One of the many benefits of voice over IP is mobility. With IP phones, employees can move their phones to a new location and retain their extension numbers, voice mail access, customized button mappings and other features.The problem is that municipalities and states have begun to mandate safety legislation requiring the ability to identify callers' locations to facilitate increased precision during 911 calls. To address these requirements, many telephone companies now offer optional E911 emergency service, which places stringent conditions on PBXs to provide location information and a callback phone number, neither of which is required with regular 911 service.But when using VoIP, administrators of large, IP-based systems have no easy means of identifying where on the Ethernet network a given station is physically located without manually keeping tabs on users, thus defeating some of VoIP's purpose.Cisco\u00a0recently introduced a product, Cisco Emergency Responder, that uses the phone's media access control address and a proprietary, Ethernet-based algorithm to identify users and station locations on the basis of terminating switch ports. In the case of large, multisite deployments, Cisco Emergency Responder can map specific switch ports to appropriate 911 trunks. We expect more IP PBX vendors to follow suit with similar Ethernet-based E911 support. Back to main feature: "Tips from the trenches on VoIP"