Twisted pair cables support a wide variety of fast modern network standards.
• Two copper conductors form a path for an electrical signal with each wire carrying an equal but opposite signal.
• The wires are twisted to reduce crosstalk, which is the absorbed signals from another pair.
• The conductors are 22- to 24-gauge in thickness and are covered in plastic insulation.
• Pairs are color coded, bundled together, and covered in a plastic jacket or sheath.
• Most cables contain four twisted pairs.
• Cables may contain 25 or 100 pairs when used in larger wiring applications.
• Each pair within a length of cable are given a different number of twists to further reduce the effects of crosstalk.
Advantages of twisted pair cabling include:
• Flexibility--you can run twisted pair cabling around tight corners and other places where other types of network cable simply cannot go without being damaged.
• Cost--twisted pair cabling is less expensive than other types of network cabling.
• Ease of use--twisted pair cabling is easy to work with, and it's much easier to install compared to other types of network cabling.
• Works with newer protocols--newer, faster network protocols and standards have been designed to run on twisted pair cabling.
Disadvantages of twisted pair cabling include:
• Susceptible to interference--the sheath around twisted pair cable is relatively thin, making it susceptible to EMI.
• Susceptible to eavesdropping--with the right equipment, you can pick up signals emanating from the wire.
There are two common abbreviations for twisted pair cables.
• UTP stands for unshielded twisted pair. UTP cables are easy to work with and less expensive than shielded cables.
• STP stands for shielded twisted pair.
◦ Shielding is electrically conductive foil or braided material that is wrapped around pairs of wires, around the overall cable, or both.
◦ Shielding helps to minimize crosstalk.
◦ The main purpose of shielding is to minimize the effects of EMI from external sources, such as fluorescent light ballasts.
◦ The shielding can be used as a ground. However, most shielded cables have a special grounding wire called a drain wire.
Specially manufactured twisted pair cables are used in plenum and riser spaces.
• A plenum space is a part of a building that provides a pathway for the airflow needed by heating and air conditioning systems, such as above a dropped ceiling or below a raised floor.
• Plenum rated cables use insulation that is fire resistant and non-toxic when burned.
• Plenum rated cables must be used in plenum spaces.
• Riser rated cables are designed for installations that run between floors.
• Riser requirements are not as strict as plenum requirements
◦ Plenum rated cables can be used in riser spaces.
◦ Riser rated cables must never be used in plenum spaces.
Twisted pair cables can be solid or stranded.
• Solid wires conduct electrical signals better, but are prone to break when they are repeatedly bent.
• Stranded cables are more flexible, but don't carry signals as well.
• Use solid cables in permanent and semi-permanent installations.
• Use stranded cables for patch cords and when cables are frequently moved.
UTP Cable Types
A phone cable is used to connect a PC to a phone jack in a wall outlet to establish a dial-up internet connection. It is also used to connect a DSL modem to a telephone network. It has two pairs of twisted cable (a total of 4 wires).
Cat 3 is designed for use with 10-megabit Ethernet (10BASE-T) or 16-megabit token ring.
Cat 5 supports 100-megabit Ethernet (100BASE-TX) and ATM networking. Cat 5 specifications also support gigabit (1000 Mb) Ethernet.
Cat 5e is similar to Cat 5 but provides better EMI protection. It supports 100-megabit (100BASE-T) and gigabit (1000BASE-T) Ethernet.
Cat 6 supports 10-Gbps Ethernet (10GBASE-T) and high-bandwidth broadband communications. In most cases, Cat 6 cables include a solid plastic core that keeps the twisted pairs separate and prevents the cable from being bent too tightly.
Cat 6a is designed to provide better protection against EMI and crosstalk than Cat 6 cabling. Cat 6a provides better performance than Cat 6, especially when used with 10-Gbps Ethernet (10GBASE-T).
Cat 7:GG45 TERA
The Cat 7 standard was ratified years before the Cat 6a standard to support 10-Gbps Ethernet (10GBASE-T). It requires shielding on each twisted pair and the cable as a whole. It also specifies the GG45 or TERA connectors.
Each type of UTP cable can be substituted for any category below it, but never for a category above. For example, Cat 6 can be substituted for a standard requiring Cat 5e. However, neither Cat 5 nor Cat 3 can be used in a situation where Cat 6 is required. The exception is Cat 7 cabling, and then only when Cat 7 is terminated with TERA connectors.
