Voice over Internet Protocol [VoIP] is rapidly replacing landline phones, providing telephone calls to homes and businesses. Common residential home VoIP providers include Vonage, Skype and now Google Voice. Enterprises are replacing today's outdated digital systems with providers that offer services such as hosted PBX, hosted VoIP, IP PBX, and virtual PBX. If you plan to implement or have implemented VoIP, you should check the quality of service [QoS] priority of your network.
There may be many reasons for VoIP voice quality issues. Loss or prioritization of voice priority or QoS is a major source of VoIP issues. This article provides some background on where you need to perform QoS in your network and explains what you need to do.
QoS provides priority for VoIP packets at the network congestion point. For most homes and businesses, the most common network blocking point is where data traffic is transmitted from the local area network [LAN] to the Internet. The LAN can operate at 10Mb to 100Mb [million bits per second] or more. DSL and cable Internet services can be as fast as 256Kb [kilobits per second]. In essence, data is transferred from a 10-lane highway to a 1-lane highway. Since VoIP is very sensitive to latency, you can't wait until your turn to get into the 1st lane.
QoS is implemented by the router, which checks each packet and sends a specific packet to the head of the line. QoS can be implemented by a service type field in IPv4 or a virtual local area network [VLAN] or a differentiated service code point [DSCP] within the traffic class Octet in IPv6. For those familiar with Layer 7 of the network, the VLAN runs on Layer 2 and DSCP on Layer 3.
VLAN
A VLAN is a virtual subnet that creates a smaller broadcast domain within the LAN to ensure efficient use of bandwidth. For example, your VoIP phone can be combined in one VLAN to combine computers into another VLAN. Separate voice and data VLANs protect VoIP traffic from service interruptions caused by viruses or worm activity, as Ethernet switches may not be able to bridge IP traffic between VLANs.
The data VLAN of the general traffic carrying the Internet has the lowest priority. Voice VLANs have the highest priority.
DSCP
The DSCP relationship of the packet is categorized and marked as associated with a particular class. The DSCP-aware router implements a per-hop behavior [PHB] that implements a packet forwarding mechanism with each type of traffic. DSCP is the primary protocol used by routers to provide different levels of service.
Although the network can implement up to 64 different types of traffic, it is recommended to use some encoding in the DSCP field. Most networks use the usual default PHB, accelerated forwarding [EF], guaranteed forwarding [AF] and class selectors. EF PHB is ideal for applications that require low latency, low jitter and low packet loss for VoIP, video and other real-time services.
The main disadvantage of DSCP is that any application can mark EF bits and compete with mission-critical traffic on your network. For example, your company might use a virtual PBX instead of a traditional phone system. Your calls to the public switched telephone network must be horizontal to the Internet with low latency, jitter, and packet loss. The router between the LAN and the Internet sends packets marked with EF priority to the Internet. Employees in your company may use personal Skype, Google Voice or other video services that may mark EF bits and compete with your mission-critical VoIP traffic. Some routers can solve this problem by checking each packet and leaving the EF bit only for packets destined for a specific IP address.
Although harder to implement, VLANs can provide excellent voice priority if your computer and VoIP phone are different devices. The VLAN will prioritize all traffic within the subnet, including call signaling. DSCP is easier to implement, but less accurate. However, for smaller networks such as small businesses and homes, it is sufficient to install a router with DSCP traffic management.
Orignal From: How to configure Quality of Service (QoS) on a VoIP application
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