Computer Networking Chapter 2

Network Data Transmission

works by modulating the properties of a transmission medium—electric current, infrared light, or radio waves—to encode a signal. One example of modulation is transitioning between low and high voltage states in an electrical circuit. These voltage pulses can encode symbols, which can be mapped to digital bits—ones and zeros.

Media Bandwidth

a frequency range measured in cycles per second or Hertz (Hz), but the term is very widely used in data networking to mean the amount of data that can be transferred, measured in multiples of bits per second (bps).

The Institute of Electrical and Electronics Engineers (IEEE) 802.3 Ethernet standards (ieee802.org/3)

describing media types, access methods, data rates, and distance limitations at OSI layers 1 and 2 using xBASE-y designations. Are very widely used on both LANs and WANs.

Ethernet Standards

provide assurance that network cabling will meet the bandwidth requirements of applications.

xBASE-y

• The speed or bit rate in megabits per second (Mbps) or gigabits per second (Gbps).

• The signal mode (baseband or broadband). All mainstream types of Ethernet use baseband transmissions, so you will only see specifications of the form xBASE-y.

• A designator for the media type.

Twisted pair

Network cable construction with insulated copper wires twisted about each other. A pair of color-coded wires transmits a balanced electrical signal. The twisting of the wire pairs at different rates acts to reduce interference and crosstalk.

High attenuation

meaning that the signal quickly loses strength over long links. (Copper cables are one of them)

Copper cable

used to transmit electrical signals. The cable between two nodes creates a low voltage electrical circuit between the interfaces on the nodes. There are two main types: twisted pair and coaxial (coax).

Category (Cat)

ANSI/TIA/EIA cable category designations, with higher numbers representing better support for higher data rates. (Twisted pair is rated to this)

Ethernet

is a multiple access area network, which means that the available communications capacity is shared between the nodes that are connected to the same media.

Media Access Control (MAC)

refers to the methods a network technology uses to determine when nodes can communicate on shared media and to deal with possible problems, such as two devices attempting to communicate simultaneously.

Collision domain

Network segment where nodes are attached to the same shared access media, such as a bus network or Ethernet hub.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

each network device competes with the other connected devices for use of the transmission media. Contention-based systems require a set of protocols that reduce the possibility of data collisions, since if the devices compete and simultaneously send data packets, neither packet will reach its intended destination. The Carrier Sense Multiple Access (CSMA) protocols allow contention-based networks to successfully communicate by detecting activity on the network media (Carrier Sense) and reacting to this (for example, if the medium is busy). CSMA/CD (Collision Detection) recognizes a signal collision on the basis of electrical fluctuations produced when signals combine.

Fast Ethernet Standard

uses the same CSMA/CD protocol as 10BASE-T but with higher frequency signaling and improved encoding methods, raising the bit rate from 10 Mbps to 100 Mbps.

100BASE-TX

refers to Fast Ethernet working over Cat 5 (or better) twisted pair copper cable with a maximum supported link length of 100 meters (328 feet). can be implemented with a hub, but the standard was created at a time that switches started to replace hubs as the connection point for end systems.

10BASE-T Ethernet

specifies that a node should transmit regular electrical pulses when it is not transmitting data to confirm the viability of the link.

Fast Link Pulse

Fast Ethernet codes a 16-bit data packet into this signal, advertising its service capabilities. A node that does not support autonegotiation can be detected by one that does and sent ordinary link integrity test signals, or Normal Link Pulses.

Gigabit Ethernet

builds on the standards defined for Ethernet and Fast Ethernet to implement rates of 1,000 Mbps (1 Gbps). When installed using Cat 5e or better copper wire, this is specified as 1000BASE-T. It does not support hubs; it is implemented only using switches. The maximum distance of 100 meters (328 feet) applies to cabling between the node and a switch port, or between two switch ports.

10GBASE-T

Cable	Maximum Distance
UTP (Cat 6) F/UTP (Cat 6A) S/FTP (Cat 7)	55 m (180 feet) 100 m (328 feet) 100 m (328 feet)

40GBASE-T

Cable	Maximum Distance
S/FTP (Cat 8)	30 m (100 feet)

Fiber optic cable

Network cable type that uses light signals as the basis for data transmission. Infrared light pulses are transmitted down the glass core of the fiber. The cladding that surrounds this core reflects light back to ensure transmission efficiency. At the receiving end of the cable, light-sensitive diodes re-convert the light pulse into an electrical signal. This cable is immune to eavesdropping and EMI, has low attenuation, supports rates of 10 Gb/s+, and is light and compact.

