Information Technology Reference
In-Depth Information
transmission on a link. It is the sum of the serialization delays of all the packets scheduled
ahead of delayed packets. LFI is used as a solution for queuing delay issues. LFI is covered
in the next section.
Figure 14-24 shows variable delays.
CUCM
Queuing
Delay
Queuing
Delay
Queuing
Delay
V
V
MPLS
V
Variable
Packet
Sizes
Dejitter
Buffer
Figure 14-24
Va r i a b l e D e l a y s
Packets might not arrive at a constant rate because they take different paths and have per-
haps experienced congestion in the network. This variable delay is called jitter. The receiv-
ing end uses dejitter buffers to smooth out the variable delay of received VoIP packets.
Dejitter buffers change the variable delay to fixed delay.
As the traffic load on a network increases, both the probability of delay and the length of
the probable delay increase. The actual queuing delay depends on the number of queues,
queue lengths, and queue algorithms. Queuing effects in VoIP networks are covered in the
next section.
Ta ble 1 4 -1 3 summarizes the fixed and variable network delays, descriptions, and possible
solutions.
Ta b l e 1 4 -1 3
Network Delays
Fixed Delay
Description
Propagation
delay
6 ms per km. No solution.
Serialization
delay
Frame length/bit rate. A faster link and smaller packets help reduce.
Processing
delay
Depends on codec used: coding, compression, and packetization. Add
hardware DSPs.
Queuing delay
Va r i a ble p ac ke t s i z e s a nd n u m b er of p ac ke t s . Us e L LQ, C BW FQ, L F I .
Jitter
Caused by variable delay. Use dejitter buffers to make delay constant;
design as much as possible for an uncongested network.
 
 
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