By Azimuth Systems
As the adage goes: "The good thing about standards is that there are so many of them." Does the world need yet another one?
Yes—to converge a few of the existing ones. Convergence of WiFi and cell technologies in a single handset will enable pervasive access to voice and data indoors and out with one standard, one network, and one device.
Most cell-phone manufacturers, including Nokia and Motorola, are introducing dual-mode handsets sporting cellular and WiFi interfaces. An April 2004 report from onworld.com forecasts 400 million WiFi-enabled handsets will ship in 2009. The prevalent standard for converged voice-data communications is UMA (Unlicensed Mobile Access).
UMA extends GSM/GPRS services to WiFi and other indoor wireless networks. It involves tunneling GSM/GPRS protocols through broadband IP networks, including unlicensed radio networks such as WiFi. Several cell-phone manufacturers and service providers developed the UMA specification (www.umatechnology.org). A device complying with UMA will select the best available network—cell or WiFi—automatically, typically choosing WiFi indoors and cellular outdoors.
Packetized voice transmission Voice over WiFi (VoWiFi) requires that a digitized voice signal be sent over a WiFi network in the form of data packets. Voice data packets must arrive at their destinations at regular time intervals (typically every 20 ms) to minimize distortion. The network infrastructure must keep packet loss, delay, and jitter (the delay variation from packet to packet that causes distortion in the reconstructed signal) within required limits.
As a voice signal travels through the WiFi infrastructure—access points (APs), Ethernet switches, routers, and gateways—packet loss, delay, and jitter can add up to reduce the signal quality, especially when the network gets congested with traffic. Several VoWiFi protocols optimize packet loss, delay, and jitter performance as the data signal travels through the handsets and the infrastructure:
- Quality of Service (QoS) protocol (defined in IEEE 802.11e) prioritizes forwarding of voice traffic,
- Admission Control protocol (802.11e) keeps the number of simultaneous active calls manageable,
- Fast Roaming protocol (802.11r) minimizes bursty packet loss as the moving handset switches from AP to AP,
- Intelligent security protocols such as pre-authentication (802.11i) reduce roaming time by enabling a handset to authenticate with neighboring APs before roaming, and
- Radio Resource Management protocol (802.11k) enables a handset to make fast roaming decisions through pre-discovery of all the neighboring APs, their distances, and call capacity.
VoWiFi test methodology
In VoWiFi testing, a manufacturer must verify proper protocol behavior of WiFi handsets and infrastructure and must also measure delay, jitter, and packet-loss performance of these products in a controlled and repeatable manner. The ITU-T (the United Nations telecommunications organization that publishes standards for voice quality) G.107 standard provides guidance on the overall limits for delay (500 ms) and packet loss rate (20%) through the Internet infrastructure, including APs, switches, and routers. Generally acceptable limits for single AP networks are latency less than 50 ms, jitter less than 5 ms, roaming time less than 50 ms, and packet loss rate less than 1%.
Another important factor determining the usability of a VoWiFi handset is the power-consumption profile of the WLAN subsystem, which must be optimized for all operating modes to provide for adequate handset battery life. The periodic nature of voice traffic, coupled with the need to maintain association with the WLAN network at all times to receive incoming voice calls, requires rapid transition between various power-saving modes and makes the power optimization of WLAN handsets particularly challenging.
The VoWiFi tests can be organized into infrastructure tests, handset tests, and system tests. Infrastructure tests measure WiFi Alliance compliance, packet loss, delay, jitter, protocol conformance, and call capacity in the presence of background load. This last requires computation of MOS (Mean Opinion Score of voice quality defined in ITU-P.800) on voice streams.
Handset tests measure roaming performance, range vs. MOS, protocol conformance, WiFi Alliance compliance, and power consumption. System tests monitor the behavior and performance of multi-AP systems with multiple handsets roaming and with controlled background traffic.
Qualifying WiFi handsets and infrastructure requires an approach that overcomes the challenges of open-air measurements. Performing open-air measurements while WiFi handsets are in motion can be physically difficult, time consuming, and error prone. One approach to WiFi test involves placing handsets and APs under test in shielded and filtered mini-isolation chambers (Ref. 1), which provide filtered Ethernet, serial, and RF connections to the DUTs inside. To exercise the full dynamic range of 802.11 radios, the cabled RF environment must provide isolation of at least 110 dB among devices in the test setup.
