/mcpIn the table, on a 64 Kbps line, a CS-ACELP voice frame with a length of 38 bytes (37+1 flag) has a
serialization delay of 4.75 ms.
Note: The serialization delay for a 53 byte ATM cell (T1: 0.275ms, E1: 0.207ms) is negligible due
to the high line speed and small cell size.
Queuing/Buffering Delay
After the compressed voice payload is built, a header is added and the frame is queued for
transmission on the network connection. Voice needs to have absolute priority in the router/gateway.
Therefore, a voice frame must only wait for either a data frame that already plays out, or for other
voice frames ahead of it. Essentially the voice frame waits for the serialization delay of any
preceding frames in the output queue. Queuing delay (ßn) is a variable delay and is dependent on the
trunk speed and the state of the queue. There are random elements associated with the queuing delay.
For example, assume that you are on a 64 Kbps line, and that you are queued behind one data frame
(48 bytes) and one voice frame (42 bytes). Because there is a random nature as to how much of the
48 byte frame has played out, you can safely assume, on average, that half the data frame has been
played out. Based on the data from the serialization table, your data frame component is 6 ms * 0.5 =
3 ms. When you add the time for another voice frame ahead in the queue (5.25 ms), it gives a total
time of 8.25 ms queuing delay.
How one characterizes the queuing delay is up to the network engineer. Generally, one needs to
design for the worst case scenario and then tune performance after the network is installed. The more
voice lines available to the users, the higher the probability that the average voice packet waits in the
queue. The voice frame, because of the priority structure, never waits behind more than one data
frame.
Network Switching Delay
The public frame relay or ATM network that interconnects the endpoint locations is the source of the
largest delays for voice connections. Network Switching Delays (ωn) are also the most difficult to
quantify.
If wide-area connectivity is provided by Cisco equipment, or some other private network, it is
possible to identify the individual components of delay. In general, the fixed components are from
Size
(bytes)
19.2
56
64
128
256
384
512
768
1024 1544 2048
38
15.83
5.43
4.75
2.38
1.19
0.79
0.59
0.40
0.30
0.20
0.15
48
20.00
6.86
6.00
3.00
1.50
1.00
0.75
0.50
0.38
0.25
0.19
64
26.67
9.14
8.00
4.00
2.00
1.33
1.00
0.67
0.50
0.33
0.25
128
53.33
18.29
16.00
8.00
4.00
2.67
2.00
1.33
1.00
0.66
0.50
256
106.67 36.57
32.00
16.00
8.00
5.33
4.00
2.67
2.00
1.33
1.00
512
213.33 73.14
64.00
32.00
16.00 10.67 8.00
5.33
4.00
2.65
2.00
1024
426.67 149.29 128.00 64.00
32.00 21.33 16.00 10.67 8.00
5.31
4.00
1500
625.00 214.29 187.50 93.75
46.88 31.25 23.44 15.63 11.72 7.77
5.86
2048
853.33 292.57 256.00 128.00 64.00 42.67 32.00 21.33 16.00 10.61 8.00
Page 8 of 18
Cisco - Understanding Delay in Packet Voice Networks
7/31/2008
http://kbase.cisco.com/paws/servlet/ViewFile/5125/delay-details.xml?convertPaths=1