Avaya IP Telephony Průvodce konfigurací stáhnout pdf (Strana 154)

154 Efficient Networking

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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. Refer to the information under the headings below for details.

• “Bandwidth requirements on half duplex links” on page 155

• “Bandwidth requirements on full duplex links” on page 156

• “LAN engineering examples” on page 157

• “WAN engineering” on page 158

• “QoS Monitoring Bandwidth Requirement” on page 159

Engineer the network for worst-case numbers to indicate the spare bandwidth a LAN 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 38 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. 7 11

(64 kb/s) 30 240 280 81.6 75.2 77.1

G.729 (8 kb/s) 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*

G.723.1 (5.3 kb/s) 30* 20* 60* 22.9* 16.5* 18.4*

Notes:

indicates payload sizes used by Business Communications Manager 3.5 for transmission. Other values listed indicate

payload sizes that the Business Communications Manager 3.5 can receive.

2) Ethernet bandwidth includes the 14 byte Ethernet frame overhead plus a 12-byte inter-frame gap.

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