Sunday, May 31, 2009

QoS Recommendations for Avaya VoIP Phones

QOS facilities are required to insure that VoIP packets take transmission precedence over data traffic.  These same facilities insure that sufficient bandwidth exists for VoIP traffic so as to minimize packet delay, jitter and discard while preserving sufficient bandwidth for data traffic. 

Layer 3 (Router) – Since it is an IETF standard and it allows the priority markings to be placed on packets by the IP telephony devices themselves, the customer should pursue options that support Differentiated Services (DiffServ) for the Layer 3 routed environment at the network edges. When implementing DiffServ, the recommended value that should be set on all Avaya telephony equipment is a 46 which equates to “EF” (Expedited Forwarding). The Avaya Servers and IP phones should be configured to mark both call control IP packets and audio IP packets with a DiffServ value consistent with the network capabilities of the data infrastructure.  Once the telephony gear has been set to mark all packets with the DiffServ values, the network routers must also be configured to prioritize packets based on this packet marking.   

In cases where IP softphone is used, it may be necessary to prioritize packets based on RTP protocol, IP addresses or destination to MedPro IP addresses.  This is a result of limitations in the operating system being able to properly mark packets with appropriate markings.

In scenario where IP Telephony will traverse the WAN, and to provide optimal audio quality, the routers should be upgraded to support QOS.

Layer 2 (Switch) – In order to mark Layer 2 Ethernet frames on the IP telephony endpoints, as is optimal, usage on switched ports should be configured to support 802.1p/Q on the CLAN/MedPro and IP phone Ethernet interfaces.  802.1p/Q is the IEEE standard for Layer 2 Quality of Service.  The 802.1p priority identifier is customer selectable and the recommended setting is 6.  Most new Ethernet LAN switches have multiple hardware queues for priority frame processing and this support typically has to be enabled on the switches.

VLAN Separation - Avaya C-LAN, MedPro board, Gateway and IP phones should be deployed on their own VLAN to isolate them from network data traffic broadcasts.  The 802.1/Q VLAN identifier is customer selectable.  For IP phones an 802.1Q trunk should be configured on the LAN switch port, with the phone being on the designated voice VLAN and a PC connected through the phone switch base being on the native or designated data VLAN.  Avaya boards and Gateways should be on their own VLAN which is not an 802.1Q trunk with this VLAN being the native one for the switch port.

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