4. Other Issues Affecting Voip - NEC UNIVERGE SV9100 Manual

24
3) Defines a Policy Map called VoIPPolicy
4) Creates a Class called VoIPClass and assigns this to the VoIPPolicy
5) Allocates 50Kbps of bandwidth to the VoIPClass
6) and 7) Determines that any data that does not match VoIPClass should be processed using the
"fair-queue" method (ie. No prioritisation)
7) Determines the amount of bandwidth available on the Serial interface – essential for the QoS
calculations
8) Applies the VoIPPolicy to any packets that exit the serial interface. This means that data being
received (input) will not use this policy.
1.4

4. Other Issues Affecting VoIP

4.1 Internet Based Connections
Internet-based connections are becoming increasingly popular. This is mainly due to the speed and
cost of xDSL and cable modem connections. For data applications, these types of connection are
generally acceptable. For Voice over IP applications there are several issues that should be taken
into consideration.
Asymmetric data rates:
On many internet based connections, there are different data rates for upstream and downstream.
For example 8Mbps down and 512Kbps up. This works well for internet access, as generally you
download files from the internet to your PC and transmit less information in the other direction. For
VoIP, speech uses the same amount of bandwidth in both directions, which means that the amount
of simultaneous calls cannot exceed the amount of "upstream" bandwidth available.
Contention:
Most internet based connections specify a "contention ratio". This is typically 50:1 for home users or
20:1 for business users. This specifies the number of users subscribed to a single connection to the
ISP. This means that you share the bandwidth with other users on the internet which means that the
speeds that you are quoted are not necessarily accurate - you will receive less than these figures.
Note that it is unlikely that all of the subscribers will be using a connection at any one time, so these
figures are not quite as bad as they first seem.
NAT:
Usually, the equipment that your ISP provides (cable modem, ADSL router, etc.) will use Network
Address Translation. This allows several devices to share one public IP address. The issues relating
to the use of NAT are outlined in "Firewalls and NAT" below.
VPN:
Due to the use of NAT, and non-routable IP addressing, it is necessary to implement a VPN solution.
This is outlined in "VPN Tunnelling" below.