Ethernet: The Definitive Guide

By Charles E. Spurgeon
1st Edition February 2000
ISBN: 1-56592-660-9

Chapter 13 Multi-Segment Configuration Guidelines

In this chapter:
Scope of Configuration Guidelines
Network Documentation
Collision Domain
Model 1 Configuration Guidelines
Model 2 Configuration Guidelines
Sample Network Configurations

The individual media chapters you've just read covered the basic configuration guidelines for a single segment media system. However, when it comes to building a more complex half-duplex Ethernet system based on repeater hubs, you need to know what the multi-segment guidelines have to say.

The official configuration guidelines provide two approaches for verifying the configuration of a half-duplex shared Ethernet channel: Transmission System Model 1 and Transmission System Model 2. Model 1 provides a set of "canned" configuration rules. As long as your half-duplex network system meets these basic rules, it will function correctly in terms of the essential timing specifications. Model 2 provides a set of calculation aids that make it possible for you to evaluate more complex network topologies that aren't covered under the Model 1 configuration rules.[1]

This chapter describes the rules for combining multiple segments with repeater hubs to build complex half-duplex Ethernet systems operating at 10-, 100- and 1000-Mbps. We begin by looking at the scope of the configuration guidelines. To help make it clear how the guidelines apply to a single Ethernet system, we then need to describe the function of a collision domain. Following that, we describe the Model 1 and Model 2 rules as they apply to each Ethernet system.

Scope of the Configuration Guidelines

The configuration guidelines apply to the Ethernet equipment described in the IEEE 802.3 standard. Further, the Ethernet media segments must be built according to the recommendations in each media system standard. If your half-duplex network system includes Ethernet equipment or media segments that are not described in the standard, you may not be able to use the configuration guidelines to verify its operation.

The engineers in the IEEE committee developed the configuration rules based on the known signal timing and electrical performance specifications of Ethernet equipment that fully conforms to the published standard. That way, they could predict what the behavior of the Ethernet equipment would be, and how the signal timing would function across multiple segments.

Using non-compliant equipment and media segments makes it impossible to evaluate the timing of the network. Linking media segments together with equipment not described in the standard also makes it impossible to evaluate the timing. In both cases, there is no way for the design engineers to know how such equipment and media segments will behave. While such an Ethernet may function perfectly well, it will be "outside the standard," and you will not be able to use the official IEEE configuration guidelines to verify that such a system meets the half-duplex timing specifications.

Network Documentation

When it comes to evaluating the configuration of your network and for later troubleshooting, you should document each network link in your system when it is installed. The documentation should include the length of each cable segment in the link, including any transceiver cables and patch cables. Also included should be the cable type used in each segment and any information you can collect on the cable manufacturer, the cable ID numbers printed on the outer sheath, and the cable delay in bit times provided by the manufacturer. The standard recommends that you create your own forms based on Table 13-1 for use in collecting information and documenting your network.

Table 13-1:Sample Cable Segment Documentation Form
Horizontal Cabling / Transceiver Cables / Wiring Closet Patch Cord(s) / Station Patch Cords
Length
Type (e.g., Category 5)
Cable Manufacturer
Cable Code/ID

Collision Domain

The multi-segment configuration guidelines apply to a half-duplex Ethernet collision domain described in Chapter 3, The Media Access Control Protocol. A collision domain is formally defined as a single Carrier Sense Multiple Access with Collision Detection (CSMA/CD) network in which there will be a collision if two computers attached to the system transmit at the same time.

An Ethernet system composed of a single segment or of multiple segments linked with repeater hubs constitutes a single collision domain. Figure 13-1 shows two repeater hubs connecting three computers. Since only repeater connections are used between the segments in this network, all of the segments and computers are in the same collision domain.

Figure 13-1.Repeater hubs create a single collision domain

Another important point is that all segments within a given collision domain must operate at the same speed. That's because repeater hubs assume that all segments connected to the repeater are operating at the same speed and have the same round-trip timing constraints. This is also why there are three sets of half-duplex configuration guidelines, one each for 10-, 100-, and 1000 Mbps Ethernet. Each of the three Ethernet speeds has its own round-trip timing constraints and its own set of configuration guidelines.

The configuration guidelines described in this chapter are taken directly from the IEEE 802.3 standard, which describes the standards for the operation of a single half-duplex Ethernet local area network (LAN). Therefore, these guidelines only apply to a single collision domain, and have nothing to say about combining multiple Ethernet collision domains with packet switching devices such as switching hubs or routers. Switching hubs enable you to create new collision domains on each port, allowing you to link many networks together. You can also link segments operating at different speeds with switching hubs. The operation and configuration of switching hubs are described in Chapter 18, Ethernet Switching Hubs.

