Copper and optical fibre structured cabling networks

Copper and optical fibre structured cabling networks

With the change of informatics’ proprietary systems to open systems, were created the conditions for the change of proprietary cable systems to open systems of common use, arising then the concept of structured cabling network.

Structured cabling networks are systems to interconnect devices allowing for sharing resources, peripherals and/or information.

A network designed and installed properly will give the speed and reliability essential to the efficiency of the system.

Once the phones were the only concern, but now we must deal with the demanding requirements of the integrated voice, data and image systems.

The wiring is a key component of any network and, as the requirements are expanding constantly, you must specify and plan networks that can grow or be modified at any time: an integrated network of voice and data must be structured, i.e. it should be a generic cabling system that uses the approach of the distribution for work areas.

The patch panels located in the racks of the technical room allow PC, printers, telephones and other equipment to be connected or disconnected easily and quickly and that all necessary changes be made only in the rack, not requiring changes in the path ways of cables and associated wiring.

The structured cabling systems have been developed to reduce the costs of these changes.

Unstructured systems usually have a lower initial cost than a well structured system, but this is a glaring example of how the cheap comes out expensive. The proof of the success of these systems is the increasing involvement of regulation entities in the normalization of new networks based on this model, since the regulations ITED (Infrastructures for Telecommunications in Buildings) and ITUR (Infrastructures for Telecommunications in Settlements, Urbanizations and Condominiums) are the best example of this practice in Portugal.

The existing structured cabling systems have a wide range of adapters that allow continuing to use old equipment and go adding the latest communication technologies that will become available.

The type of equipment to connect to the network and the amount of information to be transmitted are important aspects to consider when choosing cables, as well as the available space in the cabinet and their path ways.

The structured cabling networks are building or campus communications cabling infrastructure that consists of a set of standardized smaller elements called subsystems.

Subsystems

Structured cablings are divided into six subsystems:

  • Entrance Interfaces Are the interfaces with the outside world;
  • Equipment Room Room where the equipment which serve the users are hosted;
  • Communications Room Room that house the communication equipment which connect the backbone and the horizontal cabling subsystem (usually the same as Equipment Room);
  • Backbone Cabling Cabling to connect between the Entrance Interfaces, the Equipment Room and the Communications Room;
  • Horizontal Cabling Connect Communications Room to the terminal outlets;
  • Terminal Components Connects end-user equipments to terminal outlets of the Horizontal Cabling.

Copper cable categories

In the horizontal distribution wiring is used UTP, FTP and STP cable type, which fall under one of the following categories:

  • Cat.3 Currently defined in TIA/EIA-568-B, used for data networks using frequencies up to 16 MHz. Historically popular for telephone and 10 Mbit/s Ethernet networks using TVHV cable type. Currently in Portugal you may not use such cables in horizontal distribution, they can be used only in telephone networks’ backbones;
  • Cat.5e Currently defined in TIA/EIA-568-B for networks up to 100 MHz, is used for 100 Mbit/s Ethernet networks;
  • Cat.6 Currently defined in TIA/EIA-568-B for networks up to 250 MHz, more than double category 5e; suitable for 1000BASE-T Gigabit Ethernet networks
  • Cat.6a Currently defined in ANSI/TIA/EIA-568-B.2-10 for networks up to 500 MHz, double that of category 6. Suitable for 10GBase-T networks;
  • Cat.7 An informal name applied to ISO/IEC 11801 Class F cabling for networks up to 600 MHz. This standard specifies four individually-shielded pairs inside an overall shield;
  • Cat.7a An informal name applied to Amendment 1 of ISO/IEC 11801 Class F cabling for networks up to 1000 MHz.

Note: Categories 1, 2, 4 and 5 are not currently recognized by TIA/EIA.

Cables and its installation

Structured cabling design and installation is governed by a set of standards that specify how wiring data centers, offices and apartment buildings for data, video and voice communications using cable and modular sockets.

