> Serving GPRS Support Node (SGSN) Overview
> This chapter contains general overview information about the Serving GPRS Support Node (SGSN), including sections for:

> • | Product Description

> • | Product Specifications

> • | Network Deployments and Interfaces

> • | Features and Functionality - Basic Software

> • | Features and Functionality - Enhanced and Licensed

> • | How the SGSN Works

> • | Supported Standards

> Product Description
> The ASR 5000 provides a highly flexible and efficient Serving GPRS Support Node (SGSN) service to the wireless carriers. Functioning as an SGSN, the system readily handles wireless data services within 2.5G General Packet Radio Service (GPRS) and 3G Universal Mobile Telecommunications System (UMTS) data networks.
> Important: Throughout this chapter the designation for the subscriber equipment is referred to in various ways: UE for user equipment (common to 3G/4G scenarios), MS or mobile station (common to 2G/2.5G scenarios), and MN or mobile node (common to 2G/2.5G scenarios involving IP-level functions). Unless noted, these terms are equivalent and the term used usually complies with usage in the relevant standards.
> In a GPRS/UMTS network, the SGSN works in conjunction with radio access networks (RANs) and Gateway GPRS Support Nodes (GGSNs) to:

> • | Communicate with home location registers (HLR) via a Gr interface and mobile visitor location registers (VLRs) via a Gs interface to register a subscriber’s user equipment (UE), or to authenticate, retrieve or update subscriber profile information.

> • | Support Gd interface to provide short message service (SMS) and other text-based network services for attached subscribers.

> • | Activate and manage IPv4, IPv6, or point-to-point protocol (PPP) -type packet data protocol (PDP) contexts for a subscriber session.

> • | Setup and manage the data plane between the RAN and the GGSN providing high-speed data transfer with configurable GEA0-3 ciphering.

> • | Provide mobility management, location management, and session management for the duration of a call to ensure smooth handover.

> • | Provide various types of charging data records (CDRs) to attached accounting/billing storage mechanisms such as our SMC-based hard drive or a GTPP Storage Server (GSS) or a charging gateway function (CGF).

> • | Provide CALEA support for lawful intercepts.

> This chapter catalogs many of the SGSN key components and features for data services within the GPRS/UMTS environment. Also, a range of SGSN operational and compliance information is summarized with pointers to other information sources.
> Product Specifications
> The following information is located in this section:

> • | Licenses

> • | Hardware Requirements

> • | Operating System Requirements

> • | System Configuration Options

> Licenses
> The SGSN is a licensed product. A session use license key must be acquired and installed to use the SGSN service. As well, the SGSN supports several special features that also require license keys be acquired and installed for their use.
> The following feature licenses are available for use with the SGSN:

> • | SGSN Software License, 10K Sessions

> • | SGSN Software License, 1K Sessions

> • | Direct Tunnel Support

> • | Gd Interface (support for SMS)

> • | Lawful Intercept

> • | QoS Traffic Policing

> • | Session Recovery

> • | SGSN Pooling & Iu or Gb-Flex Flex

> Hardware Requirements
> Information in this section describes the hardware required to support SGSN services.
> Platforms
> The SGSN operates on an ASR 5000.
> ASR 5000 System Hardware Components
> The following application and line cards are required to support GPRS/UMTS wireless data services on the SGSN:

> • | System Management Cards (SMCs): Provides full system control and management of all cards within the ASR 5000. Up to two SMCs can be installed; one active, one redundant.

> • | Packet Services Cards (PSCs): Within the chassis, PSCs (either PSC or PSC2) provide high-speed, multi-threaded PDP context processing capabilities for 2.5G SGSN, 3G SGSN, and GGSN services. Up to 14 PSCs can be installed, allowing for multiple active and/or redundant cards.

> • | Switch Processor Input/Outputs (SPIOs): Installed in the upper-rear chassis slots directly behind the SMCs, SPIOs provide connectivity for local and remote management, central office (CO) alarms, and BITS timing. Up to 2 SPIOs can be installed: 1 active, 1 redundant.

> • | Line Cards: Installed directly behind PSCs, these cards provide the physical interfaces from the SGSN to various elements in the GPRS/UMTS data network. Up to 26 line cards can be installed for a fully loaded system with 13 active PSCs, 13 in the upper-rear slots and 13 in the lower-rear slots for redundancy. Redundant PSCs do not require line cards.

> Depending on the SGSN network environment, the system supports multiple types of line cards, simultaneously if needed:

> • | Various types of Ethernet line cards provide IP connections:

> • | Ethernet 10/100 line cards

> • | Ethernet 1000 line cards

> • | 4-port Quad Gig-E line cards (QGLCs)

> • | 10-Gigabit Ethernet line cards (XGLCs)

> • | Optical (ATM over SDH/SONET) Line Cards (OLC or OLC2) - ATM/POS OC-3 Single Mode or Multi-Mode optical fiber line cards providing SS7 broadband signaling, e.g., SIGTRAN over ATM via E1/DS1 (T1) signaling

> • | Channelized Line Cards (CLC or CLC2) - STM-1/OC-3 provides Frame Relay over SDH/SONET signaling

> • | Redundancy Crossbar Cards (RCCs): Installed in the lower-rear chassis slots directly behind the SMCs, RCCs utilize 5 Gbps serial links to ensure connectivity between Ethernet 10/100 or Ethernet 1000 line cards/QGLCs and every PSC in the system for redundancy. Two RCCs can be installed to provide redundancy for all line cards and PSCs.

> Additional information, for each of the application and line cards required to support GPRS/UMTS wireless data services, is located in the ASR 5000 Hardware Installation and Administration Guide.
> Operating System Requirements
> The SGSN is available for all ASR 5000s running StarOS 8.0 or higher.
> System Configuration Options
> An ASR 5000 SGSN system supports multiple GPRS Support Node (GSN) service applications, in any combination, co-located within a single chassis, for example:

> • | 2.5G SGSN & 3G SGSN - Dual Access

> • | SGSN (2.5G or 3G) & GGSN

> Benefits of Co-Located GSNs
> Integrated co-location is done without introducing proprietary protocols, thus avoiding mobility and handoff issues. Multiple network element applications, integrated as a single application within a single chassis, benefit carriers for the following reasons:

> • | Same hardware for all services

> • | Load sharing architecture ensures that all hardware is used efficiently

> • | Single software load

> • | Uniform configuration

> • | Optimal usage of the high capacity system

> • | Reduced latency in the control and data paths

> • | Simplification of network architecture

> • | Single platform-view, maintained even in the presence of multiple services

> • | Fewer IP addresses needed

> • | No internal interfaces

> • | Combined SGSN/GGSN serve other SGSNs and GGSNs with no loss of functionality

> • | Hand-offs between 2.5G and 3G networks can re-use the same SAU state; this avoids repeated exchanges with the HLR thereby reducing the number of interaction messages

> • | Operating as a combined SGSN/GGSN, the common processes host both SGSN and GGSN sessions resulting in optimized hardware usage and latency

> • | Combined with Iu-Flex and Gb-Flex, an SGSN/GGSN system enables single-hop core network routing (a given session is always routed to the same combined node)

