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Perform smart verification of GAN-enabled phones

Posted: 19 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:IP access network? UMA/GAN system? wireless network? unlicensed mobile access network? generic access network?

By Jamie Allan and John Russell
Agilent Technologies U.K. Ltd

The generic access network (GAN), originally called unlicensed mobile access (UMA) network, is a system that enables mobile phones to make seamless handovers between a cellular network (such as GERAN or GSM/Edge radio access network) and an Internet Protocol (IP) access network (such as WLAN) while carrying voice, data or both. A GAN system allows users of mobile phones to enjoy the benefits of a fixed broadband network.

The GAN system architecture and protocols are documented in 3rd Generation Partnership Project (3GPP) technical specifications 43.318 and 44.318. A mobile phone that implements GAN must be shown to conform to these standards before it is released to the market. Prior to this demonstration, however, mobile phone developers have several opportunities to test their GAN implementations against the standards. Early verification of a phone's ability to handle data and voice improves the probability of success during conformance testing.

Even for a relatively mature technology such as GAN, there are opportunities for designers to differentiate their product through the performance provided. WLAN technology in mobile phones is already extensively used, both for VoIP and as a data modem. One of the key factors influencing GAN usage will be the seamless handover between an IP and a cellular network, which standalone WLAN solutions cannot provide.

Network components
The GAN system adds an architectural component!the GAN Controller or GANC!to the GERAN/UTRAN network. The GANC is equivalent in function to a base station controller (BSC) in a typical GERAN network. Unlike a BSC, the front-end of the GANC connects to an IP access network and communicates with the phone over this interface (known as the Up interface) using GAN-specific protocols. To allow signaling and user data to pass between the phone and core network, the GANC is responsible for converting Up interface messaging into existing BSC/core network interface protocols (BSSAP/BSSGP in the case of GSM).

To receive GAN service, mobile phones must be dual-mode, with the ability to detect when a user is roaming in and out of a Bluetooth or Wi-Fi network so that the phone can switch to and from GERAN/UTRAN mode. Moreover, the phones are required to set up an IP Security (IPSec)-based virtual private network (VPN) tunnel to the serving GANC via an intermediate GAN-enabled security gateway (SEGW). Data, voice and signaling traffic essential to mobile services are appropriately protected and transported over this tunnel.

Connection setup
In a GAN deployment, several GANCs are used to divide responsibility for GAN services and provide load balancing. Each GANC in the network provides services for at least one of these logical entities: Provisioning GANC (P-GANC), Default GANC (D-GANC) or Serving GANC (S-GANC).

A mobile subscriber with a GAN-enabled phone can make use of GAN only when the phone is within range of an unlicensed wireless network to which it has permission to connect. When the phone first attempts to make a connection, it needs to identify the D-GANC. The phone thus initiates a discovery procedure to receive information about the D-GANC for use in the registration procedure.

To derive the address of the D-GANC and its associated SEGW, the phone connects to a P-GANC in the phone's Home Public Land Mobile Network (HPLMN) via the P-GANC's associated SEGW. The addresses of the P-GANC and its associated SEGW may be supplied on a phone that has been pre-provisioned with Fully Qualified Domain Name (FQDNs) or IP addresses; otherwise, the phone can derive the FQDNs based on information in the (U)SIM. Addressing requirements are covered in 3GPP TS 23.0035.

Next, the phone establishes a secure tunnel to the SEGW of the D-GANC and attempts to register with the D-GANC. The D-GANC may become the S-GANC for that connection by accepting the registration or it may redirect the phone to a different S-GANC. A phone may indefinitely maintain a registration to an S-GANC without actively being in GAN mode and while simultaneously using GERAN/UTRAN services to allow for timely speech and data handovers to GAN. Once the phone has been explicitly handed over to GAN or has switched to using GAN, the subscriber's current location information (stored in the core network) is updated by the phone and voice/data and signaling traffic can be routed to the phone via the GANC rather than the cellular network.

