WHITEPAPER SDH. Fibre Ducts ATM CAPACITAS SONET MPLS. Voice. The Challenges of Capacity Planning Multiple Services over a Single Infrastructure

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1 WHITEPAPER SDH Fibre Ducts ATM IP Wavelengths CAPACITAS Power Space SONET Voice MPLS The Challenges of Capacity Planning Multiple Services over a Single Infrastructure Dr. Manzoor Mohammed Chief Technology Officer Capacitas Ltd. 1 Introduction The advent of technologies such as MPLS promise more cost effective network solutions by using the same infrastructure to support multiple service types. However, the full savings cannot be realised without the use of more sophisticated capacity planning methodologies. The use of these sophisticated methodologies is more time consuming than those used for planning networks with a single service type and hence more expensive in terms of operational costs. This paper compares the capacity planning methodologies for multiple service types over single and multiple network infrastructures. The paper focuses on the capacity planning of links in network infrastructures. The paper doesn t take into account the capacity planning of the node resources within a network that support these links e.g. CPU, memory etc. The paper identifies the parameters that need to be derived and quantifies the complexity of deriving these parameters for service types over single and multiple network infrastructures. The paper concludes that the number of parameters that the capacity planner has to define and the derivation of these parameters are more complex in networks that carry multiple services over the same infrastructure compared with networks that carry a single service over a single infrastructure. This results in an effective capacity planning having an increased operational cost. The paper concludes that senior management needs to be aware that this increased operational cost is an investment, which reduces the network capital expenditure costs. Capacity managers also need to take into account the business needs in determining the appropriate level of capacity planning such that increases in operational cost do not exceed savings made in capital costs. (c) Capacitas

2 2 Traditional Architectures and Services This section discusses the services and architectures that exist and also the common capacity planning methodologies used for these services and architectures. The figures in this and the next section don t show the physical connectivity of the network components. Instead, the figures show a logical representation of different service types over a link. The link could be an IP, ATM or SDH link. The service types are defined as different traffic streams e.g.: Signalling traffic e.g. SS7, SIP, H.323 etc. These are used to manage streaming connections between two points in the network Real time traffic e.g. voice, video streams etc. Data traffic e.g. HTTP, FTP etc Example 1 Single Service over Single Infrastructures The example in Figure 1 shows an enterprise that runs a single service over a single link. The service is not as time sensitive as other services e.g. voice. Typically companies use these links to carry Internet traffic. Figure 1. Single Service over Single Link The most common capacity planning methodology for networks carrying a single service type is for the planner to monitor the link utilization. The planner augments links if the average utilization exceeds a predefined threshold i.e. threshold driven capacity management. The planner augments the size of the link or adds in another link. The capacity planner may set the threshold at a range of values depending on business drivers. Typically, capacity planners augment the link when the threshold figure is exceeded. These threshold figures tend to be based on guesswork rather than mathematics, business drivers or growth rates. The capacity planner may also look at other utilization measures in deciding whether to augment a link. These include the busy hour utilization, some percentile utilization or other advanced techniques. (See reference (iii)2 in section 6) 1 Data transfers can be sub-divided into differing priorities of services e.g. Bank ATM and Credit Card Authorisations are higher priority than FTP transfers

3 The important thing to note is that the capacity planner is only responsible for setting single threshold value before initiating an augment of this link. The determination of which parameter to use is relatively simple and the measurements of that parameter are also simple. Table 1 summarises the complexity of deriving and monitoring these parameters. The capacity planner is responsible for setting a threshold value for each link type in the network i.e. different link speeds E1, STM1 etc. Parameter Derivation Monitor Link Utilisation Threshold for each link type Low Low Table 1 Summary Table for Single Service over Single Infrastructure 2.2 Example 2 Multiple Services over Multiple Infrastructures The example in Figure 2 shows a company with three service types over three separate links. The service types are voice, videoconferencing and normal Internet traffic. Figure 2. Multiple Service over Multiple Dedicated Links The capacity planner uses a similar methodology for planning multiple service types over multiple links as he uses for planning a single service type over a single link. The capacity planner monitors the utilization of each link. If the link utilization exceeds a threshold then the planner augments the size of the link or adds in another link. The capacity planner may set different threshold values for the different services depending on their sensitivity to delay or packet loss. The determination of the threshold values for these service types depends not only on the business drivers but also the behaviour of the traffic. If the service type has bursty traffic characterization then the augmentation utilization threshold may be lower than that for less bursty traffic generated by other service types. The determination of the traffic characterization is difficult as it depends on the granularity of the time measurements i.e.

