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Cisco Hoot and Holler over IP
Hoot and Holler over IP Overview
VC-832
Cisco IOS Voice, Video, and Fax Configuration Guide
IP Multicast and DSP Arbitration and Mixing
When deploying Cisco hoot and holler over IP, first consider how the voice streams are going to be mixed
and how they will be distributed to other locations. This is done using a combination of two technologies:
• IPmc
• DSP arbitration and mixing
Since hoot and holler is generally used to allow many people to simultaneously talk and listen to other
people within a hoot group, by definition it requires that the same speech be delivered to multiple parties
at the same time. In an IP network, this functionality uses IPmc. IPmc allows a source to send a single
packet into the IP network and have it duplicated and sent to many listeners by the other routers within
the network. This technique is beneficial in that it does not require the source to know how many
listeners there are, and the source does not have the additional processing burden of having to send a
copy of each packet to all listeners. IPmc also allows listeners to dynamically join IPmc groups, which
eliminates the administrative burden of new users needing to be added every time a new IPmc session is
initiated.
The individual router/gateway can handle mixing and arbitrating the various voice streams that can be
initiated or terminated on its voice ports. This functionality is handled by the onboard DSPs on each
voice card (NM-1V, NM-2V, or NM-HDV). Arbitration involves identifying the various sources of the
voice stream, and mixing involves taking some of those voice streams and combining them into a
single-sourced voice stream. Cisco hoot and holler over IP can handle many inbound voice streams, but
it only arbitrates and mixes three streams to be heard within the hoot group. This value works fine in
most applications because, with more than three streams, two things happen in normal conversation:
• People are not able to distinguish the content of more than three voice streams.
• People normally stop speaking if they hear others talking ahead of them.
Note The mixing functionality does not do a summation of the voice streams.
The DSPs in the Cisco hoot and holler over IP feature can mix up to three streams. The mixing of the
three streams is important to network administrators in considering how much bandwidth they should
plan for in their Cisco hoot and holler over IP networks. This is especially crucial when planning for
WAN bandwidth, which is often much more expensive and much less available than LAN bandwidth.
The advantage to this functionality is that a network administrator never has to be concerned about
provisioning voice bandwidth for more than three times each call’s bandwidth for each WAN site, which
helps to simplify overall network planning.
Bandwidth Planning
Four main factors must be considered with regard to bandwidth planning for
Cisco hoot and holler over IP:
• Codecs used for VoIP (G.711, G.726, G.729, and G.729a are currently supported).
• Bandwidth management techniques.
• The number of voice streams to be mixed.
• The amount of guaranteed bandwidth available on the IP network. This includes both LAN and
WAN bandwidth and should take into consideration other factors, such as Frame Relay committed
information rate.