Backward GVNS

           8  7  6  5  4  3  2  1
        1  H  G  F  E  D  C  B  A

B  A    Terminating access indicator
0  0    no information
0  1    dedicated terminating access
1  0    switched terminating access
1  1    spare
bits G-C        Spare
H       Extension indicator
0       information continues through the next octet
1       last octet