Call Session Control Functions - LTE Evolved Packet Core Network

Proxy CSCF Get me home QoS

Emergency call UE’s point of contact SIP compression Security associations

Interrogating CSCF

Access authorization Gateway

Locate me (find S-CSCF) Topology hiding

Serving CSCF Registrar server Security

Access to services

Call Session Control Functions Call Session Control Functions

There are three types of CSCF, each one responsible for a number of roles and actions.

The first point of contact for the UE is the P-CSCF. The P-CSCF’s responsibilities include forwarding all SIP requests back to the user’s home network, control of QoS provisioned in the access network, and emergency calls. However, it is likely that emergency calls will be handled by the CS domain.

The SIP node at the edge of the IMS is called the I-CSCF. This node acts as a gateway for SIP messages coming into the network. There is likely to be more than one I-CSCF in an operator’s network.

It is also responsible for locating the CSCF that the user will register with or has registered with, and for access authorization. The I-CSCF may perform topology hiding by incorporating a THIG (Topology Hiding Inter-network Gateway).

The S-CSCF provides session control and registration services to the UEs. It can also perform security procedures such as mutual authentication. A network may have multiple S-CSCFs and they will not necessarily all have the same capabilities.

LT3604/v4.0 © Wray Castle Limited A.5

Appendix

Proxy CSCF UE

Visited Network

Home Network Interrogating

CSCF 1

3 Response DNS 2

Query

DNS

P-CSCF P-CSCF

The P-CSCF is the first contact point within the IM CN subsystem. In GPRS the UE discovers its address during the PDP context activation procedure. The P-CSCF forwards SIP register requests from the UE to an I-CSCF, which is determined using the home domain name as provided by the UE. It also forwards SIP requests or responses to the UE and generates CDRs. It may be used to provide emergency service breakout.

Based on the SIP URI of the REGISTER method sent from the UE (sip.register.home1.net), the P-CSCF performs the DNS queries to locate the I-CSCF in the home network.

In many IMS deployments the functions of the P-CSCF are rolled into those of the SBC (Session Border Controller). In addition to managing P-CSCF activities, the SBC is often employed to handle other network edge access functions such as the E-CSCF (Emergency CSCF), which handles the routing and local breakout of emergency call sessions, NAT Traversal functions such as NAT Keepalive, which employ STUN (Simple Traversal of UDP and NAT) techniques to ensure that heartbeat messages are transmitted at regular intervals to enable a session to maintain any NAT-provided IP addressing, and lawful interception facilities. The SBC is also the point at which network edge security functions such as firewalls, application-layer gateways and SeGW are deployed.

LTE Evolved Packet Core Network

Proxy CSCF

Visited Network

Home Network Interrogating

CSCF Serving

CSCF 1

3 Locate Serving CSCF Check user is 2

authorized for IMS

HSS

I-CSCF I-CSCF

The I-CSCF is the contact point within an operator’s network for all connections destined to a subscriber of that network or a roaming subscriber currently located within that network operator’s service area. Its responsibilities include assigning an S-CSCF to a UE during registration, obtaining from the HSS the address of the S-CSCF, routing SIP requests from other networks towards the S-CSCF, and generating CDRs.

The HSS database is utilized in S-CSCF assignment when a user requires services. The I-CSCF passes a Cx Query message to the HSS. This message contains the UE’s location and its private and public identities. The HSS responds with a list of capabilities, which are subsequently used to identify an S-CSCF that will provide the required functions.

I-CSCFs are typically deployed on the edge of an operator’s IMS that faces other operator’s networks.

The use of an I-CSCF ensures that all inter-IMS signalling traffic is concentrated through a small number of controllable interface points, which aids planning and load balancing activities. The use of I-CSCFs also provides additional security for networks in the form of ‘topology hiding’. A network operator need only publish IP address details of their I-CSCFs in public DNS directories, as all external contact will be handled via the I-CSCFs, and details of the internal structure of the network can be hidden from external view. If all egress signalling traffic is also routed via an I-CSCF external networks again only have a view of the I-CSCF IP addresses and the internal topology of the IMS can remain hidden.

Appendix

SIP SIP

I-CSCF S-CSCF

HSS

Diameter

HSS learns S-CSCF address user authentication details subscriber information downloads

S-CSCF Learns UE Address UE’s Subscribed Services AS Addresses

AS AS AS

S-CSCF S-CSCF

The S-CSCF performs the session control services for the UE and interacts with services platforms for service support. The S-CSCF functions as a SIP registrar server by accepting registration requests, and interacts with the HSS and application servers to provide services to registered users.

