Zhone 2004 ユーザーガイド
A p p l i c a t i o n N o t e s
C-1
A
P P E N D I X
A
PPENDIX
A
A
PPLICATION
N
OTES
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, Voice should be given the
highest priority within the IAD (ATM Service Category “CBR” as shown on
the “ATM Service Category Configuration Menu” on page 4-8). Also, the
PCR value should be adequate to support all configured ports at each port’s
highest bit rate, and the jitter delay parameter should be set between 10 and
20 ms (refer to “Set Jitter Delay” on page 4-52). To establish the PCR, use the
following approach for VoIP applications:
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, Voice should be given the
highest priority within the IAD (ATM Service Category “CBR” as shown on
the “ATM Service Category Configuration Menu” on page 4-8). Also, the
PCR value should be adequate to support all configured ports at each port’s
highest bit rate, and the jitter delay parameter should be set between 10 and
20 ms (refer to “Set Jitter Delay” on page 4-52). 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.
encoding option.
•
Packet overhead is 49 bytes for PPP over ATM (includes RTP, UDP, IP
header).
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.
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:
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:
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