QUALITY OF SERVICE: KEY CONCEPTS AND TESTING NEEDS By Thierno Diallo, Product Specialist With the increasing demand for advanced voice and video services, the traditional best-effort delivery model is no longer adequate to attract and retain premium service customers, who are essential to profitable carrier services. As network traffic increases, events such as congestion and data priority become an issue and can seriously affect traffic flows and delivery. Service providers must guarantee a predetermined level of service, regardless of traffic levels. To do so, a set of mechanisms known as quality of service (QoS) is used to prioritize data and guarantee performance to support customer service-level agreements (SLAs). The objective of this application note is to introduce various concepts related to QoS and the testing needs for such mechanisms. Quality of Service Drivers Interconnected networks rely on devices such as switches and routers to forward traffic via links of different rates (e.g., 10/100/1000BaseT). As packets are forwarded to the next hop, they are queued in outgoing buffers of the used port until transmission is ready to occur. However, the links are sometimes saturated when too much data uses the same link to reach the next hop. These periods of congestion are usually due to the bursty nature of traffic in interconnected networks and sometimes to the restricted capacity of the link versus the amount of traffic that needs to use it. In such cases, network congestion occurs and packet loss, latency-inducing buffering or packet jitter become inevitable. The emergence of triple-play services, a combined offering of voice, video and data, is a major driver in the establishment of QoS in networks. As carriers trend toward these new services, they must ensure that their network can handle and prioritize these data flows to properly service them. Voice and video packets are notoriously sensitive to packet jitter and packet loss, so they must be treated accordingly. With proper QoS mechanisms, carriers can ensure that voice and video packets are recognized and prioritized. Different levels of service are offered via the QoS mechanisms. With proper configuration, QoS can ensure that specific flows match the SLAs negotiated with customers. s can therefore establish pricing structures based on different priority classes (i.e., platinum, gold, silver and bronze services) while maintaining a best-effort service for other non-critical services. Why Test Quality of Service? QoS is very important for both service providers and their customers. As mentioned previously, when carriers and service providers offer a service, they must guarantee a certain level of quality. The quality of that service depends on the pre-determined service contract or SLA. For example, some customers may have critical traffic and be willing to invest more to ensure that their traffic remains the highest priority on the network. Other customers may have less-critical traffic and therefore do not require uninterruptible, 24-hour service. Regardless of the agreement, failure to respect the QoS requirements of the customer contract often results in damage compensation by the service provider. For this reason, it is vital to properly test QoS, ensuring that SLAs are met under normal and congestion conditions. Next-Generation Assessment
Events That Affect Quality QoS aims to control the following network events: 1. Bandwidth Bandwidth refers to the data rate that can be used by a particular flow at a given time. Bandwidth is usually dependent on the line rate of the link, which means that there is a limited amount of bandwidth available for all flows and that they must share this bandwidth. When flows require more bandwidth than is available, QoS mechanisms are used to prioritize and provide availability to higher-priority flows while lower-priority flows are either queued or discarded. 2. Latency Latency is a measurement of the time delay between the transmission and the reception of a packet. Typically, this is a round-trip measurement, meaning that the calculation measures both the near-end to far-end and the far-end to near-end directions simultaneously. This measurement is critical for voice applications, in which too much latency can affect call quality, leading to the perception of echoes, incoherent conversation or even dropped calls. 3. Jitter Jitter is a measurement of the variations in the time delay between packet deliveries. As packets travel through a network to their destination, they are often queued and sent in bursts to the next hop. There may be prioritization at random moments also resulting in packets being sent at random rates. Packets are therefore received at irregular intervals. The direct consequence of this jitter is stress on the receiving buffers of the end nodes where buffers can be overused or underused when there are large swings of jitter. Video applications are especially sensitive to jitter as set-top boxes (STB) use and display video packets at regular intervals. Their buffers are designed to store a certain quantity of video packets that are then processed and displayed at a regular interval to provide a smooth and error-free image and sound quality to the end user. Too much jitter will affect the quality of experience (QoE) since packets arrive at different rates; those arriving at a fast rate will cause the buffers to overfill, leading to packet loss, while packets arriving at a slow rate will cause buffers to empty, leading to still images or no sound. Constant packet rate = no jitter Variable packet rate = high packet jitter Figure 1. Graphical illustration of jitter effects 4. Packet loss Packet loss is the loss of a packet of data. Packets can be lost for a number of reasons such as errors during the transmission or network congestion. Errors due to a physical phenomenon can occur during the transmission of the frame and will result in packets being discarded by networking devices such as switches and routers, based on frame check sequence (FCS) field comparison. congestion will also cause packets to be discarded, as networking devices must drop packets in order not to saturate a link in congestion conditions.
