What does mean out-of-band?
What’s the difference with in-band?
- In-band is the normal (productive) network connection (for servers : remote desktop, pcanywhere, ... ; for swithes : telnet, http, ...)
- out-of-band is any additional connection to the device, independent from the normal network, so it's an alternate path or a ´backdoor´ access to the device
Why out-of-band is being needed?
The normal network connection usually does not fail by itself, networking today has such a high level of redundancy, that it hardly fails.
However the device itself may no longer respond over the normal network, because the OS is in an unstable condition (server, storage) or it has been misconfigured (router…). If so the only way to access and fix the problem is by accessing the device from the backdoor. If in addition the network or a part of it should fail as well, a dial-up connection could provide a complete out-of-band connectivity. So backdoor access would always be guaranteed.
Having that guaranteed remote access to the device avoids lots of hassles of the administrator and helps solving issues much quicker and easier.