Coaxial cable is a relatively old technology that is usually implemented with a bus topology. It is not suitable for ring or star topologies because the ends of the cable must be terminated. In this lesson you will learn about:
• Coaxial cable components
• Coaxial cable advantages and disadvantages
• Coaxial cable grades
Coaxial Cable Components
Coaxial cable has the following components:
• Two concentric metallic conductors
◦ The inner conductor is a solid wire made of copper or copper-coated tin.
◦ The outer mesh conductor, or shield, is made of aluminum or tin-coated copper.
• An PVC plastic insulator, which surrounds the inner conductor and insulates the signal from the mesh conductor.
• A PVC plastic cable sheath, or jacket, that surrounds and protects the wire.
Coaxial Cable Advantages and Disadvantages
Coaxial cable has the following advantages and disadvantages:
• Highly resistant to EMI (electromagnetic interference)
• Highly resistant to physical damage
• More expensive than UTP
• Inflexible construction (more difficult to install)
• Unsupported by newer networking standards
Coaxial Cable Grades:
10Base2 Ethernet networking (also called thinnet)
Cable TV and cable networking
Cable TV, satellite TV, and cable networking
To connect computers using fiber optic cables, you need two fiber strands. One strand transmits signals, and the other strand receives signals. Long-haul runs sometimes only need one fiber. The send and receive signals are transmitted over the same fiber. The following are the components of fiber optic cabling:
• The core carries the signal. It is made of glass or plastic.
• The cladding maintains the signal in the center of the core as the cable bends.
• The sheathing protects the cladding and the core.
• Totally immune to EMI (electromagnetic interference)
• Highly resistant to eavesdropping
• Supports extremely high data transmission rates
• Allows greater cable distances without a repeater
• Very expensive
• Difficult to work with
• Special training required to attach connectors to cables
Multi-Mode vs. Single-Mode
Multi-mode and single-mode fiber cables are not interchangeable. The following table describes multi-mode and single-mode fiber cables.
• Data transfers through the core using a single light path.
• The core diameter is around 8–10.5 microns.
• Cable lengths can extend a great distance.
• There is less modal dispersion, so bandwidths can be higher.
• Higher-cost electronics are required to send signals down a single path.
• Optimized for 1310 nm and 1550 nm light sources.
• Data transfers through the core using multiple light paths.
• The core diameter is around 50 to 100 microns.
• There is more modal dispersion due to the multiple paths.
• Cable lengths are limited in distance and are dependent on bandwidth.
• Higher light gathering capacity simplifies connections and allows lower-cost electronics.
• Optimized for 850 nm and 1300 nm light sources.
• Used with single-mode and multi-mode cabling.
• Has a keyed bayonet-type connector.
• Also called a push-in and twist connector.
• Each wire has a separate connector.
• Nickel plated with a ceramic ferrule to ensure proper core alignment and prevent light ray deflection.
• Mnemonics are Set-and-Twist or Straight Tip.
• Used with single-mode and multi-mode cabling.
• Has a push-on/pull-off connector that uses a locking tab to maintain connection.
• Each wire has a separate connector.
• Uses a ceramic ferrule to ensure proper core alignment and prevent light ray deflection.
• Mnemonics are Set-and-Click or Square Connector.
• Used with single-mode and multi-mode cabling.
• Composed of a plastic connector with a locking tab that is similar to a RJ45 connector.
• A single connector with two ends keeps the two cables in place.
• Uses a ceramic ferrule to ensure proper core alignment and to prevent light ray deflection.
• Is half the size of other fiber optic connectors.
• Mnemonics are Lift-and-Click or Little Connector.
• Used with single-mode and multi-mode cabling.
• Composed of a plastic connector with a locking tab.
• Uses metal guide pins to ensure that it is properly aligned.
• A single connector with one end holds both cables.
• Uses a ceramic ferrule to ensure proper core alignment and prevent light ray deflection.
• Typically used with single-mode cabling.
• Each wire has a separate connector.
• Uses a threaded connector.
• Designed to stay securely connected in environments where it may experience physical shock or intense vibration.
