How do I configure QoS settings?

The strategies discussed below pertain to traffic inside a Local Area Network (LAN) and between the hand-off from a LAN to a Wide Area Network (WAN). Any QoS set up on a WAN connection itself must be configured by your internet service provider (ISP).

Implement QoS

Physical Network Separation

  • A popular way to ensure network quality is to physically separate voice and data networks. This method involves using a dedicated WAN connection for voice only, and using separate WAN connections for data traffic.

Balancing or Policy-Based Routing

  • Another method for achieving QoS on a LAN is logical network separation. Networks can be separated into logical divisions or Virtual Area Networks (VLANs) to separate voice from lower priority traffic. This traffic balancing, or policy-based routing, can allocate bandwidth dynamically based on volume, or statically by manual assignment.
  • If you have multiple WAN connections you can configure your network to route your voice VLAN out one WAN connection and all other VLANs over another. In this scenario, saturation of the ‘data’ WAN connection is irrelevant, as all voice traffic is routed over its own WAN connection.

Class of Service / DSCP

  • Routers and gateways can be configured to honor Layer 3 DSCP values. Layer 2 802.1p/CoS values can also be used, though DSCP is preferred. As configured by GoToConnect, VoIP devices set a DSCP value in the header of each packet they generate, as shown in the following table:
    Traffic Type Traffic Purpose DSCP Value 802.1p/CoS Value
    SIP Call Control 26 5
    RTP Call Media (the media conversation) 46* 5
  • To enable QoS within a LAN and over a LAN-to-WAN hand-off, the network should be configured to prioritize traffic carrying those tags over all other traffic.
  • Implementing Downstream QoS (ingress) on your internet connection requires the cooperation of your ISP, as there is very little that can be done on a LAN to prevent saturation of the downstream link.

Test QoS

On higher-end LAN equipment, proper QoS policies can be verified by watching the traffic flows and/or queues. If this is not possible, a handful of empirical tests can also be used:

  • Saturate the LAN connection by transferring large files between computers on the LAN. Then make several concurrent phone calls.
  • Saturate the upstream portion of the WAN connection by uploading several large files. Then make several concurrent phone calls.