Today's increase in computing power, coupled with the ability to share applications and data, has driven the modern networking infrastructure to new levels of speed and sophistication. Having emerged as the leader in desktop networking, Fast Ethernet is able to bring much needed bandwidth to users, while maintaining the integrity of Ethernet.
Adopting physical-layer conventions from the fiber distributed data interface (FDDI), Fast Ethernet has leveraged existing technology into the framework of a carrier-sense multiple-access/collision-detection (CSMA/CD) network, preserving users' and administrators' knowledge of network operation. Coexisting with installed network devices, Fast Ethernet has become both legacy-compatible and capable of providing future migration to 100-Mbit/s connections as users upgrade their interconnect systems. Consequently, it's brought about a slew of testing and interoperability concerns.
The IEEE has provided the standards by which the physical-layer and physical-medium devices must perform. Mixing this with interoperability testing of existing devices provides a robust environment for determining the overall fitness of the system. Properly testing for forward migration, while maintaining backwards interoperability, shows that the network is evolving as a tool that maintains data integrity without compromise.
In addition to the IEEE standards, the University of New Hampshire Interoperability Lab, Durham, N.H., has formed a Fast Ethernet consortium. Members may have their products tested in a series of both IEEE and interoperability scenarios—testing that has become a necessity for any company developing Fast Ethernet systems. Let's review the basics of Fast Ethernet testing. Covering methodologies to real examples will provide an understanding of the challenges faced in meeting IEEE conformance.
The logical place to begin physical-layer testing is with the individual system, i.e., the network host or adapter. Testing is then done in a contained environment. Typical vehicles for testing Fast Ethernet physical-layer (PHY) and transceiver devices are media-access units (MAUs) or network interface circuits (NICs). The MAU is useful, because network test equipment frequently comes ready to test devices via a media-independent interface (MII) connection. Board-level testing should allow access to the silicon for measurement, as well as to the magnetic devices and RJ-45 connector. At the board level, other non-connection related issues may be addressed, such as power consumption, footprint, and external component count.
The basic equipment required for performing tests on Fast Ethernet physical layers includes an MII-based test platform, such as the Netcom Systems' X-1000 or Smartbits system. Testing also demands accurate voltage and current generators, as well as an oscilloscope, multimeter, and various types of cables and terminations.
The mechanism through which these multispeed devices configure the appropriate speed is auto-negotiation. It's actually a process through which the part signals the far-end station. Using 10-Mbit/s fast link pulses (FLPs), it then configures to a predetermined speed option or the highest common denominator. Auto-negotiation also lets devices distinguish between FLPs and normal link pulses (NLPs), so there's no confusing 10-Mbit/s data with the auto-negotiation process.
In addition, the part can sense "idles" in 100-Mbit/s systems. It therefore does not miss a non-auto-negotiation-capable 100-Mbit/s-only device on the other end. The end result is that a 10-Mbit/100-Mbit system with auto-negotiation should be capable of communicating with any other 10-Mbit/100-Mbit Ethernet device.
Robust physical-layer devices need to be able to operate with non-standard-compliant, as well as compliant, auto-negotiation devices. Early implementers of non-standard 10-Mbit/100-Mbit devices used a technique known as speed sensing to configure to the appropriate speed. With that technique, the host device first sends out 10-Mbit/s data and then 100-Mbit/s data alternately. That way, it can see how the far end responds. If available—many speed-sensing systems are still in place—testing should include the use of these devices. They definitely provide an added dimension of test data.
By default, it's become commonplace to perform certain tests on Fast Ethernet devices. Few people, however, understand the purpose of these tests and their relevance to the network. An example of this is the cable-length test.
It's normal to run PHY devices up to 130 m of cable. After all, the IEEE specification calls for 90 m of cable plus a patch, so 130 m guarantees this. And first-generation Fast Ethernet devices worked up to this distance, so people have been conditioned to test to that limit.
The EIA/TIA specification requires that all PHYs be able to receive a compliant signal through 90 m of CAT-5 cable, plus an additional 10 m of CAT-5 patch cable through at least four jacks in the setup. That test alone does not ensure that the part will function correctly at any other distance or temperature variations.
Every physical-layer evaluation should include tests from 0 m through at least 130 m of CAT-5 cable in 10-m increments. Some devices have deficiencies in the midrange cables. Unfortunately, these distances are common in many smaller offices. At any distance, the result should be correct auto-negotiation, a link, and an extremely low bit error rate. By understanding the test better, as well as what it exercises in the design, you will see it as a more powerful tool—not just a checklist requirement.
More often than not, there's some combination of improper and incomplete testing on these devices. Any situation in which a signal is somehow deficient has a negative effect on network performance. In the best case, that part of the network segment isn't capable of talking with the rest of the network, but doesn't interfere with its operation. If the situation is that non-standard format signals are sent across the network, other problems arise.