NATIONAL INSTITUTE OF TECHNOLOGY

KURUKSHETRA, HARYANA

DEPARTMENT OF ELECTRONICS & COMMUNICATION

ENGINEERING

A

SEMINAR REPORT

ON

POWER OVER ETHERNET

Submitted to: Submitted By:

Ms. Mahima Nikhil Pratap Garg

Roll No. - 108212

EC-3

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ACKNOWLEDGEMENT

While it is my name that appears on the cover, realization of a goal would not be possible

without supporting team members.

I sincerely convey my regards and thanks my seminar guide Ms. Mahima for her proper

guidance, and valuable suggestions. Many thanks to Mr. R. K. Sharma Prof & Head, Dept. of

Electronics & Communication Engineering, NIT KURUKSHETRA for permitting me to take up this

project.

I also convey my regards to other faculty members for giving me an opportunity to learn and

do this seminar. If not for the above mentioned people my seminar would never have been completed

successfully. I once again extend my sincere thanks to all of them.

Nikhil Pratap Garg

Roll No.-108212

Section-EC3

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ABSTRACT

Local networks today cannot be imagined without Ethernet anymore. The protocol, developed in the mid 70s in the U.S., is very popular among network adminis-trators thanks to its simple installation, hassle-free maintenance and comparably low costs. However, an increased deployment of wireless transmission tech-nologies like Wireless-LAN, Bluetooth or DECT, let dif-ficulties of additionally supplied power become more noticeable. Many manufacturers have brought their own, non-standardised, i.e. proprietary solutions to market. However, due to their lack of compatibility, none of them was very successful. The new standard IEEE 802.3af now creates the conditions for direct power supply via network cables to terminal devices in LANs.The IEEE started the standardisation process in 1999, and it resulted in the standard being officially named IEEE 802.3af DTE Power via MDI (Data Terminal Equipment Power via Media Dependant Inter-face). The powered equipment in a network is sorted into two categories:Power Sourcing Equipment (PSE) – the energy suppliers andPowered Devices (PD) – the energy usersA further distinction is made within the PSE, and relates to the power feed method.The terminal devices are powered through standard RJ45 connectors and regular Cat.5 or Cat.6 cables. Only two out of four pairs are used for signal transmission in 10Base-T and 100Base-T. The pairs 4/5 and 7/8 are idle and can therefore be used to transmit the direct voltage of 48 Volts (e.g. 4/5 positive, 7/8 negative) (alternative B). Due to the technical limitations of cabling systems the standard limits the power input to 350 mA for continuous operation, meaning that the maximum input power available amounts to 15.4 Watts. At a distance of 100 metres between PSE and Powered Device, the power available to any Powered Device (terminal device) is 13 Watts.Doubtless, power over Ethernet will establish itself in the coming years. Especially in new installations the technical advantages and cost savings thanks to the elimination of additional power supply distinctively prevail over the little additional costs of PoE-capable switches. Finally, access points can be installed where they reach best coverage – without later expenses for installation. PoE-capable devices require no separate power supply units, users worldwide no longer depend on adapters and adjustments. Ethernet is good, PoE is better.

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ETHERNETIn today's business world, reliable and efficient access to information has become an important asset in the quest to achieve a competitive advantage. File cabinets and mountains of papers have given way to computers that store and manage information electronically. Coworkers thousands of miles apart can share information instantaneously, just as hundreds of workers in a single location can simultaneously review research data maintained online.

THE ETHERNETIn 1973, at Xerox Corporation’s Palo Alto Research Center (more commonly known as PARC), researcher Bob Metcalfe designed and tested the first Ethernet network. While working on a way to link Xerox’s "Alto"computer to a printer Metcalfe developed the physical method of cabling that connected devices on the Ethernet as well as the standards that governed communication on the cable. Ethernet has since become the most popular and most widely deployed network technology in the world. Many of the issues involved with Ethernet are common to many network technologies, and understanding how Ethernet addressed these issues can provide a foundation that will improve your understanding of networking in general. The Ethernet standard has grown to encompass new technologies as computer networking has matured, but the mechanics of operation for every Computer networking technologies are the glue that binds these elements together. The public Internet allows businesses around the world to share information with each other and their customers. In this article, we will take a very close look at the Ethernet networking standard, so we can understand the actual mechanics of how all of these computers connect to one another.

Ethernet network today stem from Metcalfe’s original design. The original Ethernet described communication over a single cable shared by all devices on the network. Once a device attached to this cable, it had the ability to communicate with any other attached device. This

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allows the network to expand to accommodate new devices without requiring any modification to those devices already on the network.

ETHERNET BASICSEthernet is a local area technology, with networks traditionally operating within a single building, connecting devices in close proximity. At most, Ethernet devices could have only a few hundred meters of cable between them, making it impractical to connect geographically dispersed locations. Modern advancements have increased these distances considerably, allowing Ethernet networks to span tens of kilometers.

ETHERNET TERMINOLOGYEthernet follows a simple set of rules that govern its basic operation. To better understand these rules, it is important to understand the basics of Ethernet terminology.

