Table of Contents
Introduction: A Thought-Provoking Real Case
A factory encountered a challenging issue while deploying an IP surveillance system: the newly installed PTZ (Pan-Tilt-Zoom) cameras kept rebooting at frequent intervals. Network configurations and device compatibility were checked and ruled out, but the problem persisted. The root cause was traced to the power supply — the factory had used standard PoE switches, but the actual power draw of the PTZ cameras, which require motorized pan/tilt operation, infrared illumination, and other functions, exceeded the delivery capacity of standard PoE. After replacing them with PoE++ switches, the cameras immediately returned to stable operation.
This was not an isolated case. As IP cameras, Wi-Fi 6 access points, and industrial terminals become more prevalent, PoE (Power over Ethernet) has been widely adopted for its ability to transmit both data and power over a single Ethernet cable, greatly simplifying infrastructure. However, PoE exists in multiple standards, and selecting the wrong one can cause device instability or even create safety risks. This article outlines the different PoE standards, explains the key criteria for proper selection, and provides guidance to avoid issues such as insufficient power delivery or compatibility failures.
I. Three Generations of PoE Technology: The Evolution of Power
The essence of PoE (Power over Ethernet) is the ability to deliver both data and electrical power over a single Ethernet cable (RJ45 interface), eliminating the need for separate power adapters. Since the introduction of the first IEEE PoE standard in 2003, the technology has evolved through three generations to meet the growing power demands of increasingly capable networked devices.
1. What is PoE?
PoE is a standardized technology that enables network equipment, such as switches, to provide power to connected devices, such as IP cameras or wireless access points, over the same cable used for data transmission. Through an automated handshake between the Power Sourcing Equipment (PSE) — for example, a PoE switch — and the Powered Device (PD), the system determines whether the device requires power, how much is needed, and then supplies it safely. This intelligent negotiation prevents the risks associated with uncontrolled power delivery and is one of the core advantages of PoE.
2. Comparison of the Three Generations of PoE Standards
As end devices integrate more advanced capabilities — such as 4K/8K video streaming or Wi-Fi 6 high-throughput wireless — their power requirements continue to climb. The PoE standards have evolved in step with these needs:
| Standard | IEEE 802.3af (PoE) | IEEE 802.3at (PoE+) | IEEE 802.3bt (PoE++) |
| Maximum Port Power Output | 15.4W | 30W | Type 3: 60W Type 4: 100W |
| Output Voltage Range | 44-57V DC | 50-57V DC | 52-57V DC |
| Minimum Cable Requirement | Cat5e (≤100m) | Cat5e (≤100m) | Cat6A (≤100m) |
| Power Pairs | 2-pair (Pairs 1-2 & 3-6) | 2-pair (Pairs 1-2 & 3-6) | 4-pair (All pairs: 1-2, 3-6, 4-5, 7-8) |
| Typical Applications | VoIP phones, simple sensors, basic APs | Dual-band Wi-Fi 5 APs, cameras with small rotation angles | 4K/8K PTZ cameras, LED digital signs, industrial robots |
Key Notes
- Actual Available Power – The “Max Port Power” value shown in the table refers to the output from the PSE (Power Sourcing Equipment). Due to transmission losses in the Ethernet cable — for example, a Cat6A cable over 100 meters can lose approximately 3.2 W — the power actually received by the Powered Device (PD) will be slightly lower. When planning, always allow sufficient headroom to account for these losses.
- Technology Evolution – The original PoE standard (IEEE 802.3af) addressed the needs of low-power devices. PoE+ (IEEE 802.3at) was introduced to support medium-power devices such as multi-antenna wireless access points. PoE++ (IEEE 802.3bt) was developed to meet the high-power demands of devices like motorized PTZ cameras and high-intensity lighting systems.
II. PoE Switch Selection: Core Parameters You Cannot Overlook
The PoE switch is the central component in a PoE power delivery system, and its capabilities directly determine the stability of both data and power transmission. When selecting a PoE switch, three parameters require particular attention: total power budget, thermal design, and port compatibility.
1. Total Power Budget – Avoid the “Total Enough, Per-Port Not Enough” Trap
The total power budget defines the maximum combined output across all PoE-enabled ports. It determines the number and type of devices the switch can support.
Examples:
- 24-port PoE (802.3af) – Total budget up to ~370 W, sufficient for 24 devices at 15 W each (15 × 24 = 360 W ≤ 370 W).
- 24-port PoE+ (802.3at) – Total budget up to ~740 W, sufficient for 24 devices at 30 W each.