These standards define how to lay the cabling in a star formation, such that all outlets terminate at a central patch panel, which is normally 19 inch rack-mounted, from where it can be determined exactly how these connections will be used. Each outlet can be “patched” into a data network switch (normally also rack mounted), or patched into a “telecoms patch panel” which forms a bridge into a PABX (private automatic branch exchange) telephone system, thus making the connection to a voice port.

Lines patched as data ports into a network switch require simple straight-through patch cables at the other end to connect a computer. Voice patches to a PABX, in Portugal, do not require an adapter at the remote end because plug used with RJ11 telephone connections is physically compatible with the RJ45 (8p8c) socket and both wiring are compatible, however, in some countries, it’s required an adapter to translate RJ45 modular connectors into the local standard telephone configuration (as, for instance in UK, where an adapter must be present at the remote end as the 6-pin approved sockets are physically incompatible with RJ45 sockets).

It is common to use color patch cords to identify the type of connection, though structured cabling standards do not require it.

On the other end, cabling standards demand that all eight connectors of the cable have to be connected, resisting the temptation to “double-up” or use one cable for both voice and data.

Twisted pair’s susceptibility to the electromagnetic interference greatly depends on the pair twisting schemes staying intact during the installation – twisted pair cables have stringent requirements for maximum pulling tension as well as minimum bend radius.

This relative fragility of twisted pair cables makes the installation practices an important part of ensuring the cable’s performance.

Structured cabling networks uses the following types of cable:

  • UTP (unshielded twisted pair)
  • FTP (foiled twisted pair)
  • STP (shielded twisted pair)
  • Optical Fibre

UTP – Unshielded Twisted Pair

UTP cables are found in most Ethernet networks and telephone systems and are mostly made of 4 pairs.

Is the most common cable used in structured networking because of its relatively lower costs compared to optical fibre and coaxial cable.

UTP is also finding increasing use in video applications, primarily in security cameras – much of the middle to high-end IP cameras include a UTP output with setscrew or socket terminals (RJ45) – this is made possible by the fact that UTP cable bandwidth has improved to match the baseband of television signals.

FTP – Foiled Twisted Pair

FTP cables are screened UTP cables.

STP – Shielded Twisted Pair

STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences).

There are also S/STP cables that is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like FTP). This type of cabling offers the best protection from interference from external sources, and also eliminates alien crosstalk.

Note: FTP, STP and S/STP cable shields has to be well grounded (less then 3 ohm ground resistance) to avoid “aerial effect” – this phenomena increase external electromagnetic interferences – a shield not grounded acts as an aerial.

Optical fiber

An optical fiber is a glass or plastic fiber that carries light along its length. It is the combination of science and engineering towards the practical application of knowledge in optical sciences to communications.

Optical fibers are widely used in communications because they permit transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss and they are also immune to electromagnetic interference.

Light is kept in the core of the optical fiber by total internal reflection so the fiber acts as a waveguide.

Fibers which support many signals or propagation modes simultaneously are called multimode fibers, while those which can only support a single signal or mode are called singlemode fibers.

Multimode fibers generally have a larger core diameter, and are used for short-distance communication links (up to 550 meters).

Singlemode fibers are used for most communication links longer than 550 meters.

Joining lengths of optical fiber is more complex than joining “electrical wire” – the ends of the fibers must be carefully cleaved and then spliced together either mechanically or by fusing them together with an electric arc – special connectors are used to make removable connections.

Optical fiber can be used as a transmitting medium for communication and to networking because it is flexible and can be bundled as cables.

It is especially advantageous for long-distance communications because light propagates through the fiber with little attenuation compared to electrical cables; this allows long distances to be spanned with few repeaters; additionally, the per-channel light signals propagating in the fiber can be modulated at rates as high as 111 Gbit/s and each fiber can carry many independent channels, each using a different wavelength of light (WDM – wavelength-division multiplexing).

For short distance applications, such as creating a network within an office building, fiber-optic cabling can be used to connect the different racks on each floor and save space in cable ducts, because a single fiber can carry much more information than a lot of copper cables.

Optical fiber is immune to electrical interference; there is no cross-talk between signals in different cables and also no pickup of environmental “electrical noise”.

