CES Test An Alternative Real-World Approach

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1 WHITE PAPER Test An Alternative Real-World Approach By Tommy Cook, CEO Calnex Solutions Upgrading the wireless backhaul network to exclusively Ethernet transport is a compelling strategy for Service Providers and Network Operators looking to increase revenues from both high-speed data services and voice traffic. With soaring bandwidth-use created by new mobile broadband services being offset by little growth in the ARPU (average revenue per user) of mobile customers, packet-based Ethernet offers operators an inexpensive solution to increasing available capacity while at the same time reducing the cost per bit for transporting traffic between base stations and the core network. Ethernet is scalable too, making it a cost-effective alternative for leased-line customers who require varying bandwidth as their business needs change. And, crucially, Ethernet can support -based voice traffic using circuit emulation services () capability. The ability to support -centric services is essential while existing voice traffic still counts for a significant portion of operator revenues. enables both synchronous and asynchronous frames to be packed and time-stamped into asynchronous Ethernet frames and transported across the packet infrastructure. The receiving equipment simply unpacks the frames while maintaining clock synchronisation. This transparency to the underlying traffic is the reaon why is the preferred solution for cellular backhaul applications. While holds the key to the migration of the current backhaul network into a unified all-packet transport architecture, operators remain cautious to capital investment or widespread deployment of this technology. There are two main issues: confidence in device operation and compliance to industry standards. In the first case, after the traffic is packetised by the forwarding Inter-working Function (), the nominal packet transfer rate will be exposed to timing issues in the form of jitter and, more significantly, wander as the packets travel over the packet network. This often renders the receiving 1 Calnex Solutions Ltd, 2008

2 straining to reassemble the traffic and its associated timing correctly. Various sources generate wander including the algorithms used to recover clocks; noise introduced by Ethernet switches and routers; queuing delays; policing functions; low frequency effects due to variations in the loading patterns of the packet network; and the architecture of the data connection. To ensure clock quality and the safe delivery of timing to base station applications, the performance of clock content of the signal that is delivered at the end of the transmission path must remain within the limits set by jitter and wander performance masks (ITU-T G.823 and G.824 for the 2.048M and 1.544M hierarchy systems respectively). In the second, standards are still evolving. Indeed a number of proposals, including network reference models and measurement guidelines relating to testing devices, have been recently added to ITU-T G This includes the following note added to the start of ITU-T G.8261 Appendix VI: Although test cases in this Appendix provide useful guidance on the performance of Ethernet based Circuit Emulation techniques, evaluation in complex network scenarios that mimic the deployment profile is strongly recommended. This note succinctly sums up the operator s dilemma: how can they be certain of device performance when there is no consensus on how to accurately characterise a packet network, or how to produce a set of packet delay variation (PDV) profiles that represent a worst-case set of network conditions. Put another way, as well as testing with the Reference Network described in the standard, operators want to test devices under their real-world operating conditions to be convinced of actual performance. Challenges using the Reference Network ITU-T G.8261 provides a Reference Network (Figure 1) that aims to help manufacturers of equipment reflect real-world traffic conditions during the design and production test phases. Its topology is designed to both generate and disrupt the packet arrival times of the packet flow to produce a stressful PDV profile to the equipment under test. Inevitably, there are concerns and debate within the industry as to its suitability. Not only is it a complex test scenario to reproduce (and likely to be very expensive to build in engineering time), but more importantly, loading the Ethernet switches with additional packet streams to create realistic PDV disruptions can be problematic. In essense, 2

3 the topology will produce differences in the generated PDV each time it is run because it doesn t allow for precise control of how the setup will affect the flow under test. This lack of stimulus repeatability can be a real thorn in the side as every design engineer will testify. It s no secret that the creation of a predictable and repeatable environment is critical to reliable and conclusive testing. Only then can you truly stress-test and characterise device operation. But if there is a design defect, or worse an intermittent fault, the last thing you want is a test system that serves up something different each time a test is run. Reference Timing Signal (PRC) Packet Delay Variation Jitter, Wander, Frequency accuracy CE( traffic generator) * The Reference Timing Signal (PRC) is used to represent the service clock signal Reference point Test Equipment FE or Reference point 2 (DUT) Test Equipment Reference point 3 signal Traffic Generator Disturbance load according to traffic models Flow of interest Ethernet Switches N = 10 = 1 Gbps Ethernet FE = 100 Mbps Ethernet Source: ITU-T G.8261 Draft May 07 Figure 1. ITU-T G.8261 Reference Network Using real network equipment in the test setup is not recommended either, especially when attempting to create stress-test scenarios. The reason is that well-designed network equipment will fundamentally not try to generate stressful conditions and in fact may go out of its way to avoid doing so. In addition, test equipment needs to be designed to deliver accurate timing performance in the same way as network equipment does. Nanosecond timing accuracy and performance can only be achieved by, for example, using specially designed hardware that disables the FIFOs on the input/output stages of the Ethernet interfaces to ensure that the timing of every path through the design is known and controlled. Ethernet ports on a PC simply can not be used for accurate timing stamping it is not within their capability, it is not a valid approach, and it can lead to inaccurate and misleading test results. 3

