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Power over Ethernet - Inside or Outside the Switch?
When it is appropriate to use PoE integrated into your Switch or to use an external PoE power injector called a mid-span?
Why PoE?
Whether you are consider running IP telephony across your existing data network or extending the reach of the network with strategically placed wireless access points or just adding a physical security layer to your premises with IP cameras the risk is increased in possible failure by having remote devices that need AC power connections. Losing data during a power outage is one thing, but losing data and voice and the company security is something else entirely.
A common goal is to consistently achieve 99.999% availability with 5.3 minutes of downtime per year or less. By connecting a UPS to a PoE source in the communication room, the entire network including the remotely located resources, is capable of continuous operation during a power outage. Most UPS’s also provide for surge protection that is normally unavailable for remotely connected devices, the disruption created by power surges is more common than that created by power outages.
Manage the Risk
To understand the risk here are statistics for North America that can be considered to be generally applicable to any location.
- The average number of power outages sufficient to cause IT system malfunction per year at a typical site is 15
- 90% of the outages are less than five minutes in duration 99% of the outages are less than one hour in duration
- Total cumulative outage duration is approximately 100 minutes per year
Based on the theoretical availability and the power protection strategy to achieve five nines or higher requires the installation of Power over Ethernet sources backed up by a UPS system with a minimum of one hour battery life will protect the vast majority of systems.
The Economics
If you are installing a Wireless Access point you will tend to put them in the ceiling or high up on a wall out of the reach of the general public. This area is usually void of AC outlets therefore to power the units an AC contractor is needed to install an outlet. In a private area the installation will cost in the region of $200, some countries would be cheaper and some more expensive. If you then consider a public arena there is a health and safety issue about the provision of AC power. The power must be channelled into the wall or supplied through metal conduit, which can increase the cost by a factor of 10.
As the installer has to run an Ethernet cable from the Wireless Access point to the Switch or Router it is very simple to connect to a PoE port on the device or use a one-port power injector at the switch location. Either way no AC contactor is needed and worst-case scenario you are looking at $50 for the PoE port. You save money, installation is quicker and easier and there is no disruption while holes are cut into walls.
1.Understand the options for delivering PoE
1.a IEEE802.3af allows for switches to provide power on the Data Pair or the Spare Pair. Most switches select the Data Pair. If you are considering separate Power Injectors called mid-spans the standard states they must use the Spare Pairs. Therefore you should check your cabling installation to see if you have all pairs or only data pairs.
Note: Most mid-spans cannot be used on a Gigabit connection because there are no Spare Pairs in a Gigabit installation.
1.b Pre-standard Cisco is different from IEEE802.3af in that the polarity of the power on the connector and the signature of the end device. So if these devices have to be supported then you need to know how the cable connection can rectify the polarity of the power and the power source mid-span or Switch can recognize the Cisco signature.
1.c Pre-Standard capacitive test. There are still legacy devices in the market that were early adopters of PoE and before the IEEE ratified the current standard a vast majority of devices had a capacitive signature, which is different from today’s resistive test.
1.d Are there any non PoE capable devices that will be connected that could benefit from using PoE. If there are then you need to read the Active Splitters section to determine your support options.
2.Understand the power requirements of attached devices
2.a Devices that will be connected to PoE will have a power requirement. Approximate figures are given below for a range of common devices but you should check all device requirements on the technical specifications on the manufacturer’s data sheets.
- IP-Telephones, 3 to 5 watts
- IP Video Telephones, 10 to 12 watts
- Wireless Access/Device points, 8 to 12 watts
- Network IP-Cameras (fixed), 8 to 10 watts
- Network IP Cameras (PTZ), 20 to 30 watts
2.b IEEE802.3af power is 15.4 watts at the power source and 12.95 watts at the Powered Device. However part of the specification is to test the requirements of the end device. This is known as the Class of device. A 7 watt device is Class 2. Manufacturers marketing departments are using this as a tool instead of saying their Switch only supplies 7 watts they say their Switch is a Class 2 device. This is a problem if you are supporting Wireless, Video IP phones or many security cameras. The solution from the Switch manufacturers is to provide additional external power supplies. You should check the cost of adding these external power supplies before deciding whether to have internal or external PoE capabilities.
2.c You do not have to worry about powering non compliant devices, such as Printers, Fax machines or PC’s because these devices do not carry the IEEE802.3af signature so the ports will automatically disable power. However, if you are using integrated PoE on a 24-port networking blade and many of the ports are supporting non powered devices it might be more prudent to use a mid-span and only connect powered ports to the devices that need it. This will lower the overall cost of the installation.
