2.5G/3G Core Network Testing with G35: The Foundation of Packet Switched Connectivity and Reliable IMS Services Abstract With the advent of broadband mobile networks like UMTS This whitepaper introduces some of the key procedures and service platforms like IMS, the interaction between needed to establish sessions in the packet-switched them both is leading to entirely new test challenges. 2.5G/3G core network, as well as at the service layer. Ensuring superior end-user Quality of Experience (QoE) is In this document, we will use IMS sessions as specific adequately supported by the underlying UMTS core and example. Related test challenges and how they can be access network infrastructure is just one of these new test addressed by using Tektronix’ G35 protocol test platform challenges being faced by Network Equipment are then highlighted. Finally, the impact of new services Manufacturers and service providers alike. such as Push-to-Talk over Cellular (PoC) and Multimedia Broadcast and Multicast Services (MBMS) on the 2.5G/3G core network infrastructure are explained. White Paper I www.tektronix.com/G35 White Paper | 2.5G / 3G Core Network Testing with G35 Introduction The adoption of recently standardized UMTS R5 and R6 features like HSDPA and HSUPA is transforming mobile networks into real broadband communication systems. Declining voice revenues require carriers to seek out alternative sources of revenues by developing new services for their customers. The advent of IEEE 802.16e, commonly known as “Mobile WiMAX”, as alternative mobile broadband technology will reinforce this trend. With this new technology, competing service providers appear on the horizon, thereby enabling more and more end users to adopt and incorporate IP-based services into their daily lives. IMS and other Packet Data platforms give service providers the ability to deploy multiple services with higher QoE at reduced overall cost. Applications as diverse as Ringtone Downloads, Text Messaging, E-Mail, Picture Messaging and Information Services have shown to be equally popular. Migrating these type of services towards a common platform in order to optimize Operating Expenses related costs will be a strong driver for mobile (and fixed) operators to adopt IMS-based systems. White Paper I www.tektronix.com/G35 2 White Paper | 2.5G / 3G Core Network Testing with G35 Primary and Secondary PDP Context The one thing all these different applications have in common After sending the Activate PDP Context Request, a timer is that a certain Access Point is needed for their availability. will be started in the MS while waiting for the Activate PDP Access Points are IP routers that provide the connection Context Accept or Reject message. between the Mobile Station (MS) and the selected application. The MS must know the Access Point Name The SGSN validates the Activate PDP Context Request. (APN) in order to contact the desired Access Point via the The SGSN rejects the PDP context activation request if no GPRS subsystem. This is done through a PDP context GGSN address can be derived, if the APN cannot be activation procedure as depicted in Figure 1. resolved or if the SGSN has determined that the Activate PDP Context Request is not valid. The MS sends an Activate PDP Context Request message that consists of PDP Type, PDP Address, Access Point If the PDP context activation procedure is successful, a Name, Quality of Service (QoS) Requested and Protocol user plane tunnel between the MS and an external Packet Configuration Options to the SGSN. The SGSN uses the Data Network (PDP) will be established. It is important to APN, which has the format of a ‘Fully Qualified Domain note that this is a user plane tunnel from GPRS perspective Name’, to select a reference point (the GGSN) to the external only; from the viewpoint of the external PDN, it would be network. The APN is a logical name referring to the external possible to also carry control plane information such as Packet Data Network (PDN) that the subscriber wishes to SIP messages. connect to. The Domain Name Server function resolves the logical APN name to an IP address. This function is As stated above, the Access Point is the first contact point standard Internet functionality according to RFC 1034 [1]. for the external service platform. Examples of such services As a result, the SGSN finds a GGSN that is connected to platforms are: the desired external network. In the next step, the SGSN I Short Message Service Center (SMSC) initiates the Create PDP Context Procedure to this GGSN. I Multimedia Messaging Service Center (MMSC) I QoS Requested indicates the desired QoS profile. Protocol Wireless Application Protocol (WAP) I Configuration Options may be used to request optional IP Multimedia Subsystem (IMS) I PDP parameters from the GGSN. Those Protocol Internet Configuration Options are then sent transparently through the SGSN. Figure 1. PDP