For long cable running between floors or overhead, you might hire an experienced contractor to install the cable and the necessary connectors. Adding connectors onto a fiber optic cable takes some practice. Remember to complete the following:
• Keep the area as clean as possible.
• Cut the cable with a clean 90-degree cut.
• Polish the end of the cable prior to adding the connector. Use special polishing film and tools for polishing cable ends.
• Glue or crimp the connector onto the cable.
• Cover or cap any connectors that won't be hooked up immediately.
• If necessary, you can directly splice two cable ends together; however, this requires expensive and specialized equipment.
Fiber Optic Cable:
There are two light source technologies prevalent in fiber optic communications, diode laser and high-radiance light-emitting diode (LED). The light produced by these technologies is in the infrared region of the light spectrum.
• The most common wavelengths used in fiber optics are 850 nm, 1300 nm, 1310 nm and 1550 nm.
◦ In glass, these longer wavelengths have lower attenuation or signal loss due to scattering.
◦ Attenuation in glass due to light absorption for these wavelength is almost zero.
• Multi-mode fiber is designed to operate at 850 nm and 1300 nm.
• Single-mode fiber is optimized for 1310 nm and 1550 nm.
Wave Division Multiplexing (WDM) joins several light wavelengths (colors) onto a single strand of fiber.
• This enables light signals in both directions across a single fiber.
• Today's systems can easily multiplex 160 signals.
• WDM is mostly used by long-haul and high-speed providers.
• Most WDM systems are designed to be used with single-mode fiber.
When working with fiber optic cabling, you can use media converters to switch between different network media. For example, you can convert:
• Single-mode fiber to copper Ethernet wiring
• Multi-mode fiber to copper Ethernet wiring
• Single-mode or multi-mode fiber to coaxial wiring
• Single-mode fiber to multi-mode fiber
As a network administrator, you are often responsible for both data and telephone wiring for your organization. In this lesson, you will learn about:
• Demarcation points
• Main distribution frames (MDFs) and intermediate distribution frames (IDFs)
• Punch down blocks
• Patch or distribution panels
• MDF, IDF, and patch panel documentation
When you contract with a local exchange carrier (LEC) for data, internet, or telephone services, they install a physical cable and a termination jack onto your premises.
• The demarc (short for demarcation point) is the line that marks the boundary between the telco equipment or cable and your private network or telephone system.
• The demarc is also called the minimum point of entry (MPOE) or the end user point of termination (EU-POT).
• In businesses, the demarc is typically located on the bottom floor of a building, just inside the building, and identified by an orange plastic cover on the wiring component.
• In residential buildings, the demarc is often a small box on the outside of the house.
• The demarc may be:
◦ A box on the wall with a simple RJ45 connection
◦ A 50-pin RJ21 connector
◦ A fiber optic connection
◦ A port on a network interface device (NID)
• If needed, a demarc extension can be used to move the demarc to another location in a building. For example, if your organization uses only one floor of a building, you will want the demarc where it is not exposed to other organizations.
◦ You are responsible for installing the demarc extension, but the LEC might do it for an additional charge.
• Normally, the LEC is responsible for all equipment on one side of the demarc, and the customer is responsible for all equipment on the other side of the demarc.
While a NID can be a passive demarc that organizes the cable and connections, a more intelligent NID, known as smartjack, may be provided by the LEC.
• Smartjacks are maintained by the LEC.
• Smartjacks are typically used for more complex services, such as a T1 line.
• Smartjacks can provide signal conversions, buffer signals, and regenerate signals.
• Smartjacks may provide diagnostic capabilities for the LEC.
◦ The loopback capability can be used to test signals by transmitting them back to the LEC.
◦ Alarm indicators can report trouble to the LEC.
◦ Indicator lights can show the configuration and status of the Smartjack
Main Distribution Frames (MDFs) and Intermediate Distribution Frames (IDFs):
Strictly speaking, a main distribution frame (MDF) is a frame or rack that is used to interconnect and manage telecommunication wiring in a building. It functions like an old-time telephone switchboard, where operators used connecting wires to route telephone calls. Today's MDF describes the room that houses the traditional MDF along with networking patch panels. Often, rack mounted equipment is also housed in an MDF.
• A traditional MDF may exist in a dedicated room or a within rack space in a datacenter.
