In computing, the Preboot eXecution Environment (PXE, sometimes pronounced as pixie) specification describes a standardized client-server environment that boots a software assembly, retrieved from a network, on PXE-enabled clients. On the client side it requires only a PXE-capable network interface controller (NIC), and uses a small set of industry-standard network protocols such as DHCP and TFTP..Wikipedia
The firmware of the modem contain the DHCP responsive of the IP attribution, DNS server. But this DHCP server can define a image PXE. Combining with a TFTP server, the modem can boot an Operating System or a software (include malware).
The OLTs are control and central point to do far communication like GPON used into fiber to home (FTTH). This OLT can download the firmware, edit it and re-upload the edited firmware to do the previously mentioned.