Medium - Ethernet devices attach to a common medium that provides a path along which the electronic signals will travel. Historically, this medium has been coaxial copper cable, but today it is more commonly a twisted pair or fiber optic cabling.Segment - We refer to a single shared medium as an Ethernet segment.Node - Devices that attach to that segment are stations or nodes Frame - The nodes communicate in short messages called frames, which are variably sized chunks of information.Frames are analogous to sentences in human language. In English, we have rules for constructing our sentences: We know that each sentence must contain a subject and a predicate. The Ethernet protocol specifies a set of rules for constructing frames. There are explicit minimum and maximum lengths for frames, and a set of required pieces of information that must appear in the frame. Each frame must include, for example, both a destination address and a source address, which identify the recipient and the sender of the message. The address uniquely identifies the node, just as a name identifies a particular person. No two Ethernet devices should ever have the same address.

ETHERNET MEDIUMSince a signal on the Ethernet medium reaches every attached node, the destination address is critical to identify the intended recipient of the frame. For example, in the figure below, when computer B transmits to printer C, computers A and D will still receive and examine the frame. However, when a station first receives a frame, it checks the destination address to see if the frame is intended for itself. If it is not, the station discards the frame without even examining its contents.

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One interesting thing about Ethernet addressing is the implementation of a broadcast address.

A frame with a destination address equal to the broadcast address (simply called a broadcast,

for short) is intended for every node on the network, and every node will both receive and

process this type of frame.

CSMA/CDThe acronym CSMA/CD signifies carrier-sense multiple access with collision detection and describes how the Ethernet protocol regulates communication among nodes. While the term may seem intimidating, if we break it apart into its component concepts we will see that it describes rules very similar to those that people use in polite conversation. The term multiple access covers what we already discussed above: When one Ethernet station transmits, all the stations on the medium hear the transmission, just as when one person at the table talks, everyone present is able to hear him or her.Now let's imagine that you are at the table and you have something you would like to say. At the moment, however, I am talking. Since this is a polite conversation, rather than immediately speak up and interrupt, you would wait until I finished talking before making your statement. This is the same concept described in the Ethernet protocol as carrier sense. Before a station transmits, it listens to the medium to determine if another station is transmitting. If the medium is quiet, the station recognizes that this is an appropriate time to transmit.

COLLISION DETECTIONCarrier-sense multiple access gives us a good start in regulating our conversation, but there is

one scenario we still need to address. You and I both have something we would like to add,

and we both "sense the carrier" based on the silence, so we begin speaking at approximately

the same time. In Ethernet terminology, a collision occurs when we both spoke at once. In

our conversation, we can handle this situation gracefully. We both hear the other speak at the

same time we are speaking, so we can stop to give the other person a chance to go on.

Ethernet nodes also listen to the medium while they transmit to ensure that they are the only

station transmitting at that time. If the stations hear their own transmission returning in a

garbled form, as would happen if some other station had begun to transmit its own message at

the same time, then they know that a collision occurred. A single Ethernet segment is

sometimes called a collision domain because no two stations on the segment can transmit at

the same time without causing a collision. When stations detect a collision, they cease

transmission, wait a random amount of time, and attempt to transmit when they again detect

silence on the medium. The random pause and retry is an important part of the protocol. If

two stations collide when transmitting once, then both will need to transmit again. At the next

appropriate chance to transmit, both stations involved with the previous collision will have

data ready to transmit. If they transmitted again at the first opportunity, they would most

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likely collide again and again indefinitely. Instead, the random delay makes it unlikely that

any two stations will collide more than a few times in a row.

LIMITATIONS OF ETHERNETA single shared cable can serve as the basis for a complete Ethernet network, which is what we discussed above. However, there are practical limits to the size of our Ethernet network in this case. A primary concern is the length of the shared cable. Electrical signals propagate along a cable very quickly, but they weaken as they travel, and electrical interference from neighboring devices (fluorescent lights for example) can scramble the signal. A network cable must be short enough that devices at opposite ends can receive each other's signals clearly and with minimal delay. This places a distance limitation on the maximum separation between two devices (called the network diameter) on an Ethernet network. Additionally, since in CSMA/CD only a single device can transmit at a given time, there are practical limits to the number of devices that can coexist in a single network. Attach too many devices to one shared segment and contention for the medium will increase. Every device may have to wait an inordinately long time before getting a chance to transmit. Engineers have developed a number of network devices that alleviate these difficulties. Many of these devices are not specific to Ethernet, but play roles in other network technologies as well.

REPEATERSThe first popular Ethernet medium was a copper coaxial cable known as "thicknet." The maximum length of a thicknet cable was 500 meters. In large building or campus environments, a 500-meter cable could not always reach every network device. A repeater addresses this problem. Repeaters connect multiple Ethernet segments, listening to each segment and repeating the signal heard on one segment onto every other segment connected to the repeater. By running multiple cables and joining them with repeaters, you can significantly increase your network diameter.

SEGMENTATIONEthernet networks faced congestion problems as they increased in size. If a large number of stations connected to the same segment and each generated a sizable amount of traffic, many stations may attempt to transmit whenever there was an opportunity. Under these circumstances, collisions would

BRIDGESTo alleviate problems with segmentation, Ethernet networks implemented bridges. Bridges

connect two or more network segments, increasing the network diameter as a repeater does,

but bridges also help regulate traffic. They can send and receive transmissions just like any

other node, but they do not function the same as a normal node. The bridge does not originate

any traffic of its own; like a repeater, it only echoes what it hears from other stations.

