with enterprise network components, or they might be undesirable because they violate call admission control (CAC) policies or cause difficulties with feature interoperability. A border element device can help ease incompatibility and interoperability issues including: 142 SIP Trunking Protocol interworking: H.323, SIP, and variations of these Media interworking: DTMF and codec interworking Fax and modem traffic interworking: Negotiating between different fax and modem over IP transport mechanisms Security interworking: Connecting a network segment with encryption deployed to another network segment not yet capable of encryption Protocols All standards have variations when implemented by different vendors and by different soft- ware stacks in various applications. This is caused by variances in the interpretation of the standards, variances in vendor compliance to the standards, and by many optional elements in the standards specifications. These implementation variations cause interoperability dif- ficulties between endpoints, applications, and other protocol-aware network devices. Older standards such as H.323 and Integrated Services Digital Network (ISDN) have a maturity in the industry that has shaken out most of the interoperability issues between vendors. SIP is still a relatively new standard and is covered by many Request for Comments (RFC), and vendor compliance to, and interpretation of, these vary. Protocol interworking areas to consider as your network migrates to SIP, or begins to use SIP trunking for PSTN access, include Applications Endpoints Service provider SIP Trunk Interworking-SP User-to-Network Interface (UNI) SIP Normalization Applications Enterprise networks today widely deploy H.323, whereas SIP applications and endpoints are slowly being phased in. Users cannot upgrade their entire network overnight, SO this situation causes an almost perpetual state of networks in transition in which various pro- tocols must be connected together. As illustrated in Figure 8-1, several possible transition