P(s) (3 bits)

Protocol: X.25 X.25 PCI

Field: packet send sequence number

Length: 1 byte

Contents: packet sequence number of the sender.

The packet receive sequence number is similar to HDLC's sequence number (modulo 128 if the last two GFI bits are set to '10', modulo 8 if the last two GFI bits are 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

X.25 uses modulo 8 sequence numbers.