White PaperNovember 2016

Power Supply Concepts for FTTO Central and Decentralised Power Supply Design Guidelines

Foreword

When you start to think about IT network designs, the first thing that springs to your

mind is bandwidth for data transmission.

However, bandwidth and data transmission are only some points to consider when

planning an IT network. One must also think about an efficient, safe and reliable

power supply for the network, which also includes telephony and security services.

This is specified in electro technical standards and is particularly important for Power

over Ethernet (PoE) applications, when a data cable functions as a power line.

In many installation scenarios FTTO networks may offer greater advantages for LAN

than other structured cabling technologies because of their holistic approach and

flexibility. FTTO networks meet very high connectivity requirements regarding

redundancy, performance, scalability and network provisioning. To enjoy all benefits

of FTTO, close attention should be paid to intelligent power supply concepts.

This document is intended for network design engineers and IT managers with

interest in FTTO. It takes a closer look at various power supply concepts for FTTO

networks and presets some efficient design solutions for applications with PoE.

What is FTTO?

In contrast to classic structured cabling, Fibre To The Office (FTTO) is a

decentralised, distributed LAN cabling concept for office environments which

combines the advantages of both optical fibre and twisted pair copper cabling.

FTTO involves optical fibre cabling in the network backbone as well as in the

secondary and tertiary cabling levels. Twisted pair patch cables are used only for

short distances to connect end devices. With optical fibre, longer distances can be

bridged than with twisted pair cabling (whose distance is limited by 100 m maximum).

Moreover, optical fibre is a very flexible and scalable solution in terms of redundancy

and bandwidth. Short twisted pair patch cords in FTTO installations enable power

transmission via data cables, and the RJ45 plug offers a standardised unified

interface to connect various terminals.

The FTTO switch presents an intelligent link between optical fibre and twisted pair

copper media, i.e. the switch ensures media conversion from fibre to copper, and

vice verse. Terminal devices are connected to the FTTO switch via an RJ45 patch

cord, which also supplies end devices like VoIP telephones or WLAN access points

with power. The switch can get its power supply in various ways.

A detailed description of advantages and functionalities of FTTO networks can be

found in our LANactive brochure:

www.nexans.com/LANsystems

Figure 1. FTTO cabling concept

Power over Ethernet – Using Twisted Pairs for Power Supply

Power over Ethernet (PoE, Type 1) IEEE 802.3af was first specified in 2003. In 2009,

another standard was adopted, the IEEE 802.3at (PoE+, Type 2). Now there is

another PoE standard under development, featuring the 3rd generation of PoE

standards, i.e. the IEEE 802.3bt (PoE++, Type 3 and Type 4). It is expected to

emerge in 2017. Higher PoE standards differ from the lower ones not only in a higher

power consumption rate (in Watts), but also in the type of Ethernet protocols

supported and other features.

Number of pairs

needed

Maximum power output for PSE

Usable power

budget for the end

device (PD)

Standard In effect since

PoE 2 Class 1: 15.4 Watts 12.95 Watts IEEE 802.3af, Type 1 2003

PoE+ 2 Class 4: 30 Watts 25.50 Watts IEEE 802.3at, Type 2 2009

PoE++ 4 Class 6: 60 Watts 51 Watts IEEE 802.3bt, Type 3 expected: 2017

PoE++ 4 Class 8: 90 Watts 71 Watts IEEE 802.3bt, Type 4 expected: 2017

Class: Performance classification on the physical level

PSE: Power Sourcing Equipment = e.g. the FTTO switch

PD: Powered Device = e.g. VoIPTelephones, Wireless Access Points, IP Cameras,

PCs, printers/scanners, etc.

Table 1. Overview of PoE standards

However, a higher power budget means a temperature rise inside the data cable.

The disturbing trend can be already observed with PoE+ applications. This

development poses a risk and is an important cost factor to consider. What´s more,

the temperature rise tends to negatively affect attenuation and data transfer

properties of the cable. This is critical when several longer cables are combined into

one bundle, since heating accumulates in the center of the bundle. In contrast to

traditional network designs, FTTO networks do not face this problem as there are

only short (3-5 m) twisted pair patch cords in use. Thus the effect of heating

observed with higher PoE budgets can be completely neglected in FTTO networks.

