Session control paves the way for IP voice servicesTechnology to smooth the transition between traditional and IP voice services is becoming available for carriers looking to benefit from converged networks that can handle voice, video and data traffic.Called session controllers, these stand-alone devices help overcome some knotty problems facing voice traffic as it moves between IP networks, such as between corporate LANs and service providers' networks."Service controllers regulate bandwidth and keep service-level agreements and quality of service intact so customers can reap the benefits of IP networks," says Mark Bieberich, a senior analyst with The Yankee Group. Key vendors in this relatively new area include Acme Packets, Jasomi, Kagoor, Netrake and NexTone, he says.These processor-intensive devices peer into packets to analyze and alter them, if necessary, so payloads make their way across network boundaries intact. Acme Packets, Netrake and NexTone are focused on service providers, while the others offer gear for both providers and businesses, Bieberich says. NetRake touts its custom processors, while NexTone promotes its use of generally available processors that keep costs down, Bieberich says.The major problem that session controllers overcome is how incoming calls get through firewalls, which are designed to keep out inbound traffic unless it was solicited or headed for a firewall port that accepts all traffic of a certain type. Incoming calls would be rejected because they do not come through ports that are usually open and are not in response to outgoing traffic, as is their nature.The benefit of these devices is that they do all the needed work without upgrading or reconfiguring firewalls.Typically, session-control equipment sits outside the firewall either connected to a business LAN or in a service provider network. When users are setting up their IP phones and IP PBXs, they configure them to announce themselves on the network to the controller device. After that, they keep a session alive so the control appliance has a hole through which to initiate inbound phone sessions.These devices also handle the problems that firewalls can create by translating the private IP address of an IP phone to a public IP address that can be recognized by routers in public networks. This is a necessary function but does not alter the originating addresses on internal parts of the packets, only the external headers. When IP phone gear parses these packets and sees that the headers don't match, it discards them.These new devices fix that discrepancy for both call signaling and media packets. Outbound and inbound calls are directed to the controllers, and they alter the packets so they won't be rejected by other devices and can find their way to privately addressed devices hidden by firewalls' public IP addresses.Session controllers use back-to-back user agents that terminate calls coming out of one network, such as a customer's LAN, and establish the call across the next network it must traverse, such as a service provider's network.This is particularly useful to service providers that want to offer voice-over-IP services, but would be unable to handle resetting each customer firewall or convincing customers to buy new firewalls that can handle VoIP network address translation.Some vendors such as Jasomi support only Sessions Initiation Protocol-to-SIP translations. Others also support network address translation for Media Gateway Control Protocol and H.323 protocols used for packet voice.Yankee Group's Bieberich says that within four years, the companies selling session controllers will consolidate by being bought up by larger vendors of edge routers or by licensing their technology to them."A lot of this technology could be implemented on a router blade," he says.