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Remember how the multiple access and shared medium of Ethernet meant that every station on the wire received every transmission, whether it was the intended recipient or not. Bridges make use of this feature to relay traffic between segments. In the figure above, the bridge connects segments 1 and 2. If station A or B were to transmit, the bridge would also receive the transmission on segment 1. How should the. no need for the frame to appear on segment 2.In this case, the bridge does nothing. We can say that the bridge filters or drops the frame. If the destination address is that of station C or D, or if it is the broadcast address, then the bridge will transmit, or forward the frame on to segment 2. By forwarding packets the bridge allows any of the four devices in the figure to communicate. Additionally, by filtering packets when appropriate, the bridge makes it possible for station A to transmit to station B at the same time that station C transmits to station D, allowing two conversations to occur simultaneously! Switches are the modern counterparts of bridges, functionally equivalent but offering a dedicated segment for every node on the network (more on switches later in the article).

SWITCHED ETHERNETModern Ethernet implementations often look nothing like their historical counterparts. Where

long runs of coaxial cable provided attachments for multiple stations in legacy Ethernet,

modern Ethernet networks use twisted pair wiring or fiber optics to connect stations in a

radial pattern. Where legacy Ethernet networks transmitted data at 10 megabits per second

(Mbps), modern networks can operate at 100 Mbps!

Perhaps the most striking advancement in contemporary Ethernet networks is the use of switched Ethernet. Switched networks replace the shared medium of legacy Ethernet with a dedicated segment for each station. These segments connect to a switch, which acts much like an Ethernet bridge, but can connect many of these single station segments. Some switches today can support hundreds of dedicated segments. Since the only devices on the segments are the switch and the end station, the switch picks up every transmission before it reaches another node. The switch then forwards the frame over the appropriate segment, just like a bridge, but since any segment contains only a single node, the frame only reaches the intended recipient. This allows many conversations to occur simultaneously on a switched network.

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FULL-DUPLEX ETHERNETEthernet switching gave rise to another advancement, full-duplex Ethernet. Full-duplex is a data communications term that refers to the ability to send and receive data at the same time. Legacy Ethernet is half-duplex, meaning information can move in only one direction at a time. In a totally switched network, nodes only communicate with the switch and never directly with each other. Switched networks also employ either twisted pair or fiber optic cabling, both of which use separate conductors for sending and receiving data. In this type of environment, Ethernet stations can forgo the collision detection process and transmit at will, since they are the only potential devices that can access the medium.

same time that the switch transmits to them, achieving a collision-free environment. This allows end stations to transmit to the switch at the time that the switch transmits to them, achieving a collision-free environment

ETHERNET OR 802.3You may have heard the term 802.3 used in place of or in conjunction with the term Ethernet. "Ethernet" originally referred to a networking implementation standardized by Digital, Intel and Xerox. (For this reason, it is also known as the DIX standard.) In February 1980, the Institute of Electrical and Electronics Engineers, or IEEE, created a

committee to standardize network technologies. The IEEE titled this the 802 working group,

named after the year and month of its formation. Subcommittees of the 802 working group

separately addressed different aspects of networking. The IEEE distinguished each

subcommittee by numbering it 802.X, with X representing a unique number for each

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subcommittee. The 802.3 group standardized the operation of a CSMA/CD network that was

functionally equivalent to the DIX Ethernet. Ethernet and 802.3 differ slightly in their

terminology and the data format for their frames, but are in most respects identical. Today,

the term Ethernet refers generically to both the DIX Ethernet implementation and the IEEE

802.3 standard.

Power Over Ethernet:-

Power Over Ethernet technology allows IP telephones, wireless LAN Access Points and

other appliances to receive power as well as data over existing LAN cabling, without

needing to modify the existing Ethernet infrastructure. It has just become an international

standard, called IEEE802.3af, as an extension to the existing Ethernet standards. The freezing

of the standard will allow an explosion of Power Over Ethernet devices and installations.

Power Over Ethernet is likely to be ubiquitous in a few years, as the cost of adding the power

supplies to the Ethernet switches is going to be small. Indeed, it offers the first truely

international standard for power distribution. In the wiring cabinet existing Ethernet switch

equipment is retained and a “midspan” power source injects power into the twisted pair

LAN cables. At the other end of the cables the power is used to run phones, wireless

access points, cameras and other appliances. An Uninterruptable Power Supply (UPS)

can optionally support the installation in the case of power failures.

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10Base-T

10Base-T supports a 10 Mb/s transmission rate over two pairs of Category 3 or better

telephone twisted pair cabling, also known as "voice grade" twisted pair. The widespread use

of twisted pair wiring has made 10Base-T the most popular version of Ethernet.

10Base-T uses one pair of wires for transmitting data, and the other pair for receiving data.

The two pairs of wires are bundled into a single cable that may often include two additional

pairs of wires which are unused for 10Base-T. Each end of the cable is terminated with an 8

position RJ-45 connector, or "jack".

All 10Base-T connections are point-to-point. This implies that a 10Base-T cable can have a

maximum of two Ethernet transceivers (or MAUs), with one at each end of the cable. One

end of the cable is typically attached to a 10Base-T "repeating hub". The other end is attached

directly to a computer station's network interface card (NIC), or an external 10Base-T

transceiver. The transceiver function is integrated into most 10Base-T NICs allowing the

cable to be plugged directly into an RJ-45 connector on the NIC without any external

components or termination. The AUI interface on older NICs may be used attached to a

10Base-T network through an external transceiver.

