Support for FR over E1 in osmo-gbproxy
The existing osmo-gbproxy supports "NS over FR over GRE over IP" as stacking. This assumes there is an external device that converts from FR/E1 to FR/GRE/IP. In the past, this was a standard feature by Cisco routers, but apparently today this feature is no longer present in supported hardware/OS.
In order to continue to support users with Frame Relay based Gb interface, we should hence implement native support for FR/E1.
In terms of E1/T1 hardware, we should focus on DAHDI, as the quad/octal E1/T1 cards are still actively sold on the market.