Specifications

Performance Tuning, and Monitoring 163
5.2.1 Efficiency versus Delay
The frame packing feature enables you to determine the trade-off between
efficiency (overhead) and delay. In general, each additional voice packet being
sent adds an additional 15 milliseconds to the end-to-end delay on the voice
circuit. Although this should not have a large impact on perceived voice quality, it
can create a large improvement on the efficiency of the trunk.
For example, in a frame relay network where transmission rate is 67 pps, the
actual bandwidth usage without any packing feature will be 8533 bps choosing
4800 bps vocoder rate. When packing two packets into one frame, the bandwidth
usage is reduced to 6666 bps, which represents 22% bandwidth saving. When
packing five packets into a frame, the bandwidth usage will be cut down 35% to
5547 bps.
These settings, while important when using a standard frame relay trunk, are
especially important when tuning the network for cell-based backbones. By
ensuring that the payload fits efficiently into the appropriate cell size, this setting
ensures a minimum amount of waste when inserting the frames into cells.
5.2.2 Digital T-1/E-1 Interfaces
The IBM 9783 provides direct T-1 and E-1 interfaces for voice ports which simplify
voice port tuning so that volume levels are consistent across all ports. In addition,
they eliminate unnecessary digital-to-analog conversions, thereby reducing
distortion while enabling users to select the most cost-effective method of
voice-channel termination.
5.2.3 Making Voice Adjustments
This section provides helpful information for setting voice levels, adjusting voice
gain levels, tuning analog voice levels, and setting echo return loss.
5.2.3.1 Adjusting Voice Port Levels
IBM 9783 units are shipped from the factory with voice attenuations (both
transmit and receive) set at -4 dB. You may need to adjust the transmit and/or
receive gains/attenuations on one or more voice channels to optimize voice
quality and volume.
Adjustment to these settings must be made with care because the voice quality
achieved is dependent on signal level. If the gains are set too high, the amplifiers
may be overdriven and the signal distorted; if set too low, the signal could
become lost in background noise.
Vocoder
Rate
Frame Relay Bandwidth Usage with Voice Frame Packing
1 Packet 2 Packet 3 Packet 4 Packet 5 Packet
4800 bps 8533 bps 6666 bps 6044 bps 5733 bps 5547 bps
7470 bps 11203 bps 9336 bps 8714 bps 8403 bps 8217 bps
9600 bps 13333 bps 11466 bps 10844 bps 10533 bps 10347 bps
16000 bps 19733 bps 17866 bps 17244 bps 16933 bps 16747 bps