

- described in TS 23.228: section 5.1.1 Procedures related to Proxy-CSCF discovery and

section E.1.1.1 GPRS/EPS procedure for P-CSCF discovery

Because the procedure is pretty straight-forward, I will just copy-paste it from the spec above:

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

This is a continuation of the 4G – IMS topic I’ve started a while back.

Still, before continuing the IMS registration started in http://www.imacandi.net/windancer/2010/08/26/4g-to-ims-call-flow-register-to-ims.html

I would like to show a preview of the Specs describing this fancy IMS thinggie.

[This information is not structured by me, but it was on tech-invite, before they decided to put a price on all that information.]

The General aspects:

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

As I was describing in the previous episode - 4G to IMS call flow – Register to IMS – part 1 - the IP-CAN Session Establishment is scheduled for today.

The procedure of IP-CAN (Connectivity Access Network) establishment is described in TS 23.203.

Basically, the IP-CAN is the IP address that the User Equipment gets:

- either during Initial Attach

- or at a Dedicated Bearer creation

that connects that UE to the IMS APN. There are multiple ways of getting this IP address, ways varying from DHCP/DHCPv6, PPP and so on. With regards to IMS, the IP-CAN is given by the PGW, following the PCC rules from its local PCRF.

But, before displaying the actual IP-CAN Session Establishment, let’s take a look at the functional entities and the architectures involved.

The possible scenarios when talking about PCC (Policy and Charging Control) functionality, presented in TS 23.203. I have just copied the architecture pictures. The functional entities are described separately in the same TS 23.203 spec – section 6.2. Functional Entities. I will just underline, where the case requires, which entity from this “PCC Reference Architecture” matches which entity on the 4G architecture.

Before, let’s clarify a few things about these 3 funky pictures:

The acronyms:

HPLMN == Home Public Land Mobile Network

VPLMN == Visited Public Land Mobile Network (the user is in roaming)

SPR == Subscription Profile Repository

TS 23.203 – section 6.2.4 – SPR :

The SPR logical entity contains all subscriber/subscription related information needed for subscription-based policies and IP‑CAN bearer level PCC rules by the PCRF. The SPR may be combined with or distributed across other databases in the operator’s network, but those functional elements and their requirements for the SPR are out of scope of this document.

** The SPR is connected to the PCRF via the Sp interface.

AF == Application Function

TS 23.203 – section 6.2.3 – AF:

The Application Function (AF) is an element offering applications that require dynamic policy and/or charging control over the IP‑CAN user plane behaviour. The AF shall communicate with the PCRF to transfer dynamic session information, required for PCRF decisions as well as to receive IP‑CAN specific information and notifications about IP‑CAN bearer level events. One example of an AF is the P‑CSCF of the IM CN subsystem.

** The AF is connected to the PCRF via the Rx interface.

OCS == Online Charging System

- The OCS is described in TS 32.240. In the PCC architecture we are only interested in its component called Service Data Flow Based Credit Control - its main purpose being to perform online credit control functions.

** The OCS is connected to the PCEF via the Gy interface.

PCRF == Policy Control and Charging Rules Function

TS 23.203 – section 6.2.1 PCRF:

The PCRF encompasses policy control decision and flow based charging control functionalities.

The PCRF provides network control regarding the service data flow detection, gating, QoS and flow based charging (except credit management) towards the PCEF.

The PCRF shall apply the security procedures, as required by the operator, before accepting service information from the AF.

…………….

Here we are talking about 2 PCRF entities:

a) there is only one PCRF involved in the first case, when the UE is NOT in roaming; this is the local/home PCRF

b) in the home routed access and local breakout scenarios there is on one hand the V-PCRF (visited) – the PCRF entity from the visited network and on the other hand the H-PCRF (home) – the PCRF entity from the home network

** The PCRF is connected to the:

- AF via the Rx interface

- SPR via the Sp interface

- BBERF via the Gxx interface

- PCEF via the Gx interface

- H-PCRF and V-PCRF are connected via the S9 interface

BBERF == Bearer Binding and Even Reporting Function

TS 23.203 – section 6.2.7 BBERF:

The BBERF includes the following functionalities:

-     Bearer binding.

