GSM Voice Traffic
Q: In GSM, does voice traffic between mobiles in the same cell site pass through the MSC? I'm not concerned about signalling information.
A. (From different contributors) GSM routes all calls through the MSC, just like AMPS (internal link). That's not the case with CDMA. Not surprising -- technology evolves, and IS-95 (CDMA) is newer than GSM. In the same way, UMTS, evolved CDMA, came after IS-95 and hence operates more like IS-95. Again, not surprising.
In a CDMA network a call as you describe would not route through the switch. Any calls going through the switch must pass through the vocoder to be converted to ordinary landline type voice traffic. Every time we do that, we lose quality, of course. Therefore, for a mobile to mobile call in the same BSC, the voice traffic does not route through the switch, but instead is simply routed to the other mobile so the voice doesn't pass through the vocoder any more than necessary.