PoE classes and network design
IEEE 802.3af/at/bt PoE classes and budgets, VLANs, multicast and IGMP, and bandwidth calculations.
- Select the right PoE class and size a switch power budget
- Design VLANs and ACLs for a video network
- Explain when to use multicast and what IGMP snooping does
- Calculate aggregate bandwidth for edge and core links
Power over Ethernet
PoE delivers power and data over the same twisted-pair cable. The switch (or midspan injector) is the PSE (power sourcing equipment); the camera is the PD (powered device). Power at the PD is lower than at the port because the cable dissipates some of it.
| Class | Standard / type | Power at PSE port | Power available at PD | Pairs |
|---|---|---|---|---|
| 1 | 802.3af (Type 1) | 4 W | 3.84 W | 2 |
| 2 | 802.3af (Type 1) | 7 W | 6.49 W | 2 |
| 0 / 3 | 802.3af (Type 1) | 15.4 W | 12.95 W (13 W under 802.3bt) | 2 |
| 4 | 802.3at PoE+ (Type 2) | 30 W | 25.5 W | 2 |
| 5 | 802.3bt (Type 3) | 45 W | 40 W | 4 |
| 6 | 802.3bt (Type 3) | 60 W | 51 W | 4 |
| 7 | 802.3bt (Type 4) | 75 W | 62 W | 4 |
| 8 | 802.3bt (Type 4) | 90 W | 71.3 W | 4 |
Sources: IEEE 802.3bt overview from the Ethernet Alliance and vendor white papers (see Further reading). Ethernet including PoE is limited to a 100 m (328 ft) channel.
- Typical fixed indoor cameras are Class 2โ3. Outdoor cameras with heaters and IR, PTZs and multisensors often need Class 4 (PoE+) or 802.3bt. Use the camera datasheet's PoE class and maximum power.
- A switch that allocates by class reserves the port figure. 24 cameras at Class 3 reserve 24 ร 15.4 = 369.6 W, even if they actually draw less. Some switches can allocate by actual consumption or LLDP-negotiated power.
- Check the switch's total PoE budget, not just the per-port capability: a 24-port "PoE+" switch may only have 370 W total.
- Outdoor heaters can draw the most power on the coldest night, which is exactly when a marginal budget fails. Size for worst case.
- Put PoE switches on UPS; if the switch loses power, every camera on it goes dark.
Worked example: PoE budget for one closet
Closet IDF-2 has 20 indoor domes (Class 3) and 4 outdoor PTZs (802.3bt Class 6).
Domes: 20 ร 15.4 W = 308 W. PTZs: 4 ร 60 W = 240 W. Total reserved: 548 W.
Add ~20% headroom โ about 660 W. A switch with a 370 W budget fails; choose a switch (or pair of switches) with at least ~660 W PoE budget and 802.3bt ports for the PTZs, on a UPS sized for that load plus the switch itself.
VLANs and segmentation
- Put cameras on a dedicated video VLAN (or several, per building or zone). Put access control controllers on their own VLAN.
- Allow only the traffic needed: cameras talk to Archivers (and NTP, DHCP, DNS as required), not to the internet or the corporate LAN.
- Use 802.1X port authentication or at least port security/MAC limiting on camera ports in public areas, and disable unused switch ports.
- Time sync (NTP) for every camera, controller and server: evidence with wrong timestamps is a problem in court and makes event-to-video linkage fail.
- Plan IP addressing with room to grow; document every device's IP, MAC, location and firmware.
Unicast vs multicast
With unicast, each viewer gets its own copy of the stream. With multicast, the source sends one stream to a group address and the network replicates it only where receivers have joined. Multicast shines when many operators view the same cameras live (e.g. a SOC with video walls).
- Multicast needs IGMP snooping on switches so streams only go to ports that asked for them, and an IGMP querier (usually the L3 switch or router) per VLAN. Without snooping, multicast floods every port like broadcast and can take down a network.
- Routing multicast between VLANs/subnets requires multicast routing (e.g. PIM) on the L3 devices.
- In Security Center, the Media Router role decides how streams are delivered and can convert unicast to multicast (and IPv4 to IPv6) through redirectors. Multicast addresses are assigned in the camera and Archiver settings; Genetec's port list includes UDP 47806โ47807 for live multicast.
- Recording traffic from cameras to Archivers is usually unicast; multicast is mainly for live viewing.
Bandwidth calculation
Design capacity โฅ aggregate ร (1 + headroom), and keep sustained utilization well below link speed
Worked example: campus building uplink
Building A has 120 cameras: 80 ร 1080p at an average 3 Mbps, 30 ร 4 MP at 5 Mbps, 10 ร 4K at 10 Mbps (measured averages with smart codec on).
Recording traffic: 80 ร 3 + 30 ร 5 + 10 ร 10 = 240 + 150 + 100 = 490 Mbps.
Add live viewing: 6 operators each watching 16 tiles on the secondary (low-res) stream at ~0.5 Mbps: 6 ร 16 ร 0.5 = 48 Mbps (unicast), plus a few full-res streams for detail viewing ~40 Mbps โ total โ 580 Mbps.
With 30% headroom โ 750 Mbps. A single 1 Gbps uplink is too tight for sustained traffic plus bursts (and I-frame peaks); specify 10 Gbps uplinks or link aggregation from the building to the core, and 10 GbE NICs on the Archivers.
Try it: bandwidth and storage calculator
Decimal units (1 TB = 1,000 GB), matching how drives are sold. Operating systems often show binary TiB, which reads about 9% smaller. Use measured bitrates from a pilot camera whenever possible.
Remember that peaks matter: I-frames, busy scenes and multiple simultaneous playback requests can exceed averages. Use measured bitrates from the actual camera models and scenes wherever possible.
Common mistakes
- Sizing PoE by number of ports rather than total budget.
- Forgetting heaters and IR when choosing PoE class for outdoor cameras.
- Enabling multicast without IGMP snooping and a querier, flooding the network.
- Running cameras on the corporate user VLAN with internet access.
- Using datasheet maximum bitrates or guesses instead of measured averages.
- No UPS on PoE switches, so a power blip takes down every camera.
- Cable runs longer than 100 m without an extender or fiber.
Check your understanding
1.What is the maximum power at the PSE port for an 802.3at (PoE+) Class 4 device?
2.Which PoE classes require all four pairs?
3.Multicast is enabled but every switch port is receiving all streams. What is most likely missing?
4.40 cameras average 4 Mbps each. What is the aggregate recording bitrate?
5.Name three things you should do on camera switch ports in public areas.
6.Which Security Center role decides how live and playback streams are delivered and can convert unicast to multicast?