EPS Interfaces

This section summarizes the EPS interfaces and relevant protocols, with reference to the overall
architecture in Figure 1.2. The main protocols used inside EPS interfaces are summarized
as follows:
• S1 application protocol (S1-AP) – Application layer protocol between the eNB and the
MME.
• Stream control transmission protocol (SCTP) – This protocol guarantees delivery of signaling
messages between MME and eNB (S1). SCTP is defined in [11].
• GPRS tunneling protocol for the user plane (GTP-U) – This protocol tunnels user data
between eNB and the SGW, and between the SGW and the PGW in the backbone network.
GTP will encapsulate all end-user IP packets.
• User datagram protocol (UDP) – This protocol transfers user data. UDP is defined in [12].
• UDP/IP – These are the backbone network protocols used for routing user data and control
signaling.
• GPRS tunneling protocol for the control plane (GTP-C) – This protocol tunnels signaling
messages between SGSN and MME (S3).
• Diameter – This protocol supports transfer of subscription and authentication data for
authenticating/authorizing user access to the evolved system between MME and HSS
(home subscriber service) (S6a). Diameter is defined in [13].
1.6.1 S1-MME Interface
This interface is the reference point for the control plane between eNB andMME[5]. S1-MME
uses S1-AP over SCTP as the transport layer protocol for guaranteed delivery of signaling
messages betweenMMEand eNodeB. It serves as a path for establishing and maintaining subscriber
UE contexts.One or more S1-MME interfaces can be configured per context. Figure 1.3
illustrates the interface nodes.


One logical S1-AP connection per UE is established and multiple UEs are supported via a
single SCTP association. The following functionalities are conducted at S1-AP:
• Set up, modification and release of E-RABS.
• Establishment of an initial S1 UE context.
• Paging and S1 management functions.
• NAS signaling transport functions between UE and MME.
• Status transfer functionality.
• Trace of active UEs, and location reporting.
• Mobility functions for UE to enable inter- and intra-RAT HO.

LTE-Uu Interface
The radio protocol of E-UTRAN between the UE and the eNodeB is specified. The
user plane and control plane protocol stacks for the LTE-Uu interface are shown in Figures 1.4
and 1.5, respectively. The protocols on E-UTRAN-Uu (RRC, PDCP, RLC, MAC, and the
PHY LTE layer) implements the RRM and supports the NAS protocols by transporting the
NAS messages across the E-UTRAN-Uu interface.
The protocol stack layer and air interface functions are described in detail in next.

S1-U Interface
This interface between E-UTRAN and S-GW is used for user plane tunneling and inter-eNB
path switching during handover [15]. The user plane for S1-U is illustrated in Figure 1.6. In
addition, the end-to-end protocol stack for the user plane is shown in Figure 1.7. The S1-U
carries the user data traffic between the eNB and S-GW. S1-U also implements the DSCP
(differentiated services code point). The 6 bit DSCP value assigned to each IP packet identifies
a pre-determined level of service and a corresponding priority, which is used to implement the
appropriate QoS for the users’ data. More details on DSCP are provided in Chapter 7.
The EPS bearer service layered architecture is depicted in Figure 1.8 [14], where:
• A radio bearer transports the packets of an EPS bearer between a UE and an eNB. There is
a one-to-one mapping between an EPS bearer and a radio bearer.
• An S1 bearer transports the packets of an EPS bearer between an eNB and the S-GW.
• An S5/S8 bearer transports the packets of an EPS bearer between the S-GW and the P-GW.
• UE stores a mapping between an uplink packet filter and a radio bearer to create the binding
between SDFs (service data flows) and a radio bearer in the uplink, described later in this
chapter.
• P-GW stores a mapping between a downlink packet filter and an S5/S8 bearer to create the
binding between an SDF and an S5/S8 bearer in the downlink.
• An eNB stores a one-to-one mapping between a radio bearer and an S1 to create the binding
between a radio bearer and an S1 bearer in both the uplink and downlink.
• An S-GW stores a one-to-one mapping between an S1 bearer and an S5/S8 bearer to create
the binding between an S1 bearer and an S5/S8 bearer in both the uplink and the downlink.