> Network Deployments and Interfaces
> The following logical connections maps indicate the SGSN’s ability to connect to both 2G (GSM BSS) and 3G (UMTS RAN) radio access networks, a mobile service center (MSC) and visitor location register (VLR), a home location register (HLR), a charging gateway (CG - sometimes referred to as a charging gateway function (CGF)), a GTPP storage server (GSS), a standalone GGSN, network devices in another PLMN, an SMS server center, and a standalone SGSN.
> SGSN and Dual Access SGSN Deployments
> SGSNs and GGSNs work in conjunction within the GPRS/UMTS network. As indicated earlier in the section on System Configuration Options, the flexible architecture of the ASR 5000 enables a single chassis to reduce hardware requirements by supporting integrated co-location of a variety of the GPRS/UMTS services.
> A chassis can be devoted solely to SGSN services or the SGSN system can include any co-location combination, such as multiple instances of 2.5G SGSNs; or multiple instances of 3G SGSNs; or a combination of 2.5G and 3G SGSN to comprise a dual access SGSN.
> Dual Access 2.5G/3G SGSNs
> SGSN/GGSN Deployments
> The co-location of the SGSN and the GGSN in the same chassis facilitates handover. Again, it can be any type of SGSN, 2.5G or 3G, with the GGSN.
> Co-located SGSN and GGSN
> SGSN Logical Network Interfaces
> The SGSN provides IP-based transport on all RAN and Core Network interfaces, in addition to the standard IP-based interfaces (Ga, Gn, Gp, Iu-PS). This means enhanced performance, future-proof scaling and reduction of inter-connectivity complexity. The all-IP functionality is key to facilitating evolution to the next generation technology requirements.
> The SGSN provides the following functions over the logical network interfaces illustrated above:

> • | IuPS: The SGSN provides an IP over ATM (IP over AAL5 over ATM) interface between the SGSN and the RNCs in the 3G UMTS Radio Access Network (UTRAN). RANAP is the control protocol that sets up the data plane (GTP-U) between these nodes. SIGTRAN (M3UA/SCTP) or QSAAL (MTP3B/QSAAL) handle IuPS-C (control) for the RNCs.

> Some of the procedures supported across this interface are:

> • | Control plane based on M3UA/SCTP

> • | Up to 128 Peer RNCs per virtual SGSN. Up to 256 peers per physical chassis

> • | SCTP Multi-Homing supported to facilitate network resiliency

> • | M3UA operates in ASP & IPSP client/server and single/double-ended modes

> • | Multiple load shared M3UA ASP instances for high-performance and redundancy

> • | Works over Ethernet and ATM (IPoA) interfaces

> • | Facilitates SGSN Pooling

> • | RAB (Radio Access Bearer) Assignment Request

> • | RAB Release Request

> • | Iu Release Procedure

> • | SGSN-initiated Paging

> • | Common ID

> • | Security Mode Procedures

> • | Initial MN Message

> • | Direct Transfer

> • | Reset Procedure

> • | Error Indication

> • | Gb: This is the SGSN’s interface to the base station system (BSS) in a 2G radio access network (RAN). It connects the SGSN via UDP/IP (via an Ethernet interface) or Frame Relay (via a Channelized SDH or SONET interface). Gb-IP is the preferred interface as it improves control plane scaling as well as facilitates the deployment of SGSN Pools.

> Some of the procedures supported across this interface are:

> • | BSS GSM at 900/1800/1900 MHz

> • | BSS Edge

> • | Frame Relay congestion handling

> • | Traffic management per Frame Relay VC

> • | NS load sharing

> • | NS control procedures

> • | BVC management procedures

> • | Paging for circuit-switched services

> • | Suspend/Resume

> • | Flow control

> • | Unacknowledged mode

> • | Acknowledged mode

> • | Gn/Gp: The Gn/Gp interfaces, comprised of GTP/UDP/IP-based protocol stacks, connect the SGSNs and GGSNs to other SGSNs and GGSNs within the same PLMN (the Gn) or to GGSNs in other PLMNs (the Gp).

> This implementation supports:

> • | GTPv0 and GTPv1, with the capability to auto-negotiate the version to be used with any particular peer

> • | GTP-C (control plane) and GTP-U (user plane)

> • | Transport over ATM/STM-1Optical, Fast Ethernet, and Ethernet 1000 line cards/QGLCs)

> • | One or more Gn/Gp interfaces configured per system context

> As well, the SGSN can support the following IEs from later version standards:

> • | IMEI-SV

> • | RAT TYPE

> • | User Location Information

> • | Gr: This is the interface to the HLR. It supports SIGTRAN (M3UA/SCTP/IP) over Ethernet.

> Some of the procedures supported by the SGSN on this interface are:

> • | Send Authentication Info

> • | Update Location

> • | Insert Subscriber Data

> • | Delete Subscriber Data

> • | Cancel Location

> • | Purge

> • | Reset

> • | Ready for SM Notification

> • | SIGTRAN based interfaces M3UA/SCTP

> • | Peer connectivity can be through an intermediate SGP or directly depending on whether the peer (HLR, EIR, SMSC, GMLC) is SIGTRAN enabled or not

> • | SCTP Multi-Homing supported to facilitate network resiliency

> • | M3UA operates in ASP & IPSP client/server and single/double-ended modes

> • | Multiple load shared M3UA ASP instances for high-performance and redundancy

> • | Works over Ethernet (IPoA) interface

> • | Ga: The SGSN uses the Ga interface with GTP Prime (GTPP) to communicate with the charging gateway (CG, also known as CGF) and/or the GTPP Storage Server (GSS). The interface transport layer is typically UDP over IP but can be configured as TCP over IP for:

> • | One or more Ga interfaces per system context, and

> • | An interface over Ethernet 10/100 or Ethernet 1000 interfaces

> The charging gateway handles buffering and pre-processing of billing records and the GSS provides storage for Charging Data Records (CDRs). For additional information regarding SGSN charging, refer to the Charging section.

> • | Gd: This is the interface between the SGSN and the SMS Gateway (SMS-GMSC / SMS-IWMSC) for both 2G and 3G technologies through multiple interface mediums. Implementation of the Gd interface requires purchase of an additional license.

> • | Gs: This is the interface used by the SGSN to communicate with the visitor location register (VLR) or mobile switching center (MSC) to support circuit switching (CS) paging initiated by the MSC. This interface uses Signaling Connection Control Part (SCCP) connectionless service and BSSAP+ application protocols.

> • | Gf: Interface is used by the SGSN to communicate with the equipment identity register (EIR) which keeps a listing of UE (specifically mobile phones) being monitored. The SGSN’s Gf interface implementation supports functions such as:

> • | International Mobile Equipment Identifier-Software Version (IMEI-SV) retrieval

> • | IMEI-SV status confirmation

> Features and Functionality - Basic
> The 2.5G and 3G SGSNs support a broad range of features and functionality - all fully compliant with 3GPP standards. The following is a list of some of the basic features supported by the SGSN:

> • | All-IP Network (AIPN)

> • | SS7 Support

> • | PDP Context Support

> • | Mobility Management

> • | Location Management

> • | Multiple PLMN Support

> • | IntraInter SGSN Serving Radio Network Subsystem (RNS) Relocation (3G only)

> • | Equivalent PLMN

> • | Network Sharing

> • | Session Management

> • | Charging

> • | Overcharging Protection

> • | NPU FastPath

> • | Operator Policy

> • | Default APN

> • | VLR Pooling via the Gs Interface

> • | HSPA Fallback

> • | Local QoS Capping

> • | Tracking Usage of GEA Encryption Algorithms

> All-IP Network (AIPN)
> AIPN provides enhanced performance, future-proof scaling and reduction of inter-connectivity complexity.
> In accordance with 3GPP, the SGSN provides IP-based transport on all RAN and core network interfaces, in addition to the standard IP-based interfaces (Ga, Gn, Gp, Iu-Data). The all-IP functionality is key to facilitating Iu and Gb Flex (SGSN pooling) functionality as well as evolution to the next generation technology requirements.
> SS7 Support
> The ASR 5000 SGSN implements SS7 functionality to communicate with the various SS7 network elements, such as HLRs and VLRs.
> The SGSN employs standard SS7 addressing (point codes) and global title translation. SS7 feature support includes:

> • | Transport layer support includes:

> • | Broadband SS7 (MTP3B/SSCF/SSCOP/AAL5)

> • | SIGTRAN (M3UA/SCTP/IP)

> • | SS7 variants supported:

> • | ITU-T (International Telecommunication Union - Telecommunications - Europe)

> • | ANSI (American National Standards Institute - U.S.)