Fiber-optic cables do not conduct electricity, which makes fiber a good solution for protecting communications equipment located in high voltage environments such as power generation facilities or metal communication structures prone to lightning strikes and they can also be used in environments where explosive fumes are present, without danger of ignition.

Although optical fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance communications applications are always glass because of the lower optical attenuation.

Because of the tighter tolerances required to couple light into and between singlemode fibers, singlemode transmitters, receivers, amplifiers and other components are generally more expensive than multimode components.

In fibers, the cladding is usually coated with a tough resin buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties.

Rigid fiber assemblies sometimes put dark light-absorbing glass between the fibers to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers and reduces flare in fiber light waves.

Modern cables come in a wide variety of sheathings and armor, designed for different applications such as: direct burial in trenches, high voltage isolation, installations in conduit, lashing on aerial telephone poles, submarine installation and insertion in paved streets.

The cost of fiber cables has greatly decreased due to the high demand for fiber to the home (FTTH) installations.

Fiber cable can be very flexible, but traditional low fiber’s loss increases greatly if the fiber is bent with a radius of curvature very tight, creating a problem when the cable is bent around corners or wound around a spool, making installations more complicated.

Another important feature of cable is cable withstanding against the horizontally applied force – it’s technically called “max tensile strength” and defines how much force can be applied to the cable during the installation.

Also here, this relative fragility of these cables makes the installation practices an important part of ensuring the cable’s performance.

Some fiber optic cable versions are reinforced with armed yarns as intermediary strength member to protect the cable core against rodents.

Optical fibers are connected to terminal equipment by optical fiber connectors, usually of a standard type such as SC, ST, LC or MTRJ.

Optical fibers may be connected to each other by connectors or by splicing that join two fibers together to form a continuous optical waveguide.

The generally accepted splicing fiber method is arc fusion splicing, which melts the fiber ends together with an electric arc.

Fusion splicing is done with a specialized instrument that typically operates as follows:

  • The two cable ends are fastened inside a splice enclosure that will protect the splices and the fiber ends are stripped of their protective polymer coating (as well as the sturdier outer jacket, if present).
  • The ends are cleaved (cut) with a precision cleaver to make them perpendicular and are placed into special holders in the splicer. The splice is usually inspected via a magnified viewing screen to check cleaves before and after the splice.
  • The splicer uses small motors to align the end faces together and first emits a small spark between electrodes, at the gap, to burn off dust and moisture; then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark are carefully controlled so that the molten core and cladding do not mix minimizing optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical.

The complexity of this process makes fiber splicing much more difficult than splicing copper wire.

With multimode fiber (not acceptable with singlemode fiber) and for quicker fastening jobs, a “mechanical splice” is used:

  • Mechanical fiber splices are designed to be quicker and easier to splice, but there is still the need for stripping, careful cleaning and precision cleaving.
  • The fiber ends are aligned and held together by a precision-made sleeve, often using a clear index-matching gel that enhances the transmission of light across the joint.
  • Such joints typically have higher optical loss and are less robust than fusion splices.

All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.

Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely.

A fiber-optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be “push and click”, “turn and latch” (“bayonet”), or screw-in (threaded).

A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body.

Quick-set adhesive is usually used so the fiber is held securely and a strain relief is secured to the rear; once the adhesive has set, the fiber’s end is polished to a mirror finish.

Various polish profiles are used, depending on the type of fiber and the application:

  • The fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores – this is known as a “physical contact” (PC) polish.
  • The curved surface may be polished at an angle, to make an “angled physical contact” (APC) connection – such connections have higher loss than PC connections, but greatly reduced back reflection because light that reflects from the angled surface leaks out of the fiber core; the resulting loss in signal strength is known as gap loss – APC fiber ends have low back reflection.

Network classification

The following list presents categories used for classifying networks:

Classification as network topology:

Structured networks may be classified according to the network topology upon which the network is based, such as: bus network, star network, ring network, mesh network, star/bus network, tree or hierarchical network.