S3 Interface (SGSN-MME)
This is the interface used by the MME to communicate with Release 8 SGSNs, on the same
PLMN, for interworking between GPRS/UMTS and LTE network access technologies [6].
This interface serves as the signaling path for establishing and maintaining subscriber’s contexts.
It is used between the SGSN and the MME to support inter-system mobility, while S4
connects the SGSN and the S-GW.
S3 functions include transfer of the information related to the terminal, handover/relocation
messages, and thus the messages are for an individual terminal basis. The MME communicates
with SGSNs on the PLMN using the GTP. The signaling or control aspect of this
protocol is referred to as the GTP control plane (GTP-C) while the encapsulated user data
traffic is referred to as the GTP user plane (GTP-U). One or more S3 interfaces can be
configured per system context. User and bearer information exchange for inter 3GPP (LTE
and 2G/3G) access network mobility in an idle and/or active state. The protocol stack for the
S3 interface is shown in Figure 1.9.



S4 (SGSN to SGW)
This reference point provides tunneling and management between the S-GW and an SGSN
[6, 15]. It has equivalent functions to the S11 interface and supports related procedures for
terminals connecting via EPS. It provides related control and mobility support between the
GPRS core and the 3GPP anchor function of S-GW.
This interface supports exclusively GTPv2-C and provides procedures to enable a user plane
tunnel between SGSN and S-GW if the 3G network has not enabled a direct tunnel for user
plane traffic from RNC to S-GW. The control plane and user plane of the S4 interface are
shown in Figure 1.10.
The end-to-end protocol stack for user data of 2G subscribers that camped on the 2G
network is illustrated in Figure 1.11. Protocols on the Um and the Gb interfaces are described
in [16]. The end-to-end protocol stack for user data of 3G subscribers that camped on the
UTRAN network is illustrated in Figure 1.12a. This protocol is used between the UE and
the P-GW user plane with 3G access via the S4 interface. SGSN controls the user plane
tunnel establishment, providing a direct tunnel between UTRAN and SGW. An alternative
approach for UTRAN is via a direct tunnel between UTRAN and SGW via the S12 interface,
as illustrated in Figure 1.12b. The protocols on the Uu, the Iu, the Um, and the Gb interfaces
are described in [16].



S5/S8 Interface
This reference point provides tunneling (bearer channel) and management (signaling channel)
between the S-GW and the P-GW [6, 15]. The S8 interface is used for roaming scenarios.
The S5 interface is used for non-roaming scenarios where it provides user plane tunneling and
management between S-GW and P-GW. It is used for S-GW relocation during UE mobility
and when the S-GW needs to connect to a non-collocated P-GW for the required PDN connectivity.
Figure 1.13 illustrates this interface.
There are two protocol options to be used in the S5/S8 interface:
• S5/S8 over GTP – Provides the functionality associated with creation, deletion, modification,
or change of bearers for an individual user connected to EPS.
• S5/S8 over PMIPV6 – Provides tunneling management between the SGW and PGW.














S6a Interface (Diameter)
This is the interface used by the MME to communicate with the HSS, as illustrated in
Figure 1.14 [17]. The HSS is responsible for transferring the subscription and authentication
data for authorizing the user access and UE context authentication. The MME communicates
with the HSSs on the PLMN using the Diameter protocol. One or more S6a interfaces can be
configured per system context.
The following list summarizes the functions of S6a:
• Exchange the location information
• Authorize a user to access the EPS,
• Exchange authentication information,
• Download and handle changes in the subscriber data stored in the server,
• Upload the P-GW identity and APN (access point name) being used for a specific PDN
connection,
• Download the P-GW identity and APN pairs being stored in HSS for an already ongoing
PDN connection.

S6b Interface (Diameter)
This reference point, between a PGWand a 3GPP AAA (access authorization and accounting)
server/proxy, is used for mobility-related authentication [18]. It may also be used to request
parameters related to mobility and to retrieve static QoS profiles for UEs (for non-3GPP
access). Figure 1.15 illustrates the layout of this interface.
The S6b interface is defined between the P-GW and the 3GPP AAA server (for non-roaming
case, or roaming with home routed traffic to P-GW in home network) and between the P-GW
and the 3GPP AAA proxy (for roaming case with P-GW in the visited network).
The S6b interface is used to inform the 3GPPAAAserver/proxy about current P-GW identity
and APN being used for a given UE, or that a certain P-GW and APN pair is no longer used.
This occurs, for example, when a PDN connection is established or closed. This S6b interface
protocol is based on Diameter and is defined as a vendor specific Diameter application, where
the vendor is 3GPP.


