100BaseT LAN
Overview
As we all know with Ethernet LAN's, there are limitations on the distance by which equipment can be connected to a hub (repeaters) interface. With 10BaseT, the distance is 100 meters when using UTP cabling and 2000 meters when using fiber cabling. And we all remember the 5-4-3 rule when connecting hubs together.

Well what has changed with 100BaseT LANs? The maximum round trip delay time across a LAN still remains at 512 bit times.


For 10BaseT, this equates to 51.2 microseconds. With 100BaseT, the bits are transmitted 10 times faster. This results in a maximum round trip delay of 5.12 microseconds. To compensate for the reduction in time, the 100BaseT standard has reduced the number of repeaters and the length of cabling allowable on a LAN, and limited the distance between two repeaters to five meters with UTP cabling.

DTE Equipment Delay + Cable Delay + Repeater Delay
< 512 bit times (5.12 microseconds)

The following table is useful in figuring these values.

Device

Delay Per Meter

Maximum Delay
Two TX or FX DTEs

N/A

100
Two T4 DTEs

N/A

138
One T4 and One TX/FX DTE

N/A

127
Category 3 Cable Segment

1.14

114 (100 meters)
Category 4 Cable Segment

1.14

114 (100 meters)
Category 5 Cable Segment

1.112

111.2 (100 meters)
Shielded Twisted Pair (IBM Type 1)

1.112

111.2 (100 meters)
Fiber Optic Cable

1.00

100 (412 meters)
Class 1 Repeater

N/A

140
Class 2 Repeater - All ports TX/FX

N/A

92
Class 2 Repeater - Any port T4

N/A

67
100BaseTX to 100BaseFX Converter

N/A

50-100


The DTE station interface is the first factor in determining the round trip delay. If the stations are connected using TX, FX or a combination of TX and FX, a delay of 100 bit times needs to be used for DTE Equipment Delay. If 100BaseT4 (4 wire networking) is used, additional delay bits will be experienced.

The cable delay can easily be determined by multiplying the total cable length by the Delay Per Meter value. If a station is connected with 100 meters of UTP, the round trip delay is 111.2 bit times or 1.12 microseconds. The delay per meter value is less for fiber because the signal will travel faster on a fiber connection than on a UTP connection.

Two types of repeaters can be used with 100Mbps Ethernet. A class 1 repeater is used when combining 100BaseTX/FX and 100BaseT4 connections. On a LAN, only one class 1 repeater can be used due to the 140 bit delay. A class 2 repeater is used when using only the 100BaseTX/FX interfaces. Only two class 2 repeaters can be used on a LAN. In most networks, only the class 2 repeaters are used. The repeaters are usually connected with a five meter cable.

Figure 1

Figure 1 - Ethernet Delay

Lets review some samples of calculating the round trip delay on a 100BaseTX/FX LAN. The first example (figure 1) includes 2 stations connected to a 100BaseT hub. One of the stations is connected to the hub with 100 meters of UTP. The second station is connected to the hub with 75 meters of fiber.

Two 100Base TX/FX DTEs 100 bit time
100 meter Cat-5 Cable 111.2 bit time
75 meter Fiber Optic Cable 75 bit time
1 Class 2 Repeater 92 bit time    
Round Trip Delay 378.2 bit time

378.2 bit time is less than the maximum 512 bit time so the LAN will meet the 100BaseT specification.

Figure 2

 

Figure 2 - Ethernet Delay

Lets take a second, more complicated example. In this case (figure 2), we have two class 2 repeaters. The repeaters are connected with a five meter UTP cable. Stations are connected to each repeater with 100 meter UTP.

Two 100Base TX/FX DTEs 100 bit time
100 meter Cat-5 Cable 111.2 bit time
100 meter Cat-5 Cable 111.2 bit time
5 meter Cat-5 Cable 5.56 bit time
75 meter Fiber Optic Cable 75 bit time
1 Class 2 Repeater 92 bit time
2nd Class 2 Repeater 92 bit time     
Round Trip Delay 511.96 bit time


In this case, a 511.96 bit time is also within the 512 bit time specification. Obviously, this is on the edge of maximum delay time.

Conclusion
As you can see, calculating the round trip delay is not all that difficult. However, the thing to remember is that the length of the network has been reduced when compared with a 10BaseT network. Due to the speed of a 100BaseT LAN, it is necessary to ensure all cabling and equipment meet the IEEE standard. When installing new 100Mbps LAN equipment meets the standard. As the old saying goes, "Pay me now, or pay me later."


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