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Increasing Bandwidth Ten-fold by Installing
100BASE-TX (Fast Ethernet)
Fast Ethernet, 100BASE-TX or 100BASE-FX on the Transition Networks’
pocket switch, moves signals at 100 Mb/s, ten times faster than legacy
Ethernet.
Legacy Ethernet and Fast Ethernet
Since 100BASE-TX Fast Ethernet is an extension of the existing Ethernet
IEEE 802.3 standard that uses the existing 802.3 media access control
(MAC) layer, there is no change in the link layer frame format and no
need to recompute checksums or rebuild frames before forwarding.
Thus, the Transition Networks pocket switch moves frames easily
between Ethernet ports running at 10 Mb/s (10BASE-T, legacy Ethernet)
and 100 Mb/s (100BASE-TX, Fast Ethernet).
Since the pin numbers used in the eight-pin connector for 100BASE-TX
conform to the wiring already in use in the 10BASE-T standard, a
100BASE-TX card can replace a 10BASE-T card in an Ethernet station
that has Category 5 wiring installed without making any wiring
changes.
However, though both10BASE-T and 100BASE-TX both are limited to a
distance of 100 meters, the maximum segment length for the 10 Mb/s
link is limited to 100 meters mostly by signal strength, while the
maximum segment length for the 100BASE-TX is limited to 100 meters
to ensure that round trip timing specifications are met. Also, though
both 10BASE-T and 100BASE-TX collision domains can be measured
using the 512-Bit Rule, the 5-Segment Rule is more commonly used for
legacy10BASE-T Ethernet (See page 17). Fast Ethernet is measured using
the 100BASE-TX/100BASE-FX 512-Bit Rule (See page 18).
Connecting Legacy Ethernet to Fast Ethernet
Installing the Transition Networks’ pocket switch between existing
Ethernet workgroups or networks and a new Fast Ethernet workgroup or
network, using the cabling appropriate to each, connects technologies.
Migrating from Legacy Ethernet to Fast Ethernet
Migration from 10 Mb/s to 100 Mb/s operation can be on a station-by-
station basis, as needed, using incremental upgrades of station network
interface cards and of cable.
Transition Networks’ Pocket Switch
18
9
The 512-Bit Rule (100BASE-TX/100BASE-FX)
A bit-time (BT) is the amount of time required to send one data bit across
an Ethernet LAN. The total number of bit-times between any two DTEs
in the same Fast Ethernet 100BASE-TX or 100BASE-FX collision domain
cannot exceed 512 bit-times in the round trip.
CALCULATING RTD (ROUND TRIP DELAY)
To calculate the round trip delay in bit-times between a station (DTE)
and the Transition Networks’
pocket switch, refer to the Fast
Ethernet worst case round-trip
delays expressed in bit times.
Then:
• Find the longest and
most complicated
collision domain path
between the station and the Transition Networks’ pocket switch.
• Calculate the round trip delay for each cable segment by
multiplying the length of the cable (in meters) by the delay per
meter (in bit times (BT)) for each cable type as listed in the table
• Calculate the total round trip delay by taking the sum of all the
individual cable delay values in the path, as listed in the table,
plus station (DTE) delays and hub (repeater) delays.
• Add the segment delay values together. If the result is less than or
equal to 512 bit times, the path passes the test.
Full-Duplex Fiber/NO Collision Domain
Collision Domain
Class II
Hub
Collision Domain
Class I
Hub
100 meters TP
@ 1.11BT/meter
= 111BT
5 meters TP
@ 1.11BT/meter
= 5.55BT
DTE= 50BT
= 140BT
= 92BT
Class II
Hub
= 92BT
Switch
= 50BT
100 meters TP
@ 1.11BT/meter
= 111BT
100 meters TP
@ 1.11BT/meter
= 111BT
100 meters TP
@ 1.11BT/meter
= 111BT
Switch
= 50BT
DTE= 50BT
50.00BT
+111.00BT
+92.00BT
+5.55BT
+92.00BT
+111.00BT
50.00BT
___________
= 511.55BT
50.00BT
+111.00BT
+140.00BT
+111.00BT
50.00BT
___________
= 462.00BT
Class I Repeater 140 BT
Class II Repeater 92 BT
Class I TX/FX Media Converter 130 BT
Class II TX/FX Media Converter 92 BT
DTE 50 BT
1 meter of CAT.5 TP cable 1.11 BT
1 meter of fiber cable 1 BT
Fast Ethernet Switch 50 BT
NOTE: Actual manufacturers’ bit-time values or measured bit-time values may be greater
or less than bit-time values listed in the chart.