S6d (Diameter)
It enables transferring the subscription and authentication data for authorizing the user
access to the evolved system (AAA interface) between SGSN and HSS [17]. S6d is the
interface between S-GW in VPLMN (visited public land mobile network) and 3GPP AAA
proxy for mobility related authentication, if needed. This is a variant of S6c for the roaming
(inter-PLMN) case. Figure 1.16 illustrates the layout of this interface.



S9 Interface (H-PCRF-VPCRF)
The S9 interface is defined between the PCRF (policy and charging rules function) in the home
network policy and charging rules function (H-PCRF) and a PCRF in the visited network
policy and charging rules function (V-PCRF), as shown in Figure 1.17. S9 is an inter-operator
interface and is only used in roaming scenarios. The main purpose of the S9 interface is to
transfer policy decisions (i.e., policy charging and control, PCC, or QoS rules) generated in
the home network to the visited network and transport the events that may occur in the visited
network to the home network. The protocol over the S9 interfaces is based on Diameter. This
interface will allow the users when roamed on visited network to be treated with same QoS
and same PCC subject to the operators agreement.

S10 Interface (MME-MME)
This is the interface used by the MME to communicate with another MME in the same PLMN
or on different PLMNs, see Figure 1.18. This interface is also used for MME relocation and
MME-to-MME information transfer or handover. One or more S10 interfaces can be configured
per system context. The main function of the GTP-C layer, within this interface, is to
transfer the contexts for individual terminals attached to EPC and thus sent on a per UE basis.

S11 Interface (MME–SGW)
This interface provides communication between MME and S-GW for information transfer
using GTPv2 protocol, see Figure 1.19. One or more S11 interfaces can be configured per
system context. In the case of handover, the S11 interface is used to relocate the S-GW when
appropriate, or establish an indirect forwarding tunnel for user plane traffic and to manage use
data traffic flow.

S12 Interface
This is the reference point between UTRAN and S-GW for user plane tunneling when a direct
tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol,
as defined between SGSN and UTRAN or between SGSN and GGSN. The usage of S12 is
an operator configuration option. Figure 1.20 demonstrates the UE and P-GW user plane with
3G access via a direct tunnel on the S12 interface.






S13 Interface
This interface provides the communication between MME and the equipment identity register
(EIR), as shown in Figure 1.21. One or more S13 interfaces can be configured per system
context. This is similar to the S13’ interface between the SGSN and the EIR and they are
used to check the status of the UE. The MME or SGSN checks the UE identity by sending the
equipment identity to an EIR and analyzing the response (RES). The same protocol is used
on both S13 and S13’. This protocol is based on Diameter and is defined as a vendor specific
Diameter application. Diameter messages over the S13 and S13’ interfaces use the SCTP as
a transport protocol.

SGs Interface
The SGs interface connects the databases in the VLR and the MME to support CS fallback
scenarios [19]. The control interface is used to enable CSFB from E-UTRAN access to
UTRAN/GERAN CS domain access. The SGs-AP protocol is used to connect an MME to an
MSC server (MSS), as illustrated in Figure 1.22.
CSFB in the EPS enables the provisioning of CS-domain services (e.g., voice call, SMS,
location services (LCS), or supplementary services) by reusing the CS domain when the UE
is served by E-UTRAN.
The SGs interface connects the databases in the VLR and the MME to coordinate the location
information of UEs that are IMSI (international mobile subscriber identity) attached to
both EPS and non-EPS services. The SGs interface is also used to convey some CS related
procedures via the MME. The basis for the interworking between a VLR and an MME is the
existence of an SGs association between those entities per UE. The SGs association is only
applicable to UEs with CS fallback capability activated. The behavior of the VLR and the
MME entities related to the SGs interface is defined by the state of the SGs association for a
UE. Individual states per SGs association, that is, per UE with CS fallback capability activated,
are held at both the VLR and the MME. Chapter 4 provides more details on CSFB and it is
performance.



SGi Interface
This is the reference point between the P-GW and the PDN, see Figure 1.23. It can provide
access to a variety of network types, including an external public or private PDN and/or an
internal IMS service-provisioning network.


