Figure 4-8. Typical horizontal cabling construction
One of the most important, yet least discussed, factors in building a successful LAN is a good cable plant. Of course, by cable I mean not only copper, but also fiber optic cabling. A good cable plant design considers several factors.
First, there is a difference between vertical and horizontal cable runs. In a simple high-rise office tower with one LAN room on each floor, the difference is relatively clear. Vertical cable runs interconnect the different floors, while horizontal cabling is used to connect users to the LAN Access equipment such as hubs or switches. In a more complicated real estate plan, the terms become a bit confusing because not all horizontal runs are parallel to the ground, and not all vertical cable runs are perpendicular to it.
By horizontal cable runs, I mean Access Level wiring. I only use this term to describe cables that connect end station equipment to Access hubs and switches. Vertical cabling, on the other hand, is never used for end stations. Rather, it is used to interconnect hubs and switches. In a hierarchical design, vertical cabling usually means cabling that connects Access Level hubs and switches to Distribution switches. Vertical cabling is also often used to connect Distribution to Core devices.
For the horizontal cable runs, the important considerations are:
Type of cabling employed
Number and type of connectors at each desk
Number and type of connectors in each computer room rack
Patch panel design
Physical routing of cable
The gold standard for horizontal cabling is currently Category 5 (sometimes just called Cat5). This system was originally introduced to support 10BaseT networks, with all of the required growth capability for Fast Ethernet, which was not yet commonly available. It turns out that Category 5 is even able to support Gigabit Ethernet, so the investment in a Category 5 cable plant has been well worth the money.
One of the most fortuitous features of the Category 5 specifications for Fast and Gigabit Ethernet is that, like 10BaseT, they all specify a distance limitation of 100 meters. If the cable plant was built to respect this specification, it should theoretically be able to handle these speed upgrades.
Furthermore, Category 5 cabling can also support 4 and 16Mbps Token Ring standards with the same 100-meter distance limitation. And in all cases, the same standard RJ45 connector is used, so you can freely change your environment among any of the most popular LAN standards without needing to change your cabling. This fact is important because, although the cable itself may not be terribly expensive, the cost of rewiring an entire work area to support a new cabling standard is daunting.
However, the Category 5 standard has evolved slightly since it was introduced. To support Gigabit Ethernet, it became necessary to modify the standard, called Enhanced Category 5 or Category 5e. This enhanced standard includes limits on signal reflection properties and cross talk at junctions. While it is true that a good Category 5 cable plant supports Gigabit Ethernet, if the installation is more than a few years old, it may have trouble with Gigabit speeds. The compatibility can be tested easily by any certified cabling contractor. It is definitely a good idea to pretest any cable plant before assuming that it will support an upgrade from Fast to Gigabit Ethernet.
A new Category 6 standard is currently nearing the final stages of acceptance. Some organizations have rushed to implement the new standard in the hopes that it will provide further growth capabilities. But there have been a number of reported interoperability problems with the Category 6 cable systems, mostly caused by pushing ahead before the standard was completed. Furthermore, if these organizations implement Category 6 in the hopes that it will support similar future growth to 10 Gigabit Ethernet, they may be sorely disappointed. The 10 Gigabit Ethernet project is moving ahead quickly, but has not yet settled on any copper-based cabling standard. And it is possible that they will have to bypass Category 6 and jump directly to Category 7, which is also currently in development.
The current picture of the world of cabling standards is rather uncertain. There are no guarantees that any of today's cabling standards will support tomorrow's high-speed networking. The good news is that a good Enhanced Cat5 cable plant will readily support both 100Mbps Fast Ethernet and Gigabit Ethernet speeds.
One of the best cost-saving measures available when designing a horizontal cable plant is simply deciding how many LAN drops will be put at each user work area. In general terms, the cost of pulling one cable to a desk is the same as pulling several cables. This is because the cabling contractor simply bundles all wires together and pulls the bundle. The same amount of labor is required either way. In cabling jobs, the labor cost is commonly around 75% of the total cost. Doubling the number of LAN drops at each desk will likely increase the total cost by 25%. However, coming back to pull a new bundle of cables to every desk after the original job is done can be prohibitively expensive. It is better to slightly overestimate the number of LAN drops that will be required at each desk.