Two 10Base-T NICs may be directly attached to each other without a 10Base-T repeating

hub. In this case a special "crossover cable" is required that attaches the transmit pair of one

station to the receive pair of the other station, and vice versa. When attaching a NIC to a

repeating hub, a normal "straight through" cable is used and the cross over function is

performed inside the repeating hub.

The target segment length for 10Base-T with Category 3 wiring is 100 meters. Longer

segments can be accommodated as long as signal quality specifications are met. Higher

quality cabling such as Category 5 wiring may be able to achieve longer segment lengths in

the order of 150 meters while still maintaining the signal quality required by the standard.

The point-to-point cable connections of 10Base-T result in a "star" topology for the network.

A star topology consists of a central hub with point-to-point links that appear to radiate out

from the center like light from a star. The star topology simplifies maintenance, allows for 11

faster troubleshooting, and isolates cable problems to a single link. 10Base-T is well suited

for use in structured cabling systems. Structured cabling systems locate hubs in central wiring

closets. Cables fanout to wall outlets in each office. In the office, a "patch" cable connects the

computer station to the wall outlet.

10Base-T transceivers continually monitor the receive data path for activity to check that the

link is working correctly. During periods when the network is idle, transceivers send a "link

pulse" to each other as a means of verifying the integrity of the twisted pair connections.

10Base-T transceivers may optionally provide a "link light" that remains lit as long as the

transceiver receives frames or link pulses from the other end of the segment. If the link lights

are "on" at both ends of the segment, then you have an indication that the segment is wired

correctly. This function is known as a "link integrity test".

The independent transmit and receive paths of the 10Base-T media allow the full-duplex

mode of operation to be optionally supported. To support full-duplex mode, both the NIC and

the hub must be capable of, and be configured for, full-duplex operation.

10Base-T Advantages: 1) the star wiring topology supports easier maintenance and

troubleshooting, 2) twisted pair wiring is inexpensive and widely used, and 3) optionally

supports full-duplex operation.

10Base-T Disadvantages: 1) 10Base5 and 10Base2 support longer segment lengths.

10Base-T Illustration RJ-45 Illustration

10Base-T Facts

Transmission Rate 10 Mb/s (20 Mb/s in optional full-duplex mode)

Cable Type

two pairs of Category 3 or better unshielded twisted

pair (UTP) cabling,

also known as voice grade or telephone twisted pair

cabling,

100-ohm impedance rating

Maximum Segment Length 100 meters (328 feet)

Maximum Number of Transceivers per

Segment2

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Connecter Technology RJ-45 style modular jack (8-pins)

Signal Encoding Manchester encoding

100Base-T

The identifier "100Base-T" refers collectively to the entire set of specifications and media

standards for 100 Mb/s Ethernet, or "Fast Ethernet". Four 100 Mb/s media standards have

been defined: 100Base-TX, 100Base-FX, 100Base-T4, and 100Base-T2. Each of these

standards are described in the following sections.

All 100Base-T standards share a common "Media Access Control" (MAC) specification, but

each has its own unique "Physical Layer" (PHY), or transceiver, specification. 100 Mb/s

transceivers may be integrated directly into a network device such as a repeater or network

interface card (NIC), or located external to the device. If located externally, the transceiver is

attached to the repeater or NIC through a 40-pin "Media Independent Interface" (MII)

connector. The transceiver may be plugged directly into the MII connector, or be attached

through a MII cable that is analogous to the AUI cable defined as part of the 10 Mb/s

standard. If present, the MII cable may be a maximum of 0.5 meters in length.

The MII supports Ethernet operation at either 10 Mb/s or 100 Mb/s. Many Fast Ethernet

transceivers (PHYs) include the necessary electronics that allow them to support operation at

either speed. A device configures itself for operation at the proper speed through a protocol

known as "Auto-Negotiation".

1000Base-T

The 1000Base-T standard is was defined by the IEEE 802.3ab working group and formally

released in June 1999. The standard supports Gigabit Ethernet over 100 meters of Category 5

balanced copper cabling. It provides the following definition for 1000Base-T:

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The 1000BASE-T PHY employs full-duplex baseband transmission over four pairs of

Category 5 cabling. The aggregate data rate of 1000 Mb/s is achieved by transmission at a

data rate of 250 Mb/s over each wire pair. The use of hybrids and cancellers enables full-

duplex transmission by allowing symbols to be transmitted and received on the same wire

pairs at the same time. Baseband signaling with a modulation rate of 125 Mbaud is used on

each of the wire pairs. The transmitted symbols are selected from a four dimensional 5 level

symbol constellation. Each four dimensional symbol can be viewed as a 4-tuple (A n , B n , C

n , D n ) of one dimensional quinary symbols taken from the set {-2, -1, 0, +1, +2}. Idle mode

is a subset of code groups in that each symbol is restricted to the set {2, 0, -2} to improve

synchronization. Five level Pulse Amplitude Modulation (PAM5) is employed for

transmission over each wire pair. The modulation rate of 125 MBaud matches the GMII

clock rate of 125 MHz and results in a symbol period of 8 ns. This specification permits the

use of category 5 or better balanced cabling, installed according to ANSI/TIA/EIA-568-A.

The 1000BASE-T standard makes use of two signaling methods already used in earlier IEEE

standards: 100BASE-TX (125 Mbaud three level baseband signaling) and 100BASE-T2 (25 Mbaud

PAM5 baseband signaling.) These methods were chosen to make the 1000BASE-T PHY more

100BASE-T friendly for 100/1000 dual speed Ethernet PHY implementations, and to make the

standards development less time consuming.

