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WiMAX Forum Certified… Why Do We Need to Test Protocol?

Mon, 08/25/2008 - 7:56am
Performing automated Procotol Conformance Testing (PCT) in-house is essential for vendors who are eager to minimize time-to-market.

By James Thomsit, Aeroflex Test Solutions, Wireless Division

Dropped calls, “network busy” messages and intermittent service — these are all potential symptoms of poor device interoperation. This type of behaviour is generally caused either because the testing for certification is not rigorous enough or, in its absence, because of insufficient interoperability testing before products are released to market. Although both handsets and base stations are developed according to defined specifications, there is often ambiguity that leaves them open to interpretation. In order to iron out any creases and to ensure that any differences do not cause problems visible to the end user, such testing is vital.

Users invariably think that such problems are caused by faults with the network, making it more difficult for operators to keep customers loyal and increasing churn. Even when such problems are detected in the certification lab and caught before they reach the end user, fault investigation, resolution and re-certification are expensive and can adversely impact time-to-market.?
Challenges and Benefits of Testing
If faults are not located before devices are released onto a live network, it is the operator who will suffer. The cost of goodwill to operators due to customers experiencing poor network coverage or dropped calls is massive. Customers are more likely to change their operator at the first available opportunity, rather than try a new handset with the same operator. Therefore, any differences in implementations and software bugs must be detected and rectified as early as possible in the development cycle and certainly before product release. To do this, companies require a test bed with sufficient coverage in their laboratories in order to optimize their testing strategy; thus, minimizing development costs and time-to-market. This is especially important due to the expense of field trials, drive testing and interoperability testing, and the limited coverage they consequently afford.


click to enlarge

Table 1: PCT Functional Groups
The WiMAX Forum has mandated interoperability testing for all devices sold under the WiMAX technology by making it part of the certification program. These tests must act as a final, pre-certification sanity check since it is very difficult to find the cause of a problem when the only symptom reported from a certification lab is potentially something like ‘a dropped connection’. In a similar manner to IOT tests, Protocol Conformance Testing (PCT) is a formal part of the mobile WiMAX certification process. PCT tests, in contrast though, are able to debug behavioural problems because they tackle the cause of a problem, such as an incorrect message as opposed to detecting a symptom like a dropped call.
WiMAX Protocol Conformance Testing
Passing protocol conformance tests verifies the behaviour of the device under test (DUT) with respect to actual messages that pass between the mobile/subscriber station (MS/SS) and the base station (BS). The messages are checked for their conformance to a specific version of the IEEE 802.16e technology standard and compliance with the WiMAX Forum’s system profile. Protocol conformance tests are able to check the format of the protocol messages as well as the positive and negative behaviour (timing, message sequence and system state) of the device under test (DUT).

Protocol certification tests are specified by test purposes with a brief text explanation of the test method and are written by ETSI (European
Figure 1. WiMAX PCT system
Technology Standardization Institute). The tests aim to cover every medium access control (MAC) message that will be used by either BS or MS/SS over the air (R1) interface. Due to practical constraints, such as time and expense, not every element of every message can be tested. The optimum subset is selected by the WiMAX Forum certification working group and will be run in the laboratories. This subset can be broken down into groups related by functionality (see Table 1.) A comprehensive list of test cases and their validation status is maintained by the WiMAX Forum on their members’ website.

A protocol certification test typically tests one protocol message, mechanism or system timer at any one time. This leads to a very fine granularity of testing and the need for a vast number of protocol conformance test cases. Performing automated PCT tests in-house is essential for vendors who are eager to minimize time-to-market. This is especially important in the early stages of mobile WiMAX certification as there could be a backlog of devices in the certification program. Any failures that are found during certification will result in the additional expense of debugging the protocol fault without specialized equipment, and a delay-to-market due to re-booking and re-certifying the device in one of the WiMAX Forum’s Designated Certification Laboratories.

Using specialized test equipment, such as the Aeroflex/AT4 wireless T2230/1 AIME Protocol Test System, it is possible to test above and beyond the mandated protocol
Figure 2: PCT equipment configuration
certification. This is a must for operators in order to deliver a quality service to the market; and they can increase their test coverage by writing their own test scripts. This model has been commonly observed in other technologies such as CDMA where, for example, KDDI has adopted the Aeroflex 6402 AIME test platform to perform all of their acceptance testing.

Laboratories such as the lead WFDCL in Malaga, Spain use the T2230/1 CT Protocol Certification Test System, developed on behalf of the WiMAX Forum, to actually perform the protocol tests with guaranteed behaviour. This is achieved due to extremely tight version control and rigorous adherence to validation and system releases. Customers are able to reproduce tests exactly as they have been performed in the WiMAX Forum validation labs against the test bed of ‘golden samples’. Device vendors are able to customize test equipment to perform any variety of certification and extendable protocol testing to ensure their devices pass certification the first time with minimal costs and delay to market.
How Tests are Performed
Mobile WiMAX Protocol Certification Tests (PCT), as mandated by the WiMAX Forum for certification, are based upon approved test platforms, such as the Aeroflex/AT4 wireless T2230/1, which emulates a BS (BSE – Basestation Emulator) to test the conformance of a MS/SS or emulate a MS (MSE – Mobile Station Emulator) to test BS conformance (see Figure 2.)

