Troubleshooting Techniques for Fast Ethernet

This document will primarily discuss problems unique to Fast Ethernet.

The Collision Domain

The single biggest change in network design in Fast Ethernet is the smaller collision domain. Technically, the size of a collision domain in all flavors of Ethernet is exactly the same - 256 bits. On the wire, ten times as many 100Mbps bits can occupy the same space as an equal number of 10Mbps bits, so the collision domain in 100Mbps Ethernet can only be physically one tenth the size of a 10Mbps collision domain. What this effectively means is that, whereas up to four hubs can legally be cascaded in 10Base-T between any two stations, only one (or two) hubs can be used in a single segment in 100BASE-T without going through an interconnect device that provides link segmentation; such as a store-and-forward bridge, switch or bridge, or a router. A separate section of the Compendium discusses INTERCONNECT DEVICES in detail. If you see signs of corruption on your network that correspond to propagation delay, check to make sure that you're not cascading too many hubs. You can make some generalizations regarding the structure of corrupted data frames (as discussed in the 10 Mbps Ethernet FRAME CORRUPTION section) but remember that these corruption patterns may be quite misleading, since you have a hub or switch in the network.

Note that many hub vendors sell stackable hubs. Hubs in a single stack connected via a common backplane are usually considered to be a single hub in terms of propagation delay, but multiple stacks cascaded externally via 100BASE-TX, 100BASE-T4, or 100BASE-FX could definitely cause problems. These 100BASE standards are discussed in the INTRODUCTION to this Fast Ethernet section.

Incompatible Ethernet Jabber

Another potential problem in 100Mbps Ethernet is the use of RJ-45 jacks for more than one flavor of Ethernet. Since 100BASE-TX and 100BASE-T4 both use RJ-45 jacks, as do 10Base-T and many other network technologies, the IEEE 802.3 specified an auto-negotiation protocol to allow stations to figure out what networking technology to use. Unfortunately, they made its implementation optional. If you're using equipment that does not implement IEEE-spec auto-negotiation, the incompatible Ethernet signals could prevent one of your stations from connecting to your network, or even simulate "jabber" by constantly transmitting a TX idle stream and bringing down the network.

The possibility for this jabber is uncertain, considering that the flavors of Ethernet use different signal formats in transmission. Even if data is not exchanged, it is still possible that incompatible Ethernet flavors could assume that they have a proper connection. Ethernets using RJ-45 connections to a hub use a Link Test Pulse to verify link integrity. This pulse is the same in all flavors of Ethernet if auto-negotiation is not used. The auto-negotiation protocol itself uses a modified form of these pulses to negotiate a common Ethernet implementation.

If Ethernet incompatibility jabber were to occur between 100BASE-TX and another flavor of Ethernet, the results could be catastrophic, as 100BASE-TX transmits a continuous idle signal between frames. Although transparent to 100BASE-TX, this idle signal would completely busy out a 10Base-T or 100BASE-T4 segment. On the other hand, the 802.3 specifies that a Fast Ethernet repeater should implement jabber control, automatically partitioning off any port that is streaming information for more than 40000 to 75000 bits. If the repeater were to partition off the "jabbering" port, the symptom would be reduced to inability to connect the 100BASE-TX station to the network.

Auto-Negotiation Priorities and Alternatives

If the station and repeater both support 100BASE-TX and 100BASE-T4 and 802.3 auto-negotiation, the link will autonegotiate to 100BASE-T4 instead of 100BASE-TX. Since 100BASE-TX requires Category 5 cabling but 100BASE-T4 requires only Category 3, 100BASE-T4 is assumed to be a better default.

If the cabling is known to be UTP-5, then it is probably more efficient to turn off auto-negotiation and use 100BASE-TX wherever possible. 100BASE-T4 requires more overhead than TX because it multiplexes and demultiplexes the data stream over three wire pairs. There is also significantly less overhead in translating between 100BASE-TX and 100BASE-FX than between 100BASE-T4, as TX and FX both use 4B5B encoding instead of T4's 8B6T. 100BASE-TX and 100BASE-FX also leave open the possibility of Full Duplex communication, although full duplex is not yet part of the 802.3 spec.

On the other hand, 100BASE-TX sends an idle signal whenever it is not transmitting data. The 802.3 spec implies that it may very well be preferable to use 100BASE-T4 for battery-powered operation, since the card would only be transmitting when there is actual information to be moved.

Incompatible Cabling Specifications

One final problem with the advent of Fast Ethernet is the different cabling specifications. In classic Ethernet it was difficult to mistake 10Base-2 for 10Base-5. With Fast Ethernet, special care must be taken to verify that the entire connection between station and concentrator either supports TX's 31.25MHz signal or maintains T4's four pairs with proper twist. There are a number of good cable testers and pair scanners available to assist you in determining this for your network.

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