-     Uplink bearer binding verification.

-     Event reporting to the PCRF.

-     Sending or receiving IP‑CAN-specific parameters, to or from the PCRF.

* Note: As far as I understand, and in order to somehow “map” the names of the “PCC” entities to the names of the “EPC” entities I’ve first learned about, I believe this BBERF role is actually played by the SGW as we know it from the EPC terminology. Please correct me if I’ve got this wrong.

CORRECTION (further reading on TS 23.203): In the GTP-based 3GPP access network the BBERF entity does NOT apply.

** The BBERF is connected to the PCRF via the Gxx interface.

PCEF == Policy and Charging Enforcement Function

TS 23.203 – section 6.2.2 PCEF:

The PCEF encompasses service data flow detection, policy enforcement and flow based charging functionalities.

This functional entity is located at the Gateway (e.g. GGSN in the GPRS case, and PDG in the WLAN case). It provides service data flow detection, user plane traffic handling, triggering control plane session management (where the IP‑CAN permits), QoS handling, and service data flow measurement as well as online and offline charging interactions.

………………

*Note: The EPC entity assuming the role of the PCEF in the PCC Architecture is the PGW.

** The PCEF is connected to the:

- PCRF via the Gx interface

- OCS via the Gy interface

- OFCS via the Gz interface

OFCS == Offline Charging System

TS 23.203 – section 6.2.6 OFCS:

The Offline Charging System is specified in TS 32.240 [3].

There may be several OFCSs in a PLMN. The default OFCS addresses (i.e. the primary address and secondary address) shall be locally pre-configured within the PCEF. OFCS addresses may also be passed once per IP‑CAN session from the PCRF to the PCEF. The addresses provided by the PCRF shall have a higher priority than the pre-configured ones.

** The OFCS is connected to the PCEF via the Gz interface.

[I'll keep the pictures numbering for future referencing.]

Figure 5.1.1 Overall PCC logical architecture (non-roaming)

This architecture describes the simplest case where the User Equipment is located in his home network – in the network of the operator he subscribed to.

Figure 5.1.2 Overl PCC architecture (roaming with home routed access)

As the picture shows, here only the BBERF (which can be an SGW or a SGSN) is located in the visited network. This implies that the local (visited) PCRF is also to be used when locating the UE. The visited PCRF will contact the home PCRF via the S9 interface.

Figure 5.1.3 Overall PCC architecture for roaming with PCEF in visited network (local breakout)

This is the case where basically the entire access network is a visited network as the UE is concerned about. The BBERF (SGW or SGSN) and the PCEF (GGSN or SGW) [at least as far as the 3GPP implementations go...WiMAX guys, please help me out complete this article] are in the visited network. This implies: the use of (at least) the local (visited) PCRF, possibly the use of a local AF and the existence of a local OFCS.

This being said, let’s see how the IP-CAN Session Establishment process takes place – shamelessly copy-pasted from TS 23.203 – section 7.2 IP-CAN Session Establishment – with the consideration that, at least this spec is concise enough when describing these procedures so that I won’t feel the need to add anything else (afaik):

** Careful with the local notes, as in this single picture there are represented the IP-CAN procedures for all the 3 roaming/non-roaming scenarios described above:

This procedure concerns both roaming and non-roaming scenarios. In the roaming case when a Gateway Control Session is used, the V-PCRF should proxy the Gateway Control Session Establishment information between the BBERF in the VPLMN and the H-PCRF over S9 based on PDN-Id and roaming agreements.

For the Local Breakout scenario (Figure 5.1.3) the V-PCRF shall proxy the Indication and Acknowledge of IP‑CAN Session Establishment over S9 between the PCEF in the VPLMN and the H-PCRF

In the non-roaming case (Figure 5.1.1) the V-PCRF is not involved.

1.   The BBERF initiates a Gateway Control Session Establishment procedure as defined in clause 7.7.1 (applicable for cases 2a during initial attach and 2b, as defined in clause 7.1).

2.   The GW(PCEF) receives a request for IP‑CAN Bearer establishment. A PDN Connection Identifier may be included in the request. The GW(PCEF) accepts the request and assigns an IP address for the user.

