The IMSI is what most closely corresponds to the ESN in an AMPS network. The IMSI is the unique serial number stored in the SIM.
Successful registration on a GSM network also requires knowing an encryption key (Ki) that is stored in the SIM in addition to the IMSI. Ki is difficult to extract from the SIM and is used in a challenge-response protocol for authentication.
An AMPS (IS-136/IS-95, etc.) network requires you know a phone number, ESN, and possibly the AKEY to authenticate.
A GSM network requires you know the IMSI and Ki to authenticate.
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The IMSI is what most closely corresponds to the ESN in an AMPS network. The IMSI is the unique serial number stored in the SIM.
Successful registration on a GSM network also requires knowing an encryption key (Ki) that is stored in the SIM in addition to the IMSI. Ki is difficult to extract from the SIM and is used in a challenge-response protocol for authentication.
An AMPS (IS-136/IS-95, etc.) network requires you know a phone number, ESN, and possibly the AKEY to authenticate.
A GSM network requires you know the IMSI and Ki to authenticate.