Powering the Cisco IP Phone

Most Cisco IP Phone models are capable of using the following three options for power:

• PoE With PoE, the phone plugs into the data jack that connects to the switch, and the user PC in turn connects to the IP Phone. Power-sourcing equipment (PSE), such as Cisco Catalyst PoE-capable modular and fixed-configuration switches, insert power over the Ethernet cable to the powered device, for example, an IP Phone or 802.11 wireless access point.
• Midspan power injection Because many switches do not support PoE, the powered device must support a midspan power source. This midspan device sits between the LAN switch and the powered device and inserts power on the Ethernet cable to the powered device. A technical difference between the midspan and inline power mechanism is that power is delivered on the spare pairs (pins 4, 5, 7, and 8). An example of midspan PSE is a Cisco Catalyst Inline Power Patch Panel.
• Wall power Wall power needs a DC converter for connecting the IP Phone to a wall outlet.

Note

You must order the wall power supply separately from the Cisco IP Phone. This can increase the cost of the end devices significantly. Before you decide to use wall power rather than PoE to power the Cisco IP Phones, be sure to verify the total cost of this decision.

Two Types of PoE Delivery

Cisco provides two types of inline power delivery: the Cisco original implementation and the IEEE 802.3af PoE standard. You can refer to both inline power types as PoE.

Because Cisco was the first to develop PoE, there was not an established PoE industry standard to which Cisco could conform. Because of this, the industry considers Cisco's original implementation PoE as prestandard and proprietary. Although Cisco will eventually discontinue this prestandard implementation, Cisco devices will continue to support it for many years to come. The Cisco prestandard PoE implementation supports the following features:

• Provides 48 V DC at up to 6.3 to 7.7 watts (W) per port over data pins 1, 2, 3, and 6.
• Supports most PoE-capable Cisco devices (IP Phones and wireless access points).
• Uses a Cisco proprietary method of determining whether an attached device requires power. Power is delivered only to devices that require power.

Since first developing PoE, Cisco has been driving the evolution of this technology toward standardization by working with the IEEE and member vendors to create a standards-based means of providing power from an Ethernet switch port. The IEEE 802.3af committee has ratified this capability. The IEEE 802.3af PoE standard supports the following features:

• Specifies 48 V DC at up to 15.4 W per port over data pins 1, 2, 3, and 6 or the spare pins 4, 5, 7, and 8 (a PSE can use one or the other, but not both).
• Enables a new range of Ethernet-powered devices that consume additional power.
• Standardizes the method of determining whether an attached device requires power. The PSE delivers power only to devices that require power. This type has several optional elements, such as power classification, where powered devices can optionally support a signature that defines the maximum power requirement. PSE that supports power classification reads this signature and budgets the correct amount of power per powered device, which will likely be significantly less than the maximum allowed power.

Without power classification, the switch reserves the full 15.4 W of power for every device. This behavior might result in oversubscription of the available power supplies so that some devices will not be powered even though there is sufficient power available.

Power classification defines these five classes:

0 (default) 15.4 W reserved

1 4 W

2 7 W

3 15.4 W

4 Reserved for future expansion

All Cisco IEEE 802.3af-compliant switches support power classification.

Tip

The Cisco Power Calculator is an online tool that enables you to calculate the power supply requirements for a specific PoE configuration. The Cisco Power Calculator is available to registered Cisco.com users at www.cisco.com/go/poe.

PoE Device Detection

The Cisco prestandard PoE and industry standard IEEE 802.3af PoE have slightly different methods of detecting an inline power-capable device. Figure 8-1 illustrates how a Cisco prestandard Catalyst switch detects a Cisco IP Phone, wireless access point, or other inline power-capable device. When a switch port that is configured for inline power detects a connected device, the switch sends an Ethernet Fast Link Pulse (FLP) to the device. The Cisco powered device (IP Phone) loops the FLP back to the switch to indicate its inline power capability. The switch then delivers 48 V DC PoE (inline) power to the IP Phone or other inline power-capable endpoint.

Figure 8-1. Cisco Prestandard PoE Device Detection

A Cisco Catalyst IEEE 802.3af-compliant switch detects a Cisco IP Phone, wireless access point, or other inline power-capable device through a very similar process. The PSE (Cisco Catalyst switch) detects a powered device by applying a voltage in the range of 2.8 V to 10 V on the cable and then looks for a 25K ohm signature resistor rather than using the Cisco proprietary FLP signal. Compliant powered devices must support this resistance method. If the appropriate resistance is found, the Cisco Catalyst switch delivers power.

Catalyst Family of PoE Switches

The Cisco Catalyst LAN switching portfolio is the industry-leading family of intelligent switching solutions delivering a robust range of security and quality of service (QoS) capabilities. The Cisco Catalyst switch portfolio allows organizations to enable new business applications and integrate new technologies such as wireless and IP telephony into their network infrastructure. Here are the switches in the Cisco Catalyst family:

