Digi MIL-5008H manual Theor y of Operation

Install Guide

Theor y of Operation

The 5008H series of hubs are 100BASE-TX, half-duplex, Class II IEEE 802.3u compliant.

Fast Ethernet, 100BASE-X, supports three media types, fiber optics, four pair twisted pair, and two pair twisted pair. These are known as 100BASE-FX, 100BASE-T4 and 100BASE-TX. The timing budget for 100Mbit network is 10 times smaller than a 10Mbit network. This limits the number of Class II hubs to two and Class I hubs to one for a single collision domain. Digi’s 5008H qualify as a class II repeater. This means one can cascade up to two 5008H hubs to drive a total of 14 ports in a single collision domain (two ports are lost in the cascading process.

The 100BASE-TX differs from standard, 802.3 10BASE-T Ethernet in five ways:

•It communicates at 100 Mbps versus 10 Mbps.

•The collision domain span is limited to only one Class I or two Class II repeaters, versus four repeaters for 10BASE-T.

•It requires CAT 5 two pair wiring versus CAT 3 wiring for 10BASE-T.

•It uses a new encoding mechanism known as 4B5B in MLT-3 format versus manchester encoding.

•It uses a new attachment interface known as Media Independent Interface (MII) versus AUI.

With the exception of a dramatic improvement in throughput, the addition of 100BASE-TX to your network is transparent to your user.

The frame format and the CSMA/CD protocol are identical between 100BASE-TX and 10BASE-T. The ten fold improvement in bandwidth is attained by scaling the bit rate (the rate which data is sent) by ten times. This allows you to use your existing network software. It is up to the new hardware to take the data link information and transmit or receive it at the 100 Mbps rate.

The ten fold increase causes a ten fold decrease in the amount of time required to get data through the network. This decrease in time also reduces the turn around time for sensing collisions by ten. This causes the decrease in the physical span of the network and the limitation on the number of repeaters allowed.

The increase in speed also has the impact of requiring higher grade data cabling. Requiring CAT 5 wiring ensures that the data will get to the other end with minimal outside interference. For recent and new installations, this is a mute point in that CAT 5 is a general requirement for most building codes. This becomes an issue for older installations with CAT 3 cabling. These installations require changing to the higher grade cabling to ensure compatibility with the IEEE 802.3u standard.

Model: MIL-5008H Fast Ethernet TX Hub 6