The principal applications of 10 GbE (and better)

• Increasing bandwidth for server interconnections and network backbones, especially in datacenters and for storage area networks (SANs).

• Replacing existing switched public data networks based on proprietary technologies with simpler Ethernet switches (Metro Ethernet).

Crosstalk

is a phenomenon whereby one pair causes interference in another as a result of their proximity.

Solid cabling- twisted pair

uses a single thick wire per conductor and is used for cables that run behind walls or through ducts. uses thicker 22 to 24 AWG (American Wire Gauge)

Stranded cabling- twisted pair

uses thin filament wires wrapped around one another and is used to make flexible patch cords for connecting computers to wall ports and switch ports to patch panel ports. used for patch cords is often 26 AWG. The attenuation of the wire is higher than solid wire, so it should not be used for cables over 5 m in length.

Unshielded twisted pair

Media type that uses copper conductors arranged in pairs that are twisted to reduce interference. Typically cables are 4-pair or 2-pair. Modern buildings are often flood wired using this cabling. This involves cables being laid to every location in the building that may need to support a telephone or computer.

Shielded twisted pair

Copper twisted pair cabling with screening and shielding elements for individual wire pairs and/or the whole cable to reduce interference. Also referred to as a screened, shielded, or foiled twisted pair. is less susceptible to interference and crosstalk. This type of cable is required for some Ethernet standards and may also be a requirement in environments with high levels of interference, such as cabling that is run near motors, generators, or fluorescent lighting.

Screened cable (shielded)

has one thin outer foil shield around all pairs. Screened cable is usually designated as screened twisted pair (ScTP) or foiled/unshielded twisted pair (F/UTP), or sometimes just foiled twisted pair (FTP).

Fully shielded cabling

has a braided outer screen and foil-shielded pairs and is referred to as shielded/foiled twisted pair (S/FTP). There are also variants with a foil outer shield (F/FTP).

U/FTP cabling

has foil-shielded pairs but no outer shield.

Legacy STP cable

could be complex to install, as it required bonding each element to ground manually, but modern F/UTP and S/FTP solutions with appropriate cable, connectors, and patch panels reduce this complexity by incorporating bonding within the design of each element.

Registered Jack (J)

Series of jack/plug types used with twisted pair cabling, such as RJ45 and RJ11. There are many different types of RJ connector, identified by numbers (and sometimes letters). Some are physically different, while others are identical but wired differently for different applications. The most widely used connectors are RJ45 and RJ11.

RJ45 Connectors

are used with 4-pair copper cables. The connectors are also referred to as 8P8C, standing for 8-position/8-contact. This means that all eight "potential" wire positions are supplied with contacts, so that they can all carry signals if needed. RJ45 is used for Ethernet twisted pair cabling. have a plastic retaining clip. This is normally protected by a rubber boot. This type of cable construction is also referred to as snagless.

GG45 and TERA connectors

associated with ISO Class F and Class II cabling. GG45 has a similar form factor to RJ45 but has four conductors in the corners. TERA connectors have a completely different form factor.

RJ11 connector

is used with 2-pair copper cable. An RJ11 connector can support six positions, but only the center two contacts are wired (6P2C). In a telephone system, this pair carries the dial tone and voice circuit. These are also called the Tip and Ring wires after the way older phone plugs were wired. The other pair is usually unused but can be deployed for a secondary circuit. RJ11 connectors are used for telephone systems and to connect analog data modems to a phone jack.

Plenum space/cable

Cable for use in building voids designed to be fire resistant and to produce a minimal amount of smoke if burned. typically a false ceiling, though it could also be constructed as a raised floor. As it makes installation simpler, this space has also been used for communications wiring in some building designs. Therefore, building regulations require the use of fire-retardant plenum cable in such spaces. This cable must not emit large amounts of smoke when burned, be self-extinguishing, and meet other strict fire safety standards.

General purpose (non plenum)

cabling uses PVC (polyvinyl chloride) jackets and insulation. Plenum-rated cable uses treated PVC or fluorinated ethylene propylene (FEP). This can make the cable less flexible, but the different materials used have no effect on bandwidth. Communications cable that is plenum rated under the U.S. National Electrical Code (NEC) is marked CMP. These cables are marked CMG or CM.