Network topology signifies the way in which devices in the network see their logical relations to one another. The use of the term “logical” here is significant; that is, network topology is independent of the “physical” layout of the network, even if networked devices are physically placed in a linear arrangement, if they are connected via a hub, the network has a star topology, rather than a bus topology.

In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout.

Classification as connection method:

Structured networks can be classified according to the hardware and software technology that is used to interconnect the individual devices in the network, such as Optical fiber, Ethernet or Wireless LAN.

Ethernet uses physical wiring (copper and optical fiber cables) to connect devices and, frequently, deployed devices include: hubs, switches, bridges and/or routers.

Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium.

Wired Technologies:

  • Twisted-Pair Wire – This is the most widely used medium for communication. Twisted-pair wires ordinary consists of two insulated copper wires twisted that are than grouped into twisted pairs and are used for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed ranges from 1 Mb/s to 10 Gb/s depending on the category (from Cat.3 to Cat.7a).
  • Fiber Optics – These cables consist of one or more thin filaments of glass fiber wrapped in a protective layer. It transmits light which can travel over long distance and higher bandwidths. Fiber optic cables are not affected by electromagnetic radiation and transmission speed could go up to as high as trillions of bits per second. The transmission speed of fiber optics is thousands of times faster than twisted pair wire.

Wireless Technologies:

  • Terrestrial Microwave – Terrestrial microwaves use Earth-based transmitter and receiver. The equipment looks similar to satellite dishes. Terrestrial microwaves use low-gigahertz range and communications are limited to line-of-sight. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.
  • Communications Satellites – The satellites use microwave radio as communications medium which are not deflected by the Earth’s atmosphere. The satellites are stationed in space, typically 22,000 miles above the Equator. These Earth-orbiting systems are capable of receiving and relaying voice, data and TV signals.
  • Wireless LAN – Wireless local area networks use a high and low frequency radio-wave technology. Wireless LAN use spread spectrum technology to enable communications between multiple devices in a limited area. An example of open-standards wireless radio-wave technology is IEEE 802.11b.
  • Bluetooth It’s a short range wireless technology. Operate at approximately 1 Mbps with range from 10 to 100 meters. Bluetooth is an open wireless protocol for data exchange over short distances.

Classification as Scale:

Networks are often classified as: Local Area Network (LAN), Wide Area Network (WAN), Virtual Private Network (VPN), Storage Area Network (SAN), etc. depending on their scale, scope and purpose. Usage, trust levels and access rights often differ between these types of network – for example, LAN tend to be designed for internal use by an organization’s internal systems and employees in individual physical locations (such as a building), while WAN may connect physically separate parts of an organization to each other and may include connections to third parties.

Local Area Network

A Local Area Network (LAN) is a network covering a small physical area, like a home, an office or small group of buildings, such as a school or an airport. Current wired LAN are most likely to be based on Ethernet technology, although new standards like ITU-T G.hn also provide a way to create a wired LAN using existing wires (coaxial cables, phone cables and power cables).

The defining characteristics of LAN, compared to WAN, are: higher data transfer rates, smaller geographic range and no need for leased communication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s data transfer rate, however, IEEE has projects investigating the standardization of 40 and 100 Gbit/s.

Wide Area Network

A Wide Area Network (WAN) is a network that covers a broad area, i.e. any network whose communications links cross metropolitan, regional or national boundaries. Less formally, a WAN is a network that uses routers and public communications links, contrasting with local area networks (LAN) which are usually limited to a building.

The largest and most well-known example of a WAN is the Internet or Web.

A WAN is a data communications network that covers a relatively broad geographic area (one city to another and/or one country to another) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer and the network logical layer.

Storage Area Network

A Storage Area Network (SAN) is an architecture to attach remote computer storage devices (disk arrays, optical jukeboxes, etc.) to servers in such a way that the devices appear as locally attached to the operating system.

A SAN typically is its own network of storage devices that are generally not accessible by regular devices through the common network.

The cost and complexity of SAN has dropped in recent years due to equipment’s cost drop, features integration and speed increase, resulting in much wider adoption by small to medium sized companies.

