# Serving Gateway Overview

The Cisco® ASR 5x00 core platform provides wireless carriers with a flexible solution that functions as a Serving Gateway (S-GW) in Long Term Evolution-System Architecture Evolution (LTE-SAE) wireless data networks.

This overview provides general information about the S-GW including:

- Product Description
- Network Deployment(s)
- Features and Functionality - Base Software
- Features and Functionality - External Application Support
- Features and Functionality - Optional Enhanced Feature Software
- How the Serving Gateway Works
- Supported Standards

Product Description

The Serving Gateway routes and forwards data packets from the UE and acts as the mobility anchor during inter-eNodeB handovers. Signals controlling the data traffic are received on the S-GW from the MME which determines the S-GW that will best serve the UE for the session. Every UE accessing the EPC is associated with a single S-GW.
Figure 1. S-GW in the Basic E-UTRAN/EPC Network

The S-GW is also involved in mobility by forwarding down link data during a handover from the E-UTRAN to the eHRPD network. An interface from the eAN/ePCF to an MME provides signaling that creates a GRE tunnel between the S-GW and the eHRPD Serving Gateway.
Figure 2. S-GW in the Basic E-UTRAN/EPC and eHRPD Network

The functions of the S-GW include:

- packet routing and forwarding.
- providing the local mobility anchor (LMA) point for inter-eNodeB handover and assisting the eNodeB reordering function by sending one or more “end marker” packets to the source eNodeB immediately after switching the path.
- mobility anchoring for inter-3GPP mobility (terminating the S4 interface from an SGSN and relaying the traffic between 2G/3G system and a PDN gateway.
- packet buffering for ECM-IDLE mode downlink and initiation of network triggered service request procedure.
- replicating user traffic in the event that Lawful Interception (LI) is required.
- transport level packet marking.
- user accounting and QoS class indicator (QCI) granularity for charging.
- uplink and downlink charging per UE, PDN, and QCI.
- reporting of user location information (ULI).
- support of circuit switched fallback (CSFB) for re-using deployed CS domain access for voice and other CS domain services.

Platform Requirements

The S-GW service runs on a Cisco® ASR 5x00 Series chassis running StarOS. The chassis can be configured with a variety of components to meet specific network deployment requirements. For additional information, refer to the Installation Guide for the chassis and/or contact your Cisco account representative.

Licenses

The S-GW is a licensed Cisco product. Separate session and feature licenses may be required. Contact your Cisco account representative for detailed information on specific licensing requirements. For information on installing and verifying licenses, refer to the Managing License Keys section of the Software Management Operations chapter in the System Administration Guide.

Network Deployment(s)

This section describes the supported interfaces and the deployment scenarios of a Serving Gateway.

Serving Gateway in the E-UTRAN/EPC Network

The following figure displays the specific network interfaces supported by the S-GW. Refer to Supported Logical Network Interfaces (Reference Points) for detailed information about each interface.
Figure 3. Supported S-GW Interfaces in the E-UTRAN/EPC Network

The following figure displays a sample network deployment of an S-GW, including all of the interface connections with other 3GPP Evolved-UTRAN/Evolved Packet Core network devices.
Figure 4. S-GW in the E-UTRAN/EPC Network

Supported Logical Network Interfaces (Reference Points)

The S-GW provides the following logical network interfaces in support of the E-UTRAN/EPC network:

S1-U Interface

This reference point provides bearer channel tunneling between the eNodeB and the S-GW. It also supports eNodeB path switching during handovers. The S-GW provides the local mobility anchor point for inter-eNodeB hand-overs. It provides inter-eNodeB path switching during hand-overs when the X2 handover interface between base stations cannot be used. The S1-U interface uses GPRS tunneling protocol for user plane (GTP-Uv1). GTP encapsulates all end user IP packets and it relies on UDP/IP transport. The S1-U interface also supports IPSec IKEv2. This interface is defined in 3GPP TS 23.401.

Supported protocols:

- Transport Layer: UDP, TCP
- Tunneling: IPv4 or IPv6 GTPv1-U (bearer channel)
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

S4 Interface

This reference point provides tunneling and management between the S-GW and a 3GPP S4 SGSN. The interface facilitates soft hand-offs with the EPC network by providing control and mobility support between the inter-3GPP anchor function of the S-GW. This interface is defined in 3GPP TS 23.401.

Supported protocols:

- Transport Layer: UDP
- Tunneling:
  - GTP: IPv4 or IPv6 GTP-C (GTPv2 control/signaling channel) and GTP-U (GTPv1 user/bearer channel)
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

S5/S8 Interface

This reference point provides tunneling and management between the S-GW and the P-GW, as defined in 3GPP TS 23.401. The S8 interface is an inter-PLMN reference point between the S-GW and the P-GW used during roaming scenarios. The S5 interface is used between an S-GW and P-GW located within the same administrative domain (non-roaming). It is used for S-GW relocation due to UE mobility and if the S-GW needs to connect to a non-collocated P-GW for the required PDN connectivity.

Supported protocols:

- Transport Layer: UDP, TCP
- Tunneling: GTP: GTPv2-C (signaling channel), GTPv1-U (bearer channel)
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

S11 Interface

This reference point provides GTP-C control signal tunneling between the MME and the S-GW. One GTP-C tunnel is created for each mobile terminal between the MME and S-GW. This interface is defined in 3GPP TS 23.401.

Supported protocols:

- Transport Layer: UDP
- Tunneling: IPv4 or IPv6 GTPv2-C (signalling channel)
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

S12 Interface

This reference point provides GTP-U bearer/user direct tunneling between the S-GW and a UTRAN Radio Network Controller (RNC), as defined in 3GPP TS 23.401. This interface provides support for inter-RAT handovers between the 3G RAN and EPC allowing a direct tunnel to be initiated between the RNC and S-GW, thus bypassing the S4 SGSN and reducing latency.

Supported protocols:

- Transport Layer: UDP
- Tunneling: IPv4 or IPv6 GTP-U (GTPv1 bearer/user channel)
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

Gxc Interface

This signaling interface supports the transfer of policy control and charging rules information (QoS) between the Bearer Binding and Event Reporting Function (BBERF) on the S-GW and a Policy and Charging Rules Function (PCRF) server.

Supported protocols:

- Transport Layer: TCP or SCTP
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

Gz Interface

The Gz reference interface enables offline accounting functions on the S-GW. The S-GW collects charging information for each mobile subscriber UE pertaining to the radio network usage. The Gz interface and offline accounting functions are used primarily in roaming scenarios where the foreign P-GW does not support offline charging.
Supported protocols:

- Transport Layer: TCP
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

Rf Interface

The Diameter Rf interface (3GPP 32.240) is used for offline (post-paid) charging between the Charging Trigger Function (CTF, S-GW) and the Charging Data Function (CDF). It follows the Diameter base protocol state machine for accounting (RFC 3588) and includes support for IMS specific AVPs (3GPP TS 32.299)

Supported protocols:

- Transport Layer: TCP or SCTP
- Network Layer: IPv4, IPv6
- Data Link Layer: ARP
- Physical Layer: Ethernet

Features and Functionality - Base Software

This section describes the features and functions supported by default in the base software for the S-GW service and do not require any additional licenses to implement the functionality.

