ISSUES IN MULTIMEDIA SYSTEM DESIGN Jayabalan SathiasulanWET Kumaran KrishnanWET Nik Nor Aini Nik MahmoodWET Rajendran VignaswaranWET Santhrakala MathuvayWET ==> Presented by:

Table of Contents 1. Introduction Presented by: Rajendran Vignaswaran 2. Congestion Control Presented by: Jayabalan Sathiasulan 3. Admission Control 4. Traffic Policing Presented by: Kumaran Krishnan 5. Traffic Analysis and Prediction Presented by: Nik Nor Aini Nik Mahmood 6. Buffer Management 7. Scheduling Presented by: Santhrakala Mathuvay

Introduction ► Multimedia system design refers to the design of the network subsystem. ► Therefore, all issues that are covered will relate to the design for networks that are sensitive to the multimedia traffic.

► In general, communication can be classified as follows:- 1. Point-to-Point 2. Point-to-Multipoint 3. Multipoint-to-Multipoint ► These classifications are based on the fact that there are one or multiple sources and/or destinations. ► The functional requirements for any of these communications are:- 1. Identify the destinations 2. Find the route to the destinations 3. Transfer data

► Connection establishment refers to the process of finding a route to the destination(s). ► A connection has to be established prior to the start of data transmission. ► It is referred to as unicast if it is Point-to- Point and multicast if it is Point-to- Multipoint.

► In multimedia applications, during route finding, the QoS (Quality of Service) is considered while selecting the network service provider(s) from the source to the destination. ► By selecting a route and reserving resources along the route, a multimedia call is said to be admitted into the network. ► This is referred to as call admission control.

► In a network, many applications will be trying to establish connections at the same time. ► All the applications that have calls admitted will essentially start sharing the resources. ► When units of information start flowing through the network, it may so happen that at one or more intermediate switches queuing delays will start increasing. ► Such an occurrence is an indication of a temporary overload condition referred to as congestion.

► It is essential to monitor and control the connection after call admission, that is, during the data transfer. ► Traffic characteristics from the source can be continuously monitored, and any violation can be regulated by regulating or shaping the traffic at the source. ► This process of forcing the traffic to abide by the demanded QoS values is known as traffic policing. ► In many designs, it is combined with traffic shaping, which is a preventive measure.

► Well-behaved streams do not mean that all the QoS parameters will be satisfied through the network. ► This is because the end-to-end behavior experienced by an application largely depends on the delays incurred at the intermediate switches. ► In order to guarantee the end-to-end QoS parameter, it is essential that the media streams are scheduled using a certain service discipline at all the intermediate switches during the lifetime of the connection. ► The issues and their interactions are shown in the figure below.

Figure1: Interaction Among Different Network Functions

Congestion Control ► The unpredictable statistical of traffic flows is the main cause for congestion in high- speed networks. ► There are 2 types of congestion control mechanisms which are attempted in design. They are:- 1. Reactive congestion control 2. Preventive congestion control

► Reactive congestion control refers to the regulation of traffic flow at the access points, when congestion occurs in the network. ► For achieving this, a feedback mechanism with adequate lead time to react is required. ► Hence, this method may not be considered desirable for broadband multimedia applications.

► The preventive congestion control mechanism tries to prevent the network from reaching an unacceptable level of congestion. ► This approach is best suited for connection- oriented networks such as ATM networks. ► This is because the decision to admit a new connection can be made based on the knowledge of the state of the proposed route of the new connection.

► Congestion control for high-speed networks can be viewed as a collection of 3 independent functions. They are:- 1. Admission control and resource reservation 2. Traffic enforcement (policing and shaping) 3. Scheduling ► Admission control determines whether to accept or reject a new connection at the time of call setup. ► This decision is based on the traffic characterictics of the new connection and the current network load. ► A new connection is admitted if and only if the required resources are available and it is ensured that such an addition will not cause overuse of network resources.

Admission Control ► There are several admission control policies. ► Based on the performance metrics, they can be classified as algorithms that use link-level performance measures and algorithms that use connection-level performance measures. ► The link-level performance measures are based on the statistical behavior of cells on a link. ► Since cells on a link over high-speed networks are statistically multiplexed from many connections, it’s difficult to distinguish from which sources cells are generated.

