How to guarantee QoS for IP telephony and unified communications: a technical white paper examining the functional requirements of implementing and maintaining quality of service Published: 1 July 2007 © 2007 SAS Global Communications Limited. Reproduction of this publication in any form without express permission is forbidden. The information contained herein has been obtained from sources believed to be reliable. SAS does not accept liability for errors, omissions or inadequacies in the information or its interpretation. The opinions expressed in this document are based on judgements at the date of publication and are subject to change without notice. All Rights Reserved. Table of contents Introduction ............................................................................................................................... 3 Glossary ................................................................................................................................... 4 What is quality of service (QoS)? ............................................................................................. 5 Class of service- considerations ........................................................................................... 5 Defining class of service ....................................................................................................... 6 Implementing class of service on the network....................................................................... 7 Why is QoS important to IP telephony and unified communications? ....................................... 8 Setting class of service parameters .................................................................................... 10 How to achieve QoS throughout your network ....................................................................... 11 Switches.............................................................................................................................. 11 Routers ............................................................................................................................... 11 Testing QoS and setting benchmarks for performance .......................................................... 13 The MOS test ...................................................................................................................... 13 Maintaining QoS and policy enforcement. .............................................................................. 15 Conclusion .............................................................................................................................. 16 About SAS .............................................................................................................................. 16 List of Figures Figure 1-1 Class of service categories ............................................................................................................... 5 Figure 1-2 Class of service applications ............................................................................................................ 6 Figure 1-3 How CoS works at an IP packet level ............................................................................................... 7 Figure 1-4 QoS-enabled infrastructure .............................................................................................................. 8 Figure 1-5 Standard data ................................................................................................................................... 8 Figure 1-6 Assured data .................................................................................................................................... 9 Figure 1-7 Expedited data ................................................................................................................................. 9 Figure 1-8 Dynamic demand allocation ........................................................................................................... 10 Figure 1-9 Infrastructure components requiring QoS enablement ................................................................... 11 Figure 2-1 QoS on the WAN ............................................................................................................................ 12 Figure 2-2 MOS testing ................................................................................................................................... 13 Figure 2-3 Monitoring QoS .............................................................................................................................. 15 Published: 1 July 2007 © SAS Global Communications Limited Page 2 of 16 Introduction When voice, data and video applications had their own dedicated networks, quality of service (QoS) played a fairly small role because each type of traffic behaved in a similar way; networks could be finely tuned to support that behaviour. In the converged network, QoS is critical to the success or failure of the network because the performance of applications like voice and video can become unacceptable if subjected to many of the common features of a best-effort IP data network, such as packet loss and delays. This technical white paper provides a detailed explanation of where QoS is required within a network infrastructure, to support IP telephony and unified communications. It also provides guidance on how QoS may be achieved; offering guidelines on establishing benchmarks for testing and measuring QoS on the network, as well as providing recommendations for maintaining and proactively monitoring its delivery. Published: 1 July 2007 © SAS Global Communications Limited Page 3 of 16 Glossary AF - Assured forwarding (video / applications) CoS - Class of service (AF/EF/standard) CODEC - Coder/decoder CODEC - Compression/decompression EF - Expedited forwarding (voice) IPC - IP communications – integrating applications into IP telephony IPT - IP