Nortel VoIP Gateway, Configuration Manual

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154 Network engineering
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organizations the thresholds can be lower than those used in this example. In the event of link
failures, spare capacity for rerouting traffic is required.
Some WAN links can exist on top of layer 2 services, such as Frame Relay and Asynchronous
Transfer Mode (ATM). The router-to-router link is a virtual circuit, which is subject not only to a
physical capacity limits, but also to a logical capacity limit. The installer or administrator needs to
obtain the physical link capacity and the QoS parameters. The important QoS parameters are CIR
(committed information rate) for Frame Relay, and MCR (maximum cell rate) for Asynchronous
Transfer Mode (ATM).
The difference between the current capacity and the acceptable limit is the available capacity. For
example, a T1 link used at 48% during the peak hour with a planning limit of 85% has an available
capacity of approximately 568 kbit/s.
Network engineering
This section describes some network engineering criteria that you need to consider for your
system:
• “Bandwidth requirements on half duplex links” on page 155
• “Bandwidth requirements on full duplex links” on page 157
• “LAN engineering examples” on page 158
• “WAN engineering” on page 159
Engineer the network for worst-case numbers to indicate the spare bandwidth a LAN/WAN must
have to handle peak traffic. It is important to plan so that the LAN/WAN can handle the IP
telephony traffic using the defined codec without delay or packet loss. The installer or
administrator must select one configuration and then set up the LAN/WAN so there is more
bandwidth than the IP telephony output.
The following table provides bandwidth characteristics for the transmission of voice over IP for
various link types given codec type and payload sizes. The bandwidths provided in this table
explain the continuous transmission of a unidirectional media stream.
Table 26 VoIP Transmission Characteristics for unidirectional continuous media stream
Codec Type Payload Size IP Packet Ethernet B/W* PPP B/W FR B/W
ms Bytes Bytes kbit/s kbit/s kbit/s
G.711
(64 kb/s)
10 80 120 116.8 97.6 103.2
20 160 200 90.4 80.8 83.6
30 240 280 81.6 75.2 77.1
G.729
(8 kb/s)
10 10 50 60.8 41.6 47.2
20 20 60 34.4 24.8 27.6
30 30 70 25.6 19.2 21.1
G.723.1 (6.3 kb/s) 30 24 64 24.0 17.6 19.5