Which devices can be operated via PoE, PoE+ and PoE++ (in the future)?

Even with the first standard of PoE, IEEE 802.3af, it is possible to power very useful

applications. Many active network components already support power supplies via

twisted pairs.

PoE is very convenient for the user. A single PoE cable provides data communication

and powers end devices (like printers, WLAN access points, IP video cameras and

notebooks) at the same time. It is particularly beneficial for VoIP telephones and

other applications located in unconventional places like under the ceiling, in the

corner, etc.

General Power Supply Concepts 230V AC vs. 54 V DC

230V AC Power Supply

The easiest way to power FTTO switches is to connect them to the 230V power lines.

The advantage is that no dedicated cables or distribution equipment are required.

One should only make sure that the power lines carrying electrical current are

adequately dimensioned for the expected load maximum and that the power supply

design meets well established electrical safety regulations. However, a major

disadvantage of this power supply concept is that the switches cannot support PoE

functionality. This also means, of course, that end user devices, e.g. VoIP

telephones, must be powered via their own power supplies and require additional

power sockets.

Figure 2. Power over Ethernet applications

Figure 3. 230V AC power supply for FTTO switches

A non-redundant power supply, as described above, is acceptable only for non-

critical network services. In most cases, however, a network downtime may cause

considerable financial damages, and therefore high network reliability and availability

are a must.

Figure 4. 230V AC power supply for FTTO switches with redundancy

To ensure high network availability in the event of a power blackout or a short circuit,

the concept presented in Figure 3 has to be extended to include UPS (Uninterruptible

Power Supply) modules. These UPS modules maintain normal network operation for

a certain period of time even in case of downtime. However, even with this power

supply design FTTO switches have no PoE functionality.

230V AC/54V DC Power Supply (Alternating and Direct Current)

The mains alternating current of 230-240V AC is transformed into the lower direct

current of 48-54V DC by means of a power supply unit. The lower voltage is

important for FTTO switches to provide PoE. The use of power supply units requires

a higher planning effort as there are different cables and distribution points in use.

With the 54V DC distributed power supply concept power supply units of the switches

are distributed throughout the building and are typically found in the cable duct, under

the panel or in a separate enclosure. In contrast, in central power supply concepts

direct current is routed centrally from the control room to the switch.

Both 230V AC/54V DC power supply concepts, the central and the distributed, are

described in detail below. The big advantage of these two is that FTTO switches in

these arrangements support PoE and can supply power to connected end user

devices via a data patch cord.

The switch may consume 120W when delivering PoE+ and thus feeding several end

user devices with power. For classic office applications including VoIP and WLAN, a

power budget of 30 - 60W is enough. The maximum power consumption per switch

stands at 150W then. Table 2 shows possible planning scenarios, where each is

designed for specific applications.

Number of PoE ports in use

Maximum PoE consumption per switch, in Watts

Total power consumption per switch, in Watts

Possible applications

Scenario 1 2 20 26 VoIP, WLAN, Thin Client,

IP camera

Scenario 2 3 50 56 VoIP, WLAN ac, Thin Client, IP camera PTZ

Scenario 3 4 90 96 VoIP, WLAN ac, Thin Client, IP camera PTZ

Scenario 4 4 120 126 Full PoE+ Support

Scenario 5 5 150 156 Full PoE+ Support

Table 2. Planning scenarios for power consumption with PoE

A combined planning approach can also be adopted. For example, one can plan the

power budget for a VoIP application according to Scenario 1 and envisage a higher

power budget for WLAN installations in line with Scenario 5.

