FOA Guide 

Topic: Designing Premises Cabling Systems

Table of Contents: The FOA Reference Guide To Fiber Optics


Designing Premises Cabling Systems

Here we describe how to design a premises cabling system based on traditional structured cabling. Many new LANs are using Optical LAN designs that are a new generation of equipment based on FTTH. Here is more information on OLANs.

     The model for premises cabling standards was AT&T’s design guidelines for communications cabling developed originally  from a 1982 survey of 79 businesses located in New York, California, Florida and Arkansas involving over 10,000 cable runs. At the time, cabling was used mainly for telephones to wiring closets and PBXes (Private Branch Exchanges or local phone switches), but it established a baseline for cable length requirements for commercial customers that was used in creating TIA-568 and equivalent international standards. The AT&T survey determined that 99.9% of all stations were less than 300 feet (about 100 meters) from the wiring closet, so that became the goal of the 568 design. Much of the terminology from the telephone industry also carried over into the development of structured cabling standards.
      The communications connection to the outside world comes into the building through what is called a "service entrance" and is terminated in the main "equipment room" or "main cross connect" which houses the electronic communications equipment which connects to the outside world. There may be other equipment rooms which also contain electronics located in the building connected using what is called "backbone cable."
      The "telecommunications closet," or as it is now called "telecommunications room (TR)," is the (typically) small equipment room closest to the end user,  where the termination of the backbone cabling and connection to "horizontal cabling" which runs to the end user occurs. It will be located in proximity to the end users, officially known as the "work area" but often called the "desktop". These locations are where all switches, hubs and any other networking equipment will be located. All cabling is defined by the necessity to connect all these locations and the desktop of the end user, which is called the "work area". The facilities in which cables are run are referred to as "pathways and spaces."

TIA-568-C revision proposes to change the nomenclature of structured cabling systems. Here is an explanation of the changes.

      All equipment rooms require uninterruptible conditioned power, a separate data ground, air conditioning and an adequate amount of floor space. Proper designers and installers of these systems should be consulted if the cabling designer is not familiar with or licensed for this work.


568 architecture
     
 
     The backbone cabling can be either UTP or fiber optics. In larger networks today, fiber is most often used for its longer distance capability and higher bandwidth. TIA 568 specifies two multimode fibers, 62.5/125 (OM1) - the most common MM fiber until network speeds exceeded 1 Gb/s, and 50/125 (OM2 or OM3 - laser optimized) - a higher bandwidth fiber compatible with all fiber equipment. OM3 50/125 fiber is rated for use with lasers for >gigabit networks and is more often the choice now.  Singlemode fiber is also specified for longer backbone links, as in a campus, for very high speed networks or where distance exceeds even OM3 fiber capability. 
     Horizontal connections have traditionally been UTP copper, compatible the network port provideded on practically every type of computer gear. With the advent of wireless communications of adequate bandwidth and the inclusion of wireless adapters in all laptops and many other portable devices, wireless has become the user connection of choice. 
     Virtually every corporate network now includes wireless, which is, of course, not wireless, since access points are connected into the network with copper or fiber cabling. Provision is necessary in the design of a facility for adequate numbers of access points, cabling to the access points and proper power, also conditioned and uninterruptible.
     Fiber optics is also a horizontal option in TIA 568, but not often used because of the higher cost of electronics.  The exception is where high bitrate networks or future upgrades are expected. However, a properly designed centralized fiber network that connects the desktop directly to the computer room with no intermediate electronics, only passive interconnections, does not need a telecom room and saves the cost of conditioned power, data ground, AC and the floor space of the telecom room, which may offset the additional cost of the fiber electronics. 
       The telecom closet, or telecom room (TR) as it is now called, houses the hubs for the computers in the work areas. These hubs are interconnected on "backbone" wiring which is mostly fiber optics, as it usually carries higher speed signals over longer distances and provides isolation from ground loops, another problem with copper cabling in LANs. The main cross-connect (MXC) or equipment room contains the network and telco hardware. For traditional POTS (plain old telephone service) telephones, their lower bandwidth requirements allow longer runs, so they are usually simply connected to backbone cables in the telecom room with a punchdown and run straight to the PBX. Many users are now using VoIP (voice over Internet protocol) phones which share network cables with LANs.

Cabling Design Criteria 

Cable Types
Cabling for premises networks has many options, mostly determined by performance of the cabling. Today, one can choose UTP cabling of grades, called categories, commonly referred to as Cat 5 (Category 5), Cat 5e (enhanced Category 5), Cat 6 and Cat 6A (augmented Category 6). Higher ratings are for cables with higher bandwidth and other performance specifications, explained here. Likewise, fiber is rated as OM1, OM2, OM3 and some called OM3+ by manufacturers until OM4 standards are ratified. Again, higher ratings are for fiber with higher bandwidth capability, explained here.