IMPORTANT:

To configure the basic service and functionality on the system for the S-GW service, refer to the configuration examples provided in the Serving Gateway Administration Guide.

The following features are supported and brief descriptions are provided in this section:

- ANSI T1.276 Compliance
- APN-level Traffic Policing
- Bulk Statistics Support
- Circuit Switched Fall Back (CSFB) Support
- Closed Subscriber Group Support
- Congestion Control
- Downlink Delay Notification
- Dynamic GTP Echo Timer
- Event Reporting
- Idle-mode Signaling Reduction Support
- IP Access Control Lists
- IPv6 Capabilities
- Location Reporting
- Management System Overview
- Multiple PDN Support
- Online/Offline Charging
- Operator Policy Support
- QoS Bearer Management
- Rf Diameter Accounting
- Subscriber Level Trace
- Threshold Crossing Alerts (TCA) Support

ANSI T1.276 Compliance

ANSI T1.276 specifies security measures for Network Elements (NE). In particular it specifies guidelines for password strength, storage, and maintenance security measures.

ANSI T1.276 specifies several measures for password security. These measures include:

- Password strength guidelines
- Password storage guidelines for network elements
- Password maintenance, e.g. periodic forced password changes

These measures are applicable to the ASR 5x00 Platform and the Web Element Manager since both require password authentication. A subset of these guidelines where applicable to each platform will be implemented. A known subset of guidelines, such as certificate authentication, are not applicable to either product. Furthermore, the platforms support a variety of authentication methods such as RADIUS and SSH which are dependent on external elements. ANSI T1.276 compliance in such cases will be the domain of the external element. ANSI T1.276 guidelines will only be implemented for locally configured operators.

APN-level Traffic Policing

The S-GW now supports traffic policing for roaming scenarios where the foreign P-GW does not enforce traffic classes. Traffic policing is used to enforce bandwidth limitations on subscriber data traffic. It caps packet bursts and data rates at configured burst size and data rate limits respectively for given class of traffic.

Traffic Policing is based on RFC2698- A Two Rate Three Color Marker (trTCM) algorithm. The trTCM meters an IP packet stream and marks its packets green, yellow, or red. A packet is marked red if it exceeds the Peak Information Rate (PIR). Otherwise it is marked either yellow or green depending on whether it exceeds or doesn't exceed the Committed Information Rate (CIR). The trTCM is useful, for example, for ingress policing of a service, where a peak rate needs to be enforced separately from a committed rate.

Bulk Statistics Support

The system's support for bulk statistics allows operators to choose to view not only statistics that are of importance to them, but also to configure the format in which it is presented. This simplifies the post-processing of statistical data since it can be formatted to be parsed by external, back-end processors.

When used in conjunction with the Web Element Manager, the data can be parsed, archived, and graphed.

The system can be configured to collect bulk statistics (performance data) and send them to a collection server (called a receiver). Bulk statistics are statistics that are collected in a group. The individual statistics are grouped by schema. Following is a partial list of supported schemas:

- System: Provides system-level statistics
- Card: Provides card-level statistics
- Port: Provides port-level statistics
- MAG: Provides MAG service statistics
- S-GW: Provides S-GW node-level service statistics
- IP Pool: Provides IP pool statistics
- APN: Provides Access Point Name statistics

The system supports the configuration of up to four sets (primary/secondary) of receivers. Each set can be configured with to collect specific sets of statistics from the various schemas. Statistics can be pulled manually from the system or sent at configured intervals. The bulk statistics are stored on the receiver(s) in files.

The format of the bulk statistic data files can be configured by the user. Users can specify the format of the file name, file headers, and/or footers to include information such as the date, system host name, system uptime, the IP address of the system generating the statistics (available for only for headers and footers), and/or the time that the file was generated.

The Cisco Web Element Manager is capable of further processing the statistics data through XML parsing, archiving, and graphing.

The Bulk Statistics Server component of the Web Element Manager parses collected statistics and stores the information in its PostgreSQL database. It can also generate XML output and can send it to a Northbound NMS or an alternate bulk statistics server for further processing.

Additionally, the Bulk Statistics server can archive files to an alternative directory on the server. The directory can be on a local file system or on an NFS-mounted file system on the Web Element Manager server.

IMPORTANT:

For more information on bulk statistic configuration, refer to the Configuring and Maintaining Bulk Statistics chapter in the System Administration Guide.

Circuit Switched Fall Back (CSFB) Support

Circuit Switched Fall Back (CSFB) enables the UE to camp on an EUTRAN cell and originate or terminate voice calls through a forced switchover to the circuit switched (CS) domain or other CS-domain services (for example, Location Services (LCS) or supplementary services). Additionally, SMS delivery via the CS core network is realized without CSFB. Since LTE EPC networks were not meant to directly anchor CS connections, when any CS voice services are initiated, any PS based data activities on the EUTRAN network will be temporarily suspended (either the data transfer is suspended or the packet switched connection is handed over to the 2G/3G network).

CSFB provides an interim solution for enabling telephony and SMS services for LTE operators that do not plan to deploy IMS packet switched services at initial service launch.
The S-GW supports CSFB messaging over the S11 interface over GTP-C. Supported messages are:

- Suspend Notification
- Suspend Acknowledge
- Resume Notification
- Resume Acknowledgement

The S-GW forwards Suspend Notification messages towards the P-GW to suspend downlink data for non-GBR traffic; the P-GW then drops all downlink packets. Later, when the UE finishes with CS services and moves back to E-UTRAN, the MME sends a Resume Notification message to the S-GW which forwards the message to the P-GW. The downlink data traffic then resumes.

Closed Subscriber Group Support

The S-GW supports the following Closed Subscriber Group (CSG) Information Elements (IEs) and Call Detail Record:

- User CSG Information (UCI) IE in S5/S8
- CSG Information Reporting Action IE and functionality in S5/S8
- An SGW-CDR that includes a CSG record

Congestion Control

The congestion control feature allows you to set policies and thresholds and specify how the system reacts when faced with a heavy load condition.

Congestion control monitors the system for conditions that could potentially degrade performance when the system is under heavy load. Typically, these conditions are temporary (for example, high CPU or memory utilization) and are quickly resolved. However, continuous or large numbers of these conditions within a specific time interval may have an impact the system’s ability to service subscriber sessions. Congestion control helps identify such conditions and invokes policies for addressing the situation.

