The "per link" distance is shorter. Digital signals can be regenerated ( ie multiple links) , so can be sent large distances - ie around the world. Analogue amplifiers introduce nose and distortion so number of repeaters is limited before signal degradation causes the signal to be poor.
Arlin, good question / thought. Yes, in those situations (specifically network outages), storage is often moved inside of the camera or with smaller recorders at the edge. Unreliable power supply requires some form of battery backup / UPS, which we did not cover in this 101 but is certainly part of many systems - related, UPS Backup Power for Surveillance Guide
Most higher-end NVRs that accept multiple drives are capable of running them in RAID. A NAS would be unnecessary unless you need massive storage capacity beyond what the NVR can support. A NAS would also be used where you have a server/small PC running an independent VMS without sufficient storage in itself.
As someone brand new to the video surveillance and VMS industry, very informative lecture and interesting to see the different architectures, and how they are based on 5 fundamental decisions. Great to see the applicability as it pertains to a companies needs, budget, constraints, nature and scope of work, etc.
In SMB scenarios, when the communication links between cameras and recorders are not very reliable, you can rest on a redundant recording strategy. Cameras (IP) can have micro SD cards installed, and when the connection to NVR is lost, they record in the local cards. When the comms are restored, the camera updates the NVR by sending, in parallel with the current video, the stored minutes. This is called ANR (Automated Network Replenishment) and it´s very useful for instance where cameras use 4G/LTE or Wifi data uplinks which may have temporary service drops.
I have seen ANR also used between remote NVRs at branches, and central NVR. You can save in both recorders (sometimes just the lower quality substream in the central site) and if there is a IP link service drop, the remote NVR will sync when IP link is back.