Voice over IP (VoIP)
Fundamentals
104
9.1.4 Optimisation
If you detect a large fluctuation in the propagation delay during measurement,
this may also cause the voice quality to deteriorate. This may indicate a defective
or overloaded line caused by bit-error or collision correction resulting from
retransmission by the transmission procedure.
An existing star-topology ethernet-network may uses a Hub as the central distri-
butor of ethernet packets. A Hub repeats all ethernet packets received on all con-
nected lines. This can cause substantial collisions and result in a high fluctuation in
the propagation delay.
If this is the case, use a modern switch component. Selective forwarding of
ethernet packets (“Layer 2 switching”) avoids collisions. Modern switch compo-
nents also evaluate the TOS byte of IP packets, thereby providing the optimal pre-
requisites for VoIP telephony.
Note:
The OpenCom 100 uses a TOS byte (“Type of Service”)
value of 0xB8 for IP packets with VoIP data. This requests “Mi-
nimise Delay” and “Maximise Throughput” for IP packets
marked with this value.
9.1.5 Call set-up
Various IP-based protocols are used for system telephony via the Internet protocol
(“IP”) (see also
Start Procedure
starting on page 112). Multiple TCP connections are
made between an IP telephone and OpenCom 100 for the telephone’s start pro-
cedure, registration and signalling.
Call data is directly exchanged between IP telephones using the RTP (“Realtime
Transport Protocol”) protocol.
Channels on a
Media Gateway (MGW)
are allocated for making a telephone con-
nection with an ordinary terminal or for dial tones. The MGW converts IP voice
data into PCM data streams used with conventional telephony and vice versa. For
this, IP voice data are exchanged between the IP telephone and the gateway.
Tip:
Switching between voice data channels may cause a slight
delay in some circumstances. For example: when accepting a
call on an IP telephone, headset users should wait about one
second before answering.
Summary of Contents for OpenCom 130
Page 1: ...130 OpenCom100 131 150 Mounting and Commissioning User Guide ...
Page 10: ...8 ...
Page 202: ...Index 200 Notes ...
Page 203: ......