Security gateway
An important element of the GAN system is the SEGW, which provides a secure link between the mobile phone and the operator's network at the other end of the 'unsecured' IP access network being used to make the connection.

In a typical GAN deployment, an SEGW is responsible for setting up and maintaining secure (encrypted IPSec/IKEv2) connections between phones and the core mobile network on which the GANC is located. When a GAN connection is established, the security gateway must be authenticated by the phone through the use of signed public key security certificates. This ensures that subscribers are not 'lured' unwittingly onto other, nefarious networks when GAN is used over a public IP access network.

The SEGW also plays a role in authenticating subscribers for use of the network's GAN services by acting as an intermediary in communication between the phone and a connected accounting, authentication and authorization (AAA) server. The AAA server bases authentication decisions on the result of request/response challenges sent to the phone, which force both the phone and the network to prove their knowledge of shared, secret 'k' values stored on the subscriber SIM or USIM, and the network home location register to which the AAA server is connected. In addition to authenticating both the phone and the network, results from authentication algorithms are used to derive the keying material required at both ends to encrypt the IPSec connection between the phone and the SEGW.

The authentication protocols used between the phone and AAA server vary between 2G and 3G networks. The protocols used in both cases are based on existing 2G and 3G authentication technologies but strengthened greatly to provide increased protection on a more vulnerable IP access network. The Extensible Authentication Protocol for Subscriber Identity Module (EAP-SIM) is used for authentication in 2G networks while the Extensible Authentication Protocol Method for UMTS Authentication and Key Agreement (EAP-AKA) is used in 3G networks.

In test scenarios where GAN security features cannot be disabled in the phone, SEGW and AAA server components with the appropriate IPSec and EAP capabilities must be deployed in the test system to allow the phone to connect to and use GAN services.

Test considerations
Before attempting to verify the design parameters of a GAN phone, a design engineer must answer several questions. Which parameters must be tested for conformance to the specifications? What real-life usage scenarios can be tested to best determine the fitness of the phone for its purpose? Where in the design process is the optimal test point for minimizing rework and unnecessary retest?

Just as in a purely cellular network, the call setup, tear down and handover functions are likely to be the weak points in the system, introducing many opportunities for service problems and customer dissatisfaction. To thoroughly exercise these functions, a design engineer ideally will have test capability on the bench and will be located near other members of the design team. This facilitates efficient decision-making based on test results.

The GAN system introduces new signaling procedures and is an access stratum technology used by many existing upper layer functions. The 3GPP has specified a number of conformance tests to which GAN-enabled phones must comply. These are specified in 3GPP TS 51.010-1. While these test specifications provide a strong focus on the testing of new GAN specific procedures, they offer relatively little coverage of the use and end user experience of existing cellular technologies deployed over GAN. Many operating scenarios are thus candidates for testing including:

  • Success or failure of the GAN discovery procedures
  • Success or failure of the GAN registration procedures including register updates
  • Initial mobile station mode selection between GAN and GERAN/UTRAN operation
  • Mobile Originated (MO) and Mobile Terminated (MT) GAN-based speech calls
  • GAN-based data connections and the end user experience of applications making use of these underlying connections
  • GAN-based Dual Transfer Mode (DTM) or SS (UTRAN) connections
  • GSM-based MO and MT point-to-point SMS messaging over GAN

  • GPRS-based MO and MT point-to-point SMS messaging over GAN
  • All applicable to-and-from cell transition scenarios including Rove-in (G/U to GAN); Rove-out (GAN to G/U); Handover-in (G/U to GAN); Handover-out (GAN to G/U); Cell Change Order (G/U to GAN); Handover-in/out during DTM or SS connection (GAN to G/U and G/U to GAN).

Setup for GAN testing
Using a wireless communications test set such as the Agilent 8960, various scenarios for testing GAN functions can be emulated in a controlled, repeatable manner. Such testing is important across the development cycle of a GAN phone from the early design phase through system integration and verification, simulation of field testing, interoperability testing and conformance testing.

