Recording at the edge is a distributed or de-centralized approach to storage—the video is spread across a number of edge-storage devices as opposed to centralized on one.
SD cards in these cameras are continuously filled with data, written over, and over-worked -They burn out very quickly, compared to a normal Hard Drive.
Pre-alarm recording is offered by introducing a buffer in the encoder so that the seconds or minutes of video before and after an alarm can be automatically transmitted to the centralized storage.
For instance, if you continuously transmit a 1 Mbit/s stream (30 IPS at 4 CIF) to a central recorder in the anticipation that an alarm event will occur, you will consume 1 Mbit/s of network bandwidth, or send about 11 Gigabytes of video data.
The ideal solution is to seamlessly combine both approaches—when the network bandwidth is available and reliable then use it, but when it is severely limited or intermittent, then use recording at the edge.
These include SNMP compatibility to network management systems such as Tivoli and HP Open View, and the ability to reconfigure and upgrade multiple units simultaneously.
This is because the analog cameras must first record to a central DVR; the video is then compressed into a digital format, and then stored typically on a hard drive inside the unit.