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Cisco Hoot and Holler over IP
Hoot and Holler over IP Overview
VC-833
Cisco IOS Voice, Video, and Fax Configuration Guide
Codecs
By default, Cisco IOS sends all VoIP traffic (media, using RTP) at a rate of 50 packets per second. The
packets include not only the voice sample, but also an IP, User Datagram Protocol (UDP), and RTP
header. The IP/UDP/RTP header adds an additional 40 bytes to each packet. The amount of bandwidth
each VoIP call consumes depends on the codec selected. The resulting bandwidths can be as follows:
• G.729 or G.729a = 3,000 bytes * 8 bits = 24 Kb/call
• G.726 = 6,000 bytes * 8 bits = 48 Kb/call
• G.711 = 10,000 bytes * 8 bits = 80 Kb/call
In addition to these calculations, network administrators must consider the Layer 2 headers
(Frame Relay, PPP, Ethernet, and so on) and add the appropriate number of bytes to each packet.
In Table 58, the assumption is that the payload size (in bytes) is 20-millisecond samples per packet with
50 packets per second.
The value of n is equal to the number of voice streams in a session.
The uncompressed bandwidth includes IP/UDP/RTP headers (40 bytes) in the bandwidth calculation.
Compressed RTP (cRTP) reduces the IP/UDP/RTP headers to between 2 to 4 bytes per packet. The
calculation of compressed bandwidth below uses 4 bytes for a compressed IP/UDP/RTP header per
packet.
Maximum RTP Control Protocol (RTCP) bandwidth is 5 percent of the total RTP traffic in a hoot and
holler session. Since the Cisco hoot and holler over IP application supports mixing of a maximum of
three voice streams, the RTCP bandwidth is limited to 5 percent of three-voice-stream traffic.
In addition to the above, Layer 2 headers (Frame Relay, Point-to-Point Protocol, Ethernet, and so on)
should be considered and added to the bandwidth calculation.
cRTP, Variable-Payload Sizes and VAD
Some network administrators may consider this amount of bandwidth per call unacceptable or outside
the limits of the bandwidth they can provide, especially in the WAN. There are several options that
network administrators have for modifying the bandwidth consumed per call:
• RTP header compression (cRTP)
• Adjustable byte size of the voice payload
• Voice activity detection (VAD)
IP/UDP/RTP headers add an additional 40 bytes to each packet, but each packet header is basically
unchanged throughout the call. Compressed RTP can be enabled for the VoIP calls, which reduces the
IP/UDP/RTP headers from 2 bytes to 4 bytes per packet.
Table 58 Bandwidth Consideration Table
Codec
Payload Size
(byte)
Bandwidth/ Voice Stream
(Kbps)
RTCP Bandwidth per
Cisco Hoot and Holler over IP
Session (Kbps)
Example—One Voice Stream in a
Session (Bandwidth in Kbps)
Uncompressed Compressed =(1)*n+(3) =(2)*n+(3)
g.729
20 24 9.6 3.6 27.6 13.2
g.726
80 48 33.6 7.2 55.2 40.8
g.711
160 80 65.6 12.0 92.0 77.6