A SAN alone does not provide the file abstraction, only execute block-level operations, however, file systems built on top of SAN do provide this abstraction and are known as SAN file systems or shared disk file systems.

Virtual Private Network

A Virtual Private Network (VPN) is a network in which some of the links between nodes are carried by virtual connections or virtual circuits instead of by physical wires (as in some larger network as the Internet).

The data link layer protocols of the virtual network are tunneled through the larger network when needed.

One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption; VPN, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

Generally, a VPN has a topology more complex than point-to-point so a best-effort performance may have been done and may have been defined a minimum service level agreement between the customer and the external VPN service provider.

Internetwork:

The connection of two or more distinct networks, or network segments, via a common technology is called an internetwork.

Two or more networks or network segments connect to each other using a device that operates at layer 3 (the logic network layer) of the OSI Basic Reference Model, such as, for example, a router.

Any interconnection among or between public, private, commercial, industrial, or governmental networks may be defined as an internetwork.

In practice, interconnected networks use the IP (Internet Protocol). There are at least three variants of internetworks, depending on who administers and who participates in them: Intranet, Extranet and Internet.

  • Intranet An intranet is a set of networks using the Internet Protocol and IP-based tools, such as web browsers and file transfer applications that are under the control of a single administrative entity. Access to the intranet is restricted to all but specific, authorized users, thus, an intranet is the internal network of an organization. A large intranet will typically have at least one dedicated web server to provide organizational information.
  • Extranet An extranet is a network or internetwork that is limited in scope to a single organization or entity that can also have limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g.: a company’s customers may be given access to some part of its intranet, creating in this way an extranet, while at the same time the customers may not be considered ‘trusted’ from a security standpoint). Technically, an extranet may also be categorized as a WAN, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.
  • Internet The Internet is a global system of interconnected governmental, public, academic and private networks which is based upon the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by Defense Advanced Research Projects Agency (DARPA) of the U.S. Department of Defense. The Web is the communications backbone underlying the World Wide Web (www).

Intranet and extranet may or may not have connections to the Web – if connected to the Web, the intranet or extranet is normally protected from being accessed from the Web without proper authorization. The Web is not considered to be a part of the intranet or extranet, although it may serve as a portal for access to portions of an extranet.

Participants in the Web use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet Protocol Suite, an addressing system (IP Addresses) administered by the Internet Assigned Numbers Authority and address registries. Service providers and enterprises exchange information about the reachability to their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.

Basic hardware components

All networks are made up of basic hardware building blocks to interconnect network nodes, such as: Network Interface Cards, Bridges, Hubs, Switches, and Routers.

In addition, some method of connecting these building blocks is required, usually in the form of copper cable (most commonly on Cat.5e/Cat.6), microwave links (IEEE 802.12) or optical fiber cable.

Network Interface Cards


A network card, network adapter, or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a network. It provides physical access to a networking medium and often provides a low-level addressing system through the use of MAC (Media Access Control) addresses.

Repeaters


A repeater is an electronic device that receives a signal, cleans it from the unnecessary noise, regenerates it and retransmits it at a higher power level so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable which runs longer than 100 meters (90 m between end sockets).

Hubs

A network hub contains multiple data ports. When a data packet arrives at one port, it is copied unmodified to all ports of the hub for transmission. The destination address in the frame is not changed to a broadcast address.

Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as a hub do, but learns which MAC addresses are reachable through specific ports.

Once the bridge associates a port to an address, it will manage that address’ traffic only through that port.

Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports – once a frame arrives through a port, its source address is stored and the bridge assumes that that MAC address is associated with that port – the first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived to obtain the MAC address of the destination port and can save it.

Bridges come in three basic types:

  • Local bridges: Directly connect local area networks (LAN);
  • Remote bridges: Can be used to create a wide area network (WAN) link between one or more local area networks (LAN). Remote bridges where the connecting link is slower than the end networks largely have been replaced with routers;
  • Wireless bridges: Can be used to join LAN or connect remote stations to LAN.