• An MDF is usually located on the bottom floor or basement of a building.
• All internet or WAN demarcation points are normally near or within the MDF.
A traditional intermediate distribution frame (IDF) is a smaller wiring distribution frame or rack within a building. Like an MDF, the room that houses the IDF along with networking patch panels and rack-mounted equipment is called an IDF.
• IDFs are typically located on each floor directly above the MDF, although additional IDFs can be added on each floor as necessary.
• IDFs located above the MDF are connected using a vertical cross connect (VCC), or wire bundles that run vertically between the MDF and an IDF.
• If a floor has more than one IDF, the IDFs are connected with a horizontal cross connect (HCC).
Punch Down Blocks:
Punch down blocks are the predecessors to patch panels. They are commonly used to support low-bandwidth Ethernet and telephony wiring.
A 66 block is a punch down block used to connect individual copper wires together.
• The 66 block has 25 rows of four metal pins. Pushing a wire into a pin pierces the plastic sheath on the wire, making contact with the metal pin.
• There are two different 66 block configurations:
◦ In the 25-pair block (also called a non-split block), all four pins are bonded (electrically connected). Use the 25-pair block to connect a single wire with up to three other wires.
◦ With the 50-pair block (also called a split block), each set of two pins in a row are bonded. Use the 50-pair block to connect a single wire to one other wire.
• With a 50-pair block, use a bridge clip to connect the left two pins to the right two pins. Adding or removing the bridge clip is an easy way to connect wires within the row for easy testing purposes.
66 blocks are used primarily for telephone applications. When used for data applications:
• Be sure to purchase 66 blocks rated for Cat5.
• When inserting wires in the block, place both wires in a pair through the same slot to preserve the twist as much as possible.
A 110 block is a punch down block used to connect individual wires together.
• The 110 block comes in various sizes for connecting pairs of wires (for example 50, 100, or 300 pair).
• The 110 block has rows of plastic slots. Each plastic slot connects two wires together.
◦ Place the first wire into the plastic slot on the 110 block.
◦ Insert a connecting block over the wire and slot. The connecting block has metal connectors that pierce the plastic cable sheath.
◦ Place the second wire into the slot on the connecting block.
• C4 connectors connect four pairs of wires; C5 connectors connect five pairs of wires.
• When connecting data wires on a 110 block, you typically connect wires in the following order:
◦ White wire with a blue stripe followed by the solid blue wire.
◦ White wire with an orange stripe followed by the solid orange wire.
◦ White wire with a green stripe followed by the solid green wire.
◦ White wire with a brown stripe followed by the solid brown wire.
110 blocks are used primarily for telephone applications. They are preferable over 66 blocks in high-speed networks because the introduce less crosstalk. When used for data applications:
• Be sure to purchase 110 blocks that are certified for Cat 5, Cat 6 and Cat 6a.
• When inserting wires, preserve the twist as much as possible.
Use a punch down tool to insert wires into 66 or 110 blocks.
• The punch down tool pushes the wire into the block and cuts off the excess wire.
• Be sure to position the blade on the side of the clip toward the end of the wire.
• The blade for a 66 block is straight, while the blade for a 110 block is notched.
Patch or Distribution Panels:
In an MDF or IDF, punch down blocks are rarely used for network cabling. Instead, twisted pair cables are terminated at a patch panel.
• Typically, individual four-pair cables are used rather than 25-pair or 100-pair cables. This takes advantage of cable shielding and minimizes cross-talk.
◦ In large applications, bundles of 25- and 100-pair cables can be used for VCCs and HCCs. However, they should be certified to support the desired network speed.
• Twisted pairs are connected at the rear of the panel with connections similar to punch down blocks. A special tool is usually required.
•At the front of the panel, patch cables are used between the patch panel and network devices.
•A patch panel for fiber optic cabling is called a fiber distribution panel.
Keeping an MDF or IDF organized is a major challenge. One key to doing so is proper documentation. Here are some guidelines:
• Develop a naming convention and use it to label cables, wall jacks, patch panel ports, network devices, and racks.
• Record names in tables and diagrams.
◦ Include location, installation dates, cable lengths, and cable grades.
◦ Consider using documentation software. Perform an internet search for cable plant documentation software or cable management software to view available options.
