What Are Traversal Calls - Cisco TelePresence Administrator's Manual

What are traversal calls?

A traversal call is any call passing through the VCS that includes both the signaling (information about the
call) and media (voice and video). The only other type of call is a non-traversal call, where the signaling
passes through the VCS but the media goes directly between the endpoints (or between one endpoint and
another VCS in the call route, or between two VCSs in the call route).
A call is "traversal" or "non-traversal" from the point of view of the VCS through which it is being routed at
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the time. A call between two endpoints may pass through two or more VCSs. Some of these VCSs may
just take the signaling, in which case the call will be a non-traversal call for that VCS. Other VCSs in the
route may need to take the media as well, and so the call will count as a traversal call on that particular
VCS.
The following types of calls require the VCS to take the media. They are classified as traversal calls, require
a traversal call license, and will always pass through the Traversal Subzone:
firewall traversal calls, where the local VCS is either the traversal client or traversal server
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calls that are gatewayed (interworked) between H.323 and SIP on the local VCS
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calls that are gatewayed (interworked) between IPv4 and IPv6 on the local VCS
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for VCSs with Dual Network Interfaces enabled, calls that are inbound from one LAN port and outbound on
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the other
a SIP to SIP call when one of the participants is behind a NAT (unless both endpoints are using
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NAT traversal)
encrypted calls to and from Microsoft OCS Server 2007 / Lync Server 2010 (requires the Enhanced OCS
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Collaboration option key) if the Microsoft OCS/Lync B2BUA is not being 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
Traversal calls use more resource than non-traversal calls, and the numbers of each type of call are licensed
separately. The VCS has one license for the maximum number of concurrent traversal calls it can take, and
another for the maximum number of concurrent non-traversal calls. You can increase the number of each
type of call available on your VCS (or VCS cluster, see
by purchasing and installing the appropriate
Note that a non-traversal call on a VCS Expressway will consume a traversal license if there are no non-
traversal call licenses available (in this situation, the call will remain a non-traversal call — the VCS
Expressway will not take the media, even though it is using a traversal license).
Cisco VCS Administrator Guide (X7.1)
Resource usage within a cluster
option key.
Reference material
ICE for
for more information)
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