What is bandwidth and why is it important?
Modern-day CCTV systems increasingly use IP networks to send video from one point to another (for example, from a camera to a Video Management System typically running on a PC).
IP networks are an attractive medium for transporting video because one cable can carry the video from many cameras. These networks also serve a multi-functional purpose because the same cable can carry video as well as two-way audio, alarm signals, relay signals, PTZ commands, and serial data. With Power over Ethernet, the cable can even carry power to the camera. This greatly simplifies the multitude of cables usually required.
However, the network is limited by the amount of traffic it can carry - known as bandwidth, which is measured in bits per second (bps). The table below shows the three kinds of star-topology Ethernet networks in common use today.
| Name | Also known as | Bandwidth |
|---|---|---|
| 10Base-T | Standard Ethernet10 | 10 Mbps (Megabits per second) |
| 100Base-T | Fast Ethernet | 100 Mbps |
| 1000Base-T | Gigabit Ethernet | 1,000 Mbps or 1 Gbps |
Video over IP consumes a large amount of bandwidth. As a rough guide, one camera might consume between 100 kbps and 2 Mbps - compare this to audio, which typically consumes only 80 kbps. This explains the immediate success of Voice over IP. The factors that affect bandwidth consumption are described in the table below.
| Factor | Description | Examples |
|---|---|---|
| Video compression method | Typically temporal or spatial compression | MPEG-4, M-JPEG, Wavelet, and MPEG-2 |
| Frame rate | Frames per second | 1-30 FPS |
| Image resolution | The number of horizontal and vertical pixels | QCIF, CIF, 2CIF, 4CIF (also known as full D1) |
| Scene activity level | The amount of activity in the camera's field of view | Low, medium, and high |
| Quiet time | The fraction of time where there is no movement (important for temporal compression algorithms like MPEG-4 because negligible bandwidth is consumed during quiet time) | 8:00 p.m. - 6:00 a.m. Monday -Friday, all day Saturday and Sunday, equates to about 50% quiet time |
If you run out of bandwidth on your network, you will start to experience the following:
- Video artifacts (e.g., blocks in MPEG and M-JPEG, and increased fuzziness in Wavelet)
- Frames may get dropped, making the video appear choppy
- The video resolution may drop from 4CIF to 2 CIF or even CIF, making the picture less clear
- The video may freeze entirely and lose the connection temporarily