Specifications

Engineering Guidelines
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Solutions that come from this example can then be covered by:
• Compression (G.729a) to the remote phones can be used to increase the voice channel
capability. However, it also reduces voice quality, which may not be acceptable.
• The WAN link bandwidth can be increased.
• The blocking ratio can be changed to P.01, and such a link would handle 68.8 CCS.
• The number of remote phones or the overall number of phones can be reduced.
• Ensure that QoS/Priority mechanisms are in place and active.
These are all potential solutions and each has to be investigated to understand the nature of
the installation. Doing this calculation before equipment is bought and installed ensures that
such issues are highlighted.
Assume that the routers have the capability of prioritizing traffic and the customer does not
want to use compression for trunk or internal calls. Thus, all calls will use the G.711 coding
scheme. The standard trunk blocking, P.01, is acceptable. The WAN link is over Frame Relay
and is a routed VPN (Layer 3).
• ErlangB compensation will be used to estimate the maximum expected number of “chan-
nels” required to handle the expected peak rate. (An under-provisioned link could result in
voice quality degradation.)
• 56.5 CCS results in 6 channels for voice at P.01 (using standard Erlang Tables).
• 6 channels at 83 kbps requires 499 kbps.
• Signalling adds an overhead of 10% taking the total to 550 kbps.
•The CIR for Frame Relay would be 550 kbps or 576 kbps, if rounded to the nearest 64
kbps. Rounding down to 512 kbps leaves minimum bandwidth during the busy period and
may result in slower signalling and degraded voice, as packets will be marked for discard
at the router if the CIR is exceeded.
• Ideally, the link should not be more than 70% utilized so the bit rate should be 784 kbps,
or 832 kbps when rounded to the nearest 64 kbps. Since fractional T1/E1 is often based
on larger boundaries, it is likely this would be rounded to 1.024 Mbps.
• The calculations are all based on the expected busy hour call traffic, and CIR is specified
to ensure adequate bandwidth is guaranteed during this period should the Frame Relay
network also be busy (people tend to make phone calls and answer e-mails at the same
time). Other (smaller) values of CIR could be specified, and may well work, but during busy
periods the response to features may be slow and voice quality may be affected.
IP networking and Use of Compression
IP networking allows the construction of larger systems, and the combining of systems in
different geographic locations into a single system.
If LAN/WAN connections exist between nodes, this medium can be used to pass traffic. A limit
on the number of conversations for this connection is programmed at installation. If the limit is