Help with WLAN cabling
When the WLAN started to become popular, the Telecommunications Industry Association (TIA) proposed a guide to help people plan the cabling infrastructure to support wireless LANs. That proposal, starting in June 2004, led to a task group’s investigation resulting in a new guidance, the Telecom Services Bulletin (TSB), instead of a formal standard with hard requirements. At the beginning, the work was misinterpreted. Some reacted as though it was going to be a formal standard telling people how to design WLANs, when in fact, it was developed as guidance to help design WLAN cabling.
This guidance document is almost finished; it will soon be published through Global Engineering Documents and identified as TSB 162, Telecommunications Cabling Guidelines for Wireless Access Points. The full TSB discusses the cell, antenna and power options, horizontal cabling, labeling, separation from power branch circuits, wireless access point (WAP) mounting options, and test guidelines.
In the last quarter of 2005, final changes were made to TSB 162 that could influence the way this work is designed.
The recent changes cover three critical areas.
1. Changes to the cabling TSB that affect WLAN coverage—Many issues involved figuring out WLAN coverage and how to make it effective. Before getting deep into the TSB, the TIA Ad Hoc Group discovered that the guide’s purpose description should explain it and could be used to appropriately size the cells needed for wireless access. They did not want people to think that it was imperative that they follow TSB 162 to size a wireless cell.
Radio frequency (RF) coverage was affected by different styles of grids, so they added that WLAN RF coverage was affected by the building’s design (e.g., adjacent floors, directional antennas) and by the number of occupants or users.
Another coverage issue was that some smaller spaces or cells needed to be configured to contain the wireless signal in that smaller space, such as in hospital rooms, classrooms and conference rooms. This was mentioned in the beginning of the TSB. Since the telecom outlet (TO) was not emitting the signal, the WAP needed to be placed in the center of the cell. The illustration was changed to show that placement ( see Figure 1).
2. Changes to the cabling TSB that affect design/cell layout—The effectiveness of wireless LAN coverage, was dependent on the layout of the cell (a coverage area in which devices can roam freely with a wireless connection).
Because a cell may not always be in the form of a square—it is usually visualized as a circular space—the TSB explained how the square shape and round-shaped cell worked together. A cell may be formed in the shape of a square by having the four corners of the square touch the outer boundary of a circle that is made by the maximum length of a copper patch cord.
Since the telecom enclosure (TE) could contain a WAP because it was allowable electronic equipment, the TIA Ad Hoc Group made the concession of listing a TE as an option to the telecom room (TR) when linking between the TR and the TO. To figure out the horizontal cabling needed for this setup, they suggested to first find the maximum patch cord distance between the TO and the WAP. Then, determine the maximum horizontal length from the TE to the TO and, finally, save 20 feet for the cross connects/equipment cords in the TR or TE, allowing the maximum length of the horizontal channel to be 327 feet.
The designer would not be the one deciding the number of wireless access points in a building because building spaces were dynamic. Remember, the TSB was a guide. Any hard number of access points could not be recommended because it was too subjective. But, what TIA did do was tell people involved in the building design to consider the coverage area, bandwidth, types of applications, number of users and quality of service when deciding on the number of wireless access points.
There is an annex at the end of the TSB for “other wireless access point options.” The first option involved connecting the WAP while integrated into a single enclosure, such as a TE. In that situation, the horizontal cabling should not be connected directly to the wireless access point. Connection between the WAP and the TO should be made using a short patch cord and that the standard horizontal permanent link length of 90 meters would apply.
3. Changes to the cabling TSB on methods for powering the WAP—There were options for powering a WAP that needed explanation. The wireless access point could be powered either by a local power supply or remotely from a power supply, located in the telecommunications room.
When remote powering was chosen, there was no need for a power outlet within the vicinity of the WAP. Along with that information, if using local power, the user needed assistance. The TIA Ad Hoc Group ended up stating that any local power used should be provided by a 120-volt, nominal, nonswitched branch circuit dedicated to the telecommunications function, and if any standby power was available, an automatic switchover of power should be provided.
Since not all WAPs were IEEE 802.3af-compliant (DTE Power via MDI), the TIA Ad Hoc Group added a new section to the TSB on Remote Power, stating that power to IEEE 802.3af-compliant WAPs could be delivered through balanced twisted-pair cabling, and that the power could be supplied by the IEEE 802.3af- compliant switch or a mid-span power insertion device.
It was agreed that no specific category of copper cable (5, 5e, 6, 6a) or type of fiber (50/125 m or 62.5/125 m) should be specified for the horizontal cabling to the WAP. Instead of specifying any particular category of copper or fiber cabling for the horizontal cabling to the WAP, the Ad Hoc Group agreed that it should read as Category 5e or higher as in TIA 568-B.2 or two-fiber multimode fiber as in TIA 568-B.3. The group worded it that way so the user could verify any and all types of cabling mentioned in 568-B.2 and B.3.
The Ad Hoc Group tried to get the guide to state that most WAPs would be remotely powered and that the use of fiber should be limited to cases where local power was available. That was not accepted.
Because an above-ceiling WAP could have an integrated or external antenna depending on the application requirements, some felt that should be illustrated in the document’s drawings. This was rejected because both figures were labeled as “typical” with remote power from a switch or power from a mid-span device (see Figure 2).
All in all, the TSB is extremely valuable and important for people working with WLANs. It may seem easy to set up an access point in a conference room and have people sit around a table and wirelessly communicate, but it can be more complex to set up wireless coverage for offices with cubicles where wireless communications are expected while moving throughout the building.
Although the TIA TSB is primarily for new buildings that are in the planning stage—where precabling in a grid approach is recommended—it can also be used to plan wireless support for existing buildings when applicable.
The importance of site surveys show up here as very important for existing buildings and those being built, occupied and furnished. RF or radio waves don’t actually travel the same distance in all directions, so walls, doors, elevator shafts, people and other obstacles offer varying degrees of attenuation (loss that makes for irregular and unpredictable coverage).
Large facilities, such as an office complex or apartment building, can use an extensive RF site survey. This will avoid ending up with inadequate coverage and low performance in some areas. EC
MICHELSON, president of Jackson, Calif.-based Business Communication Services and publisher of the BCS Reports, is an expert in TIA/EIA performance standards. Contact her at www.bcsreports.com or firstname.lastname@example.org.