An organization that expects every user to have only one network device on his or her desk should probably consider pulling two LAN drops to each desk. This way, if several users suddenly need a second workstation or a printer, it is easily accommodated with minimal expense. Similarly, if it is known from the start that a lot of users will have two network devices on their desks, then pulling at least three LAN drops would be wise.
Some organizations, particularly investment banks with large computerized trading floors, opt to pull both fiber and copper cabling to every desk. This way, they know that they will be able to support future speed increases, even if the standards for copper cabling change. But this option is probably overkill for most office applications.
Another common cost-saving measure in horizontal cabling is combining LAN and telephone cable runs, terminating them all on the same termination block at the workstation. This measure is particularly useful if the telephone system uses the same wiring closet as the LAN does.
The same considerations apply to the cabling of server racks in the computer room. Some organizations take advantage of their raised computer room floor to do all of their server cabling in an ad hoc, as needed fashion. However, this can make troubleshooting problems extremely difficult because there will be no reliable pattern associating particular racks with patch panels.
I generally recommend precabling every rack in the computer room in a rational pattern. Then, if additional cables are required, they can be pulled with minimal disruption. Remember that every time somebody works under the floor tiles, they risk disrupting power or network cables that are already present. For this reason, many organizations have strict rules prohibiting any such work during business hours. Thus, precabling the racks can result in significant time savings when connecting new systems.
Patch panels are critical to maintaining the required signal loss and reflection characteristics of LAN cabling systems. Generally, the horizontal runs that terminate under user desks are collected on patch panels in a wiring closet near the work area. The total distance limit on any twisted pair copper cable run should be kept below 100 meters because this length is the upper limit to most 802.3 and 802.5 specifications. This length restricts the service area of any given wiring closet.
The most common method for terminating the horizontal cable runs in the wiring closet is to use a patch panel of RJ45 connectors. This panel is called the station field, as shown in Figure 4-8. It was once common to also have a similar second patch panel called the equipment field that connected to LAN Access hubs or switches. However, with the advent of higher speed LAN technology, simply using the RJ45 connectors on the front of the Access equipment as the equipment field is usually preferable. You generally do not want to introduce an additional connection point, as it can result in too much signal loss and reflection.
Also, to reduce signal loss and reflection, it is generally preferable to run directly from the back of the station field patch panel to the user's desk, with no intermediate terminations or patch panels. Again, in earlier implementations, using an intermediate BIX style (that is, punch-down connectors with individual wires rather than RJ45 connector jacks) panel was relatively common, since this style gave the cabling contractor extra flexibility in running the cabling.
The patch cords, both at the user's desk and connecting from the station field to the Access equipment, must be compatible with the cabling specification used for the horizontal runs and the patch panels. This compatibility becomes particularly important in Gigabit applications where signal reflections and cross talk between wire pairs can destroy the signal. In fact, existing cable plant problems are almost invariably in the patches and terminations.
One last important thing to consider when designing a cable plant is the physical routing of the cables. For horizontal cabling, this routing generally means avoiding sources of electrical noise. Fluorescent lights are some of the worst noise sources in an office building, and they are often difficult to fully avoid. However, the noise radiated from any such source decreases rapidly with distance, so an extra few feet can make a huge difference.
Usually, it is better to pull horizontal LAN cabling through the floor or walls rather than the ceiling. In an open-concept office, using walls may be impossible, however. The relatively common practice of running cables through the ceiling and down into each user cubicle by means of a hollow pole is unattractive and tends to age poorly; over time the floor layout will inevitably change. If the LAN drops come up out of a panel in the floor, it is often easy to move cubicle walls by several feet in any direction. However, with the hollow pole systems, the pole generally has to line up perfectly with the cubicle wall. Even shifting the wall by a few inches can result in a mess of bizarre angles.
Some buildings were never designed for cabling through the floor. Some building designs use thick floors of solid concrete. The only way to run cable through the floor is actually to drill through to the false ceiling of the floor below and run the cables through those holes. Drilling holes through cement (called "coring") can be extremely expensive. In these cases, it may be necessary to run the horizontal cables through the ceiling and down hollow poles, but I recommend this option as a last resort.
