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Quality of Service (QoS)

What is QoS?

Quality of Service (QoS) refers to the prioritization of traffic throughout a network. In the context of Covoda Voice, this term refers to giving priority to voice and video transmissions over other traffic types like general internet browsing or file transfers. Particularly in scenarios where bandwidth is limited, properly configured QoS is crucial if maximum call quality is to be obtained. Properly implemented, QoS can reduce congestion, latency, and packet loss—all of which negatively impact call quality.

Configure QoS

There are several different ways to implement QoS, and network admins should use the option that best fits their network environment. 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). This is one of the primary motivators for purchasing internet access from Covoda Communications (Covoda Broadband), as QoS is applied across the WAN connection automatically.

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 Covoda Communications, 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

* DSCP value 46 is sometimes called EF (expedited forwarding) in some devices.

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. This is a major reason why many Covoda clients purchase their internet connection through Covoda Communications. Covoda is one of only a handful of providers that offer “end-to-end” QoS data solutions.

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:

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


Do I really need to implement QoS?

It is usually recommended, but whether or not it is needed depends on the nature of your network and how likely you are to saturate bandwidth on any one of your links.

For example:

  • A switched network environment typically means that QoS implementation on the LAN would be needed for large campus environments, but would be less likely needed for smaller offices.
  • Implement QoS on the egress WAN connection (hand-off) if upload bandwidth has the likelihood of being saturated. If not, avoid QoS as it could cause other traffic that is not voice to bottleneck.

What’s the best way to implement QoS?

The best way to implement QoS is to implement as many of the following as possible:

  1. Logically separate phones and other connections (data) through the use of voice VLANs.
  2. Use CoS (layer 2) and DSCP (layer 3) values. These are already tagged—it’s just a matter of implementing them on your equipment.

What QoS enabled router would Covoda recommend?

SimpleWan Router. Most other enterprise grade routers would work about as well, but implementation on a SimpleWan is easy, elegant, and it just plain works.