Optimized VT1.5, STS-1 grooming lowers MAN costs

June 1, 2004

In an era when many companies can take advantage of optical Ethernet and fiber to the premises for their broadband services, most businesses are still served by T1 (1.554-Mbit/sec) lines over copper facilities, which are typically trunked as channelized DS-3s (44.736 Mbits/sec) on interoffice or long-haul networks. In local-exchange networks, VT1.5 (virtual tributary 1.5) and STS-1 (52-Mbit/sec) grooming is required to efficiently use bandwidth and switching equipment.

Most carriers rely on manual patch panels and large centrally located digital crossconnect systems (DCSs) for VT1.5 and STS-1 grooming. However, this equipment is expensive to install, operate, and maintain. In addition, DCSs are usually deployed in non-optimal locations, thereby reducing network efficiency and resulting in higher operational costs.

The integration of STS-1 and VT1.5 grooming functionality into an emerging class of crossconnects called multiservice core aggregation systems (MCASs) effectively changes the network paradigm. Instead of continuing to expand the capacity of legacy 3/1 DCS in the metro core, carriers can cap the growth of legacy 3/1 DCS and deploy grooming throughout the network as needed. This approach facilitates a pay-as-you-grow deployment strategy with optimized VT1.5 and STS-1 grooming fabrics and lowers network costs.

DS-1 (1.544-Mbit/sec) and DS-3 grooming has traditionally been supported by manual digital-signal patch panels (DSX-1 and DSX-3) at the network edge and large monolithic DCSs in the metro core. DCSs are usually only deployed in large central offices (COs) and points of presence due to their high cost and lack of modularity.

All DS-1s, whether destined for local or distant destinations, are typically backhauled to a 3/1 DCS in the metro core for grooming and test access. Test access is usually not supported from the equipment deployed in edge offices, such as add/drop multiplexers (ADMs), thus requiring all DS-1s to be backhauled to the centrally located 3/1 DCS. In an attempt to maximize utilization of transmission resources and 3/1 DCS ports, network technicians use DSX-1 patch panels and M13 multiplexers to manually aggregate DS-1s in small COs.

But due to high DS-1 services growth and churn, it has become difficult to administer DSX-1 patch panels and M13 multiplexers in many COs. As a result, transmission resources and 3/1 DCS ports are not being well used in the current network. The manual intervention time and equipment required for this type of network infrastructure equates to high capital equipment expenditures and operational expenditures (opex) and causes longer service delivery and troubleshooting times.
All DS-1s, whether destined for local or distant destinations, are backhauled in channelized DS-3s to a 3/1 digital-crossconnect system in the metro core for grooming. The DS-3s are manually crossconnected via DSX-3 patch panels between network equipment. Growth and churn have caused a proliferation of DSX-3 patch panels within metro core and edge central offices, making DS-3 interconnects difficult and expensive to operate.

DS-1s are transported in channelized DS-3s across the network and for handoff between carriers. DS-3s are used as intra-office interconnects between transport network elements (NEs) and between NEs and 3/1 DCSs within edge and metro core COs. Between network equipment, the DS-3s are crossconnected manually via DSX-3 patch panels. Over the years, DS-3 growth and churn has caused a proliferation of DSX-3 patch panels within COs. As a result, DS-3 intra-office interconnects have become difficult to administer and expensive to operate.

With DS-3 growth, the number of SONET ADMs within COs has also grown rapidly. Since a SONET ADM typically supports only one transport ring or protected pair of facilities, even a small CO may contain bays of stacked SONET ADMs.

This proliferation of DSX panels and SONET ADMs in edge and metro core COs has led to high opex and capital expenditures (capex). The main capex contributors are the SONET ADMs and DSX panels themselves as well as the associated tie trunks and repeaters. The main opex contributors are the truck rolls required to provision intra-office connections due to growth or churn, real estate costs, and power consumption. Additionally, there is a high cost associated with service failures resulting from human error, lack of redundancy, and reliability issues inherent in coaxial cables and metallic connections.

In response to carrier need to resolve these operational issues, several equipment vendors have introduced MCASs. These aggregation systems combine SONET ADM and DS-3/

DS-1 crossconnect functionality into a modular scalable platform. The MCASs' integrated STS-1 and VT1.5 switching fabrics cost significantly less than those in legacy DCS because of technology advances enabling greater modularity, density, and capacity. These innovations make it possible to cost-effectively deploy STS-1 and VT1.5 switching capabilities at the edge as well as in the metro core.

The STS and VT1.5 grooming is integrated into remotely configurable, electronic-switching fabrics. This feature allows network operators to remotely control STS and VT1.5 crossconnections via a command-line interface such as TL1 or via an element or network management system. This remote capability minimizes the need for expensive manual intervention and reduces human error in provisioning and troubleshooting circuits. The aggregation systems also support integrated test access, so there is no need to backhaul all circuits to a centrally located DCS for fault isolation and performance monitoring.

By replacing legacy equipment in current CO architectures with MCASs, carriers can save as much as 60%–80% on capex and opex from STS grooming alone. The capex savings are primarily from the use of MCAS line cards instead of additional SONET ADMs and electronic VT1.5/STS-1 fabrics instead of additional manual DSX panels, tie trunks, and repeaters.
In next-generation metro networks, multiservice core aggregation systems, which combine SONET add/drop-multiplexer and DS-3/DS-1 crossconnect functionality into a single platform, will perform grooming at the edge as well as in the metro core.

In terms of opex, MCASs offer several advantages to help lower costs. Because the crossconnects in MCASs can be controlled remotely from the network operating center, the number of truck rolls can be substantially reduced to save labor costs and shorten time to revenue. The integration of multiple SONET ADMs onto a single MCAS line card reduces real estate costs and power consumption. MCASs can also help eliminate service failures resulting from human error, lack of redundancy, and reliability issues inherent in coaxial cables and metallic connections, bringing higher end-customer retention by increasing satisfaction with the services provided by the carrier.

With the deployment of grooming at the edge, partially utilized DS-3s can be aggregated into a smaller number of fully utilized DS-3s before they are terminated on a legacy 3/1 DCS in the metro core, thereby "freeing up" legacy 3/1 DCS ports and transmission resources. As a result, the "life" of legacy 3/1 DCS is extended, and any planned capacity upgrades of legacy 3/1 DCS can be postponed. In effect, the carrier can cap the growth of legacy 3/1 DCS, reaping substantial capex and opex savings. ..

Bill McDonald is product marketing manager at Mahi Networks (Petaluma, CA).