For vertical cabling the considerations are similar, except that you should never connect end stations directly to vertical cable runs. The important considerations are:
Type of cabling employed
Patch panel design
Physical routing of cable
The distances involved in vertical runs are often larger than the distances used in horizontal runs. This is particularly true when talking about "vertical" runs between buildings. Consequently, for vertical cabling you generally want to use fiber optic cabling instead of copper, although there are still times where copper cabling is required between floors. Vertical copper cabling, often called "house pairs," is usually run in large bundles with 25 pairs of wires twisted together, which retain the Category 5 specifications. These cable bundles are normally terminated either on an Amphenol connector or on a BIX-style punch-down block. They can be used either for standard LAN protocols over shorter distances or for legacy serial standards such as X.25 or SDLC connections.
There are two main types of fiber optic cable—single mode and multimode. Multimode fiber is less expensive, and devices that use it have lower optical power requirements, making them less expensive. However, this lower power generally means that multimode fiber is useful only for shorter distances. Most vertical cable runs use multimode fiber optic cabling. For longer distance requirements the power of the injected signal has to increase, which usually requires single mode fiber cable.
The rule has historically been that multimode fiber is used for LANs and any short distance requirements while single mode is used by WAN and MAN service providers. This rule may need to be altered because of multimode distance restrictions on Gigabit Ethernet.
The current Gigabit Ethernet specification restricts multimode fiber cable runs to 500 meters. This length is enough to reach from the top to the bottom of the world's tallest office buildings, but it is not sufficient to cross even a modest-sized campus. Thus, some organizations will probably need to pull new single-mode fiber runs between buildings to allow them to take full advantage of Gigabit Ethernet trunks.
Fiber patch panels are similar in concept to patch panels for twisted pair cabling. Usually, a bundle of fibers is run from any given LAN wiring closet to a central Distribution LAN room. Multimode fiber comes in a variety of different bundles. The smallest bundles generally include only a single pair of fibers. As with horizontal LAN cabling, the main expense in pulling a bundle of fiber optic cable is in the labor, not the cable. Thus, it is usually wise to pull a larger bundle of fibers, even if there is no immediate requirement for more than one pair. Remember that fiber is almost always used in pairs, so it is easy to use up all available strands quickly when new requirements emerge.
The usual method for running vertical fiber cabling is to designate a few Distribution LAN rooms where the Distribution Level switches will be housed. Then all Access devices that use this Distribution Area will be housed in local wiring closets. You need to run at least one bundle of fibers from each local wiring close to the Distribution LAN room. Then you can simply use fiber patch cords to connect the Access equipment to the patch panel on one end and the Distribution equipment to the patch panel on the other end.
Fiber optic cabling is not susceptible to electrical interference. It is, however, far more susceptible to cutting and breaking than copper wire. You can use two common methods to help protect against these problems.
First, though the fiber bundles themselves are held together in a protective sheath, this sheath is not sufficient to protect the delicate fiber from damage. The bundles are usually passed through long metal conduits, which helps protect them against damage from accidental bumping or crushing.
Second, and most important to a stable LAN design, running two sets of fiber bundles through different conduits is a good idea. It is even better if these conduits follow completely different physical paths. For example, in many cases, vertical cabling runs through the elevator shafts of a building. The preference here would be to run two bundles through separate conduits located in different elevator shafts. This way, even if a fire or other similar disaster in the building destroys one physical path, it doesn't destroy your only way of connecting to a remote area.
In this case, you would also carefully construct your trunks so that you always run redundant pairs of trunks, one from each conduit. Then if you have a physical problem that damages one fiber bundle, Spanning Tree or some other mechanism will activate the backup trunk and there will be no service outage.
Another good reason to use physically separate fiber conduits is that fiber cable itself is susceptible to low levels of background radiation. If one conduit happens to pass through an area that has unusually high radiation (a radiology office, or perhaps some impurity in the concrete), then over time the fiber could become cloudy and start showing transmission errors. In this case, the other conduit will probably not have the same problem. Thus, you can simply switch over to the backup link.