CHAPTER 7

Outside Plant

Оutside plant is the collection of cables, poles, conduit, and fiber optics that
Interconnect central offices and connect the central office to the subscribers’ premises.
The local loop is the most expensive and the least technically complex portion
оf the entire telecommunications system. Wide-bandwidth signals travel
аcross the country in ribbons of fiber-optic cable, are digitally routed and
switched, but finally must be converted to analog and piped to the customer over
a pair of wires that may cut off any frequency higher than 4 kHz. Even though the
local loop is less than ideal, together with its supporting conduit it is arguably the
most valuable asset the ILECs have, and one that cannot easily be duplicated or
replaced by their CLEC competitors. Furthermore, technologies such as DSL,
which Chapter 8 discusses, increase the value of the local loop.
The local loop is often referred to as the “last mile,” and it consists largely of
twisted-pair copper wire enclosed in cables that are routed through conduit, buried
in the ground, or mounted on poles to reach the end user. Except for metropolitan
areas where many buildings are served by fiber optics, the local loop has not
changed much over the years. Insulation has improved, cable sheaths have evolved
from lead to nonmetallic, and improved splicing techniques have increased cable splicing productivity. Much of the outside plant that once was aerial has migrated
underground, which makes it less vulnerable to damage. Otherwise, cable today is
technically little changed from that placed a century ago.
As the telephone network developed, central offices were linked with high quality
copper cables that were enclosed in an underground conduit. Manholes were
placed to provide splice points and to hold load coils, which were placed at 6000-ft
(1800 m) intervals to improve voice frequency response. As conduits filled to capacity,
the next step was to remove load coils and install analog carrier, which typically
multiplexed 12 channels on two copper pairs. In the 1960s digital carrier began to
replace analog, doubling the capacity of the cable. Twenty years later, fiber optics
changed the character of interoffice trucking completely. Its small size and enormous
capacity has now all but eliminated copper cable in the plant between central offices.
Eventually, the local loop will undoubtedly migrate to fiber optics. The large
ILECs, SBC and Verizon, have announced plans to place fiber loops gradually
over an indefinite period. For now, copper cable will serve the majority of subscribers
for several reasons. First is the matter of economics. Fiber-optic cable has
enormous bandwidth, but it must be multiplexed, and multiplexing equipment is
expensive for the subscriber’s premises and requires local powering. Today’s
copper cable carries power to the customer, and with backup batteries in the central
office, telephone service is effectively immune to commercial power failures.
Conversion to fiber is also deterred by the magnitude and cost of the task. LECs
install fiber as needed to meet service demands, but the benefits are insufficient to
justify extending it to millions of residences and small businesses. Competition, however, is changing the mix. Cable providers have fiber to network nodes now, and are inspiring the ILECs to follow suit. Many observers believe that video-on-demand
will drive the conversion to fiber in the local loop, but so far the service has not
caught on. The industry has even given fiber-in-the-loop technology the name FITL,
but it has yet to make a significant impact except for feeds to large businesses.
This chapter discusses how outside plant is designed and constructed. Our
focus is LEC plant, but the principles are equally applicable to private organizations
that use copper cable to link their buildings. Interbuilding fiber applications
are somewhat more complex, and are discussed in Chapter 17.
OUTSIDE PLANT TECHNOLOGY

Outside plant (OSP), diagrammed in Figure 7-1, links the central office to subscribers’
premises. Feeder cables enclosed in conduit connect the central office to office
buildings or neighborhood centers. Feeder pairs are terminated directly on terminals
in office buildings, usually in enough quantity to provide Centrex service, which
requires a pair per station. In neighborhood centers feeder cables are cross-connected
to smaller distribution cables, which are placed along streets and alleys to provide connections to homes and smaller commercial buildings. Terminals provide access to the cable pairs, which are connected to the subscribers’ premises with drop wire.
Supporting Structures
Cable is classified according to its supporting structure:
-  Aerial cable supported on pole lines
-  Underground cable enclosed in buried conduit
-  Buried cable placed directly in the ground
Aerial cable is lashed to a galvanized metallic strand known as a messenger,
which is attached to the poles. Self-supporting aerial cable contains an internal
messenger. Down guys and anchors are placed at bends and at the ends of cable
runs to relieve strain on the poles. Aerial cable is unsightly and vulnerable to
storm damage, but it is less expensive than placing cable underground.
Underground cable is placed in conduit. Cost aside, conduit is always the preferred
method of placing cable under ground. Where future additions and rearrangements
will be required, LECs provide empty ducts for expansion. Manholes are
placed at intervals for splice points and to house T-carrier repeaters and load coils.
Direct burial is often less expensive than conduit. Buried cable is either
placed in an open trench or plowed with a tractor-drawn implement that feeds the
cable underground through a guide in the plow blade.Cable Characteristics