Congestion control operation is based on configuring the following:

- Congestion Condition Thresholds: Thresholds dictate the conditions for which congestion control is enabled and establish limits for defining the state of the system (congested or clear). These thresholds function in a way similar to operational thresholds that are configured for the system as described in the Thresholding Configuration Guide. The primary difference is that when congestion thresholds are reached, a service congestion policy and an SNMP trap, starCongestion, are generated.
  A threshold tolerance dictates the percentage under the configured threshold that must be reached in order for the condition to be cleared. An SNMP trap, starCongestionClear, is then triggered.
  - Port Utilization Thresholds: If you set a port utilization threshold, when the average utilization of all ports in the system reaches the specified threshold, congestion control is enabled.
  - Port-specific Thresholds: If you set port-specific thresholds, when any individual port-specific threshold is reached, congestion control is enabled system-wide.
- Service Congestion Policies: Congestion policies are configurable for each service. These policies dictate how services respond when the system detects that a congestion condition threshold has been crossed.

IMPORTANT:

For more information on congestion control, refer to the Congestion Control appendix in the System Administration Guide.

Downlink Delay Notification

This feature is divided between the following:

- Value Handling
- Throttling

Value Handling

This feature provides for the handling of delay value information elements (IEs) at the S-GW. When a delay value is received at the S-GW from a particular MME, the S-GW delays sending data notification requests for all idle calls belonging to that particular MME. Once the timer expires, requests can be sent. The delay value at the S-GW is determined by the factor received in the delay value IE (as a part of either a Modify Bearer Request or a Data Downlink Notification Request) and a hard-coded base factor of 50 ms at the S-GW

Throttling

This feature provides additional controls allowing the S-GW to set factors that “throttle” the continuous sending and receiving of DDN messages. A single command configures the throttling parameters supporting this feature,

A description of the ddn throttle command is located in the S-GW Service Configuration Mode Commands chapter in the eHRPD/LTE Command Line Interface Reference.

Dynamic GTP Echo Timer

The Dynamic GTP Echo Timer enables the eGTP and GTP-U services to better manage GTP paths during network congestion. As opposed to the default echo timer which uses fixed intervals and retransmission timers, the dynamic echo timer adds a calculated round trip timer (RTT) that is generated once a full request/response procedure has completed. A multiplier can be added to the calculation for additional support during congestion periods.

Event Reporting

The S-GW can be configured to send a stream of user event data to an external server. As users attach, detach, and move throughout the network, they trigger signaling events, which are recorded and sent to an external server for processing. Reported data includes failure reasons, nodes selected, user information (IMSI, IMEI, MSISDN), APN, failure codes (if any) and other information on a per PDN-connection level. Event data is used to track the user status via near real time monitoring tools and for historical analysis of major network events.

The S-GW Event Reporting appendix at the end of this guide describes the trigger mechanisms and event record elements used for event reporting.

The SGW sends each event record in comma separated values (CSV) format. The record for each event is sent to the external server within 60 seconds of its occurrence. The session-event-module command in the Context Configuration mode allows an operator to set the method and destination for transferring event files, as well as the format and handling characteristics of event files. For a detailed description of this command, refer to the Command Line Interface Reference.

A sample configuration sequence for enabling S-GW event reporting is provided in the Serving Gateway Configuration chapter of this guide.

Idle-mode Signaling Reduction Support

The S-GW now supports Idle-mode Signaling Reduction (ISR) allowing for a control connection to exist between an S-GW and an MME and S4-SGSN. The S-GW stores mobility management parameters from both nodes while the UE stores session management contexts for both the EUTRAN and GERAN/UTRAN. This allows a UE, in idle mode, to move between the two network types without needing to perform racking area update procedures, thus reducing the signaling previously required. ISR support on the S-GW is embedded and no configuration is required however, an optional feature license is required to enable this feature.

ISR support on the S-GW is embedded and no configuration is required, however, an optional feature license must be purchased to enable this feature.

IP Access Control Lists

IP access control lists allow you to set up rules that control the flow of packets into and out of the system based on a variety of IP packet parameters.

IP access lists, or Access Control Lists (ACLs) as they are commonly referred to, control the flow of packets into and out of the system. They are configured on a per-context basis and consist of “rules” (ACL rules) or filters that control the action taken on packets that match the filter criteria. Once configured, an ACL can be applied to any of the following:

- An individual interface
- All traffic facilitated by a context (known as a policy ACL)
- An individual subscriber
- All subscriber sessions facilitated by a specific context

IMPORTANT:

The S-GW supports interface-based ACLs only. For more information on IP access control lists, refer to the IP Access Control Lists appendix in the System Administration Guide.

IPv6 Capabilities

IPv6 enables increased address efficiency and relieves pressures caused by rapidly approaching IPv4 address exhaustion problem.

The S-GW platform offers the following IPv6 capabilities:

IPv6 Connections to Attached Elements

IPv6 transport and interfaces are supported on all of the following connections:

- Diameter Gxc policy signaling interface
- Diameter Rf offline charging interface
- Lawful Intercept (X1, X2 interfaces)

Routing and Miscellaneous Features

- OSPFv3
- MP-BGP v6 extensions
- IPv6 flows (Supported on all Diameter QoS and Charging interfaces as well as Inline Services (for example, ECS, P2P detection, Stateful Firewall, etc.)

Location Reporting

Location reporting can be used to support a variety of applications including emergency calls, lawful intercept, and charging. This feature reports both user location information (ULI).
ULI data reported in GTPv2 messages includes:

- TAI-ID: Tracking Area Identity
- MCC: MNC: Mobile Country Code, Mobile Network Code
- TAC: Tracking Area Code

The S-GW stores the ULI and also reports the information to the accounting framework. This may lead to generation of Gz and Rf Interim records. The S-GW also forwards the received ULI to the P-GW. If the S-GW receives the UE time zone IE from the MME, it forwards this IE towards the P-GW across the S5/S8 interface.

Management System Overview

The system's management capabilities are designed around the Telecommunications Management Network (TMN) model for management - focusing on providing superior quality Network Element (NE) and element management system (Web Element Manager) functions. The system provides element management applications that can easily be integrated, using standards-based protocols (CORBA and SNMPv1, v2), into higher-level management systems - giving wireless operators the ability to integrate the system into their overall network, service, and business management systems. In addition, all management is performed out-of-band for security and to maintain system performance.

Cisco's O+M module offers comprehensive management capabilities to the operators and enables them to operate the system more efficiently. There are multiple ways to manage the system either locally or remotely using its out-of-band management interfaces.

These include:

- Using the Command Line Interface (CLI)
- Remote login using Telnet, and Secure Shell (SSH) access to CLI through SPIO card's Ethernet management interfaces
- Local login through the console port on the SPIO card via an RS-232 serial connection
- Using the Web Element Manager application
- Supports communications through 10 Base-T, 100 Base-TX, 1000 Base-TX, or
- 1000Base-SX (optical gigabit Ethernet) Ethernet management interfaces on the SPIO
- Client-Server model supports any browser (such as, Microsoft Internet Explorer v6.0 and above or others)
- Supports Common Object Request Broker Architecture (CORBA) protocol and Simple Network Management Protocol version 1 (SNMPv1) for fault management
- Provides complete Fault, Configuration, Accounting, Performance, and Security (FCAPS) capabilities
- Can be easily integrated with higher-level network, service, and business layer applications using the Object Management Group's (OMG’s) Interface Definition Language (IDL)

The following figure demonstrates these various element management options and how they can be utilized within the wireless carrier network.
Figure 5. Element Management Methods

IMPORTANT:

P-GW management functionality is enabled by default for console-based access. For GUI-based management support, refer to the Web Element Manager section in this chapter.

