The coding of Information Elemenent IMSI in GTP v1 according to 3GPP 29.060 is as follows:
bits 8765 4321
Octects 1 type=2
2~9 IMSI
The IMSI is TBCD-coded with a fixed length of 8 octets. Bits 8765 of octet n+1 encodes digit 2n, bits 4321 of octet
n+1 encodes digit 2n-1. Unused half octets shall be coded as binary "1 1 1 1". Digits are packed contiguously with no
internal padding.
So for string in the trace which I captured running on GTPv1 "02 13 20 06 02 22 22 00 F3"
the IMSI was wrongly decoded as 310060202222003, but it should be decoded as 310260202222003
The coding of IMSI in GTPv0 according to GSM 03.60
bits 8765 4321
Octects 1 type=2
2 MCC(digit 2) MCC(digit1)
2 1111 MCC(digit3)
2 MNC(digit 2) MCC(digit1)
2 MSN(digit 2) MSN(digit1)
2 MSN(digit 4) MSN(digit3)
2 MSN(digit 6) MSN(digit5)
2 MSN(digit 8) MSN(digit7)
2 MSN(digit 10) MSN(digit9)
So apparently in the above example, GTPv0 decoding was wrongly used to decode IMSI in GTPv1 format.
Please feel free to contact me for any questions.
Thanks, JW
