As carriers continue looking for ways to reduce capital and operating expenses while delivering higher customer satisfaction, the metro core aggregation system (MCAS) has evolved as a way to streamline aggregation and provisioning in metro-core central offices (COs). MCAS products from Cisco, Lucent, Nortel, Mahi Networks and others can streamline the metro-core CO architecture by consolidating the functions of add-drop multiplexers (ADMs), broadband content delivery systems, optical node interconnections and multiservice provisioning platforms into a single system. An MCAS can terminate hundreds of fiber rings and groom thousands of circuits, thereby speeding provisioning of digital signals in T-3 (DS3s), reducing provisioning errors and eliminating CO clutter, while lowering both capital and operating expenses.

A typical metro network consists of multiple central offices that support services interfaces to the customer via access rings along with interconnections to other central offices via inter-office facility (IOF) rings. Each CO must terminate the Sonet rings and must aggregate and switch traffic. A single central office might support a handful of rings or hundreds of them, and each ring terminates at an ADM. COs typically contain banks of Sonet ADMs.

Ring aggregation and optical node interconnection with MCAS.

Multiple types of network elements support switching and aggregation functions, including Layer 1 transport (such as Sonet ADM and MSPP); Layer 1 switching (such as wideband digital cross connects); and Layer 2 switching (i.e., Ethernet switching). The Layer 1 network elements are interconnected with DS3 tie lines. In larger central offices, repeaters regenerate DS3 signals that travel long distances (such as cable runs between floors). Finally, the central office uses digital signal (DSX) panels to provide for the administration of and test access for the DS3 tie lines.

The DS3 tie lines, repeaters and DSX panels bind together the various components of the network. These elements. however, also represent a large and growing component of the service provider's capital and operating expenditures. They are the source of many service failures that result from human error, lack of redundancy, and reliability issues inherent to coaxial cables and metallic connections.

Metro core aggregation systems eliminate multiple central-office components along with the problems associated with them. The MCAS is a new category of product tailored to deliver cost-effective Sonet ring aggregation, optical-node interconnection and service provisioning.

One system

The MCAS improves Sonet ring aggregation by replacing stacks of discrete access and long-haul ADMs with a single system. Within the CO, an MCAS provides high-density STS-1 switching and eliminates BBDCS, DSX-3 panels, tie cables and repeaters, thereby lowering the possibility of service outages due to manual connection errors. The MCAS also reduces intra-office Ethernet connectivity requirements.

For service provisioning, the MCAS supports multiple, multiservice line cards, each of which is effectively an MSPP. Each card can be configured for TDM, Ethernet or Ethernet-over-Sonet services, and as an IOF or access ring node. By integrating as many as hundreds of multi-service provisioning platforms, a BBDCS, and a Layer 2 VLAN switch into a single network element, the MCAS dramatically decreases the number of intra-office DS3 tie trunks and associated DS3 interface cards, patch panels and repeaters. In addition, the MCAS improves transmission efficiencies by eliminating the need for service providers to back-haul Ethernet traffic to a centralized Ethernet switch. As a result, the MCAS cuts the space, power and maintenance requirements of CO interconnection to reduce capital and operating expenses by as much as 60 percent to 70 percent.

The metro core aggregation system also simplifies central-office and inter-office facility engineering and planning. With its high scalability in regard to both ring "fan-out"and switching capacity, plus with a high degree of interoperability, the MCAS can support ring additions and bandwidth upgrades well into the foreseeable future. As such, service providers no longer need to plan for switching matrix upgrades or the addition of additional boxes every couple of years. Extra capacity can now be added by simply inserting additional line cards into the existing MCAS chassis. Additionally, since a single network element can provide connectivity among all IOF rings terminating at the CO, the IOF network can now be viewed as a large mesh supporting any-to-any node connectivity, thereby simplifying circuit routing and provisioning.

Ideally, central offices should be able to deploy an MCAS and then gradually eliminate or redeploy legacy equipment over time. To support this migration, MCAS equipment must satisfy three key requirements: interoperability, management functionality and scalability.

MCAS must terminate multivendor Sonet rings. Replacing Sonet ADMs in a metro core office with an MCAS requires the use of open, standards-based Sonet protection schemes such as APS, UPSR and BLSR. 1+1 APS and UPSR are the first protection schemes to achieve widespread interoperability. In the future, carriers will demand that vendors cooperate to achieve BLSR interoperability.

Datacom channel

The other key area of interoperability is in the data communications channel (DCC). Sonet equipment vendors use either the OSI-over-DCC or the Internet Protocol (IP)-over-DCC protocol. Carriers selecting an MCAS must ensure that it operates with the DCC protocol or protocols used in the various Sonet rings to be aggregated.

MCAS must also operate with edge MSPP platforms to manage Ethernet efficiently in its native form or in Sonet facilities. This requires interworking using today's standard Ethernet-over-Sonet framing protocols such as X.86 and GFP.

Because an MCAS may aggregate dozens or hundreds of Sonet rings comprising hundreds or thousands of discrete nodes, the MCAS must support gateway network element functionality for all subtending network elements. As a gateway network element, it must consolidate all management traffic between the network operations center and the SNEs. Thus the MCAS must support the routing of management traffic between the SNEs and the network operations center using open shortest path for SNEs employing the IP-over-DCC protocol or using IS-IS-the intermediate system-to-intermediate system protocol-for SNEs employing the OSI-over-DCC protocol. For SNEs using the OSI-over-DCC protocol, the MCAS must also support protocol interworking between the IP suite supported by the Data Communications Network (DCN) and the OSI protocol suite supported by the SNEs. This interworking function must include support for the Target Identifier Address Resolution Protocol and the File Transfer, Access and Management Protocol.

Current CO Architecture

Additionally, the MCAS and its management system should support the auto-discovery, surveillance and provisioning of multivendor network elements. These features provide a topology view of the entire network, including the MCAS and all subtending network elements, as well as the ability to quickly diagnose network faults and provision circuits across the network from a single management console.

Because carriers have different organizational structures, the MCAS management system should also support user access and privilege management using a role-based access control (RBAC) model that allows the establishment of carrier-defined access models based on organizational, functional or geographical boundaries and responsibilities. That allows carriers to continue to support traditionally separate management "silos" and smoothly transition to a converged silo over time. Alternatively, greenfield deployments can be managed within a single network operations center with complete and secure flexibility.

RBAC also enables distinct groups to access MCAS transport, switching and data capabilities without conflict while supporting Customer Network Management-enabled product offerings, such as private rings.

As a gateway network element, the MCAS' IP- and OSI-over-DCC routing protocol implementations must support a network of subtending network elements that could number into the thousands. As such, its routing protocol implementations must support hierarchical network architectures (such as Level 1 and Level 2 IS-IS routing for the OSI-over-DCC protocol and backbone and nonbackbone areas for the IP-over-DCC protocol) and scale to support tens or hundreds of routing domains or areas. The MCAS must support gateway network element functionality for thousands of subtending network elements.

Bill McDonald is manager of product marketing for Mahi Networks Inc., (Petaluma, Calif.).

See related chart Streamlined CO connections with MCAS.