/mcp• Function as a traversal client and establish a secure connection to Expressway-E through the firewall • Establish secure or non-secure connection to Cisco Unified CM • Integrate with an existing internal video network that uses H.323 • Enable business-to-business calls to external entities that communicate using SIP or H.323 • Provide interworking between H.323 and SIP protocols for H.323 business-to-business communications. • Enable mobile and remote access capabilities and call signaling for Cisco supported endpoints, directing them to Cisco Unified CM for SIP registration and/or the IM and Presence Service (See Table 5 for information on which endpoints support mobile and remote access.) Cisco Expressway-E Because Expressway-E is reachable directly from the untrusted external network, it should be placed in a DMZ for security. The organization's firewall policies control communications to and from this server. Deploy Expressway-E to: • Function as a traversal server and allow secure communications to and from Expressway-C • Enable audio, video, and IM and Presence connections to other organizations using SIP or H.323 on the Internet • Provide DNS SRV lookup service to resolve outbound calls and to receive inbound calls over the Internet. • Process registration and IM and Presence information from Cisco endpoints on the external network and use secure traversal communications to pass the information to Expressway-C • Provide interworking between protocols (between SIP and H.323, and between IPv4 and IPv6) for business-to- business communications. PSTN Gateway Because landlines and mobile phones use the PSTN for local and international calls, external connectivity to the PSTN from an organization's IP telephony network is a requirement (Figure 13). Figure 13. PSTN Connectivity