Chapter 3 - Before You Deploy
© Polycom, Inc. 53
Capacity Planning
H.323 traffic uses more bandwidth than the selected call quality or
H.320 equivalent. Polycom recommends that you allow 20%
overhead for the H.323 signaling traffic. ISDN networks do not
include signaling in the payload calculations, but in TCP/IP
networks all signaling must also be accounted for. For example, a
384-Kbps video call would require approximately 384 Kbps + 20% =
460 Kbps of bandwidth on a TCP/IP network. These figures assume
a full-duplex network.
If H.323 traffic starts out on a half-duplex network segment, it will
require twice the bandwidth indicated by the bandwidth
calculations described above. It will, however, take advantage of
full-duplex segments as soon as it reaches them. For full-duplex
segments, the calculations above remain valid. WAN segments (T1,
Frame Relay, ATM) are typically full-duplex.
The following equations will help in determining the bandwidth
required for H.323 traffic across various network segments:
❑ Full-duplex Ethernet = (Call Speed + 20%) x 1
❑ Half-duplex Ethernet = (Call Speed + 20%) x 2
❑ Wide Area Network = (Call Speed + 20%) x 1
❑ ATM (Using LANE) = (Call Speed + 35%) x 1
The table below provides a comparison between H.320 and H.323
point-to-point calls.
Table 3-2. Bandwidth Requirements
Call Quality
(Speed)
Bandwidth Required over
ISDN (H.320)
Bandwidth Required
over IP (H.323)
128 Kbps 1 Basic Rate ISDN (BRI) line 153 Kbps
256 Kbps 2 BRI lines 307 Kbps
384 Kbps 3 BRI lines 460 Kbps
512 Kbps 4 BRI lines 614 Kbps
768 Kbps Fractional T1 or full Primary
Rate ISDN (PRI) line
922 Kbps
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