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Fine-grain switching goes from core to edge

Posted: 01 Oct 2004 ?? ?Print Version ?Bookmark and Share

Keywords:vt switching? carrier? telecom? ppp? ip?

Carriers today are focused on providing solutions that offer more flexible services, enable box consolidation and reduce overall capital expenditures as well as operations and costs. Requested carrier services include efficient, flexible and mixed voice/data services; bandwidth on demand; QoS; and various protection services for carrier-class reliability. The emergence of enterprise data services and the continuing proliferation of wireless applications are driving the end-user applications that carriers are trying to service.

Lowering capital expenditures and operating costs while providing mixed-voice/data services requires finer traffic grooming closer to the edge of the network. This has forced telecom equipment vendors to provide virtual tributary (VT) functions in network elements that traditionally had little, if any, VT capability. With the current technologies and the proliferation of newer services, such as enterprise data services and wireless applications, VT functions at the edge can be realized.

Historically, carriers have provided data services by encapsulating IP packets into point-to-point protocol (PPP) packets, mapping them into DS-1 signals and then transporting them over T1 lines from the enterprise to the central office. The DS-1s would then be M13 multiplexed into DS-3s, mapped into STS-1s and then SONET multiplexed up into various SONET ratesfor example, STS-3s, STS-12s, STS-48s and STS-192s. This entire process of mapping data services led to inefficient utilization of SONET links, effectively translating into wasted bandwidth over entire networks.

As enterprise customers' data bandwidth requirements grew, the carriers would simply add T1 lines or do forklift upgrades to provide T3 service between the enterprise and the central office. That used core and metro networks inefficiently, which was reflected in the high tariffs carriers charged to profitably deploy this service.

Carriers today are addressing the data-traffic explosion by developing more efficient ways to map and transport the data. One solution involves mapping DS-1 traffic that carries IP packets directly into VT signals. VT signals are natively mapped into STS-1 signals, and thus, better utilize existing SONET pipes. A second approach involves developing a standard called virtual concatenation (VC).

VC enables direct mapping of nonstandard SONET/SDH payload-size data services into the right-size SONET/SDH payload containers. The multiple standard SONET/SDH payload containers are linked into groups via a standard manner to allow transport through the established SONET/SDH network. Multiple VC groups (VCGs) are tagged to be transmitted divergently across a network and then recovered and regrouped at the far end of the network. Through a protocol called the link capacity adjustment scheme (LCAS), the size and number of VCGs can be dynamically changed to support flexible bandwidth requirements enabling better traffic management and QoS. VC mapping is implemented at the high-order (HO) (STS-1 or STS-3c/STM-1) and low-order (LO) (VT/TU) levels.

Consider the LO-(VT/TU) level payloads. With both solutions, VT payload mappings are fundamental to more efficient data services and reduced backhauling costs. With VT mappings, data services with native rates that are sub-STS-1 (51.44Mbps) bandwidth, for example, 10Mbps Ethernet or fractional Ethernet, can more efficiently use SONET networks. The growth in data traffic and these solutions for supporting it has led to the rapid growth in VT traffic and the need for finer switching at the edge of the network.

Frame relay is another important data service for enterprises. As with other data service implementations, frame relay operators use DS-1 and DS-3 services to backhaul traffic. In the frame-relay operators' case, however, traffic is backhauled from the enterprise to the centralized frame relay switch data centers.

Backhauling is one of the fundamental driving factors in the need for finer level switching at the edge of the network. As described, data services were traditionally mapped into less efficient HO (STS-1) payload types through a convoluted process of PPP IP encapsulation and DS-1/DS-3 to STS-1 mappings. With limited data service offerings (as well as limited competition), the wasted T1/T3 bandwidth was considered undesirable, but acceptable.

Today, however, with the high demand for data services, competition among the carriers and alternative provider solutions, backhauling costs have reached unacceptable levels for both carriers and their customers. The end customers are unwilling to pay for additional high-priced, inefficiently used T1/T3 backhauling services to support their growing data requirements.

To address the demand in data services and reduced backhauling costs, telecom equipment providers have begun to develop VT-level mapping and switching/grooming functions in boxes at the edge of the network. With these new boxes, equipment providers can offer carriers flexible data services that have not traditionally fit in standard SONET/SDH payload types, and save 30 percent to 50 percent on backhauling costs.

VT switching and LO VC functions, coupled with traffic-management protocols such as LCAS and multiprotocol label switching (MPLS), are the foundation of these telecom boxes.

Voice traffic, particularly wireless, has been a key application and contributor to the growing demand for finer granularity switching at the edge of the network. Along with a move toward SONET/SDH-based ring networks for basestations, there's also development underway for next-generation add-drop multiplexers. Called multiservice provisioning platforms, these boxes will minimize backhauling and quickly allow carriers to enable and disable added services.

- Mark Donovan

Sr. Product Marketing Manager

Applied Micro Circuits Corp.





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