4 every second or every minute etc. This is dependent on the service type. If the capacity planner is interested in looking at the performance of a particular real time service type such as voice then he may look at measurements of the traffic rate at very fine time intervals e.g. seconds or milliseconds in order to spot any transient peaks in traffic rates and the duration of these peaks. However, if the capacity planner is looking at less time sensitive service types e.g. Internet traffic then he may look at the traffic rates over more granular time measurements e.g. 1 minute or 5 minute intervals. The important thing to note is that the capacity planner is only responsible for deriving a single threshold value which when reached will trigger an augment for the link. The complexity of deriving this threshold depends on the traffic characterization of the service type and the service levels of the service type. These derivations can be based on analytical models or they can be based on capacity planners experiences from measurements on the live network. The capacity planner is responsible for setting a threshold value for each service type and each link type in the network (i.e. different link speeds E1, STM1 etc.) Parameter Derivation Monitor Traffic Characterisation for each service type High High Link Utilisation Threshold for each service type and each link Low - Medium Low size type Table 2 Summary Table for Single Service over Single Infrastructure 3 New Architecture Enterprises are moving to a single infrastructure with multiple service types. This is driven by the need for lower costs. A single infrastructure for multiple services reduces the cost of supporting different services. Figure 3 shows a company with a single network infrastructure supporting three service types. The service types are voice, signalling and Internet traffic. We have chosen these service types as they have different sensitivities to delay and packet loss. Signalling is sensitive to both delay and packet loss, voice is sensitive to delay but is less sensitive to packet loss while Internet traffic is less sensitive to delay and more sensitive to dropped packets. Figure 3. Multiple Service over Single Link

5 3.1 Example 1 Multiple Services over Single Infrastructures The capacity planner is now faced with a new challenge. The capacity planner must manage the different service types so that each service type meets its own service level agreement. The capacity planner may just continue to look at the overall utilization of the link. However, this increases the chance that priority time sensitive traffic will be drowned by non-time sensitive traffic. This would result in a breach of service level agreements for individual services even though the overall link utilisation is low. Technologies such as ATM, MPLS have a rich set of features to manage this problem. However, the features can only be used effectively if the capacity planner starts using more complex analysis to determine the values of the parameters for these different features. The capacity planner needs to define parameters such as: Sensitivity of service type to delay and packet loss. Priority of each service type. In the example shown in Table 3, the capacity planner may decide that the signalling service has a higher priority than the voice service and the voice service has a higher priority than the data service. Traffic characterization or profiling of each of these service types e.g. the burstiness of the traffic. This is a highly complex area. The capacity planner needs to understand the traffic profile of each service type. The traffic profile for Internet traffic will be burstier than voice traffic. The capacity planner needs to understand these in order to define the queue thresholds for each service type such that the dropped traffic doesn t impact the service quality. Queue sizes at which packets are dropped for each service type. The capacity planner needs to define the queue thresholds for each service type when traffic is discarded. This depends on the service types sensitivity to delay and discard. Voice services will tend to have smaller queue sizes because delayed voice packets are not useful Class of Service (CoS) features e.g. Weighted Random Early Discard and setting the parameters for these CoS features. Vendors give the customers the ability to allocate queue-servicing resource for each service type. Inappropriate allocation may cause resource starvation for some service types and result in excessive delays or dropped packets for these service types. Overall Link threshold utilization for augmentation for each link type. This list is not comprehensive but shows that the capacity planning of multiple services over a single infrastructure is more technically challenging. Parameter Derivation of Parameter Monitor Service type sensitivity Low Med Service priority for each service type Low N/A Traffic characterization for each service type High High Queue Threshold for Each Service Type and each High High link type CoS Parameters for each link type High High Overall link utilization for each type of link High Low Table 3 Summary Table for Multiple Services over Single Infrastructure Table 3 shows that the number of parameters that the capacity planner has to determine has increased. Also, the complexity in deriving each of these parameters has also increased. The capacity planner wants to meet the service level agreements for each service type. In order to do this he has to priorities the allocation of the links resource to the different resources. In doing this he still has take into account the traffic characterization of the service types such that higher priority traffic doesn t get starved of access to resource without adversely impacting other service types such that service level agreements are broken. The capacity planner is looking at solving problem of the conflicting demands of many service level

6 agreements and many parameters simultaneously rather than a single service level agreement and single parameter. Analytical models can be used to solve simple queuing systems where the traffic arrives with random nature and there is only a one or two service types. Analytical models cannot capture transient system problems. However, the network systems which use multiple services over a single infrastructure using complex CoS features are to complex to model using analytical models. These systems are more likely to be solved using simulation models. However, as links go to higher rates then the simulation times of these systems increases making the building and running of simulation models a less attractive solution for defining these parameters. In addition, building these complex simulation models is likely to be time consuming and hence increase the operational costs associated with the capacity planning function. 4 Capital Costs against Operating Costs The previous section stated that the capacity planning for multiple services over a single infrastructure is more complex because of the number of parameters that need to be defined. This requires more of the capacity planners time and results in increased operational costs. Developing reusable parameterised models of the network can reduce these operational costs. Deriving approximate answers for determining parameters using simpler models can also reduce the operational costs although the accuracy of these models may not be as good as simulation models. The capacity manager needs to make a judgement on the most appropriate solution for his/her business taking into account the business needs. 5 Conclusions This author makes the following conclusions: (i) (ii) (iii) The capacity planners task is becoming more technically challenging because of the demands of meeting service level agreements using new technologies. In order to get the cost savings from carrying multiple services over a single infrastructure, the enterprise needs to invest more time and effort in capacity planning. This leads to an increased operational cost in managing this shared infrastructure. However, enterprises should recognize this increased operational cost as an investment. The time and money spent on capacity planning should result in improved network utilisation rates without impacting service level agreements. 6 References 1. Computer Networks. Andrew Tanenbaum Automated Threshold Alerting Using Statistical Filtering. Danny Quilton