The Cisco 3600 voice/fax network modules enable voice and fax traffic to be routed over IP networks. Integrating voice applications onto IP networks is a natural extension of Cisco's IP networking expertise. This effort also leverages Cisco's proven track record in effectively handling time-sensitive traffic such as Systems Network Architecture (SNA) over IP networks. Cisco IOSTM software, with its built-in quality-of-service (QoS) features, is the primary enabler of business-quality voice and fax, even over low-bandwidth WAN connections. With the ubiquity of IP networks, many customers will benefit from the attractive economics of IP-based voice/fax solutions, including companies that use a mix of WAN technologies such as Frame Relay, leased lines, and Integrated Services Digital Network (ISDN), and companies that intend to extend voice capabilities to desktop applications.The Cisco 3600 voice/fax network modules slide into Cisco 3620 and 3640 chassis slots and contain either one or two voice interface card (VIC) slots. The VICs are daughter cards that slide into the voice/fax network modules. Just as the Cisco 3600 WAN interface cards can be swapped with other WAN interface cards, the Cisco 3600 VICs can be deployed interchangeably with other VICs in the voice/fax network modules. These built-in flexibility and investment protection features are key reasons for the increasing popularity of the Cisco 3600 platform.