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20 Engineering guidelines

Enterprise Edge 2.0 IP Telephony Configuration Guide P0911590 Issue 02

LAN engineering

Engineering the network for worst case numbers indicates the spare bandwidth a

LAN must have to handle peak traffic. It is important the LAN be planned to handle

the IP telephony traffic using the defined codec, without Ethernet delay or packet

loss. The installer or administrator must select one configuration and then set up the

LAN so there is more bandwidth than the IP telephony output.

Refer to standard Ethernet engineering tables for passive 10BaseT repeater hubs.

Refer to the manufacturer’s specification for intelligent 10BaseT layer switches.

Table 1 LAN and WAN IP bandwidth usage per Enterprise Edge Gateway (loaded to 36 CCS

per port per hour) with silence compression

Silence compression

If an IP gateway acts as a tandem switch in a network where circuit-switched trunk

facilities have a large amount of low audio level, enabling silence compression (also

known as Voice Activity Detection) degrades the quality of service, causing broken

speech. Under tandem switching conditions, with a large amount of low audio level,

disable the silence compression using the IP telephony interface.

With silence compression disabled, the bandwidth use of the LAN/WAN

approximately multiplies by two. Table 2 LAN and WAN IP bandwidth usage per

Enterprise Edge Gateway (loaded to 36 CCS per port per hour) without silence

compression on page 21 shows the full-duplex bandwidth requirements with

silence compression disabled.

Fax calls use a G.711 codec which does not support silence compression. Fax calls

require 64 kbit/s bandwidth.

Codec Type Packet

duration

in ms

(payload)

Voice/fax

payload in

bytes

IP packet

in bytes

Ethernet

frame

bytes

Bandwidth

usage

on LAN

in kbit/s

Bandwidth

usage on

WAN in

kbit/s

G.729

(8 kbit/s)

10 10 50 76 60.8 20.0

20 20 60 86 34.4 12.0

30 30 70 96 25.6 9.3

G.723.1

(5.3 kbit/s)

30 20 60 86 22.9 8.0

G723.1 (6.3

kbit/s)

30 24 64 90 24.0 8.5

Note 1:

LAN data rate is the effective Ethernet bandwidth use.

Note 2:

LAN kbit/s = Ethernet frame bytes*8*1000/Frame duration in ms

Note 3:

50% voice traffic reduction due to silence compression; no compression for fax.

Note 4:

Overhead of (RTP+UDP+IP) packet over voice packet is 40 bytes; overhead of

Ethernet frame over IP packet is 26 bytes.

Note 5:

Keep Ethernet bandwidth to support an Interframe gap of at least 12 bytes per

frame. This gap is not included in the above bandwidth calculation.

Note 6:

IP telephony uses a frame duration of 20 ms for G.729.

Note 7:

If interworking with an M1-ITG, other frame durations are supported (config-

ured on the M1-ITG).

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 75 76

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