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AIX Versions 3.2 and 4 Asynchronous Communications Guide

Appendix D: Wiring and Interconnection

This appendix describes how to:

Structured Wiring

For many communications applications, the wiring is the most troublesome and expensive aspect of the installation. You can minimize wiring costs and problems by adopting a standard methodology and using it consistently throughout your installation. In recent years, several organizations have proposed standardized wiring systems that specify the types of cables and interconnects to use and the rules for using this equipment.

The following information presents a recommendation for wiring, derived from the Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) standard 568. If you follow these recommendations, you can utilize your investment in wiring for a variety of communications applications including serial, local area network (LAN), and voice communications. The recommendations presented here have been used for many years for commercial telephone installations. In this case, EIA/TIA Standard 568 recommends a better grade of cable than that typically used for telephone installations.

A structured wiring system includes the following components:

Station Termination

In wiring language, a station is the device with which you are trying to communicate, such as a terminal on your desk. Station termination is the cable from the wall outlet to the station and its associated adapter, if necessary. Refer to the "7318 Feature Codes and Part Number Information" for information about available adapters.

Standard Information Outlet

The standard information outlet is the wall outlet of the system. If it is installed and used properly, this outlet can be used for many types of data depending on the signals carried over the wires. You should use the RJ-45 jack as the standard information outlet. This jack is available from any wiring equipment vendor.

Horizontal Distribution

Horizontal distribution is the wire that runs from the wall jack to a central wiring closet. It is called horizontal because that is the main direction that it runs, although the first part is normally vertically up the wall, then horizontally through the ceiling to the wiring closet. Special types of wiring and supporting structures, such as conduit or raceways, may be needed to satisfy local building or fire safety codes. Some of the options and requirements are described in "Twisted Pair Cable".

Distribution Frames

In most installations, the horizontal distribution cables from a region of the building come together in a central location, the wiring closet. At this point, the horizontal distribution cables are spliced together with other cables such as the building main distribution or cables that attach directly to some communications equipment such as the 7318. The interconnection equipment that ties the cables together is called the intermediate distribution frame. Recommendations on the type of equipment to use are presented in "Interconnection Systems".

Building Main Distribution

Building main distribution refers to the equipment that ties together all the wiring closets in the building. In a telephone installation, it is usually one or more 25-pair cables that terminate at the private branch exchange (PBX) or leave the building to the phone company central office. In data applications, the building main distribution is usually a LAN that ties together the hubs or serial concentrators such as the 7318.

Twisted Pair Cable

Communications wiring uses many types of cable; however, all new installations should use 100 ohm unshielded twisted-pair (UTP) cable conforming to EIA/TIA standard 568, Category 3 or higher. This type of cable is widely available and relatively inexpensive and offers performance as good as or better than many of the other cables used for serial communications. This same type of cable is recommended for 10BaseT applications and is suitable for voice telephones.

Each pair in this cable consists of two solid copper conductors enclosed in a polyvinyl chloride (PVC) sheath. The conductors are twisted around one another to minimize interference and crosstalk with other electrical signals. All installations should use 4-pair cable for horizontal distribution unless a large number of signals are being routed together to an intermediate distribution frame, in which case a 25-pair or larger bulk cable can be used.

The recommended cable is not shielded. Shielded cable is more difficult to install and more expensive and provides no technical benefit except in cases of extreme electrical interference.

For station termination, telephones generally use a flat-, 4-, 6-, or 8-conductor untwisted cable in a silver or gray PVC sheath, known as silver satin. This type of cable should not be used for data cables, even for short patch cables. It has the wrong electrical characteristics and will severely degrade the performance of your installation.

  1. At high data rates, the type of cable you use is critical. You should use UTP cable specified at level 3 grade or higher according to EIA specification 568.
  2. German radio interference regulations require the use of shielded cables for 7318 serial communications and 10BaseT ports to satisfy the emissions limits of Verband Deutscher Elektrotechniker (VDE) class A. The cable shield of cables attached to the 7318 serial ports should be connected to ground at both ends of the cable. Ground for the 7318 serial port is pin 7 of the RJ-45 connector. For cables attached to the 7318 10BaseT ports, use shielded RJ-45 connectors with the cable shield connected to the shield contact of the RJ-45 connector.

Color Codes

Adopting and maintaining a consistent wiring pattern will simplify the maintenance of your installation. The communications industry has adopted a standard naming and numbering convention for twisted-pair cables. For historical reasons, one conductor in each pair is the tip conductor and the other is the ring conductor. The terms tip and ring originated when phones were switched by operators using phone-jack plug-and-socket patch cables. The tip conductor attached to the tip of the plug. The ring conductor attached to the metal ring in the plug.

