Increased demand from consumers through cloud computing has helped drive more demand from Ethernet backplane servers, routers and switches. To help increase bandwidth requirements 10GBASE-KR and 40GBASE-KR4 have helped provide the additional bandwidth in the Ethernet backplane arena. The higher bandwidth, however, has created some challenges in interoperability between these devices. The result is a very precise physical layer developed from the IEEE (News - Alert) 802.3ap Ethernet Backplane specification. Oscilloscopes play a critical role in testing the physical-layer compliance testing. What are the challenges in selecting the right oscilloscope to ensure that the 10GBASE-KR specification is being met?
10GBASE-KR Technology Basics
10GBase-KR is backplane Ethernet. The full definition comes from 10G (speed), BASE (BASEband), K (bacKplane) and R (Random signaling). Backplanes are used in an Advanced Telecommunication Computing Architecture shelf. The ATCA shelf can hold a mixture blades consisting of servers, processors, and routers. The fabric backplane, as it is called, is the communication channel that all of these blades use to talk to one another.10GBASE-KR (or, KR for short) is one of these technologies that is supported in this backplane architecture. 40GBASE-KR4 is simply four lanes of 10GBASE-KR and is typically referred to as KR4 for short. Let’s focus now on the physical layer of this technology. 10GBASE-KR is a single differential channel running at 10.3125 GBd and uses 64B/66B encoding. However, there is a bit more going on underneath the hood. For example, KR is a closed eye specification. This means that if an engineer probes the signal at the receiver, the user will not see an open eye on the oscilloscope making the measurement. The equalizer inside of the receiver is what opens the eye. So transmitters and receivers must work around this using equalization and forward error correction techniques to recover the signal. It is therefore critical that the KR signal be compliant with the specification.
10GBASE-KR Compliance Application – Easy to setup and run
Oscilloscope vendors have engineers who translate the physical layer test specification into an automated application that runs on a Windows-based oscilloscope. For example, Agilent (News - Alert) offers a N8814A 10GBASE-KR compliance application that runs on all currently shipping 90000X/Q Infiniium oscilloscope families. The application takes the IEEE 802.3ap specification and puts it into an easy-to-use application that runs on an Infiniium oscilloscope. The application can be purchased initially with a new scope, or can be added to an existing Infiniium oscilloscope as a software upgrade.
The 10GBASE-KR compliance application runs all of the required tests to verify that the device is working properly, or a user can choose to run a subset of the tests as shown in figure 2. The results are shown as pass/fail and also show margins of how close the test passed or failed. Details will show the limits of the test, actual values as well as the name of the test. Multiple trials as well as different devices can be compared.
When appropriate the application will display a screenshot of the measurement. The report is generated using HTML format so that reports can be shared using Internet Explorer. This makes it easy to document or share testing results as no oscilloscope is required to view the report as shown in figure 3.
Here are some oscilloscope capabilities that are required and others that should be considered when choosing an oscilloscope for testing 10GBASE-KR standards. The oscilloscope will need to be one that runs Microsoft (News - Alert) Windows as compliance applications do not run on scopes with embedded operating systems. For example, the basic setup requirement is a 25gHz or faster Infiniium oscilloscope, the N8814A 10GBASE-KR compliance application, a differential probe, and typically a solder-in probe head. A starting configuration will run about $260,000.
If you are considering incorporating 10GBASE-KR in your next-generation Ethernet Backplane design, discuss your upcoming needs with your oscilloscope vendor. They can talk to you about what standards they support, what configuration is needed, and should be able to do a quick demo for you, or provide loaner equipment to evaluate.
Alex Bailes is product manager Ethernet and wireline applications for the oscilloscope and protocol division at Agilent Technologies (News - Alert) Inc. (www.agilent.com).
Edited by Stefania Viscusi