telephony – voice traffic is 100% IP QoS - Quality of service UC - Unified communications – integrating all multimedia communications VoIP - Voice over IP – analogue traffic converted to IP at the router Published: 1 July 2007 © SAS Global Communications Limited Page 4 of 16 What is quality of service (QoS)? Quality of service is best defined as the capability of the network to guarantee priority and performance for different data types, ensuring smooth delivery of applications across the network. QoS is primarily achieved by applying class of service (CoS) functionality to the network infrastructure. Figure 1-1 Class of service categories EF AF Standard Figure 1-1 shows an example of six categories of CoS being used across a single network. The diagram incorporates three important categorisations: expedited forwarding (EF), used for voice-specific traffic. assured forwarding (AF), for primary applications standard, used for generic data. Class of service- considerations Currently, in most network environments, class of service is not used. The implications of this are that each network treats all traffic as generic data, according no priorities and, therefore, offering no time guarantee. The absence of such parameters results in the data being broken down into packets and potentially arriving in a non-sequential manner. This process takes time and causes delays. Occasionally, data is lost and has to be retransmitted. The timescale of the delay is, of course, miniscule, and of no consequence to many users. Depending on the application in use at the time, they may not even notice the delay. With voice and other primary applications, however, such delay can be a serious performance issue. Whilst a one-second delay in the delivery of an email may cause no problems, since the despatch of the email is never so precisely time-critical, a similar delay in voice traffic is, by any yardstick, unacceptable. It leads to callers talking across one another and creates a barrier to conversation clarity in a way that was common on transatlantic calls ten to fifteen years ago. Fluctuations in network activity can also cause problems when class of service is not present. Published: 1 July 2007 © SAS Global Communications Limited Page 5 of 16 Email and web browsing applications that may have been categorised as standard data, can generate sudden bursts of traffic capable of flooding the network for a short period of time. This can result in the delay of primary applications when routing through the network at the same time. Defining class of service Figure 1-2 Class of service applications EF AF . Standard Figure 1-2 shows an example of service categorisations working in a live environment, as follows: Expedited forwarding- used for voice traffic. Assured forwarding- used for primary applications; shown for illustrative purposes are a CRM application, a database and an ERP application plus multimedia in the form of video conferencing. Standard- used to describe traffic other than voice and primary applications; in this example, web-browsing and email. Class of service facilitates efficient and timely delivery of all data traffic through the network by assigning each application to a specified CoS category, thereby ensuring that delay, jitter and packet loss is kept to a minimum. The alternative simply does not offer such high network delivery standards. Traditional networks are able to differentiate between standard and primary applications through the use of packet shapers. Such products are not dynamic and typically divide the network into different segments and permanently allocate these segments to specific applications, whether or not they are used. Also in common usage are local area network and wide area network accelerators, often referred to as expand boxes; of limited functionality however, able to recognise different systems but only useable within the specified network. They become less effective when the traffic is handed off to a carrier’s wide area network, where there is no in-transit control mechanism. Expedited forwarding class of service (EF CoS) provides the highest level of priority, so is particularly suitable for voice traffic. There is negligible packet loss and data delivery is optimal. Published: 1 July 2007 © SAS Global Communications Limited Page 6 of 16 Assured forwarding class of service (AF CoS) has a high level of network performance but can experience some low level, and basic packet, interruption. Standard data does not need any priority; it fits into the background around high priority class of service traffic. Implementing class of service on the network. Figure 1-3 How CoS works at an IP packet level Layer 2 – 802.1Q/p Pream. SFD DA SA Type PRI CFI TAG (4 bytes) PT DATA FCS VLAN ID Three bits used for CoS Layer 3 – IPv4 Version Length ToS (1 byte) Len ID Offset TTL Proto FCS 7 6 5 4 3 2 1 0 IP-SA IP-DA DATA Unused bits; Flow control for DSCP IP Precedence DSCP Figure 1-3 represents the composition of a data packet at both Layer 2 and Layer 3 levels. At any one time, millions of data packets are moving around every network. The application itself automatically provides the relevant packet information, whilst configuration of the network infrastructure delivers the class of service as well as the quality of service mechanism. In the diagram, header information is carried at the front of the packet, for Layer 2 packets. The actual size of the data within the packet is relatively small compared to the overall size of the information associated with it. Of particular interest here, with regards to class of service, is the tag. (The tag is shown in blue and expanded in the middle of the diagram). The tag provides a simple identifier for class of service, recognised by routing devices to route the data to the correct location. This is made up from basic information, including the VLAN address identifying the data destination. At Layer 