For a GSM network the grade of service on the UM interface is 2% and on the Abis interface it may varry from 0.1% to 1%. The total number of trunks required is calculated from the Erlang table, the only inputs required is the GOS and the Erlangs.
For 100000 Subscriber at a rate of 0.1% the number of E1s on the Abis interface required comes to 172.