Copper Wire Troubleshooting:
The following table describes several conditions that can affect the performance and functions of copper wiring.
Electromagnetic Interference (EMI):
Radio Frequency Interference (RFI):
Electromagnetic interference and radio frequency interference are external signals that interfere with normal network communications. Common sources of EMI/RFI include nearby generators, motors (such as elevator motors), radio transmitters, welders, transformers, and fluorescent lighting.
To protect against EMI/RFI:
• Use fiber optic instead of copper cables. Fiber optic cables are immune to EMI/RFI.
• Use shielded twisted pair cables. Shielded cables have a metal foil that encloses all of the wires. Some cables might also include a drain wire that is a bare wire outside of the foil, but within the cable jacket. The drain wire can be grounded to help absorb EMI/RFI.
• Avoid installing cables near EMI/RFI sources.
Crosstalk is interference that is caused by signals within the twisted pairs of wires (for example, current flow on one twisted pair causing a current flow on an adjacent pair).
• The twisting of wires into pairs helps reduce crosstalk between pairs.
• Each pair of wires is twisted at a different rate to reduce crosstalk between pairs.
• Crosstalk is often introduced within connectors, where the twists are removed to add the connector. Crosstalk can also occur where wires are crushed or where the plastic coating is worn.
There are several forms of crosstalk:
• Near-end crosstalk (NEXT) is measured on the same end as the transmitter. For example, when a signal is sent on one wire pair, near-end crosstalk measures the interference on an adjacent wire pair at the same connector end.
• Far-end crosstalk (FEXT) is measured on the end without the transmitter. For example, when a signal is sent on one wire pair, far-end crosstalk measures the interference on an adjacent wire pair at the opposite connector end.
• Alien crosstalk is introduced from adjacent, parallel cables. For example, a signal sent on one wire pair causes interference on a wire pair that is within a separate twisted pair cable bundle.
Attenuation is the loss of signal strength from one end of a cable to the other. This is also known as dB loss.
• The longer the cable, the more attenuation. For this reason, it is important never to exceed the maximum cable length defined by the networking architecture.
• Cables at a higher temperature experience more attenuation than cables at a lower temperature.
• A repeater regenerates the signal and removes the effects of attenuation.
Open Impedance Mismatch (Echo):
Impedance is the measure of resistance within the transmission medium.
• Impedance is measured in ohms (Ω).
• All cables must have the same impedance rating. The impedance rating for the cable must match the impedance of the transmitting device.
• Impedance is mostly a factor in coaxial cables used for networking. Be sure to choose cable with the correct rating (50 or 75 ohm) based on the network type. Never mix cables with different ratings.
• When signals move from a cable with one impedance rating to a cable with another rating, some of the signal is reflected back to the transmitter, distorting the signal. With video (cable TV), impedance mismatch is manifested as ghosting of the image.
• Cable distance does not affect the impedance of the cable.
An electrical short occurs when electrical signals take a path other than the intended path. In the case of twisted pair wiring, a short means that a signal sent on one wire arrives on a different wire. Shorts occur when two wires touch; this can be caused by worn wire jackets, crushed wires, or a metal object piercing two or more wires.
An open circuit is when a cut in the wire prevents the original signal from reaching the end of the wire. An open circuit is different from a short in that the signal stops (electricity cannot flow because the path is disconnected).
A miswired cable is caused by incorrect wire positions on both connectors. Several wiring problems might exist:
• A reverse connection is when a cable is wired using one standard on one end and another standard on the other end, creating a crossover cable. While this condition might be intentional, it can cause problems when a crossover cable is used instead of a straight-through cable. This will reverse the transmit and receive match up; the transmit pins on one end maps to the transmit pins on the other end when they should map to the receive pins on the other end.
• Wiremapping is matching a wire with a pin on one end with the same pin on the other end. For example, an error in the wiremapping results when the wire at pin 1 connects to pin 4.
• A split pair condition is when a single wire in two different pairs is reversed at both ends. For example, if instead of the solid green wire, the solid brown wire is matched with the green/white wire in pins 1 and 2. With a split pair configuration, the cable might still work (especially if it is short), but it could introduce crosstalk.
◦ When the T568A/B standards for making drop cables are followed, one pair is split to meet the standards. In this case, a common split pair error is simply placing all wire pairs in order in the connector instead of splitting the pair according to the standard.