3.Caution regarding Full Power and Forced Power
3a. The question of using managed power or full power has to be considered. Full power is where the power delivered by every port is the 15.4 watts specified by the IEEE802.3af specification. This looks like an attractive option. However, the issue to consider is the requirement of the organization. If the installation is mainly IP telephony where the IP phones are in the range 3-5 watts then to put a 24-port unit in the computer room delivering 400 watts when all that is required is 120 watts is false economy. Today’s power supply technology is not linear. If you purchase a 200 watt PSU and you spend $500 it does not follow that the 400 watts PSU is going to be $1000. It is nearer to $2000.
All systems installation teams have to calculate the BTU’s and Airflow requirements in the computer room to ensure the air conditioning systems can perform adequately. A better solution is to use power injectors that use the class of service information to manage the delivery of power to the ports that require the higher levels. Power management allows the automation of efficient power distribution from a lower powered Injector.
3b. A number of manufactures are using the term Forced Power. This is where they leave the power on. This is outside the standard. The IEEE802.3af requires that power is removed within 47milliseconds of disconnect. The reason is that an engineer might be swapping leads at a ‘cross connect panel’ and by accident connect a powered port into a PC connection. Today PC’s are not supported and 48 volts can burn out the Ethernet port. The issue is that if it is a Lap Top the user will now try to get a network connection from another Ethernet port and runs the risk of the damaged Lap Top damaging Switch ports.
4.Evaluate the data requirements of end device
4a. Adding power into the Ethernet cable is adding a possible source of EMI from noise generated by the power source. Therefore an important consideration is conformance to the cable specifications of Cat5 and Cat5E. Among the crucial tests is cross talk. Any device that does not conform to the specification will generate excessive errors into the data path.
4b. There is a defined limit to the current that can be sent down a pair of twisted wires currently the ITA specification is 175 milliamps on one pair of cables. The IEEE802.3af specification, for delivering the required power, is 350 milliamps on two pairs. PoE is running the cable at its maximum allowable capacity. Therefore there is a real need for effective current protection to prevent failing end devices drawing too much current and causing cable faults. (Cable faults are usually very difficult for engineers to trace in reasonable amount of time)
5.Evaluate internal power or separate mid-span units
5.a How old are your switches and do they have the features such as QOS that fits your requirements. mid-spans were designed to be used with any switch and save on the expense and time involved in doing an upgrade to the switch to implement a PoE solution. Mid-spans can be installed while the network is live zero down time.
5.b Will the proposed Switch have the correct power level to support all connected devices or are additional power supplies needed.
5.c What is your history of up grades. Companies keep their network
Switches any where between 3, 5 and 10 years. Over the last 5 years how many adaptors did you swap out of the switch for new ones, most probably, with new features? If you are doing 2 – 3 upgrades then you will buy power 2 – 3 times if it is integral to the switch. Therefore how much will integrated PoE cost you over 5 years compared to buying external mid-spans which are designed to be part of the cabling scheme and last 10 years.
5.d Assess whether it makes sense to consider PoE part as of the cable infrastructure or whether it should be part of the Switching infrastructure. It might be a better strategy to use Integrated PoE for the network adaptors that can be supported by the normal switch power supply. But when an external power supply is required for the power levels or to support more adaptors then a mid-span solution might be your cost effective choice.
6.Managed or unmanaged PoE
6.a PoE mid-spans can be SNMP managed or unmanaged the areas of relevance to most enterprises are power consumption and the ability to remotely control Power On – Power Off. Management data and control functions need to be secured against unauthorized personnel. A hacker intruding into your network could use the power to disable all peripheral PoE devices.
6.b Wireless Access Points and Security Cameras tend to be installed out of reach of the public. This means they are also not very accessible to engineers. The ability to remotely perform a power on reset could be extremely useful.
6.c IPT requires that all IP phones be powered. SNMP management might be useful as the controlling tool for applications that can power telephones on and off when the relevant staff has passed security into the building and powering the phones off when they leave. It might be interesting for energy conservation to use batch files to enable telephones during work hours and disabling telephones in the evenings, weekends and holidays.
6.d Enterprise users might need to account for power usage by the different departments inside the company. Therefore consideration should be given to the ability to of the PoE system to monitor power consumption of the remote devices. Also on power failure when the UPS kicks in the management could use a priority scheme to ensure that as the power outage continues the power is directed to key resources. This management of failed power distribution could enable cost savings in the size of UPS needed for any given scenario.
7 Conclusions
- PoE is a must. Savings in power outlets, the speed of installation and the lack of disruptions to office environments are sufficient justification for PoE.
- Use Integrated PoE switches unless you need higher power than is delivered. Use a mid-span for all additional power and high power requirements.
- Choose units that conform to the IEEE standard and legacy support where needed. Ensure that they conform to EMI and cross talk specifications and have current protection.
- It is desirable to have power management (internally using the power efficiently) and SNMP v3 for secure control, monitoring and management of power distribution.
- Use central UPS’s in conjunction with a mid-span to deliver the UPS service to remote locations.
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