Network Load

 

The MOBOTIX concept is designed to minimize the network load from the outset. The primary contributors to this are the camera's integrated image processing and the camera's internal event detection which the device uses to detect motion in the image. With this design, the MOBOTIX cameras allow the user to wait until images have been recorded before they are transmitted to the network, for example if an event occurs. The decision on this is taken by the actual camera. The big advantage of this process is that MOBOTIX cameras do not require continuous contact and data transfer with the network. Since they conduct the storage independently, this relieves the burden on the central storage computer considerably.

Disadvantages of the Classical Technology

In contrast to the MOBOTIX concept, image analysis in classical systems takes place on a central PC. Its analysis software has to read the image data continuously via the network. Another disadvantage of these systems is that all the camera activities are threaded through a central PC, the eye of the needle for the whole system so-to-speak. Given the fact that images are being recorded by several cameras, this doesn't just place a high burden on the network, the PC capacity is also quickly exhausted or even exceeded.

Operated in live mode using MxPEG compression, the MOBOTIX camera takes up between 1 and 1.5 Mbit on a network, yielding good quality images at a full frame rate. However, this value may double when images are saved. In order to relieve the burden on the network here too, MOBOTIX cameras offer the option of selecting a different frame rate during storage than that for the live image. In the case of motion-JPEG, the frame rate doubles or trebles because the individual frames are always transmitted as full images.

Networks Don't Represent a Video Bottleneck Nowadays

Data transmission from a camera to the PC is no longer a problem because star-cabled 100 MBit networks are normally used in which a video signal only accounts for about two percent of cable capacity. The old 10-Mbit coax ring circuits have now largely been replaced. During this conversion, not only was the bandwidth increased by the factor of ten, the structure of the network was also changed from a ring to a star.

The old 10-Mbit networks were ring-shaped in structure and each ran from one user to the next. Whilst this may have saved on cable, whenever there were lots of users in communication with one another, all the user data was available at each place in the cable simultaneously, which meant that the network capacity was quickly exhausted.

However, the new 100-Mbit networks are connected in a star configuration via a distributor, the switch. It analyses the data packages on each line and sends them solely to the recipient's location. Generally speaking, the data for a camera only occupies two lines as a result; the cable from the camera to the switch and the cable from the switch to the PC. None of the other lines are affected and, consequently, nor is their capacity.

A Single Gbit Line Suffices

A MOBOTIX camera with MxPEG, recording at a rate of 25 fps in CIF format including audio, occupies around 1 to max. 2 Mbit on the network, i.e. only causing a 2 percent load on a standard 100-Mbit network. Consequently, there is practically no load on a single line from the camera to the switch. However, the load from all the cameras is accumulated on the "collective line" from the switch to the PC.

In order to avoid delays in display, the practical network load should not exceed 30 to 40 percent, i.e. 30 to 40 Mbit in a 100-Mbit network. This corresponds to 15 to 20 live cameras at full frame rate. For this reason it is advisable to use a Gbit switch which provides ten times greater bandwidth capacity at two of its terminals, at the connection to the display PC and, possibly, at the connection to the storage PC as well. This type of system can easily transfer images from 100 cameras live to a PC and is also very cost-efficient because you can now buy Gbit switches with 48 100 Mbit inputs and 2 Gbit lines for around $1,000 (AT-8350GB.