A setup for functional testing is shown in Figure 1. An application running on a PC is used to emulate the GANC.

Figure 1: An application running on a PC is used to emulate the GANC for functional testing.

In this setup, an IP network connection can be provided by any device capable of connecting to the phone and obtaining an IP address. In Figure 2 an ordinary, commercially available WLAN access point is used.

Figure 2: An ordinary, commercially available WLAN access point is used to provide n IP network connection in this example.

Other factors
In addition to verifying that a mobile phone performs in accordance with the GAN standards, the design engineer should consider testing in areas not covered by the standards but relevant to everyday use. The following three scenarios are examples of usage testing that are especially helpful in predicting real world phone performance.

Current drain analysis!Unlike cellular standards, WLAN standards do not give much consideration to the battery capacity of user devices, which can be voracious in their current consumption. A single-mode GSM phone, which runs continuously on standby, may still have 5x the battery life of a phone operating on WLAN. When talk time is calculated, the differences are even greater. Because a GAN-capable phone has to monitor the GERAN, UTRAN and GAN networks simultaneously, battery life may be a major design consideration.

A comprehensive analysis can be conducted to assess the impact of current drain at key usage points; for example, during packet data transfer (Figure 3). In this graph, the vertical axis quantifies time-related changes and the horizontal axis quantifies amplitude-related changes.

Figure 3: A comprehensive analysis can be conducted to assess the impact of current drain at key usage points, such as during packet data transfer.

Voice quality!Voice over GAN is similar to VoIP in that IP packets are the underlying transport used to carry the speech data. In GAN, the IP packets contain encapsulated, GSM-encoded voice information. Voice over GAN differs from a GERAN/UTRAN voice call mainly in the determination of codec selection, in which the GAN is determined by the percentage of lost packets rather than the perceived power levels or relative levels of carrier to interference.

No receiver sensitivity tests have been mandated thus far for the codec rate changes or any hysteresis associated with number of lost packets, yet these factors can impact the perceived performance of the phone because even the most subtle changes of the AMR codec may be detected by the listener. Voice quality testing makes a valuable contribution in assessing how well a GAN-enabled phone design overcomes these challenges (Figure 4).

Figure 4: Voice quality testing makes a valuable contribution in assessing how well a GAN-enabled phone design overcomes design challenges.

Data throughput testing!A third and basic staple of usage testing is evaluating end-to-end data throughput rates and uncovering any latency or delay that can degrade the user experience. Emulating a variety of usage scenarios is a must for design engineers, who need to measure end-to-end IP data rates on a WLAN connection and any changes that occur as the call is handed over!for example, to an GERAN serving cell!and back again.

This is important due to the order of magnitude increase in the theoretically achievable data rates when using a typical GAN system (such as WLAN) compared to the much slower data rates usually achieved via GERAN/UTRAN (Figure 5).

Figure 5: Evaluating end-to-end data throughput rates is important due to the order of magnitude increase in the theoretically achievable data rates when using a typical GAN system compared to GERAN/UTRAN.

In verifying a design implementation prior to conformance testing, several factors are considered. The list of test scenarios, while not exhaustive, provides a solid basis for understanding the performance of a phone. Performing these tests in a setup that emulates the GAN system allows an engineer to evaluate the impact of various decisions and thus minimize the number of iterations that may be required in a design cycle. This approach ultimately saves time, effort and cost.

Likewise, realistic usage testing allows the engineer to predict the likely customer perception of phone performance and thus the quality of the phone design. Usage testing is a valuable tool to add to the design engineer's toolkit and improves the prospects of a successful commercial launch.

Using a bench-top wireless communications test set provides a simple and cost-effective means of adding capability to evaluate new and emerging technologies such as UMA/GAN.

Gerry Fitzpatrick and Sandy Fraser contributed to the article.

About the authors
Jamie Allan
is a R&D software engineer and John Russell is the business development manager at the mobile broadband division of Agilent Technologies U.K. Ltd.





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