IMPORTANT:

For more information on command line interface based management, refer to the Command Line Interface Reference and P-GW Administration Guide.

Multiple PDN Support

Enables an APN-based user experience that enables separate connections to be allocated for different services including IMS, Internet, walled garden services, or offdeck content services.

The Mobile Access Gateway (MAG) function on the S-GW can maintain multiple PDN or APN connections for the same user session. The MAG runs a single node level Proxy Mobile IPv6 (PMIP) tunnel for all user sessions toward the Local Mobility Anchor (LMA) function of the P-GW.

When a user wants to establish multiple PDN connections, the MAG brings up the multiple PDN connections over the same PMIPv6 session to one or more P-GW LMAs. The P-GW in turn allocates separate IP addresses (Home Network Prefixes) for each PDN connection and each one can run one or multiple EPC default and dedicated bearers. To request the various PDN connections, the MAG includes a common MN-ID and separate Home Network Prefixes, APNs and a Handover Indication Value equal to one in the PMIPv6 Binding Updates.

IMPORTANT:

Up to 11 multiple PDN connections are supported.

Online/Offline Charging

The Cisco EPC platforms support offline charging interactions with external OCS and CGF/CDF servers. To provide subscriber level accounting, the Cisco EPC platform supports integrated Charging Transfer Function (CTF) and Charging Data Function (CDF) / Charging Gateway Function (CGF). Each gateway uses Charging-IDs to distinguish between default and dedicated bearers within subscriber sessions.

The ASR 5x00 platform offers a local directory to enable temporary file storage and buffer charging records in persistent memory located on a pair of dual redundant RAID hard disks. Each drive includes 147GB of storage and up to 100GB of capacity is dedicated to storing charging records. For increased efficiency it also possible to enable file compression using protocols such as GZIP.

The offline charging implementation offers built-in heart beat monitoring of adjacent CGFs. If the Cisco P-GW has not heard from the neighboring CGF within the configurable polling interval, it will automatically buffer the charging records on the local drives until the CGF reactivates itself and is able to begin pulling the cached charging records.

Online: Gy Reference Interface

The P-GW supports a Policy Charging Enforcement Function (PCEF) to enable Flow Based Bearer Charging (FBC) via the Gy reference interface to adjunct Online Charging System (OCS) servers. The Gy interface provides a standardized Diameter interface for real-time content-based charging of data services. It is based on the 3GPP standards and relies on quota allocation. The Gy interface provides an online charging interface that works with the ECS Deep Packet Inspection feature. With Gy, customer traffic can be gated and billed. Both time- and volume-based charging models are supported.

Offline: Gz Reference Interface

The Cisco P-GW and S-GW support 3GPP Release 8 compliant offline charging as defined in TS 32.251,TS 32.297 and 32.298. Whereas the S-GW generates SGW-CDRs to record subscriber level access to PLMN resources, the P-GW creates PGW-CDRs to record user access to external networks. Additionally when Gn/Gp interworking with SGSNs is enabled, the GGSN service on the P-GW records G-CDRs to record user access to external networks.

To provide subscriber level accounting, the Cisco S-GW supports integrated Charging Transfer Function (CTF) and Charging Data Function (CDF). Each gateway uses Charging-IDs to distinguish between default and dedicated bearers within subscriber sessions.

The Gz reference interface between the CDF and CGF is used to transfer charging records via the GTPP protocol. In a standards based implementation, the CGF consolidates the charging records and transfers them via an FTP or SFTP connection over the Bm reference interface to a back-end billing mediation server. The Cisco EPC gateways also offer the ability to transfer charging records between the CDF and CGF serve via FTP or SFTP. CDR records include information such as Record Type, Served IMSI, ChargingID, APN Name, TimeStamp, Call Duration, Served MSISDN, PLMN-ID, etc.

Offline: Rf Reference Interface

Cisco EPC platforms also support the Rf reference interface to enable direct transfer of charging files from the CTF function of the S-GW to external CDF or CGF servers. This interface uses Diameter Accounting Requests (Start, Stop, Interim, and Event) to transfer charging records to the CDF/CGF. Each gateway relies on triggering conditions for reporting chargeable events to the CDF/CGF. Typically as EPS bearers are activated, modified or deleted, charging records are generated. The EPC platforms include information such as Subscription-ID (IMSI), Charging-ID (EPS bearer identifier) and separate volume counts for the uplink and downlink traffic.

Operator Policy Support

The operator policy provides mechanisms to fine tune the behavior of subsets of subscribers above and beyond the behaviors described in the user profile. It also can be used to control the behavior of visiting subscribers in roaming scenarios, enforcing roaming agreements and providing a measure of local protection against foreign subscribers.

An operator policy associates APNs, APN profiles, an APN remap table, and a call-control profile to ranges of IMSIs. These profiles and tables are created and defined within their own configuration modes to generate sets of rules and instructions that can be reused and assigned to multiple policies. In this manner, an operator policy manages the application of rules governing the services, facilities, and privileges available to subscribers. These policies can override standard behaviors and provide mechanisms for an operator to get around the limitations of other infrastructure elements, such as DNS servers and HSSs.

The operator policy configuration to be applied to a subscriber is selected on the basis of the selection criteria in the subscriber mapping at attach time. A maximum of 1,024 operator policies can be configured. If a UE was associated with a specific operator policy and that policy is deleted, the next time the UE attempts to access the policy, it will attempt to find another policy with which to be associated.

A default operator policy can be configured and applied to all subscribers that do not match any of the per-PLMN or IMSI range policies.

The S-GW uses operator policy to set the Accounting Mode - GTPP (default), RADIUS/Diameter or none. However, the accounting mode configured for the call-control profile will override this setting.

Changes to the operator policy take effect when the subscriber re-attaches and subsequent EPS Bearer activations.

QoS Bearer Management

Provides a foundation for contributing towards improved Quality of User Experience (QoE) by enabling deterministic end-to-end forwarding and scheduling treatments for different services or classes of applications pursuant to their requirements for committed bandwidth resources, jitter and delay. In this way, each application receives the service treatment that users expect.

An EPS bearer is a logical aggregate of one or more Service Data Flows (SDFs), running between a UE and a P-GW in case of GTP-based S5/S8, and between a UE and HSGW in case of PMIP-based S2a connection. An EPS bearer is the level of granularity for bearer level QoS control in the EPC/E-UTRAN. The Cisco P-GW maintains one or more Traffic Flow Templates (TFTs) in the downlink direction for mapping inbound Service Data Flows (SDFs) to EPS bearers. The P-GW maps the traffic based on the downlink TFT to the S5/S8 bearer. The Cisco P-GW offers all of the following bearer-level aggregate constructs:

QoS Class Identifier (QCI): An operator provisioned value that controls bearer level packet forwarding treatments (for example, scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc). Cisco EPC gateways also support the ability to map the QCI values to DiffServ codepoints in the outer GTP tunnel header of the S5/S8 connection. Additionally, the platform also provides configurable parameters to copy the DSCP marking from the encapsulated payload to the outer GTP tunnel header.

