Question

When the VLR/SGSN/MME "decides" to perform the MS Authentication procedure?

I am referring to the 3GPP 43.020, it is done on Location update. Is it also done on every MS radio contact (call/SMS)? What regarding the data traffic?

Comments

I don't think it would be done on every radio contact, UE is already authenticated why it should be authenticated again.

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Thank you for your response.

When VLR/SGSN/MME generates SAI/AIR to the HLR/HSS it may require an amount of vectors (amount of triplets/quintuplets).
The VLR/SGSN/MME uses one of them in order to authenticate the UE.
Once the vector is used and shall not be reused again (unless on exceptional circumtances).
In reference to the 3GPP 43.020 specifications, the VLR/SGSN/MME uses the others vectors for subsequent authentications but I did not find any reference on "when" it is done.
Based on my obervation on signaling traffic, VLR/SGSN generates SAI to the HLR without any relation to the update location.
I come to the conclusion that it performs it to request for new vectors because those provided in the last SAI transaction is already used.
From your response, I understand that since the MS is authenticated only once, the VLR/SGSN shall ask for Authentication vectors duing either attach procedure or handover which is reflected by a new update location to the HLR.
The question is why multiple vectors are required on each SAI dialog and why SAI is generated without any relation to the update location.

Answer 1

I am not sure but want to share some thoughts. Kasme is derived at HSS from KDF (Key derivation function), SQN xor AK, CK , IK and SN Id (serving network id) . Serving network ID is provided by MME to HSS in authentication request named as visited-network. I think not in every MS radio contact authentication need to be performed but if MS select different serving network then only authentication procedure need to performed again since a new Kasme need to be generated.

Looking for others to respond or comment.