4 Finally, there are doubts about how representative the ITU-T G.8261 Reference Network is of a real-world network. The device under test may function correctly under the test setup conditions, but what happens if the device results in inferior network performance when deployed in a real network? Worse still, how do you prove otherwise? This scenario is likely to lead to an unwelcome and potentially damaging finger-pointing exercise and is a genuine concern for network operators. So, to avoid applying even more engineering resources, time and effort as a consequence of the above factors, how do you create test scenarios that represent real-world situations in a lab environment and still allow engineers to control that environment with a reliable and repeatable stimulus? Alternative test topology One way to achieve this goal is to replace the Reference Network with a test setup that can produce the same net impact on the PDV of the flow under test, and deliver the kind of repeatability that eliminates the need to constantly monitor for subtle differences in the PDV being produced by the Reference Network. A typical test setup is shown in Figure 2. source tester ~ Synchronisation source Figure 2. Alternative test approach 4

5 A key capability for such a test approach is the ability to capture and replay real-world PDV profiles. Even if a Reference Network has already been constructed, the ability to capture, save and replay the PDV rather than having to re-construct the trial setup significantly reduces the engineering effort required in repeat testing. Furthermore, a capture and replay facility removes the repeatability issue when using the Reference Network. This test approach allows engineers to easily identify corruption and disruption events, plus introduce packet corruption events, alarms, and systematic delays. It also provides the ability to edit a real-world PDV profile and merge it with a G.8261 effect such as a step-function before replay. Stress-testing in this manner not only proves that the device under evaluation is operating to G.8261 standards, but provides confidence that it will function as expected within the network. What s more, should a problem arise in post deployment, the ability to capture live traffic allows engineers to bring the problem flow back into their lab environment where it can be replayed and analysed to determine whether or not its PDV really is degrading the service. Ethernet Core Network Base Station Live or trial network Lab source tester ~ Figure 3. Capturing real-world PDVs from the network for replay in the lab 5

6 Embedded time-stamps While the test set-up shown in Figure 2 uses the Adaptive method of clock recovery that is, where the timing is recovered directly from the packet arrival time methods that utilise embedded timestamps to transfer timing are also under consideration. Once again, the test approach needs to be as adaptable; delivering the same level of controllability and repeatability to fully stress-test the device s sensitivity to both forward and return path impairments. The test setup shown in Figure 4 illustrates how PDVs can be synchronised to identify sensitivities in the system. source tester Figure 4. Testing timestamp-based systems Next steps in the migration to Ethernet With pressure on Service Providers and Network Operators to increase backhaul capacity and reduce the cost of transporting traffic between their base stations and the core network, it is clear that the industry is on a path to packetise the backhaul network. It is also clear that while implementing Circuit Emulation over Ethernet offers significant cost and capacity benefits, it will be an evolutionary migration rather than a revolutionary change to the network. Of course this is just the beginning. The ultimate goal is a completely packetbased network where, naturally, each incremental step will be taken only when it has been proven to continue to support the current service infrastructure. 6

7 While the advantages of are apparent, the need for comprehensive evaluation of devices is simple economics: network QoS must be sufficient to both minimise revenue leakage and meet customer expectations. Drawing on past SDH/Sonet experience where impairments created by pointer movements and mapping jitter encroached on the PDH network operators will need to be convinced that the latest devices on offer are in a state of real-world readiness when timing issues again play such a critical role. And it is likely that this trend will continue until the ITU-T G.8261 standard is truly representative of real network conditions. In the meantime, simplifying the G.8261 Reference Network as described above is the first step in creating a stable, repeatable test environment for design engineers and evaluation teams to assess device performance. The second step is to agree a range of PDV profiles that will accurately evaluate and qualify devices and reassure operators of the device s ability to work flawlessly in the real world. Based on that criterion, only then will Service Providers and Network Operators feel confident in investing their backhaul expenditure in network equipment. Tommy Cook is founder and CEO of Calnex Solutions Ltd, a UKbased company specialising in test solutions for the emerging Packet Transport and Carrier Ethernet network technologies. He has 23 years telecoms experience with Agilent Technologies/Hewlett-Packard where he was formerly R&D Engineering Manager and Business Team Leader. For more information on testing and the Calnex Paragon Test Solution, please visit or contact Calnex Paragon test solution Fully stress test the operation of components and network elements that perform the circuit emulation of streams into any packetbased network. Capture then replay real-world PDV profiles in a repeatable, controllable environment Quickly identify corruption and disruption events with the simple-to-use graphical interface Edit and change monitored PDV profiles before replay Introduce packet corruption events, lost packets, mis-ordered packets, and repeated packets Calnex Solutions Ltd. Telephone: +44 (0) info@calnexsol.com 7