Information Technology Reference
In-Depth Information
speed of light is the theoretical limit. A reasonable planning figure is approximately 10 ms
per 1000 miles, or 6 ms per 1000 m (6 ms per km). This figure allows for media degrada-
tion and devices internal to the transport network. Propagation delay is noticeable on
satellite links.
CUCM
Serialization
Delay
Serialization
Delay
MPLS
V
V
V
Processing
Delay
Propagation
Delay
Processing
Delay
Propagation
Delay
IP Phones
Figure 14-23
Fixed Delays
Processing delay includes coding, compression, decoding, and decompression delays.
G.729 has a delay of 15 ms, and G.711 PCM has a delay of 0.75 ms. The delay created by
packetization is also a processing delay. Packetization delay occurs in the process of wait-
ing for a number of digital voice samples before sending out a packet. Packetization delay
is the time taken to fill a packet payload with encoded/compressed speech. This delay is a
function of the sample block size required by the coder and the number of blocks placed
in a single frame.
Serialization delay is how long it takes to place bits on the circuit. Faster circuits have less
serialization delay. Serialization delay is calculated with the following formula:
Serialization delay = frame size in bits / link bandwidth in bps
A 1500-byte packet takes (1500 * 8) / 64,000 = 187 ms of serialization delay on a 64 Kbps
circuit. If the circuit is increased to 512 kbps, the serialization delay changes to (1500 * 8)
/ 512,000 = 23.4 ms. Data-link fragmentation using link fragmentation and interleaving
(LFI) or FRF.12 mechanisms reduces the serialization delay by reducing the size of the
larger data packets. This arrangement reduces the delay experienced by voice packets as
data packet fragments are serialized and voice packets are interleaved between the frag-
ments. A reasonable design goal is to keep the serialization delay experienced by the
largest packets or fragments on the order of 10 ms at any interface.
Va r i a ble del ay s a re
Queuing delay
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Jitter buffer delay
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As packets cross a network, they pass through several devices. At every output port of
these devices, it is possible that other voice and data traffic is sharing the link. Queuing