> • | B-ICI (B-ISDN Inter-Carrier Interface)

> • | China

> • | TTC (Telecommunication Technology Committee - Japan)

> • | NTT (Japan)

> • | SS7 protocol stack components supported:

> • | MTP2

> • | MTP3

> • | SCCP with BSSAP+ and RANAP

> • | ISUP

> • | TCAP and MAP

> PDP Context Support
> Support for subscriber primary and secondary Packet Data Protocol (PDP) contexts in compliance with 3GPP standards ensure complete end-to-end GPRS connectivity.
> The SGSN supports a total of 11 PDP contexts per subscriber. Of the 11 PDP context, all can be primaries, or 1 primary and 10 secondaries or any combination of primary and secondary. Note that there must be at least one primary PDP context in order for secondaries to establish.
> PDP context processing supports the following types and functions:

> • | Types: IPv4, IPv6, and/or PPP

> • | GTPP accounting support

> • | PDP context timers

> • | Quality of Service (QoS)

> Mobility Management
> The SGSN supports mobility management (MM) in compliance with applicable 3GPP standards and procedures to deliver the full range of services to the mobile device. Some of the procedures are highlighted below:
> GPRS Attach
> The SGSN is designed to accommodate a very high rate of simultaneous attaches. The actual attach rate depends on the latencies introduced by the network and scaling of peers. In order to optimize the entire signaling chain, the SGSN eliminates or minimizes bottlenecks caused by large scale control signaling. For this purpose, the SGSN implements features such as an in-memory data-VLR and SuperCharger. Both IMSI and P-TMSI based attaches are supported.
> The SGSN provides the following mechanisms to control MN attaches:

> • | Attached Idle Timeout - When enabled, if an MN has not attempted to setup a PDP context since attaching, this timer forces the MN to detach with a cause indicating that the MN need not re-attach. This timer is particularly useful for reducing the number of attached subscribers, especially those that automatically attach at power-on.

> • | Detach Prohibit - When enabled, this mechanism disables the Attached Idle Timeout functionality for selected MNs which aggressively re-attach when detached by the network.

> • | Prohibit Reattach Timer - When enabled, this timer mechanism prevents MNs, that were detached due to inactivity, from re-attaching for a configured period of time. Such MNs are remembered by the in-memory data-VLR until the record needs to be purged.

> • | Attach Rate Throttle - It is unlikely that the SGSN would become a bottleneck because of the SGSN’s high signaling rates. However, other nodes in the network may not scale commensurately. To provide network overload protection, the SGSN provides a mechanism to control the number of attaches occurring through it on a per second basis.

> Beside configuring the rate, it is possible to configure the action to be taken when the overload limit is reached. See the network-overload-protection command in the “Global Configuration Mode” chapter in the Command Line Interface Reference. Note, this is a soft control and the actual attach rate may not match exactly the configured value depending on the load conditions.
> GPRS Detach
> The SGSN is designed to accommodate a very high rate of simultaneous detaches. However, the actual detach rate is dependent on the latencies introduced by the network and scaling of peers. A GPRS detach results in the deactivation of all established PDP contexts.
> There are a variety of detaches defined in the standards and the SGSN supports the following detaches:

> • | MN Initiated Detach - The MN requests to be detached.

> • | SGSN Initiated Detach - The SGSN requests the MN to detach due to expiry of a timer or due to administrative action.

> • | HLR Initiated Detach - The detach initiated by the receipt of a cancel location from the HLR.

> Mass detaches triggered by administrative commands are paced in order to avoid flooding the network and peer nodes with control traffic.
> Paging
> CS-Paging is initiated by a peer node - such as the MSC - when there is data to be sent to an idle or unavailable UE. CS-paging requires the Gs interface. This type of paging is intended to trigger a service request from the UE. If necessary, the SGSN can use PS-Paging to notify the UE to switch channels. Once the UE reaches the connected state, the data is forwarded to it.
> Paging frequency can be controlled by configuring a paging-timer.
> Service Request
> The Service Request procedure is used by the MN in the PMM Idle state to establish a secure connection to the SGSN as well as request resource reservation for active contexts.
> The SGSN allows configuration of the following restrictions:

> • | Prohibition of services

> • | Enforce identity check

> • | PLMN restriction

> • | Roaming restrictions

> Authentication
> The SGSN authenticates the subscriber via the authentication procedure. This procedure is invoked on attaches, PDP activations, inter-SGSN routing Area Updates (RAUs), and optionally on configurable periodic RAUs. The procedure requires the SGSN to retrieve authentication quintets/triplets from the HLR (AuC) and issuing an authentication and ciphering request to the MN. The SGSN implements an in-memory data-VLR functionality to pre-fetch and store authentication vectors from the HLR. This decreases latency of the control procedures.
> Additional configuration at the SGSN allows for the following:

> • | Enforcing ciphering

> • | Retrieval of the IMEI-SV

> P-TMSI Reallocation
> The SGSN supports standard Packet-Temporary Mobile Identity (P-TMSI) Reallocation procedures to provide identity confidentiality for the subscriber.
> The SGSN can be configured to allow or prohibit P-TMSI reallocation on the following events:

> • | Routing Area Updates

> • | Attaches

> • | Detaches

> • | Service Requests

> The SGSN reallocates P-TMSI only when necessary.
> Identity Request
> This procedure is used to retrieve IMSI and IMEI-SV from the MN. The SGSN executes this procedure only when the MN does not provide the IMSI and the MM context for the subscriber is not present in the SGSN’s data-VLR.
> Location Management
> The SGSN’s 3GPP compliance for location management ensures efficient call handling for mobile users.
> The SGSN supports routing area updates (RAU) for location management. The SGSN implements standards based support for:

> • | Periodic RAUs

> • | Intra-SGSN RAUs

> • | Inter-SGSN RAUs.

> The design of the SGSN allows for very high scalability of RAUs. In addition, the high capacity of the SGSN and Flex functionality provides a great opportunity to convert high impact Inter-SGSN RAUs to lower impact Intra-SGSN RAUs. The SGSN provides functionality to enforce the following RAU restrictions:

> • | Prohibition of GPRS services

> • | Enforce identity request

> • | Enforce IMEI check

> • | PLMN restriction

> • | Roaming restrictions

> The SGSN also provides functionality to optionally supply the following information to the MN:

> • | P-TMSI Signature and Allocated P-TMSI

> • | List of received N-PDU numbers for loss less relocation

> • | Negotiated READY timer value

> • | Equivalent PLMNs

> • | PDP context status

> • | Network features supported

> Multiple PLMN Support
> With this feature, the 2.5G and 3G SGSNs now support more than one PLMN ID per SGSN. Multiple PLMN support facilitates MS handover from one PLMN to another PLMN.
> Multiple PLMN supportalso means an operator can 'hire out' their infrastructure to other operators who may wish to use their own PLMN IDs. As well, multiple PLMN support enables an operator to assign more than one PLMN ID to a cell-site or an operator can assign each cell-site a single PLMN ID in a multi-cell network (typically, there are no more than 3 or 4 PLMN IDs in a single network).
> This feature is enabled by configuring, within a single context, multiple instances of either an IuPS service for a single 3G SGSN service or multiple GPRS services for a 2.G SGSN. Each IuPS service or GPRS service is configured with a unique PLMN ID. Each of the SGSN and/or GPRS services must use the same MAP, SGTPU and GS services so these only need to be defined one-time per context.
> Intra/Inter SGSN Serving Radio Network Subsystem (RNS) Relocation (3G only)
> Implemented according to 3GPP standard, the SGSN supports both inter- and intra-SGSN RNS relocation (SRNS) to enable handover of an MS from one RNC to another RNC.
> The relocation feature is triggered by subscribers (MS/UE) moving from one RNS to another. If the originating RNS and destination RNS are connected to the same SGSN but are in different routing areas, the behavior triggers an intra-SGSN Routing Area Update (RAU). If the RNS are connected to different SGSNs, the relocation is followed by an inter-SGSN RAU. This feature is configured through the Operator Policy Configuration Mode.
> Equivalent PLMN
> This feature is useful when an operator deploys both GPRS & UMTS access in the same radio area and each radio system broadcasts different PLMN codes. It is also useful when operators have different PLMN codes in different geographical areas, and the operators’ networks in the various geographical areas need to be treated as a single HPLMN.
> This feature allows the operator to consider multiple PLMN codes for a single subscriber belonging to a single home PLMN (HPLMN). This feature also allows operators to share infrastructure and it enables a UE with a subscription with one operator to access the network of another operator.
> Network Sharing
> In accordance with 3GPP TS 23.251, the SGSN provides an operator the ability to share the RAN and/or the core network with other operators. Depending upon the resources to be shared, there are 2 network sharing modes of operation: the Gateway Core Network (GWCN) and the Multi-Operator Core Network (MOCN).
> Benefits of Network Sharing
> Network sharing provides operators with a range of logistical and operational benefits:

> • | Enables two or more network operators to share expensive common network infrastructure.

