While commonly playing the role of a Forwarder for VoIP traffic, there are possible issues that can arise from putting a firewall in line for SIP or H.323 Sessions.
This article will detail the common issues as well as how to resolve them on the SonicWall.
Issue - One Way Audio or No Audio
TIP:If the Public Branch Exchange (PBX) that the SIP Server communicates with is located behind the SonicWall then SIP transformations should be disabled in most deployments. Consult with your VoIP vendor.
TIP:If the PBX is located outside the SonicWall, usually on the public Internet, then SIP transformation should be enabled in most deployments. Consult with your VoIP vendor.
In order to enable or disable SIP transformations navigate to Manage|VoIP | Settings and examine the Enable SIP Transformations setting.
Issue - Audio Quality Degradation
NOTE: Both SIP and H.323 have poor tolerance for latent connections. Because of this it is often necessary to optimize latency related settings on the SonicWall and other in-line network devices.
Exempt the PBX and all IP Phones from Content Filtering.
Navigate to Security Services | Content Filter and exclude the necessary IP address ranges from Content Filtering.
Exempt the PBX and all IP Phones from Single Sign-On.
Navigate to Manage|Users | Settings and click Configure SSO.
On the pop-up window navigate to Enforcement. Select Add and include address object or service object for exclusion from SSO. TIP: It may be necessary to utilize an address or service object group in many deployments.
Select Full Bypass (Don't Trigger SSO) and Add. This will populate the entry in the SSO Bypass field. NOTE: It is also possible to Bypass SSO for an entire Zone, this is appropriate for deployments with a dedicated VoIP subnet.
Ensure that all VoIP Traffic flows over a single WAN Interface.
If multiple WANs are in use along with WAN load balancing, navigate to Manage|Network | Failover & LB. On the default load balancing group ensure that Source and Destination IP Address Binding is selected.
If multiple routes are used for VoIP Traffic, ensure that all routes are setup with appropriate probes to denote when the relevant route is up or down.
Configure Bandwidth Management and Quality of Service on the SonicWall