As you might expect higher performance cable supports higher bit rate systems, and in the case of fiber, longer link distances (copper is limited by the standards and the laws of physics) but at a higher cost. Users generally choose the highest bandwidth cabling they can justify as it provides more "headroom" for networks installed today and more likelihood of supporting faster networks in the future.

A general restriction for structured cabling is the permissible distances for cable runs. The table below lists cable distances for various types of permitted cabling. The restrictions on fiber links in the horizontal are arbitrary to be equal to copper cabling and may be exceeded for many network uses. Fiber lengths in the backbone may be restricted by the bandwidth of the fiber when used with high speed networks.

Cable Distances
Cable Type Distance (M) Distance (F)
UTP copper (data) 100 330
UTP copper (voice-POTS) 800 2625
UTP Copper
(ADSL) 9/12.9/52.8 Mb/s
5000/1500/300 16404/4900/1000
MM fiber (horizontal) 100 330
MM fiber (centralized) 300 1000
MM fiber (backbone) 2000 6560
SM fiber (backbone) 3000 9840

Copper cabling designed into a network is allowed 100 meters total length, comprised of 90m of permanently installed cable (the "permanent link") and up to 10m of patchcords used to interconnect cabling or connect active networking equipment. As long as the installed length does not exceed 90m, it meets the standards, and, if properly installed, should pass certification tests.

Fiber has much more leeway in premises cabling, with longer lengths possible and more options in termination. When designing fiber networks, one must design the paths for the installed cables, estimate the lengths and do an analysis of the losses incurred in that section, called a loss budget, to determine if the link will meet the standards and support the network electonics proposed as well as create pass/fail criteria for testing.

All premises cables, copper or fiber, must be rated per electrical codes for flammability in order to be used indoors. Copper cables are rated differently than fiber, which may be non-conductive or conductive if metallic elements are included in the cable design. Refer to the cable sections for more detail on cable ratings.

Designing Pathways and Spaces
Industry standards cover designing pathways and spaces in great detail. Certainly providing adequate space and proper design of cabling systems is needed for a correct installation. However, the design of pathways and spaces is hardly the sole responsibility of the cabling designer, contractor or installer. It is a cooperative effort that should involve a the owner and/or lessor of the facility, the architects and engineers, information technology personnel and other contractors: mechanical, electrical, plumbing, etc. Familiarity of the needs of cabling and industry standards is important to all these parties in order to complete a successful installation.

Proper installation depends on proper design. The facility in which cabling is being installed should be designed according to industry standards which include all the pathways and spaces in which cabing and equipment will be installed. Even where the cables are routed in the building is important. Cables need to be kept dry and in moderate temperatures. Above the ceiling in some buildings in hot climates can get very hot, causing UTP cable to have higher attenuation so it will not support full standard link distances.

Supports for the cables should be wide enough to support the cables without distortion, spaced closs enough so the weight of cable supported (copper cable bundles are heavy!) and have no sharp edges to cut or kink the cables.
 
The performance of the cabling network is also heavily dependent on the installation. The components used in structured cabling installation have been carefully designed and exhaustively tested to meet or exceed the requirements of EIA/TIA 568 for performance at 100-250 MHz. If the cabling is not properly designed and installed, performance will be degraded.


Pathways 

Horizontal  pathways are facilities for the installation of telecommunications cable from  the
telecommunications closet to the work area telecommunications outlet/connector.
Backbone pathways consist of intra- and interbuilding pathways that provide the means for  placing  backbone  cables between the entrance room or space, telecommunications closets, equipment rooms, or the main terminal space.  Backbone pathways  may be either vertical or horizontal depending on the building layout. Interbuilding backbone pathways extend between buildings. Intrabuilding backbone pathways  are contained within a building.
Pathways encompass underfloor, access  floor, conduit,  tray and wireway, ceiling, and perimeter facilities. The pathway and cable type will determine the maximum number of cables that can be accomodated and vice versa. The design shall provide a suitable means and method for supporting cables from equipement room to the telecommunications closet (backbone) and from  the telecommunications closet  to  the  work areas to  be served (horizontal.) Cable shall not be laid directly on the  ceiling  tile or rails. All pathways shall be installed, grounded and bonded per applicable building, fire and electrical codes.


Work Areas 
A work area is where a user is sited. The work area my be an office or area divided by modular office furniture. For planning purposes, a user should be allocated approximately 10 m2 or 100 square feet of floor space.  Each work area should have at least one cabling outlet with one voice and one data jack.  This outlet should be placed within 1 m ( 3 ft) of a power outlet.
Work areas may be divided into zones encompassing several work areas which are served by multi-user telecom outlet assemblies (MUTOA) mounted on building walls or columns (not above ceilings) and use short drop cables to each user.