Twisted-pair cables are classified by the wire gauge, sheath material, protective
outer jacketing, and the number of pairs contained within the sheath. Sizes range
from one- or two-pair drop wire to 3600-pair cable used for central office building
entrance. Cable size is limited to the size of conduit ducts, which are up to 4 in.
(10.5 cm) in internal diameter. The sheath diameter that a conduit will hold
depends on wire gauge and the number of pairs. Cables of larger sizes, such as
2400 and 3600 pairs, are used primarily for entrance into telephone central offices.
Wire gauges of 26, 24, 22, and 19 AWG are used in loop plant. Cost considerations
dictate the use of the smallest wire gauge feasible, consistent with technical
requirements. Finer gauges are used close to the central office to feed the largest
concentrations of users. Coarser gauges are used at greater distances from the
central office as needed to reduce loop resistance.
Cable sheath materials are predominantly high-durability plastics such as
polyethylene and polyvinyl chloride (PVC). Cable sheaths are constructed to protect
against damage from lightning, moisture, induction, corrosion, rocks, and
rodents. In addition to the sheath material, submarine cables are protected with
coverings of Kevlar and steel armor. Besides the outer sheath, cables are shielded
with metallic tape that is grounded on each end.
Cable pairs are precisely twisted to preserve their electrical balance.
Unbalanced pairs are vulnerable to induced noise, so the twist is designed to
ensure that the coupling between cable pairs is minimized. Cable is manufactured
in layers of pairs twisted around a common axis, with each pair in a unit given a
different twist length. These units are called complements.
Cable pairs are color coded within 50-pair complements. A color-coded
binder is wrapped around the pairs to identify each complement. At splice points,
the corresponding binder groups and pairs are spliced together to ensure end-to end
pair identity and continuity. Cables can be manually spliced with compression
sleeves or ordered from the factory cut to the required length and equipped
with connectors.
Splicing quality is an important factor in preserving cable pair balance.
Older cables are insulated with paper and were spliced by twisting the wires
together. These older splices are often a source of imbalance and noise because of
insulation breakdown and splice deterioration. To prevent crosstalk, it is also
important to avoid splitting cable pairs. A split occurs when a wire from one pair
is spliced to a corresponding wire in another pair. Although electrical continuity
exists between the two cable ends, an imbalance between pairs exists, and
crosstalk may result.
Cable splices and terminations are placed in aerial or aboveground closures
such as the ones shown in Figure 7-2. Cables must be manufactured and spliced
to prevent water from entering the sheath because moisture inside the cable is the
most frequent cause of noise and crosstalk.
Loop Resistance Design
Outside plant engineers select the wire gauge to achieve an objective loop resistance
commensurate with the characteristics of the switching system. All telephone
switching systems, including central office switches, PBXs, and key telephone
systems are designed to support a maximum loop resistance range. The loop
resistance includes the following elements:
_ Battery feed resistance of the switching system (usually 400 Ω)
_ Central office wiring (nominally 10 Ω)
_ Cable pair resistance (variable to achieve the design objective of the central office or PBX)
_ Drop wire resistance (nominally 25 Ω)
_ Station set resistance (nominally 400 Ω)
Central office switches can signal over loop resistance ranges in the order of
1300 to 1500 Ω. If the fixed elements listed above total 835 Ω, that leaves some
500 Ω for cable pair resistance. This method of design is called resistance design.
Most PBXs have less range, typically 400 to 800 Ω. The range can be extended with
subscriber carrier or range extension devices, which boost the nominal –48-V central
office battery to –72 V. The range limitation of subscriber loops depends on the current
required to operate PBX trunk circuits, dual tone multi-frequency (DTMF) dials,
and the telephone transmitter. Designers aim for loop current between 23 and 50 mA.
A further consideration in selecting cable is the capacitance of the pair,
expressed in microfarads (μF) per mile (1.6 km). Ordinary subscriber loop cable
has a high capacitance of 0.083 μF/mile. Low capacitance cable, used for trunks
because of its improved frequency response, has a capacitance of 0.062 μF/mile.
Special 25-gauge cable used for T carrier has a capacitance of 0.039 μF/mile.
Special types of cable are used for cable television, closed circuit video,
LANs, and other applications. Some types of cable are constructed with internal
screens to isolate the transmitting and receiving pairs of a T-carrier system.
Feeder and Distribution Cable

