Centre National d'Etudes des Telecommunications, France Telecom, CNET/PAA/ATR .... Layer 1: Signalling and call connection layer. call connection signals ...
TELETRAFFIC AND DATATRAFFIC in a Period of Change. ITC-13 A. Jensen and Y.B. Iversen (Editors) Elsevier Science Publishers B.Y. (North-Holland) © lAC. 1991
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MULTI.LAYER TRAFFIC CONTROL ARCHITECTURE FOR THE WORLDWIDE INTELLIGENT NETWORK Prosper Chemouil and Janusz Filipiak§ Centre National d'Etudes des Telecommunications, France Telecom, CNET/PAA/ATR 92131 Issy-Ies-Moulineaux, France
The paper provides a framework for the analysis of interworking between the real-time traffic management systems of domestic networks and their international gateways. It presents the traffic control architecture for a Worldwide Intelligent Network (WIN). The proposed design takes into account costs which are incurred by the implementation of Operation Support Systems associated with particular algorithms. The rudiments of the Multi-layer Operation Support System (MOSS) architecture are developed and used to define entities subject to future standardization. Theoretical issues related to the multi-layer framework are briefly discussed to set up a scene for further research. The paper takes into account practical problems of WIN design as well as for research directions in that area which were discussed during the Seminar on WIN Design and Control held in Paris in June 1990.
1. INTRODUCTION Many PTT administrations and network operating companies consider at present the introduction of adaptive routing and other intelligent resource and service management techniques in their networks [1, 3-4, 6, 9-13]. Furthermore, the concept of the Worldwide Intelligent Network (WIN) has recently been developed and proposed for future implementation [2, 14]. In this paper we aim at the preliminary analysis of internetworking between the real-time traffic management systems of domestic and international networks. The task is by no means easy if the intricacy of currently implemented algorithms and a variety of new network capabilities are taken into account Resources of the global network considered in this paper are composed of several domestic networks and one (or more) WINs. From the point of view of the WIN, the domestic networks are treated as ingress/egress networks which also serve the local traffic. We then deal with a twolevel network structure.The lower level domestic networks serve both the long distance and the international traffic. Usually, it is assumed that the network administration aims at maximizing its revenue. In a market economy, however, the revenue maximization is a complex task. It must take into account various factors related to the competition between service providers such as, for instance, the quality of service seen by the users, network reliability, and others. Therefore, various economical and technical objectives and constraints are imposed on the designed network. The question remains open to what extent the network designer can directly take into account various criteria of network efficiency. Also, it is questionable if the developed network should be oriented towards a specific set of objectives. It seems rather that the new projects .should aim at developing a universal architecture having a modular structure and being capable to
meet different technical and economical objectives.
Another important issue is that the communication network evolution is to a large extent technology driven. To survive in a market place, the network administration has to keep pace with the current progress and implement new solutions in spite of costs being involved. Finally, to conclude the set up of a background for network design, we note that the trade-offs between the network costs and the quality of service are subject to constant changes. The service quality requirements are more stringent while at the same time the transmission costs per capacity unit decrease. In our opinion this requires that the network design philosophy be changed. Rather than tightly sizing the network according to some assumed grade of service, it seems useful (and cost effective) to : • provide some extra capacity in the network, and • develop a flexible traffic management system, so as to efficiently cope with traffic fluctuations, unexpected traffic demands, and link and node failures. In this paper we propose a traffic management and control architecture which can be used to efficiently manage network resources. The proposed architecture has the following important features : (i) It is modular so that it can easily encompass functional changes. (ii) It gives a global picture of the NM (network management) system. This feature facilitates the analysis of control complexity, the comparison of centralized and decentralized solutions, etc. (ill) It provides a framework for the design and interworking of heterogeneous traffic management and control systems. . (iv) It applies to future integrated services networks (broadband and narrowband). (v) It is secure from the network administration point of view in the sense that the higher level sophisticated functions can be switched off at any stage without affecting the basic mode of network operation.