• Cisco Catalyst modular switching The Cisco Catalyst 6500 Series delivers a 96-port 10BASE-T/100BASE-T line card and 48-port 10BASE-T/100BASE-T and 10BASE-T/100BASE-T/1000BASE-T line cards. The Catalyst 6500 Series offers a modular PoE daughter card architecture for the 96-port card and the 48-port 10/100/1000 card. The Cisco Catalyst 4500 Series delivers 48-port 10/100 and 10/100/1000 line cards. All line cards support both IEEE 802.3af and Cisco prestandard inline power. The cards are compatible with any Cisco Catalyst 6500 or 4500 chassis and Supervisor Engine. The Cisco Catalyst modular chassis switches can deliver 15.4 W per port for all 48 ports on a module simultaneously.
• Cisco Catalyst stackable switching The Cisco Catalyst 3750 Series offers 48- and 24-port Fast Ethernet switches that comply with IEEE 802.3af and Cisco prestandard PoE. The Cisco Catalyst 3560 Series offers 48- and 24-port Fast Ethernet switches that support both the industry standard and Cisco standard PoE.
• Cisco EtherSwitch modules The Cisco 36- and 16-port 10/100 EtherSwitch modules for Cisco 2600 and 3700 Series routers offer branch office customers the option to integrate switching and routing in one platform. These modules can support Cisco prestandard PoE and provide straightforward configuration, easy deployment, and integrated management in a single platform. The Cisco 2600 Series requires a separate external PoE power supply; the Cisco 3700 Series integrates the power supply. Cisco has also released EtherSwitch modules for the newer Integrated Service Routers (ISRs). These new EtherSwitch modules support both the Cisco prestandard PoE and the IEEE 802.3af PoE.

Table 8-1 lists the Cisco Catalyst PoE options.

Tip

The switches that are listed here also support multiple VLANs per port and QoS capabilities.

Note

The pace of change in IP telephony is intense. By the time you are reading this text, Cisco will have most likely introduced multiple new product lines and switch models that support PoE and many other features. Always be sure to check the Cisco website for the latest product information.

Configuring PoE

By default, PoE is enabled on all Cisco devices that support the PoE feature. The default mode for PoE is auto, which means the switch will automatically detect if a device is PoE capable and supply power, if necessary. If you are using a switch that is running Cisco Catalyst Operating System software (CatOS), use the following syntax to modify the default PoE settings:

CatOS>(enable) set port inlinepower <mod/port> ? auto Port inline power auto mode off Port inline power off mode

The two modes are auto and off. In the off mode, the switch does not power up the port even if an unpowered phone is connected. In the auto mode, the switch powers up the port only if the switching module has discovered the phone. Examples of devices running Cisco CatOS include the Cisco Catalyst 6500, 4500, and 4000 Series.

Tip

It can be useful to turn PoE capabilities off on ports that you are sure will never use the feature. If the power supply your switch is equipped with is unable to extend power to all ports, you can specify ports that should receive power. Otherwise, the switch will allocate power to ports with lower port numbers until it exhausts the available power supply leaving the higher port numbers unpowered. The switch allocates power to ports configured with the "auto" setting regardless of whether the port is using the power.

If you are using a switch that is running Cisco IOS (NativeIOS), use the following syntax to modify the default PoE settings:

NativeIOS(config-if)# power inline [auto | never]

Use the power inline command on switches that are running native Cisco IOS software (examples include the Catalyst 6500, 4500, 3750, and 3560 switches). The powered device-discovery algorithm is operational in the auto mode. The powered device-discovery algorithm is disabled in the never mode. Other modes exist for allocating power, depending on the version of Cisco IOS, for example, the ability to allocate power on a per-port basis with the allocation milliwatt command.

Note

The Catalyst 6500 Series can run either Cisco Catalyst Operating System software or native Cisco IOS software if the switch Supervisor Engine has a Multilayer Switch Feature Card (MSFC). Otherwise, these switches can run only Cisco Catalyst software. The Cisco Catalyst 4500 and 4000 Series can also run Cisco Catalyst software or native Cisco IOS software, depending on the Supervisor Engine. Generally, late-edition Supervisor Engines run native Cisco IOS software; however, you should check the product documentation to determine the Supervisor Engine and the operating system that is supported on your specific model.

Verifying PoE

You can use the following command to display a view of the power allocated on Cisco Catalyst switches running the CatOS:

CatOS>(enable) show port inline power 7 Default Inline Power allocation per port: 10.000 Watts (0.23 Amps @42V) Total inline power drawn by module 7: 75.60 Watts (1.80 Amps @42V) Port InlinePowered PowerAllocated Admin Oper Detected mWatt mA @42V ---- ----- ---- -------- --------- ----------- 7/1 auto off no 0 0 7/2 auto on yes 6300 150 7/3 auto on yes 6300 150 7/4 auto off no 0 0 7/5 auto off no 0 0 7/6 auto off no 0 0 7/7 auto off no 0 0

You can use the following command to display a view of the power allocated on Cisco Catalyst switches running the NativeIOS:

NativeIOS#show power inline Available:360(w) Used:22(w) Remaining:338(w) Interface Admin Oper Power Device Class Max --------- ------ ---------- ------- ------------------- ----- ---- Fa0/1 auto off 0.0 n/a n/a 15.4 Fa0/2 auto off 0.0 n/a n/a 15.4 Fa0/3 auto off 0.0 n/a n/a 15.4 Fa0/4 auto off 0.0 n/a n/a 15.4 Fa0/5 auto off 0.0 n/a n/a 15.4 Fa0/6 auto off 0.0 n/a n/a 15.4 Fa0/7 auto off 0.0 n/a n/a 15.4 Fa0/8 auto on 10.3 IP Phone 7970 15.4 Fa0/9 auto off 0.0 n/a n/a 15.4 Fa0/10 auto on 6.3 IP Phone 7960 n/a 15.4 Fa0/11 auto off 0.0 n/a n/a 15.4 Fa0/12 auto off 0.0 n/a n/a 15.4 Fa0/13 auto off 0.0 n/a n/a 15.4 Fa0/14 auto on 6.3 IP Phone 7960 n/a 15.4

Table 8-2 provides a brief description of the output.