IP Trunk Ports

The first generation of converged IP-PBXs could support only private network IP trunk calls. VoIP calls are not currently used for local trunk connections to and from the central office. Off-premises network calls are routed across IP routers over the customer WAN and are typically under the control of a policy management server. A policy management server uses a customer-defined program of IP telephony policies to support VoIP QoS levels across the customer WAN. The server can extend QoS differentiation for content networking through network-based application recognition support. The customer-based policies are managed through interaction with a directory server to provide different network classes of service to different station users. Through the server, end-to-end service can be provisioned, and service level agreements are maintained. Access control policies can be applied to maintain network and application security levels.

Converged IP-PBX systems support two types of outgoing IP trunk calls:

• IP station generated calls

• Non-IP station generated calls

For IP station calls, if the ARS program selects an IP WAN trunk circuit to route a private network call, the IP line port circuit card providing control signaling to the desktop station will direct the IP telephone to transmit voice packets to an assigned IP WAN router. The IP WAN router will then handle the networking operations as programmed. Incoming IP trunk calls from the WAN can be routed directly to an IP station based on the control signaling directions from the terminating IP-PBX system gatekeeper.

An outgoing IP trunk call placed by an IP station is commonly handled without local TDM connectivity, except in cases requiring a circuit switched connection. A direct IP trunk call not requiring TDM connectivity must satisfy a set of conditions similar to that listed for IP station calls. If any of these conditions are not satisfied, a TDM switched connection may be required to place or receive a trunk call.

In a converged IP-PBX system, a non-IP station can also place an IP trunk call if the ARS program selects a route using customer IP WAN facilities instead of traditional PSTN trunk circuits. An integrated IP trunk port interface card provides a gateway between the circuit switched TDM bus network and the LAN/WAN facilities. An external VoIP gateway server is not required, as do circuit switched PBXs that do not support integrated IP trunk interface capabilities.

The IP trunk card functions similarly to a DS1 trunk circuit interface card in support of private line tie trunk control signaling and bearer communications channels. It connects to a local TDM bus and packets/compresses PCM voice communications signals for transmission over the Ethernet LAN to an IP router for access to the customer WAN. The card can also modulate and demodulate incoming and outgoing fax calls. IP trunk cards typically support ISDN protocols, such as ISDN D channel, for private network signaling, H.323 signaling, and a standard VoIP protocol stack.

IP trunk cards also monitor QoS parameters, such as latency, jitter, and packet loss to ensure that acceptable voice communications quality is maintained throughout the call. A customer can define a delay threshold, such as 150 ms, for monitoring purposes; if transmission delay exceeds this level, the call is automatically rerouted over circuit switched PSTN trunk carrier facilities. Packet loss measurements also can be thresholded and monitored for call rerouting purposes. In all cases call rerouting is performed transparently and seamlessly to the call parties. There are several methods used by IP-PBX system designers to monitor QoS transmission delay parameters, including analysis of packet time stamps embedded in the RTP signaling stream or calculating delay based on a self-generated signaling ping across the network.

IP trunk circuit card gatekeeper and gateway functions are similar to those of the IP station card. IP trunk cards typically support 24 (T1 carrier interface) or 30 (E1 carrier interface) gateway channels. There is always a 1:1 ratio between the number of trunk channel equivalents and gateway channel connections. Some cards can be configured to support increments of eight channels to support the maximum T1/E1 carrier channel capacity. Like IP station cards, the most common voice codecs supported by IP trunk cards are G.711, G.723.1, and G.729/A. Although IP-PBX control signaling may be proprietary across a private network of like IP-PBX systems, to support feature transparency functions, almost all systems attempt to comply with the latest version of ITU-T H.323 standards. H.323 compliance supports network connectivity within a multivendor network of different IP-PBX systems.

An IP trunk circuit card can be used for private networking configurations as an alternative to other PBX circuit cards: analog E&M tie trunk, T1/E1 interface, and ISDN PRI interface. It handles outgoing IP trunk calls and is used for incoming IP trunk calls to non-IP stations. Incoming IP trunk calls directed by the terminating IP-PBX gatekeeper to non-IP stations, such as analog telephones, require TDM connectivity. The IP trunk circuit card supporting outgoing IP calls by non-IP stations performs a reverse gateway function for incoming IP calls to non-IP stations. Incoming IP call voice packets are decompressed, reformatted into PCM signals by the IP port interface card, and transmitted across available gateway channels onto a local TDM bus for circuit switched connections to the called non-IP station.

The first release of each supplier’s converged IP-PBX system was designed with dedicated IP station and IP trunk interface cards, the same approach used for traditional TDM/PCM ports. Avaya was the first converged IP-PBX system designer to develop a universal IP port interface for IP stations and IP trunk circuit connections. The Avaya IP Media Processor Interface’s gateway resources are available for station and trunk calls. Pooling the gateway resources for all types of IP endpoints provides greater flexibility in traffic engineering design and can reduce customer hardware costs. Alcatel’s OmniPCX 4400 originally required different IP interface cards for station, trunk, and remote cabinet support, but the latest system release supports a universal INT-IP card that replaces the three dedicated INT-IP cards. A universal IP port interface card likely will be a design trend followed by most converged IP-PBX system suppliers.

To summarize, there are four categories of IP trunk calls:

1. IP station to IP station (IP trunk circuit cards are not usually required)

2. IP station to non-IP station (IP trunk circuit card required at terminating IP-PBX to perform gateway functions)

3. Non-IP station to IP station (IP trunk circuit card required at originating IP-PBX to perform gateway functions)

4. Non-IP station to non-IP station (IP trunk circuit card required at originating and terminating IP-PBXs)

The major benefit of an IP trunk call is potential cost savings due to the reduced requirement for private line carrier facilities. If the IP call is implemented with a codec using compressed voice packets, such as G.729/A, less bandwidth is required for the IP trunk call than when placed over circuit switched PSTN carrier facilities. Although the LAN/WAN requires continual traffic engineering to maintain an acceptable QoS level and may experience less than optimal QoS levels due to higher than anticipated transmission delays, a customer balances lowerquality voice communications service with reduced telecommunications expense. If QoS levels exceed a defined benchmark, IP calls can be overflowed to PSTN carrier facilities.

Some customers first test IP trunk calls for only the last category of calls, when the only non-IP station equipment placing and receiving calls are facsimile terminals, before using IP networking for real-time voice calls. The ARS software can be programmed to select IP trunk routes for only certain types of calls between specific station users. For example, conference calls typically require a very high QoS level and may be set up only with PSTN trunk circuits. Call types classified as less important may use an IP trunk route as the first choice, accepting the possibility that a diminished QoS level occurs during the call.