Riser cabling

Cabling that passes between two floors. This means that fire cannot spread through the opening created by the conduit. Riser cabling (in conduit or in spaces such as lift shafts) should also conform to the appropriate fire safety standards. These are similar to the requirements for plenum spaces but not quite as strict. Data cable that is riser rated under the NEC is marked CMR. You can use plenum-rated cables in place of riser-rated cables, but never use riser-rated cables in place of plenum-rated cables. Both of these typically include a rope or filament that helps support their weight when they're installed vertically.

Coaxial cable

Media type using two separate conductors that share a common axis categorized using the Radio Grade (RG) specifications. The core conductor is made of solid or stranded copper wire and is enclosed by plastic insulation. A wire mesh wrapped around the plastic constitutes the second conductor. This serves as shielding from interference.

Bayonet Neill-Concelman (BNC) connector

Twist and lock connector for coaxial cable.

F-type connector

Screw down connector used with coaxial cable.

Radi Grade (RG) designations

represent the thickness of the core conductor and the cable's characteristic impedance. RG6 is 18 AWG cable with 75 ohm impedance typically used as drop cable for Cable Access TV (CATV) and broadband cable modems. Thinner, more flexible RG59 cable is used for audio/video and closed-circuit television (CCTV).

Twinaxial

Media type similar to coax but with two inner conductors to improve performance. used for datacenter interconnects working at 10 GbE (unofficially referred to as 10GBASE-CR) and 40 GbE (40GBASE-CR4). The maximum distance is up to about 5 meters for passive cable types and 10 meters for active cable types.

Direct attach copper (DAC)

Factory-terminated twinax patch cords used for 10+ Gbps Ethernet connections, typically between rack-mounted appliances. These transceivers can be installed as modules in switch, router, and server appliances.

Structured cabling scheme

is a standard way of provisioning cabled networking for computers in an office building. The best known is the ANSI/TIA/EIA 568 Commercial Building Telecommunications Wiring Standard.

Work Area

• The space where user equipment is located and connected to the network, usually via a patch cable plugged into a wall port. 

Horizontal Cabling

Connects user work areas to an intermediate distribution frame (IDF). Horizontal cabling is so-called because it typically consists of the cabling for a single floor and so is made up of cables run horizontally through wall ducts or ceiling spaces. When using copper cabling, the IDF must be within 90 m (295 feet) cabling distance of each wall port. If this is not possible, multiple IDFs must be provisioned. Multiple IDFs on the same floor are linked by horizontal cross connects.

intermediate distribution frame (IDF)

Passive wiring panel providing a central termination point for cabling. An IDF is an optional layer of distribution frame hierarchy that cross-connects "vertical" backbone cabling to an MDF to "horizontal" wiring to wall ports on each floor of a building or each building of a campus network. Smaller facilities might not require IDFs. If distance limitations are not exceeded, wall ports can be terminated directly to a single main distribution frame.

Telecommunications Room

Room or closet that houses an intermediate distribution frame and networking equipment, such as switches. Essentially, this is a termination point for the horizontal cabling along with a connection to backbone cabling. This wiring closet must be used only for networking equipment (not general storage) and should ideally be secured by a lockable door.

Backbone Cabling

Connects IDFs to a main distribution frame (MDF). Backbone cabling is also referred to as vertical cabling, as it is more likely to run up and down between floors.

Main distribution frame (DMF)

Passive wiring panel providing a central termination point for cabling. A MDF distributes backbone or "vertical" wiring through a building and connections to external access provider networks.

Entrance Facilities/Demarc

• Special type of telecommunications room marking the point at which external cabling is joined to internal cabling, via the MDF. Entrance facilities are required to join the local exchange carrier's (LEC's) network and for inter-building communications. The demarcation point is where the access provider's network terminates and the organization's network begins.

Demarcation point

Location that represents the end of the access provider’s network (and therefore their responsibility for maintaining it). The demarc point is usually at the Minimum Point of Entry (MPOE). If routing equipment cannot be installed at this location, demarc extension cabling may need to be laid.

