Polycom VS4000 Manuale Utente

Chapter 3 - Before You Deploy

© Polycom, Inc.

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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

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