§ Dr. J. Filipiak is with the Telecommunications Department, Cracow University, Poland.
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2. LAYERED MANAGEMENT ARanTECIURE
The breakdown of NM functions into layers which we have presented above is based more on a concept of different time scales according to which network status and control information are exchanged than on the logical grouping of NM functions. The use of instantaneous traffic data in Layer 2 and carried traffic estimates in Layer 3 finds a theoretical support in [7]. Figure 1 illustrates how the control modules corresponding to routing schemes developed so far by various organizations can be encompassed within the proposed framework. To explain the interworking of various schemes within one node, we note the following :
The proposed approach to the design of heterogeneous traffic management systems is based on the observation that actions taken by traffic management and control algorithms refer to different time scales. In fact, we assume that the traffic management and control functions can be broken down into the following layers, see Figure 1. Layer 1: Signalling and call connection layer. call connection signals are exchanged to establish a path from origin to destination according to the routing tables and flow control theresholds memorized in the network node. Layer 2: Control updating layer. Routing tables and flow control parameters are updated with a cycle length of several seconds. Control actions of this layer depend on the instantaneous occupancy of trunk groups and switches. Layer 3: Real-time management layer. This layer performs time-dependent routing and actions like temporary extensionlreduction of routes, calculation of call gapping or code blocking parameters, as well as others actions, all being based on estimates of carried traffic performed on the time horizon of the order of minutes. Layer 4: Servicing and provisioning layer. Network resources (trunks, bandwidth) are allocated to different traffic types : POTS, Switched Digital Services, and others. This layer operates on a time horizon of hours.
.The control modules exchange information through interface files According to this rule, the routing plan obtained from the network planning center is stored in the file "Routing Plan". The DNHR [3] or STR [12] highest level module reads the routing plan from that me and makes .use of the available information about the network status to determine the routing domain. The control module of layer 2 (OCR [13], STAR [9] or SDR [11] algorithms) works out the routing sequence and memorizes it in the me "Routing Sequence". This update procedure is performed each several seconds according to the trunk group and switch states. Finally, the DAR [10] algorithm or the lower level module of the STR algorithm, acting on a very short time horizon determined by the call interarrival time, further improves the previous choices. Similar considerations for flow and congestion control algorithms can be found in [5].
Higher layers referring to longer time horizons as well as service provisioning layers can also be defined. However, they are beyond the scope of this paper which focuses on dynamic network management.
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Figure 1. - Multi-Layered Management Architecture -
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In the case of homogeneous network, the foregoing example has a theoretical value and can serve mainly as a reference model. In the case of interactions between heterogeneous systems, however, the proposed framework becomes practically important Figure 2 illustrates the interworking between two heterogeneous systems. To discuss this example, we need to consider in greater detail the flow of network status information. We have already classified the network status information patterns according to the update cycle length. We now assume that the information processing (JP) modules are implemented in the layers performing the control functions. The info processing modules use data received from lower layers and from distant systems to work out the traffic estimates and statistics needed for control. As in the case of decision modules the info processing modules read from and write to the interface files. According to Figure 2 the interworking systems exchange information within the layers and then, each system works out the estimates needed for operation of implemented modules. IT in the actual srstem, the n~th layer decision decision module does not eXIst, the routmg data worked out by layer n+ 1 are directly used by layer n-l.
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While routing algorithms like OAR, OCR, DNHR, SDR, STAR STR and others well adapt to traffic fluctuations around the designed values, human-operated actions may still be needed to deal with major problems in the network. Human decisions can be automated by implementing expert systems with knowledge databases storing information about preplanned actions, based on previous experience of network operators, as well as on simulation and mathematical models of specific situations. In the proposed architecture, expert systems are implemented as additional decision modules. Actions of the expert systems are triggered by events in the network (alarm messages). According to a general concep~ of the layered architecture, expert modules operate on different time horizons.