Terminating cable

Twisted pair must be properly terminated. Patch cords are terminated with RJ45 plugs, while structured cabling is terminated to insulation displacement connectors (IDCs) in wall ports and distribution frames. When terminating cable, an organization should use a consistent wiring scheme across all sites. Each conductor in a 4-pair data cable is color-coded. Each pair is assigned a color (blue, orange, green, or brown). The first conductor in each pair has a predominantly white insulator with strips of the color; the second conductor has an insulator with the solid color. The ANSI/TIA/EIA 568 standard defines two methods for terminating Ethernet cabling: T568A and T568B.

T568A and T568B

Twisted pair termination pinouts defined in the ANSI/TIA/EIA 568 Commercial Building Telecommunications Standards.

T568A

the green pairs are wired to pins 1 and 2, and the orange pairs are wired to pins 3 and 6. is mandated by the residential cabling standard (TIA 570)

T568B

these pairs swap places, so orange is terminated to pins 1 and 2 and green to 3 and 6. is probably the more widely deployed of the two.

Cable management

techniques and tools ensure that cabling is reliable and easy to maintain. Structured copper wiring runs from a wall port in the user's work area to some type of distribution frame in the network closet. At both ends, it is terminated at a punch down block with insulation-displacement connection (IDC) terminals.

insulation-displacement connection (IDC)

Block used to terminate twisted pair cabling at a wall plate or patch panel available in different formats, such as 110, BIX, and Krone. An IDC contains contacts that cut the insulation from a wire and hold it in place. This design allows large numbers of cables to be terminated within a small space.

Patch panel

Type of distribution frame used with twisted pair cabling with IDCs to terminate fixed cabling on one side and modular jacks to make cross-connections to other equipment on the other. Also called a patch bay. In data networks, numerous moves, adds, and changes (MACs) would require re-terminating the wiring. To simplify MACs, a distribution frame is normally implemented as a patch panel. This has punch down blocks on one side and pre-terminated RJ45 modular ports on the other. This allows incoming and outgoing connections to be reconfigured by changing the patch cable connections, which is much simpler than re-terminating punch down blocks. The structured cabling running from the work area or forming a backbone is terminated at the back of the patch panel on the IDCs, using either T568A or T568B wiring order. An RJ45 patch cord is used to connect the port to another network port, typically a switch port housed in the same rack. This greatly simplifies wiring connections and is the most commonly installed type of wiring distribution where connections need to be changed often.

Structured cable installation

Installing structured cable from a bulk spool is referred to as pulling cable because the cable must be pulled, carefully, from the telecommunications closet to the work area. Cable is normally routed through conduits or wall spaces, avoiding excessive bends and proximity to electrical power cables and fittings, such as fluorescent lights, as these could cause interference. The main fixed cable run can be up to 90 m (295 feet). Starting at the patch panel, label the end of the cable with the appropriate jack ID, then run it through to the work area. This is also referred to as a drop, as in most cases you will be dropping the cable from the ceiling space through a wall cavity. If several cables are going to roughly the same place, you can bundle them and pull them together. Leave enough slack at both ends (a service loop) to make the connection and to accommodate future reconnections or changes, cut the cable, and label the other end with the appropriate ID. Electrician's scissors (snips) are designed for cutting copper wire and stripping insulation and cable jackets. Alternatively, there are dedicated cable stripper tools that have replaceable blades for different data cable types. Cable-cutting blades should be rounded to preserve the wire geometry. Stripping tools should have the correct diameter to score a cable jacket without damaging the insulation wires.

Stranded wire patch cords

can be up to 5 m each (16 feet) and no more than 10 m (33 feet) in overall length. This is because the attenuation of stranded cable is higher than solid cable.

Cable stripper

Tool for stripping the cable jacket or wire insulation.

Termination tools

To terminate a cable, untwist the ends of the wire pairs and place them into the punch down block in the correct order for the wiring configuration (T568A or T568B) you want to use.

You must not untwist the wires too much. Cat 6 is demanding in this respect and requires no more than 0.375" (1 cm) of untwisting.

Punch down tool

Tool used to terminate solid twisted pair copper cable to an insulation displacement connector. Fixed cable is terminated using a punch down tool. This tool fixes conductors into an IDC. There are different IDC formats (66, 110, BIX, and Krone), and these require different blades. Many punch down tools have replaceable blades, though. Blades are double sided; one side pushes the wire into the terminal while the other side cuts the excess. Make sure the blade marked "cut" is oriented correctly to cut the excess wire.

Block tool

terminates a group of connectors in one action. For a 110 format panel, a four position block is suitable for terminating 4-pair data cabling.