Context Activation Procedure (MS initiated) White Paper I www.tektronix.com/G35 3 White Paper | 2.5G / 3G Core Network Testing with G35 One GGSN may offer different services that can be The QoS of any active primary or secondary PDP context accessed through different APNs. From the viewpoint of can be modified by the MS or network using the PDP the external PDN, the usage of primary and secondary PDP Context Modification procedure [2]. Figure 3 depicts the contexts must be distinguished (see Figure 2). A primary negotiated QoS parameters simulated by the G35 protocol PDP context will always be established for the first tester. For detailed information concerning QoS parameters connection between an MS and a specific APN. Up to 10 used in UMTS networks, refer to [3; chapter 6.4.3: “UMTS secondary PDP contexts may then be established if further Bearer Service Attributes”]. connections with the same APN are needed. Usually, every IMS media stream will be routed independently over a The maximum number of supported PDP contexts depends separate tunnel. This is necessary because the GPRS on the MS’s capabilities. Typical test mobiles support up to charging function in Release 5 is not able to generate six PDP contexts. SGSN/GGSN nodes can support up to multiple charging records for one data stream. Additionally, eleven PDP contexts per MS. For thorough stress testing, different IMS user plane sessions may have different QoS the G35 simulates up to 6 million mobile subscribers requirements. whereas every simulated MS may have up to 11 PDP contexts activated. With a total capacity of more than 60 Thus, PDP contexts can be categorized as following: million simultaneous GPRS tunnels, the G35 can simulate mobility scenarios and subscriber load ranging from a small 1. Primary PDP contexts: They provide connections to rural area up to the largest metropolitan city. different APNs. The SGSN assigns a unique IP address to the MS for each primary PDP context. 2. Secondary PDP contexts: They provide connections to the same APN, but with different QoS. A secondary PDP context is always associated with one primary PDP context. Both IP addresses and Access Points are re-used from the primary PDP context. Figure 3. Quality of Service IE (Screenshot from G35 Protocol Simulator) Figure 2. PDP Context Types White Paper I www.tektronix.com/G35 4 White Paper | 2.5G / 3G Core Network Testing with G35 Traffic Flow Template (TFT) and Protocol Configuration Options (PCO) Since a PDP address (e.g. IP address) is common for the The size of the packet filter content field itself is variable primary and for all associated secondary PDP contexts, the and contains exactly one of the following filter types: questions arises as to how user plane content (SIP signaling, voice, video, etc.) can be mapped to the correct GPRS I IPv4 source address tunnel (see Figure 2). As the IP Address alone is not an I IPv6 source address adequate indicator, another information element needs to I Next header type be exchanged during the Secondary PDP Context Activation I Single destination port procedure, the Traffic Flow Template (TFT) [2; chapter I Destination port range 9.2.2.1.1]. The TFT is provided by the MS, stored in the I Single source port GGSN and is examined when downlink user plane data I Source port range needs to be delivered to the MS. The TFT can be modified I IPSec Security parameter index or deleted by using the MS-initiated PDP Context I Traffic Class / Type of Service Modification procedure. By means of the MS initiated PDP I Flow Label Context Modification procedure, a TFT may also be assigned to a primary PDP context [2; chapter 9.2.3.3]. More information about usage and structure of the TFT Information Element can be found in [2; chapter 15.3] and The TFT Information Element is an array of packet filters [4; chapter 10.5.6.12: “Traffic Flow Template”]. and is used by the GGSN to inject IP packets (SIP, RTP, MPEG, etc.) into the correct GPRS tunnel. Each packet The relationship between PDP contexts, TFTs and packet filter has a variable length and consists of the following filters is depicted in Figure 4. elements: I packet filter identifier I packet filter evaluation precedence I the length of packet filter content I the packet filter content itself The packet filter identifier is used to identify each packet filter inside the TFT array. The packet filter evaluation precedence specifies the precedence class among all packet filters associated with a given PDP address (IP address). Signaling may have higher precedence than user plane, and streaming services a higher precedence than background services. Figure 4. PDP contexts, TFTs and Packet Filters White Paper I www.tektronix.com/G35 5 White Paper | 2.5G / 3G Core Network Testing with G35 The Evaluation Precedence Index specifies the order in The structure of the PCO IE is defined in [4; chapter which packet filters are examined.