The functions of the SGi interface include access to the Internet, Intranet, or an ISP (Internet
service provider) and involve functions such as IPv4 address allocation, IPv6 address auto
configuration, and may also involve specific functions such as authentication, authorization,
and secure tunneling to the intranet/ISP.
When interworking with the IP networks, the packet domain can operate IPv4 and/or IPv6.
The interworking point with the IP networks is at the Gi and SGi reference points. Typically
in the IP networks, the interworking with subnetworks is done via IP routers. The Gi reference
point is between the GGSN and the external IP network while the SGi is between the P-GW
and the external IP network. From the external IP network’s point of view, the GGSN/P-GW is
seen as a normal IP router. Interworking with user-defined ISPs and private/public IP networks
is subject to interconnect agreements between the network operators.
The access to the Internet, Intranet, or ISP may involve specific functions, such as user
authentication, user’s authorization, end-to-end encryption between UE and intranet/ISP, allocation
of a dynamic address belonging to the PLMN/intranet/ISP addressing space, and IPv6
address autoconfiguration. For this purpose the packet domain may offer either direct transparent
access to the Internet; or a non-transparent access to the intranet/ISP. In this case the
packet domain, that is, the GGSN/PGW, takes part in these functions.

Gx Interface
TheGx reference point lies between the PCRF and the PCEF (policy and charging enforcement
function) as illustrated in Figure 1.24. This signaling interface supports the transfer of policy
control and charging rules information (QoS) between the PCEF in the P-GW and a PCRF
server. The Gx application has an own vendor specific Diameter application [20].With regard
to the Diameter protocol defined over the Gx interface, the PCRF acts as a Diameter server, in
the sense that it is the network element that handles PCC rule requests for a particular area.



The PCEF acts as the Diameter client, in the sense that is the network element requesting
PCC rules in the transport plane network resources. The main purpose of the Gx interface is
to support PCC rule handling and event handling for PCC. PCC rule handling over the Gx
interface includes the installation, modification, and removal of PCC rules. All these three
operations can be made upon any request coming from the PCEF or due to some internal
decision in the PCRF. The event handling procedures allows the PCRF to subscribe to those
events. The PCEF then reports the occurrence of an event to the PCRF.

Gy and Gz Interfaces
The Gy reference interface enables online accounting functions on the P-GW in accordance
with 3GPP Release 8 specifications. The Gy reference point for online flow-based bearer charging
(i.e., OCS, online charging system). On the other hand, the Gz reference point is for offline
flow-based bearer charging (i.e., OFCS, offline charging system), see Figure 1.25.
The Gz reference interface enables offline accounting functions on the P-GW. The P-GW
collects charging information for each mobile subscriber UE pertaining to the radio network
usage. The Gz reference point enables transport of SDF-based offline charging information.
The Gz interface is specified in [21].

DNS Interface
MME supports the DNS (domain name system) interface for MME, SGW, PGW, and SGSN
selection in the EPC CN. The MME uses the tracking area list as a fully qualified domain
name (FQDN) to locate the address relevant to the call. One or more DNS interfaces can be
configured per system context (refer to the addresses in Table 1.8).

Gn/Gp Interface
Gn interfaces facilitate user mobility between 2G/3G 3GPP networks. They are used for
intra-PLMN handovers [16, 22]. The MME supports pre-Release 8 Gn interfaces to allow
interoperation between EPS networks and 2G/3G 3GPP networks. Roaming and inter-access
mobility between Gn/Gp 2G and/or 3G SGSNs and an MME/SGW are enabled by:
• Gn functionality, as specified between two Gn/Gp SGSNs, which is provided by the MME
and
• Gp functionality, as specified between Gn/Gp SGSN and Gn/Gp GGSN that is provided by
the P-GW.


SBc Interface
The SBc application part (SBc-AP) messages are used on the SBc-AP interface between the
MME and the cell broadcast center (CBC) [23]. According to Figure 1.26, the SBc-AP interface
is a logical interface between the MME and the CBC. All the SBc-AP messages require
an SCTP association between the MME and the CBC.
The MME and the CBC support IPv6 [24] and/or IPv4 [25]. The IP layer of SBc-AP only
supports point-to-point transmission for delivering SBc-AP messages. SBc-AP consists of
elementary procedures (EPs). An EP is a unit of interaction between the MME and the CBC.
These EPs are intended to be used to build up complete sequences in a flexible manner.
Examples of using several SBc-APs together with each other and EPs from other interfaces
can be found [26].

Sv Interface
The Sv is the interface between the MME/SGSN and MSC Server to provide SRVCC (single
radio voice call continuity) [27]. The Sv interface, as shown in Figure 1.27, is between
the MME or the SGSN and 3GPP MSC server enhanced for SRVCC.1 The Sv interface is used to support inter-RAT handover from VoIP/IMS over EPS to a CS domain over 3GPP
UTRAN/GERAN access. The Sv messages are based on GTP protocol.






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