Figure 5. 54V DC power supply for FTTO switches using installed power

networks

Figure 6. 54V DC power supply for FTTO switches using central power supplies

and dedicated electrical wiring (power lines)

Both the central and decentralised (or distributed) power supply concepts with 54V

DC can be redundantly configured. This is achieved in decentralised 54V DC power

supply concepts via central UPS modules, and in central 54V DC power supply

concepts via additional power supply units which ensure safe and secure network

operation. The modules monitor the entire network and detect critical operating

conditions at an early stage, before outages or short circuits occur, and ensure

network availability in the event of downtime.

Figure 8. 54V DC power supply for FTTO switches using a central redundant

power supply and dedicated electrical wiring

Figure 7. 54V DC power supply for FTTO switches using power supply

installations connected to a 230V AC power line with UPS redundancy

Central vs. Decentralised Power Supplies

Central Power Supplies for FTTO Switches

48V DC is a well established voltage value in telecommunications, which is also

found in the first generation of PoE standards, i.e. IEEE 802.3af. The PoE voltage

ranges from 44 to 57V DC. To allow for a possible voltage drop, additional 4V is

required and has to be added overhead, hence 48V DC for PoE according to IEEE

802.3af.

The voltage for PoE+ according to IEEE 802.3at ranges from 50 to 57V DC.

Additional 4V has to be added overhead to allow for a voltage drop, so power

supplies for PoE+ stand at 54V DC.

In central power supply concepts FTTO switches get their power from a central

power source with a voltage range from 50 to a maximum of 57V DC. The central

power source consisting of several 1,500 or 2,500W power supply modules is

located, for example in the switchgear cabinet in the technical room, and the power

cables run from there to distributed FTTO switches, just like optical fibre cables. The

switches are connected to power outlets which are fed by power lines.

The voltage level of the power source has a great impact on the current level and the

amount of power loss. It should be as high as possible but must not exceed the

normative limit of 57V DC. The higher voltage level of the source helps to reduce the

overall current level and to avoid significant power losses. The voltage level of 60V

DC is defined as low according to the European Low Voltage Directive and is

harmless for humans and animals.

Figure 9 shows how power supply efficiency of a cabling network decreases with

increasing power consumption. The typical loss is about 5 to 10%. With a balanced

power supply concept power losses can be reduced by 1 to 2%.

Figure 9. Energy efficiency depending on the power consumption and the

voltage level at the source

The cross-section/diameter of the cable conductor also determines the maximum

length of the power cable. Of course, it is possible to bridge a longer distance with

larger wire cross sections. However, in this case the costs for the entire installation

will significantly increase, which will also incur adjusting safety protection circuits to

account for higher current.

Figure 10 shows the performance of two wire cross sections in terms of power

consumption per point of supply/power distribution point (in W) and the length of the

power supply line (in m).

Figure 10. The length of the power supply cable depending on the power

consumption rate at distribution points

The larger the power consumption is, the shorter is the cable length, and the thicker

is the wire cross section to ensure the same power budget at a longer distance. For

example, the maximum possible length of a single power supply cable with a wire

cross section of 6 mm2 and a power load of 60W per distribution point is circa 85

metres. For a `thicker` version of the cable with a cross section of 10 mm² and the

same power load, a length of over 150 metres is realisable.

Components of a Central Power Supply System

The component make-up of a central power supply system includes:

• Power supply units with 1,500 - 2,500 Watt modules positioned in 19"cabinets with a total output of 8 to 25kW

• Accumulator units for special requirements

• Five core low-voltage power lines with wire cross sections of 4, 6 or 10 mm² each (4 wires form two bipolar channels to ensure direct forwarding of current from the central power supply)

• Pre-assembled distribution boxes with two power supply cables and plugs each

• Up to 20 FTTO switches can be powered from one power distribution point

• 1: n distribution connectors

• Pre-assembled power supply cables for supplying power to individual switches

Figure 11. Components of a central power supply

Benefits

• Centralised 230V AC / 48V DC power supplies have a modular design and can be managed via Ethernet

• The solution is energy efficient for distances shorter than 50 meters

• Scalable central redundancy

• Monitoring and control of the power supply

Disadvantages

• Complex planning required to provide for worst case scenarios

• Separate power supplies required for FTTO switches

• Higher operating costs due to power losses in case of larger distances and smaller wire cross-sections

• Over-dimensioning of power cabling is mandatory, and thus higher investment is needed

Distributed Power Supply for FTTO Switches

Distributed power supply concepts can be divided into two main groups.

