he possibility that several alternative substructure solutions may exist and must be maintained. Such alternatives are called variants. Names should be unique over all levels. The name of the sysnet (sn1 in Fig. 4.9) should be unique in a database that stores many sysnet models. Each physical network model fragment has an identifier. The top-level fragment has the name of the sysnet. It identifies no level and no variant. The top-level model is assigned to level 0. Physical nodes and network inter- faces of this model have suffix 0, indicating that they are defined on network level 0. Fragments on level 1 show subnetworks for physical nodes from level 0. Physi- cal nodes in this model have suffix 1, as do internal network interfaces. If vari- ants exist, a variant suffix is included. Physical nodes on level 1 can be further expanded to subnetworks on level 2, and SO forth. This hierarchical structure of models is open to any number of levels (N) and variants (M), but normally the sysnet manager defines a maximum number of levels. 4 Physical Network Modelling Syntax: Identifier::=CHOICE{Sysnetld, Otherld} SysnetId::= SysnetName Otherld::=SEQUENCE{Name," ".", Level, ".", Variant OPTIONAL} Name::=CHOICE{NetworkOrNodeName, InterfaceName} Level::=INTEGER (0..N) where Nis defined by the sysnet manager Variant::=INTEGER (0..M) where M is defined by the sysnet manager Example: Physical network level 0 Physical network Physical network level 1, variant 1 level 1, variant 1 a2.1.1 a1.1.1 {Q.931} {Q.931,X.25} c2.1.1 b2.1.1 {LAPB,IP} k1.1.1 1.1.1 {1.411,LAPD,M} k2.1.1 c3.1.1 b1.1.1 {1.411,LAPD,M} c1.1.2 b3.1.1 j2.1.1 {LAPB,IP} {X.25} k3.1.1 c4.1.1 Fig. 4.9 Identification system for physical networks General Packet Radio Service, a Case Study 4.4.1 Introduction In this section we will use the principles for designing physical substructures to analyze a new network concept (GPRS) that is standardized by ETSI. An overview of this concept is given by ETSI recommendation GSM 03.60. GPRS is an enhancement of the existing GSM conce