> • | A single operator with multiple MCC-MNC Ids can utilize a single physical access infrastructure and provide a single HPLMN view to the UEs.

> • | Facilitates implementation of MVNOs.

> GWCN Configuration
> With a gateway core network configuration, the complete radio access network and part of the core network are shared (for example, MSC/SGSN) among different operators, while each operator maintains its own separate network nodes (for example, GGSN/HLR).
> GWCN-type Network Sharing
> With the GWCN configuration, the SGSN supports two scenarios:

> • | GWCN with non-supporting UE

> • | GWCN with supporting UE

> MOCN Configuration
> In the multi-operator core network configuration, the complete radio network is shared among different operators, while each operators maintains its own separate core network.
> MOCN-type Network Sharing
> With the MOCN configuration, the SGSN supports the following scenarios:

> • | MOCN with non-supporting UE

> • | MOCN with supporting UE

> Implementation
> To facilitate network sharing, the SGSN implements the following key features:

> • | Multiple virtual SGSN services in a single physical node.

> • | Sharing operators can implement independent policies, such as roaming agreements.

> • | Equivalent PLMN configuration.

> • | RNC identity configuration allows RNC-ID + MCC-MNC instead of just RNC-ID.

> Configuration for network sharing is accomplished by defining:

> • | NRI in the SGSN service configuration mode

> • | PLMN IDs and RNC IDs in the IuPS configuration mode

> • | IMSI ranges in the Operator Policy configuration mode

> For commands and information on network sharing configuration, refer to the Service Configuration Procedures section in the SGSN Administration Guide and the command details in the Command Line Interface Reference.
> Session Management
> Session management ensures proper PDP context setup and handling.
> For session management, the SGSN supports four 3GPP-compliant procedures for processing PDP contexts:

> • | Activation

> • | Modification

> • | Deactivation

> • | Preservation

> PDP Context Activation
> The PDP context activation procedure establishes a PDP context with the required QoS from the MN to the GGSN. These can be either primary or secondary contexts. The SGSN supports a minimum of 1 PDP primary context per attached subscriber, and up to a maximum of 11 PDP contexts per attached subscriber.
> The PDP context types supported are:

> • | PDP type IPv4

> • | PDP type IPv6

> • | PDP type PPP

> Both dynamic and static addresses for the PDP contexts are supported.
> The SGSN provides configuration to control the duration of active and inactive PDP contexts.
> When activating a PDP context the SGSN can establish the GTP-U data plane from the RNC through the SGSN to the GGSN or directly between the RNC and the GGSN (one tunnel).
> The SGSN is capable of interrogating the DNS infrastructure to resolve the specified APN to the appropriate GGSN. The SGSN also provides default and override configuration of QoS and APN.
> PDP Context Modification
> This procedure is used to update the MN and the GGSN. The SGSN is capable of initiating the context modification or negotiating a PDP context modification initiated by either the MN or the GGSN.
> PDP Context Deactivation
> This procedure is used to deactivate PDP contexts. The procedure can be initiated by the MN or the SGSN. The SGSN provides configurable timers to initiate PDP deactivation of idle contexts as well as active contexts.
> PDP Context Preservation
> The SGSN provides this functionality to facilitate efficient radio resource utilization. This functionality comes into play on the following triggers:

> • | RAB (Radio Access Bearer) Release Request

> This is issued by the RAN to request the release of RABs associated with specific PDP contexts. The SGSN responds with a RAB assignment request, waits for the RAB assignment response and marks the RAB as having been released. The retention of the PDP contexts is controlled by configuration at the SGSN. If the PDP contexts are retained the SGSN is capable of receiving downlink packets on them.

> • | Iu Release Request

> The RAN issues an Iu release request to release all RABs of an MN and the Iu connection. The retention of the PDP contexts is controlled by configuration at the SGSN. When PDP contexts are retained the SGSN is capable of receiving downlink packets on them.
> When PDP contexts are preserved, the RABs can be restored on a service request from the MN without having to go through the PDP context establishment process again. The service request is issued by the MN either when it has some data to send or in response to a paging request, on downlink data, from the SGSN.
> Charging
> To provide efficient and accurate billing for calls and SMS passing through the SGSN, the system:

> • | allows the configuration of multiple CGFs and GSSs and their relative priorities.

> • | implements the standardized Ga interface based on GTPP over UDP and all relevant charging information as defined in 3GPP TS.32.251 v 7.2.0.

> SGSN Call Detail Records (S-CDRs)
> These charging records are generated for PDP contexts established by the SGSN. They contain attributes as defined in TS 32.251 v7.2.0.
> Mobility Call Detail Records (M-CDRs)
> These charging records are generated by the SGSN’s mobility management (MM) component and correspond to the mobility states. They contain attributes as defined in 3GPP TS 32.251 v7.2.0.
> Short Message Service CDRs
> SGSN supports following CDRs for SMS related charging:

> • | SMS-Mobile Originated CDRs (SMS-MO-CDRs)

> • | SMS Mobile Terminated CDRs (SMS-MT-CDRs)

> These charging records are generated by the SGSN’s Short Message Service component. They contain attributes as defined in 3GPP TS 32.215 v5.9.0.
> Overcharging Protection
> In releases 9.0 and higher, Overcharging Protection enables the SGSN to avoid overcharging the subscriber if/when a loss of radio coverage (LORC) occurs.
> When a mobile is streaming or downloading files from external sources (for example, via a background or interactive traffic class) and the mobile goes out of radio coverage, the GGSN is unaware of such loss of connectivity and continues to forward the downlink packets to the SGSN.
> Previously, upon loss of radio coverage (LORC), the SGSN did not perform the UPC procedure to set QoS to 0kbps, as it does when the traffic class is either streaming or conversational. Therefore, when the SGSN did a Paging Request, if the mobile did not respond the SGSN would simply drop the packets without notifying the GGSN; the G-CDR would have increased counts but the S-CDR would not, causing overcharges when operators charged the subscribers based on the G-CDR.
> Now operators can accommodate this situation, they can configure the SGSN to set QoS to 0kbps upon detecting the loss of radio coverage. The overcharging protection feature relies upon a proprietary private extension to GTP LORC Intimation IE messages. This LORC Intimation IE is included in UPCQ, DPCQ, DPCR and SGSN Context Response GTP messages. One of the functions of these messages, notify the GGSN to prevent overcharging.
> The following table summarizes the SGSN's actions when radio coverage is lost or regained and LORC overcharging protection is enabled.
> Overcharging Protection - SGSN Actions