Cable crimper

Tool to join a network jack to the ends of a network patch cable. A patch cord is created using a cable crimper. This tool fixes a plug to a cable. The tools are specific to the type of connector and cable, though some may have modular dies to support a range of RJ-type plugs.

Shielded and screened termination

termination must be made to shielded IDCs or modular plugs. On an IDC, a metal clip placed over the exposed foil or braided shield bonds the cable to the housing. A shielded modular plug has a metal housing and is not terminated using a standard crimper. There are several different designs, but all follow the principle of connecting the cable shield to a bonding strip.

Fiber Optic Cable Considerations

The electrical signals carried over copper wire are subject to interference and attenuation. Fiber optic signaling uses pulses of infrared light, which are not susceptible to interference, cannot easily be intercepted, and suffer less from attenuation. Consequently, fiber optic cabling supports higher bandwidth over longer cable runs. Fiber optic cabling can be many kilometers long. 

A single optical fiber is constructed from three elements: 

• Core provides the transmission path, or waveguide, for the light signals. 

• Cladding reflects signals back into the waveguide as efficiently as possible. The core and cladding can be made from glass or plastic. The cladding is applied as a thin layer surrounding the core. While made of the same material, the cladding has a different refractive index than the core. The effect of this is to create a boundary that causes the light to bounce back into the core, facilitating the process of total internal reflection that guides the light signal through the core. 

• Buffer is a protective plastic coating. It may be of a tight or loose configuration, with the loose format using some form of lubricant between the strand and the sheath. 

In basic operation modes, each fiber optic strand can only transfer light in a single direction at a time. Therefore, multiple fibers are often bundled within a cable to allow simultaneous transmission and reception of signals or to provide links for multiple applications.

Fiber Protection

There are many different outer jacket designs and materials suited for different installations (indoor/plenum, outdoor, underground, undersea, and so on). Kevlar (Aramid) strands and sometimes fiberglass rods (strength members) are often used to protect the fibers from excessive bending or kinking when "pulling" the cable to install it. For exposed outdoor applications, a steel shield (armor) may be added to deter rodents from gnawing the cable. 

Fiber Optic Mode

Fiber optic cables are specified using the mode, composition (glass/plastic), and core/cladding size; for example, 8.3 micron core/125 micron cladding single mode glass or 62.5 micron core/125 micron cladding multimode plastic. Fiber optic cables fall into two broad categories: single mode and multimode. 

Single Mode Fiber (SMF)

Fiber optic cable type that uses laser diodes and narrow core construction to support high bandwidths over distances of over 5 km. has a small core (8 to 10 microns) and a long wavelength. It uses a laser to generate a near infrared (1,310 nm or 1,550 nm) light signal. Single mode cables support data rates up to 100 Gbps and cable runs of many kilometers, depending on the quality of the cable and optics. There are two grades of SMF cable; OS1 is designed for indoor use, while OS2 is for outdoor deployment. Optical transceivers for SMF are now only slightly more expensive than ones for MMF. Consequently, SMF is often used for short-range applications in datacenters, as well as for long-distance links. SMF still comes at a slight price premium, but it provides better support for 40 Gbps and 100 Gbps Ethernet standards.

Multimode Fiber (MMF)

Fiber optic cable type using LED or vertical cavity surface emitting laser optics and graded using optical multimode types for core size and bandwidth. has a larger core (62.5 or 50 microns) and shorter wavelength light (850 nm or 1,300 nm) transmitted in multiple waves of varying length. MMF uses less expensive optics and consequently is less expensive to deploy than SMF. However, it does not support such high signaling speeds or long distances as single mode and so is more suitable for LANs than WANs. MMF is graded by optical multimode (OM) categories, defined in the ISO/IEC 11801 standard

Optical multimode (OM)

Classification system for multimode fiber designating core size and modal bandwidth.

OM1/OM2

• 62.5-micron cable is OM1, while early 50-micron cable is OM2. OM1 and OM2 are mainly rated for applications up to 1 Gbps and use LED transmitters. 

OM3/OM4

These are also 50-micron cable, but manufactured differently, designed for use with 850 nm vertical-cavity surface-emitting lasers (VCSEL), also referred to as laser optimized MMF (LOMMF). A VCSEL is not as powerful as a laser type used for SMF, but it supports higher modulation (transmitting light pulses rapidly) than LED-based optics.

