Figure 2 — Approach 2 uses the path model.

APPROACH 2

For a detailed analysis on the restrictions for cascading hubs, approach 2 can be used. With this approach two parameters are calculated. First the worst case round trip delay is calculated. Second the interframe gap (IFG) shrinkage is calculated. The IEEE 802.3 standard provides tables for these calculations. This approach is used for situations not covered in the more generalized approach number 1.

The model used consists of two DTEs interconnected by repeaters as shown in Figure 2. There is a left end DTE and a right end DTE. The middle segments are for the repeaters. The round-trip bit times for all devices can be found in the table. This calculation is done first. The total round trip delay of all devices or components cannot exceed 575 bit times. This number is based upon the 64-bit preamble followed by 511 bits of frame. You should include some margin (up to 5 bits) and the standard recommends not exceeding 572. Some technical references will say the limit is actually 512 bit times since this is the slot time. This ignores the 64-bit preamble. IEEE 802.3 considers the preamble as well. Table 1 provides a listing of segment delay values (SDV) for the various media.

As an example let us assume an all twisted-pair network consisting of six segments and five repeaters. Since both ends are 10BASE-T segments, there is no significance to left end and right end. If the end segments are indeed different, you would need to do the calculation twice since the left and right end delays are different. Use the worst case calculation.

For our example, we want to use the maximum allowable segment length of 100 meters. Therefore, from the chart select 26.55 and 176.3 for the ends and 53.3 for the four mid-segments. Adding them all up yields 416.05 which is less than the 572 limit. There seems to be much margin. If you do not use the maximum length of the segments in the calculation, you will need to calculate the actual delay value for a particular length of cable using the following equation: SDV = Base + [Length * (RT delay/meter)]. Do not forget to add the base value which represents the inherent delay within the transceiver. These base values include the effect of 2 m of AUI cables and are therefore conservative since AUI cables are seldom used today. However, if more than 2 m of AUI is to be used, this additional length must be included in the calculation.

Segment Type	Max length	Left End		Mid-sement		Right End		RT delay / meter
		Base	Max	Base	Max	Base	Max
10BASE5 Coax	500	11.75	55.05	46.5	89.8	169.5	212.8	0.0866
10BASE2 Coax	185	11.75	30.731	46.5	65.48	169.5	188.48	0.1026
FOIRL	1000	7.75	107.75	29	129	152	252	0.1
10BASE-T	100	15.25	26.55	42	53.3	165	176.3	0.113
10BASE-FP	1000	11.25	111.25	61	161	183.5	284	0.1
10BASE-FB	2000	N/A	N/A	24	233.5	N/A	N/A	0.1
10BASE-FL	2000	12.25	212.25	33.5	233.5	156.5	356.5	0.1
Excess Length AUI	48	0	4.88	0	4.88	0	4.88	0.1026

Table 1 — Segment round-trip delay values in bit times. The total round-trip cannot exceed 572 bit times.

The next calculation is to determine the IFG shrinkage. As frames are processed through repeaters, there may be loss of bits that must be compensated for by the repeaters. The result is that the time between frames might be reduced below the minimum stated in the standard. Therefore, a path variability value (PVV) calculation must be made on the worst-case path. Table 2 provides the values. Note that you do not need to consider the receiving end. Therefore, there are only five effected segments—the transmitting segment and the four mid-segments. For the transmitting segment use 10.5 and for the mid-segments use 8. The total would be 42.5 which is less than the 49 maximum that is allowed by the standard. This example shows there is a situation where five repeaters can be used in a row which contradicts approach 1. However, if you want to remain conservative, limit your network to four repeaters. Notice that if we added one more repeater in our example (one more mid-segment) that the additional 8 bit times in the PVV calculation would exceed 49. Therefore, we could still pass the delay calculation but fail the IFG test.

Segment Type	Transmitting End	Mid-segment
Coax	16	11
Link except 10BASE-FB	10.5	8
10BASE-FB	N/A	2
10BASE-FP	11	8

Table 2 — Segment variability values in bit times. The total cannot exceed 49.

 

Summary

Shared Ethernet networks can be extended with repeaters as long as the network diameter does not exceed the limit of a single collision domain. The IEEE 802.3 standard mentions two approaches in determining the limit. Approach 1 provides a set of rules which has resulted in the short form 5-4-3 rule. Approach 2 is more analytical and should be used when the network topology is inconsistent with the rules. What should be remembered is that in a shared Ethernet network, repeaters should not be applied without thought.

References

Practical Networking With Ethernet, Charles E. Spurgeon, 2000, International Thomson Computer Press

Switched and Fast Ethernet, Second Edition, Robert Breyer and Sean Riley, 1996, Macmillan Computer Publishing USA

International Standard ISO/IEC 8802-3 ANSI/IEEE Std 802.3, 2000, The Institute of Electrical and Electronic Engineers, Inc.