3 there is more segmentation of information on the packet and the proportion of the packet represented by data is even smaller. The header, or service identifier, used in Layer 3 provides more detailed IP information on how the data should be managed. The type of service (ToS) header ensures that each application has a unique identifier which, in turn, facilitates a far more granular level of management and routing capability. Published: 1 July 2007 © SAS Global Communications Limited Page 7 of 16 Why is QoS important to IP telephony and unified communications? A short sequence of diagrams serves to answer this question in a relatively self-explanatory fashion. Figure 1-4 QoS-enabled infrastructure Figure 1-4 shows a London/ New York office set-up with identical product sets in use at each location: voice runs over an IP platform and assured data is deployed for video conferencing; there is also a CRM application, a database application and an ERP application. Standard data is represented by web browsing and email applications. With a wide area network, when London users are in communication with each other, nothing passes across the network connection to New York. This means that two levels of quality of service are required, one on the local area network and the other on the 2MB wide area network which connects the two sites. Figure 1-5 Standard data If nothing other than standard data were being routed across the network, the traffic would simply utilise the available bandwidth; in such a situation, a large email being routed between the two offices, with no control mechanism, would use as much bandwidth as were available. Published: 1 July 2007 © SAS Global Communications Limited Page 8 of 16 Figure 1-6 Assured data As other applications are brought on line, class of service mechanisms are introduced to prioritise each of the application types; shown here are four assured data services, voice conferencing, CRM, database and an ERP application. The implication for standard data is that it is then condensed down to the remaining bandwidth. In this scenario, video conferencing is using 512K, and each of the other three applications (CRM, database and ERP) is using 256K. Each has an independently allocated amount of bandwidth. All assured data receives priority over standard data. Figure 1-7 Expedited data As the voice application is brought on line, it receives the highest level of priority, allocated by the expedited forwarding parameters for class of service. The voice in this scenario might be using 512K. This produces an aggregated total through the 2MB pipe of 512K for voice, 512K for video and 256K for each of the remaining three services. Voice and video together fill half the pipe. Web-browsing and email applications are throttled to a bandwidth allowance of just 256K, preventing any large email distribution from consuming all of the network bandwidth. Published: 1 July 2007 © SAS Global Communications Limited Page 9 of 16 Figure 1-8 Dynamic demand allocation Both the management of class of service over the network, and delivery of quality of service, are dynamic. As voice traffic drops, the other applications automatically take up the available bandwidth according to their class of service categorisation. Setting class of service parameters There are a number of areas where, in traditional networking, the requirements for class of service do not necessarily match a pure IP environment. Video conferencing demonstrates this point, where a significant difference exists between the old system and the new. Video conferencing over ISDN, using a system of 35 frames a second, would traditionally require four ISDN2 circuits, which is 8 channels or 512K bandwidth. The same performance using the H.323 video standard, the requirement for transmission over an IP network, would need 768K bandwidth. There is a big difference in the way in which the bandwidth is allocated between ISDN and IP. Because the class of service and the way data is handled across the network is dynamic, IP is much more flexible than ISDN, which locks out all channels for the duration of the video conference, If, in a video conferencing scenario, all participants were sitting fairly still with not much conversation occurring, then no more than 128K would be needed for the majority of the session. As people become more animated and there is more conversation, then the network will flex and push the bandwidth up to 768K as required. It is important to note that standard data can flood the network and stop other applications from working. This observation does not apply to email and web usage alone. On a wide area network with no quality of service, other applications, such as back-up systems running over the wide area network or servers sending file data to other servers, can also flood the network. This potential danger also arises through continual anti-virus updating, causing considerable problems that impede the other applications for which quality of service needs to be available. Published: 1 July 2007 © SAS Global Communications Limited Page 10 of 16 How to achieve QoS throughout your network Figure 1-9 Infrastructure components requiring QoS enablement WAN service Carrier router Firewall PSTN / BRI / PRI VPN Voice gateway router WAN router Default gateway Application server SAN Application server Email server Switch Printer Switch gateway WiFi switch WIP phone UM server Call control server Switch gateway IP phone Laptop Figure 1-9 shows the typical components of a network infrastructure. At the most basic layer is the switch infrastructure. This is where the packet information is processed; the detail and break-down between the Layer 3 packet and the Layer 2 packet. Switches The ideal switch infrastructure is a Layer 3 switch. This ensures a faster path from the device through to the ultimate destination. Reliance on a Layer 2 switch configuration, if the IP subnet, or even