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