Cable plant is divided into two categories in the local loop—feeder and distribution.
Feeder cables extend from the central office to a serving area. Main feeders
are large backbone cables that exit the central office and are routed, usually
through conduit, to intermediate branching points. Branch feeders are smaller
cables that route pairs from the main feeders to a serving area. Distribution cable
extends from a serving area interface to the users’ premises. Figure 7-3 shows the
plan of a typical serving area.
Where 25 pairs or more are needed in a building, the LEC splices distribution
cable directly into the building. Otherwise, the feeder cable may terminate in
a crossconnect cabinet similar to the one shown in Figure 7-4, which serves as a
junction point between the feeder and distribution cable. Terminals provide access
to cable pairs, either on binding posts or direct connection. Terminals may be
mounted on the ground in pedestals, in buildings, on aerial cable messengers, or
underground. Aerial or buried drop wire runs from the terminal to a protector at
the user’s premises.
To the degree feasible, feeder and distribution cables are designed to avoid
bridged tap, which is any portion of the cable pair that is not in the direct path
between the user and the central office. Bridged tap has the electrical effect of
connecting a capacitor across the pair and impairs the frequency response of the
circuit. Bridged tap is inherent in the way drop wire is often connected to the
subscriber’s premises. The drop wire is bridged across the distribution cable pair,
and the unterminated pair continues on to the end of the cable. Bridged tap often
must be removed (i.e. the pair cut off beyond the drop) before the loop can be used
for DSL or high-speed modems.
The frequency response of long subscriber loops is improved by loading.
Load coils are small inductors wound on a powdered iron core as shown in
Figure 7-5. They are normally placed at 6000-ft (1800 m) intervals on loops longer
than 18,000 ft (5.5 km). Load coils are contained in weatherproof cases that are
mounted on poles or in manholes. Load coils flatten the frequency response in the
voice band, but attenuate higher frequencies. They must be removed from cable
pairs that support high-frequency applications such as T carrier and DSL.
ELECTRICAL PROTECTION

Whenever communications conductors enter a building from an environment that
can be exposed to a foreign source of electricity, it is essential that electrical protection
be used. Protection is required for two purposes—to prevent injury or
death to personnel and to prevent damage to equipment in case of lightning strike
or cross with electrical power. Common carriers protect both ends of cables
between their central offices and the subscribers’ premises. The type of protection
provided is designed to prevent injury or death to personnel, but may not be
sufficient to prevent damage to delicate telecommunications and computer
equipment. Interbuilding cables on customer premises may require protection,
and the carrier will supply it only if it owns the cable. LECs normally wire a building
to their point of demarcation and leave it to the subscriber to place any cable
needed beyond that point.
Protection requirements are based on the National Electrical Safety Code.
ANSI T1.316-2002, Electrical Protection of Telecommunications Outside Plant, and
much of the overview in this section is more stringent than the code.
Determining Exposure
The first issue in determining protection requirements is whether the cable is
considered exposed. An exposed cable is one that is subject to any of the following
hazards:
_ Contact with any power circuit operating at 300 V r ms (root mean
square) or more from ground
_ Lightning strike
_ Induction from a 60 Hz source that results in a potential of 300 V r ms
or more
_ Power faults that cause the ground potential to rise above 300 V r ms
All aerial cables are considered exposed. Even though a short section of aerial
cable may not be in proximity to power at the time it is constructed, aerial power
may be added later and expose the cable. A cable is considered to be exposed if any
pairs within are exposed. All buried and underground cables should be considered
exposed unless one or more of the following conditions exist:
_ The region experiences five or fewer thunderstorm days per year
and the earth resistivity is less than 100 m Ω.
_ A buried inter building cable is shorter than 140 ft (43 m) and has
a shield that is grounded on both ends.
_ A cable is totally within a cone of protection because of its proximity
to buildings or other structures that are grounded.

