It is pointed out that the interworking between the ISDN (integrated services digital network) user-network interface layer 3 protocol and the SS7-ISUP (ISDN user part) protocol is not straightforward, in spite of the fact that ISUP was developed to support ISDN. Three basic criteria for designing two protocols, such that their interworking is simplified, are presented, and it is shown that the ISDN-ISUP interworking does not follow these criteria.
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