Quality of Service – Why Do We Need It?

Understand CoS, QoS and VLAN interactions

Quality of Service – Why Do We Need It?

By the end of this blog post ‘Quality of Service – why do we need it’ you will understand how important and vital it’s the implementation of these elements in a VoIP Topology, and how VLANs help the voice traffic traverse from point A to point B.

Why is it important to configure QoS whenever implementing voice and video in conjunction with your data traffic?. When optimizing your voice/data infrastructure CoS (Class of Service), and QoS (Quality of Service) are a must. Here, I introduce you to the basics of QoS, and why we need it.

  • 1.-Introduction of QoS
  • 2.-Understanding CoS vs QoS
  • 3.-Things to watch for
  • 4.-Segregate your Voice and Data
  • 5.-Benefits and Drawbacks of VLANs


1.-Introduction of Quality of Service.

QoS was defined by the UTI in 2004. Having QoS is important not only in your local network (LAN), but in the cloud as well (service provider/WAN). Quality of Service is obtained only after a Class of Service (CoS) mechanism tags the voice traffic. When using both QoS/CoS policies, it will improve the voice traffic specially when congestion occurs, the voice traffic will be less affected. Voice packets get a tag of 802.1Q Trunking also known as 802.1p/Q, from the 46xxxSetting.txt file,  or the DHCP scope options 242 or 176.

If you are not using DHCP Options or pushing the 46xxxSettings file out the phones the only option left is the manual configuration, where the phones need to be statically be configured

2.-Understanding CoS versus QoS

Class of Service (CoS) is a classification method only. CoS does not ensure a level of Quality of Service (QoS), but is the method used by queuing mechanisms to limit delay and other factors to improve QoS. Most CoS strategies assign a priority level, usually 0–7 or 0-63, to a frame or packet respectively. Common CoS models include the IP ToS (Type Of Service) byte, Differentiated Services Code Point (DiffServ or DSCP, defined in RFC 2474 and others) and the IEEE 802.1p/Q.

Quality of Service (QoS) involves giving preferential treatment through queuing, bandwidth reservation, or other methods based on attributes of the packet, such as CoS priority. A service quality is then negotiated. Examples of QoS are CBWFQ (Class Based Weighted Fair Queuing), RSVP (RESERVATION Protocol – RFC 2205), MPLS, (Multi Protocol Label Switching – RFC 1117 and others).

CoS, or tagging, is totally ineffective in the absence of QoS because it can only mark data. QoS relies on tags or filters to give priority to data streams.

Things to watch for:

  • Latency  = Under 50 ms one-way
  • Jitter= Under 20 ms variation.
  • Packet Loss= less than 0.2% of packets
  • MOS (Mean Opinion Score) greater than 4.0
  • MOS:4.3= G.711 (64kb/s)
  • MOS:4.0= G.729 (8 kb/s)

3.-Segregate the voice and data traffic


Like the name estates, it is a Virtual LAN that partitions your Network Switch dividing your traffic between subnets. It also has the functionality of Tagging your Ethernet Frames. It creates a smaller broadcast domains in your L2 Network causing better processor performance.

4.-Benefits and Drawbacks of VLANs

I have listed 6 benefits of VLANs

  1. Easier to Troubleshoot
  2. When troubleshooting networking issues having VLANs will make easier to get to the root cause of the problem.
  3. Minimize your Broadcast Traffic
  4. Creates a new broadcast domain; broadcasting only on that subnet Not the entire LAN.
  5. Prioritize your traffic based on CoS levels
  6. Security – Directing traffic to designated VLANs

And Finally the three Drawbacks of VLANs are=

  1. Limited to switch capacity
  2. it needs L3 Routing
  3. High Maintenance/Management



Avaya IP Voice Quality Network Requirements


Question – When implementing VoIP networks, Do you use Access Ports or Trunks for the VLAN configuration?


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