Host software is software that runs on the host PC, and not inside the phone itself.
Location (source and binary): src/host/layer23/*
Layer 3 applications implement various functionality based on GSM Layer 3 in combination with Layer 2 (LAPDm).
It implements most of the behavior of a regular GSM telephone, but is extended in many ways with features interesting to researchers.
The cell_log application scans through valid available carrier frequencies, attempts to sync to them and dumps information gathered from the BCCH.
It is usually used to create a list of used ARFCNs and information such as their reception levels, MNC, MCC, and System Information.
The ccch_scan application can sync to a carrier ARFCN and logs power measurement and CCCH information (paging requests and Immediate Assignments).
bcch_scan is basically a predecessor of cell_log and logs information as observed on the BCCH (System Information).
cbch_sniff dumps cell broadcast channel information such as, e.g., GPS location of the cell.
virtphy is a virtual L1 implementation based on GSMTAP over UDP multicast. You can use it to run layer2/3 programs such as mobile without any actual hardware or real PHY/L1. For more information see Virtual_Um.