Zone of Protection

Structures extend a zone of protection that diverts lightning strikes and shields the
cable from damage. As Figure 7-6 shows, if a mast or building is at least 25 ft (7.6 m)
high, it offers lightning protection to objects within a radius equal to the mast
height. If the structure is 50 ft (15.2 m) high, it protects within a radius of twice the
mast height. To illustrate, assume that a cable runs between two buildings, each of
which is 50 ft high. Each building extends a zone of protection of 100 ft, which
means that a buried cable 200 ft long would not be considered exposed to lightning.
For structures higher than 50 ft, the zone of protection concept can be
pictured with the “rolling ball” concept. Visualize a ball 300 ft (91 m) in diameter
rolled up against the side of the structure as in Figure 7-7. The zone of protection
is shown as the shaded area. Note that the zone of protection applies only to lightning,
not to power exposures. If a cable rises above the elevation of surrounding
terrain such as on a hilltop or tower it should be considered exposed to lightning
even though it is in an area that would otherwise be excluded by the earth
resistivity and thunderstorm frequency requirements.
Normally, ground is considered to be at zero potential, but current flow from
a power fault or lightning strike can cause the ground potential to rise. Induction
occurs when power lines and telephone cables operate over parallel routes. Under
normal conditions the magnitude of the induction is not so great as to constitute
a hazard, but when telecommunications lines are unbalanced or when a fault
occurs in the power line, the amount of induced voltage can rise to hazardous
levels. This is not a concern in most private networks, but designers should be
alert to the possibility of induction whenever power and telecommunications
circuits share the same route, though they may not share a pole line.
Although a circuit is protected to eliminate hazards to users, the equipment
attached to it may be sensitive to foreign voltages. Since the equipment provider is
in the best position to know of this sensitivity, all requests for proposal and purchase
orders should require the vendor to specify the level of protection required.
Protection Methods Personnel and equipment can be protected from the hazards of unexpected contact with a foreign source of electrical potential by the following methods:
- Insulating telecommunications apparatus
- Shielding communications cables
- Grounding equipment
- Opening affected circuits
- Separating electrical and telecommunications circuits
This section discusses how each preventive measure is applied to telecommunications
circuits.
Insulating Telecommunications Apparatus

The first line of defense against contact with foreign voltages is insulation.
Polyethylene, which is the insulation used with most copper cables, has a conductorto-
conductor breakdown value of from 1000 to 4000 V. Although this is enough to
guard against high voltages, a lightning strike or power cross may destroy the insulation, so it alone is not enough to satisfy electrical protection requirements.

Shielding Communications Cables

Cables can be shielded from lightning strikes by placing a grounded conductor
above the cable so it intercepts the lightning strike. A grounded shield wire can be
placed above aerial cable to attract the lightning strike to itself. Shield wires can
also be buried above a communications cable. If there is enough separation to
prevent arcing between the shield and the cable, this method is effective.

Grounding Equipment

An important principle of electrical protection is to provide a low-impedance path to
ground for foreign voltage. Both carbon and gas tube protectors, which are illustrated
in Figure 7-8 operate on the principle of draining the foreign voltage to ground.
The simplest form of protector is the carbon block. One side of the carbon block
is connected to a common path to ground. It is essential that the ground path be a
known earth ground. In most buildings the grounding point for the power entrance
is suitable. A metallic cold-water pipe may be a satisfactory ground, but only if
the entire water system is metallic. To ensure an effective water-pipe ground, the pipe
should be bonded to the power ground with a copper wire of at least #6 AWG.
The conductors are connected to the other side of the carbon block. When
voltage rises to a high enough level to arc across the gap, current flows, the block
fuses, and the cable pair is shunted to ground. When a carbon block protector is
activated, it is destroyed and must be replaced.
A gas tube protector connected between the communications conductors
and ground is a better, but more expensive alternative. Like the carbon block
protector, its purpose is to provide a low-impedance path to ground for foreign
voltage. The electrodes of the gas tube are contained in a glass envelope that is
filled with an inert gas. When the breakdown voltage is reached, the gas ionizes
and current flows until the voltage is removed. When the voltage is removed, the
tube restores itself. Although gas tubes are more expensive than carbon blocks,
the self-restoring effect may repay the additional cost. They are particularly
effective in sensitive apparatus that is easily damaged by relatively low voltages.
Another type of protector is the heat coil, which is a spring-loaded device that
grounds conductors when it operates. Heat coils protect against sneak currents,
which are currents that flow from voltages that are too low to activate a carbon block
or gas tube protector. The heating effect of the sneak current is sufficient to melt a low melting-point metal that keeps the electrodes separated. When the metal is melted,
the spring forces the electrodes together and the circuit is grounded until the heat coil
is replaced. Heat coils are normally installed on central office protector frames.
At the user’s end of the circuit, protectors range from a simple single-pair
device to multiple-pair protected terminals. Although station protectors are
adequate to prevent injury to users, they are often inadequate to prevent damage
to delicate electronic equipment.