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3. WORlDWIDE INTELLIGENT-NE1WORK DESIGN The design of management and control systems for WIN involves many.issues, including the choice of the routing strategy. To illustrate different cost-performance trade-offs which may occur in that case, consider two examples. Example 1 (Network with small and medium trunk groups). It is commonly agreed at present that a OCR-like state-dependent routing scheme gives the best performance. Updating routing tables with a cycle of several seconds helps to follow the changes of trunk group state. This is of special importance in the case of small and medium trunk groups when the traffic changes abruptly on the time scale of seconds. However, such control requires that a large amount of network state and control information be exchanged. In a large network, transmission and processing of that information may become very costly. Example 2 (Network with large trunk groups). In ' contrast to the above situation traffic variations on large trunks groups are small as compared to the mean. Moreover, in the case of the non-coincidence of busy hours and during periods of congestion spread out (after a link or a node failure) the mean carried traffic evolves slowly and can be tracked on a time scale of minutes. Thus, in some cases, in order to profit from the non-coincidence of busy hours and to effectively counteract the negative congestion effects, it is sufficient to exercise controls on a time scale of minutes. Because the amount of processed and exchanged information can be then relatively small, such a solution may give a good cost-performances trade-off. The analysis of cases presented in the examples allows us to make the following assumption. Assumption 1. The cost of the real-time NM control is determined by the cost of transmitting and processing the infonnation on the network state. Moreover, it is intuitive that the quality of control relates to the amount of information about the controlled system. Assumption 2. The best network performance is obtained when the routing and flow control decisions are based.on the global and detailed knowledge about the current network state. According to the assumptions, if the data exchange and processing are expensive, then the decisionmaking takes place in the higher layers. However, with the progress in the information technology, the intelligent time-dependent controls move down the multi-layer hierarchy. The latter comment is consistent with the evolution of NM systems which we have observed so far. Using the developed framework, we can assert that, from the performance point of view, therouting in WIN should be implemented in the lowest possible layer. However, practical implementation of low-layer algorithms may encounter difficulties due to various approaches of countries to tariffs, differences in network management and maintenance rules, previous investments, economic objectives, competition on the international market, heterogeneity of equipment, and others. Some operators may resist investing into new management systems as regards undefmed returns. The system which transmits and processes the network status and control information is the Operations Support System (OSS). Regarding the comments made above, it is purposeful to analyze the OS systems in a greater detail. To that aim, reference is made once again to the multi-layer architecture shown in Figure 2.
Information patterns shown in that figure suggest that, in order to obtain a cost effective structure, the OS system must be modular and possibly standardized. One way to achieve modularity of the operation support system is to physically separate the network state info processing entities and the switch. The info processing and transmission modules belonging to layers 2, 3, and 4 (or some combination of them) are then implemented in an external device. They communicate with the switch by means of the interface module. The set of modules can be easily designed to interface homogeneous (standardized) OS systems with different switch designs. It provides a cost effective environment for implementation of different real-time network traffic management schemes (centralized vs isolated, state- vs time-dependent, and others). The described system shall be called the Multi-layer Operation Support System (MOSS). Let us now again consider the routing problem. The complexity of the economical and technical environment in which the international network is operated implies that a flexible NM system for such a network will evolve slowly. As previously mentioned, the most simple and least controversial are management systems operating in the higher layers. Once such systems are implemented and national administrations assert their profits, the way will be paved for an introduction of lower layer algorithms. This suggests the following scenarios: Stage 1. OS Systems are developed interfacing international switches in Layers 3 and 4 according to the concepts shown in figure 3a. The interface implementation in Layers 3 and 4 is cheap since man/machine input/output channels (available in switches and management systems) can be used. The routing algorithm of Layer 3 uses measurements of carried traffic, which are updated with a cycle length of minutes. Such control profits from the noncoincidence of busy hours. It can bring fast and significant returns on investments as discussed in Example 2.