Fiber Connector Types

Fiber optic connectors are available in many different form factors. Some types are more popular for multimode and some for single mode.

Straight Tip (ST)

Bayonet-style twist-and-lock connector for fiber optic cabling. an early bayonet-style connector that uses a push-and-twist locking mechanism. ST was used mostly for multimode networks, but it is not widely used for Ethernet installations anymore.

Subscriber Connector (SC)

Push/pull connector used with fiber optic cabling. allowing for simple insertion and removal. It can be used for single- or multimode. It is commonly used for Gigabit Ethernet.

Local Connector (LC)

Small form factor push-pull fiber optic connector; available in simplex and duplex versions. (also referred to as Lucent Connector) is a small-form-factor connector with a tabbed push/pull design. LC is similar to SC, but the smaller size allows for higher port density. LC is a widely adopted form factor for Gigabit Ethernet and 10/40 GbE.

Fiber Optic Cable Installation

Fiber optic can be installed in the same topology as copper cable using distribution frames and switches. Long-distance cables are typically laid as trunks or rings with repeaters or amplifiers between cable segments to strengthen the signal.

With duplex fiber, strands are installed in pairs, with one strand for transmit (Tx) and one strand for receive (Rx). 

Fiber Optic Patch Cord

Patch cables for fiber optic can come with the same connector on each end (LC-LC, for instance) or a mix of connectors (LC-SC, for instance). Duplex patch cords must maintain the correct polarity, so that the Tx port on the transmitter is linked to the Rx port on the receiver and vice versa. The TIA/EIA cabling standard sets out a system of A to B polarity. Each element in the link must perform a crossover, and there must be an odd number of elements, such as two patch cords and a permanent link (three elements). Most connectors are keyed to prevent incorrect insertion, but if in doubt, an optical power meter can be used to determine whether an optical signal is being received from a particular fiber.

Transmitted optical signals are visible as bright white spots when viewed through a smartphone camera. This can be used to identify which adapter on an optical interface is transmitting and which fiber patch cord is receiving a signal from the other end of the cable.

Finishing Type

The core of a fiber optic connector is a ceramic or plastic ferrule that holds the glass strand and ensures continuous reception of the light signals. The tip of the ferrule can be finished in several formats. It is important to match the finishing type when you are selecting a connector type. APC finishing is often not supported by the patch panels, transceivers, and switch ports designed for Ethernet. 

Also, by convention, cable jackets and connectors use the following color-coding:

Type	Jacket Color	Connector Color
OM1	Orange	Beige
OM2	Orange	Black
OM3/OM4	Aqua	Aqua
SMF PC/UPC	Yellow	Blue
SMF APC	Yellow	Green

Ultra Physical Contact (UPC)

Fiber optic connector finishing type that uses a slightly curved polish for the ferrule. The faces of the connector and fiber tip are polished so that they curve slightly and fit together better. 

Angled Physical Contact (APC)

Fiber optic connector finishing type that uses an angled polish for the ferrule. The faces are angled for an even tighter connection. APC cannot be mixed with PC or UPC.  

Fiber Distribution Panel

Type of distribution frame with pre-wired connectors used with fiber optic cabling. A modern build or refurbishment might replace copper wiring with fiber optic cabling. Structured cabling links are installed in a manner similar to copper cabling. However, to avoid the wear and tear damage associated with continually reconnecting fiber optic cables, it's essential not to frequently replace cable runs through conduit. Permanent cables are therefore routed through conduit to wall ports at the client access end, and to a fiber distribution panel at the switch end. To complete the connection, fiber patch cables are used to link the wall port to the network interface card (NIC) and the patch panel to the switch port.

Multi-Fiber Push On Connectors

Fiber optic cable type that terminates multiple strands to a single compact connector, supporting parallel links. termination allows for low-footprint backbone or trunk cabling. An MPO backbone ribbon cable bundles 12 or more strands terminated to a single compact ferrule. MPO cables are usually prefabricated and not typically field terminated. There are MMF and SMF variants.

MPO is mostly used to aggregate 10 Gbps or 25 Gbps lanes into a 40 Gbps, 100 Gbps, or 400 Gbps parallel optical link. Each lane normally requires two fiber strands (send and receive). A 40 Gbps link comprising 4 x 10 Gbps lanes therefore requires eight strands. MPO can terminate this type of parallel optical link more efficiently than separate LC-terminated strands. An MPO connector capable of carrying 24 or 32 fibers has the same footprint as a duplex LC pair. Where there are multiple strands within a single cable, the strands are color-coded (TIA/EIA 598) to differentiate them.

