ce expres- sive models of logical networks, we also need symbols for routes. The large varia- 3 Node Structures tion of route types makes it necessary to select a few criteria to affect the route symbol, and add all other properties of routes as attributes in logical-network mod- els. We normally put the name of the hosting network that realizes the route as an annotation, as exemplified in Fig. 3.15. This annotation also servers as a reference to the transfer properties (rate, quality of service, etc.) of the route. If nw1 type is ISDN a route in, e.g., the ISDN media service logical network, realized by the ISDN switching stratum. If nw1 type is LAPD -> a route in, e.g., the ISDN call-handling logical network (in the access part). If nw1 type is IP -> a route in, e.g., the TCP logical network. If nw1 type is TCP -> a route in an application logical network of an IP-based network. Fig. 3.15 Defining route properties in logical networks A number of other criteria may influence the design of route symbols: 1. One criterion may be the position in the layer structure of the layer that realizes the route. 2. Another is how the route is addressed (as mentioned previously). 3. Some routes are realized on-demand, others are realized for permanent or semi- permanent use. 4. Another property of interest is if the data flow over the route is a single flow, or separated into multiple flows (connections or channels). 5. A property that must be represented in a route symbol is its structure, i.e., whether it is point-to-point, point-to-multipoint or broadcast. 6. A property of interest might be if the data flow is synchronous (as in PSTN/ ISDN/PDH/SDH) or asynchronous, as in all routes realized by packet-switching systems (adaptation layers such as AAL1 in ATM are disregarded). We do, how- ever, not use this criterion in the route symbols that are defined here. 7. Some routes are duplex, some are simplex. Duplex routes may be symmetric (the same transfer rate in both directions) or asymmetric. These criteria are also not used