1000Base-T Facts

Transmission Rate1000 Mb/s (2000 Mb/s in optional full-duplex

mode)

Cable Types4-pairs of Category 5 or better cabling,

100-ohm impedance rating

Maximum Segment Length 100 meters (328 ft)

Maximum Number of Transceivers per

Segment2

Connecter Technology 8-Pin RJ-45 connector

Signal Encoding PAM5

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5.4.1 RJ-45

An "RJ-45" connector is used on Ethernet twisted pair links. This includes the 10Base-T,

100Base-TX, 100Base-T4, 100Base-T2, and 1000Base-T physical layer types. An RJ-45

connector has 8-pins, and may also be referred to as an "8-pin Modular Connector". A male

RJ-45 "plug" is mounted on each end of the twisted pair cable. A female RJ-45 "jack" or

"receptacle" is integrated into the Ethernet hub or NIC.

power over Ethernet (PoE) brings a host of benefits to the design, implementation and long-

term usability of wired Ethernet local area networks (LANs). Cost, flexibility and even safety

are all enhanced. It overcomes the major limitation that system designers often encounter

whereby they must locate powered network devices within close proximity to AC power

outlets.

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With PoE, both data and power at a safe nominal 48VDC are carried over the same Ethernet

cable. If network devices can be configured to run from 48V, the need for devices on the

network to be supplied with intrinsically unsafe AC power from a separate power cable

connected to a building’s AC ring main is eliminated. The freedom this gives to position

devices where they are needed rather than where power cords dictate, and the long-term

flexibility to move devices around to suit the changing operational needs of the business, are

both highly desirable benefits for any organisation. As an example, networked security

cameras benefit from PoE as they normally need to be sited high up on walls, away from AC

outlets. Typical devices used on a powered Ethernet such as VoIP phones, cameras and Wi-

Fi access points may need to be specially designed or adapted to run from Ethernet 48VDC.

DC/DC converters in the network devices will normally be required to drop and isolate the

48V supply down to a lower voltage such as 5V, suitable for the device circuitry. To take full

advantage of PoE, the network devices will also be required to handle the hand-shaking

process that allows the network hub to recognise a device as being PoE enabled and

understand and manage its specific power needs.

THE BASICS OF A POE NETWORKThere are three main elements to a PoE system, the power sourcing equipment (PSE), the

powered device (PD) and cable. The PSE is connected to the Ethernet switch and

often itself powered by an uninterruptible power supply (UPS) providing 48V, the PSE feeds

data to all devices on the network, and data plus power to all PoE compatible devices.

Although the nominal output is 48VDC, the allowed range is 44 to 57VDC. The range of PoE

powered devices is continually growing as the adoption of – and market for PoE continues to

expand rapidly. Current types of enabled devices include VoIP phones, EPOS systems and

Wi-Fi access points. The PD gives a ‘signature’ to the PSE to indicate that it is PoE enabled

and how much power it requires. The management and conversion of the incoming power is

normally handled by a DC/DC converter in the PD, described in more detail later in this

article.

Ever since CAT5 cable was invented for the purpose of carrying data, the spare conductors

have been utilized to additionally carry power to the other end of the cable. The PoE standard

IEEE802.3af defines the detail of how this is done and provides the platform for industry to

adopt a single approach for the design and implementation of PoE.

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TRANSFERRING POWERA standard CAT5 Ethernet cable has four twisted pairs, only two of which are required for

the transmission of data. With PoE, there is a high degree of flexibility with the IEEE802.3af

standard allowing DC power of either polarity to be transferred using either the two spare

pairs or superimposed on the two data pairs. Fully compliant PoE Devices must be able to

accept power from either option. Currently, the IEEE standard allows nominal 48V and

approximately 15W of power to be transferred over a single powered Ethernet cable with a

minimum of about 13W available to the powered device. The restricted available power does

limit the types of peripheral devices that can be used with PoE. However, future changes to

the standard are being proposed which will allow for much higher power levels and

consequently a wider variety of devices to be operated via Cat5 cables on the Ethernet. PoE-

enabled laptops for example might be charged directly from network connections without

cumbersome AC adapters. In order to prevent damage to existing Ethernet equipment, which

may not be compatible, the IEEE802.3af standard requires that a ‘discovery process’ be

instigated by the PSE. This examines the cables looking for PoE enabled devices. It checks

for the presence of a 25KΩ parallel resistor in the remote devices and only if this is found is

the full 48V applied. The supply is current-limited to prevent any damage to the equipment

and cables should a fault condition occur. Before connection, the PD also signals back to the

PSE what current it expects to draw in four categories. If the powered device does not draw a

minimum current – which may occur for example when the device is unplugged from the

network – then the PSE removes the power from that particular cable and is then aware that

the power is available for other devices. The PSE might also remove power from selected

non-critical devices on a power outage, to maximize the run time of a UPS, maintaining more

critical security devices.

POWER CONVERSIONIsolated DC/DC converters are usually required within PoE enabled devices to transform the

48V supply to a lower voltage appropriate for the PD. IEEE802.3af defines stringent low-

noise, start-up and isolation requirements for the converters to be used in PoE applications.

