Project properties

This box is used to specify the options associated with each project and is opened using the Projects | Properties command on the main menu.

There are three areas to the box. They can be selected by clicking on the tabs at the top of the box.

General

Blocking target

This is the blocking figure which Westplan uses when working out how many channels are required on each link. A reasonable blocking target is 0.01 which means that 1% of calls would be lost because not enough lines had been provided.

Busy hour factor

This is the percentage of the daily traffic which occurs during the busiest hour. A normal figure is 17%, although this can vary as business days lengthen beyond the normal eight hours. This parameter is only important if traffic figures are entered as daily minutes rather than Erlangs of busy hour traffic.

Default link type

This is the default transmission medium which new links are assumed to use. You can select between analogue, various aggregate formats and Voice over IP. After a link is added, its transmission medium can be changed.

Voice over IP

Voice activity

The percentage of time when active conversation is in progress. The parameter is only available for certain voice compression schemes which support the transmission of SID frames (silence frames) to reduce the bandwidth required.

CODEC

The voice compression scheme (or CODEC) in use. Several different CODECs are available. They differ in terms of their quality and bandwidth requirements.

Packet interval

The interval between the generation of IP datagrams. The lower the packet intervals, the more packets will be sent per seconds, and therefore, the higher the packet header overhead. However, high intervals should be avoided as they have a detrimental effect on voice quality.

Transport protocol

Westplan assumes that voice samples are sent using the IP, UDP and RTP protocols. The combined headers for these three protocols is 40 octets. This is a considerable overhead. Header compression schemes exist to reduce this overhead to 2 or 4 octets (depending on whether a checksum is required). The header compression scheme in use is selected using this parameter.

Control Protocol

RTCP is a protocol associated with RTP which provides feedback to voice packet senders concerning quality. RTCP payloads are normally sent within IP and UDP headers. The normal overhead of 28 octets can be reduced to either 6 or 2 octets (depending on whether a checksum is required) using header compression. The header compression scheme can be selected using this parameter.

Default datalink

This is the default layer 2 datalink protocol which new Voice over IP links are assumed to use. After a link is added, its datalink protocol can be changed.

Traffic

When a new node is added to a project, you are prompted to enter whether it generates any traffic and, if so, whether its traffic levels are low, medium or high. The default traffic figures used for new nodes can be altered by entering busy hour traffic figures in Erlangs into the three boxes in this section.