SHARED FAST ETHERNET

Application Note - Understanding Class I, Class II, and Beyond

What does it mean to have a Class I Repeater?

In simplified terms, a Class I repeater is one that permits a configuration to have only one repeater or hub in a collision domain. In other words, there can be no cascading. (See IEEE definition below). The maximum configuration would look like this:

So, a Class II must mean that TWO units can be configured in a collision domain, right ?

Right ! One level of cascading is permitted with Class II repeaters or hubs. The maximum configuration would look like this:

This looks like it severely limits Fast Ethernet installations. I am used to 10MB with 4 hops.

Compared to the simple expansion by multi-level cascading in the 10MB Ethernet world that we all know and love, the standards for shared Fast Ethernet configurations are quite daunting. Having stackable Fast Ethernet hubs expands the port count and helps some, but stackables still require that the wiring for every node goes back to the central wiring closet where the big stack is installed. This usually means expensive re-wiring, even if you have used already Cat 5 cable to connect to the users. Of course, you can use Fast Ethernet switches if you have a big enough budget.

Doesn’t someone out there have a more practical solution for shared Fast Ethernet?

The new Fast Ethernet Magnum 800E Workgroup Hubs and 40E Personal Hubs from Garrett Communications are designed to enable three levels of cascading. This is unique in the industry at this time. They have much lower internal signal delays than the IEEE Fast Ethernet standards require. This really opens up the flexibility of your installation, making it possible to configure wiring connections to the users’ computers much more like 10Mb Ethernet. And with these Magnum hubs, you have a clear path to expand your shared Fast Ethernet network whenever you need to.

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From the IEEE Std 802.3u-1995 Publication SH94306 dated October 26, 1995, Section 1.4 Definitions:

1.4.47 Class I repeater: A type of 100BASE-T repeater set with internal delay such that only one repeater set may exist between any two DTEs within a single collision domain when two maximum length copper cable segments are used.

1.4.48 Class II repeater: A type of IEEE 802.3100BASE-T repeater set with internal delay such that only two or fewer such repeater sets may exist between any two DTEs within a single collision domain when two maximum length copper cable segments are used.

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With three levels of cascading, these sound like "Class 3" Fast Ethernet repeaters to me?

Well, the IEEE standard does not define a "Class 3" repeater yet. But it certainly seems logical that they will as the technology advances because the need for something better than Class II for better installation flexibility is a big issue for users of shared Fast Ethernet systems.

What does the maximum configuration look like for Magnum "Class 3" Fast Ethernet repeaters?

You can build a tree-structure with the Magnum hubs, cascading them to two levels. The maximum Cat 5 cable distance between DTEs cannot exceed about 215 meters. This means you have a lot more installation and expansion flexibility.

You can use the "Class 3" flexibility to get more network expandability, like this:

Or, you can use the Magnum "Class 3" advantage to get more flexibility into the location of the hubs, so that you can distribute them closer to the users like this:

You say the Magnum hubs have lower internal signal delay times. What are the numbers exactly?

For Fast Ethernet hubs, the signal delay is measured in Bit Times (BT). The numbers are:

****************************************************************************** Ordinary Class II repeater that meets standard 95 BT may be 92 BT, 3 BT margin

Magnum 40E, 4-port Personal Hub 60 BT port-to-port, front ports

Magnum 800E, 8-port Workgroup Hub 60 BT port-to-port, front ports *

Magnum 8012E, 12-port Stackable Hub 80 BT port-to-port, any ports

* for front-port-to-bonus-port, the internal signal delay is 95 BT

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The Magnum 8012E Stackable at 80 BT is slightly higher than the others. What does this mean?

The practical result is that the Magnum 8012E Stackables can be used with Magnum "Class 3" Fast Ethernet hubs in two-level cascaded applications with a maximum Cat 5 cable length of only 90 meters (295 ft), rather than the standard maximum limit of 100 meters. The Magnum 8012Es are therefore "Class 2.9" hubs ! They substantially exceed the Class II standard requirements, and they are much better than ordinary hubs that merely meet the standards. When cascaded with other Magnum hubs such as the Magnum 40E Personal Hubs, the network configuration flexibility is unmatched in the industry.

To learn more about PDVs and collision domains, please consult our PDV Sales Note