As an example, the C&D Technologies NPH10 range is ideally suited. C&D also offers a full

interface solution in its NMPD product which integrates the data isolation transformers, PoE

handshaking and DC/DC conversion with customer-specified output voltage. The SIP module

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provides the full PoE-compliant data and power interface to the Ethernet line. The 48V

Ethernet power source is also defined by the IEEE standard, (actually 44 to 57VDC) at the

hub. However with significant voltage drops along the Ethernet cables, the PoE standard

allows the minimum voltage at the powered devices to be as low as 30V. The nominal 48V

can be derived from an AC/DC converter but also from an existing system 48V. In this case

another high power isolated DC/DC converter is required in the PSE. Although not defined

by the PoE standards, this DC/DC converter, designed to be incorporated into the PSE,

should ideally possess a number of characteristics and features to make it more suited to the

task. As an example, C&D Technologies’ HHS04-520 has been specifically designed with

PoE in mind. As well as complying with the requirements of the standard, it has extremely

high power density enabling it to manage and deliver 200W in a very small package. This is

important as many non-PoE hubs may be re-engineered to make them suitable for use on a

powered network. Engineers will often be tasked with squeezing any PoE related circuitry

into the existing enclosure – so clearly the smaller the converter the better. The HHS04

operates from a nominal 48V input and delivers an isolated 52.5V and 200W at its output

with a high efficiency of 92% at full load. IEEE802.3af specifies an open frame design that

allows for better thermal management of the product when fitted in the end system.

It also decreases component stress levels to provide increased reliability and module

longevity. C&D also has a 400W part in development, the HHS08, for larger PoE

systems.

Other useful features of a DC/DC converter that engineers may wish to consider when

designing PoE enabled devices include over-temperature protection, remote sense and remote

on/off and output trimming. Next generation DC/DC converters for PoE enabled network

devices are likely to include functions such as device to PSE handshaking and Ethernet data

isolation within more integrated packages. This will help simplify the task of developing PoE

devices by decreasing the amount of electronics design work required and reducing the PCB

real-estate needed within the device to give PoE compatibility.

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How Power is Transferred Through the CableA standard CAT5 Ethernet cable has four twisted pairs, but only two of these are used for

10BASE-T and 100BASE-T. The specification allows two options for using these cables for

power:

• The spare pairs are used. Figure 2 shows the pair on pins 4 and 5 connected together

and forming the positive supply, and the pair on pins 7 and 8 connected and forming the

negative supply. (In fact, a late change to the spec allows either polarity to be used).

• The data pairs are used. Since Ethernet pairs are transformer coupled at each end, it is

possible to apply DC power to the center tap of the isolation transformer without upsetting

the data transfer. In this mode of operation the pair on pins 3 and 6 and the pair on pins 1 and

2 can be of either polarity.

Power Through the Cable on the Spare Pairs

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Power Through the Cable on the Data Pairs

The spec does not allow both sets of wires to be used – a choice must be made. The Power

Sourcing Equipment (PSE) applies power to either set of wires. The Powered Device (PD)

must be able to accept power from both options. The Midspan Hub is the Power Sourcing

Equipment, and the VoIP Phone, Wireless Access Points, and Network Camera are the

Powered Devices. Newer Ethernet Switches include the PSE funciton internally, so the

Midspan Hub is not required. The voltage is nominally 48V, and about 13W of power is

available at the Powered Device. An isolated DC-DC converter transforms the 48V to a lower

voltage more suitable for the electronics in the Powered Device, while maintaining 1500V of

isolation for safety reasons. An obvious requirement of the spec is to prevent damage to

existing Ethernet equipment. A “discovery process”, run from the Power Sourcing Equipment

(PSE), examines the Ethernet cables, looking for devices that comply with the specification.

It does this by applying a small current-limited voltage to the cable and checks for the

presence of a 25k ohm resistor in the remote device. Only if the resistor is present is the full

48V applied, but this is still current-limited to prevent damage to cables and equipment in

fault conditions.

The Powered Device must continue to draw a minimum current. If it does not (for example,

when the device is unplugged) then the PSE removes the power and the discovery process

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begins again. As an optional extension to the discovery process, a Powered Device may

indicate to the Power Sourcing Equipment its maximum power requirements. The Power

Sourcing Equipment may optionally provide a level of system management, using, for

example, the Simple Network Management Protocol (SNMP). This allows for mangement of

actions such as devices to be powered off at night, or remotely reset. More PoE equipment

continues to emerge due to the ease-of-use of existing standard Cat5 cable infrastructures.

Hence, no modification or tampering with existing Ethernet infrastructures is needed.

The PoE technology has many benefits. For example, only one set of wires is brought to your

appliances, thus simplifying installation and saving space. It also allows continued service

during power outage by using the same centralized UPS that backs up the network.

There is also no need to pay an expensive electrician or to delay installation to meet the

electrician's schedule, thus saving time and money. Furthermore, the appliance can be easily

moved to wherever you can lay a LAN cable. Since there are no mains voltages present

anywhere, it's safe. Using a UPS can guarantee power to their clients during mains-power

failures.

The Simple Network Management Protocol infrastructure can be used to monitor and control

the appliance, and the data transfer to and from the appliance. Hence, the appliances can be

managed, shut down or reset remotely in a centralized manner.

802.3af standardModern Ethernet networks and traditional telephone systems have much in common. Both

typically send data or voice over an unshielded twisted pair cable, usually using a form of star

network.

The difference between the two is that traditional phones are powered from the same wire as

the data or the voice wires. Meanwhile, Ethernet devices require a local power source.