Telecommunications Room (Closet)
The telecommunications room on each floor is the location of the common access point for backbone and horizontal pathways. The telecommunications closet is designed to contain telecommunications equipment, cable terminations, and associated cross-connect cable/wiring. The telecommunications closet shall be  located as close as practicable to the center of the  area  served. Telecom rooms should not share space with other facilities including electrical with the exception of electrical services required for the equipment placed in the room.

There should be one telecom room per floor or more where the areas served are greater than 1000 m2 or 10,000 ft2 or the distance to work areas will exceed 90m, the maximum length of permanently installed cable.

Telecom room space should be large enough for the served area. In new buildings, requirements  should be coordinated with the architect and mechanical contractors. In existing structures, rooms may need to be constructed.
   
Telecom Room Floor Area
Served Area (Floor Space) Room Size (Dimensions)
m2 ft2 m ft
1000 10,000 3 X 3.4 10 X 11
800 8,000 3 X 2.8 10 X 9
500 5,000 3 X 2.2 10 X 7
 
Traditionally, telecom rooms have had two walls covered by 20 mm (3/4 in) plywood to support attached equipment, including large arrays of punchdown blocks for POTS cables. With the widespread usage of racks of patch panels, such may not be needed and should be decided with the cooperation of the user.

Telecom rooms need conditioned, uninterrupbible power and a data ground. Lighting and other electrical equipment should be run off the normal building power. Adequate AC must be provided to maintain the temperature the same as the surrounding area or at least within the operating limits of the equipment.


Equipment Room 
The equipment room is a centralized  space  for telecommunications equipment (e.g., PBX, computing  equipment, servers, switches, routers, storage devices, video switches, etc. that serve occupants of the facility. Any or all of the functions of a telecommunications  closet or entrance facility may alternately be provided by an equipment room. The equipment room should be restricted to telecom equipment and adequate security provided.
The room should be sized for current and future usage. Guidelines for floow area of the equipment room are given inthe table below.  Accessibility for large equipment is often necessary and should be provided.

Equipment Room Floor Area
# Work Areas Room Area ( m2) Room Area (ft2)
 <100 14 150
101-400 37 400
401-800 74 800
801-1200 111 1200

Like telecom rooms, the equipment room needs conditioned, uninterrupbible power and a data ground. Lighting and other electrical equipment should be run off the normal building power. Adequate AC must be provided to maintain the temperature the same as the surrounding area or at least within the operating limits of the equipment.

Entrance Facility 
The entrance facility  consists of the telecommunications service entrance to the building,  including
the entrance through the building wall, and  continuing to the  entrance room  or space. The entrance facility may contain the backbone pathways  that link to the main  terminal space and  to
other buildings in  campus situations. Antenna entrances may also constitute part of the entrance facility.
All carriers and telecommunications providers involved in providing service to the  building  shall  be contacted  to establish their requirements and explore alternatives for delivering service. The  location of other utilities, such as  electrical, water, gas, and sewer, shall be considered in the selection of the telecommunications entrance facility location. A service entrance pathway shall be provided, The  basic methods for provisioning  are  underground, buried, aerial pathways, and tunnels.
The entrance room or space is the component of the entrance facility that provides  space
for the placing and termination of protectors on the entrance cable and may contain network interface devices. If network interface devices and telecommunications equipment  are  required
in the entrance room, additional space will be needed. 

Other Design Considerations 
Numerous issues must be addressed inthe construction of a cabling system, many of which involve other parties, e.g. the end user, IT personnel, architects, engineers, electricians and other contractors. Cabling pathways and spaces must maintain separation from electrical conductors per electrical codes, provide protection from lightning and other power surges, be protected from electromagnetic interference (EMI) and be properly grounded and bonded. All penetrations must be properly firestopped per building codes.

Removal and Recycling of Abandoned Cables 
Unless directed by the owner or other agency that unused cables are reserved for future use and the cables are marked accordingly, it may be required to remove abandoned optical fiber cable (cable that is not terminated at equipment other than a connector and not identified for future use with a tag) as required by the National Electrical Code or local codes.
At the discretion of the owner of the site, the contractor may be requested to remove other cables (e.g. copper communications or power cables) in addition. Removal of cables is much more time consuming than installation, as each cable must be identified and carefully removed to prevent damaging other cables. No cable should be cut for removal unless it is positively identified as one to be removed.
All cables removed should be recycled properly.

Documentation 
One cannot emphasize strongly enough the value of documentation. Documentation should be part of the design process, creating the nomenclature and database used throughout the installation process. Installation should be done according to the documentation created in design and every cable, patch panel or other equipment marked properly. TIA 606 is the US standard for cable plant administration which includes marking and documentation.Nothing facilitates proper installation, testing, upgrades, moves/adds/changes or restoration more than proper documentation.

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Table of Contents: The FOA Reference Guide To Fiber Optics




 


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