> Condition | Triggered by | SGSN Action | LORC Intimation IE - private extension payload
> Loss of radio coverage (LORC) | RNC sends Iu release request with cause code matching configured value | Send UPCQ to GGSN Start counting unsent packets/bytes Stop forwarding packets in downlink direction | No payload
> Mobile regains coverage in same SGSN area | MS/SGSN | Send UPCQ to GGSN Stop counting unsent packets/bytes Stop discarding downlink packets | New LORC state and unsent packet/byte counts
> Mobile regains coverage in different SGSN area | MS/SGSN | Send SGSN Context Response message to new SGSN Stop counting unsent packets/bytes | Unsent packet/byte counts
> PDP deactivated during LORC | MS/SGSN | Send DPCQ to GGSN Stop counting unsent packets/bytes | Unsent packet/byte counts
> PDP deactivated during LORC | GGSN | Send DPCR to GGSN Stop counting unsent packets/bytes | Unsent packet/byte counts

> ‌
> Refer to the SGSN APN Policy Configuration Mode chapter of the Command Line Interface Reference for the command to configure the GTPC private extension and refer to the IuPS Service Configuratioin Mode chapter of the Command Line Interface Reference to configure the LORC Cause IE.
> NPU FastPath
> NPU FastPath’s proprietary internal direct tunnel optimizes resource usage and reduces latency when processing GTP-U packets. This proprietary feature is only available on the ASR 5000 SGSN.
> Incoming traffic passes through the switch fabric and the routing headers are changed to re-route traffic from the incoming network processing unit (NPU) of the ingress PSC directly to the outgoing NPU of the egress PSC. This means that intervening NPUs and CPUs are by-passed. This provides the SGSN with router-like latency and increased node signaling capacity.
> SGSN NPU FastPath
> FastPath is established when both ends of a tunnel are available. Two FastPath flows are established, one for the uplink and one for the downlink direction for a given PDP context. FastPath will temporarily go down or be disengaged so that packets temporarily do not move through FastPath when either an Intra-SGSN RAU or an Iu-Connection Release occurs.
> If FastPath cannot be established, the NPU forwards the GTP-U packets to a CPU for processing and they are processed like all other packets.
> FastPath can not be established for subscriber PDP sessions if:

> • | Traffic Policing & Shaping is enabled.

> • | Subscriber Monitoring is enabled.

> • | Lawful Intercept (LI) is enabled,

> • | IP Source Violation Checks are enabled.

> • | GTP-v0 tunnel is established with an GGSN.

> For NPU fast path configuration, refer to Enabling NPU Fast Path for GTP-U Processing section of “Service Configuration Procedures” chapter of SGSN Administration Guide.
> Operator Policy
> The non-standard operator policy feature is unique to the ASR 5000 SGSN. This feature empowers the carrier with unusual and flexible control to manage functions that aren’t typically used in all applications and to determine the granularity of the implementation of any operator policy: to groups of incoming calls or to simply one single incoming call.
> What an Operator Policy Can Do
> An SGSN operator policy enables the operator to define a policy with rules governing the services, facilities and privileges available to subscribers depending on factors such as:

> • | roaming agreements between operators,

> • | subscription restrictions for visiting or roaming subscribers, and/or

> • | provisioning of defaults to over-ride standard behavior.

> These policies can override standard behaviors and provide mechanisms for an operator to circumvent the limitations of other infrastructure elements such as DNS servers and HLRs. By configuring an operator policy, the operator fine-tunes any desired restrictions or limitations needed to control call handling and this can be done for a group of callers within a defined IMSI range or per subscriber.
> For example, on APN resolution, DNS servers can be configured to return a list of IP addresses of GGSNs. However, this only allows the implementation of an equal-weight round-robin scheme for distribution of load. The operator policy configuration can provide finer control.
> In another example, it is not unusual for a blanket configuration to be implemented for all subscriber profiles stored in the HLR. This results in a waste of resources, such as the allocation of the default highest QoS setting for all subscribers. The operator policy provides the opportunity to address such issues by allowing fine-tuning of certain aspects of profiles fetched from HLRs and if desired overwrite QoS settings received from HLR.
> How the Operator Policies Work
> The specific operator policy that is applied is selected on the basis of the subscribers IMSI at attach time, and optionally the PLMN ID selected by the subscriber or the RAN node’s PLMN ID.
> The following flow diagram maps out the logic for applying operator policies.
> Logic Diagram for Policy Selection
> Unique, non-overlapping, IMSI + PLMN-ID ranges create call filters that are used to distinguish between the configured operator-policies. These filtering ranges are defined using the mcc command documented in the “SGSN Operator Policy Configuration Mode” chapter of the Command Line Interface Reference
> The system supports up to 1000 operator policies, including the operator policy named default. All operator policies must be configured by the user to define limitations to be applied but for the default policy there is no mcc-command defined IMSI range filter to determine implementation - the default policy applies to any IMSIs that are not covered by any other defined operator policy.
> Some Configurable Features for Operator Policies
> The following is a list of some of the features and functions that can be controlled via configuration of SGSN Operator Policies:

> • | Operator Determined Barring (ODB) - similar in function to roaming restrictions, but applied on a per service basis, such as SMS.

> • | Roaming Restrictions - control subscriber’s access to the network based on policy configuration. The policies can be retrieved from the HLR or locally from the SGSN.

> • | SuperCharger - helps to reduce MM signaling associated with inter-SGSN location updates.

> • | Network Sharing

> • | Authentication

> • | Equivalent PLMNs

> • | Extended APN configuration for charging characteristics and QoS control

> Default APN
> Operators can configure a “default APN” for subscribers not provisioned in the HLR. This feature is available in releases 8.1 and higher.
> The Default APN feature will be used in error situations when the SGSN cannot select a valid APN via the normal APN selection process. Within an operator policy, a default APN can be configured for the SGSN to:

> • | override a requested APN when the HLR does not have the requested APN in the subscription profile.

> • | provide a viable APN if APN selection fails because there was no "requested APN" and wildcard subscription was not an option.

> In either of these instances, the SGSN can provide the default APN as an alternate behavior to ensure that PDP context activation is successful.
> Refer to the SGSN Operator Policy Configuration Mode in the Command Line Interface Reference for the command to configure this feature.
> VLR Pooling via the Gs Interface
> VLR Pooling, also known as Gs Pooling, helps to reduce call delays and call dropping, when the MS/UE is in motion, by routing a service request to a core network (CN) node with availbale resources.
> VLR pools are configured in the Gs Service, which supports the Gs interface configuration for communication with VLRs and MSCs.
> A pool area is a geographical area within which an MS/UE can roam without the need to change the serving CN node. A pool area is served by one or more CN nodes in parallel. All the cells, controlled by an RNC or a BSC belong to the same one (or more) pool area(s).
> VLR hash is used when a pool of VLRs is serving a particular LAC (or list of LACs). The selection of VLR from this pool is based on the IMSI digits. From the IMSI, the SGSN derives a hash value (V) using the algorithm: [(IMSI div 10) modulo 1000]. Every hash value (V) from the range 0 to 999 corresponds to a single MSC/VLR node. Typically many values of (V) may point to the same MSC/VLR node.
> For commands and information for VLR pooling configuration, refer to the “Gs Service Configuration Mode” chapter in the Command Line Interface Reference and the VLR Pooling in Service Configuration Procedure section in the SGSN Administration Guide.
> HSPA Fallback
> Besides enabling configurable support for either 3GPP Release 6 (HSPA) and 3GPP Release 7 (HSPA+) to match whatever the RNCs support, this feature enables configurable control of data rates on a per RNC basis. This means that operators can allow subscribers to roam in and out of coverages areas with different QoS levels.
> The SGSN can now limit data rates (via QoS) on a per-RNC basis. Some RNCs support HSPA rates (up to 16 Mbps in the downlink and 8 Mbps in the uplink) and cannot support higher data rates - such as those enabled by HSPA+ (theoretically, up to 256 Mbps both downlink and uplink). Being able to specify the QoS individually for each RNC makes it possible for operators to allow their subscribers to move in-and-out of coverage areas with different QoS levels, such as those based on 3GPP Release 6 (HSPA) and 3GPP Release 7 (HSPA+).
> For example, when a PDP context established from an RNC with 21 Mbps is handed off to an RNC supporting only 16 Mbps, the end-to-end QoS will be re-negotiated to 16 Mbps. Note that an MS/UE may choose to drop the PDP context during the QoS renegotiation to a lower value.
> This data rate management per RNC functionality is enabled, in the radio network controller (RNC) configuration mode, by specifying the type of 3GPP release specific compliance, either release 7 for HSPA+ rates or pre-release 7 for HSPA rates. For configuration details, refer to the RNC Configuration Mode chapter in the Command Line Interface Reference.
> Local QoS Capping
> The operator can configure a cap or limit for the QoS bit rate.
> The SGSN can now be configured to cap the QoS bit rate parameter when the subscribed QoS provided by the HLR is lower than the locally configured value.
> Depending upon the keywords included in the command, the SGSN can:

> • | take the QoS parameter configuration from the HLR configuration.

