4G – IMS

Posted: July 16, 2010 in technical
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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.


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