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Stage 2. The second stage of MOSS evolution shall involve an implementation of routing schemes in Layer 2 and possibly in Layer I, Fig. 3b. As already mentioned, the introduction of such schemes will be fostered by the progress in the design of operations support system and in the standardization activities. Moreover, the strong interplay between network elements which takes place in such systems, will have to be accepted by national network operators.
4. RESEARCH DIRECTIONS The sketched picture of the NM architecture suggests several research topics. Before discussing them, we stress the twofold meaning which can be attached to the developed concepts. First of all, the proposed framework can be seen as a technical concept with the Multilayer Operation Suppon System as an actual instant of its implementation. However, the multilayer NM architecture can also be seen as a general model of information collection and processing system. In the sequel, we want to concentrate on the latter aspect of the developed approach. More precisely, we make an attempt to demonstrate how the developed model can be used to work out the management system design rules. Let us first address the structural design issues. Example 3. Consider an international network with · two subsets of domestic networks (DNl and DN2) and a set of common resources (WIN). The simplest solution to the NM problem in such environment, presently in use, is characterized by strong information coupling in Layer 1 (call connection signals) and weak (off-line) coupling in Layer 4. The network operation efficiency can be improved by implementing in Layer 2 decentralized controllers (DC) associated with each subset of domestic subnetworks. The DC controllers collect locally available information about the WIN state and use that information for traffic routing. The next step consists of implementing a system with direct information exchange between DC controllers. Finally, we may think about centralized control. Similar decisionmaking structures can be developed for Layer 3.
IN1ERFACE
Figure 3b. - International-Routing Evolution It is to be no~ that the implementation of state-dependent routing in Layer I, based on the exchange of global information about the network state, requires some unification in the switch design which may be difficult to achieve. The considerations of this section indicate that in order to speed up an international cooperation in the design of WIN, it is purposeful to standardize the layered archi-
tecture concept along with the associated information flows and dataformats. Currently, the CCITf signalling system nry and Telecommunications Management Network (TMN) standards are being specified. In a natural way, the CCS 7 signalling network can be used to carry information needed for messaging and control in Layer 1. The TMN can be adopted as a medium for info exchange in Layers 3 and 4. The question remains open what means can be used to transmit messages needed to operate the Layer 2 modules (one may try to use the TMN ressources). In any case, the resulting solution consists of developing the separate data
formats for messaging in each layer.
The basic problem in designing state-dependent controls consists of defining the adaptation layer in which the control decision are taken. It has been already mentioned that the layer chosen must correspond to the best costperformance trade-off. However, defining that trade-off is not easy, especially in the WIN environment where each decision agent has its own objectives. However, some factors influencing the economical and technical feasibility of particular choices can be discussed. We have previously suggested that, due to limited processing and memory resources of computing devices, the information used in higher layers is : • updated less frequently, and • aggregated, as compared to the information being processed in the lower layers. At the same time, the higher-layer decision are more complex. We have also noted that the choice of the decision layer depends to a large extent on the traffic concentration (Examples 1 and 2). In many cases, the traffic concentration is closely related to the network hierarchy. To give an example, the concentration of traffic destined to Country B is lower on links of Country A than in the WIN. This brings us to the · concept of a decision stratum defined as a couple (network
level, decision layer). We shall focus our attention on two levels: • Level 1 Systems = Domestic networks, WIN • Level 2 System = A set of domestic networks +
WIN Note that the concept of levels refers to the physical network resources.
