Next-generation crossconnect supports Sonet and ATM technologies

June 1, 1996

Next-generation crossconnect supports Sonet and ATM technologies

Called a multirate transport node, a proposed network element handles different crossconnect fabrics, standard Sonet and ATM interfaces and is managed by a common administration system

V. John Joseph

DSC Communications corp.

The multiplicity of components, the advantages of fiber-optic cable over coaxial cable for interconnection, and the universal acceptance of Synchronous Optical Network (Sonet) and Synchronous Digital Hierarchy (SDH) standards suggest an integrated, distributed multiple-rate architectural concept for the network

transport node. An all-optical reference network element, known as the multirate transport node (MTN), is a possible implementation of the concept.

In the MTN design, components are interconnected by optical links that carry information in Sonet format. The distributed software and processing architectures are based on local area networks employing Sonet at the data link layer. In addition to the recursive elegance and symmetry of this architectural concept, economic and environmental benefits are realized by the flexibility and expandability of the node.

The incorporated optical links allow interface components to be placed far apart; for example, node termination points may be collocated with network termination equipment and optically linked to the crossconnect fabrics. The different component bays of the MTN network element need not be in close proximity, thereby allowing spatially challenged central offices to adapt. The distributed software and processing architecture fosters application development based on integrated data management and standard communications protocols.

The proposed MTN is needed to support the extensive telecommunications network of fiber, cable and wireless media that are expected to carry future transport network signals at various bandwidths and in different formats. The network elements needed to reformat, multiplex and crossconnect these signals under automated control are the mix-masters of the network.

These network elements or nodes currently consist of Asynchronous Transfer Mode (ATM), broadband, wideband and narrowband crossconnect fabrics, optical and electrical termination equipment, multiplexers, and interconnect devices.

Digital crossconnects

Today`s electronically controlled digital crossconnect system (DCS) evolved from manual equipment used to implement digital distribution frames. The centrally controlled DCS reduces both labor costs and service-provisioning time. It was developed to meet the needs of network restoration and network providers who wanted more control when managing and reconfiguring network connections. Most DCSs can continuously and nonintrusively monitor the performance of all circuits, isolate faults and help fix problems before a service outage occurs. In addition to its basic functions of grooming, filling and crossconnecting, the DCS brings an additional level of robustness, fault tolerance and flexibility to the network.

Currently, a fundamental requirement imposed on network providers is a DCS with the ability to evolve with changing network requirements. The DCS, a critical component in the growth of the network, facilitates the coexistence and cooperation of different network signal protocols, formats (optical and electrical) and bandwidths. Consequently, a next-generation DCS or MTN is specified with the following features:

Multirate supports a combination of narrowband, wideband, broadband and ATM crossconnect fabrics to promote central office efficiency, reliability and cost reduction.

Transport manages the prevailing North American and international transport interfaces (such as DS-1, DS-3, STM-1e, E1) with migration to higher-order Sonet and ATM transport levels at OC-N, STM-No and STS-Nc.

Node is a network platform incorporating various network elements (such as crossconnect fabrics, optical and electrical termination equipment) under a common administration system, and presenting itself as a monolith to the backbone network. The platform is necessarily a distributed system of hardware and software heterogeneity.

The MTN also functions as a Sonet/SDH gateway and provides the transport infrastructure for the ATM switching function. The MTN would have to support these payloads:

Sonet (ANSI)--VT1.5, VT2, VT6, ST-1 synchronous payload envelope (SPE), STS-3c SPE and STS-12c SPE

SDH (ETSI)--VC11, VC12, VC2, VC3, VC4 and VC4-4c

A computing platform is designed as a distributed system so it can expand as needed. This principle may also be applied to telecommunications networks and network elements. Clearly, the ability to evolve and grow is a basic requirement for the success of the MTN. Also, some hardware bays of the network element have to be physically distant from each other for the following reasons:

Network interfaces should be physically located near facility termination points, thereby reducing office wiring and cable congestion.

The MTN will typically be installed in existing central offices. Trying to place all MTN matrix fabrics and network interfaces in one location (for example, on one floor of an office) may challenge the spatial and physical-plant limitations of the office.

The use of fiber-optic cable in the MTN internal design provides several benefits. Fiber connections are less expensive than those of coaxial cable, take up less space, simplify testing, provide enhanced electromagnetic compatibility, and ease system growth. System planners need not be concerned about rigid bay lineups, and fiber may be diversely routed within the office. In particular, node elements linked by fiber may be kilometers, rather than meters, apart.

For these reasons, fiber is preferred as the interconnect medium for the node elements of the MTN. In a typical distributed office using fiber-optic links, the MTN components may be located on any building floor or in nearby buildings (see Fig. 1).

Fiber-optic office links

The fiber-optic office links are optical links used to interconnect major subsystems of the MTN. Enabling of these interconnects as the transport medium for both control and data within the network element is needed. Data transport on the links is preferably performed in a uniform format and without remapping the data paths from the format in which it enters and leaves the MTN.