> • | take the QoS parameter configuration from the local settings for use in the APN policy.

> • | during session establishment, apply the lower of either the HLR subscription or the locally configured values.

> Refer to the SGSN APN Policy Configuration Mode chapter of the Command Line Interface Reference for the qos command.
> Tracking Usage of GEA Encryption Algorithms
> GPRS encryption algorithm (GEA) significantly affects the SGSN processing capacity based on the GEAx level used - GEA1, GEA2, or GEA3.
> Operators would like to be able to identify the percentages of their customer base that are using the various GEA encryption algorithms. The same tool can also track the migration trend from GEA2 to GEA3 and allow an operator to forecast the need for additional SGSN capacity.
> New fields and counters have been added to the output generated by the show subscribers gprs-only|sgsn-only summary command. This new information enables the operator to track the number of subscribers capable of GEA0-GEO3 and to easily see the number of subscribers with negotiated GEAx levels.
> Features and Functionality - Enhanced and Licensed
> Enhanced features add or expand the capabitilies of the SGSN beyond basic levels of operation. All of these features comply with relevant 3GPP specifications. All of these features require the purchase of an additional license to implement the functionality on the SGSN.
> The following is an alphabetical list of the enhanced features:

> • | Direct Tunnel

> • | Lawful Intercept

> • | QoS Traffic Policing per Subscriber

> • | Session Recovery

> • | SGSN Pooling and Iu-Flex Gb-Flex

> • | Short Message Service (SMS over Gd)

> 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 GGSN.
> 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 (e.g., 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 GGSN. This means a significant increase in control plane load on both the SGSN and GGSN components of the packet core. Hence, deployment requires highly scalable GGSNs since the volume and frequency of Update PDP Context messages to the GGSN will increase substantially. The system’s 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 SGSN Direct Tunnel Configuration chapter in the System Enhanced Feature Configuration Guide.
> Lawful Intercept
> The SGSN supports lawful interception (LI) of subscriber session information to provide telecommunication service providers (TSPs) with a mechanism to assist law enforcement agencies (LEAs) in the monitoring of suspicious individuals (referred to as targets) for potential criminal activity.
> How LI Works
> Law enforcement agencies (LEAs) provide one or more telecommunication service providers (TSPs) with court orders or warrants requesting the monitoring of a particular target. The targets are identified by information such as their mobile station Integrated Services Digital Network (MSISDN) number, or their International Mobile Subscriber Identification (IMSI) number, or their International Mobile Equipment Identity (IMEI-SV).
> Once the target has been identified, the SGSN serves as an access function (AF) and performs monitoring for either new PDP contexts (“camp-on”) or PDP contexts that are already in progress. While monitoring, the system intercepts and duplicates Content of Communication (CC) and/or Intercept Related Information (IRI) and forwards it to a Delivery Function (DF) over an extensible, proprietary interface.
> So, when a target establishes multiple, simultaneous PDP contexts, the system intercepts CC and IRI for each of them. The DF, in turn, delivers the intercepted content to one or more Collection Functions (CFs).
> Some commands for lawful intercept configuration and operations are described in the Command Line Interface Reference. For detailed information, please contact your account representative.
> QoS Traffic Policing per Subscriber
> Traffic policing enables the operator to configure and enforce bandwidth limitations on individual PDP contextsf for a particular traffic class.
> Traffic policing typically deals with eliminating bursts of traffic and managing traffic flows in order to comply with a traffic contract.
> The SGSN conforms to the DiffServ model for QoS by handling the 3GPP defined classes of traffic, QoS negotiation, DSCP marking, traffic policing, and support for HSDPA/HSUPA.
> QoS Classes
> The 3GPP QoS classes supported by the SGSN are:

> • | Conversational

> • | Streaming

> • | Interactive

> • | Background

> The SGSN is capable of translating between R99 and R97/98 QoS attributes.
> QoS Negotiation
> On PDP context activation, the SGSN calculates the QoS allowed, based upon:

> • | Subscribed QoS - This is a per-APN configuration, obtained from the HLR on an Attach. It specifies the highest QoS allowed to the subscriber for that APN.

> • | Configured QoS - The SGSN can be configured with default and highest QoS profiles in the Operator Policy configuration.

> • | MS requested QoS - The QoS requested by the UE on pdp-context activation.

> DSCP Marking
> The SGSN can perform DSCP marking of the GTP-U packets according to allowed-QoS to PHB mapping. The default mapping matches that of the UMTS to IP QoS mapping defined in 3GPP TS 29.208.
> Traffic Policing
> The SGSN can police uplink and downlink traffic according to predefined QoS negotiated limits fixed on the basis of individual contexts - either primary or secondary. The SGSN employs the Two Rate Three Color Marker (RFC2698) algorithm for traffic policing. The algorithm meters an IP packet stream and marks its packets either green, yellow, or red depending upon the following variables:

> • | PIR - Peak Information Rate (measured in bytes/second)

> • | CIR - Committed Information Rate (measured in bytes/second)

> • | PBS - Peak Burst Size (measured in bytes)

> • | CBS - Committed Burst Size (measured in bytes)

> The following figure depicts the working of the trTCM algorithm:
> trTCM Algorithm Logic for Traffic Policing
> For commands and information on traffic policing configuration, refer to the Traffic Policing and Shaping and Dynamic QoS Renegotiation chapter in the System Enhanced Feature Configuration Guide.
> Session Recovery
> Session recovery provides a seamless failover and reconstruction of subscriber session information in the event of a hardware or software fault that prevents a fully attached user session from having the PDP contexts removed or the attachments torn down.
> Session recovery is performed by mirroring key software processes (e.g., session manager and AAA manager) within the system. These mirrored processes remain in an idle state (in standby-mode) until they may be needed in the case of a software failure (e.g., a session manager task aborts). The system spawns new instances of “standby mode” session and AAA managers for each active control processor (CP) being used.
> As well, other key system-level software tasks, such as VPN manager, are performed on a physically separate packet processor card (PSC or PSC2) to ensure that a double software fault (e.g., session manager and VPN manager fail at the same time on the same card) cannot occur. The PSC used to host the VPN manager process is in active mode and is reserved by the operating system for this sole use when session recovery is enabled.
> The additional hardware resources required for session recovery include a standby System Management Card and a standby packet processor card (PSC/PSC2).
> There are two modes for Session Recovery.

> • | Task recovery mode: One or more session manager failures occur and are recovered without the need to use resources on a standby packet processor card. In this mode, recovery is performed by using the mirrored “standby-mode” session manager task(s) running on active packet processor cards. The “standby-mode” task is renamed, made active, and is then populated using information from other tasks such as AAA manager.

> • | Full packet processor card recovery mode: Used when a PSC/PSC2 hardware failure occurs, or when a packet processor card migration failure happens. In this mode, the standby packet processor card is made active and the “standby-mode” session manager and AAA manager tasks on the newly activated packet processor card perform session recovery.