The first one features a power supply for short distances of less than 10 metres,

where a single power supply with a power budget of less than 100W is provided by

one or maximum two switches.

Figure 12. The structure of a distributed power supply

The second group includes the so-called power supply islands with a power budget

of 500 to 1,500W, which ensure the power supply for several FTTO switches.

The distributed power supply is an efficient way to distribute power, since there are

short distances to be bridged between the point of AC to DC conversion (to 54V DC)

and FTTO switches (in contrast to central power supplies with long distances and

larger wire cross sections as shown above). This effectively reduces power losses

through transfer. Power is transmitted over a longer distance at a higher level of

voltage (230V AC), thereby reducing the current level and therefore the amount of

power loss.

Figure 13. The structure of distributed power supplies using power supply

islands

The power line connection between the power supply and the switch is short, usually

between 1 and 3 m, so the losses through DC transmission are respectively low. For

a typical consumption of 65W the losses can be neglected. For higher requirements

(with 120W per point of power supply) DIN rail mounted power supplies are a great

solution. These can be equipped with up to 500W of usable power and cover areas

with distances of 10 to 50 metres between the source (i.e. the power supply unit) and

the consumer (i.e. the FTTO switch).

Components of an Individual Power Supply System

The design of a distributed power supply system can include the following

components:

• 65 - 70W power supply units and/or 100W DIN rail power supplies

• Distribution cable with 3 wires optionally with a 0.75 or 1.5 mm² copper cross-section

• Optionally, pre-terminated power supply with Wago, Wieland or Ensto connectors for a quick on-site installation

• 1:n – power distribution connectors for 230V AC

Advantages

• The most efficient type of energy transfer (as AC to DC transformation takes place very close to the consumer)

• Flexible implementation for both new and existing infrastructures

• Simplified planning

• Longer power lines with smaller copper cross-sections (less expensive cabling infrastructure)

Disadvantages

• Monitoring options only via the switch management

• Limited power budget of 65 to 100W per switch

• Redundancy only for connections with a likewise redundant 230V AC power supply

• Hardware redundancy is only possible to a limited extent

Figure 14. Components of a distributed 54V DC power supply design featuring

individual power supplies

Components of a Power Supply Island

The distributed power supply concept featuring power supply `islands` can include

the following components:

• 500 - 1,500W DIN rail power supply units

• A distribution cable with 3 wires optionally with a 0.75, 1.5, 2.5 or 4 mm² copper cross-section

• Optionally, pre-terminated cables with Wago, Wieland or Ensto connectors for a fast on-site installation

• 1: n - distribution connectors for 230V AC installations

• 1: n - distribution connectors for 54V DC installations

Figure 15. Components of a distributed 54V DC power supply system with

individual power supplies

Advantages

• Improved energy efficiency while transfer (transformation takes place very close to the consumer)

• Flexible implementation for both new and existing infrastructures

• Simplified planning

• Longer lines with smaller copper cross-sections (cheaper cabling infrastructure)

• Scalability through parallel connection of power supplies

• Hardware redundancy

Disadvantages

• Restricted monitoring functionalities

• Requires special installation space for power supplies, e.g. housing modules in the wall

• Redundancy only for connections with a likewise redundant 230V AC power line

Tips & Tricks

When designing a tailored power supply system for an FTTO network, planning

errors are easy to make. However, many mistakes can be avoided with the

appropriate know-how and targeted approach.

First of all, the maximum power load of the whole system, electrical safety measures

and the resulting power losses have to be taken into account and checked in order to

ensure a smooth operation of connected devices. Secondly, one should determine

the efficiency of the power supply concept as well as all associated costs.