Because Sonet/SDH is the prevailing network standard, a Sonet transport-like signal can be used as the internal data format. For STS and OC line terminations, it is possible to reuse the Sonet line and section overheads to piggyback control information on the incoming data without using additional rates and data path formats. In this concept, the 622-megabit-per-second OC-12 signal rate could thus provide the necessary bandwidth.

Using an OC-12 signal, an office link could carry 12 STS-1 (52-Mbit/sec) SPE signals. The 27 bytes of Section and Line overhead in the STS-1 frame add up to 324 (27 ¥ 12) overhead bytes per office link every 125 microseconds. These bytes may be used to carry MTN or office link specific information.

This internal data path rate and format may be defined as an STS-1MTN frame. The population and interpretation of the STS-1MTN frame overhead are implementation-specific. Then, the STS-1MTN frames can be used as the framing method for all signals on the office links from line terminating equipment to the connection fabrics, as well as between different connection fabrics. Data and control between the main administration system processor and the subsystem controllers on the interface bays also flow along the office links.

Functional view

From a high-level functional view, the MTN logically integrates the physically separated subsystems interconnected by the office links. The administrative subsystem provides the transport node with operations, administration, maintenance and provisioning (OAM&P) functions. Its duties include monitoring component status, performance monitoring, fault isolation and fault reporting. The office-link interconnects between subsystems could functionally be any hardware/software interconnect for a particular implementation. It has been established that there are architectural, environmental and economic reasons to realize the links as optical carriers transporting Sonet frames.

Signal processors are generically referred to as resources in the MTN functional view (see Fig. 2) and are used to overcome the inflexibility of current crossconnect systems. Resources are developed when needed to accommodate future growth. For example, an ATM packetizing resource may be introduced into the broadband matrix of an MTN to provide 44.736-Mbit/sec DS-3 to ATM mappings.

Resources are also used to provide capabilities such as multiplexing, format conversion, cell assembly and disassembly, and mapping between various Sonet and asynchronous signals. The MTN thereby serves as a gateway between asynchronous and Sonet networks, and resources provide the necessary mappings. In an implementation of the MTN reference model, resources may be embedded in the connection fabrics.

Distributed software

As a critical component in the network, the MTN`s performance, correctness and ability to monitor network facilities are paramount to the robustness of the overall telecommunications network. The administrative system needs to be fault-tolerant so that the MTN will not be left without its OAM&P capabilities. To ensure reliability, the administrative subsystem is implemented with full redundancy using active replication techniques.

The distributed nature of MTN subsystems implies that the processing elements (subsystem controllers) managing this node may also be remotely located. Therefore, the software architecture of the network element is necessarily distributed.

The data collection, performance monitoring and surveillance functions of the administrative subsystem dictate that it have a real-time picture of the state of all node elements at all times. The subsystem`s software contains a map of the state of the network facilities, equipment and connections. The subsystem is an integrated distributed-information manager of data residing on the main administrative processor, the various subsystem controllers and the individual circuit packs.

In addition, the administrative subsystem needs to provide a user interface (both character-oriented and graphics-based) for craft personnel and operational support systems. Different languages--TL1 and CMISE, for example--for implementing different user interfaces are required as the network evolves.

The administrative software may be implemented by various high-level programming languages. Interoperability between different user languages is facilitated by object-oriented, high-level languages interchanging object content and agreeing upon message protocols and network information models (see Fig. 3).

Industry standards and commodity hardware and software for implementing common functions let the developer concentrate on core functionality. A reference model does not dictate implementation choices. However, the following guidelines are recommended:

Software platforms should consist of industry-standard operating system and communications facilities supporting applications running in remote bays.

Integrated real-time data management layer should provide uniform support of distribution, replication and equalization of all the data in the node.

Distributed object technology should support a multilingual design and development environment.

Integrated solution

The MTN is an integrated solution for bandwidth and signal-format management. This integrated approach provides a low-cost solution because it requires fewer components to support multiple telecommunications technologies and avoids duplicating network components such as input/output processors. A common network platform guarantees fast, flexible bandwidth allocation and supports mixed end-user payloads and services. Furthermore, the MTN also acts as a central point to handle both Synchronous Transfer Mode (STM) and ATM traffic and provides smooth evolution from STM to ATM, eliminating the need for overlay networks.

The MTN can support terminations for asynchronous, Sonet, plesiochronous and SDH rates, including synchronous and asynchronous transport modes (see table).

A distributed hardware and software architecture is key to the success of the MTN. A heterogeneous multilingual software architecture based on standard operating systems is recommended for the administrative subsystem. An integrated, distributed real-time data manager administers the MTN`s operation and performance monitoring. The Sonet-based network element uses fiber optics to interconnect the network element subsystems, enabling the system to be implemented in a distributed office. u

V. John Joseph is senior manager at DSC Communications Corp., Plano, TX.