In order to answer this question, I tracked down Michael Lammers who the Flight Director for the ISS and he kindly offered to speak to me on the phone about ISS communications. (He is also a super nice guy)
He said that almost all of the ISS Command and Control, Voice Communications, as well as science downlink, goes through TDRS with the Ku-Band handling HDEV, Voice channels 3 and 4 and other high bandwidth communications. The S-Band handles critical systems, command uplink, and voice channels 1 and 2.
TDRS is a shared resource and is mostly computer scheduled and he said that HDEV is a good proxy for when the S-Band is up (S-Band has a little higher availability), they share it with Hubble and other resources as well as some launches.
Nominally TDRS is available 80-85% of the time with 10-15min dropouts due to random blockages by station parts, and other users (each TDRS satellite can support 2 high bandwidth users). The longest such dropout in normal operation is around 15 mins. This depends on scheduling (i.e. you could schedule no comms for a whole day if you wanted)
During critical times (such as the spacewalk on 5th October 2017 during Expedition 53) they will look at the outage schedule that is computer generated and if something lines up poorly then they can request that the TDRS schedule be readjusted; on that occasion they only had one outage all day which was 5 mins long.
There are other communications platforms on the ISS, but they are all there to support TDRS in Science downlink not providing primary or (as far as he knew) contingency communications.
Other Comm systems onboard:
- Russian Luch SDRN (Satellite Data Relay Network) He was unsure how much it was used, mostly the Russians share TDRS
- ICS (Interorbital Communications System), on Kibō used for additional science downlink capability
- Ham radio, I didn't ask specifically but I assume this is only for its outreach purpose and is not a primary backup system
- EDRS (European Data Relay System) Mostly just for technology development, not seriously used for Science downlink, definitely not for Command and Control
Cool picture from This PDF of ISS info showing all the Comms systems onboard (I only talked about the ISS to ground ones here):
- Wikipedia ISS article
- Phone Call and an email or two with Michael Lammers (on 10/5/17), who kindly took some time out of his day to answer my questions.
- Many other odd cool NASA resources on the web