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Engineering guidelines 23

P0911590 Issue 02 Enterprise Edge 2.0 IP Telephony Configuration Guide

Determining WAN link resources

For most installations, IP telephony traffic is routed over WAN links within the

intranet. WAN links are the most expensive recurring expenses in the network and

they often are the source of capacity problems in the network. Different from LAN

bandwidth, which is almost free and easily installed, WAN links, especially inter-

LATA and international links require time to receive financial approval, provision

and upgrade. For these reasons, it is important to determine the state of WAN links

in the intranet before installing the IP telephony.

Each voice conversation, (G.729, Annex B codec, 20 ms payload) uses 12 kbit/s of

bandwidth for each link that moves across in the intranet; a DS0 can support below

5 simultaneous telephone conversations.

Link utilization

The starting point of this evaluation is to get a current topology map and link

utilization report of the intranet. A visual inspection of the topology map indicates

which WAN links are expected to deliver IP telephony traffic. Also use the

Traceroute tool (see Measuring Intranet QoS on page 27).

The next step is to find out the current utilization of those links. Note the reporting

window that appears in the link utilization report. For example, the link utilization

can be an average of a week, a day, or one hour. To be consistent with the

dimensioning considerations (see Enterprise Edge VoIP Gateway bandwidth

engineering on page 17), get the peak utilization of the trunk. Also, because WAN

links are full-duplex that data services show asymmetric traffic behavior, get the

utilization of the link representing traffic flowing in the heavier direction.

The third step is to determine the available spare capacity. Enterprise Edge intranets

are subject to capacity planning controls that ensure that capacity use remains

below some determined utilization level. For example a planning control can state

that the utilization of a 56 kbit/s link during the peak hour must not exceed 50%; for

a T1 link, the threshold is higher, for example at 85%. The carrying capacity of the

56 kbit/s link can be 28 kbit/s, and for the T1 1.3056 Mbit/s. In some organizations

the thresholds can be lower than that used in this example; in the event of link

failures, there needs to be spare capacity for traffic to be re-routed.

Some WAN links can be provisioned on top of layer 2 services such as Frame Relay

and ATM; the router-to-router link is a virtual circuit, which is subject not only to

a physical capacity, but also a “logical capacity” limit. The installer or administrator

needs to get 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 ATM.

The difference between the current capacity and its 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 about 568 kbit/s.

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Avaya BCM 2.0 IP Telephony Manual de usuario Pagina 23