destISDN NA. ISDN NAs The ISDN ISDN access PABX signal transport signal transport service service (LAPD) Fig. 2.11 Supporting multiple network addresses through interworking Nevertheless, individual NAs could be used for discrimination as well. The hypothetical cases of assigning multiple NAs to terminating layer elements in IP and X.25 networks (in order to use them for discrimination) are depicted in Fig. 2.12. Multiple network addresses in an X.25 terminal (a DTE): xna1-LE1 .,destXNA{xna1,xna2},...) X.25 L3 xna2-LE2 LAPB service Multiple network addresses in an IP terminal (a "host"): (UDP) (UDP) (TCP) (TCP) If Ipa1: If ipa2: {destIPA(ipa1,ipa2},destPRN{TCP=6,UDP=17}) 6-LE1 6-LE3 17-LE2 17->LE4 IP transmission service Fig. 2.12 Discrimination by multiple network addresses. XNA=X.25 network address; IPA=IP network address Note, however, that from all aspects except for routing, switching, and transmis- sion, layer elements in X.25 terminals and IP hosts that rely on different NAs belong to completely separated networks. 2.2.5 Discrimination in Circuit-Switching Layers Discrimination by a terminating layer element can also be provided without using any particular identifier. This is specially SO in circuit-switching layers. Figure 2.13 depicts how termination is performed in an ISDN terminal of the BRA type. 2 Layer Structures handling 64 or 128 kbps Media resources A BRA frame 8 bits 1 bit 8 bits 8 bits bit 8 bits 1 bit BRA layer element Symbol for multiplexing Fig. 2.13 Discrimination in the ISDN circuit-switching layer. B1,B2=bits of B-channels; D=bits of the D-channel; OH=overhead bits Synchronous frames, as depicted in Fig 2.13, arrive continuously from the net- work. The bits in this frame represent three channels: D (16 kbps), B1, and B2 (each 64 kbps). Since the number of bits of respective channels is the same in all frames, and always have the same position, there is no need for any special param- eter to identify each channel. We call this discrimination by position. The D channel terminates in the LA