For example, when testing a MS/SS, the PCT acts as a BSE which appears to be a real, live base station attached to a real network. This allows the MS/SS to synchronize, range and attempt to connect. The PCT system interacts with the MS as if it were a real base station replying to all of the MS’ MAC management messages and connection attempts to set up the initial conditions specified in the test procedure. A document produced by ETSi and the WiMAX Forum™ defines the expected behaviour of the DUT for all of the test cases. When the PCT system reaches the message being tested, it verifies the expected behaviour by applying a template to the received messages and comparing them against specific values or ranges. If the received message is within a certain time frame, matches the template and values fall within the desired criteria, then that test will pass. In order to certify a device and be allowed to use the WiMAX brand, all validated Category-A (reference in the WiMAX Forum CRSL, certification requirements status list) test cases must pass in a WiMAX Forum Designated Certification Laboratory (WFDCL).

The two biggest challenges for PCT test platforms are feature support and rigorous adherence to the standard. Major features of Sprint’s Mobile WiMAX™, Xhom-branded, rollout are features such as HARQ, compressed MAPs and MIMO. In order for PCT test systems to reach a sufficient level to test the MS/SS or BS, these features must be fully supported. Even if the PCT and DUT interoperate successfully, the benefits of protocol certification rapidly decline if the test equipment does not strictly adhere to the standard, as this is the only testable reference implementation. The 802.16e standard explicitly states that for interoperation if an incorrect message is received, it should be discarded.

The Aeroflex/AT4 wireless T2230/1 AIME/CT is unique in its flexibility to optionally enable/disable that statement and to flag any incorrect messages, fields or values (TLVs) that are received from the MS/SS or BS under test. This is possible as it is not based upon a real device. The picoChip’s software physical layer ensures excellent IOT system compatibility, and the in-house developed MAC layer and MAC codec allows the test system to stick rigidly to the specified test procedure regardless of the IEEE 802.16e version that the device is being certified against.
Summary
Protocol Conformance Testing for WiMAX is vital for minimizing time-to-market for vendors by ensuring that any problems, such as differences in implementation and software bugs, are detected and rectified as early as possible in the development cycle.

James Thomsit is WiMAX PCT specialist, Aeroflex Test Solutions, Wireless Division. www.aeroflex.com. wireless@aeroflex.com.

Q: What role do macro base stations play in the deployment of WiMA X?

Macro Base Stations Provide a Quick Inservice Solution

Jim Orr, Principal Network Architect, Fujitsu Network Communications

Fujitsu believes that WiMAX will be an enabling technology for personal broadband wireless services across the world. With mobile WiMAX, service providers will be able to build their wireless broadband infrastructure at a lower cost and offer end-users service at more reasonable rates.

Macrocell base stations are designed for wide-area transmission over a radius of almost two miles, supporting several thousands of users with a rich set of features. The wireless service providers we spoke with said they wanted a mobile WiMAX macrocell base station that delivers superior performance that consumes less power and space than other available products while providing a full set of features and various redundancy options.

Such base stations are ideal for operators looking for a quick in-service solution, with easy installation and maintenance. Fujitsu designed its macro base station with many features to meet the global requirements for mobile WiMAX networks. The Fujitsu amplifier provides industry leading efficiency allowing for very tight packaging, excellent systems performance and a long life span. This advanced implementation reduces the costs associated with installing and operating the WiMAX network. The Fujitsu mobile WiMAX base station will provide fully certified mobile WiMAX macrocell functionality in a picocell package.

Because our BroadOne WX300 macrocell base station weighs approximately 40 pounds, a single person can install it on a tower, pole or building-top. This type of mobile WiMAX base station gives service providers the means to build the ubiquitous broadband wireless infrastructure that consumers have envisioned.

The Role of Macro Base Stations in WiMAX Deployment


Wolfgang Mack, Chief Marketing Officer, Telsima

WiMAX networks offer broadband connectivity to subscribers; mobile and stationary, indoor and outdoor. The challenge facing network planners is to provide high quality connections to subscriber devices enabling them to communicate at high modulation rates, thereby making efficient use of the available spectrum wherever they are in the network.

In order to overcome these challenges, the network planners must employ a mix of macro-, micro-, pico- base stations and femtocells to ensure network coverage and high frequency reuse. The fundamental requirement of the network is to connect subscribers with as high a modulation rate as possible and avoid low modulation rate connections as much as possible.

Macro-, micro- base stations are usually deployed in three sector configurations on towers and rooftops with high power amplifiers and high gain panel antennas. Their coverage area is typically that of one to five kilometers depending on the capacity and coverage requirements, and the RF clutter resulting from terrain elements such as hills, buildings and trees.

Pico base stations are defined by their small form factor and low power, typically covering an area between 100 meters and 500 meters. Their use in a WiMAX network increases the spectral reuse and the overall capacity of the network. Their small form factor and enclosed designs makes them easy to deploy for areas in need of coverage not sufficiently serviced by the macro base station sites.

Femtocells are inexpensive low power network access points, typically deployed indoors, connected to Ethernet or DSL backhaul. The use of these short range access points reduces the burden on the outdoor network by servicing indoor subscribers with high modulation rates who would otherwise, because of the path loss resulting from penetrating walls, communicate over low modulate rates if connected to the outdoor network. This increases the spectral efficiency and frequency reuse which are vital to a profitable business model.

Networking technologies are increasingly pushing functionality to the network edge; in the case of WiMAX networks, the base station element. This trend toward a distributed environment favors the use of inexpensive, small form factor access points that enable subscribers to communicate using the highest modulation rates and that offer the network operator the highest spectral reuse. The result is a network that makes the best use of the valuable frequency resource and provides the best return of the operator’s CAPEX and OPEX investments.

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