Guaranteed Bit Rate (GBR): A GBR bearer is associated with a dedicated EPS bearer and provides a guaranteed minimum transmission rate in order to offer constant bit rate services for applications such as interactive voice that require deterministic low delay service treatment.

Maximum Bit Rate (MBR): The MBR attribute provides a configurable burst rate that limits the bit rate that can be expected to be provided by a GBR bearer (e.g. excess traffic may get discarded by a rate shaping function). The MBR may be greater than or equal to the GBR for a given dedicated EPS bearer.

Aggregate Maximum Bit Rate (AMBR): AMBR denotes a bit rate of traffic for a group of bearers destined for a particular PDN. The Aggregate Maximum Bit Rate is typically assigned to a group of Best Effort service data flows over the Default EPS bearer. That is, each of those EPS bearers could potentially utilize the entire AMBR, e.g. when the other EPS bearers do not carry any traffic. The AMBR limits the aggregate bit rate that can be expected to be provided by the EPS bearers sharing the AMBR (e.g. excess traffic may get discarded by a rate shaping function). AMBR applies to all Non-GBR bearers belonging to the same PDN connection. GBR bearers are outside the scope of AMBR.

Policing and Shaping: The Cisco P-GW offers a variety of traffic conditioning and bandwidth management capabilities. These tools enable usage controls to be applied on a per-subscriber, per-EPS bearer or per-PDN/APN basis. It is also possible to apply bandwidth controls on a per-APN AMBR capacity. These applications provide the ability to inspect and maintain state for user sessions or Service Data Flows (SDF's) within them using shallow L3/L4 analysis or high touch deep packet inspection at L7. Metering of out-of-profile flows or sessions can result in packet discards or reducing the DSCP marking to Best Effort priority. When traffic shaping is enabled the P-GW enqueues the non-conforming session to the provisioned memory limit for the user session. When the allocated memory is exhausted, the inbound/outbound traffic for the user can be transmitted or policed in accordance with operator provisioned policy.

Rf Diameter Accounting

Provides the framework for offline charging in a packet switched domain. The gateway support nodes use the Rf interface to convey session related, bearer related or service specific charging records to the CGF and billing domain for enabling charging plans.

The Rf reference interface enables offline accounting functions on the HSGW in accordance with 3GPP Release 8 specifications. In an LTE application the same reference interface is also supported on the S-GW and P-GW platforms. The Cisco gateways use the Charging Trigger Function (CTF) to transfer offline accounting records via a Diameter interface to an adjunct Charging Data Function (CDF) / Charging Gateway Function (CGF). The HSGW and Serving Gateway collect charging information for each mobile subscriber UE pertaining to the radio network usage while the P-GW collects charging information for each mobile subscriber related to the external data network usage.

The S-GW collects information per-user, per IP CAN bearer or per service. Bearer charging is used to collect charging information related to data volumes sent to and received from the UE and categorized by QoS traffic class. Users can be identified by MSISDN or IMSI.

Flow Data Records (FDRs) are used to correlate application charging data with EPC bearer usage information. The FDRs contain application level charging information like service identifiers, rating groups, IMS charging identifiers that can be used to identify the application. The FDRs also contain the authorized QoS information (QCI) that was assigned to a given flow. This information is used correlate charging records with EPC bearers.

Subscriber Level Trace

Provides a 3GPP standards-based session level trace function for call debugging and testing new functions and access terminals in an LTE environment.

As a complement to Cisco's protocol monitoring function, the S-GW supports 3GPP standards based session level trace capabilities to monitor all call control events on the respective monitored interfaces including S1-U, S11, S5/S8, and Gxc. The trace can be initiated using multiple methods:

- Management initiation via direct CLI configuration
- Management initiation at HSS with trace activation via authentication response messages over S6a reference interface
- Signaling based activation through signaling from subscriber access terminal

Note: Once the trace is provisioned it can be provisioned through the access cloud via various signaling interfaces.

The session level trace function consists of trace activation followed by triggers. The EPC network element buffers the trace activation instructions for the provisioned subscriber in memory using camp-on monitoring. Trace files for active calls are buffered as XML files using non-volatile memory on the local dual redundant hard drives on the ASR 5x00 platform. The Trace Depth defines the granularity of data to be traced. Six levels are defined including Maximum, Minimum and Medium with ability to configure additional levels based on vendor extensions.

All call control activity for active and recorded sessions is sent to an off-line Trace Collection Entity (TCE) using a standards-based XML format over an FTP or secure FTP (SFTP) connection. In the current release the IPv4 interfaces are used to provide connectivity to the TCE. Trace activation is based on IMSI or IMEI.
Once a subscriber level trace request is activated it can be propagated via the S5/S8 signaling to provision the corresponding trace for the same subscriber call on the P-GW. The trace configuration will only be propagated if the P-GW is specified in the list of configured Network Element types received by the S-GW. Trace configuration can be specified or transferred in any of the following message types:

- S11: Create Session Request
- S11: Trace Session Activation
- S11: Modify Bearer Request

As subscriber level trace is a CPU intensive activity the maximum number of concurrently monitored trace sessions per Cisco P-GW or S-GW is 32. Use in a production network should be restricted to minimize the impact on existing services.

Threshold Crossing Alerts (TCA) Support

Thresholding on the system is used to monitor the system for conditions that could potentially cause errors or outage. Typically, these conditions are temporary (i.e high CPU utilization, or packet collisions on a network) and are quickly resolved. However, continuous or large numbers of these error conditions within a specific time interval may be indicative of larger, more severe issues. The purpose of thresholding is to help identify potentially severe conditions so that immediate action can be taken to minimize and/or avoid system downtime.

The system supports Threshold Crossing Alerts for certain key resources such as CPU, memory, IP pool addresses, etc. With this capability, the operator can configure threshold on these resources whereby, should the resource depletion cross the configured threshold, a SNMP Trap would be sent.

The following thresholding models are supported by the system:

- Alert: A value is monitored and an alert condition occurs when the value reaches or exceeds the configured high threshold within the specified polling interval. The alert is generated then generated and/or sent at the end of the polling interval.
- Alarm: Both high and low threshold are defined for a value. An alarm condition occurs when the value reaches or exceeds the configured high threshold within the specified polling interval. The alert is generated then generated and/or sent at the end of the polling interval.

Thresholding reports conditions using one of the following mechanisms:

- SNMP traps: SNMP traps have been created that indicate the condition (high threshold crossing and clear) of each of the monitored values.
  Generation of specific traps can be enabled or disabled on the chassis. Ensuring that only important faults get displayed. SNMP traps are supported in both Alert and Alarm modes.
- Logs: The system provides a facility called threshold for which active and event logs can be generated. As with other system facilities, logs are generated Log messages pertaining to the condition of a monitored value are generated with a severity level of WARNING.
  Logs are supported in both the Alert and the Alarm models.
- Alarm System: High threshold alarms generated within the specified polling interval are considered outstanding until a the condition no longer exists or a condition clear alarm is generated. Outstanding alarms are reported to the system's alarm subsystem and are viewable through the Alarm Management menu in the Web Element Manager.
  The Alarm System is used only in conjunction with the Alarm model.

