'01' Bits (2 bits)

Protocol: X.25 X.25 PCI

Field: 01 bits

Length: 2 bits

Contents: modulo indicator / fixed value.

The packet send/receive sequence number of X.25 follows the sequence number scheme of HDLC (modulo 128 with 7-bits sequence numbers [modulo indicator bits set to '10'] or modulo 8 with 3-bits sequence numbers [modulo indicator bits set to '01']).

GFI bits 3/4 = 01 >> modulo 8 >> 3-bit sequence number

GFI bits 3/4 = 10 >> modulo 128 >> 7-bit sequence number

With X.25, the modulo indicator bits have a fixed value of '01'.