Specifications

Network Requirements and Preparation
Planning and Installation Guide 9 – 11
call, the user is informed that there is insufficient network bandwidth to complete the
call. The user can try again later or dial the external number of the other user.
NOTE If PSTN failover is enabled for a user extension, the user’s extension-to-
extension calls are automatically routed to the public switched telephone
network (PSTN) when there is insufficient bandwidth for an IP connection to
phone. If the call cannot be made it goes to the backup, which is usually voice
mail.
Spanning Tree Protocol
Spanning Tree Protocol (STP) is used by Ethernet switches and routers to determine if
there are multiple paths on the network between any two endpoints. You must disable
STP on any network port that has a ShoreGear or ShoreWare server connected.
Traffic Shaping to Reduce Bottlenecks
Given that more applications are requiring WAN bandwidth, the need to optimize is
increasingly important. This is particularly true for enterprises that want to deploy
voice over virtual networks where quality of service and traffic shaping are required.
With traffic shaping, it is possible to set policies that determine who or what gets top
priority. For example, by prioritizing the various flows of traffic, an administrator can
make sure that UDP (voice) traffic gets a higher priority than HTTP (web surfing)
traffic.
Echo Cancellation
Echo in a voice communication system is caused by signal reflections generated by the
electrical circuits called hybrids that convert between two-wire (shared transmit and
receive pair) and four-wire circuits (separate transmit and receive pairs). These
reflections cause the speaker’s voice to be heard in the speaker’s ear as delayed by many
milliseconds. Echo is present even in the traditional circuit-switched telephone
network, but since the delay in a local circuit-switched call is so low, the echo is not
perceivable. On a packet-based voice network, there is more delay, and the speaker will
perceive the echo if it is not properly cancelled.
The DSP software on the ShoreGear voice switches provides dynamic echo
cancellation. When a user places an extension-to-trunk call using an analog trunk on a
ShoreGear-120/24, ShoreGear-60/12, or ShoreGear-40/8 voice switch, the user’s voice
bounces off the initial four-wire to two-wire conversion in the analog trunk circuit,
then off the two-wire to four-wire in the central office, and finally off the called party’s
telephone. This echo returns from the central office and is cancelled by the echo
canceller on the trunk port of the voice switch. The echo from the called party’s phone,
however, is usually cancelled or suppressed by the central office. If this echo is not
cancelled, the user will hear himself or herself talking.
In the opposite direction, the external person’s voice bounces off the user’s telephone.
This echo returns from the telephone and is cancelled by the echo canceller on the
telephone port of the voice switch. If this echo is not cancelled, the external party hears
himself or herself talking. This same process of echo cancellation applies to extension-
to-extension as well as trunk-to-trunk calls.
The ShoreGear-12 (IPBX) and ShoreGear-Teleworker can cancel echo received up to 7
msecs after being sent. The ShoreGear-120/24, ShoreGear-60/12, ShoreGear-40/8,
ShoreGear-T1, and ShoreGear-E1 can cancel echo received up to 16 msecs after being
sent.