Verilink 8100A (34-00237) Product Manual - Manual (Page 294)

D-2 8 0 0 0 S e r i e s
Peak Cell Rate (PCR) Considerations and Recommendations
Voice-only Applications
The primary requirement to ensure toll-quality voice is to keep packet loss as
close to zero as possible and keep the round-trip-delay rate through the
network below 150 ms. You must employ judicious network design
engineering to control round-trip delay. To avoid packet loss, enforce timely
delivery, and thereby maintain high voice quality, Verilink recommends that
Voice be given the highest priority within the IAD (ATM Service Category
“CBR” as shown on the “ATM Service Category Configuration Menu” on
page 4-26). Verilink also recommends that the PCR value be adequate to
support all configured ports at each port’s highest bit rate, and that the jitter
delay parameter be set between 10 and 20 ms (refer to “Set Jitter Delay” on
page 4-79). To establish the PCR, use the following approach for VoIP
applications:
• Each ATM cell has (requires) 424 bits (53 bytes per cell x 8 bits per byte).
• Voice Payload is 160 bytes per packet for G.711 (derived by using 20 ms
encoding option.
•
Packet overhead is 49 bytes for PPP over ATM (includes RTP, UDP, IP
header).
•
A packet is generated every 20 ms (50 packets per second) to carry the
Voice payload plus overhead at the encoding rate for each PCM channel.
With this approach, the bandwidth requirement for an attached POTS port is
the following:
BW = 50 packets per second x 1672 bits per packet
= 83,600 bps
For an IAD with eight attached POTS ports, allocate 8 x 83,600 bps of
bandwidth on the link, or 668,800 bps. The PCR in this case would be the
following:
668,800 bps/424 bits per cell = 1577 cells per second
Voice and Data Applications
Set the PCR for the Voice PVC to the maximum for the link’s transmission
rate and set the PCR for the data PVC to zero. This allows the IAD to
dynamically allocate the bandwidth with Voice having the highest priority.
For example, on an SDSL link running at 512 kbps, set the PCR at 1208 for
the Voice PVC, and set the PCR for Data to 0 (zero).
Network Address Translation (NAT)
In an IP network, all devices must have unique IP addresses. NAT allows
multiple devices on the LAN port of an IAD to share Internet access via the
IP address of the WAN port of the IAD. With this configuration, Verilink
recommends that the LAN port of the IAD and all LAN devices use one of
the private IP address ranges as defined in Section 3, Page 4 of RFC 1918: