Quality of service or QoS is the ability to provide better or special service on a network to a group of users and applications at the cost of other users and applications.  QoS is not a tool , but a group of many tools which aim at giving complete control over how traffic flows in a network.  So at times a single QoS tool is deployed to decrease traffic delays or sometimes multiple QoS tools could be used to control delays, conserve bandwidth and do compression of voice data flowing over WAN.  Which QoS tool to be deployed would depend on network requirements of traffic such as bandwidth, delays of the traffic supported on networks.  The goal of QoS is to provide consistent bandwidth to voice traffic so we need to deploy QoS in some form at any point of the network where congestion exists.

QoS deployment could be complex, to ease use of QoS, Cisco applies AutoQoS, which allows and enables to use a variety of QoS mechanisms with much lesser complexity and knowing the technical intricacies of deploying QoS. AutoQoS uses a template based QoS configuration based on Cisco QoS best practices based on the bandwidth and encapsulation we have configured under router or switch interfaces.  Template reduces the deployment time, provides configuration consistency, and reduces cost of deployment and manual steps.

Cisco supports various queuing tools that enable to specify what type of network traffic is important and queue the traffic based on that information instead of concept of FIFO (First in – First out).   In Cisco Unified Communications, QoS components are provided through IP traffic management, queuing and traffic shaping of Cisco IP network infrastructure.

Some of the Cisco IOS features which address the requirements of end to end QoS for voice packet delivery are described as under:

  • Header Compression – It is used along with Real time Transport protocol (RTP) and Transmission control protocol (TCP) , it does compression of excessive RTP or TCP header resulting in reduced consumption of voice bandwidth.
  • Frame Relay Traffic Shaping (FRTS) – is another technique which delays excessive traffic using a buffer/queuing technique to hold packets and shape the flow during the time when data rate is higher than expected
  • FRF.12 (and higher) – to ensure voice traffic predictability and to provide better throughput on low speed frame relay network links by interleaving delay sensitive traffic on one virtual circuit (VC) with fragments of a long frame on another VC using the same interface.
  • Public Switched Telephone Network (PSTN) fallback – This does monitoring of calls based on status of congestion on the network and either routes the call to PSTN network or rejects them
  • IP RTP priority and Frame Relay RTP Priority – This gives a PQ scheme which permits delay sensitive data such as voice, to be sent on priority before other packets in the queue.  This is quite useful for WAN links such as frame relay, Multilink PPP and T1/ATM links.
  • IP to ATM Class of Services (CoS) – This maps the QoS characteristics between IP and ATM.  It gives differential service classes across WAN links and not just the routed portion.
  • Low Latency Queuing (LLQ) – This provides priority queuing on ATM VCs and serial interfaces. This enables to configure the priority status for a class within CBWFQ.
  • MLP – this permits large packets to be multilink encapsulated and fragmented, to be small enough to fulfill delay requirements of real time traffic.

Resource Reservation Protocol (RSVP) – provides reservation of resources across the network so that end systems can request QoS guarantees from the networks. RSVP provides call admission for voice traffic.  It also provides support for LLQ and frame relay.

QoS implementation ensures the ability to provide quality service to selected network traffic, VOIP guarantees high quality voice traffic only if signaling and audio channel packets have priority over other kinds of network traffic. QoS provides better service by implementing the following:

  • Support guaranteed bandwidth – design network in such a way that required bandwidth for voice and data is always available.
  • Improve loss characteristics – design the frame relay network and keeping voice below the committed information rate (CIR)
  • Advice and manage network congestion – ensure that LAN and WAN infrastructure support volume of data and voice calls
  • Shape network traffic – use Cisco traffic shaping tools to ensure consistent delivery of frames in WAN
  • Set traffic priority across the network – mark voice traffic as high priority and queuing it at priority

Voice features that support QoS are deployed at various network points to minimize jitter and delay.  Cisco IOS software has a complete set of features to deliver QoS on networks.  Cisco recommended mechanism.  For QoS on VOIP is LLQ on a router output interface.  It gives strict priority queuing (PQ) in addition with CBWFQ.  Priority status for class is set within CBWFQ in LLQ to give voice packets priority over all other network traffic.  We will take an example to illustrate this in figure 1.

Figure 1: Low latency queuing example

ccna voice qos voice quality

Voice traffic receives 256kbps of “priority” bandwidth and web traffic receives 128 kbps of bandwidth (if required). Voice traffic is transmitted first and voice traffic is also limited to 256kbps to avoid bandwidth starvation for web traffic.

We will take an example to illustrate how QoS is configured on a Cisco device.  In the said example we assume that we are using VLANs to separate voice and data traffic (voice – VLAN 101 and data on VLAN 100).

Step 1: Define access-lists to match our voice traffic. We will create 2 extended ACLs, one for the voice RTP traffic, and one for the voice signaling traffic.

For Skinny, H.323, MGCP:
!signaling traffic
access-list 101 permit tcp any any range 2000 2002
access-list 101 permit tcp any any eq 1720
access-list 101 permit tcp any any range 11000 11999
access-list 101 permit udp any any eq 2427

!RTP traffic
access-list 102 permit udp any any range 32767

!signaling traffic
access-list 101 permit udp any any eq 4569
access-list 101 permit udp any any eq 5036
access-list 101 permit udp any any eq 5060

!RTP traffic
access-list 102 permit udp any any range 16384 32767

Step 2: Create class maps for each type of traffic:
class-map match-all voice-traffic
match access-group 102

class-map match-all voice-signaling
match access-group 101


Step 3: Create a policy map for the classes:
policy-map qos-voice
class voice-traffic
priority 240
class voice-signaling
bandwidth 16
class class-default


The policy-map on the router places all voice traffic into the Priority Queue, which is given 240kbps of bandwidth.  All signaling traffic is in a class-based queue with 16kbps of bandwidth.  And all other traffic is queued using weighted fair queueing.

Step 4: Apply the policy to the interface:
interface FastEthernet0/0
service-policy output qos-voice

This concludes CCNA Voice session on how quality of service or QoS helps to improve the quality of voice traffic on VOIP networks. We learned about different ways/ and techniques to improve voice quality / real time traffic.