Below you will find our tips for a better power supply design:

1. Nexans DIN rail solutions offer an adjustment possibility of 48 to 57V. To avoid

power losses, keep the voltage level below 57V.

2. Always think of electrical safety and electrical protection of the cabling

infrastructure.

3. Do not experiment: opt for reliable, well tested solutions on the market. The

installer is very familiar with their installation, and the compatibility with other

system components is thus ensured.

4. Use pre-terminated cables! Pre-terminated cables offer several advantages:

transparent price per piece, tested quality from manufacturer as well short

installation time.

5. Use pre-terminated power supplies that are equipped with standardised

connectors and plugs suitable for the installation. Such systems provide

mechanical coding and latching according to IEC 61535. This also allows people

without any special knowledge to replace or rearrange the power supplies.

6. Ensure parallel connection for DIN rail power supplies. Please make sure that the

cabling system is also designed for parallel connection.

7. For flat cables with a central voltage supply, use all 4 copper wires for power

transmission! This allows you to double the wire cross-section, thus increasing the

maximum available power budget and reducing power losses.

Concept Comparison / Evaluation Matrix

The following table sums up the three power supply concepts presented above. The

assessment is based on efficiency, monitoring, redundancy, project costs and ease

of planning. The rating features 1 - 5 stars, whereby five stars represent the best

possible ranking.

Type of power supply

Efficiency Monitoring Redundancy Costs Planning

Central power supply

** ***** ***** ** **

Distributed units as power islands

**** *** **** **** ****

Distributed units as power supply for individual switches

***** * * ***** *****

Table 3. Assessment matrix for power supply concepts

Summary

With modern, efficient power supply technologies, it is easy to design flexible and

scalable FTTO infrastructures. Cost, reliability and energy efficiency are among key

factors which have to be considered when choosing the power supply infrastructure

for FTTO. Each power supply approach, both central and decentralised, has its

advantages and disadvantages. And because the requirements may vary greatly,

there is no one fits all solution.

While the central power supply has proven very efficient for FTTO infrastructures with

PD devices according to IEEE 802.3af, the decentralised power supply concept

offers a better performance in scenarios with PoE+ according to IEEE 802.3at.

However, the central power supply has advantages in terms of global availability

through advanced redundancy, which is enhanced by UPS modules. However, the

efficiency of power supplyfor active devices is limited in this case by the cabling

length and the number of connection points.

Also, meeting the growing power needs may become challenging with time,

especially in view of the rapidly changing technology and IP convergence. The

system may grow and encompass more modules and distribution points, reaching its

saturation point too soon. In order to counteract this, cabling should be over-

dimensioned, so that the power supply system of the FTTO network does not fizzle

out too early. Otherwise the weaknesses of the central power supply will become

apparent, at least when migrating from PoE to PoE+ or PoE++.

On the other hand, limiting the voltage drop to only 7V (50-57V DC) clearly reduces

the maximum achievable length of the power cable. This and the increasing

operating current lead to an inefficient power supply. Finally, the fist rule is that power

(W) should always be transmitted with the highest possible voltage (V). Summing up

it can be stated that the central power supply is a good alternative for special

application scenarios, e.g. when redundancy is important, or when global monitoring

options are desired.

On the other hand, the distributed power supply concept is the best approach when it

comes to flexibility and energy efficiency. Due to the comparatively short distances

between the powered end user devices and the power source, significant losses can

be avoided and important power savings can be achieved.

When a higher power budget is required, e.g. over 65W per FTTO switch, power

supplies with a higher power output can be used. Power supply monitoring can be

set up via internal switch management which includes PoE monitoring and SNMP.

For example, Nexans LANactive switches capture and transmit alarms related to

voltage and PoE consumption thresholds. The redundancy of the distributed power

supply system can be built up by separate power supply circuits each with a UPS.

This could then be part of a 230V AC UPS concept for the entire IT infrastructure.

Case Studies

Nexans supports you with planning and design of your IT infrastructure projects.

For more information, please visit

www.nexans.com/LANsystems/Powersupply.

