Back in October 2009, I blogged about the incredibly deep protocol stack on the GPRS Gb interface between a BSS and the SGSN.
Today I had the pleasure of implementing an even more odd variant of the Gb interface, where NS does not get encapsulated in UDP/IP/Ethernet, but in FrameRelay/GRE/IP/Ethernet. The total protocol stack thus then looks like: HTTP/TCP/IP/SNDCP/LLC/BSSGP/NS/FR/GRE/IP/Ethernet, with an optional PPP between IP and SNDCP. If anyone does the math to calculate the total protocol overhead, please let me know[...
The reason for that oddity is apparently that there are Cisco and other routers that can encapsulate Frame Relay in GRE. So using a old circuit-switched SGSN with E1 interfaces and such a router, you can convert from Frame Relay on E1 to Frame Relay on GRE/IP/Ethernet.
Both the Gb Proxy as well as the upcoming OsmoSGSN use the same NS implementation, i.e. they can now both talk NS/FR/GRE and the NS/UDP variants - even at the same time, as the encapsulation is a property of each individual NS-VC.