> Session/Call state information is saved in the peer AAA manager task because each AAA manager and session manager task is paired together. These pairs are started on physically different packet processor cards to ensure task recovery.
> 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

> For more information on session recovery use and session recovery configuration, refer to the Session Recovery chapter in the System Enhanced Feature Configuration Guide.
> SGSN Pooling and Iu-Flex / Gb-Flex
> This implementation allows carriers to load balance sessions among pooled SGSNs, to improve reliability and efficiency of call handling, and to use Iu-Flex / Gb-Flex to provide carriers with deterministic failure recovery.
> The SGSN, with its high capacity, signaling performance, and peering capabilities, combined with its level of fault tolerance, delivers many of the benefits of Flex functionality even without deploying SGSN pooling.
> As defined by 3GPP TS 23.236, the SGSN implements Iu-Flex and Gb-Flex functionality to facilitate network sharing and to ensure SGSN pooling for 2.5G and 3G accesses as both separate pools and as dual-access pools.
> SGSN pooling enables the following:

> • | Eliminates the single point of failure between an RNC and an SGSN or between a BSS and an SGSN.

> • | Ensures geographical redundancy, as a pool can be distributed across sites.

> • | Minimizes subscriber impact during service, maintenance, or node additions or replacements.

> • | Increases overall capacity via load sharing across the SGSNs in a pool.

> • | Reduces the need/frequency for inter-SGSN RAUs. This substantially reduces signaling load and data transfer delays.

> • | Supports load redistribution with the SGSN offloading procedure.

> Short Message Service (SMS over Gd)
> The SGSN implements a configurable Short Message Service (SMS) to support sending and receiving text messages up to 140 octets in length. The SGSN handles multiple, simultaneous messages of both types: those sent from the MS/UE (SMS-MO: mobile originating) and those sent to the MS/UE (SMS-MT: mobile terminating). Short Message Service is disabled by default.
> After verifying a subscription for the PLMN’s SMS service, the SGSN connects with the SMSC (short message service center), via a Gd interface, to relay received messages (from a mobile) using MAP-MO-FORWARD-REQUESTs for store-and-forward.
> In the reverse, the SGSN awaits messages from the SMSC via MAP-MT-FORWARD-REQUESTs and checks the subscriber state before relaying them to the target MS/UE.
> The SGSN will employ both the Page procedure and MNRG (mobile not reachable for GPRS) flags in an attempt to deliver messages to subscribers that are absent.
> The SGSN supports

> • | charging for SMS messages, and

> • | lawful intercept of SMS messages

> .
> For information on configuring and managing the SMS, refer to the SMS Service Configuration Mode chapter in the Command Line Interface Reference.
> How the SGSN Works
> This section illustrates some of the GPRS mobility management (GMM) and session management (SM) procedures the SGSN implements as part of the call handling process. All SGSN call flows are compliant with those defined by 3GPP TS 23.060.
> First-Time GPRS Attach
> The following outlines the setup procedure for a UE that is making an initial attach.
> imple First-Time GPRS Attach
> This simple attach procedure can connect an MS via a BSS through the Gb interface (2.5G setup) or it can connect a UE via a UTRAN through the Iu interface in a 3G network with the following process:
> First-Time GPRS Attach Procedure

> Step | Description
> 1 | The MS/UE sends an Attach Request message to the SGSN. Included in the message is information, such as:
> • | Routing area and location area information
> • | Mobile network identity
> • | Attach type
> 2 | Authentication is mandatory if no MM context exists for the MS/UE:
> • | The SGSN gets a random value (RAND) from the HLR to use as a challenge to the MS/UE.
> • | The SGSN sends a Authentication Request message to the UE containing the random RAND.
> • | The MS/UE contains a SIM that contains a secret key (Ki) shared between it and the HLR called a Individual Subscriber Key. The UE uses an algorithm to process the RAND and Ki to get the session key (Kc) and the signed response (SRES).
> • | The MS/UE sends a Authentication Response to the SGSN containing the SRES.
> 3 | The SGSN updates location information for the MS/UE: a) The SGSN sends an Update Location message, to the HLR, containing the SGSN number, SGSN address, and IMSI. b) The HLR sends an Insert Subscriber Data message to the “new” SGSN. It contains subscriber information such as IMSI and GPRS subscription data. c) The “New” SGSN validates the MS/UE in new routing area: If invalid: The SGSN rejects the Attach Request with the appropriate cause code. If valid: The SGSN creates a new MM context for the MS/UE and sends a Insert Subscriber Data Ack back to the HLR. d) The HLR sends a Update Location Ack to the SGSN after it successfully clears the old MM context and creates new one
> 4 | The SGSN sends an Attach Accept message to the MS/UE containing the P-TMSI (included if it is new), VLR TMSI, P-TMSI Signature, and Radio Priority SMS. At this point the GPRS Attach is complete and the SGSN begins generating M-CDRs.

> ‌
> If the MS/UE initiates a second call, the procedure is more complex and involves information exchanges and validations between “old” and “new” SGSNs and “old” and “new” MSC/VLRs. The details of this combined GPRS/IMSI attach procedure can be found in 3GPP TS23.060.
> PDP Context Activation Procedures
> The following figure provides a high-level view of the PDP Context Activation procedure performed by the SGSN to establish PDP contexts for the MS with a BSS-Gb interface connection or a UE with a UTRAN-Iu interface connection.
> Call Flow for PDP Context Activation
> The following table provides detailed explanations for each step indicated in the figure above.
> PDP Context Activation Procedure

> Step | Description
> 1 | The MS/UE sends a PDP Activation Request message to the SGSN containing an Access Point Name (APN).
> 2 | The SGSN sends a DNS query to resolve the APN provided by the MS/UE to a GGSN address. The DNS server provides a response containing the IP address of a GGSN.
> 3 | The SGSN sends a Create PDP Context Request message to the GGSN containing the information needed to authenticate the subscriber and establish a PDP context.
> 4 | If required, the GGSN performs authentication of the subscriber.
> 5 | If the MS/UE requires an IP address, the GGSN may allocate one dynamically via DHCP.
> 6 | The GGSN sends a Create PDP Context Response message back to the SGSN containing the IP Address assigned to the MS/UE.
> 7 | The SGSN sends a Activate PDP Context Accept message to the MS/UE along with the IP Address. Upon PDP Context Activation, the SGSN begins generating S-CDRs. The S-CDRs are updated periodically based on Charging Characteristics and trigger conditions. A GTP-U tunnel is now established and the MS/UE can send and receive data.

> ‌
> Network-Initiated PDP Context Activation Process
> In some cases, the GGSN receives information that requires it to request the MS/UE to activate a PDP context. The network, or the GGSN in this case, is not actually initiating the PDP context activation -- it is requesting the MS/UE to activate the PDP context in the following procedure:
> Network-Initiated PDP Context Activation
> The table below provides details describing the steps indicated in the graphic above.
> Network Invites MS/UE to Activate PDP Context

> Step | Description
> 1 | The GGSN receives a PDU with a static PDP address that the GGSN ‘knows’ is for an MS/UE in its PLMN.
> 2 | The GGSN uses the IMSI in place of the PDP address and sends an SRI (send routing information for GPRS) to the HLR. The HLR sends an SRI response back to the GGSN. The response may include the access of the target SGSN and it may also indicate it the MS/UE is not reachable, in which case it will include the reason in the response message.
> 3 | The GGSN sends a PDU Notification Request to the SGSN (if the address was received). If the address was not received or if the MS/UE continues to be unreachable, the GGSN sets a flag marking that the MS/UE was unreachable. The notified SGSN sends a PDU Notification Response to the GGSN.
> 4 | The SGSN determines the MS/UE’s location and sets up a NAS connection with the MS/UE. The SGSN then sends a Request PDP Context Activation message to the MS/UE.
> 5 | If the MS/UE accepts the invitation to setup a PDP context, the MS/UE then begins the PDP context activation process indicated in the preceding procedure.