The 802.3af standard has changed all this by allowing the central switch to provide 48Vdc of

up to 13W through the RJ-45 connector. This standard addresses all of the issues in supplying

power to devices via Ethernet cabling. Simply put, the standard defines the functional and

electrical characteristics of two optional power entities: the power device (PD) and the power

sourcing equipment (PSE).

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This is to be used for the PHYs defined in the standard. The standard is about how these

devices or entities can supply and draw power using the same generic cabling, as used for

data transmission.

The data terminal equipment powering is intended to provide a 10Base-T, 100Base-TX or

1,000Base-T device with a single interface to both the data it requires and the power to

process these data.

Earlier PoE implementations will not conflict with 802.3af, and the available power levels are

expected to address future applications. We are already seeing discussion on 30W power

transmission.

To meet the 802.3af standard, the PSE output voltage needs to be 44-57V; the maximum

output current in normal mode is 350mA; and the continuous output power is around 15.4W.

For the PD end, which is the client, the input voltage through your PD needs to be 37-57V. The

average input power is 12.95W, and the input inrush current is 400mA. The following are the other

specifications:

Figure 1. An endpoint PoE implementation, with the end application

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hooked up to the switch.

Alternative architecturesTwo particular implementations are defined. One is the endpoint, which is applicable to a

PoE-enabled switch (Figure 1). Note that the end application is hooked up to your switch and

it derives power from the switch.

The other application or implementation is called the mid span (Figure 2). Designers or users

can appreciate that in between the switch and their end application, there is a port for the mid span.

Figure 2. Mid-span implementation of PoE, wherein the power is

inserted to the network

This is where power is inserted to their network. In turn, the powered devices that are hooked

up to this mid span will get its power from it.

To illustrate how a PoE system works, consider the block diagram of a simple one-port PoE

system (Figure 3). It comprises a PSE, which is a hub or a switch shown on the left-hand side, and

the PD, which is on the right side and the end application side.

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Figure 3. A simple one-port PoE system comprises a PSE, which is

a hub or a switch shown on the left-hand side, and the PD, which is

on the right side and the end application side.

The PD side would typically drive applications such as IP phones or Web cameras. From the

left, the PSE provides power via data lines or spare lines that would go through the Cat5

cable and then to the Ethernet appliance side (PD).

Designers would extract this power (through a center tap transformer, if data lines are used)

before feeding this through the PD, such as the Freescale MCZ34670.

Figure 4 shows the two options for endpoint PSE application. The alternative A architecture

uses the actual data pairs of Cat5 LAN cable to transmit power.

The power is, in fact, superimposed on the data pairs. In 802.3af, powering is implemented using the

secondary winding center taps of the transmitter and receiver transformers at each end of the link.

Meanwhile, the alternative B option makes use of spare wires, with designers transmitting power on

a spare pair.

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Figure 4: The alternative A architecture uses the actual data pairs of

Cat5 LAN cable to transmit power. Meanwhile, the alternative B

option makes use of spare wires, with designers transmitting power

on a spare pair.

In summary, there are two options for inserting or extracting power. One is the use of a data

pair, where you superimpose power on top of the data. The second is the use of spare pairs to

transmit power.

The mid-span configuration is very useful because it allows power supply to be external to

the Ethernet. Moreover, it provides data and power on the twisted pair linked segment

without burdening each port with the Ethernet equipment when you need to provide power.

This allows the addition of PoE to your older systems without replacing switches or hubs.

The current specification only allows power in two of the four wire pairs in the Cat cable. For

the endpoint applications, we considered using the data pair or the spare pair.

In the standard for the mid span applications, however, PSEs are restricted to using only the

spare pair of the Cat5 wires. Thus, the PD extracts power from the PSE using the spare pair.

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PSE, PD operationDetecting and powering PoE devices require PSE power sourcing to work out, identify or

distinguish between a PoE and a non-PoE device.

This prevents the use of powering devices that are not PoE-compliant, and do not need or do

not want to be sent power. For obvious safety reasons, this also prevents you from blowing

up devices that are not PoE-compliant.

For a safe and reliable operation of PoE systems, the 802.3af standard mandates that the PSE

determine whether or not to supply power to the PD by applying test voltages.

The test voltages are used to determine the load characteristics of the PD. The load

characteristics of the PD are called the PD detection signature.

The PSE reads the PD detection signature to determine whether to supply power and how

much power to supply. If it doesn't see the signature in the device or the client end, it does not

deliver power from the network to that particular device.

The important functions of the PSE are to identify the PDs that are enabled to receive power,

to provide required power levels and to remove power when the PD is disconnected from the

link.

The detection mechanism is an extremely important function of the PSE to circumvent the

application of power to various devices that can be plugged into the eight-position modular

jack.

Signature detectionWhat makes a valid signature? The detection impedance of the resistor that is attached to the

power devices is all that's required for valid detection. The detection impedance requires only

one transistor with a value of 23.75-26.25KΩ.

We use a resistor called Rsig that is attached to the Rsig pin of our device. It draws current

that is close to what is expected of a valid impedance, thus minimizing power consumption.

The tolerance for such a resistor is very tight, between 23.75KΩ and 26.25KΩ. You also need

to consider the serial resistance of the diode.

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To understand the standard's power interface, the PD connected to the Ethernet keyboard

goes through a series of steps that must be compliant with the standard. Otherwise, no power

will be sent to your PD.

