that the user creates a symbol with identifiable route endpoints, to indicate the boundary of a logical node. 3 Node Structures Route Type Examples 3.4.1 Physical Routes Since physical routes abstract paths through physical-media strata, they terminate in nodes that are hosting nodes only. Such nodes frequently terminate more than one physical route, possibly of different types. Figure 3.21 shows an example taken from GSM: the model shows part of the functionality of a GSM base station ("base transceiver station" (BTS)) in the L-N plane. Physical network: SAPI value LAPDm 64 kbps LAPDm Signalling Radio transmission Wireline transmission user data protocol protocol phys. phys. Combiner, divider, antenna, radio medium Fig. 3.21 Part of a GSM base station (BTS). AC=actor; BSC=base station controller; LM=layer management; MS=mobile station; O&M=operation and maintenance; TRX=transceiver A BTS performs multiplexing of two types of streams between many mobile stations (MS, to the right) and a "base station controller" (BSC, to the left). These functions are performed by two logical nodes in a BTS: a physical node that multi- plexes user data and that is also a hosting node for a logical node that multiplexes signaling data. The latter also acts as a hosting node for several logical nodes (e.g., O&M and LM) that deal with management. The physical layer on the radio network side is built by a number of agent lay- ers, with one per carrier. The standard defines an agent as a part of a TRX module (that also includes a LAPDm agent, see page 135 for definition of LAPDm). A sin- gle agent layer on the fixed network side handles all physical layer communication 3.4 Route Type Examples between BSC and the BTS. The physical node includes a multiplexing actor that connects all agent layers and multiplexes user-data channels. A physical route on the radio network side is a wireless point-to-multipoint route. The radio transmission protocol uses the capacity of this route to create eight timeslots (a call occupies one timeslot). A 