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VoIP Network Issues

VoIP Setup- Routers and Switches

Having more than one NAT/router in a network can
create issues that will affect your VoIP connection. You may experience issues right away, or they
may develop sporadically over a period of time. These issues can include
one-way audio and calls dropping for no apparent reason. Setting up your
network correctly can be beneficial for your VoIP quality and connection, as
well as the overall
stability of your LAN. We have added information on best practices,
including how to configure your second wireless router after adding a VoIP
Linksys RT31P2 ATA/router.

Routers, switches and other Devices:

Starting with the Internet connection you will usually find a DSL modem,
cable modem or another firewall/router device. These devices can have both
firewall capabilities and be NAT/routers, supplying DHCP addresses to the
internal LAN devices. The problem for VoIP is that they may use an
unfriendly NAT transversal or SPI (stateful packet inspection),
preventing the voice packets from making it back to the ATA correctly. (Netgear
routers use both a problematic NAT and SPI. You may need to place the ATA
into a
DMZ zone on a Netgear.

In most cases, if more than one NAT/router is between the
ATA and the Internet, this double NAT issue will create connection problems and
one-way audio. Add to that the fact that double NAT is not
considered good network practice, you will want to make sure to limit the amount
of devices supplying DHCP.

Many of our featured business VoIP providers below include a free setup and router with their service plans:

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Having problems with your VoIP connection then look at these possibilities:

More than one device with DHCP enabled between
the ATA and the public IP.

A device that uses NAT unfriendly SIP or SPI.

Firewalls blocking the UDP packets needed for the
ATA.

Most DSL modems act as a routers for the internal
LAN, while most cable modems do not. (This is the primary
reason why a cable VoIP installation will be much easier than a DSL install.) Some DSL modems and certain types of NAT/routers can use a SIP unfriendly
NAT. The result can be one way-audio, where the other party may hear you,
but you cannot hear any incoming audio. This is an issue caused by the
return voice packets stopping at the router, firewall or double NAT router and
never finding their way back to the ATA. If you suspect this, then:

Make your DSL a bridge and authenticate on the router or ATA/router using
PPPoE. If connecting a separate IAD device, then connect it directly to
the router.

Change any second NAT/routers to a switch and disable DHCP.

If you need to add a wireless or second router to your LAN and are using a
Linksys router/ATA, then the best configuration would be as follows.

Connect the Linksys/ATA (address 192.168.15.1) to the modem using the
Internet port of the ATA.

Connect a PC to your second router (address 192.168.1.1) and access the
internal setup pages. Disable DHCP and save settings.

Connect the second router via an Ethernet cable from a numbered port on
the ATA VoIP router to the second router's Uplink port (if this second router
does not have an uplink port then try a regular port).

One-way Audio:

VoIP protocols often will not function correctly in networks that include
some types of NAT transversal or firewall components. The result is usually
that calls can have one-way audio or fail.

One-way audio is usually the result from a double NAT issue or an
unfriendly NAT transversal being used on the network. VoIP itself
presents a problem for NAT, in that the normal protocols it uses deal with the
signaling and establishing of the connection. These packets are generally
identified by the NAT/router with the ATA device and will be forwarded
correctly. But, once the connection is established, the audio, which is an
entirely different protocol and can be on a port initiated from an outside
source, confuses the router. It does not recognize the incoming packets as
belonging to the ATA, and so they get dropped.

To alleviate some of these NAT issues some VoIP providers run Session Border
Control servers. These devices both handle traffic and shape the packets
to get past NAT transversal and firewalls.

How to troubleshoot one-way audio.

The first step would be to try to limit the devices between the ATA and the
Internet. To do this connect the ATA directly to the first device,
which would usually be a modem. (You may have to do a power cycle of the
modem to get the VoIP ATA to connect. more)
If after the ATA connects and establishes dial tone you have two way audio, then
you know that it works at that point in the network and you will need to locate
the device that is blocking the transmission further down line (usually a second
router).

If the VoIP ATA does not establish two-way audio connected directly to the
modem (first device), then look at these possible issues and solutions:

A DSL modem that uses an unfriendly NAT transversal and does not pass VoIP
protocols easily. In this case you should consider bridging the modem
and configuring the ATA/router or a VoIP protocol friendly router to use
PPPoE, with username and password.

Determine where the public IP address is sitting and if possible configure
the network so the ATA/router or IAD receives a public IP address.

Configure any firewalls to allow the correct ports needed to pass directly
to the ATA.

Place the ATA VoIP device in a DMZ or perimeter zone, where it will sit
outside the firewall.

networking tips

Linksys BEFSR41 Version 4
router

How to set QoS for VoIP
on this popular router. Although the QoS capabilities are not
as geart as we would like, having the ability to prioritize traffic to benefit
your VoIP connection is a big benefit.
[instructions]

One-way audio is usually the result of two or more NAT
routers or a router that uses a SIP unfriendly NAT.Other
possibilities can be a firewall that creates havoc with the voice packets and
they do not make it back to the phone.

VoIP itself presents a problem for NAT. The
normal protocols it uses (SIP) deal with the signaling and establishing of a
connection of the ATA. The router or firewall will not recognize the incoming voice
packets as belonging to the ATA, and so they get dropped.

Netgear routers can present problems for some VoIP
because of the type of NAT translations and SPI that it does. To
resolve issues with Netgear routers place the ATA into a DMZ zone.

If you are using two devices on the same network make sure that they do
not share the same IP address. This will lead to routing problems on your
network. For example if the two devices each have a LAN IP address of
192.168.1.1, the change the second device to 192.168.1.254, thus keeping it on
the same subnet, but stopping the IP conflict.

The most stable and best configuration for connecting a router/IAD would be
to have it connected directly to the modem, and any other router connected to it
with DHCP disabled. For full instructions see the following:

If you are using more than one ATA behind the same
router then you may want to set the port ranges on each ATA device so that they
will not conflict with each other.

Using multiple ATA's in the same location is usually not a problem if the
following conditions exist:

Each ATA/router has a public IP address.

Each ATA has a good Internet connection with sufficient bandwidth.

It's when multiple ATA's are sitting on the same internal network sharing one
Internet connection that special configurations and considerations are needed.
(For small businesses the first limitation will usually be sufficient
bandwidth.)