Wavelength Division Multiplexing (WDM)

A duplex fiber channel link uses one transmit lane and one receive lane and requires two fiber strands. Parallel fiber uses bundles of lanes working at 10 Gbps or 25 Gbps to implement 40 Gbps or 100 Gbps links. These channel links require between eight and twenty strands.

Wavelength Division Multiplexing (WDM) is a means of using one or two strands to provision multiple channels.

Bidirectional Wavelength Division Multiplexing

System that allows bidirectional data transfer over a single fiber strand by using separate wavelengths for transmit and receive streams. Also called wavelength division multiplexing (WDM). Bidirectional (BiDi) transceivers support transmit and receive signals over the same strand of fiber. This uses WDM to transmit the Tx and Rx signals over slightly shifted wavelengths, such as 1,310 nm for Tx and 1,490 nm for Rx. BiDi transceivers must be installed in opposite pairs, so the downstream transceiver would have to use 1,490 nm for Tx and 1,310 for Rx. Bidirectional wavelength division multiplexing (BWDM) links are documented in Ethernet standards (1000BASE-BX and 10GBASE-BX).

Coarse Wavelength Division Multiplexing (CWDM)

Technology for multiplexing up to 16 signal channels on a single fiber using different wavelengths. supports up to 16 wavelengths and is typically used to deploy four or eight bidirectional channels over either a single fiber strand or unidirectional channels over dual fiber strands (one strand for transmit, the other for receive). CWDM and DWDM transceivers support multi-channel 1 G, 10 G, and 40 G Ethernet links. The transceivers must be installed in opposite pairs.

Dense Wavelength Division Multiplexing (DWDM)

Technology for multiplexing 40 or 80 signal channels on a single fiber using different wavelengths. provisions greater numbers of channels (20, 40, 80, or 160). This means that there is much less spacing between each channel and that it requires more precise and expensive lasers.

Datacenter

Networking equipment should be installed within secure areas. Within a building, these can be referred to as telecommunications closets, equipment rooms, or server rooms. A whole facility dedicated to provisioning servers is called a datacenter. All these spaces should be dedicated to appliance and server installation and not used for other kinds of storage. They need physical access controls so that only authorized persons are allowed entry.

Rack systems

Storage solution for server and network equipment. Racks are designed to a standard width and height (measured in multiples of 1U or 1.75"). Racks offer better density, cooling, and security than ordinary office furniture. Within a telecommunications closet, server room, or datacenter, equipment is installed in racks. A rack is a specially configured steel shelving system designed for standard-size equipment. Using a rack allows equipment to be stored more securely and compactly than ordinary desks or shelving would allow for. The concept of installing more computing appliances in a smaller space is referred to as density.

Network appliances and server hardware designed for rack-mounting are EIA standard 19" / 48.26 cm width. Each appliance can be screwed into the rack directly. Nonstandard components, such as a tower server or monitor, can be installed on shelves. If there is little need to remove it for upgrades or maintenance, an appliance can be screwed directly into the rack. However, devices are often mounted on rail kits so that they can be slid out of the rack for hardware maintenance and upgrades.

Rack height is measured in "U" units of 1.75" / 4.45 cm. Racks are sold in heights from 8U to 48U. Rack-compatible equipment is designed with a vertical height quoted in U so you can plan exactly how much vertical space you require.

Most racks are designed to be freestanding, though smaller wall-mounted cabinet units are also available. Freestanding racks can be bolted together in rows. There should be about 3 feet (1 meter) clearance aisle for service access and airflow. Multiple rows should be placed back-to-back not front to back to maximize cooling. This is referred to as a hot aisle/cold aisle layout.

Port-side exhaust

Feature of switches that allows fans to switch between expelling hot air and drawing in cool air from the side with ports. Rack-mounted appliances are usually designed with intake fans on the front to draw in cool air and exhaust fans on the back to expel warm air. Some switch models can be configured between port-side exhaust, where hot air is expelled on the same side as the port interfaces, and port-side intake. Port-side intake allows a switch to be installed with ports facing the front of the rack, which might be better for some cable management scenarios. Side panels and blanking plates should cover unused rack slots to improve airflow. Each rack can be installed with lockable doors (front and rear) to prevent unauthorized access to the equipment.