► Even though global performance objectives can be achieved using link-level performance measures, individual connections may experience a higher loss rate and queuing delay than desired. ► As satisfactory link-level performance does not guarantee satisfactory user-level performance, it may not be possible to satisfy user-level QoS using link-level performance measures. ► Connection-level performance measures give the performance for individual connections and are better suited to manage QoS.

Traffic Policing ► Multimedia information covers traffic characteristics ranging from low bit-rates to broadband bit-rates. ► These heterogeneous traffic sources also differ widely in the degree of burstiness, correlations and expected QoS from the network. ► Access control algorithms is required to enforce and maintain a specific QoS for each traffic. ► Access control algorithms enforce specific QoS by either bit or cell dropping mechanisms or feedback preventive control mechanisms.

► In the bit dropping method, the cells or packets that violate the negotiated traffic call admission parameters are dropped. ► The feedback preventive control uses a control signal that is a function of the buffer occupancy level to control the source coder sampling rate. ► Frequent decisions to drop increase the cell or packet loss probability.

► In networks where packets or cells are transferred without flow control between the user and network, a need to control the individual cell or packet streams arises automatically. ► The control must be enforced during the entire duration of the call in order to ensure an acceptable QoS for all outgoing calls that are sharing the network resources. ► Such a control is exercised by introducing policing and (or) shaping.

Traffic Analysis and Prediction ► Ideally, performance evaluation should be based on measurements taken directly from an actual, fully operating multimedia network. ► Since such a network does not exist yet, two other approaches to performance evaluation have been used by network analysts. ► The first and most common approach is based on a presumed traffic model that encapsulates some of the stochastic characteristics of the actual input stream(s). ► Such a model can be used in subsequent queuing analysis or simulations of buffers at nodes.

► The second approach is based on traces of actual traffic streams. ► These traces can be used as traffic inputs to simulations. ► Indeed, this latter approach relies heavily on the availability of such traces. ► Whatever assumptions are used to characterize the arrival process of the traffic will have a significant impact on the predicted performance. ► Therefore, it is necessary when studying the performance to use traffic models that capture the most important characteristics of the actual traffic.

► Another strategy is to build models at a functional level, and use them to predict the traffic by appropriately combining the models with values measured from actual networks. ► For instance, the functional behavior of an orchestrated multimedia presentation describes the events that happen in time domain. ► These events specify the objects to be part of the presentation in different media streams that make up the presentation. ► This functional behavior of an orchestrated presentation, represented by its synchronization characteristics, can be effectively used for understanding the performance behavior of the application. ► For characterizing the performance behavior, we need to describe the multimedia application in terms of tangible system- related parameters such as the QoS discussed earlier.

Buffer Management ► A traffic model of an application, which is based on the functional behavior, is represented as a sequence of objects to be presented at different time instants with an associated duration of presentation. ► Such a sequence gives an implicit description of the traffic associated with the orchestrated presentation. ► But the actual traffic generated by an application also depends on the object retrieval schedule adopted by the client.

► ► The retrieval schedule basically determines the time instant(s) at which the client wants to receive the object(s). ► ► This schedule depends on the buffering that can be done at the client side. ► ► In a similar manner, the QoS requirements of a client carrying out the application depends on the size of different media objects to be presented, the duration available for retrieving them, and the buffering strategies adopted by the client.

Scheduling ► The heart of a QoS architecture is the multiplexing policy used at the switching nodes. ► Multiplexing is the allocation of link capacity to competing connections. ► The manner in which multiplexing is performed has a profound effect on the end-to-end performance of the system. ► Since each connection might have different traffic characteristics and service requirements, ► it is important that the multiplexing discipline treats them differently, in accordance with their negotiated QoS.

► However, this flexibility should not compromise the integrity of the scheme, that is, a few connections should not be able to degrade service to other connections to the extent that the performance guarantees are violated. ► Also, the scheme should lend itself to analysis since performance guarantees are to be given. ► Finally, it should be simple enough for implementation in high-speed switches.