Cisco TelePresence Video Communication Server (VCS)

VCS Traversal and Non-Traversal Call License Usage

Document ID: 118872

Updated: Mar 31, 2015

Contributed by Cisco TAC Engineers.



This document describes the difference between a traversal and a non-traversal call on the Cisco TelePresence Video Communications Server (VCS) and the use of call licenses.

VCS Traversal Call License Usage

When a call is made and the VCS takes the media as well as the signaling, it is a traversal call and uses a traversal call license on that VCS. Here are some examples of traversal calls that require the VCS to take the media:

  • For a VCS Control, calls to or from a traversal server (known as Firewall traversal calls).

  • For a VCS Expressway, calls to or from a traversal client (Firewall traversal calls). Traversal clients include other VCSs, gatekeepers, Border Controllers, or traversal-enabled endpoints.

  • Calls that are gatewayed (interworked) between H.323 and Session Initiation Protocol (SIP) on the local VCS.

  • Calls that are gatewayed (interworked) between IPv4 and IPv6 addresses on the local VCS.

  • For VCSs with Dual Network Interfaces enabled, calls that are inbound from one LAN port and outbound on another.

  • An SIP-to-SIP call when one of the participants is behind a Network Address Translation (NAT), unless both endpoints use Interactive Connectivity Establishment (ICE) for NAT traversal.

  • Calls that have a media encryption policy applied.

  • Encrypted calls to and from the Microsoft Office Communications Server (OCS) Version 2007 or Microsoft Lync Server Version 2010, where the OCS/Lync back-to-back user agent (B2BUA) is not used. If the B2BUA is used, the B2BUA application always takes the media, but the call is not classified as a VCS traversal call and does not consume a traversal call license (it might still consume a non-traversal license if the VCS takes the call signaling).

Note: The Enhanced OCS Collaboration option key is required for encrypted calls to the OCS or Lync Server.

All other calls are non-traversal (local) calls.

Note: A call might be a traversal call on one VCS but a local (non-traversal) call on another VCS in the call route, dependent upon whether each VCS in the route is required to take the media.

Traversal calls use more resources than non-traversal calls, and the number of each type of call is licensed separately. The VCS has one license for a maximum number of concurrent traversal calls that it can take, and another for the maximum number of concurrent non-traversal calls. In order to increase the number of each type of call that is available on your VCS (or VCS cluster), you can purchase and install the appropriate option key. While every deployment is different, as a guideline, Cisco recommends that your system has a 10:1 ratio of registrations to concurrent call licenses.

VCS Expressway Non-Traversal Call License Usage

Usually, a VCS Expressway requires a traversal call license to route calls, even for calls between devices that are directly registered to it. This is because the endpoints that are behind Firewalls need the VCS Expressway to receive and forward the media in order to guarantee that the media is routable between the endpoints in the call.

The only situation where a call that passes through a VCS Expressway is classified as a local (non-traversal) call is when it matches any of these scenarios:

  • There is no SIP-to-H.323 interworking required.

  • There is no IPv4-to-IPv6 address interworking required.

  • The call is not routed from the VCS Expressway through a Traversal Zone.

  • Neither of the endpoints are traversal-enabled (neither of the endpoints has registered with Assent or H.460.18/19, and neither has made the call with a request for Assent or H.460.18/19).

  • The call is received from and sent to:

    • An endpoint or a Neighbor Zone that is directly connected to the WAN.

    • An endpoint or a Neighbor Zone behind a Firewall that has an SIP or H.323 Application-Level Gateway (ALG) that supports video calls, which makes the signaling appear as though the endpoint is directly connected to the WAN.

    • Locally registered, ICE-enabled endpoints (if VCS Version X5 or later is used).

Note: In order to aid Firewall traversal, all Cisco TelePresence endpoints are traversal-enabled, so a traversal license is always required when at least one of the endpoints that are involved in the call is a TelePresence endpoint.

Note: A non-traversal call on a VCS Expressway consumes a traversal license if there are no non-traversal call licenses available (if VCS Version X5 or later is used). In this scenario, the call remains a non-traversal call, and the VCS Expressway does not take the media, even though it uses a traversal license.

Updated: Mar 31, 2015
Document ID: 118872