- 24-port PoE++ (802.3bt Type 4) – Total budget up to ~1440 W, sufficient for 14 devices at 100 W each (100 × 14 = 1400 W ≤ 1440 W).
Important: A sufficient total budget does not guarantee that each individual port can deliver the required power. For example, a PoE+ switch with a total budget of 740 W but a per-port limit of 30 W cannot power a 40 W device, even if most of the total budget is unused.
2. Thermal Design – Higher Power Means More Heat
Converting and delivering electrical power generates heat. As PoE power output increases, so does the thermal load. Insufficient cooling can lead to overheating, performance degradation, or even hardware damage.
- PoE (802.3af) – Lower per-port output (15.4 W) generates minimal heat; compact models with ≤8 ports often use fanless, passive cooling.
- PoE+ (802.3at) – Mid-range output (30 W per port) in 16+ port configurations typically requires active cooling with built-in fans.
- PoE++ (802.3bt) – High per-port output (up to 100 W) demands industrial-grade heatsinks combined with intelligent fan control; some models are rated for -40°C to 75°C to support harsh environmental conditions.
3. Port Compatibility – Matching Standards with Device Needs
PoE standards differ in both voltage and maximum deliverable power. The switch must match the requirements of connected devices.
A switch supporting IEEE 802.3bt (PoE++) generally provides full backward compatibility with PoE+ (802.3at) and PoE (802.3af) devices, enabling a mix of low-, medium-, and high-power endpoints on the same platform.
By contrast, a switch supporting only IEEE 802.3af cannot power higher-demand PoE+ or PoE++ devices, even if it has unused total power capacity.
III. Three-Step Selection Method: From Devices to Cabling, Done Right
Selecting a PoE switch does not require complex calculations. Following a structured “device power → switch parameters → cable matching” process ensures reliable performance and future-proofing.
Step 1 – Match Device Power Requirements to the Appropriate PoE Standard
| Device Power Range | Recommended PoE Standard | Typical Devices |
| < 15 W | PoE (IEEE 802.3af) | VoIP phones, access control sensors, single-band APs |
| 15–30 W | PoE+ (IEEE 802.3at) | Dual-band Wi-Fi 5 APs, fixed-view IP cameras |
| > 30 W | PoE++ (IEEE 802.3bt) | 4K PTZ cameras, LED digital signage, industrial robots |
Example:
A supermarket plans to deploy 10 PTZ cameras with pan/tilt capability, each consuming 22 W. Since 22 W falls in the 15–30 W range, the appropriate choice is a PoE+ or PoE++ switch.
(CX102S-16GT-M-SWP, which supports both PoE+ (802.3at) and PoE++ (802.3bt)).
Step 2 – Calculate Switch Power Budget and Port Count
Once the PoE standard is determined, calculate the total power budget and port requirements:
Formula:
Total Required Power = (Per-Device Power × Device Quantity) × 1.2
(The 20% margin accounts for cable losses and future expansion.)
Port count should be at least equal to the number of powered devices, with an additional 2–3 spare ports recommended for scalability.
Example:
10 cameras × 22 W each × 1.2 = 264 W total
→ Select a PoE+ switch with a total budget of at least 264 W and ≥10 PoE ports.
(CX204Y-24GT-M-SWP2, 24 x 1G PoE+ (ports 1 to 8 support up to PoE++), PoE Budget 370W).
Step 3 – Select the Correct Cabling to Ensure Power Delivery and Safety
The Ethernet cable is effectively the “power artery” in a PoE system. Its conductor size, shielding, and build quality directly affect both stability and safety.
| PoE Standard | Minimum Cable Category | Recommended | Risks if Downgraded |
| PoE / PoE+ | Cat5e | Cat6 for improved EMI resistance | Cat5e meets spec but offers less noise immunity |
| PoE++ (≥60 W) | Cat6A | Certified Cat6A | Cat5e increases voltage drop and heat buildup; potential fire risk |
Technical Note:
PoE++ operates at higher current levels. Cat6A cables have thicker conductors (approx. 0.57 mm) and enhanced shielding, which reduce resistance and electromagnetic interference, ensuring that power loss stays within 5% over a 100 m run.
IV. Frequently Asked Questions (Q&A)
Q1 – How can I confirm the PoE capability supported by a switch?
CLI Method (Example: AsterNOS)
- Log in to the device.
- Run the command: show interface poe detail
- In the output, check:
- poe_status – Indicates whether PoE is enabled on the port. 4P denotes 4-pair power delivery, equivalent to PoE++ interfaces. It is backward compatible with PoE/PoE+ devices now.
- AssignedPower – Shows per-port allocated power (e.g., 30 W).