3.  The PCEF determines that the PCC authorization is required, requests the authorization of allowed service(s) and PCC Rules information. The PCEF includes the following information: UE Identity (e.g. MN NAI), a PDN identifier (e.g. APN), the IP‑CAN type and the IP address(es), if available, the PDN Connection Identifier received for IP‑CAN Bearer establishment and, if available, the default charging method and the IP‑CAN bearer establishment modes supported. The PDN identifier, IP address(es) and UE identity enables identification of the IP‑CAN session. The IP‑CAN Type identifies the type of access from which the IP‑CAN session is established. If the service data flow is tunnelled at the BBERF, the PCEF shall provide information about the mobility protocol tunnelling encapsulation header. The PCEF may also include the Default Bearer QoS and APN-AMBR (applicable for case 1, as defined in clause 7.1). In case 2a the PCEF may also include charging ID information.

4.   If the PCRF does not have the subscriber’s subscription related information, it sends a request to the SPR in order to receive the information related to the IP‑CAN session. The PCRF provides the subscriber ID and, if applicable, the PDN identifier to the SPR. The PCRF may request notifications from the SPR on changes in the subscription information.

5.   The PCRF stores the subscription related information containing the information about the allowed service(s) and PCC Rules information.

6.   The PCRF makes the authorization and policy decision.

7.  The PCRF sends the decision(s) , including the chosen IP‑CAN bearer establishment mode, to the PCEF. The GW(PCEF) enforces the decision. The PCRF may provide the default charging method and may include the following information: the PCC Rules to activate and the Event Triggers to report. The Policy and Charging Rules allow the enforcement of policy associated with the IP‑CAN session. The Event Triggers indicate to the PCEF what events must be reported to the PCRF.

8.   If online charging is applicable, and at least one PCC rule was activated, the PCEF shall activate the online charging session, and provide relevant input information for the OCS decision. Depending on operator configuration PCEF may request credit from OCS for each charging key of the activated PCC rules.

9.   If online charging is applicable the OCS provides the possible credit information to the PCEF and may provide re-authorisation triggers for each of the credits.

In cases 2a and 2b if the OCS provides any re-authorisation trigger, which can not be monitored at the PCEF, the PCEF shall request PCRF to arrange those to be reported by the BBERF via the PCRF.

10. If at least one PCC rule was successfully activated and if online charging is applicable, and credit was not denied by the OCS, the GW(PCEF) acknowledges the IP‑CAN Bearer Establishment Request.

11. If network control applies the GW may initiate the establishment of additional IP-‑CAN bearers. See Annex A and Annex D for details.

12.  If the PCRF in step 7 requested an acknowledgement based on PCC rule operations, the GW(PCEF) sends the IP‑CAN Session  Establishment Acknowledgement to the PCRF in order to inform the PCRF of the activated PCC rules result.

Many more information on this on TS 23.203. Insist on the QoS parameter interaction.

Specially: Annex 4. 3GPP Accesses (GERAN/UTRAN/E-UTRAN) GTP-based EPC

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

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

Doing some more reading in the TS 29.061, I ended up in some other dilemmas.

The following information results from this spec so far (as per my understanding):

1. the PGW acts as a “proxy” for the SIP-IMS messages, encapsulating them in GTPv1-U

2. in order for the PGW to locate the P-CSCF, this PGW can have a pre-configured list of P-CSCFs

3. when the UE connects to that APN, the PGW must look through the list of pre-configured Ps, verify which ones are still up (by using ICMP, for example) and send to the UE a list of Ps; if there are multiple Ps in the list, the PGW will use the PCO IE to provide to the UE a prioritized list of Ps

Now, the dilemma comes. As the PGW is a control-plane entity and a user-plane entity in the 4G world, it can send both 4G control-plane messages (to the SGW – that may propagate or not till the MME – GTPv2-C messages) and user-plane messages (which are GTPv1-U messages encapsulating SIP, DHCP, whatever protocol).

TS 29.061 states the following, about how the PGW sends the IP / IPs of the P-CSCF to the UE: – section 13a.2.2  IMS Specific Procedures in the GGSN/P-GW:

The GGSN/P-GW shall then provide only those P-CSCF address(es) that are available in a Create PDP Context Response/Create Bearer Response.