Wireless devices placed in these chambers connect into a network of programmable RF attenuators, combiners, and switches that allow for automated emulation of device motion relative to other clients and APs installed in the system. Devices can be "moved" and precisely "positioned" under software control to allow test scripts to exercise the new voice protocols and others while the DUTs are subjected to controlled motion, traffic, and channel conditions.
Infrastructure tests
The infrastructure tests should measure the ability of APs, switches, and other infrastructure devices to forward and prioritize voice traffic in the presence of background data traffic. One example of an infrastructure test is a measurement of voice quality as a function of call load and background traffic. The test configuration (Figure 1) includes two WiFi client emulators that can emulate traffic from multiple WiFi PCs or handsets. The client emulators must support the IEEE 802.11 QoS protocols so calls can be prioritized over the background load.
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| Figure 1. (a) An AP call-capacity test increments the number of calls going through the infrastructure in a controlled way and simultaneously varies the background traffic load. (b) It measures delay, jitter, and packet loss on each active call and computes the MOS voice quality metric for each call. |
While sending traffic, the client emulators can measure forwarding rate, packet loss, delay, and jitter on the packet streams going through the infrastructure under test. To measure call capacity, one emulator emulates multiple voice clients, each generating a voice packet stream; another emulator emulates background traffic from conventional PC clients or other handsets.
Handset tests
The important tests for a handset are the measurement of voice quality as a function of path loss and the measurement of roaming time. Roaming performance is a serious issue for VoWiFi because in WiFi networks, where the AP densities can be as high as one AP per 6 ft2, roaming can occur every few seconds as the caller walks.
During a roam, a WiFi handset discontinues communicating through the source AP and begins communicating through the destination AP. This process causes bursts of lost packets and is highly detrimental to voice quality. The IEEE and the WiFi Alliance are discussing a 50-ms limit on roaming time.
The roaming test measures the roaming time and analyzes the roaming behavior of the handset. The handset connects between two APs via programmable attenuators (Figure 2a), which vary so as to force the handset to roam in a controlled way from one AP to another. One set of attenuators is initially set to minimum and the other to maximum so the handset receives a strong signal from AP1 and associates with it, with AP2 initially being out of the handset's range. The attenuators between AP1 and the handset gradually increase while the attenuators between the handset and AP2 gradually decrease, "moving" AP1 out of range and AP2 within range, thus forcing a roam.
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| Figure 2. (a) In a handset roaming speed test, a handset connects to two attenuators. (b) The roaming data collected during the test shows the duration of each phase of the roaming process. |
Roaming data collected simultaneously on source and destination channels during the test (Figure 2b) shows the duration of each phase of the roaming process. As the emulated motion progresses, the data rate of the handset falls (tTRANSITION) as the connection between the handset and AP1 deteriorates. The handset starts to scan for another AP (tSCAN). The measured time it takes for it to associate with AP2 is tASSOCIATE, and the time it takes to resume data transitions is tDATA. The roaming time (tROAM) equals the time between the last data transmission prior to roam and the first data transmission following the roam.
Range vs. quality
The operating range of a WiFi handset must be measured under controlled conditions while the attenuation between the handset and the AP is varied. The handset range test configuration (Figure 3a) is a subset of the roaming test configuration. It involves varying the attenuation between a handset and an AP while measuring voice quality as a function of path loss. The range test configures the handset to receive a voice stream as the attenuators between the handset and the AP step through the dynamic range of the handset. A data monitor collects packet loss, delay, and jitter statistics and uses the ITU-T G.107 standard to compute the R-Factor. The R-Factor is then converted to MOS and plotted vs. path loss (Figure 3b).
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| Figure 3. (a) A handset range test involves varying attenuation between the handset and the AP while (b) measuring voice quality as a function of path loss. |
The success of the WiFi-cell convergence trend depends on how well voice works over the WiFi data networks. QoS, fast roaming, power optimization, and other demanding requirements of the WiFi voice application have prompted the IEEE to start defining a number of new 802.11 protocols. Typically, the early implementations of complex technologies such as VoWiFi do not work as expected. Savvy IT managers and service providers would not deploy new services without thorough and systematic test. Ensuring reliability of budding technologies such as VoWiFi through test is the only way to ensure their success.
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