Power over Ethernet cable

TCP / IP protocol on the Ethernet platform is very popular in today's communication market. The success of Ethernet-based technologies is in part due to the very active cooperation in the development of common standards. This success will also create new strengths on existing infrastructures such as cabling, network architecture, packet format and drivers that are already installed in existing Ethernet networks.

Revolution in network technology?

Ethernet networks transmit data at current speeds up to 1Gbit / s (1000BaseT) using twisted pair UTP 5/6. Power over Ethernet (PoE), a self-powered Ethernet connection, uses the availability of this copper cable connection to transmit both data and power from one point to the other via UTP cable. This technology is increasingly widely accepted in the network market.

Supplying power via an Ethernet UTP cable is derived from the function of a traditional telephone system (POTS), in which analog phones are powered from the remote operator (CO or PBX) via UTP cable .

Available Ethernet connections are already present in many devices on both the network infrastructure as well as on the user side. Using UTP cables to provide both data and power reduces the costs of installing and deploying applications such as wireless LAN access points, remote control security cameras, VoIP phones and security devices. security and fire alarm access control.

The process comes to consistency

IEEE began its standardization process in 1999. The companies that participated in the beginning included 3COM, Intel, PowerDsine, Nortel, Mitel and National Semiconductor. The standardization process has attracted many leading experts in many companies, so it can be said that the issues raised have been considered in many different ways. The IEEE 802.3af standard is now completed and approved by the IEEE Standards Committee on June 12, 2003.

The last minute changes of the standard may make some products currently on the market not fully meet the final version of this standard.

IEEE 802.3af Power over Ethernet application - New generation of power

The 802.3af standard may not sound promising, but this is the first international standard for a new category of power supplies.

This is just an example, laptops can only last for a few hours if they only use spare batteries. This means that anyone who carries a laptop will need a power supply cable and a suitable plug. To maintain the laptop's performance, people will have to search for supplies to plug it in before connecting the computer to the network to exchange data. With this new standard, you only need to connect your computer to the network without having to worry about finding the power outlet.

Power supply and Ethenet make a perfect combination

This technology will spark the birth of many new kinds of devices and applications such as fully connected 'smart buildings'.

Power over Ethernet can overcome some location restrictions. For example, in repositioning the access point in the wireless LAN without concern about installing a new power outlet at the new location.

Power over Ethernet - Ready?

Currently, PoE can provide 48VDC power with a capacity of 12.95W via Cat5 cable for standard Ethernet 10/100 / 1000Mbit / s services. In order to be compliant with the IEEE 802.3af PoE standard, power supplies on Ethernet must be defined, classified, controllable during start-up and meet isolation requirements between power and data. material.

Figure 1 shows a typical system. Ethernet switches are maintained and additionally an 'intermediate' supply to 'inject' the source into the LAN cable. at the other end of the cable, the source is used to run the phone, wireless LAN access point, camera and other devices. A system of UPS (UPS) will also assist in the event of a major power failure.

How is energy transmitted via LAN cable?

Standard Ethernet Cat5 cable has 4 twisted-pair cables, but only two pairs are used for data transfer according to 10BaseT and 1000 BaseT standards. This creates two possibilities for using cables to transmit energy.

Use backup cables. Figure 2 shows a pair of cables in pins 4 and 5 are connected together to create the source anode; A pair of cables on pin 7 and pin 8 are connected to each other and create a negative pole of the source. (In fact, the final changes in the standard do not require any extreme positive or negative source).

Use of data cables.Because the paired transformer cables (transformer coupled) at each end can power the position between the primary and the center tap without affecting data exchange.In this mode of operation, the cables on pins 3 and 6 and the cable in pins 1 and 2 will be the supply terminals.

 

Keep in mind that the standard allows only one of the two options to be selected (do not use both alternatives at the same time).The Powered Device must be able to accept both options.

In the first picture, the 'intermediate' Hub is a Power Sourcing Equipment, VoIP phone, radio access point, and camera are powered devices.Newer Ethernet switches may already include this function, and therefore the 'intermediate' hub may not be necessary.The rated voltage is 48V DC and the capacity of about 13W is suitable for powered devices.A separate DC-DC converter to turn the voltage of 48V down to a lower voltage will be more suitable for electronic circuits in powered devices, while maintaining insulation at 1500V because safety reasons.According to Amir Lehr, vice president of PowerDsine, the leading developer of this technology, the biggest problems are the power loss through the connection prices and the length of the cable.Therefore, 802.3af standards only set a power level of 15.4W per port, when the device is powered, this power level is only 13W.This power level is also sufficient for most IP phones (from 3W to 5.5W), for Web cameras and WLAN access points (from 6W to 10W).Even for the simplest laptops that need about 20W, users can increase the backup battery's time to about double.

There is a standard requirement to prevent harm to existing network devices. A 'detection cycle', running from the power supply device (PSE), will check the Ethernet cables, searching for devices that are compatible with the standard. It does this by supplying a low voltage with a limited current to the cable and checking for the presence of a 25 k resistor (at the remote terminal. Only when this resistor is detected, is it new 48V power supply but with a limited current intensity to avoid possible damage to the cable and equipment under fault conditions.

The powered device (PD) must continue to maintain a minimum current. Otherwise (for example, when the device is removed from the network) the power supply device (PSE) will stop powering and the detection cycle will start again.

To enhance the detection cycle, the powered device (PD) can instruct the power supply device (PSE) to know that it needs the maximum power.

Power supplies can also provide a level of system management using the SNMP protocol. This allows to manage operations such as night power off or remote reset.

Price

The difference in the price of equipment that uses this technology compared to conventional equipment is negligible. The real cost is that the power supply units are added to the network in the form of conventional switches with an additional power supply component. Currently the cost for this item is about 50% more expensive. This number may be reduced to 20% in the near future.

Peter Doggart, the UK market leader of the leading company in this field, 3COM, has confirmed that switches capable of supplying power will be 20 to 40% more expensive despite a realistic example today. The switch is capable of providing a price of 2500 USD while the price of a conventional switch is the same as 1600 USD.

However, with the cost of the high-power system part, the above cost is perfectly acceptable, especially when the technology is put into commercial exploitation in the next few years. There is no doubt that this new technology is changing the way electricity is supplied to today's electronic devices.

Some applications of technology

Devices using this technology are already on the market. There are no restrictions on devices 'intermediate' PSE. Major applications have used this technology:

VoIP phone
- IEEE 802.3af wireless LAN access points
- Bluetooth access points
- Web cameras

And for upcoming applications:

- Smart signs / web signs.
- Selling machine.
- Computer game.
- Audio / video juke boxes.
- Retail point of information systems.
- Construction of access control systems.
- Loader for mobile devices and PDAs
- Electronic musical instruments

(Pham Chuong)