Now, there are 2 issues with this statement:

1. the Create Bearer Response message is actually sent FROM the MME/SGW TO the PGW; the PGW is the one sending the Create Bearer Request message

2. disregarding item 1 and only thinking about the fact that the PGW will send the IP of the P-CSCF as part of the GTPv2-C signaling (rather the proxying it via the GTPv1-U tunnel), TS 29.274 leaves no room for more IEs in the Bearer Context grouped IE:

Table 7.2.3-2: Bearer Context within Create Bearer Request

So, if the 3GPP guys actually claim to configure multiple P-CSCF addresses in the above grouped IE, where are they putting those IP addresses?

where:

IMS – IP Multimedia Subsystem

P-CSCF – Proxy Call Session Control Function

PGW – PDN Gateway

PDN – Packet Data Network

SGW – Serving Gateway

MME – Mobility Management Entity

UE – User Equipment

IE – Information Element

GTP – GPRS Tunneling Protocol

Tags: 4G, HSS, IMS, LTE, P-CSCF, passion, PGW, SAE, techie

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

Cea mai recenta achizitie a Ixia este produsul Catapult, o solutie high-end pentru cei mai pretentiosi customeri in ale telecomunicatiilor mobile. Eu am apucat de ceva timp sa ma joc cu unealta asta, pentru a face testare de 4G core-networking. Si, desi lucrul cu ea e prohibitiv la inceput, o data ce te obisnuiesti cu particularitatile, constati ca este un produs pentru INGINERI, pentru PROFESIONISTI. Si asta imi place.

Sunt constienta ca multi clienti sunt manageri si cu ei trebuie sa discuti in alti termeni, dar cand vine vorba de negrisorul care se apuca sa purice comportamentul unui device care va fi echipamentul de baza in reteaua 4G de anul viitor, atunci vorbim de adevarate skill-uri ingineresti. De aceea imi place Catapulta, cum o alintam noi, un tool care te lasa sa-ti creezi mesaje si flow-uri de protocoale asa cum iti doreste inimioara.

Iar inimioara mea nu se multumeste cu orice De aceea, de la generarea unui flow corect si simplu, am trecut la simularea cauzelor de eroare specificate de TS 29.274 pentru fiecare mesaj de raspuns, pana la implementarea unui PCRF, pana la simularea comportamentului unui bundle de SGW+PGW+PCRF pentru a putea testa MME-ul (interfetele S11 si S1-U), precum si emularea de UE, eNB, MME pentru a putea testa comportamentul unui dispozitiv SGW/PGW real, chiar si in conditii de handover – care, dupa cum povesteam in alt post, nu sunt chiar putine si nici usoare.

Iar ca sa arat modularitatea jucariei, am ales un mesaj, Create Bearer Request, aratat in forma “disecata”, utilizatorul putand adauga si sterge IE-uri – dar cu validarea continutului si valorilor de catre disectoarele Catapultei, cat si in format DCPL (Digital Communication Programming Language™), where the sky is the limit pentru utilizator. Aici pot modifica pana si codarea hexa a fiecarei bucati de header si/sau IE din mesaj.

#54 e codarea hexa pentru IE-ul de TFT (Traffic Flow Template), pe cand #55 e codarea pentru TAD (Traffic Aggregate Description). Primul apare in mesajul de Create Bearer Request cand se creeaza un bearer dedicat – initiat de retea sau ca raspuns la o cerere. Al doilea se foloseste in mesajul de Bearer Resource Command cand MME cere retelei crearea/stergerea/modificarea unui bearer.

Binenteles, daca esti un tester picky de la un producator de echipamente renumit, vrei sa fii sigur ca masina ta trateaza corect orice minune ce poate veni din retea. Si nici nu am ajuns la testarea de LTE, pentru ca jucaria asta stie si interfata radio, si nici pe departe la implementarile de PCRF sau Diameter – more to come

Mwell, pentru ca toti who’s who ca producatori de echipamente de core-networking pana la provideri se pare ca au in lab o jucarie din asta, am zis ca vreau si eu una :>

Tags: 4G, EPC, LTE, passion, SAE, techie