Specifications

IPmux-1/1E Installation and Operation Manual Chapter 4 Troubleshooting and Diagnostics
Performance Monitoring and Troubleshooting Statistics 4-21
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. Underflow events initiated by the device (after an overflow
event) are not counted by the jitter buffer underflow counter.