Controller GUI (Example: Asterfusion Controller)
- The interface list will display overall PoE/PoE+/PoE++ capability.
- Click on a specific interface’s PoE status to view detailed parameters.
Q2 – Can PoE and non-PoE switches coexist in the same network?
Yes. Use uplink fiber modules to connect them:
- Uplink: PoE switch to non-PoE switch via optical modules, with PoE disabled (data only).
- Downlink: PoE switch ports connect to powered devices, enabling both data and power transmission.
Q3 – Why should PoE power supply be disabled for uplinks?
| Reason | Explanation |
| Peer device has its own PSU | Core switches and routers are powered separately and do not require PoE. |
| Reduce energy waste | Avoids ~3–5 W per-port idle power consumption. |
| Minimize interference | Prevents potential electromagnetic interference on high-speed (10G+) uplinks. |
| Safety protection | Reduces the risk of damage to non-PoE devices in case of miswiring. |
Q4 – Can a PoE++ switch be connected to a PoE device?
Yes. IEEE PoE is backward compatible. For example, connecting a PoE++ (100 W) port to a PoE (15.4 W) device will automatically negotiate a 15.4 W output, ensuring safe operation.
Q5 – Can a regular PC be connected to a PoE port?
- Non-PoE PC NIC (most cases): The switch detects no PoE signature and transmits only data—safe to connect.
- PoE-capable NIC (rare industrial PCs): Verify the PC’s PoE power requirements before connecting. Over-supply could damage NIC hardware.
Recommendation: Use non-PoE ports for ordinary PCs to avoid accidental power negotiation.
Q6 – Why do devices still reboot if the total PoE budget is sufficient?
Check per-port power limits. For example, a PoE+ switch with a total budget of 740 W (30 W max per port) connected to a 35 W device will cause reboots despite having budget headroom—because the per-port limit is exceeded.
Ⅴ. Summary and Action Suggestions
The core principles for selecting a PoE switch are “power matching, reserved margin, and cable synchronization”:
1. Power should be higher than needed: Select the standard according to the maximum power consumption of the device, and reserve 20% power for future upgrades (such as upgrading cameras from 2 million pixels to 4K);
2. Cables must be matched: Cat6A must be used for PoE++; do not use old cables to save costs;
3. Monitor the load after actual measurement: After deployment, use CLI or controller to monitor the real-time power consumption of ports to ensure that it does not exceed the single-port and total budget.
If you would like to learn more detailed specifications about our products, you can visit the Asterfusion PoE switch product page. The following sheet is for your reference:
| PoE Standard | Asterfusion PoE Switch Model | Port | Switch Capacity | PoE Budget | PSU | Fans |
| 802.3at | CX102S-8GT-M-SWP | 8x1GBase-T, 2x10Gb SFP+ ports | 28Gbps | 150W | 1 | 2 |
| 802.3at | CX102S-8MT-M-SWP | 8×2.5GBase-T, 2x10Gb SFP+ ports | 72Gbps | 150W | 1 | 2 |
| 802.3at/bt | CX102S-16GT-M-SWP | 16x1GBase-T,2x10Gb SFP+ ports | 36Gbps | 150W | 1 | 2 |
| 802.3at/bt | CX102S-16GT-DPU-M-SWP | 16x1GBase-T, 2x10Gb SFP+ ports | 36Gbps | 150W | 1 | 2 |
| 802.3at/bt | CX204Y-24GT-M-SWP2 | 24x1GBase-T,4x25Gb/10Gb SFP28 ports | 124Gbps | 370W | 1 | 3 |
| 802.3at/bt | CX204Y-24GT-M-SWP4 | 24x1GBase-T,4x25Gb/10Gb SFP28 ports | 180Gbps | 740W | 1 | 3 |
| 802.3at/bt | CX204Y-48GT-M-SWP4 | 48 x 1G base-T , 4 x 25GE SFP28 ports | 148Gbps | 740W | 1 | 3 |
| 802.3at/bt | CX206Y-48GT-M-HWP4 | 48x1G BASE-T ,6x25Gb/10G SFP28 ports | 198Gbps | 740W | 2 | 3 |
| 802.3at/bt | CX206Y-48GT-M-HWP8 | 48x1G BASE-T ,6x25Gb/10G SFP28 ports | 198Gbps | 1440W | 2 | 3 |
Through the above guide, it is believed that you have mastered the selection logic of PoE switches. Remember: choosing the right PoE switch can not only avoid device failures but also reserve space for device upgrades in the next 3-5 years, truly achieving “one deployment, long-term benefit”.
