How PoE can make your IP telephony application as reliable as a POTS device - Embedded.com

How PoE can make your IP telephony application as reliable as a POTS device

Power over Ethernet (PoE) is atechnology of transferring power and data using a LAN cable. It allows safe andreliable power transmission -15W with 48V – for telecom applicationsover existing Cat5, Cat5e and Cat6 cables.

For example, it can be used to power IP phones, WLAN access points, networkcameras and various other network terminals in the allowed power rangeof 13W, as measured by the powered device side. PoE is also known aspower-over-LAN and is based on the IEEE 802.3afstandard.

More PoE equipment continues to emerge due to the easeof- use ofexisting standard Cat5 cable infrastructures. Hence, no modification ortampering with existing Ethernet infrastructures is needed.

The PoE technology has many benefits. For example, only one set ofwires is brought to your appliances, thus simplifying installation andsaving space. It also allows continued service during power outage byusing the same centralized UPS that backs up the network.

There is also no need to pay an expensive electrician or to delayinstallation to meet the electrician's schedule, thus saving time andmoney.

Furthermore, the appliance can be easily moved to wherever you canlay a LAN cable. Since there are no mains voltages present anywhere,it's safe. Using a UPS can guarantee power to their clients duringmains-power failures.

The Simple Network ManagementProtocol infrastructure can be used to monitor and control theappliance, and the data transfer to and from the appliance. Hence, theappliances can be managed, shut down or reset remotely in a centralizedmanner.

802.3af standard
Modern Ethernet networks and traditional telephone systems have much incommon. Both typically send data or voice over an unshielded twistedpair cable, usually using a form of star network.

The difference between the two is that traditional phones arepowered 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 centralswitch to provide 48V DC of up to 13W through the RJ-45 connector. This standard addresses all of the issues in supplying power to devicesvia Ethernet cabling. Simply put, the standard defines the functionaland electrical characteristics of two optional power entities: thepower device (PD) and the power sourcing equipment (PSE).

This is to be used for the PHY layers defined in thestandard. The standard is about how these devices or entities cansupply and draw power using the same generic cabling, as used for datatransmission.

The data terminal equipment powering is intended to provide a 10Base-T, 100Base-TX or 1,000Base-Tdevice with a single interface to both the data it requires and thepower to process these data.

Earlier PoE implementations will not conflict with 802.3af, and theavailable power levels are expected to address future applications. Weare already seeing discussion on 30W power transmission.

To meet the 802.3af standard, the PSE output voltage needs to be44-57V; the maximum output current in normal mode is 350mA; and thecontinuous output power is around 15.4W.

For the PD end, which is the client, the input voltage through yourPD needs to be 37-57V. The average input power is 12.95W, and the inputinrush current is 400mA. The following are the other specifications:

PSEoutput voltage: 44-57V;
PSE maximum output current in normalmode: 350mA;
PSE continuous output power: 15.4W;
PD input voltage: 37-57V;
PD input average power: 12.95W;
PD input inrush current: 400mA.

Figure1. Shown is an endpoint PoE implementation, with the end applicationhooked up to the switch.

Alternative architectures
Two particular implementations are defined. One is the endpoint, whichis applicable to a PoE-enabled switch (Figure1, above ). Note that the end application is hooked up to yourswitch, and it derivespower from the switch.

The other application or implementation is called the mid span (Figure 2, below ). Designers or userscan appreciate that in between the switch and their end application,there is a port for the mid span.

Figure2. Shown is the mid-span implementation of PoE, wherein the power isinserted to the network

This is where power is inserted to their network. In turn, thepowered devices that are hooked up to this mid span will get its powerfrom it.

To illustrate how a PoE system works, consider the block diagram ofa simple one-port PoE system (Figure 3below ). It comprises a PSE, which is a hub or a switch shown onthe left-hand side, and the PD, which is on the right side and the endapplication side.