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A crucial question in the design of NM systems is whether the WIN MOSS can be conceived as a system coordinating the operations of the Level 2 (global) network as it is shown in Figure 4. Though this solution is attractive because it permits the end-to-end allocation of transmission and switching resources, it seems that it can be implemented in the higher layers only. In general, taking into account the economical and technical feasibility of different solutions we note the following:
In the low level systems, the traffic management controls are implemented in the low layers while in the high level systems, interactions mainly take place in the high layers For instance, though DCR performs well in small and medium networks, it may not be cost effective in large networks. Then, according to the observation made above, the algorithm can possibly be decentralized, so that the local decisions are based on a knowledge of the global network state or; if it is not feasible, on some part of the gobal network status information pattern. We stress that the progress in technology will change the cost-performance trade-offs allowing for the implementation of statedependent controls in lower and lower layers;
In contrast to domestic networks which can be developed separately, the WIN design requires a common platform for integration of systems currently in use or planned for implementation. The Multilayer Operation Support System provides such platform because of its modular architecture with clear separation of transport, signalling and management functions in different layers. In particular, it can be used to create coalitions of operating companies in which each participant retains its autonomy, but shares information among the members to optimally design and administer the Worlwide Intelligent Network.
BmLIOGRAPHY [1] [2]
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II
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Figure 4. - WIN Management Architecture Another example refers to the situation in which national administrations agree to implement the second level management system to coordinate end-to-end traffic flows. It seems that in view of currently available processing and data storage capabilities, interaction of such management centers with national Operation Support Systems can only take place in layers 3 and 4. Similar rules can be formulated and used as practical hints by network designers after analysis asserting their validity.
[9] [10] [11] [12]
5. CONCLUDING REMARKS [13] In this paper a multilayer approach to the design of realtime NM systems has been developed. The proposed architecture provides an environment in which various solutions to routing and flow control may be easily conceived. It also provides a unified framework for the analysis of different concepts and for standardization activities.
G.R. Ash: "Use of a Trunk Status Map for RealTime DNHR". Proc. ITCll, Kyoto, Japan, 1985. G.R. Ash, P. Chemouil, A.N. Kashper, S.S. Katz, K. Yamazaki, and Y. Watanabe : "Robust Design and Planning of a Worldwide Intelligent Network". IEEE J. SAC, Vot. 7, No 8, 1989. G.R. Ash and E. Oberer : "Dynamic Routing in the AT&T Network - Improved Service Quality at Lower Cost". Proc. Globecom'89, Dallas, USA, 1989. F. Caron : "Results of the Telecom Canada High Performance Routing". Proc. ITC'12, Turin, Italy, 1988. P. Chemouil and J. Filipiak : "Integrated Network Management and Control". ITC'89 Specialist Seminar, Adelai'de, Australia, 1989. A. Di Benedetto, P. La Nave, C. Sisto : "Dynamic Routing of the Italcable Telephone Traffic : Experience and Perspectives". Proc. Globecom'89, Dallas, USA, 1989. J. Filipiak : "Analysis of Automatic Network Management Controls". IEEE Trans. Commun., to appear. J. Filipiak : "M-Architecture: A Structural Model of Traffic Management and Control in Broadband ISDN". IEEE Communications Magazine, Vot. 27, No. 5, May 1989. . P. Gauthier and P. Chemouil : "A System for Testing Adaptive Traffic Routing in France". Proc. Globecom'87, Tokyo, Japan, 1987. RJ. Gibbens, F.P. Kelly, and P.B. Key: "Dynamic Alternative Routing - Modelling and Behaviour". Proc. ITCI2, Turin, Italy, 1988. K.R. Krishnan : "Network Control with StateDependent Routing". ITC'89 Specialist Seminar, Adelai'de, Australia, 1989. K. Mase and H. Yamamoto : "Advanced Traffic Control Methods for Network Management". IEEE Communications Magazine, Vol. 28, nO 10, 1990. . I. Regnier and W.H. Cameron : "State-Dependent · Dynamic Traffic Management for Telephone Networks". IEEE Communications Magazine, Vol.
28, n01O, 1990. [14] Y. Watanabe and H. Mori : "Dynamic Routing
Schemes for International ISDN's". ITC Specialist Seminar, Como Lake, Italy, 1987.
