Cisco SPA2102 Administration Guide - Page 20

Introducing Cisco Small Business Analog Telephone Adapters ATA Connectivity Requirements 1 ATA Connectivity Requirements An ATA device can be connected to a local router, or directly to the Internet. Each phone connected to an RJ-11 (analog) port on the ATA device connects to other devices through SIP, which is transmitted over the IP network. In order to ensure connectivity between the devices connected to its FXS ports, the ATA device requires the following functionality to be supplied on the network connected to its Ethernet port: • Connection to an IP router with hairpinning support • Connection to an outbound Proxy server When a phone connected to the ATA device communicates with another phone, it sends a SIP packet onto the internal LAN. The packet is then forwarded to the external LAN or directly to the Internet. The source address and source port on the original packet are assigned by the ATA device DHCP server. The address and port are translated by the ATA device using Network Address Translation (NAT) and Port Address Translation (PAT). The packet is then routed back to the internal network on the ATA device by the local router or the ISP router. Problems can occur with calls between phones connected to the ATA device when an outbound proxy or a router with hairpinning support is not available. The ATA device cannot directly connect the two telephone devices, but requires a local or remote router to route the packet back to its destination on the local network from which it originated. The necessary routing can be provided by a router with hairpinning support, or by an outbound SIP proxy, which is typically provided by the Internet Telephony Service Provider (ITSP). When relying on the ITSP for interconnecting phones on the ATA device, local phones connected to the ATA device are unable to communicate with each other if the Internet connection is not available for any reason. It is recommended you connect the ATA device to a local router that provides hairpinning support to prevent this problem. Cisco Small Business ATA Administration Guide 20