Transition Networks E-PSW-TX-02 The 512-Bit Rule 100BASE-TX/100BASE-FX, Transition Networks’

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

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	Class I Repeater	140	BT
Ethernet worst case round-trip	Class II Repeater	92	BT
delays expressed in bit times.	Class I TX/FX Media Converter	130 BT
	Class II TX/FX Media Converter	92	BT
Then:	DTE	50 BT
• Find the longest and	1 meter of CAT.5 TP cable	1.11	BT
	1 meter of fiber cable	1	BT
most complicated	Fast Ethernet Switch	50	BT

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.

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

100 meters TP

@1.11BT/meter

=111BT

Class II

Hub = 92BT

5 meters TP

@1.11BT/meter

=5.55BT

Class II

Hub = 92BT

100 meters TP	Collision Domain
	50.00BT
@ 1.11BT/meter
	+111.00BT
= 111BT
	+92.00BT
	+5.55BT
	+92.00BT
DTE= 50BT	+111.00BT
	50.00BT

___________

= 511.55BT

Full-Duplex Fiber/NO Collision Domain					Switch
					= 50BT
Switch		100 meters TP
= 50BT
		@ 1.11BT/meter
		= 111BT	Collision Domain
					50.00BT
			Class I	+111.00BT
			Hub	+140.00BT
	100 meters TP		= 140BT	+111.00BT
					50.00BT
@ 1.11BT/meter				___________
		= 111BT		= 462.00BT

DTE= 50BT

interface cards and of cable.

NOTE: Actual manufacturers’ bit-time values or measured bit-time values may be greater or less than bit-time values listed in the chart.

9 Transition Networks’ Pocket Switch 18