Model 1 Configuration Guidelines for 10 Mbps

The first configuration model provided in the 802.3 standard describes a set of multi-segment configuration rules for combining various 10 Mbps Ethernet segments. Bold text is taken directly from the IEEE standard.[2]

  • Repeater sets are required for all segment interconnection. A "repeater set" is a repeater and its associated transceivers (i.e., medium attachment units, or MAUs) and attachment unit interface (AUI) cables, if any. Repeaters must comply with all IEEE specifications in clause 9 of the 802.3 standard, and are used for signal retiming and reshaping, preamble regeneration, etc.
  • MAUs that are part of repeater sets count toward the maximum number of MAUs on a segment. Twisted-pair, fiber optic and thin coax repeater hubs typically use internal MAUs located inside each port of the repeater. Thick Ethernet repeaters use an outboard MAU to connect to the thick coax.
  • The transmission path permitted between any two DTEs may consist of up to five segments, four repeater sets (including optional AUIs), two MAUs, and two AUIs. The repeater sets are assumed to have their own MAUs, which are not counted in this rule.
  • AUI cables for 10BASE-FP and 10BASE-FL shall not exceed 25 m. (Since two MAUs per segment are required, 25 m per MAU results in a total AUI cable length of 50 m per segment.)
  • When a transmission path consists of four repeaters and five segments, up to three of the segments may be mixing and the remainder must be link segments. When five segments are present, each fiber optic link segment (FOIRL, 10BASE-FB, or 10BASE-FL) shall not exceed 500 m, and each 10BASE-FP segment shall not exceed 300 m. A mixing segment is defined in the standard as one that may have more than two medium dependent interfaces attached to it (e.g., a coaxial cable segment). A link segment is defined as a point-to-point full-duplex medium that connects two and only two MAUs.[3]
  • When a transmission path consists of three repeater sets and four segments, the following restrictions apply:
  • The maximum allowable length of any inter-repeater fiber segment shall not exceed 1000 m for FOIRL, 10BASE-FB, and 10BASE-FL segments and shall not exceed 700 m for 10BASE-FP segments.
  • The maximum allowable length of any repeater to DTE fiber segment shall not exceed 400 m for 10BASE-FL segments and shall not exceed 300 m for 10BASE-FP segments and 400 m for segments terminated in a 10BASE-FL MAU.
  • There is no restriction on the number of mixing segments in this case. In other words, when using three repeater sets and four segments, all segments may be mixing segments if desired.

Figure 13-2 shows an example of one possible maximum Ethernet configuration that meets the canned configuration rules. The maximum packet transmission path in this system is between station 1 and station 2, since there are four repeaters and five media segments in that particular path. Two of the segments in the path are mixing segments, and the other three are link segments.

Figure 13-2.A maximum Model 1 10 Mbps configuration

While the canned configuration rules are based on conservative timing calculations, you shouldn't let that lead you to believe that you can bend these rules and get away with it. Despite the allowances made in the standards for manufacturing tolerances and equipment variances, there isn't a lot of engineering margin left in maximum-sized Ethernets. If you want maximum performance and reliability, then you need to stick to the published guidelines.

In addition, while the configuration guidelines emphasize the maximum limits of the system, you should beware of stretching things as far as they can go. Ethernets, like many other systems, work best when they aren't being pushed to their limits.

The "5-4-3" Rule

An over-simplified version of the 10 Mbps Model 1 rules, called the "5-4-3" rule, has been circulating for some years. Various forms of the 5-4-3 rule have been published, and some of them include misleading terms that are incorrect. To quote from one widely distributed configuration guide, the 5-4-3 rule means that there may be as many as five segments connected in series in a network. This guide further states that up to four repeaters may be used, and up to three "populated segments." A populated segment is defined as a segment that is "attached to PCs."
While this may sound like an easy to remember rule of thumb, the "5-4-3" rule is an over-simplification of the actual configuration rules described above. Worse, the use of the term "populated segment" is misleading. This definition means that a coax segment could be regarded as an "unpopulated" segment in a network system as long as two conditions were met. First, the coax segment was not used to support PCs and, second, the segment was only used as a link segment to connect to a repeater at each end. However, this is incorrect.
A link segment is specifically defined in the 802.3 standard as a segment based on a point-to-point full-duplex media type that connects two--and only two--MAUs. A full-duplex medium means that the medium provides separate transmit and receive data paths. This is important, since collision detection occurs faster on a full-duplex medium than it does on coaxial segments. This difference in timing is factored into the total round-trip timing delays that are incorporated in the Model 1 configuration guidelines. That's why the notion of an "unpopulated" coax segment that could be used as a link segment is misleading and incorrect.
To recast the 5-4-3 rule into something closer to reality, we can define it to mean that you can have up to five segments in series, with up to four repeaters, and no more than three "mixing" segments. If three mixing segments are used, then the remaining two segments must be link segments as defined in 802.3. Actually, you can have up to four mixing segments under some circumstances as described in the real 802.3 rules above, so even our corrected 5-4-3 rule is still an over-simplification.

