One of the main frequency bands for mid-band 5G networks in the US.
It is a relatively large band that includes C Band, the 3.45 GHz band, and the 3.5 GHz band (CBRS / band 48).
See: C Band
Each of these smaller bands covered by band 77 have their own layout and rules, but all are unpaired bands using TDD.
Because some of these smaller bands operate by very different and complicated rules, there is unfortunately not just one "band 77" that ensures compatibility, as with other bands. A phone that supports 5G in band 77 may support just the C-Band, but not the 3.45 GHz band, for example, even though the 3.45 Ghz band is technically within the frequencies band 77 is intended to cover.
To denote these differences, the 3.45 GHz band is sometimes written as band "77-A", or "77 (DoD)", (since these frequencies were previously used by the US Department of Defense.) Similarly, the C-Band is sometimes indicated as band "77-C".
Band 77 spans 3.3 – 4.2 GHz, although only 3.45 – 3.98 GHz is used in the US.