An ATM switching unit architecture for BISDN

The paperpresents the architecture of an ATM switching unit for broadband networks. Fundamental architectural requirements are considered and system solution is described to be used for early applications in field trials. The different entities of the network are explained in detail in addition to the mechanism which allow the implementation of the architecture to provide the features of future ATM networks and meet the requirements of National ATMprograms. 1.INlRODUCllON This paper describes the architecture of an ATM switching unit, taking the local exchange as an example. It proposes a set of subsystems to be used as building blocks during the definition of a specific configuration. The proposed system solution is limited in size and is intended for early field trials and National programs. 2. OVERALL ARCHITECTURE DESCRIPTlON AND CONCEPTS During the fmt introduction of broadband services in an ATM switching environment, the ATM switches will be required to: to increase the capacity of interconnected MAN networks. to provide switched broadband services (e.g. videotelephony, individual videoconferencing, high quality image retrieval, distribution services, ...). The proposed ATM architecture takes into account the fact that broadband service demand will be scattered over the country on a low density basis. The important components of such a network are: ATM Subscriber concentrators with a capacity of up to 150 subscribers. Concentration is done on a call per call basis onto a small number of highly loaded links to the ATM switching unit (see later). An ATM switching unit supporting about 3000 subscriirs. They perform the call processing functions for the broadband services. Subscriber concentrators can be local or remote to the ATh4 switching unit. When remote, they are linked to the ASU with standard transmission equipment. ?b offer N-ISDN services and interworking with N-ISDN, each broadband exchange is connected to an E10612 local exchange which is in charge of the N-ISDN call handling. When no E10/S12 exchange is available, transparent 2Mb/s1.5Mb/s channels can be made available between the customer narrowband equipment and the NE3 local exchange through the broadband exchange. The presented architecture has been designed taking into account the specific properties of broadband services such as: multiple service components (e.g. image, voice) point to point, point to multipoint variable bitrate source coding various qualities of service The resulting architecture of the ATM switching unit is based on: the clear separation of functions either belonging to the network transport or the network control functions. These functions are grouped in the so-called "transport network (TN) " and in the "control network (CN) ". The main characteristic of such a separation is the concept of services delivered by one type of network to the other. These services are the only interaction defied between both network types. This concept allows a separate evolution of both network types, depending on e.g. technology evolution. The organisation of one network is, as far as possible, unknown to the other and optimisation of each network in an independent way is possible. Fig.1 depicts an example of a broadband network configuration consisting of two ATM switching units an their links to the supporting hosts inside the N-ISDN network. From a local exchange point of view the system comprises: either a S12 or Alcatel E10 host, switch nodes, called ATM Switching Unit ASU, which can be located locally or remotely with respect to the host. Concentrators (e.g. ATh4 Subscriber Concentrator ASC) which can be installed either locally (internally into the ASU) or remotely with respect to the ASU. Proceedings Vol IV p.7 AN ATM SWITCHING UNIT ARCHITECTURE FOR BISDN. Figure 1 General Reference Configuration Figure 2 shows the relation between the entities of the reference configuration and the logical concepts of " transport network" and "control network". The transport network (TN) comprises all functions from all reference entities (e.g. ASC, ASU) which are together responsible for the end to end data transfer capabilities of the system. The control network (CN) is made up from all functions required to manage calls and connections, together with the functions responsible for the control of the operation of the overall network. 'Itrble 1 presents some of the functions of both TN and CN and their situation in the reference configuration. Section 2.1 and 2.3 describe in detail the intemal structure of the ASU and the ASC and the components they are made from. Figure 2 Mapping of 'TW and 'CW on Reference Configuration m CN Table 7 Examples of Functions, their Functional and Physical Location General description of the transport network 2.1 m The transport network comprises all functions required to transport information between entities connected to or contained within it. These entities can be user access entities, processing entities, additional servers, etc. 2.1.1 hperties of the TN. Objectives The TN: allows peripheral entities to be connected to it on both optical and electrical interfaces. One standard interface is used intemally in the TN. supports in an integrated and unique way the services requested by the subscribers. allows modular network growth: 10 to 3000 broadband subscribers up to 64 PCM links up to 64 ATM trunks. allows ATM connections to be set up in a few milliseconds. requires a minimum number of boards. provides ATM connections with a predefined QOS. offers services to the control network applications. These services may be specific for the type of application. Control network applications considered are: call control applications O&M applications (maintenance, initialisation, observations, ... The transport network is used to exchange all information (user traffic and control messages) between network entities. A standard interface between the transport network entities has been identified. This interface has a tranfer rate of 600 Mb/s and is called the ATM Electrical interface (AEI). 2.13 Main characteristics of the TN The switching actions of the TN are performed by switching elements (SE). These SEs have the following basic characteristics: Proceedings p.8 Vol IV Session A5 Paper # 2 The basic mode of operation is a combination of storeand-fonvurd and spoce switching. The queueing discipline adopted is 0qu1 Queueing. The switching network is connection oriented for the following reasons: all current public services are connection oriented. the ATM interfaces are defined as connection oriented. a close mapping of the switch mechanism and the network mode of operation was favoured. easy implementation of both point to point and multicast connections in the switching network. Applications requiring a connectionless network service will be supported by an emulation of this service. The connections through the transport network are controlled by messages generated at the edges of the network. Selection of the path through the switching network is entirely done internally to the switching network itself. Multiple connections can be established simultoneously. The TN is considered, as far as possible, as a self-contained blackbox. As a result, internal maintenance mechanisms have been defied to control and monitor the performance of the transport network. 2.13 Entities of the TN Figure 3 presents the architecture of the transport network and the relation between the different entities. These entities are intelligent and can be addressed to perform a specific task.