Nortel RLC, Installation And Administration Manual

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RLC Installation and Administration Guide 65
June 2005 Planning for RLC installation
The following table indicates one-way delay in ms for the common
configurations. It can seem unusual that the ISDN delay is larger than the IP
delay. However, this is due to the serialization delay of sending the packets
across the relatively low-speed (64K) channels provided by ISDN. (The IP
Network delay assumes high-speed Internet access as part of the intranet.)
Excessive delay results in users talking over each other (that is, both users
speaking at the same time). Studies vary as to when this becomes a voice quality
problem, but most of them suggest that this occurs between 120 and 180 ms.
Real-time data transmission produces two types of packet loss:
„
network-related packet loss (that is, packets lost due to either errors or
delays in transmission)
„
jitter buffer-related packet loss (that is, packets lost due to their being
thrown away when the jitter buffer is full)
In effect, there is a trade-off between jitter buffer delay (jitter buffer size) and
packet loss delay at the Remote Gateway 9100 Series unit. The relationship
between jitter buffer size and packet loss has a direct impact on the user’s
perception of voice quality.
Customer feedback and experience indicate that configuring the jitter buffer to
reduce delay produces better results. Delay, as introduced by the size of the jitter
buffer, is inversely proportional to lost packets. A jitter buffer configuration of
30 ms results in less delay, but a greater likelihood of lost (thrown away)
packets. Most customers find packet loss of 1% far more acceptable than an
increase in one-way delay beyond 180 ms.
IP Network
Delay
G.729A
Over
PSTN G.726
Over
PSTN G.711
Over
PSTN
Minimum recommended
jitter buffer setting
30 30 60 90
Algorithmic delay 30 30 30 30
Reasonable network delay 10 20 20 20
Total One Way Delay 70 80 110 140