The Mobisense testbed was explicitly deployed to assess the user perceived quality of mobility in NGNs using VoIP as the case application. In order to do this, the Mobisense testbed should support experiments involving the collection of speech samples, the execution of network handovers and codec changeovers, gathering of network traces, and the modification of network conditions. The following functional requirements can be derived from these processes:
- The mobile terminal should have connectivity to at least two different wireless access technologies like WiFi, Flash-OFDM, WiMAX, UMTS/HSDPA or GPRS/ EDGE.
- The mobile terminal should be able to seamlessly hand off a connection between these networks, as it can be done with Mobile IPv4 or SIP (IETF RFC 3261, 2002).
- Network traces should be collected during the voice calls to allow further evaluation of the network conditions and codec changeovers.
- There should be the possibility to degrade the network conditions artificially, i.e., to increase delay, jitter, packet loss, and additional traffic to the network connection in use.
- It is desired to use a broadband sharing environment (using IEEE 802.11 technology).
- A VoIP call has to be established between the mobile terminal and a fixed host with a stable Internet connection to measure only the impact of the wireless path on audio quality.
- The audio stream of the VoIP call must be recorded for further analysis and for the listening and conversational tests.
- The VoIP client should support wideband and narrowband speech codecs for different transmission bandwidths.
- The VoIP client must be able to switch between different speech codecs during an ongoing call.
- A jitter buffer monitoring to track the codec frames in the VoIP application should be implemented.
Some of these requirements were realized using off-the-shelf hardware and software components. However, other parts were specially implemented for the Mobisense testbed. For example, the PJPROJECT client (PJSIP, n.a.) was extensively modified to cope with the demanded features such as codec changeover and jitter buffer monitoring.
Hardware and Software Components
The Mobisense testbed uses Mobile IPv4 as a solution to enable seamless handovers between different radio access technologies. Mobile IPv4 requires a Home Agent, a Mobile Node, and a Correspondent Node to build a working system. The access networks emulate an integrated NGN conformed by the following technologies: LAN, WiFi, Flash-OFDM, and UMTS/HSDPA. The CN and MN were deployed on laptops with Linux 2.6.18.2, and the HA is a Cisco 2620XM router with Cisco IOS 12.2(8r), supporting MIPv4.
Mobile IPv4 was deployed as the mobility management protocol because all involved access networks support IPv4. As Mobile IP client, the SecGo implementation (SecGo, n.a.) was chosen because it provides a telnet interface for remote control (Telnet, n.a.) and supports NAT traversal (IETF RFC 3519,2003). As the VoIP framework, the PJPROJECT was selected and extensive modifications have been made to fulfill the testbed requirements. The tcpdump (tcpdump, n.a.) and Wireshark (Wireshark, n.a.) tools are used for trace collection and evaluation, netem (netem, n.a.) is used to enable changing the network characteristics in terms of adding delays, packet loss, packet duplication, packet corruption, and packet re-ordering. Finally, a TCP-based client/server application has been implemented to centralize the overall control of the test settings. In addition, an UDP sender has been implemented to enable the remote control of the VoIP client.
Testbed Deployment
Figure 1 shows the Mobisense testbed network architecture and its hardware components. The Mobisense testbed supports connectivity to six networks as attachment points to the Internet, which are based on different (wireless) technologies: two LAN networks (one home and one foreign network), two foreign WLAN networks, one foreign UMTS/HSDPA network, and one foreign Flash-OFDM network. The home LAN network is directly connected to the Internet and constitutes the fixed attachment point for the HA and CN; the MN can roam between the five remaining networks. The MN could connect to the foreign LAN and can then roam to any of the two foreign WLAN networks, supplied by a DSL line as the backhaul. Moreover, the MN can log on to a public (foreign) UMTS/HSDPA network, as well as communicate with the Flash-OFDM RadioRouter that forwards packets to the Internet via the Flash-OFDM internal Home Agent. The Flash-OFDM access network is provided by the BIB3R testbed (Steuer et al., 2006) and it is based on Flarion RadioRouter version 1.1 (a testing license in the UMTS frequency band has been granted by the German regulator enabling operational measurements).
The Mobile Node may roam between LAN, WLAN, UMTS/HSDPA, and Flash-OFDM foreign networks. All of them apply IPv4 protocol stack. The mobility is served by the Mobile IPv4 protocol providing transparent mobility for overlying protocols. The deployed Mobile IPv4 infrastructure consists of a HA and a MN and does not involve additional Foreign Agents. Therefore, the MN individually obtains IP addresses and acts directly as one end of the Mobile IPv4 tunnel (reverse tunneling).
Arising from the complex setting described in this section, two further requirements were posed to the design: (1) the CN, acting as the counterpart of the VoIP call generated by the MN, has to be remotely controlled and (2) all software components should have an interface for remote management, connecting to the MN and CN. The central point of the architecture is a controlling script run on the MN that synchronizes the execution of all other software components. For example, the controlling script starts tcp dump to collect network trace s, telnet to communicate with the Mobile IP client, a UDP sender to control the PJPROJECT client, and the TCP client/server. The TCP client/server is employed twofold. At first, it runs the PJPROJECT client as a background process. Secondly, it controls the software installed on the Correspondent Node. Thus, the TCP client/ server is also integrated to the CN, where it starts tcpdump and the PJPROJECT client, and controls the UDP sender.
The Mobisense project aims to evaluate the effects of mobility on the perceived quality of real time services. Therefore, the testbed was designed to provide the following features:
- Access to different network technologies
- Ability to change the network connection (network handover)
- Possibility to artificially change network conditions
- Record network traces
- Share available bandwidth using DSL and WLAN technologies
- Perform VoIP calls over the networks
- Support for different voice codecs (e.g. narrowband and wideband)
- Possibility to change voice codecs (codec changeover)
- Ability to record voice samples
- Ability to record internal parameters of the VoIP application
With these features there is much potential to evaluate different effects and their impact on the quality of VoIP calls such as different network, different codecs, network handovers and codec changeovers. In order to evaluate the perceived quality, experiments with the above listed phenomena can be conducted. The resulting audio samples are recorded and then used to run auditory tests with human listeners. During these listening tests, people are asked to rate the quality of different samples. These results can be used as an input for quality prediction models. Beyond this, the network-centric effects of the network handovers and codec changeovers can be evaluated by means of the network trace recording ability of the testbed. Through its open nature, the testbed can be easily extended towards different directions such as the evaluation of multimedia applications.
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