In the current VoIP market, ITSPs who provide wholesale VoIP services use their own IP-to-TDM gateways to exchange calls with the PSTN. Problems occur when a wholesaler receives a call from an originating ITSP and decides to terminate the call to another ITSP. Because it does not own the PSTN gateways, the wholesaler does not receive call setup or release information and therefore cannot bill for the call. Wholesalers are forced either to forbid these connections, thereby foregoing a potential revenue source, or to set up the call through a combination of back-to-back IP-to-TDM gateways. This solution results in reduced quality due to double media coding and decoding, and it wastes TDM port resources.
Cisco Unified Border Element allows the wholesaler to terminate the call from the originating ITSP and then reoriginate it, thereby providing a point at which accurate call detail records (CDRs) can be collected for billing.
The superior interconnect capability provided by the Cisco Unified Border Element enables service providers to conceal their internal network and business relationships while improving call admission control, flexible routing, and protocol interworking capabilities.
The Cisco Unified Border Element includes the following changes to gateways and gatekeepers to allow Cisco UBE call legs:
- Support for H.323-to-H.323, H.323-to-SIP, and SIP-to-SIP connection types
- Support for transparent codec on H.323-to-H.323 connection types
- Support for H.323 call capacities
- Introduction of gatekeeper via-zones.
Via-zone is a Cisco term for a zone that contains Cisco Unified Border Elements and via-zone-enabled gatekeepers. A via-zone-enabled gatekeeper is capable of recognizing via-zones and sending traffic to via-zone gateways. Cisco via-zone-enabled gatekeepers include a via-zone command-line interface (CLI) command.
Via-zones are usually located on the edge of an ITSP network and are like a VoIP transfer point, or tandem zone, where traffic passes through on the way to the remote zone destination. Gateways in this zone terminate requested calls and reoriginate traffic to its final destination. Via-zone gatekeepers operate as usual for applications that are not Cisco UBE gatekeepers in via-zones support resource management (for example, gateway selection and load balancing) using the Capacities field in the H.323 Version 4 RAS messages.
The figure below shows a simple topology example of the Cisco Unified Border Element using via-zone gatekeepers.
Figure 1. Cisco Unified Border Element Feature Sample Topology
The gatekeeper in Domain A and the gatekeeper in Domain B are connected to the via-zone gatekeeper. GK408 and the via-zone gatekeeper exchange Registration, Admission, and Status (RAS) messages for the originating side. Then the connection is made between the originating gateway and the Cisco Unified Border Element. The via-zone gatekeeper exchanges RAS messages with GK919 for the terminating side. If the call is accepted, the Cisco Unified Border Element completes the connection from GW408 to GW919, and the media flows through the Cisco Unified Border Element.
In a basic call scenario, on receiving a location request (LRQ) message from the originating gatekeeper (GK408), the via-zone-enabled gatekeeper (GKVIA) processes the message and determines that the call should be set up using the Cisco Unified Border Element. After the originating gateway receives its admission confirmation (ACF) message, it sets up the call.
Cisco Unified Border Element feature, instead of the originating gateway signaling the terminating gateway directly, the Cisco Unified Border Element controls the call set-up both the signaling and media channel. The Cisco Unified Border Element is terminating the signaling and media channels, but the information associated with the media is propagated through to the opposite call leg. This process allows the endpoints to determine what media channel capabilities to use for the call. When the call is established, the audio stream flows through the Cisco Unified Border Element, meaning that the gateway terminates the audio channel on one call leg and then reorginates it to the other leg.
The following scenario illustrates a basic call from the originating gateway to the terminating gateway, using the Cisco Unified Border Element and gatekeepers.
- GW408 (the originating gateway) calls someone in the 919 area code, which is serviced by GW919 (the terminating gateway).
- GW408 sends an ARQ with the called number having the 919 area code to a gatekeeper in its zone (GK408).
- GK408 resolves 919 to belong to a via-zone gatekeeper (GKVIA). GK408 then sends an LRQ to GKVIA.
- GKVIA receives the LRQ for the 919 number. GKVIA resolves the 919 prefix to belong to the Cisco Unified Border Element. GKVIA is configured to route requests for 919 prefix calls through its Cisco Unified Border Element. GKVIA sends an LCF to GK408.
- GK408 returns an ACF specifying Cisco Unified Border Element to GW408.
- GW408 sends a SETUP message to Cisco Unified Border Element for the 919 number.
- Cisco Unified Border Element consults GKVIA with an ARQ message with the answerCall=true parameter to admit the incoming call.
- GKVIA responds with an ACF to admit the call. From the perspective of the gatekeeper, the first call leg has been established.
- Cisco Unified Border Element has a dial peer specifying that RAS messages should be sent to GKVIA for all prefixes. Cisco Unified Border Element initiates the resending of the call by sending the ARQ message with the
answerCall parameter set to, false to GKVIA for 919.
- GKVIA knows that prefix 919 belongs to GK919, and since the source zone is the via-zone, the GKVIA sends an LRQ to GK919.
- GK919 sees prefix 919 as a local zone and sends an LCF pointing to GW919.
- GKVIA returns an ACF specifying GW919.
- Cisco Unified Border Element sends a SETUP message to GW919 for the 919 call.
- GW919 sends an ARQ to GK919 to request admission for the call.
- GK919 sends an ACF with the answerCall=true parameter.
All other messages (for example, Proceeding, Alerting, and Connect) are created as two legs between GW408, and GW919, with the Cisco Unified Border Element acting as an intermediate gateway.