Voice over Internet Protocol (VoIP) is rapidly replacing landlines for delivering phone calls to homes and businesses. Common residential home VoIP providers include Vonage, Skype and now Google Voice. Businesses are replacing their now antiquated digital systems with providers delivering services called Hosted PBX, Hosted VoIP, IP PBX, Virtual PBX and many more. If you have plans to implement or have already implemented VoIP, you should review your network for Quality of Service (QoS) prioritization.
VoIP voice quality problems can have many causes. Missing or inadequate voice prioritization or QoS is a primary source of VoIP problems. This article provides some background on where QoS needs to be enforced in the network and gives an explanation on what needs to be done.
QoS provides priority to VoIP packets at network choke points. The most common network choke point, for most homes and businesses, is where the data traffic transitions from the Local Area Network (LAN) to the Internet. The LAN can be running at speeds of 10Mb to 100Mb (million bits per second) or more. DSL and cable Internet services may have speeds out to the Internet as low as 256Kb (thousand bits per second). In essence, the data is transitioning from a 10 lane superhighway to a 1 lane road. As VoIP is very sensitive to delays, it must not wait for its turn to enter the 1 lane road, your Internet connection.
QoS is put into effect by a router that examines each packet and ships specific ones to the head of the line. QoS can be accomplished through Virtual Local Area Networks (VLANs) or Differentiated Service Code Point (DSCP) inside the Type of Service field for IPv4 or Traffic Class Octet in IPv6. For those familiar with the 7 layers of the network, VLAN operates in layer 2 and DSCP is layer 3.
A VLAN is a virtual subnet that creates smaller broadcast domains within the LAN to ensure efficient use of bandwidth. For example, your VoIP telephones can be grouped together in one VLAN and your computers into another. Separate voice and data VLANs can protect VoIP traffic from service disruptions caused by virus or worm activity because Ethernet switches may not bridge IP traffic between VLANs.
The data VLAN carrying general traffic to the Internet is given the lowest priority. The voice VLAN is given the highest priority.
DSCP relies on packets being classified and marked as belonging to a specific class. DSCP aware routers implement Per Hop Behaviors (PHBs), which implement packet forwarding mechanisms with each class of traffic. DSCP is the primary protocol routers use to provide different levels of service.
Although a network can implement up to 64 different classes of traffic, certain encodings in the DSCP field are recommended. Most networks use the commonly defined PHBs of default, Expedited Forwarding (EF), Assured Forwarding (AF) and Class Selector. The EF PHB is for applications that require low delay, low jitter and low packet loss which makes it suitable for VoIP, video and other realtime services.
The primary disadvantage of DSCP is any application can mark the EF bit and compete with your network’s mission critical traffic. For example, your company may use a Virtual PBX instead of a traditional telephone system. Your calls to the Public Switched Telephone Network must transverse the Internet with low delay, jitter and packet loss. A router between the LAN and Internet gives packets marked with EF priority out to the Internet. Employees within your company may use personal Skype, Google Voice or other video services which may mark the EF bit as well and compete with your mission critical VoIP traffic. This problem is solved by certain routers that can inspect each packet and only keep the EF bit set for packets destined to specific IP addresses.
Although more difficult to implement, VLANs can provide superior voice prioritization if your computers and VoIP telephones are different devices. VLANs will prioritize all traffic within the subnet including call signaling. DSCP is easier to implement but is less accurate. However, smaller networks such as small businesses and homes, installing a router with DSCP traffic management is enough.