IMPORTANT:

For more information on threshold crossing alert configuration, refer to the Thresholding Configuration Guide.

Features and Functionality - External Application Support

This section describes the features and functions of external applications supported on the S-GW. These services require additional licenses to implement the functionality.

- Web Element Management System

Web Element Manager

The Web Element Manager (WEM) provides a graphical user interface (GUI) for performing fault, configuration, accounting, performance, and security (FCAPS) management of the ASR 5x00 platform.

The Web Element Manager is a Common Object Request Broker Architecture (CORBA)-based application that provides complete fault, configuration, accounting, performance, and security (FCAPS) management capability for the system.

For maximum flexibility and scalability, the Web Element Manager application implements a client-server architecture. This architecture allows remote clients with Java-enabled web browsers to manage one or more systems via the server component which implements the CORBA interfaces.

The following figure demonstrates various interfaces between the Cisco Web Element Manager and other network components.
Figure 6. Web Element Manager Network Interfaces

IMPORTANT:

For more information on WEM support, refer to the WEM Installation and Administration Guide.

Features and Functionality - Optional Enhanced Feature Software

This section describes the optional enhanced features and functions for the S-GW service.

Each of the following features require the purchase of an additional license to implement the functionality with the S-GW service.

This section describes following features:

- Always-On Licensing
- Direct Tunnel
- Inter-Chassis Session Recovery
- IP Security (IPSec) Encryption
- Lawful Intercept
- Layer 2 Traffic Management (VLANs)
- Session Recovery Support

Always-On Licensing

Use of Always On Licensing requires that a valid license key be installed. Contact your Cisco account representative for information on how to obtain a license.

Traditionally, transactional models have been based on registered subscriber sessions. In an “always-on” deployment model, however, the bulk of user traffic is registered all of the time. Most of these registered subscriber sessions are idle a majority of the time. Therefore, Always-On Licensing charges only for connected-active subscriber sessions.

A connected-active subscriber session would be in “ECM Connected state,” as specified in 3GPP TS 23.401, with a data packet sent/received within the last one minute (on average). This transactional model allows providers to better manage and achieve more predictable spending on their capacity as a function of the Total Cost of Ownership (TCO).

Direct Tunnel

In accordance with standards, one tunnel functionality enables the SGSN to establish a direct tunnel at the user plane level - a GTP-U tunnel, directly between the RAN and the S-GW.
Figure 7. GTP-U with Direct Tunnel

In effect, a direct tunnel reduces data plane latency as the tunnel functionality acts to remove the SGSN from the data plane and limit the SGSN to the control plane for processing. This improves the user experience (for example, expedites web page delivery, reduces round trip delay for conversational services). Additionally, direct tunnel functionality implements the standard SGSN optimization to improve the usage of user plane resources (and hardware) by removing the requirement from the SGSN to handle the user plane processing.

Typically, the SGSN establishes a direct tunnel at PDP context activation using an Update PDP Context Request towards the S-GW. This means a significant increase in control plane load on both the SGSN and S-GW components of the packet core. Hence, deployment requires highly scalable S-GWs since the volume and frequency of Update PDP Context messages to the S-GW will increase substantially. The ASR 5x00 platform capabilities ensure control plane capacity will not be a limiting factor with direct tunnel deployment.

For more information on Direct Tunnel configuration, refer to the Direct Tunnel Configuration appendix in this guide.

Inter-Chassis Session Recovery

The ASR 5x00 platform provide industry leading carrier class redundancy. The systems protects against all single points of failure (hardware and software) and attempts to recover to an operational state when multiple simultaneous failures occur.

The system provides several levels of system redundancy:

- Under normal N+1 PSC hardware redundancy, if a catastrophic PSC failure occurs all affected calls are migrated to the standby PSC if possible. Calls which cannot be migrated are gracefully terminated with proper call-termination signaling and accounting records are generated with statistics accurate to the last internal checkpoint
- If the Session Recovery feature is enabled, any total PSC failure will cause a PSC switchover and all established sessions for supported call-types are recovered without any loss of session.

Even though Cisco provides excellent intra-chassis redundancy with these two schemes, certain catastrophic failures which can cause total chassis outages, such as IP routing failures, line-cuts, loss of power, or physical destruction of the chassis, cannot be protected by this scheme. In such cases, the MME Inter-Chassis Session Recovery (ICSR) feature provides geographic redundancy between sites. This has the benefit of not only providing enhanced subscriber experience even during catastrophic outages, but can also protect other systems such as the RAN from subscriber re-activation storms.

ICSR allows for continuous call processing without interrupting subscriber services. This is accomplished through the use of redundant chassis. The chassis are configured as primary and backup with one being active and one in recovery mode. A checkpoint duration timer is used to control when subscriber data is sent from the active chassis to the inactive chassis. If the active chassis handling the call traffic goes out of service, the inactive chassis transitions to the active state and continues processing the call traffic without interrupting the subscriber session. The chassis determines which is active through a propriety TCP-based connection called a redundancy link. This link is used to exchange Hello messages between the primary and backup chassis and must be maintained for proper system operation.

Interchassis Communication

Chassis configured to support ICSR communicate using periodic Hello messages. These messages are sent by each chassis to notify the peer of its current state. The Hello message contains information about the chassis such as its configuration and priority. A dead interval is used to set a time limit for a Hello message to be received from the chassis' peer. If the standby chassis does not receive a Hello message from the active chassis within the dead interval, the standby chassis transitions to the active state. In situations where the redundancy link goes out of service, a priority scheme is used to determine which chassis processes the session. The following priority scheme is used:

- router identifier
- chassis priority
- SPIO MAC address

Checkpoint Messages

Checkpoint messages are sent from the active chassis to the inactive chassis. Checkpoint messages are sent at specific intervals and contain all the information needed to recreate the sessions on the standby chassis, if that chassis were to become active. Once a session exceeds the checkpoint duration, checkpoint data is collected on the session. The checkpoint parameter determines the amount of time a session must be active before it is included in the checkpoint message.

IMPORTANT:

For more information on inter-chassis session recovery support, refer to the Interchassis Session Recovery appendix in System Administration Guide.

IP Security (IPSec) Encryption

Enables network domain security for all IP packet switched LTE-EPC networks in order to provide confidentiality, integrity, authentication, and anti-replay protection. These capabilities are insured through use of cryptographic techniques.

The Cisco S-GW supports IKEv1 and IPSec encryption using IPv4 addressing. IPSec enables the following two use cases:

- Encryption of S8 sessions and EPS bearers in roaming applications where the P-GW is located in a separate administrative domain from the S-GW
- IPSec ESP security in accordance with 3GPP TS 33.210 is provided for S1 control plane, S1 bearer plane and S1 management plane traffic. Encryption of traffic over the S1 reference interface is desirable in cases where the EPC core operator leases radio capacity from a roaming partner's network.