The interaction with the HSS uses the Diameter protocol. These messages include updating the user’s location (with which S-CSCF the UE is registered), distributing authentication information and subscriber profiles.

The interaction with the application servers uses SIP. The S-CSCF may also forward SIP requests and responses for customers of a different operator to an I-CSCF of the network operator.

LTE Evolved Packet Core Network

SGW

The LTE IM CN subsystem uses SIP to manage IM sessions and IP as the transport mechanism for SIP session signalling and media transport. The configuration of IM CN subsystem entities is presented in the diagram. Only the interfaces specifically linked to the IM subsystem are shown.

3GPP has defined three different functional behaviours that the CSCF will exhibit. The P-CSCF provides a first point of contact for the UE within the IMS domain. All signalling to and from the handset goes through the P-CSCF. The I-CSCF is the contact point for all IM subsystem connections destined to a subscriber of that network operator. The S-CSCF performs the session control services for the UE.

The IM-MGW provides real-time communication between bearer channels and media channels to and from CS networks. It may support media conversion, bearer control and payload processing. The IM-MGW interacts with the MGCF for resource control and may support the H.248/Megaco protocol.

The MGCF (Media Gateway Control Function) controls the parts of the Call Session that pertain to connection control for media channels in an IM-MGW.

The MRFP (Multimedia Resource Function Processor) mixes incoming media streams for multiparty conferences, performs audio transcoding and media analysis and sources media streams for multimedia announcements.

The MRFC (Multimedia Resource Function Controller) controls the media streams in the MRFP, interprets information coming from an application server and an S-CSCF, and generates CDRs.

The SLF (Subscription Locator Function) is queried by the CSCF during subscriber registration to get the name of the HSS containing the required subscriber data.

LT3604/v4.0 © Wray Castle Limited A.9

IM CN Subsystem Entities (continued) IM CN Subsystem Entities (continued)

The BGCF (Breakout Gateway Control Function) selects the network in which PSTN breakout is to occur and forwards the SIP signalling to the BGCF of that network. If the breakout is to occur in the same network then it forwards the SIP signalling message to an MGCF responsible for the interworking with the PSTN.

The HSS is the master database of users. It contains all subscription-related information to support the network entities handling calls and sessions.

A.10

LTE Evolved Packet Core Network

UE 1 UE 2

E-UTRAN E-UTRAN

PDN-GW

S-GW

PDN-GW

S-GW P-CSCF S-CSCF I-CSCF

P-CSCF S-CSCF I-CSCF

SIP Signalling

User Data

SIP Signalling and User Data Flow Paths SIP Signalling and User Data Flow Paths

When the UE has obtained a signalling channel through the access network, it performs IMS registration by sending a REGISTER message to the P-CSCF in the visited network. Upon receipt of the REGISTER message the P-CSCF examines the home domain name to discover the I-CSCF of the subscriber’s home network and then forwards the REGISTER message to it.

The I-CSCF obtains a list of capabilities from the HSS to determine the name of the S-CSCF. It will then send the register message to the S-CSCF.

In the IMS, SIP signalling and media do not traverse the same paths. SIP signalling goes through the CSCFs, while the media goes via the PDN-GW after the SIP signalling has been processed.

Appendix

TCP UDP

IPv4, IPv6

H.323 SIP RTSP

Signalling Signalling

RSVP RTCP

Quality of Service Quality of Service

RTP Media encaps (H.261, MPEG) Media Transport Media Transport

PPP AAL 3/4 AAL5 LLC PPP

SONET ATM ^Ethernet V.34

Internet Multimedia Protocol Stack Internet Multimedia Protocol Stack

If the EPS is being used in conjunction with an IMS to deliver real-time services, the set of protocols used to establish and manage the connections created are drawn mostly from the IETFs Internet Multimedia Protocol Stack.

Not all of the protocols shown in the diagram are employed in the EPS, but the main ones that are used include SIP, RTP and RTCP.

SIP translates application-layer addresses into IP addresses to allow the establishment of new sessions and management of existing ones.

RTP is a media transport protocol for carrying real-time traffic types such as voice and video codecs.

RTP ensures that data is delivered with as little delay as possible to ensure a real-time experience for the users.

RTCP provides feedback on session quality to allow RTP to adjust its functionality if required.

RTP and RTCP are the protocols used to manage the flow of real-time session data between hosts once SIP and SDP have been used to initiate the connection.

Further Reading: IETF RFC1889/3550 (RTP & RTCP); RFC3286/4960 (SIP)

A.12

LTE Evolved Packet Core Network

Initiates, modifies and terminates

In document LTE Evolved Packet Core Network (Page 178-186)