◦ When cables are connected using a punch down block, pairs are not split.
An incorrect termination occurs when an incompatible or incorrect connector is used. This can result in reduced performance or complete connection loss.
A bad connector is a damaged connector that is causing connectivity issues. For example, a broken locking tab on an RJ45 connector can cause intermittent connection problems. Another common connector is when there is a bent or damaged pin, especially on female RJ45 connections and the center wire of a coaxial cable.
Well-manufactured network devices have interfaces that can be tailored to different cable types, protocols, and speeds. This is done by connecting a hot swappable transceiver to the interface. When connecting one network device to another, matching transceivers must be used. For twisted pair cabling, a mismatch in speed is a common issue.
Troubleshooting Fiber Optic:
Troubleshooting fiber optic wiring is more complex than troubleshooting copper network wiring. To function properly, fiber optic cabling must be created, installed, and maintained very carefully. Several important factors that can affect fiber optic performance are listed in the following table.
Several issues can occur when you are working with fiber optic connectors.
• For light to pass through a fiber optic connector, the fiber within the jack must line up perfectly with the fiber in the connector. Using the wrong connector will result in misaligned fibers, disrupting the light signal even if the connector is successfully locked into the jack.
• Dirty connectors can also impede or disrupt the light signal, so it's important that they are kept clean. Several cleaning methods can be used with fiber optic connectors:
◦ For connectors where the ferrule protrudes out of the connector, such as the FC connector, you can wipe the end of the ferrule with a lint-free cloth that has a small amount of denatured alcohol applied. Immediately wipe the ferrule dry with a dry lint-free cloth.
◦ For fiber optic connectors where the end of the ferrule is less accessible, you must use a specialized cleaning tool. Some cleaning tools allow you to plug in the fiber optic cable and then clean it by pumping the tool's handle.
◦ To clean the jacks on fiber optic network interfaces, you can purchase a specialized fiber optic cleaning stick to remove foreign material.
Whenever a connector is installed on the end of fiber optic cable, a degree of signal loss occurs. This is called insertion loss. Additionally, some of the light that is lost is reflected directly back down the cable, toward the source. This is called back-reflection, or optical return loss (ORL). It can corrupt the data being transmitted and even damage the transmitter.
For a connector to work properly, there must be as little insertion loss and ORL as possible. The better the polish on the connector, the better the light will pass through without reflection. Fiber optic equipment manufacturers rate their connectors using the following polish grading designations:
• Physical contact (PC) polishing is usually used with single-mode fiber. The ends of the fiber are polished with a slight curvature so that when the cable end is inserted into the connector, only the cores of the fiber actually touch each other.
• Super physical contact (SPC) and ultra physical contact (UPC) polishing use a higher grade of polish and have more of a curvature than PC polishing, further reducing ORL reflections.
• Angled physical contact (APC) polishing is used to reduce back reflection as much as possible. An APC connector has an eight-degree angle cut into the ferrule, which prevents reflected light from traveling back down the fiber. Any reflected light is bounced out into the cable cladding instead. You can only use angle-polished connectors with other angle-polished connectors. Using an angle-polished connector with a non-angle-polished connector causes excessive insertion loss.
APC connectors are colored green to prevent them from being mixed with non-APC connectors.
Damaged and Mismatched Cables:
Several issues can occur when you are working with fiber optic cabling.
• Fiber optic cabling is much less forgiving of physical abuse than copper wiring. The fiber core is fragile and can be easily damaged by rough handling. For example, bending a fiber cable at too tight of a radius will break the core.
• Wavelength mismatch will cause serious issues with fiber optic cables. You cannot mix and match different types of cable. For example, if you connect single-mode fiber to multi-mode fiber, you will introduce a catastrophic signal loss of up to 99%. Even connecting cables of the same type that have different core diameters can cause a loss of up to 50% of the signal strength.
Media Adapters and Transceivers:
Many network switches and routers allow you to insert a transceiver such as a gigabit interface converter (GBIC) in an empty slot to convert the interface from copper wiring to fiber optic. Other devices use a small form-factor pluggable (SFP) transceiver to accomplish the same goal. Several issues can occur when using these and other fiber optic media adapters:
• Some GBIC/SFP modules use multimode fiber, while others use single-mode. Make sure that you use the correct type of fiber optic cable and connector required by the specific adapter.