IMPORTANT:

You must purchase an IPSec license to enable IPSec. For more information on IPSec support, refer to the IP Security appendix in this guide.

Lawful Intercept

The Cisco Lawful Intercept feature is supported on the S-GW. Lawful Intercept is a licensed-enabled, standards-based feature that provides telecommunications service providers with a mechanism to assist law enforcement agencies in monitoring suspicious individuals for potential illegal activity. For additional information and documentation on the Lawful Intercept feature, contact your Cisco account representative.

Layer 2 Traffic Management (VLANs)

Virtual LANs (VLANs) provide greater flexibility in the configuration and use of contexts and services.

VLANs are configured as tags on a per-port basis and allow more complex configurations to be implemented. The VLAN tag allows a single physical port to be bound to multiple logical interfaces that can be configured in different contexts. Therefore, each Ethernet port can be viewed as containing many logical ports when VLAN tags are employed.

IMPORTANT:

For more information on VLAN support, refer to the VLANs appendix in the System Administration Guide.

Session Recovery Support

Provides seamless failover and reconstruction of subscriber session information in the event of a hardware or software fault within the system preventing a fully connected user session from being disconnected.

In the telecommunications industry, over 90 percent of all equipment failures are software-related. With robust hardware failover and redundancy protection, any card-level hardware failures on the system can quickly be corrected. However, software failures can occur for numerous reasons, many times without prior indication. StarOS has the ability to support stateful intra-chassis session recovery (ICSR) for S-GW sessions.
When session recovery occurs, the system reconstructs the following subscriber information:

- Data and control state information required to maintain correct call behavior
- Subscriber data statistics that are required to ensure that accounting information is maintained
- A best-effort attempt to recover various timer values such as call duration, absolute time, and others

Session recovery is also useful for in-service software patch upgrade activities. If session recovery is enabled during the software patch upgrade, it helps to preserve existing sessions on the active packet services card during the upgrade process.

IMPORTANT:

For more information on session recovery support, refer to the Session Recovery appendix in the System Administration Guide.

How the Serving Gateway Works

This section provides information on the function of the S-GW in an EPC E-UTRAN network and presents call procedure flows for different stages of session setup and disconnect.

The S-GW supports the following network flows:

- GTP Serving Gateway CallSession Procedures in an LTE-SAE Network

GTP Serving Gateway Call/Session Procedures in an LTE-SAE Network

The following topics and procedure flows are included:

- Subscriber-initiated Attach (initial)
- Subscriber-initiated Detach

Subscriber-initiated Attach (initial)

This section describes the procedure of an initial attach to the EPC network by a subscriber.
Figure 8. Subscriber-initiated Attach (initial) Call Flow
Table 1. Subscriber-initiated Attach (initial) Call Flow Description

Step | Description
--- | ---
1 | The UE initiates the Attach procedure by the transmission of an Attach Request (IMSI or old GUTI, last visited TAI (if available), UE Network Capability, PDN Address Allocation, Protocol Configuration Options, Attach Type) message together with an indication of the Selected Network to the eNodeB. IMSI is included if the UE does not have a valid GUTI available. If the UE has a valid GUTI, it is included.
2 | The eNodeB derives the MME from the GUTI and from the indicated Selected Network. If that MME is not associated with the eNodeB, the eNodeB selects an MME using an 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 an indication of the E-UTRAN Area identity, a globally unique E-UTRAN ID of the cell from where it received the message to the new MME.
3 | If the UE is unknown in the MME, the MME sends an Identity Request to the UE to request the IMSI.
4 | The UE responds with Identity Response (IMSI).
5 | If no UE context for the UE exists anywhere in the network, authentication is mandatory. Otherwise this step is optional. However, at least integrity checking is started and the ME Identity is retrieved from the UE at Initial Attach. The authentication functions, if performed this step, involves AKA authentication and establishment of a NAS level security association with the UE in order to protect further NAS protocol messages.
6 | The MME sends an Update Location (MME Identity, IMSI, ME Identity) to the HSS.
7 | The HSS acknowledges the Update Location message by sending an Update Location Ack to the MME. This message also contains the Insert Subscriber Data (IMSI, Subscription Data) Request. The Subscription Data contains the list of all APNs that the UE is permitted to access, an indication about which of those APNs is the Default APN, and the EPS subscribed QoS profile for each permitted APN. If the Update Location is rejected by the HSS, the MME rejects the Attach Request from the UE with an appropriate cause.
8 | The MME selects an S-GW using the Serving GW selection function and allocates an EPS Bearer Identity for the Default Bearer associated with the UE. If the PDN subscription context contains no P-GW address the MME selects a P-GW as described in clause PDN GW selection function. Then it sends a Create Default Bearer Request (IMSI, MME Context ID, APN, RAT type, Default Bearer QoS, PDN Address Allocation, AMBR, EPS Bearer Identity, Protocol Configuration Options, ME Identity, User Location Information) message to the selected S-GW.
9 | The S-GW creates a new entry in its EPS Bearer table and sends a Create Default Bearer Request (IMSI, APN, S-GW Address for the user plane, S-GW TEID of the user plane, S-GW TEID of the control plane, RAT type, Default Bearer QoS, PDN Address Allocation, AMBR, EPS Bearer Identity, Protocol Configuration Options, ME Identity, User Location Information) message to the P-GW.
10 | If dynamic PCC is deployed, the P-GW interacts with the PCRF to get the default PCC rules for the UE. The IMSI, UE IP address, User Location Information, RAT type, AMBR are provided to the PCRF by the P-GW if received by the previous message.
11 | The P-GW returns a Create Default Bearer Response (P-GW Address for the user plane, P-GW TEID of the user plane, P-GW TEID of the control plane, PDN Address Information, EPS Bearer Identity, Protocol Configuration Options) message to the S-GW. PDN Address Information is included if the P-GW allocated a PDN address Based on PDN Address Allocation received in the Create Default Bearer Request. PDN Address Information contains an IPv4 address for IPv4 and/or an IPv6 prefix and an Interface Identifier for IPv6. The P-GW takes into account the UE IP version capability indicated in the PDN Address Allocation and the policies of operator when the P-GW allocates the PDN Address Information. Whether the IP address is negotiated by the UE after completion of the Attach procedure, this is indicated in the Create Default Bearer Response.
12 | The Downlink (DL) Data can start flowing towards S-GW. The S-GW buffers the data.
13 | The S-GW returns a Create Default Bearer Response (PDN Address Information, S-GW address for User Plane, S-GW TEID for User Plane, S-GW Context ID, EPS Bearer Identity, Protocol Configuration Options) message to the new MME. PDN Address Information is included if it was provided by the P-GW.
14 | The new MME sends an Attach Accept (APN, GUTI, PDN Address Information, TAI List, EPS Bearer Identity, Session Management Configuration IE, Protocol Configuration Options) message to the eNodeB.
15 | The eNodeB sends Radio Bearer Establishment Request including the EPS Radio Bearer Identity to the UE. The Attach Accept message is also sent along to the UE.
16 | The UE sends the Radio Bearer Establishment Response to the eNodeB. In this message, the Attach Complete message (EPS Bearer Identity) is included.
17 | The eNodeB forwards the Attach Complete (EPS Bearer Identity) message to the MME.
18 | The Attach is complete and UE sends data over the default bearer. At this time the UE can send uplink packets towards the eNodeB which are then tunnelled to the S-GW and P-GW.
19 | The MME sends an Update Bearer Request (eNodeB address, eNodeB TEID) message to the S-GW.
20 | The S-GW acknowledges by sending Update Bearer Response (EPS Bearer Identity) message to the MME.
21 | The S-GW sends its buffered downlink packets.
22 | After the MME receives Update Bearer Response (EPS Bearer Identity) message, if an EPS bearer was established and the subscription data indicates that the user is allowed to perform handover to non-3GPP accesses, and if the MME selected a P-GW that is different from the P-GW address which was indicated by the HSS in the PDN subscription context, the MME sends an Update Location Request including the APN and P-GW address to the HSS for mobility with non-3GPP accesses.
23 | The HSS stores the APN and P-GW address pair and sends an Update Location Response to the MME.
24 | Bidirectional data is passed between the UE and PDN.