The Basics of QoS QoS implementation can be summarized in three stages: 1. Classification Classification is the first stage of a QoS implementation. It is a process by which incoming packets are identified and prioritized based on specific trigger characteristics. At this stage, packets are classified by flows, where a flow refers to a group of packets sharing the same attribute defined by specific triggers. These triggers can include: Layer 1 information such as incoming ports Layer 2 information such as VLAN ID or VLAN P-bit user-priority fields Layer 2.5 multiprotocol label switching (MPLS) information such as MPLS label or EXP/COS bits Layer 3 information such as IPv4 type of service (ToS) or differentiated services code points (DSCP), IPv6 traffic class and flow label fields Deep inspection for specific protocols such as transport control protocol (TCP) A combination of different trigger points such as specific VLAN and specific IPv4 DSCP 2. Shaping and policing Shaping and policing are stages where flows are manipulated in order to conform to the desired bandwidth and burst requirements. Policing will create a bandwidth ceiling where packets from a flow that has superseded its allocated bandwidth will be discarded. Shaping, on the other hand, regulates the number of consecutive packets from the same flow that can be transmitted and buffers packets exceeding the burst size requirements to allow other flows to use the link. Shaping creates a better overall use of the link but may lead to transmission delay due to the buffering process. 3. Scheduling The scheduling stage determines how a packet from a flow exits the QoS device. Its process queues packet and schedules when packets can exit the device according to their priority. Typically, scheduling functions are only used during congestion periods. Flow 1 Classifier Flow 2 Flow 3 Scheduling Classification of packets in flows according to specific triggers: - Port - VLAN ID - IP ToS Policing and shaping: Bandwidth limits and burst buffer Scheduling: Packet queuing during congestion phases Figure 2. Graphical illustration of each QoS stage
QoS Devices Different devices are used to implement QoS on a network. Devices such as switches and routers are typically used to perform QoS, as they handle traffic based on Layer 2 or Layer 3 information. A new class of device, the Ethernet demarcation device, provides dedicated QoS implementations in small units that can be installed at the edge of the customer network. These devices can inspect packets and enforce QoS policies without the constraints of performing extensive routing and switching functions. SONET/ SDH ing QoS implementation via a Layer 3 device INTERNET QoS implementation via a Layer 2 device QoS implementation via an Ethernet demarcation device Figure 3. QoS device implementation Testing Needs Testing a network s QoS mechanism requires specific tools in order to simulate real-life scenarios, including multistream traffic generation and monitoring, per-flow analysis and measurement of QoS controlled parameters. 1. Multistream capabilities Testing using a single test stream is appropriate to ensure that a flow cannot exceed a specific bandwidth rate. However, the true use of QoS lies in its ability to handle multiple flows simultaneously and in diverse situations. Any QoS testing must be performed with a certain number of flows in order to stress the QoS implementing device to scenarios that are as close as possible to real-life implementations. 2. Per-flow analysis A global picture is not the ideal tool for QoS testing, as results do not guarantee that parameters are properly enforced per flow. Per-flow analysis provides the ability to ensure that QoS profiles are properly enforced and helps determine the performance that can be attained for each flow under congestion conditions. 3. QoS statistics The ideal test tool should measure bandwidth, jitter, latency and packet loss, as these characteristics are usually controlled in QoS implementations. Testing should also provide service-affecting statistics such as out-of-sequence counts and any errors in the packets such as checksum errors.
