802.3at vs 802.3af: Which PoE Standard to Use
| technology | networking - David Chapman

802.3at vs 802.3af: Which PoE Standard to Use

Power over Ethernet (PoE) can be a confusing topic. It is one of the avenues of IT that cross with the need to understand basic electricity concepts. Many times we just think about plugging something in and it just works. PoE is extremely helpful and some of the original use cases for it are Voice over IP Phones (VoIP) but today that has started to expand a bit.

Why PoE At All?

PoE is a very useful standard that helps prevent the need for power adapters and network cables for devices that require both but have a minimal power requirement. Phones were a great use case for this due to falling into those needs. Most phones consumed power minimally, needed network access and also provided a switched network port for a computer. This synergy helps reduce and ease management of the end user's workstation.

Power capacity planning is not specific to either standard but it is important to ensure you have adequate power to your switches to supply PoE. Many switches with modular power supplies have higher end supplies with higher wattage. It is important to size these power supplies to help meet your needs and any redundancy requirements. If you lose a power supply, do you still have sufficient power to keep all devices powered or is it acceptable that some lose power? This may help determine if you need N+1 power capacity or not.

Because this power is delivered over copper twisted pair, it is important to ensure your cables are up to spec. If you have older cables or damaged cables, rolling out PoE may cause you to find network drops that need to be replaced or your cabling may need to be updated, particularly if you are running CAT3 or CAT 5 (non-CAT 5e or higher).

VoIP Phones are one of the original use cases for PoE. They have minimal power requirements and need network and power. As devices became more energy efficient and PoE switches started being able to supply more power, more use cases started to come up such as security cameras. Wireless Access Points (WAPs) are another great use case. Many times these are dropped in the ceiling which is very easy to run network cabling in offices with drop ceiling tiles.

A huge benefit to all of these devices using PoE is that a network admin can remotely reboot the device simply by shutting down the port and turning it back on again. An interesting use case is for small mini-switches that are powered via the uplink port.

For those that do not have PoE(+) capable switches, PoE injectors exist although they are less than ideal. They are simply a power supply that takes a non electrified ethernet cable as input and outputs the data signal along with power to the appropriate cables. These can be bulky but offer an alternative for equipment such as home office equipment or home security equipment where there may not exist a PoE(+) capable switch.

What is 802.3af?

The 802.3af standard is also referred to as simply PoE. This is because it is the initial PoE standard. It allows for a maximum of 15.4 watts. Devices that consume at this wattage or less are type 1 devices. This standard was a great start and at the time provided more than sufficient power for most devices. A huge benefit for type 1 devices are the power requirements. Because these have the lowest power requirements, that equates to lower capacity power supplies and circuits to the switches.

In other words, it can be more economical. 802.3af devices are compatible with 802.3af and 802.3at switches. The downside to these devices is that functionality and features may be limited due to the limited power constraints.

What is 802.3at?

The 802.3at standard increased the maximum power to 25.5 watts. It is commonly referred to as PoE+ or PoE Plus. Devices that consume at this wattage or less down to the wattage of type 1 devices are considered type 2 devices. The major benefit for this is that this standard supports higher power needs for devices (type 2). For example, VoIP phones with LCD and video conference ability may fit into this.

Another benefit for this is that at the switch level, it supports type 1 and 2 devices as it is backwards compatible with 802.3at. From a device perspective, 802.3at devices are likely to be more feature rich, including color screens where applicable. The downside is that with the increased power requirements, it could cost a lot more in power related costs. Many times power supplies require enough power that you may need one or more dedicated circuits for these power supplies. If you use a battery backup, higher end backups are also needed. It can become difficult to supply the power in the way that's needed if you have many devices that need this increased power.

802.3at vs. 802.3af

The standards are not mutually exclusive. It is not one versus the other. With 802.3at being the newer standard, on the switch side, it is backwards compatible. If a port can only negotiate 802.3af due to the end device only supporting that, there will be no issues. On the other hand if the end device requires 802.3at but the switch only supports 802.3af, it will not work since the device requires a newer standard and more power than the switch can accommodate. 802.3at compatible switches work quite well with 802.3af devices.

802.3at or 802.3af: Which Should I Use?

The decision point is fairly simple. If you already have PoE capable switches, many times determine the standard supported. In some cases you can replace/upgrade line cards to support newer PoE standards. It is important to determine the standards that your existing switching gear support. Next determine the standards and power requirements of the devices you need to use.

Because phones are the most common example, we'll use that. If your phones only require 10W, 802.3af should suffice. If on the other hand, your phones require 19W, you will need 802.3at. At this point if your switch is only capable of supporting 802.3af you would need to investigate upgrading/replacing or finding a different phone.

Beyond 802.3at

Beyond these, standards are being drafted for 802.3bt type 3 and type 4. As the convenience of this takes off and more use cases are found, we will find that increased power will be needed for some of the devices. Be mindful when upgrading your switching gear so that you can try to support the latest devices.

Final Thoughts

If you have the option, on the switch side, choose 802.3at — as it supports type 1 and 2 devices. On the device side, try to find the most efficient device possible to minimize the power requirements while also meeting any business requirements you have. If you do have to purchase devices with higher power needs, try to minimize those to select employees that require them.

PoE can be great but if you hit the tipping point of power consumption, line cards and switches can start to power down to conserve power so it is important to be power conscious in your decisions.



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