Quality of Service (QoS) gives you control over how your bandwidth is used, prioritizing critical network traffic (such as streaming media) over less important communication (such as e-mail) and limiting the bandwidth used by specific applications. It can also help cut costs by getting more use of existing bandwidth and enabling network consolidation by combining your phone, videoconferencing and wide area network connections with your Internet connection while minimizing performance problems.
Routers forward traffic between networks. One of the biggest causes of network performance problems (specifically latency) is router queuing. Queuing occurs when a router needs to forward more traffic to a destination network than the network can handle at one time.
By default, most routers work on a first-in, first-out basis. This works well for Web browsing and e-mail, but if real-time communication sits in a queue for more than a few milliseconds, the quality degrades significantly. For example, if the total latency is more than 150 milliseconds, a Voice over Internet Protocol conversation will cut out.
Consider a small business with a 512-kilobits-per-second Internet connection. If a user makes a VoIP phone call while sending an e-mail, the router won’t be able to immediately forward all the traffic across the DSL connection. With first-in, first-out, some of the VoIP packets would need to wait behind the e-mail packets — adding latency and degrading the quality of the VoIP phone call. With QoS, as shown in the following figure, the router forwards all the high-priority VoIP traffic first, and sends the lower priority e-mail traffic only after the high-priority queue is empty.
Routers can distinguish between high-priority and low-priority packets by examining the Differentiated Services Code Point (DSCP) value in each packet. Unfortunately, previous versions of Windows didn’t give systems administrators control over the DSCP values, and almost all traffic had the default DSCP value of 0.
With Windows Vista and Windows Server 2008, you can use Group Policy settings to mark outgoing network traffic with DSCP values. With this capability, a QoS routing infrastructure, and a few QoS policies, database queries from your critical customer service application can soar past e-mail and Web browsing traffic.
You can also use Group Policy settings to throttle traffic from specific applications. For example, if your outgoing e-mail server is filling up your Internet connection every time you send a mass-mailing, you can throttle the e-mail server to use only half the bandwidth — even if your network infrastructure doesn’t support QoS. Policy-based QoS can be very specific, so you could throttle traffic bound for the Internet but allow traffic bound for computers on your high-speed local area network to use all available bandwidth.
QoS policies are focused on controlling outgoing traffic. While you have much less control over incoming traffic, you can also use Group Policy settings to throttle all incoming Transmission Control Protocol traffic for a computer. So, if downloads to your client computers are using too much bandwidth, you can tune Windows to slow down that incoming traffic.
QoS requires careful planning and coordinated efforts from systems and network administration teams. Before you create your policies, follow these high-level steps to create a plan to add QoS to your network:
Typically, QoS will work only on your internal network. Most Internet Service Providers either remove or ignore DSCP values.
To configure QoS policies, follow these steps:
While you don’t have direct control over incoming traffic, Windows can use TCP’s traffic control capabilities to slow down incoming traffic. Unlike the policy-based QoS you just read about, this change affects all applications on a computer (including any critical applications). Because of this limitation, you should leave this setting at the default unless you have a computer that’s slowing down your network by downloading large files.
To specify the inbound TCP throughput level, follow these steps: