e used for operation and maintenance of the BTS. 3.4.2 Link Routes We will use LAPD ("link access procedure over the D channel")4 as an example. LAPD was originally designed for realizing multichannel link routes for many con- current hosting networks in ISDN access networks. LAPD therefore defines an access-point identifier (called SAPI, "service access point identifier") and a con- nection identifier (called TEI, "terminal identifier"). The LAPD logical network is an asynchronous, duplex, symmetric, non-switch- ing hosting network that realizes link routes with multiple connections. It is used as a general hosting network in, e.g., the access part of an ISDN network. Hosted nodes belong to diverse control networks, including nodes of the LAPD manage- ment logical network. Discrimination between hosted nodes is done by SAPI according to the layer APi method. Figure 3.22 shows the link routes that LAPD realizes and the tables that are used by LAPD layer elements. 4 "D channel" is an ISDN concept. Since LAPD is used also within other network systems that define no D channels (e.g., in GSM), the name LAPD sounds inadequate today. In this model we have assumed a LAPD application that uses a point-to-point route to connect a LAPD node in a terminal to a LAPD node in a network. When LAPD is used in ISDN, a point- to-multipoint route called the D channel is used instead to connect a number of terminals to the network. 3.4 Route Type Examples LAPD node in terminal Node in local exchange SAPI=63 SAPI=0 SAPI=0 management element element te5, te9,te 127 element LAPD routes {sap0,te5,. sap63 No addrPar Fig. 3.22 Link routes realized by LAPD. TEI table=list of connections that the terminal can use A single link route interconnects two LAPD nodes. It is assumed to be a single- channelled, permanent route (in ISDN called "D channel"), i.e., it has no REi and no REdef, which is why no T-RE table is needed in LAPD layer elements of termi- nals. A LAPD hosting network realizes multichannelled, permanent point-to-point routes for 