 from the frames of the wireline protocol and delivers them to the LAPD agent. Thus, the physical layer realizes a single channel link route for the hosted node on this side. Let's also briefly look at how the hosted network operates for signaling data received from the radio network side: The LAPDm agent performs a typical link protocol function on every received LAPDm frame (error detection, etc.). The protocol is based on LAPD. The pos- sibilities that LAPD gives to define connections (through SAPI and TEI identifi- ers) are not needed, however, since the hosting network already has identified each type of signaling channel. When LAPDm receives a correct frame, it takes 3 Node Structures its user data and adds a signaling channel identifier (PCH, RACH, etc.) and sends the resulting datum as user data to the multiplexer over the actor-agent interface. The multiplexer must handle a T-L table that relates each actor-agent interface (i.e., each TRX) on the radio network side to a a link identifier (Li) on the fixed network side. The multiplexer delivers the user data with a link identifier value for the actual TRX to the LAPD agent. The LAPD agent associates everything coming from the multiplexer as data belonging to access-point identifier 0 (i.e. SAPI=0). It also uses the TEI parame- ter to identify links (i.e., TRXs). The agent therefore puts the user data in LAPD frame and sets SAPI=0 and TEI=Li. The data of the frame is then sent over the layer interface to the physical layer agent on the fixed network side. This agent puts the data into the single 64-kbps channel over the physical route to the BSC. As figure 3.21 shows, the LAPD agent layer is also used for making this layer a hosting network for other networks. Other SAPI values are used for that. For exam- ple, all received LAPD frames with SAPI=63 are delivered to a logical network for layer management (LM), e.g., for maintaining the T-L table in the multiplexer. The BSC is responsible for the operation of the BTSs it connects. Frames referring SAPI=62 ar