The EXFO Solution EXFO, a leader in test and measurement, proposes different solutions to help test and troubleshoot networks with QoS implementations: 1. The AXS-200/850 Testing scenarios The AXS-200/850 Ethernet Test Set, in combination with other devices in loopback mode or another AXS-200/850 as a sink unit, can be used in typical test scenarios. In loopback mode, one AXS-200/850 generates and analyzes streams that are looped back by the far end unit in loopback mode. In a source-sink scenario, one AXS-200/850 is used as a generator while analysis is performed on another AXS-200/850. In this type of scenario, measurements are performed on the sink unit allowing for unidirectional testing and latency is not measured as this is a roundtrip measurement and traffic must be analyzed on the generator. Loopback testing Device in Smart Loopback returning traffic to the AXS-200/850 Ethernet demarcation device set in loopback mode and returning traffic to the AXS-200/850 Source-sink testing The AXS-200/850 as a stand-alone unit generating and analyzing traffic Figure 4. Testing scenarios with the AXS-200/850
The AXS-200/850 as a QoS testing tool Designed as a powerful handheld solution, the innovative AXS-200/850 provides easy and comprehensive QoS testing in the palm of the technician s hand. Multistream support: The AXS-200/850 can generate up to four flows simultaneously, with independent VLAN and IP parameters, as well as IPv4 ToS/DSCP fields. This allows a tester to simulate up to four services typically offered by service providers (i.e., platinum, gold, silver and bronze). The generator also includes a profile selector through which multiple predefined voice and video traffic settings can be selected to reduce configuration steps. Independent background stream selection and configuration Predefined profile selector IPTV and VoIP profile Full control over IP ToS/DSCP field for all streams Full VLAN control up to two layers Figure 5. Screenshots displaying the AXS-200/850 s multistream support
Simultaneous analysis of QoS parameters: From a single page, a user can quickly assess compliance to a QoS profile, as the test set can provide simultaneous jitter, latency, bandwidth, out-of-sequence and frame loss conditions. A quick glance at the AXS-200/850 s exclusive graphical multistream traffic generation gauges allows any technician to immediately and accurately interpret real-time thresholds. The innovative GUI uses speedometer-like images to display real-time test results that eliminate errors in data interpretation with clear, easily recognizable colorcoded graphics. Full line rate throughput, packet jitter, real-time latency, frame loss, out-of-sequence statistics and on-the-fly adjustment of throughput and frame size give technicians the power to quickly and easily determine if Ethernet and IP services are meeting their SLAs. Simultaneous throughtput, jitter and latency measurement with graphical pass/fail thresholds for quick determination if streams meet their QoS profiles - Frame loss - Out of sequence - Pause frame detected Real-time frame size and bandwidth modification for troubleshooting purposes Figure 6. Screenshot displaying the AXS-200/850 s simultaneous QoS parameter analysis Per-flow analysis: Users can quickly assess if policing and shaping per flow is correctly implemented by monitoring bandwidth per flow. Per-stream bandwidth analysis with min/max and average results Jitter and latency measurement Figure 7. Screenshot displaying the AXS-200/850 s per-flow analysis functions
2. The Packet Blazer FTB-8510B/8510G Testing scenario The Packet Blazer FTB-8510B/8510G Ethernet Test Modules can be used in loopback testing, in a source-sink scenario or in a monitor type test. Loopback and source-sink scenarios follow the same principles described above, that is using either a looping device or two devices. With the monitor mode, the Packet Blazer does not generate any traffic on the network but analyzes all traffic seen on the network. This type of scenario is ideal for a monitoring situation using the advanced traffic filter tool. Loopback testing Device in Smart Loopback returning traffic to the AXS-200/850 Ethernet demarcation device set in loopback mode and returning traffic to AXS-200/850 Source-sink testing The AXS-200/850 as a stand-alone unit generating and analyzing traffic Monitoring Packet Blazer is used as a monitoring probe via either a tap or a monitor port and analyzes traffic Figure 8. Testing scenarios with the Packet Blazer FTB-8510B/8510G Packet Blazer as a QoS testing tool EXFO s compact and portable Ethernet test solutions aid in the following QoS testing scenarios: Multistream support: Packet Blazer Ethernet test modules allow users to configure up to ten flows, each with independent addressing, VLAN and IP parameters. Users can simulate different flows will full control over QoS classification triggers such as VLAN (up to three layers) and IP ToS/DSCP fields. Figure 9. Screenshot displaying the Packet Blazer s multistream support
Advanced technology support: Packet Blazer Ethernet test modules support stacked VLAN up to three layers, MPLS and IPv6 protocol, ideal for next-generation network QoS testing. QoS measurement: Packet Blazer Ethernet test modules support bandwidth, packet jitter and latency measurement (via RFC-2544) for troubleshooting QoS-related issues. Advanced traffic filters: The advanced traffic filter, a powerful QoS tool, can be used to differentiate and collect statistics on flows. Up to ten different filters are available and each one can be composed of up to four triggers. Filter definition and selector: Up to 10 filters available with up to four trigger values per filter. Results per filter: Comprehensive results per filter including: - Real-time bandwidth measurement - Ethernet, IP error analysis - Frame counter Figure 10. Screenshot displaying the Packet Blazer s advanced traffic filter functions Conclusion Testing a network for QoS can seem like a daunting task, but with the proper tools, network operators can qualify and ensure that they meet the service that they sell to their customer. APPNOTE209.1AN 2009 EXFO Electro-Optical Engineering Inc. All rights reserved. Printed in Canada 09/02