Each pair in the cable is numbered sequentially, starting at one. Stripes of color in the sheath of each conductor uniquely identify that conductor and its pair number. The stripes consist of a primary color and a secondary color. A 4-pair cable is numbered as shown in the following table.

4-Pair Cable Color Code
Pair Primary Secondary
1 White Blue
2 White Orange
3 White Green
4 White Brown

In the tip conductor, the primary color stripes are wide and the secondary color stripes are narrow. In the ring conductor, the primary color stripes are narrow and the secondary wide.

Cable Grades

The EIA/Telecommunications Industries Association (TIA) standard 568 defines different categories or levels of cable for different applications, as shown in the following table.

Cable Applications
Level Typical Bandwidth Application
2 < 1 MHz Telephone
3 < 10 MHz LAN (10BaseT)
4 < 20 MHz LAN (16 Mbps UDP)
5 < 100 MHz LAN (FDDI)

The cost increment of using a high grade of cable generally isn't a significant part of your overall installation costs, so you should standardize on Level 3 or higher. This permits you to use any of your installed wiring for telephone, serial communications, or 10BaseT LANs as needed. If you select Level 5 cable, you may be able to use your cables with the emerging high-performance LAN standards such as fiber distributed data interface (FDDI) on copper twisted pair and 100 Mbps Ethernet.

Cable Distances

Cable distances have been optimized around common baud rates. The following table shows maximum baud rates and optimal slew rate settings.

Slew Rate Settings
Distance (ft) RS-423/EIA 422 Distance (ft) EIA 232 Maximum Baud Rate (bps) Slew Rate Setting
< 2300 < 200 9600 slow
< 625 < 200 38400 medium
< 150 < 50 115,200 fast
< 130 N/A Reserved super

Plenum Cables

If you plan to route your horizontal distribution cables through a plenum, that is, a duct used for circulating air in a building, you should use plenum-rated cable. This twisted-pair cable uses a sheath material that is more fire retardant than PVC, such as Teflon. plenum-rated cable is more expensive but is much safer than PVC cable. If the building were to catch fire, PVC would give off toxic fumes, which would spread quickly through the plenum.

Note: Fire officials in many communities regard the open space above raised ceiling tiles as a plenum and require plenum-rated cables, unless the cable is enclosed in electrical conduit.

Interconnection Systems

The horizontal distribution cable connects your information outlet to a central point such as a wiring closet, where it attaches to an active concentrator like the 7318 or a 10BaseT hub, or attaches to other cables for routing to another point in the building. A distribution frame ties the cables together in the wiring closet.

There are two types of distribution frames. When splicing cables together in a stable configuration environment, the preferred method uses a punch-down block, also known as a cross-connect block. When tying equipment together in an environment subject to frequent configuration changes, the preferred method uses a patch panel constructed from RJ-45 connectors.

For high-bandwidth applications such as 10BaseT, you need to minimize the number of cable splices and connectors between your LAN adapter and the hub. Each time you go through a plug-and-jack connector or cable splice, you disturb the electrical characteristics of the cable by introducing signal reflections that increase the probability of data errors. Using proper, good-quality connectors and punch-down blocks minimizes these disturbances, but they cannot be eliminated. Each such disturbance effectively reduces the maximum distance of a cable span. Exceeding this distance increases the error rate to unacceptable levels.

Note: For many applications, the 7318 itself makes a very effective distribution frame. Simply terminate the horizontal distribution cables with RJ-45 plugs and insert the plugs into the client port RJ-45 jacks on the front of the 7318. The 10BaseT link from the 7318 to your host computer or 10BaseT hub can be just another horizontal distribution cable constructed using the wiring you use for the serial ports.

Main Building Distribution

Telephone systems usually tie all phones to a centrally located PBX. Multipair cables tie intermediate distribution frames to the PBX located in a phone room or directly to the telephone central office if a PBX is not used.

A similar approach can be used for 7318s and LAN systems, but the distance limitations on serial lines and especially on LAN wiring make it more practical to locate the 7318 or 10BaseT hub at the intermediate distribution frame. For these systems, the LAN replaces multipair cables as the main building distribution.

The same reliability and maintenance considerations that make 10BaseT LANs the wiring of choice for horizontal distribution apply to main building distribution as well. However, the 100 meter distance limitation on 10BaseT wiring may cause problems in tying together intermediate distribution frames. When this is the case, use standard Ethernet with repeaters. Standard Ethernet becomes more practical when it ties together a limited number of 10BaseT hubs at the intermediate distribution sites.

For longer runs between the intermediate distribution sites, particularly if some of the runs are outdoors, you should use fiber optic Ethernet. The use of fiber optics for your main distribution network will make your whole building much less susceptible to electrical disturbances.

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