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Choose by the pixel density you need on the target, not by the megapixel number alone. 2 MP is 1920×1080, 4 MP is usually 2560×1440 (some models use 2688×1520) and 8 MP is 3840×2160, and with the same lens the pixels per metre grow in proportion to the horizontal pixel count. With a 90° lens, for example, a face reaches the identification density of 250 px/m at about 3.8 m with 2 MP, 5.1 m with 4 MP and 7.7 m with 8 MP. As a starting point, 2 MP is enough for a single doorway or a small room, 4 MP suits most shops, offices and yards, and 8 MP pays off in wide open areas or when you want to zoom into recordings, at the cost of a higher bitrate and a little more noise at night.

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Add up the bitrates of all the cameras and apply the formula: 16 cameras at 4 Mbps each (typical for a 2 MP camera with H.264 at 25 fps) make 64 Mbps, which uses 64 × 86,400 ÷ 8 ÷ 1,000 = 691.2 GB per day. Over 30 days that is 691.2 × 30 = 20,736 GB, about 20.7 TB. With the free-space margin of about 10% used by the KDT website's storage calculator you need 20.736 ÷ 0.9 = 23.04 TB, for example 2 × 12 TB disks. With H.265 (about 2 Mbps per camera) the same system needs half: 10.37 TB, or 11.52 TB with the margin, so a single 12 TB disk.

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The bandwidth a link needs is about the number of cameras × the bitrate per camera × 1.3, where 1.3 is a 30% margin for live viewing and bitrate peaks, the same margin the KDT website calculator uses. For example, 16 cameras of 4 MP with H.265 at 25 fps need about 3 Mbps each: 16 × 3 = 48 Mbps, and 48 × 1.3 = 62.4 Mbps with the margin, which a 100 Mbps link can carry. The same 16 cameras with H.264 need about 6 Mbps each, so 96 Mbps, or 124.8 Mbps with the margin, which calls for a gigabit link. Do this calculation separately for every link on the path: the switch uplinks, the port and incoming bandwidth of the recorder (NVR) and, for remote viewing, the site's internet upload speed.

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Work out three numbers: ports, PoE budget and uplink speed. For ports, add one or two ports for the NVR and the uplink to the number of cameras and allow about 20% spare for growth; for 16 cameras a 24-port model fits. For power, assume 12 W per fixed camera (typically 6 to 16 W): 16 × 12 = 192 W at the cameras, divided by 0.85 for cable loss is about 226 W, and with 20% headroom about 271 W at the switch, so you need a PoE budget of roughly 300 W or more. Camera ports can be 100 Mbps for typical cameras, but the switch's link to the NVR should be Gigabit, and if you need VLANs or remote monitoring, choose a Lite-Managed or managed model. KDT examples are the KP-1606J3SFT (16 PoE ports, 400 W) if you do not plan to expand, and the KP-2408H4SMU (24 Gigabit PoE ports, 600 W) if you do.

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Use surveillance-grade disks, or at least NAS-class disks rated for 24/7 operation. Desktop disks are commonly specified for about 2,400 power-on hours per year (24/7 operation is 8,760 hours) and a workload of around 55 TB/year, while surveillance disks are designed for continuous operation and typically rated at 180 TB/year, with higher-end surveillance and AI models often rated at up to about 550 TB/year. Surveillance firmware is optimized for many simultaneous write streams, typically up to 64 HD cameras per disk, and keeps the stream flowing instead of retrying a weak sector for a long time, which helps prevent dropped frames; many larger models also have vibration sensors for multi-bay chassis. A desktop disk may work at first, but under 24/7 recording it usually develops gaps, errors and a shorter life.

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Work from the bottom up, one layer at a time, because each layer depends on the one below it. First check power and link: the screen is lit, the switch port shows link (and, if the phone runs on PoE, PoE is enabled on that port with budget left) and the cable is no longer than 100 meters. Then check that the phone has a valid IP address (no address, or 0.0.0.0, means DHCP failed, often because of a wrong VLAN or DHCP scope) and that its SIP account shows Registered; if not, verify the server address, extension, password and transport, and whether the PBX has banned the phone's IP. If calls ring but the audio is missing, one-way or choppy, look at NAT and firewall settings, RTP ports, codecs, jitter and packet loss. Finally, swap in a known-good phone, or move this phone to a known-good port, to learn whether the fault is in the phone or in the network.

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