HVAC (Heating, Ventilation, Air Conditioning)

Control systems that maintain an optimum heating, cooling, and humidity level working environment for different parts of the building. Environmental controls mitigate the loss of availability through mechanical issues with equipment, such as overheating. Building control systems maintain an optimum working environment for different parts of the building. The acronym HVAC (Heating, Ventilation, Air Conditioning) is often used to describe these services. An HVAC uses temperature sensors and moisture detection sensors (to measure humidity).

Servers and appliances are fitted with internal sensors to monitor conditions within the device chassis. These can report problems such as excessive temperatures within the device chassis, fan speeds, component failure, and chassis intrusion to a monitoring system.

Environmental Factors

Sensors can also be installed to measure ambient environmental conditions for a network rack or enclosure or within a server room or equipment closet. The following environmental factors need monitoring:

• Temperature—High temperature will make it difficult for device and rack cooling systems to dissipate heat effectively. This increases the risk of overheating of components within the device chassis and consequent faults.

• Humidity—More water vapor in the air risks condensation forming within a device chassis, leading to corrosion and short circuit faults. Conversely, very low humidity increases risks of static charges building up and damaging components.

• Electrical—Computer systems need stable power supply, free from outages (power failures), voltage dips (under-voltage events), and voltage spikes and surges. Sensors built into power distribution systems and backup battery systems can report deviations from a normal power supply.

• Flooding—There may be natural or person-made flood risks from nearby watercourses and reservoirs or risks from leaking plumbing or fire suppression systems. Electrical systems need to be shut down immediately in the presence of any significant amount of water.

Power Management

All types of network appliances require a stable power supply to operate. Electrical events, such as voltage spikes or surges, can crash computers, switches, and routers, while loss of power from under-voltage events or power failures will cause equipment to fail. An under-voltage event is where the voltage drops briefly, while a power failure is a complete loss of power lasting seconds or more. Power management means deploying systems to ensure that equipment is protected against these events and that network operations can either continue uninterrupted or be recovered quickly.

Power Load and Voltage

The circuits supplying grid power to a rack, network closet, or server room must meet the load capacity of all the installed equipment (plus room for growth). Consequently, the alternating current (AC) circuits to a server room will typically be higher capacity than domestic or office circuits (30 or 60 amps as opposed to 13 amps, for instance). They might also be run at a higher voltage (240 VAC, rather than 120 VAC).

The power supply for each appliance has a wattage rating. For example, a basic switch might be 20 watts, while a 1U server might be 200 watts. Wattage is calculated as V(olts) * Current (Amps). To calculate the maximum load for a rack, add up the watts used by each appliance power supply and divide by the circuit voltage. For example, if a rack contains equipment that draws 2,000 watts in total, and the circuit VAC is 240, the amperage is 8.3. A single 30 amp circuit could supply three such racks.

If the circuits were 120 VAC, the amperage would be double. This is why equipment room and datacenter facilities tend to use high voltage circuits.

Power Distribution Units (PDU)

Advanced strip socket that provides filtered output voltage. A managed unit supports remote administration. Each circuit might be run through a power distribution unit (PDU). A PDU has circuitry to "clean" the power signal, provides protection against spikes, surges, and under-voltage events, and can integrate with an uninterruptible power supply (UPS).

On a smaller scale, PDUs are also available as "strip" sockets that can take a higher load than a typical 13 amp rated strip. Such sockets are rack mounted and can be oriented horizontally or vertically to allow for different cabling and layout options. PDUs also often support remote power monitoring functions, such as reporting load and status, switching power to a socket on and off, or switching sockets on in a particular sequence.

Uninterruptible power supply (UPS)

Battery-powered device that supplies AC power that an electronic device can use in the event of power failure. If there is loss of power, system operation can be sustained for a few minutes or hours (depending on load) using battery backup. Battery backup can be provisioned at the component level for storage device or array cache. The battery protects any read or write operations cached at the time of power loss.

At the system level, an uninterruptible power supply (UPS) will provide a temporary power source in the event of a power failure. UPS runtime may range from a few minutes for a desktop-rated model to hours for an enterprise system. In its simplest form, a UPS comprises a bank of batteries and their charging circuit plus an inverter to generate AC voltage from the direct current (DC) voltage supplied by the batteries. Different UPS models support different power outputs and form factors—from desktop to rack mounted depending on your needs.