• Media adapter modules malfunction on occasion. If you have lost connectivity on one of these links, ensure that the adapter module is working correctly.
Light signals being transmitted through a fiber optic cable experience attenuation, or signal loss, as they pass through the cable due to:
• Reflection: A measurable amount of light is reflected when it hits the ends of the cable. Much of a cable's reflection loss occurs at each cable connection. When the light hits the boundary between the core and the cladding, it is reflected back into the core. There is minor loss to the signal when this occurs, but it contributes to overall signal loss.
• Refraction: If the light hits the boundary between the core and the cladding at too steep of an angle, the light is refracted into the cladding instead of reflected back into the core, causing signal loss. Some fiber optic cables are doped with impurities near the edge of the fiber so that the signals are bent instead of reflected back to the center of the core. The loss due to this refraction is minor when compared with the benefits of confining the light to the center of the core.
• Scattering: Impurities in the fiber core can cause light to scatter. Some of the light continues down the fiber. The light that is scattered backwards contributes to the signal loss.
• Absorption: Impurities in the fiber can also absorb the light, converting it to another form of energy, like heat. This is a major cause of signal loss.
Several physical cable attributes can contribute to signal loss:
• Cable length: while higher quality cables will carry light signals further, the longer the cable, the more signal absorption and the greater the signal loss.
• Connectors: every connector will cause some level of signal loss, mostly due to reflection. While patch cables at each end of a run are to be expected, you should minimize any other connections.
• Splices: there are tools that you can use to splice a cut fiber optic cable. However, the signal loss from a splice is comparable to the signal loss from a connector.
• Bends: micro bends in the cable due to things such as temperature change or manufacturing anomalies can cause signal loss. While you have little control over micro bends, even macro bends, those detected by the human eye, can contribute to signal loss. The straighter the fiber optic cable, the less the signal loss.
You can estimate how much signal loss (measured in dB) you should reasonably expect in a given run of fiber optic cabling. Signal loss is calculated by summing the average loss of all the components used in the cable run to generate an estimate of the total attenuation that will be experienced end-to-end. This estimate is called a loss budget.
When calculating a loss budget for a segment of fiber optic cable, use the following guidelines:
• Connectors: 0.3 dB loss each
• Splices: 0.3 dB loss each
• Multi-mode cabling:
◦ 3 dB loss per 1000 meters when using an 850 nm light source
◦ 1 dB loss per 1000 meters when using a 1300 nm light source
• Single-mode cabling:
◦ 0.5 dB loss per 1000 meters when using a 1310 nm light source
◦ 0.4 dB loss per 1000 meters when using a 1550 nm light source
The total attenuation should be no more than 3 dB less than the total power at the transmission source. This is called the link-loss margin. For example, if the total power output at the transmission source of a cable run is 15 dB, then the total attenuation over the cable run should not exceed 12 dB. This ensures that the cable will continue to function as its components (such as the LED light transmitters and connectors) degrade with age and use.
A loopback plug, or loopback adapter, reflects a signal from the transmit port on a device to the receive port on the same device. Use the loopback plug to verify that a device can both send and receive signals.
• There are loopback plugs for both copper and fiber connections.
• A failure in the loopback test indicates a faulty network card.
• A successful loopback test means the problem is in the network cabling or another connectivity device.
You can purchase pre-made loopback plugs, or you can make an inexpensive one by cutting the end off a cable and manually connecting the transmit wires to the receive wires (connect the wire from pin 1 to the wire at pin 3, and the wire at pin 2 to the wire at pin 6).
A smartjack is an intelligent loopback device installed at the demarcation point for a WAN service. Technicians at the central office can send diagnostic commands to the smartjack to test connectivity and performance between the central office and the demarc. When you contact a WAN service provider for assistance, they might execute a test using the smartjack. A successful test indicates that the problem is within your customer premises equipment (CPE).
Known Good Spares:
One valuable troubleshooting method is to keep a set of components that you know are in proper functioning order. If you suspect a problem in a component, swap it with the known good component. If the problem is not resolved, troubleshoot other components. Examples of using this strategy include the following:
• Changing the drop cable that connects a computer to the network
• Replacing a NIC with a verified working NIC
• Moving a device from one switch port to another
A cable tester (or line tester) verifies that the cable can carry a signal from one end to the other and that all wires are in the correct positions.