Switches

A network switch is a device that forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in the packets.

This is distinct from a hub in that it only forwards the frames to the ports involved in the communication rather than all ports connected.

A switch normally has numerous ports, facilitating a star topology for devices and cascading additional switches.

Some switches are capable of routing based on Layer 3 addressing or additional logical levels; these are called multi-layer switches. Switches allow for a dedicated connection to each terminal and allows for many communications to occur simultaneously.

By using a switch the network is able to maintain full-duplex Ethernet – this means that data can be transmitted in both directions at the same time.

The core function of a switch is to allow terminals to communicate only with the switch instead of with each other and, as a switch can maintain full-duplex Ethernet, this means that data can be sent from the terminal to the switch and from the switch to the terminal simultaneously.

The core purpose of a switch is to decongest the network data flow to the terminals so that the connections can transmit more effectively, allowing terminals to receive only specific transmissions to their network address.

With the network decongested and transmitting data in both directions simultaneously, in fact, when two workstations are trading information this can double network speed and capacity.

Routers

A router is a networking device that forwards packets between networks using information in protocol headers and forwarding tables to determine the best next router for each packet.

Routers work at the Network Layer (layer 3) of the OSI model and the Internet Layer of TCP/IP.

Power over Ethernet (PoE)

Power over Ethernet or PoE technology describes a system to safely pass electrical power along with data on Ethernet network cabling. Not to be confused with Power Line Communication (PLC) or Point-to-Point Protocol over Ethernet (PPPoE). Standard versions of PoE specify Cat.5e cable or higher for best performance.

Power can come from a power supply within a PoE enabled networking device such as an Ethernet switch or from a device built for “injecting” power onto the Ethernet cabling.

The IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (44-57 V DC, though the nominal voltage is 48 V, and 10-350 mA), over two of the four available pairs on a Cat.5e/Cat.6 cable, to each device, but only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.

A “phantom power” technique is also used to allow the powered pairs to also carry data – this technique permits PoE use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (gigabit Ethernet), which uses all four pairs for data transmission.

This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling – each pair operates in “common mode” as one side of the DC supply; so two pairs are required to complete the circuit.

The powered device must operate with both spare pairs 4-5 and 7-8 or data pairs 1-2 and 3-6.

The IEEE 802.3at-2009 PoE standard, sometimes called “POE+” provides up to 25 W of power.

Powering devices

There are two types of PSE (power sourcing equipment) specified by IEEE 802.3-2008: Endspans and Midspans.

Endspans are Ethernet switches that include the Power over Ethernet transmission circuitry, are commonly called PoE switches.

Midspans are power injectors that stand between a regular Ethernet switch and the powered device, injecting power without affecting the data, are normally used on installations where there is no desire to replace and configure a new Ethernet switch and only PoE capability needs to be added to the network.

Advantages over other integrated data and power standards

This technology is especially useful for powering IP telephones, wireless LAN access points, IP cameras with pan tilt and zoom (PTZ), remote Ethernet switches, embedded computers, thin clients and LCD monitors.

All these require more power than USB offers and very often must be powered over longer runs of cable than USB permits. In addition, PoE uses only one type of connector (8p8c RJ45) whereas there are four different types of USB connectors.

PoE is presently deployed in applications where USB is unsuitable and where AC power would be inconvenient, expensive or infeasible to supply.

However, even where USB or AC power could be used, PoE has several advantages over either, including the following:

  • Cheaper cabling — even Cat.5e cable is cheaper than USB repeaters;
  • A Gb/s of data to every device is possible, which exceeds newer USB and the AC powerline networking capabilities;
  • Global organizations can deploy PoE everywhere without concern for any local variance in AC power standards, outlets, plugs or reliability of AC power;
  • Direct injection from standard 48 V DC battery power arrays; this enables critical infrastructure to run more easily in outages and make power rationing decisions centrally for all the PoE devices;
  • Symmetric distribution is possible – unlike USB and AC power, power can be supplied at either end of the cable or outlet – this means the location of the power source can be determined after cables and outlets are installed.

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