amateur radio rules, these addresses derive from the station call signs of the source, destination and repeater stations. Media access control follows the Carrier sense multiple access approach with collision recovery (CSMA/CR). AX.25 supports both virtual-circuit connected and datagram-style connectionless modes of operation. The latter is used to great effect by the Automatic Packet Reporting System (APRS). A simple source routing mechanism using digipeaters is available at the datalink level. Digipeaters act as simplex repeaters, receiving, decoding and retransmitting packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly. The digipeaters use and modify the information in the frame's address field to perform this function. The AX.25 specification defines a complete, albeit point to point only network layer protocol, but this has seen little use outside of keyboard-to-keyboard or keyboard-to-BBS connections. NET/ROM, ROSE, and TexNet exist to provide routing between nodes. In principle, a variety of layer 3 protocols can be used with AX.25, including the ubiquitous Internet protocol. This approach is used by AMPRNet, which is an amateur radio TCP/IP network using AX.25 UI-frames at the datalink layer. Implementations Traditionally, amateur radio operators have connected to AX.25 networks through the use of a terminal node controller, which contains a microprocessor and an implementation of the protocol in firmware. These devices allow network resources to be accessed using only a dumb terminal and a transceiver. AX.25 has also been implemented on personal computers. For example, the Linux kernel includes native support for AX.25. The computer connects to a transceiver via its audio interface or via a simple modem. The computers can also interconnect to other computers or be bridged or routed to TNCs and transceivers located elsewhere using BPQ over Ethernet framing, which is also natively supported by the Linux kernel to facilitate more modern setups with the actual transceivers directly placed under or in the antenna mast, creating a 'low loss', shorter RF wiring need, and replacing expensive and long and thick coax cables and amplifiers with cheap fiber (RFI (both ways)/EMP/lightning resistant) or copper Ethernet wiring. BPQ Ethernet framing allows to connect entire stacks of TNC+transceiver pairs to any existing network of computers which then can all access all radio links offered simultaneously (transparently bridged), communicate with each other internally over AX.25, or with filtered routing select specific TNCs/radio frequencies. Note that the Linux kernel AX.25 implementation does contain some