Product Overview of Nexans Power Supplies and Accessories

Product Overview

Power Supply

88646200 Click-In Power Supply 54VDC/70W 90x45

88646066 Installation Power Supply 54VDC/65W

88646077 Desk Power Supply with Schuko Connector 54VDC/65W

DIN Rail Power Supply:

88646182 iPowerSupply S 115-230VAC/48VDC 60W

88645960 iPowerSupply S 115-230VAC/48VDC 100W

88646072 iPowerSupply S 115-230VAC/48VDC 500W

Connectors for FieldMounting

8645986 Wieland Connector GST18I3S, female type

88645987 Wieland Plug GST18I3S, male type

88646098 Wago Winsta Connector, female type

88646099 Wago Winsta Plug, male type

Cable Sets and Pre-terminated Power Supply Units:

88646280 Cable Set 1x1m H03VV-F 3 G0.75 open wires + 1x1.5m H03VV-F 3 G0.75 open wires (useable for power supplies with < 240W)

88646281 Cable Set 1x1m H03VV-F 3 G0.75 open wires + 1x1.5m H03VV-F 3 G0.75 with Wieland GST18 Plug on one side, pre-terminated (useable for power supplies with < 240W)

88646282 Cable Set 1x1m H03VV-F 3 G0.75 open wires + 1x1.5m H03VV-F 3 G0.75 with WAGO WINSTA Plug on one side pre-terminated (useable for power supplies with < 240W)

88646080 Pre-termination with Wieland GST18 Plug (for 88646066)

88646093 Pre-termination with WAGO Winsta Plug (for 88646066)

88646264

Cable Set with 54V DC Plugs on both sides and 230V AC Plug for

Click-In Power Supply Unit and Wieland GST18 Plug (for

88646200)

About the author:

Fjodor Lamm, Head of Product Marketing, Nexans Deutschland GmbH, studied

electrical engineering while majoring in communications engineering at the University

of Applied Sciences of Aachen. After successful graduation, he was initially Product

Manager Data Cables and Systems with KERPEN GmbH & Co. KG. Since his move

to Nexans Advanced Networking Solutions in 2005, he has been responsible for

LANactive portfolio. As Product Manager, Mr. Lamm initiates and supports the

development of new products and concepts for FTTO solutions. Lamm is also

responsible for planning, designing and supporting various network design projects

for public administrations, universities, hospitals, airports, transport, industry, military

& defence.

This document is for guidance only. Although the accuracy of the information

contained in this document is believed to be correct, Nexans discards any liability for

actions taken based on the information provided.

This document is protected by copyright © 2016 Nexans Deutschland. The products

shown are protected by copyright. Any unauthorised copy of this document or our

products is prohibited.

If you have any questions, please contact us by phone:

Tel: +49 (0) 2166/27-2220

If you have any questions regarding the document (suggestion, corrections, criticisms

or praise), please send an e-mail to: sales.ans@nexans.com

Internet: http://www.nexans.com/LANsystems

Support Portal: http://www.nexans-ans.de/support

Offices

Nexans Advanced Networking SolutionsBonnenbroicher Str. 2-1441238 MoenchengladbachGermanyTel: +49 (0)2166 27-2220Fax:+49 (0)2166 27-2499

Nexans Cabling SolutionsAlsembergsesteenweg 2, b3 B-1501 BuizingenBelgiumTel: +32 (0)2 363 38 00Fax: +32 (0)2 365 09 99

Nexans Cabling Solutions FranceRue Mozart 4-1092587 Clichy CedexFrance

Nexans Cabling Solutions UK 2 Faraday Office ParkFaraday Road - BasingstokeHampshire RG24 8QQUnited KingdomTel: +44 (0)1256 486640Fax: +44 (0)1256 486650

Nexans Middle EastOffice 1703, Jumeirah Bay Tower - X3Jumeirah Lake TowersPO Box 634339, Dubai - UAETel: +971 4 369 7007Fax: +971 4 368 9258

www.nexans.com/LANsystems info.ans@nexans.com

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