the device, is not present, on that switch, means that the data could end up at a router and require further multiple hops before reaching its final destination; the whole process being noticeably slower than directly switching out through the Layer 3 device. Attention should also be paid to WiFi switches, particularly with regard to voice. With the increasing use of mobile voice WiFi 802.11 standard WiFi technology, a WiFi access point is also a switch. Not all wireless access points support the voice standard required for WiFi voice. Radial distance, or bandwidth, required for voice is shorter than that required for standard data. If access points were in place to provide coverage for the whole office, and voice traffic was routed over that area, additional access points may be required; or at least tuning of those in place to provide better coverage. Routers Figure 1-9 shows three routers indicated by the Voice, default and WAN gateways. The routers manage any information which cannot be handled by the switches; in the case of a Layer 3 switch, the gateway will pass the information to the intended destination. Published: 1 July 2007 © SAS Global Communications Limited Page 11 of 16 The two most likely routes for that information are either via a voice gateway router to the outside world, or via the wide area network router to other company locations. In the former situation the information would be passed to the PSTN dial network through a primary rate or basic rate ISDN. At this point any control over quality of service is handed off to the carrier network. In the latter situation, where the router passes information to the wide area network router, it is necessary for users to put their own control mechanisms in place. This practice ensures that the router devices support the auto class of service recognition. It also ensures that, as they pass the information out across the wide area network, the relevant wide area network carrier also supports the same class of service parameters. This means that, as the traffic flows from the local area network, across the WAN, to the final destination, class of service is maintained and equipment at the intended destination can receive the information, maintaining quality of service throughout transmission. Figure 2-1 QoS on the WAN Cisco voice-enabled router DSL or Fractional T1 Catalyst switch with Catalyst switch with Secure LAN features Secure LAN Gigabit or 10/100 Ethernet features PSTN Catalyst switch with inline power and Secure LAN features IP WAN Internet PSTN Desktops/laptops with third-party anti-virus software Cisco distributed CallManager platform Cisco voice application servers Main business location Cisco IP Phone XML apps or IPCC Express agent POTS phones Corporate servers Cisco voice-enabled router with firewall and VPN Dedicated connection Teleworker/remote access Branch Office DSL or Cable Broadband access modem Laptops/desktops w/Cisco VPN Client, Cisco Secure Agent and PC-based SoftPhone Fax Catalyst switch with inline power and Secure LAN features Cisco IP Desktops/laptops with Phone third-party anti-virus XML apps or software IPCC Express agent Figure 2-1 uses three scenarios to illustrate how quality of service might be achieved within a business environment. The main business location is where most of the local area network infrastructure would be located; where call control servers are installed for an IP telephony environment or where the main switch infrastructure is located and where the main outside routers would be connected. For the shortest path, the best possible network configuration, the switches will, ideally, be running Layer 3, the routers running auto-CoS and the shortest path is available from end device to the outside world and to the ultimate destination. A branch office can be connected in a number of ways. The optimum configuration would be either through an IP WAN, with class of service invoked, and across the carrier PSTN network, where quality of service can be guaranteed. With teleworkers or mobile workers, class of service is not possible over the Internet; quality of calls over the network can therefore not be guaranteed for those users. Published: 1 July 2007 © SAS Global Communications Limited Page 12 of 16 Testing QoS and setting benchmarks for performance Even if Layer 2 switches are replaced with Layer 3 switches, and devices are configured to offer the optimal routing path, ensuring that the quality of service is being achieved on the network is an ongoing challenge. The first step to overcoming this challenge is to establish a baseline for monitoring the performance and delivery of QoS on the network. The most effective way to establish a baseline is using a testing system known as mean opinion score (the MOS test) which covers all aspects of the network and is not limited to specific devices or components. The MOS test Figure 2-2 MOS testing Figure 2-2 illustrates the results from the MOS test report for a company with six offices, each of which has two IP addresses, shown on the left, which represent the wide area network router and the wide area network switch. From the subnet at the beginning, it can be seen that the address starts at 10.1.1, then 241 for the wide area network router and 253 for the switch. The rankings range from 5 for ‘excellent’ to 1 for ‘bad’; these are used to judge voice quality. The very top result is the overall score for the MOS test on that particular wide area network and network infrastructure. The performance is seen to be improving over time. A score of 4.34 is judged to be ‘carrier quality’ and users are always very satisfied with this level of service. Published: 1 July 2007 © SAS Global Communications Limited Page 13 of 16 3.6 is judged to be ‘business quality’ but some users might be dissatisfied with this score. 