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How to Play By the Rules of Fast Ethernet

The truth is everyone who uses Ethernet has wished from time to time that their network was faster. When Ethernet was first designed in the late 1970s, 10Mbps was like a ride on the autobahn.

With today’s bandwidth-intensive multimedia applications that number is barely adequate. For example, full motion video for video conferencing can require up to 25 Mbps. That means that classic Ethernet, at 10 Mbps, can only deliver poor quality real-time video for a single session. Fast Ethernet, which runs at 100 Mbps, allows for watching a broadcast presentation in one window while running a conference with three people in three other windows, while still leaving enough margin for network-based application usage.

Below are two primary areas to think about, if you want to play by the rules, when it comes to upgrading your network from 10Mbps to 100Mbps:

1. Cabling
A common problem with Fast Ethernet is the different cabling specifications. In Fast Ethernet, twisted pair cabling either needs to be category 5 or category 3 with proper twist on all four pairs. In classic Ethernet, it was easy to distinguish ¬†between 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 help in determining this for your network.

2. Hubs
The problem with hubs is the number allowed in a single collision domain. Classic Ethernet allows hubs to be cascaded up to four deep between any two stations. In Fast Ethernet, the number of hubs allowed in a collision domain is drastically reduced to only a single hub. Sometimes it may be possible to have more than one hub in a collision domain, but it will probably be easier over the long term to design a Fast Ethernet network assuming that only one hub is allowed.

What the IEEE 802.3 spec does not explicitly state is that this limitation only applies to shared 100BASE-T, not switched 100BASE-T. Because switches act like bridges in defining a separate collision domain, installing Fast Ethernet switches will allow you to work around the single-hub problem. Even if it is not necessary to deliver dedicated switched Fast Ethernet to each desktop, Fast Ethernet hubs can be connected to switches. Connecting a number of repeaters to a switch will provide shared Fast Ethernet and allow you to maintain the size of your network.

The increase in speed and quality is well worth the transition to Fast Ethernet, however the number of hubs, along with the length and the type of cabling, need to be considered when upgrading your network to make sure it’s an easy switch and has an overall positive impact on your organization.

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