IPmux-1/1E Installation and Operation Manual Chapter 4 Troubleshooting and Diagnostics
Performance Monitoring and Troubleshooting Statistics 4-23
Table 4-6. Bundle Connection Status Parameters (Cont.)
Parameter Description
Jitter Buffer
Underflow
The IPmux is equipped with a “Packet Delay Variation Tolerance” buffer, also called a “Jitter Buffer”, responsible for compensating for IP networks delay
variation (IP jitter). The Jitter Buffer is configured in milliseconds units and exists for each bundle independently.
Explanation:
Packets leave the transmitting IPmux at a constant rate, but the problem is that they are reaching the opposite IPmux at a rate which is NOT constant, due to
network delay variation (caused by congestion, re-routing, queuing mechanisms, wireless media, half-duplex media, etc.). The TDM devices at both ends
require a constant flow of data, so they can’t tolerate delay variation. Therefore the Jitter Buffer is required in order to provide the TDM equipment with a
synchronous and constant flow.
This is done as follows:
Upon startup, the Jitter Buffer stores packets up to its middle point (the number of packets correlates to the buffer’s configured depth in milliseconds). Only
after that point it starts outputting the E1/T1 flow towards its adjacent TDM device. The stored packets assure that the TDM device will be fed with data even
if packets are delayed by the IP network. Obviously, if packets are delayed too long, then the buffer is gradually emptied out until it is underflowed. This
situation is called Buffer Starvation. Each underflow event increases the Jitter Buffer underflow counter by one and indicates a problem in the end_to_end
voice/data integrity.
The second functionality of the Jitter Buffer is that in Adaptive mode the Jitter Buffer is also a part of a mechanism being used to reconstruct the clock of the
far end TDM side.
An Underflow situation can be a cause of:
Buffer starvation: Packets delay variation causes the buffer to empty out gradually until it is underflowed.
Continuous Sequence Errors. The sequence error means a halt in the valid stream of packet arrival into the Jitter Buffer.
Packets are being stopped/lost/dropped.
Too small Jitter Buffer configuration that can’t compensate for the network delay variation.
When all system elements are not locked on the same master clock, it will lead to a situation in which data is clocked out of the Jitter Buffer at a rate different
from the one it is clocked into. This will gradually result in either an Overflow or Underflow event, depending on which rate is higher. The event will repeat
itself periodically as long as the system clock is not locked.
When an Overflow (see below) situation occurs, the IPmux instantly flashes the Jitter Buffer, causing a forced Underflow. So when you need to calculate the
real Underflow events and not the self-initiated ones, subtract the number of Overflows from the total number of Underflows counted by the device.