Delay; Jitter; Jitter Buffer - Avaya T3 Service Manual

General explanations
If sent via a G.729 Codec every 20 ms, the user data are 20 bytes per datagram plus
additional layer 2 bytes (Ethernet plus 18 bytes).

Delay

(delay between packet or speech packet length)
Delay times can be set using the IP telephone. These times indicate the time delay
before the transmitter dispatches the data into the network. Of course, the Codec can
continue to collect data during this delay time.
The transmitter generates user data of 160 bytes with a 64 kb/s Codec and a delay
of 20 ms.
Nominal Data Rate (bits/s) x Delay (ms)
------------------------------------------------------- =
8 (bits/byte) x 1000 (ms/s)
240 bytes of user data are sent ever 30 ms at a delay time of 30 ms.
Increasing the delay reduces the required bandwidth because the ratio of management
data to user data is less. Voice quality however is reduced because recipient obtains the
information later.

Jitter

The recipient should get every voice packet at the same time. Network topology could
cause the packets to arrive later however. This variation is known as inter arrival time or
jitter.

Jitter buffer

Using the management data contained in the RTP header, the jitter buffer
•
corrects the time interval between received packets
•
corrects the sequence of received packets
•
detects packets received twice
•
initiates algorithms in the event of packet loss
The packet loss rate in any given time interval should be lower than 1%. (This requires
network specialists.)
The transfer process selected on the Ethernet of the IP telephone is an asynchronous
data transfer process. The voice data transferred via the network should be heard
synchronously however. A jitter buffer is installed to bridge the gap between
asynchronous data transfer and synchronous listening. The jitter buffer delays the
arriving data packets so that they can be transferred to end-users synchronously. Jitter
buffers can usually offset runtime variations of up to 100 ms. Larger variations cause
the buffer to empty (buffer underflow). This can adversely affect the service to the same
extent as a packet loss. As a result, the network is required to limit packet losses in the
data network and runtime variation greater than 100 ms to considerably less than 1 %.
64000 x 20
---------------- = 160
8 x 1000
10