Opening Affected Circuits

Everyone is familiar with the next method of protecting circuits and equipment:
a fuse or circuit breaker. If the communications conductors are opened before they
enter the building or before they reach the protected equipment, current cannot
flow and damage the equipment or reach the operator. The LECs often install
a fuse cable between its distribution cable and the building entrance. A fuse cable
is a short length of fine-gauge cable, usually 27-gauge. In the case of sustained
high current the finer gauge fuse cable will open before the protected cable.
A fuse, by its nature, takes time to operate. Current flow during lightning
strikes tends to be short, lasting less time than it takes to open the cable. Therefore,
fuse cables are effective against power crosses, but not against lightning.

Separating Electrical and Telecommunications Circuits

Another method of protecting from accidental cross with electrical power is
adequate spacing. Buried power and telecommunications cables share a joint
trench to many buildings. The minimum acceptable separation between power
and telecommunications circuits in a joint trench is 1 ft; more separation gives an
additional measure of protection. The sharing of a joint trench with at least the
minimum separation does not, of itself, create an exposure condition.
DIGITAL LOOP CARRIER (DLC)

DLC is increasingly used to deliver telecommunications service to concentrations
of subscribers. The DLC unit is fed from the central office with either fiber optics
or copper cable. The feed from the DLC to the subscriber is usually copper cable.
DLCs are available in two configurations. One alternative uses matching central
office and remote units with analog interfaces to the central office line circuits. The
other uses a standard interface known as GR-303 in the central office. The GR-303
interface matches the remote unit to any compatible digital switch. The degree to
which the DLC increases the channel-carrying capacity of a cable pair is known as
its pair gain. For example, a 24-channel DLC operating over separate transmit and
receive pairs has a pair gain of 22. DLC provides better transmission quality than standard cable facilities. The
transmission loss is fixed, normally at 5 dB or less, and channel noise is lower than
cable pairs. The remote terminal is contained in a pole-mounted cabinet or in an
environmentally controlled vault that is mounted either above or below ground.
The enclosure provides space for multiple racks of equipment, battery backup,
alarms, and other facilities to provide reliability that is equivalent to cable pairs.
The feed from the central office is often a self-healing fiber ring. In addition to
POTS service, the complex may be equipped for DSL. A variety of plug-in units
are available to support services such as ISDN and DSL to subscriber locations
that are outside the cable-pair range for these services.
OUTSIDE PLANT APPLICATION ISSUES

The focus of this chapter has been on LECs’ outside plant applications, but the
same principles apply to private cable systems. Private campus networks in
the past have had little choice but to use copper cable to link buildings because
fiber-optic equipment was too expensive for voice applications. Today, VoIP
and fiber-connected remote switching units often make it feasible to eliminate
copper cable between buildings except for alarm systems. Wireless also offers an
alternative to both copper and fiber in small locations. Nevertheless, copper
cabling will survive on campuses for many decades to come.
Any new cable construction should include plans for both fiber and copper
cable. For telephone systems the break-even point between running copper cable
directly to the PBX as opposed to using a fiber remote depends on the number of
stations. For a few stations, perhaps 20 or fewer, copper cable is usually less
expensive and more reliable than a separate remote unit. Fiber will be part of
almost any plan, either initially or in the future, so where fiber is run anyway,
larger concentrations of stations can be connected to the main PBX with fiber. One
major advantage of fiber is the fact that electrical protection is not needed.
Cable can be buried directly in a campus environment, but conduit should be
used if possible. Conduit not only offers physical protection, it also facilitates
future rearrangements. Flexible conduit can be plowed underground. That method
is often used with small conduits intended for fiber alone. Copper cable in sizes
above 25 pairs requires conduit too large and inflexible to be plowed. Trenching is
usually more expensive and disruptive than plowing, but it is easy to place additional
empty conduits while the trench is open. If the conduit is trenched, there is
usually little reason to use than the full 4-in. size. Copper cable and fiber can share
the conduit if it is large enough. In a shared conduit the fiber should be protected
by installing an inner duct, which is a 1-in. flexible sub-conduit.
An investment in cable is far more durable than the apparatus that uses it.
Fiber, copper cable, and conduit have service lives measured in decades. With the
right design, the service life of cables connecting between buildings should
approach the lives of the buildings themselves.