Digi MIL-5008H - Manual (Page 6)

Install Guide
Model: MIL-5008H Fast Ethernet TX Hub 6
T h e o r y o f O p e r a t i o n
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.