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Installation and Upgrade Guide for Cisco Unified Videoconferencing 3522 BRI Gateway and 3527 PRI Gateway Release 5.6
OL-17010-01
Chapter 1 Functionality
About Cisco Unified Videoconferencing 3500 Gateway Functionality
About Gateway Call Bandwidth Overhead
According to the H.320 standard, the available bandwidth allocated to a call at any given bit rate will
always be slightly less than the stated maximum for the following reasons:
• All stated maximum call bandwidths include provision for control, audio, video, and data traffic.
• Video traffic on the ISDN side contains additional bits for error correction purposes which also
consume bandwidth. Video traffic on the IP side does not include this additional load.
• Opening an audio channel further reduces the bandwidth available to the video traffic.
For example, a call at 384 Kbps actually has only 363 Kbps available to it. Control and error correction
account for the remaining 21 Kbps.
Resource Allocation across E1/T1 Lines
The gateway can allocate bandwidth resources to calls across separate E1 or T1 connections to maximize
bandwidth capacity in cases where there is not enough capacity for a call on a single E1 or T1
connection, but where sufficient capacity does exist when remaining capacity on both E1/T1 lines is
combined.
For example, a gateway using two T1 lines can support three 6B calls on each T1 line, with 320 Kbps
spare capacity per line:
• Each T1 line provides 23 B channels.
• Each B channel supports 64 Kbps
• Each T1 line supports 23 x 64 = 1472 Kbps
• Each 6B call requires 6 x 64 = 384 Kbps
• Each T1 line supports 1472/384 = 3 6B calls + 320 Kbps spare
The gateway processes an additional 6B call requiring a further 384 Kbps by taking bandwidth resources
from each of the two T1 lines, both of which have 320 Kbps available. In this way, the gateway spreads
the call over both T1 lines.
About Peer-to-Peer Connectivity
The gateway supports the following types of connectivity to the IP network
• Through a gatekeeper
• Directly to a peer device such as Cisco Unified Communications Manager without the need for a
gatekeeper.