Figure3. A simple one-port PoE system comprises a PSE, which is a hub or aswitch shown on the left-hand side, andthe PD, which is on the right side and the end application side.

The PD side would typically drive applications such as IP phones orWeb cameras. From the left, the PSE provides power via data lines orspare lines that would go through the Cat5 cable and then to theEthernet appliance side (PD).

Designers would extract this power (through a center taptransformer, if data lines are used) before feeding this through thePD, such as the Freescale MCZ34670.

Figure 4, below shows thetwo options for endpoint PSE application. Thealternative A architecture usesthe actual data pairs of Cat5 LAN cableto 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 thetransmitter and receiver transformers at each end of the link.Meanwhile, the alternative B option makes use of spare wires, with designers transmitting power on aspare pair.

Figure4: The alternative A architecture uses the actual data pairs of Cat5LAN cable to transmit power. Meanwhile, Meanwhile, thealternative B option makes use of spare wires, with designerstransmitting 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 ofthe data. The second is the use of spare pairs to transmit power.

The mid-span configuration is very useful because it allows powersupply to be external to the Ethernet. Moreover, it provides data andpower on the twisted pair linked segment without burdening each portwith the Ethernet equipment when you need to provide power.

This allows the addition of PoE to your older systems withoutreplacing switches or hubs. The current specification only allows powerin two of the four wire pairs in the Cat cable. For the endpointapplications, we considered using the data pair or the spare pair.

In the standard for the mid span applications, however, PSEs arerestricted to using only the spare pair of the Cat5 wires. Thus, the PDextracts power from the PSE using the spare pair.

PSE, PD operation
Detecting and powering PoE devices require PSE power sourcing to workout, identify or distinguish between a PoE and a non-PoE device.

This prevents the use of powering devices that are notPoE-compliant, and do not need or do not want to be sent power. Forobvious safety reasons, this also prevents you from blowing up devicesthat are not PoE-compliant.

<>For a safe and reliable operation of PoE systems, the 802.3afstandard mandates that the PSE determine whether or not to supply powerto the PD by applying test voltages.

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

The PSE reads the PD detection signature to determine whether tosupply power and how much power to supply. If it doesn't see thesignature in the device or the client end, it does not deliver powerfrom the network to that particular device.

The important functions of the PSE are to identify the PDs that areenabled to receive power, to provide required power levels and toremove power when the PD is disconnected from the link.

The detection mechanism is an extremely important function of thePSE to circumvent the application of power to various devices that canbe plugged into the eight-position modular jack.

Signature detection
What makes a valid signature? The detection impedance of the resistorthat is attached to the power devices is all that's required for validdetection. The detection impedance requires only one transistor with avalue of 23.75- 26.25 k-ohms.

We use a resistor called Rsig that is attached to the R sig pin of our device. It drawscurrent that is close to what is expected of a valid impedance, thusminimizing power consumption. The tolerance for such a resistor is verytight, between 23.75 k-ohms and 26.25 k-ohms. You also need to considerthe serial resistance of the diode.

To understand the standard's power interface, the power de- viceconnected to the Ethernet keyboard goes through a series of steps thatmust be compliant with the standard. Otherwise, no power will be sentto your PD.

The power-sourcing end provides a voltage of 2-10V. This feeds thePDs, and this voltage range is good enough to detect a valid signaturefrom your resistor (e.g. a nominal 25 k-ohms). Once this voltagedetects the presence of that signature, it increases its power to15-20V.

Here, it measures the particular class of required power. Uponsuccessful signature detection and classification, it ramps up itspower for normal operation. Because it's for a telecom application,it's 48V. We have a voltage of 37-57V for normal application.

Norman Chan is Marketing Managerin the Transportation and Standard Products Group at Freescale Semiconductor Inc. wherehe is responsible for the Power over Ethernet product line, includingthe MCZ34670. To view a PDF version of this article, go to “Transmitpower safely with PoE tech.

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