3.1 is where users are really dissatisfied. The example in Figure 2-2 shows an overall ‘fail’ ranking due to the 10.8.1. location, where both devices failed on separate tests. The first device, 1.240, failed on jitter, (refer to the third column, in green), and the second device, 253, failed due to loss of service. Looking above that at the 240, it will be noticed that there were delays caused by jitter and loss. On further analysis, it becomes evident that these failures were caused by the switch running at half duplex which, in turn, was linked with other devices running at full duplex; a mismatch in their configuration. Due to that simple error on one particular switch, the entire network failed the test and quality of service thus failed as well. When looking at quality of service across the infrastructure, every node throughout the infrastructure needs to be optimally configured and enabled for quality of service routing. This means that adjustments to the infrastructure may be required to reduce data paths, and network equipment may need to be upgraded to ensure that QoS can be delivered. Published: 1 July 2007 © SAS Global Communications Limited Page 14 of 16 Maintaining QoS and policy enforcement. Maintaining quality of service, and ensuring policy enforcement, are two prime responsibilities in ensuring an efficient and effective system. One of the most successful techniques is that of conducting long-term trending of quality of service across the network, so that MOS testing is deployed on a daily basis to maintain quality of service running across the network. Figure 2-3 Monitoring QoS Figure 2-3 shows the overall testing results for a network over a four-day period. At the top, in blue, is the overall MOS test result for the network. In green are shown the results for delay, in yellow the results for jitter and in aqua, the results for packet loss. The test was conducted over a four-day period (from 27th April), at the commencement of which the system failed, dropping below the acceptable threshold for quality of service delivery. Configuration changes were made through the first day, with demonstrable improvements; packet loss, jitter and delay were all reduced. By the end of the day a suitable level of service had been established. Nonetheless there were still peaks, outside the threshold parameters. On the afternoon of the 28th there was a spike in the delay which went above and beyond the threshold. By monitoring such occurrences longer term, user’s expectations can be managed; interim adjustments can be made to correct problem issues before the overall quality of service is lost on the network. Ongoing monitoring also permits remedial investigations if users complain of voice quality problems after the event. Long-term trending also serves as a useful tool for capacity planning for the future. If the inclusion of additional users on the network is being considered, or if a new CoS-dependant application is being rolled out, trending will enable realistic impact analysis. Published: 1 July 2007 © SAS Global Communications Limited Page 15 of 16 Conclusion There are nine foolproof reasons why quality of service simply cannot be ignored. Poor QoS configuration is the major cause of IP telephony and unified communications project failure. Under-optimised QoS causes end-user dissatisfaction. It has frequently proved to be the case, with organisations undertaking an IP telephony pilot or a small project, that the system does not performing to anticipated levels. Upon investigation it invariably emerges that the infrastructure is not supporting quality of service to deliver the voice quality required. Critical applications need a guaranteed packet delivery time. As well as voice, this can also apply to other critical applications such as the CRM, ERP, database and video conferencing systems. Quality of service may well require the replacement of the core infrastructure and/or at least upgrades to components and certainly the configuration of the network. QoS is not possible over the Internet. In the future, with IP6, this will be possible, however, the estimated delivery date for IP6 is currently the year 2015. QoS acceptance testing must be done prior to deployment. Acceptance testing is essential to ensure that the quality of service is there; simply having an up-to-date and correctly configured infrastructure is no guarantee that it will work. Change control is a pre-requisite for maintaining QoS. All networks evolve and change and the organisation must be fully and operationally cognisant of the fact. IT staff must fully understand QoS and its significance to performance. If separate personnel maintain the server infrastructure, with no responsibility overlap in the area of IP telephony, they may not realise the impact of plugging a particular device into a particular port on a switch. Quality of service will definitely be affected everywhere else. The duplex mismatch mentioned earlier in this white paper is a classic example of such an event. Supporting and maintaining QoS requires specialist tools. This was demonstrated with the MOS test results and also needs some comprehensive research and planning on how to support it within the infrastructure. About SAS SAS Global Communications is a major provider of managed and professional network services. The company provides consultancy and implementation services to design, build and manage converged IP networks for enterprises of all sizes. Since its inception in 1989, the company has successfully delivered more than 2000 highly commended solutions, to a broad spectrum of clients including many renowned brands, such as Coca Cola Enterprises, The Body Shop International and Millennium and Copthorne Hotels. For more information contact: SAS Global Communications Limited SAS House Blackhouse Road Colgate, Horsham West Sussex RH13 6HS Tel: +44 (0) 1293 851951 Fax: +44 (0) 1293 852200 Email: info@sas.co.uk www.sas.co.uk Published: 1 July 2007 © SAS Global Communications Limited Page 16 of 16