Model 2 Configuration Guidelines for 10 Mbps

The second configuration model provided by the IEEE provides a set of calculation aids that make it possible for you to check the validity of more complex Ethernet systems. We will be describing the network models and segment timing values provided in the standard for making the Model 2 calculations.

While the detailed description of this calculation method may seem complex, in reality the calculation method is a very straightforward process based on simple multiplication and addition. You may find the following description of the network models and timing values confusing at first glance. If so, you may wish to skip ahead to the section, Simple 10 Mbps Model 2 Configuration, to see how easy the actual calculations can be.

There are two sets of calculations provided in the standard that must be performed for each Ethernet system you evaluate. The first set of calculations verifies the round-trip signal delay time, while the second set verifies that the amount of interframe gap shrinkage is within normal limits. Both calculations are based on network models that evaluate the worst-case path through the network.

Network Models and Delay Values

The network models and the delay values provided in the Model 2 guidelines were deliberately designed to hide a lot of complexity while still making it possible for you to calculate the timing values for any Ethernet system. Each component in an Ethernet system provides a certain amount of delay, all of which are listed in the 802.3 standard in excruciating detail.

As the Ethernet signal moves through the system, it also encounters startup delays that vary depending on which kind of equipment is involved. If you were an expert, you could use a calculator and a copy of the 802.3 standard to calculate the total of all of the bit time delays. You could also calculate the complex timing delays involved in detecting and signaling collisions, all of which differ depending on the media type involved and even the direction in which the signal travels. Fortunately, the IEEE standard has provided a better way to do things.

In the Model 2 configuration guidelines, the standard provides a set of network models and segment delay values that incorporate all of the complex delay calculations and other considerations we've just mentioned. All you need to do is to use the network models provided and follow the rules for the calculations involved. Although the network models and the rules for using them may seem arbitrary at first glance, by following the rules you can quickly and simply evaluate the round-trip timing for a complex Ethernet system.

If you are curious as to exactly what goes into the segment delay values and the path delay calculations, you can refer to the IEEE 802.3 standard. The standard lists every delay component used, and explains why the calculation rules are set up the way they are.

Figure 13-3 shows the network model which is used in the standard for calculating the round-trip timing of the worst-case path. The worst-case path is the path through your network system that has the longest segments and the most repeaters between any two stations. The calculation model includes a left and right end segment, and as many middle segments as needed.

Figure 13-3.Network model for round-trip timing

To check the round-trip timing on your network, you make a similar model of the worst-case path in your system. We will show how the round-trip timing model is used by evaluating two sample networks later in this chapter. The network model used for interframe gap shrinkage is very similar to the round-trip timing model, as you will see in the section on calculating interframe gap shrinkage.

Finding the Worst-Case Path

You begin the process of checking an Ethernet system by finding the path in the network with the maximum delay. This is the path with the longest round-trip time and largest number of repeaters between two stations. In some cases, you may decide that you have more than one candidate for worst-case path in your system. If that's the case, then you should identify all the paths through your network that look like they meet the worst-case definition. Following this, you can do the calculations for each worst-case path you have found, and if any path exceeds the limits for round-trip timing or interframe gap, then the network system does not pass the test.

You should have a complete and up-to-date map of your network on hand that you can use to find the worst-case path between two stations. However, if your system is not well documented then you will have to investigate and map the network yourself. The information you need includes:

  • The type of segments being used (twisted pair, fiber optic, coax).
  • How long the segments are.
  • The location of all repeaters in the system.
  • How the segments and repeaters in the system are laid out.

Once you have this information then you can determine what the maximum path between any two stations is, and what kinds of segments are used in the maximum path.