Windows 7 cannot find Brother network scanner

The scanner is a Brother mfc6890cdw (a printer/scanner hybrid), connected through wireless. The Windows 7 is a laptop. Whether wireless or wired, the 'PaperPort' program on the laptop sometimes doesn't find the scanner. On other days, it does.

A portscan shows that the scanner and printer are online. Wireshark shows that Windows goes looking for the scanner using SSDP but gets no reply.

I don't bother fixing that. I just use telnet to access the scanner (default passwd is 'access') and assign it a fixed IP. (The LinkSys wireless router seems to be unable to deal out fixed IP numbers based on MAC.) Then I reinstall the Brother software on Windows and make sure to specify that the printer/scanner is at said IP.

Now it works.

As to why Windows insists on SSDP, or why the scanner doesn't listen to that (it doesn't), I don't know.