The power-sourcing end provides a voltage of 2-10V. This feeds the PDs, and this voltage

range is good enough to detect a valid signature from your resistor (e.g. a nominal 25KΩ).

Once this voltage detects the presence of that signature, it increases its power to 15-20V.

Here, it measures the particular class of required power. Upon successful signature detection

and classification, it ramps up its power for normal operation. Because it's for a telecom

application, it's 48V. We have a voltage of 37-57V for normal application.

POISEDA gentle squall has been gathering over the prospect of carrying power over the same copper

used to transmit data. But the technique, also known as Power over Ethernet (PoE), is about

to generate an electrical storm. PoE technology has been with us for about four years, initially

with numerous proprietary and custom solutions. But in June 2003, the IEEE ratified a

standard, dubbed the 802.3af. This was conceived to support the way switches, routers and

hubs should deliver power over standanrd Ethernet cabling to end user devices, such as

Internet Protocol (IP) phones,security systems and wireless Local Area Network

(LAN)access points (see 'Detection and handshakes', below). But, even as indnstry coalesced

round this standard, it was clear that the power output available to the power device (PD) - a

realistic 13W once the inevitable losses through the cabling and power conversion

inefficiencies were taken into account - was just not high enough. So, just as inevitably, a

new 802.3 task group was initiated in November 2004 to extend the power output and come

up with a revised version of the standard, the 'PoEplus'. Initial soundings suggest power will

be doubled to 3oW per port, although some of the companies that have shown interest in

participating in the Study Group favour going as high as 40W. But why the need for more

and more power? The motivation is obvious; to support many more devices and end user

applications. Typical devices that will make use of PoEplus include video phones, point tilt

zoom (PTZ) security network cameras and multi-channel as well as outdoor wireless LAN

access points. The PoE technology will also be extended to new markets, such as using the

LAN to power laptops, PDAs, workgroup switches, radio frequency identification (RFID)

readers, industrial controllers and base-stations for the emerging wireless broadband,

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'WiMAX. Indeed, each of these markets already needs more than the 13W currently

available (fip 1). Clearly, backward compatibility to existing Ethernet cabling and the current

802.M specifications is essential. But concerns exist over moving towards higher power

levels, since twisted pair cabling was never intended to carry serious power loads.

Interference, safety and heat build-up in large bundles of cable are all real worries.

Issues aside, the latest initiative seems to have the backing of most of the major players in the

sector. Switch and router makers such as Cisco, 3Com, Nortel and F'njitsu are all on board as

are component suppliers, Texas Instruments, Linear Technology Corporation, National

Semiconductor, Maxim Integrated Products, Philips Semiconductors, Fairchild and Supertex.

One company playing a significant promotional role with the 802.3af standard is Israeli group

PowerDsine. Collaborating with Freescale Semiconductor, US, the company has developed

and is selling what is arguably the most highly integrated PoE controller chip on the market.

PowerDsine is developing so-called end-span and mid-span products, which refer to the two

main implementations of PoE. The former is a PoE-enabled Ethernet switch in which power

is supplied directly from the data ports. Meanwhile, the mid-span option is a standalone

plug-and-play device that sits between an ordinary Ethernet switch and the terminals, or

power hubs .As has now become the norm, PowerDsine has just released a pre-standard

version of its mid-span, dubbed the High Power Series 8000, ahead of a final standard

ratification. This provides up to 39.5W of power while supporting the 802.3af standard with

regards to detection and safety. The series incorporates a small DC-DC stepdown converter

that provides a simple way to power legacy 12.5Y 2.5A devices with high power PoE.

SUMMARYPoE is undoubtedly going to change the way that many pieces of electrical equipment are

powered. The benefits and flexibility that the approach provides are huge and make it

impossible to ignore for businesses of all types. For companies building new premises or

those refitting their existing locations, it is well worth them considering becoming early

adopters of PoE.

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Some Possible Uses of the Technology (Future Scope):Already manufacturers have products on the market. Many of these are described in the

“Products” section of the www.PowerOverEthernet.com website. There is no shortage of the

Power Sourcing Equipment Midspan Hubs, so you can start playing with the technology now.

The big market players are using it for these applications:

• VoIP (Voice over Internet Protocol) Telephones.

• IEEE802.3af Wireless LAN Access Points

• Bluetooth Access Points.

• Web Cameras.

But Power Over Ethernet will enable many more appliances. Here’s a list – but you can

imagine many others!

• Smart signs/web signs.

• Vending machines.

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• Gaming machines.

• Audio and video juke boxes.

• Retail point of information systems.

• EPOS systems.

• Building access control systems.

• Time and attendance systems.

• Battery chargers for mobile phones and PDAs.

• Electronic musical instruments

PoE is a technology of transferring power and data using a LAN cable. It allows safe and reliable power transmission—15W with 48V—for telecom applications over existing Cat5, Cat5e and Cat6 cables.

For example, it can be used to power IP phones, WLAN access points, network cameras and

various other network terminals in the allowed power range of 13W, as measured by the

powered device side. PoE is also known as power-over-LAN and is based on the IEEE

802.3af standard.

REFRENCES:-

WWW.WIKIPEDIA.ORG

WWW.IEEE.COM

WWW.HOWSTUFFWORKS.COM

IET COMPUTING AND CONTROL UNIT

COMPUTER NETWORKS –TANENBAUM, ANDREWS

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