• High-end cable testers can check for various miswire conditions (wire mapping, reversals, split pairs, shorts, or open circuits).
• You can use a cable tester to quickly tell the difference between a crossover and a straight-through cable..
• Most testers have a single unit that tests both ends of the cable at once. Many testers come with a second unit that can be plugged into one end of a long cable run to test the entire cable.
Time-Domain Reflectometer (TDR):
A time-domain reflectometer is a special device that sends electrical pulses on a wire in order to discover information about the cable. The TDR measures impedance discontinuities (the echo received on the same wire in response to a signal on the wire). The results of this test can be used to identify several variables:
• Estimated wire length
• Cable impedance
• The location of splices and connectors on the wire
• The location of shorts and open circuits
Optical Time-Domain Reflectometer (OTDR):
An optical time-domain reflector performs the same function as a TDR, but is used for fiber optic cables. An OTDR sends light pulses into the fiber cable and measures the light that is scattered or reflected back to the device. The information is then used to identify specifics about the cable:
• The location of a break
• Estimated cable length
• Signal attenuation (loss) over the length of the cable
A cable certifier is a multi-function tool that verifies that a cable or an installation meets the requirements for a specific architecture implementation. For example, you would use a certifier to verify that a specific drop cable meets the specifications for 1000BaseT networking.
• A certifier is very important for Cat 6 cable used with bandwidths at or above 1000 Mbps. Slight errors in connectors or wires can cause the network to function at 100 Mbps instead of the desired 1000 Mbps or 10 Gbps.
• Certifiers can also validate the bandwidth capabilities of network interface cards and switches. Many can detect the duplex settings of network devices.
• Most certifiers include features of a toner probe, TDR, and cable tester.
• Certifiers are very expensive and are typically used by organizations that specialize in wiring installations.
A toner probe is composed of two devices that are used together to trace the end of a wire from a known endpoint to the termination point in the wiring closet. To use a toner probe:
• Connect the tone generator to one end of the wire. It will send a signal on the wire.
• In the wiring closet, touch the probe to wires or place the probe close to wires. A sound at the probe indicates that the generated tone has been detected and the wire that you are touching is the termination point for the wire you are tracing.
A multimeter is a device used to test various electrical properties. A multimeter can measure several parameters:
• AC and DC voltage
• Current (amps)
• Resistance (ohms)
Voltage Event Recorder:
A voltage event recorder keeps track of voltage conditions on a power line. Basic recorders simply keep track of the occurrence of undervoltage or overvoltage conditions, while more advanced devices track conditions over time and create a graph, saving data from a program running on a computer.
Some UPS systems include a simple voltage event recorder. Use a voltage event recorder to identify periods of low or high voltage that can adversely affect network components.
An environmental monitor does what its name implies—it monitors the environmental conditions of a specific area or device.
• Monitors are often used to track the conditions within server rooms (such as temperature, humidity, water, smoke, motion, and air flow).
• Typically computers, especially servers, have an internal monitor that measures fan speed and CPU temperature.
• Many monitors will sound an alarm if a specified temperature or other environmental condition is reached.
Wire Stripper, Snips, and Crimpers:
Wire strippers remove the protective sheath of a cable in order to expose the conductive wire.
• Wire strippers are rated to specific gauge (cable width) ranges.
• Most wire strippers are combination tools (they can strip, cut, and crimp cables).
• Almost all wire strippers have multiple holes or can be adjusted for specific cable sizes.
A crimping tool is used to attach connectors to wires. Some crimpers are designed for power connections. A special crimper is used to attach RJ45 connectors to twisted pair cables.
Snips are cutting tools used to cut cables or wires to a specific length or to remove damaged sections. A diagonal cutter is an example of a snip tool.
Whenever possible, use a wire stripper instead of snips to strip a cable. Wire strippers are specifically designed to cut only the protective sheath without cutting the internal wire.
Speed Test Website:
A speed test website is an online tool that is used to test the bandwidth of your internet connection. There are countless speed test websites available, all of which provide essentially the same information:
• Connection latency (ping)
• Download speed
• Upload speed