474 APPENDIX D: CONNE XTIONS H.323 GATEWAYPOTS AdaptersYou can purchase circuit boards that plug into a personal computer andadapt an analog telephone (POTS) for use with an H.323 connection.H.323 GatekeepersThe gatekeeper is an H.323 entity on the network that provides addresstranslation and controls access to the network for H.323 terminals,Gateways, and MCUs. The gatekeeper also provides services to theterminals, Gateways, and MCUs, such as managing bandwidth andlocating Gateways.The H.323ConnectionH.323 calls between local and remote 3Com Business and BasicTelephones are transparent to users, except for the IP dial plan. The CallProcessor sets up the local end of the H.323 call as though it were settingup a call through a line card. However, this connection actually goes to anetwork interface card (NIC) in a dedicated Windows 2000 system that isrunning the ConneXtions software.The Call Processor requests an H.323 port in the ConneXtions softwareby sending a frame, with a simulated Ethernet address, that contains arequested IP address. The ConneXtions gateway uses this address torequest a level three connection between the local router and the remoterouter associated with another PBX or NBX system.After an IP connection has been established, the ConneXtions softwarebegins a series of H.323 exchanges by using TCP packets on the IPconnection.These H.323 exchanges set up the call and negotiate the type of voicecompression that is used. They also cause the remote NBX (or PBX)system to begin setting up the remote end of the connection.ConnectionConsiderationsAs soon as an end-to-end connection has been set up, all three networks(local LAN, WAN, and remote LAN) are ready to pass voice packets. The3Com Business and Basic Telephones use their DSP to convert spokenwords into digital voice packets. The voice packets are transferred acrossthe Ethernet to the local H.323 gateway. The gateway strips off theEthernet frames, compresses the voice, and encapsulates it within UDPpackets which are delivered to the router, again via the Ethernet. The UDPpackets are placed on the WAN for IP delivery to a remote H.323
