Virtual concatenation, LCAS benefit next-gen Sonet/SDH nets

To support the massive Internet expansion of recent years, carriers significantly increased the capacity of backbone and core networks. Metropolitan area networks (MANs), however, have not kept pace with this growth, creating a bottleneck in the overall network infrastructure. Carriers built the metro using mature Sonet technologies, which, while optimal for voice or other jitter- and delay-sensitive applications, lack fast circuit-provisioning capabilities, scalability and bandwidth efficiency. This makes the MAN inefficient for the cost-effective transport of data.

Carriers looking to reduce capital expenditures, while meeting the demands of data traffic growth and new service offerings, need to extract maximum value from their existing networks. Emerging mapper/framer technologies, such as virtual concatenation and link capacity-adjustment scheme (LCAS), enable carriers to upgrade their existing Sonet networks with minimal investment. These technologies can help increase carriers' profitability by enabling new services through greater scalability, faster provisioning and much higher efficiency when transporting Ethernet over Sonet and packet over Sonet data.

Virtual concatenation significantly improves the efficiency of data transport, as well as the scalability of legacy Sonet networks, by grouping the synchronous payload envelopes (SPEs) of Sonet frames in a nonconsecutive manner to create virtual concatenation groups. Traditionally, layer capacity formats were available only in contiguous concatenated groups of specific size. SPEs belonging to a virtual concatenated group are called members of that group. This virtual concatenation method allows finer granularity for provisioning of bandwidth services and is an extension of an existing concatenation method, contiguous concatenation, in which groups are presented in a consecutive manner and with gross granularity.

Different granularities of virtual concatenated groups are required for different parts of the network, such as the metro core or metro edge. Virtual concatenation applies to low order (VT-1.5) and high order (STS-1) paths. Low order virtual concatenated groups are suitable at the edge while the high order virtual concatenated groups are suitable for the core of the MAN.

Virtual concatenation allows for the efficient transport of Gigabit Ethernet. Traditionally, Gigibit Ethernet is transported over Sonet networks using the nearest contiguous concatenation group size available, an OC-48c (2.488 Gbits/second), wasting approximately 60 percent of the connection's bandwidth. Some proprietary methods exist for mapping Ethernet over Sonet, but they, too, are inefficient. With virtual concatenation, 21 STS-1s of an OC-48 can be assigned for transporting one Gigibit Ethernet. The remaining 27 STS-1s are still free to be assigned either to another Gigibit Ethernet, to any other data client signal, for example, Escon, Ficon or Fibre Channel, or to any PDH or SDH signal.

The Link Capacity Adjustment Scheme (LCAS) adds a new member, in this case an STS-1, to an existing virtual concatenation group. LCAS operates at the end points of the connection only. To deploy LCAS, carriers just install new tributary cards.
Source: Multilink Technology Corp.

Virtual concatenation improves bandwidth efficiency more than 100 percent when transporting clients such as Gigabit Ethernet using standard mapping or around 25 percent when compared to proprietary mapping mechanisms (for example, Gigibit Ethernet over OC-24c). This suggests carriers could significantly improve their existing networks' capacity by using virtual concatenation. Further, carriers gain scalability by increasing the usage of the network in smaller incremental steps. And, the signals created by virtual concatenation framers are still completely Sonet, so a carrier need only upgrade line cards at the access points of the network, not the elements in the core.

While virtual concatenation provides the ability to "right size" Sonet channels, LCAS increases the flexibility of virtual concatenation by allowing dynamic reconfiguration of virtual concatenation groups. Together, the technologies allow for much more efficient use of existing infrastructure, giving service providers the ability to introduce new services with minimal investment.

LCAS allows carriers to move away from the slow and inefficient provisioning process of classical Sonet networks and offers a means to incrementally enlarge or shrink the size of a Sonet data pipe without impacting the transported data. To hitlessly provision more bandwidth over a Sonet link with virtual concatenation, LCAS can add or remove members (STS-1s) of a virtual concatenation group. LCAS uses a request/acknowledge mechanism that allows for the addition or removal of STS-1s without disrupting traffic. The LCAS protocol works unidirectionally, enabling carriers to provide asymmetric bandwidth.

The LCAS protocol uses the H4 control packet, which consists of the H4 byte of a 16-frame multiframe. The H4 control packet contains information of the member's sequence (sequence indicator - SQ#) and alignment (multiframe indicator - MFI) of a virtual concatenated group. The information carried on H4 control packet includes commands such as: LCAS not supported on this STS-1; add this STS-1 to a virtual concatenated group; this STS-1 is in use; last STS-1 of this virtual concatenated group; and this STS-1 is not part of a channel.

LCAS operates at the end points of the connection only, so it does not need to be implemented at the nodes where connections cross or in trunk line cards. This allows carriers to deploy LCAS in a simple manner, by installing new tributary cards. Likewise, they can scale LCAS implementations by adding more tributary cards, without requiring hardware upgrades, for example, to add/drop multiplexers, throughout the entire network.

One of the greatest benefits of LCAS for carriers is the ability to "reuse" bandwidth to generate more revenue, and/or offer enhanced services that allow higher bandwidth transmission in times of need. This will be a key reason for carriers to implement next-generation Sonet gear, and the potential extra revenue stream from such services and bandwidth efficiency could prove crucial for the health of the industry.

Bandwidth on demand (BoD) service allows carriers, Internet service providers and business customers to purchase virtual leased lines for Internet Protocol bandwidth as their traffic grows. LCAS makes it possible for carriers to offer reliable BoD services over a Sonet infrastructure, by enabling hitless bandwidth increases or decreases, which was previously impossible. This benefits the customer by providing Sonet resiliency with service contract options such as a "purchase-as-you-grow" option. A BoD service derivative, "time-of-day service," creates a traffic profile that allocates different bandwidths for the same end-to-end connection at different periods of the day, giving customers better ways of spending their IT budgets. For example, a typical enterprise customer would pay for more bandwidth during the day, using four STS-1s for the hours 8:00 a.m. to 8:00 p.m., and then would use only one STS-1 for the remainder of the night. In a situation such as this, carriers could sell capacity during the day to one customer with heavy daytime traffic, and then reuse the same capacity (sell it again) to a second customer with heavy nighttime traffic.

Graceful degradation is the "degradation of a system in such a manner that it continues to operate, but provides a reduced level of service rather than failing completely," as defined by the Institute for Telecommunication Sciences (Boulder, Colo.). Since the members of a virtual concatenation group don't need to use the same physical links of the network (path diversity), some of its members could experience a failed link along their path while the remaining members do not.

With LCAS controlling the path, only the affected members of that virtual concatenation group would be removed while the link failure exists. This keeps the service up and running but at degraded levels, providing additional network resiliency. Since classical Sonet technology does not have this capability, the feature of graceful degradation is a service by itself, differentiating carriers and increasing the availability time of customers' connections.

These are just a few examples of services carriers will be able to offer through virtual concatenation and LCAS. Mapper/framer devices with virtual concatenation and LCAS have the potential to increase the revenue and profitability of existing Sonet networks, by allowing carriers to offer more services and make more efficient and flexible use of existing network resources.