Subscriber-initiated Detach

This section describes the procedure of detachment from the EPC network by a subscriber.
Figure 9. Subscriber-initiated Detach Call Flow
Table 2. Subscriber-initiated Detach Call Flow Description

Step | Description
--- | ---
1 | The UE sends NAS message Detach Request (GUTI, Switch Off) to the MME. Switch Off indicates whether detach is due to a switch off situation or not.
2 | The active EPS Bearers in the S-GW regarding this particular UE are deactivated by the MME sending a Delete Bearer Request (TEID) message to the S-GW.
3 | The S-GW sends a Delete Bearer Request (TEID) message to the P-GW.
4 | The P-GW acknowledges with a Delete Bearer Response (TEID) message.
5 | The P-GW may interact with the PCRF to indicate to the PCRF that EPS Bearer is released if PCRF is applied in the network.
6 | The S-GW acknowledges with a Delete Bearer Response (TEID) message.
7 | If Switch Off indicates that the detach is not due to a switch off situation, the MME sends a Detach Accept message to the UE.
8 | The MME releases the S1-MME signalling connection for the UE by sending an S1 Release command to the eNodeB with Cause = Detach.

Supported Standards

The S-GW service complies with the following standards.

- 3GPP References
- 3GPP2 References
- IETF References
- Object Management Group (OMG) Standards

3GPP References

Release 9 Supported Standards

- 3GPP TS 23.216 V9.6.0 (2010-12): 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Single Radio Voice Call Continuity (SRVCC); Stage 2 (Release 9)
- 3GPP TS 29.274 V9.5.0 (2010-12): 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 (Release 9)

Release 8 Supported Standards

- 3GPP TR 21.905: Vocabulary for 3GPP Specifications
- 3GPP TS 23.003: Numbering, addressing and identification
- 3GPP TS 23.007: Restoration procedures
- 3GPP TS 23.107: Quality of Service (QoS) concept and architecture
- 3GPP TS 23.203: Policy and charging control architecture
- 3GPP TS 23.401: General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access
- 3GPP TS 23.402: Architecture Enhancements for non-3GPP accesses
- 3GPP TS 23.060. General Packet Radio Service (GPRS); Service description; Stage 2
- 3GPP TS 24.008: Mobile radio interface Layer 3 specification; Core network protocols
- 3GPP TS 24.229: IP Multimedia Call Control Protocol based on SIP and SDP; Stage 3
- 3GPP TS 29.210. Gx application
- 3GPP TS 29.212: Policy and Charging Control over Gx reference point
- 3GPP TS 29.213: Policy and Charging Control signaling flows and QoS
- 3GPP TS 29.214: Policy and Charging Control over Rx reference point
- 3GPP TS 29.274 V8.1.1 (2009-03): 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 (Release 8)
- 3GPP TS 29.274: Evolved GPRS Tunnelling Protocol for Control plane (GTPv2-C), version 8.2.0 (both versions are intentional)
- 3GPP TS 29.275: Proxy Mobile IPv6 (PMIPv6) based Mobility and Tunnelling protocols, version 8.1.0
- 3GPP TS 29.281: GPRS Tunnelling Protocol User Plane (GTPv1-U)
- 3GPP TS 32.251: Telecommunication management; Charging management; Packet Switched (PS) domain charging
- 3GPP TS 32.295: Charging management; Charging Data Record (CDR) transfer
- 3GPP TS 32.298: Telecommunication management; Charging management; Charging Data Record (CDR) encoding rules description
- 3GPP TS 32.299: Charging management; Diameter charging applications
- 3GPP TS 33.106: 3G Security; Lawful Interception Requirements
- 3GPP TS 36.107: 3G security; Lawful interception architecture and functions
- 3GPP TS 36.300: Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description
- 3GPP TS 36.412. EUTRAN S1 signaling transport
- 3GPP TS 36.413: Evolved Universal Terrestrial Radio Access (E-UTRA); S1 Application Protocol (S1AP)
- 3GPP TS 36.414: Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 data transport

3GPP2 References

- X.P0057-0 v0.11.0 E-UTRAN - eHRPD Connectivity and Interworking: Core Network Aspects

IETF References

- RFC 768: User Datagram Protocol (STD 6).
- RFC 791: Internet Protocol (STD 5).
- RFC 2131: Dynamic Host Configuration Protocol
- RFC 2460: Internet Protocol, Version 6 (IPv6) Specification
- RFC 2698: A Two Rate Three Color Marker
- RFC 2784: Generic Routing Encapsulation (GRE)
- RFC 2890: Key and Sequence Number Extensions to GRE
- RFC 3319: Dynamic Host Configuration Protocol (DHCPv6) Options for Session Initiation Protocol (SIP) Servers
- RFC 3588: Diameter Base Protocol
- RFC 3775: Mobility Support in IPv6
- RFC 3646: DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
- RFC 4006: Diameter Credit-Control Application
- RFC 4282: The Network Access Identifier
- RFC 4283: Mobile Node Identifier Option for Mobile IPv6 (MIPv6)
- RFC 4861: Neighbor Discovery for IP Version 6 (IPv6)
- RFC 4862: IPv6 Stateless Address Autoconfiguration
- RFC 5094: Mobile IPv6 Vendor Specific Option
- RFC 5213: Proxy Mobile IPv6
- Internet-Draft: Proxy Mobile IPv6
- Internet-Draft: GRE Key Option for Proxy Mobile IPv6, work in progress
- Internet-Draft: Binding Revocation for IPv6 Mobility, work in progress

Object Management Group (OMG) Standards

- CORBA 2.6 Specification 01-09-35, Object Management Group