This field enables the router to best manage QoS (Quality of Service Assurance) so that adequate bandwidth is reserved for VoIP to ensure best voice quality. You should set the field to approximately 85% of the actual upstream speed (upstream is the speed which you can send to the internet as opposed to receive/downstream).
This setting is not necessary on ADSL VoIP routers as the router can tell the linespeed directly - it is mostly available on routers which connect to the Internet via some secondary modem or other connection.
NOTICE : This document and sub-sections are © SEG Communications and neither it or its component parts may be distributed in part or full without specific written consent. Information and products subject to change at any time without notice.