BTS may terminate one or several routes of this type (one per TRX or "carrier"), depending on the expected traffic intensity in the cell it serves. All carrier routes are realized over a single physical medium layer that consists of antennas for reception and sending, filters ("combiner" and "divider" that allow the same antennas to be used for several carriers), and the radio medium. The single physical route on the fixed network side is realized by a wireline physical medium (as depicted) or a point-to-point wireless medium (a "radio link"). The transmission protocol uses the capacity of the physical route to trans- port several 64-kbps channels (normally 24 or 32), most of which are used for user- data streams, and at least one for signaling streams. Since a voice call is com- pressed to 13 kbps in this physical network, a single channel on the network side can carry four calls. No configuration tables are needed in the physical layer multiplexer, since user- data channels have fixed positions in physical routes on both sides. The physical node also acts as a hosting node for a hosted node that performs multiplexing of signaling data. The physical node performs its hosting node role by separating signaling channels from user-data channels and deliver signaling data to LAPD and LAPDm hosted nodes. The discrimination method is "by position" on both sides: The physical layer agent on the radio network side extracts a number of different types of signaling channels from the frames of the radio transmission protocol. There are special channels for paging (PCH), access (RACH), call setup SDCCH), etc. The physical layer agent delivers data of these channels to the LAPDm agent over the layer interface, also defining to which signaling channel the data belong (as a kind of connection-endpoint identifier). Thus, the physical layer realizes a multichannel link route for the hosted node, with one route per carrier. The physical layer agent on the fixed network side extracts the data on the single 64-kbps signaling channel