> ‌
> MS-Initiated Detach Procedure
> This process is initiated by the MS/UE for a range of reasons and results in the MS/UE becoming inactive as far as the network is concerned.
> MS-Initiated Combined GPRS/IMSI Detach
> The following table provides details for the activity involved in each step noted in the diagram above.
> MS-Initiated Combined GPRS/IMSI Detach Procedure

> Step | Description
> 1 | The UE sends a Detach Request message to the SGSN containing the Detach Type, P-TMSI, P-TMSI Signature, and Switch off indicator (i.e. if UE is detaching because of a power off).
> 2 | The SGSN sends Delete PDP Context Request message to the GGSN containing the TEID. The GGSN sends a Delete PDP Context Response back to the SGSN. The SGSN stops generating S-CDR info at the end of the PDP context.
> 3 | The SGSN sends a IMSI Detach Indication message to the MSC/VLR.
> 4 | The SGSN sends a GPRS Detach Indication message to the MSC/VLR. The SGSN stops generating M-CDR upon GPRS Detach.
> 5 | If the detach is not due to a UE switch off, the SGSN sends a Detach Accept message to the UE.
> 6 | Since the UE GPRS Detached, the SGSN releases the Packet Switched Signaling Connection.

> ‌
> Supported Standards
> The SGSN services comply with the following standards for GPRS/UMTS wireless data services.
> IETF Requests for Comments (RFCs)

> • | RFC-1034, Domain Names - Concepts and Facilities, November 1987; 3GPP TS 24.008 v7.8.0 (2007-06)

> • | RFC-1035, Domain Names - Implementation and Specification, November 1987; 3GPP TS 23.003 v7.4.0 (2007-06)

> • | RFC-2960, Stream Control Transmission Protocol (SCTP), October 2000; 3GPP TS 29.202 v6.0.0 (2004-12)

> • | RFC-3332, MTP3 User Adaptation Layer (M3UA), September 2002; 3GPP TS 29.202 v6.0.0 (2004-12)

> • | RFC-4187, Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA), January 2006

> • | RFC-4666, signaling System 7 (SS7) Message Transfer Part 3 (MTP3) - User Adaptation Layer (M3UA), September 2006; 3GPP TS 29.202 v6.0.0 (2004-12)

> 3GPP Standards
> Release 6 and higher is supported for all specifications unless otherwise noted.

> • | 3GPP TS 22.041 v8.1.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Operator Determined Barring (ODB) (Release 8)

> • | 3GPP TS 23.060 v7.4.0 (2007-03), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; General Packet Radio Service (GPRS); Service description; Stage 2

> • | 3GPP TS 23.107 v7.0.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Quality of Service (QoS) concept and architecture

> • | 3GPP TS 23.236 v7.0.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes (Release 7)

> • | 3GPP TS 23.251 v7.0.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Network Sharing; Architecture and functional description

> • | 3GPP TS 24.008 v6.16.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobile radio interface Layer 3 specification; Core network protocols; Stage 3; some features support v7.8.0 (2007-06) and v7.12.0 (2007-06)

> • | 3GPP TS 25.410 v6.5.0 (2006-03) and v7.0.0 (2006-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iu Interface: general aspects and principles

> • | 3GPP TS 25.411 v7.0.0 (2006-03) and (2007-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iu interface layer 1

> • | 3GPP TS 25.412 v7.1.0 (2006-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iu interface signaling transport

> • | 3GPP TS 25.413 v6.14.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iu interface RANAP signaling; some features support v7.6.0 (2007-06)

> • | 3GPP TS 25.414 v7.1.0 (2006-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iu interface data transport & transport signaling

> • | 3GPP TS 25.415 v6.3.0 (2006-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iu interface user plane protocols

> • | 3GPP TS 29.002 v6.15.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobile Application Part (MAP) specification

> • | 3GPP TS 29.016 v6.0.0 (2004-12), 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Serving GPRS Support Node SGSN - Visitors Location Register (VLR); Gs Interface Network Service Specification

> • | 3GPP TS 29.018 v6.5.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; General Packet Radio Service (GPRS); Serving GPRS Support Node (SGSN) - Visitors Location Register (VLR) Gs interface layer 3 specification

> • | 3GPP TS 29.060 v6.17.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface

> • | 3GPP TS 29.202 v8.0.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Core Network; SS7 signaling Transport in Core Network; Stage 3

> • | 3GPP TS 32.215 v5.9.0 (2007-10), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Telecommunication management; Charging management; Charging data description for the Packet Switched (PS) domain

> • | 3GPP TS 32.251 v7.4.0 (2007-10), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Telecommunication management; Charging management; Packet Switched (PS) domain charging

> • | 3GPP TS 32.298 v7.4.0 (2007-10), 3rd Generation Partnership Project; Technical Specification Group Service and System Aspects; Telecommunication management; Charging management; Charging Data Record (CDR) parameter description

> • | 3GPP TS 33.102 v6.5.0 (2005-12), Technical Specification 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects;3G Security; Security architecture

> • | 3GPP TS 33.107 v6.4.0 (2004-12), 3rdGeneration Partnership Project; Technical Specification Group Services and System Aspects; 3G security; Lawful interception architecture and functions

> • | 3GPP TS 44.064 v7.1.0 (2007-03), 3rd Generation Partnership Project; Technical Specification Group Core Network; Mobile Station - Serving GPRS Support Node (MS-SGSN); Logical Link Control (LLC) layer specification

> • | 3GPP TS 48.014 v7.3.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group GSM EDGE Radio Access Network; General Packet Radio Service (GPRS); Base Station System (BSS) - Serving GPRS Support Node (SGSN) interface; Gb Interface

> • | 3GPP TS 48.016 v7.3.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group GSM EDGE Radio Access Network; General Packet Radio Service (GPRS); Base Station System (BSS) - Serving GPRS Support Node (SGSN) interface; Network Service

> • | 3GPP TS 48.018 v7.10.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Base Station System (BSS) - Serving GPRS Support Node (SGSN); BSS GPRS Protocol (BSSGP)

> • | Appendix 1: SGSN-TRS_QoS-3GPP Standards

> ITU Standards

> • | Q711; 3GPP TS 29.002 v6.15.0 (2007-12), 3GPP TS 29.016 v7.0.0 (2007-08), and 3GPP TS 25.410 v7.0.0 (2006-03)

> • | Q712; 3GPP TS 29.002 v6.15.0 (2007-12), 3GPP TS 29.016 v7.0.0 (2007-08), and 3GPP TS 25.410 v7.0.0 (2006-03)

> • | Q713; 3GPP TS 29.002 v6.15.0 (2007-12), 3GPP TS 29.016 v7.0.0 (2007-08), and 3GPP TS 25.410 v7.0.0 (2006-03)

> • | Q714; 3GPP TS 29.002 v6.15.0 (2007-12), 3GPP TS 29.016 v7.0.0 (2007-08), and 3GPP TS 25.410 v7.0.0 (2006-03)

> • | Q715; 3GPP TS 29.002 v6.15.0 (2007-12), 3GPP TS 29.016 v7.0.0 (2007-08), and 3GPP TS 25.410 v7.0.0 (2006-03)

> • | Q716; 3GPP TS 29.002 v6.15.0 (2007-12), 3GPP TS 29.016 v7.0.0 (2007-08), and 3GPP TS 25.410 v7.0.0 (2006-03)

> • | Q771; 3GPP TS 29.002 v6.15.0 (2007-12)

> • | Q772; 3GPP TS 29.002 v6.15.0 (2007-12)

> • | Q773; 3GPP TS 29.002 v6.15.0 (2007-12)

> • | Q774; 3GPP TS 29.002 v6.15.0 (2007-12)

> • | Q775; 3GPP TS 29.002 v6.15.0 (2007-12)

> Object Management Group (OMG) Standards

> • | CORBA 2.6 Specification 01-09-35, Object Management Group

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