Power over Ethernet Technology For Access Control

Power over Ethernet (PoE) technology describes a system to pass electrical power and network data over Category Type cable.

By eliminating the requirement for a local power source for the Ethernet endpoint device, it represents a significant savings in materials and labor, PoE has made its entrance into the lexicon of electronic security with compelling cutting edge appliances such as:

• PoE enabled IP cameras,
• PoE enabled Intercoms
• PoE enabled IP based electronic access control modules.

Although there is a lot of buzz about wireless video surveillance, wireless has certain constraints, such as the security of information transmitted wirelessly. It certainly cannot be assumed that anything wireless is secure. Of course there are measures which can be taken that will go a long way towards ensuring the security of wirelessly transmitted data, but you cannot jump to a conclusion that wireless means secure.

Another problem is power. Cameras generally require continuous power that is not typically available using batteries or solar. For the overwhelming percentage of applications, batteries or solar are not practical at this point in time.

A number of technologies pass power down the same wire along with data which is not actually Ethernet. To be technically correct, you first must understand the different technologies then choose your words when discussing and specifying. Possibly contributing to the confusion is the adaptation of Category type cable for use in non-network applications.

The IEEE standard for PoE requires Category 5 cable or higher, (example: Category 6)

Power is passed over two or more pairs of wires in the cable and is used to operate the PoE device itself and possibly peripheral accessories connected to the PoE device. For example in a PoE electronic access control deployment, peripheral accessories could include the electric door lock and the credential reader.

In a PoE configuration power comes from one of these devices:

• PoE-enabled network switch referred to as an endspan,
• A special purpose hub, referred to as a midspan
• A standalone injector.

PoE Standards

The original IEEE 802.3AF-(2003) PoE standard provides up to  15.4 Watts of DC power (minimum 44 V DC and 350 mA) to each device with 12.95 W available at the powered device due to loss in the cable.

The updated IEEE 802.3AT-(2009) PoE standard (also known as PoE+) provides up to 25.5 W of power. The 802.3AT standard provides 51 W of power over a single cable by utilizing all four pairs in the Cat.5 cable.

Numerous non-standard PoE schemes exist to send power through the Category cable for non-PoE enabled equipment. In these applications, an adapter is used to inject power and data at the head end, and the companion module is located on the other end of the cable where a RJ45 data jack provides the Ethernet connection. A voltage jack or terminals are used to provide output voltage to the end of line device.

A PoE port on a switch cannot be connected to a non-PoE appliance, since the power present in the cable will be improperly terminated, resulting in issues with the data function of the switch, and no power being delivered to the end-of-line device.

Another variation is where the signal is not Ethernet, but instead audio or video, but the cabling used is network Category wire. To differentiate this from network PoE, it is often referred to as UTP (Untwisted Pair).

IP & POE Over Coax

Since we spent decades pulling coax for our analog video surveillance systems, much of it is still installed and now can be enlisted to transmit Ethernet and PoE for analog to IP system upgrades. Hardware adapters are installed at each end of the coax, and cameras and network equipment connect to the adapters. Re-using the existing coax scores a big cost savings for clients and a time saver for installers.

A PoE setup is comprised of a power source (referred to as the Power Sourcing Equipment or PSE); the network cable, and the network appliance being powered referred to as the Powered Device or PD.

There are two categories of Power Sourcing Equipment (PSE). The PSE is a device, such as a switch, that provides the power source referred to as an Endspan; and an intermediary device located on the network cable somewhere between a non PoE switch and a PoE device; referred to as a Midspan.

Midspans

One type of midspan is an injector which is an interface placed in series between the network switch and the powered device.

An injector has three external connections: an in network connector, an out network connector, and a power input. The network power supply or power source connects to the injector, and sends the power in the direction of the powered device.

Another type of midspan is a hub which manages the data and injects the power onto the network cable. The midspan hub is a PoE switch which has an input for a cable from the network, network connections for devices, and input power. Typically the network connections are divided into PoE and non-PoE or may be individually switched.

A powered device (PD) is the object of the injection such as a EAC module. IP Camera, PoE phone or an access point.

Many Powered Devices have an integral auxiliary power connector for an optional, direct external power supply input.

Having multiple ways to power a PoE PD can enable the system designer flexibility in the system layout and also to be able to provide for continued Powered Device operation even if there is a power failure.

PoE Modes

Two PoE modes, A and B, are available.

Mode A delivers phantom power on the data pairs of 100BASE-TX or 10BASE-T.

Mode B delivers power on the spare pairs. PoE can also be used on 1000BASE-T Ethernet in which case, there are no spare pairs and all power is delivered using the phantom technique.

Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities.

In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48 V DC, and pins 3 and 6 (pair #3 in T568B) form the other side.

These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks.

The free polarity allows PoE to accommodate for crossover cables as well as patch cables.

In mode B, pins 4–5 (pair #1 in both T568A and T568B) form one side of the DC supply and pins 7–8 (pair #4 in both T568A and T568B) provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.

The PSE, not the powered device (PD), decides whether power mode A or B shall be used. PDs must accommodate either mode in order to be compliant with the Standard.

To stay powered, the PD must continuously use 5–10 mA for at least 60 ms with no less than 400 ms since last use or else it will be unpowered by the PSE.

Endspans Vs. Midspans

Endspans (switches) are normally used on new installations or when the switch has to be replaced for other reasons (such as moving from 10/100 Mbit/s to 1 Gbit/s or adding security protocols), which makes it convenient to add the PoE capability.

Midspans are used when there is no desire to replace and configure a new Ethernet switch, and only PoE needs to be added to the network.