CUCM Call Routing (Part 5): Gateways and gatekeepers

The NANP dial plan has been discussed in the past four blogs. By default, Cisco Unified Communications Manager (CUCM) does not know how to route any calls to the PSTN based on the NANP or any other International dial plan. All phone numbers that are external to the CUCM cluster must be programmed in CUCM. PSTN phone numbers are configured as Route Patterns in CUCM. These phone calls are normally routed out a gateway for PSTN access.

A gateway serves a very special purpose in Unified Communications. The Cisco gateway router converts the VoIP RTP (Real Time Protocol) to a TDM (Time Division Multiplexing) format. If the interface to the PSTN was an ISDN PRI (Primary Rate Interface), DSPs (Digital Signal Processors) on the router would convert the VoIP RTP media into a 64kbps B channel on the ISDN PRI. The call signaling event would be communicated to the service provider using the industry standard Q.931 signaling in the D channel of the PRI. Gateway routers support a variety of digital and analog telephony voice ports used to connect to the PSTN and traditional equipment including T1-CAS, T1-PRI (CCS), FXO, analog trunk, E&M, CAMA, and FXS.

Phone calls can also be routed over the IP network using a trunk interface configuration in CUCM. The word trunk in telephony has historically been used to indicate a trunk card on a PBX. The trunk card would normally connect to the PSTN using one of the traditional voice ports mentioned in the last paragraph. CUCM uses the word trunk in a completely different context. Trunk in CUCM refers to an IP Telephony destination. CUCM trunks allow toll bypass and least cost routing (tail end hop off) scenarios between locations that aids the company in saving money on long distance and International PSTN charges. These topics will be discussed in more detail in future blogs.

The four trunks types used in CUCM are as follows:

• Non-Gatekeeper Controlled Inter-Cluster Trunk

• Gatekeeper Controlled Inter-Cluster Trunk

• H.225 Trunk

• SIP Trunk

Non-Gatekeeper Controlled Inter Cluster Trunks (ICT) represent a logical binding between multiple CUCM clusters. A Non-Gatekeeper Controlled Inter-Cluster Trunk may point to up to 3 CUCM servers in the destination cluster. These trunks represent a very easy way to allow call routing over IP networks between multiple clusters, but they are unidirectional. If a customer has two CUCM clusters at two separate locations, each location must have trunks configured to point to the other site. The configuration of this trunk type can be quite cumbersome to setup and maintain in large environments with many CUCM clusters.

Gatekeeper Controlled Inter Cluster Trunks (ICT) are similar to Non-Gatekeeper Controlled ICTs, but they run this industry standard H.323 protocol so they can register to an H.323 gatekeeper as a gateway. Each CUCM cluster will only need one Gatekeeper Controlled ICT to get to each gatekeeper (multiple gatekeepers are recommended for redundancy purposes). The Gatekeeper configuration will provide the call routing information to route the call to the other CUCM clusters. The gatekeeper will provide the call routing capabilities, but the gatekeeper also performs the very important task of Call Admission Control (CAC). CAC is a mechanism that limits the number of calls that can be routed over the WAN in distributed call processing architectures. CUCM is not aware of link speeds or QoS priority queue (PQ) configurations in the WAN (wide area network). The CUCM server assumes that bandwidth is infinite. A T1 interface running at 1.544 Mbps looks like a giant 10 gigabit Ethernet interface to CUCM. The GK CAC configuration should align to the PQ configuration in the data network.

The next blog will cover the H.225 trunks, SIP trunks, and continue the call routing discussion.

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