Service providers today face an increasing number of challenges in addressing customer demand for new services while continuing to deliver traditional voice and private line services. Until recently, new services such as Ethernet and storage area networks (SANs) could not efficiently be delivered over existing SONET/SDH networks despite the proven value of, and considerable investment in this infrastructure. However, the maturity of three new technologies-Generic Framing Procedure (GFP), Virtual Concatenation (VCAT), and Link Capacity Adjustment Scheme (LCAS)-can transform this infrastructure to make it both an efficient and flexible transport network to meet the demand for Layer 2 and Layer 3 services.

VCAT Virtual Concatenation
VCAT enables transport pipes to be "right-sized" for various data payloads by allowing SONET/SDH channels to be multiplexed in arbitrary arrangements. VCAT breaks down data packets and maps them into the base units of TDM frames; e.g., STS-1 (51 Mb/s) for SONET, and AU4 (155 Mb/s) for SDH. This data is then grouped in multiple data flows of varying size to create larger, aggregate payloads optimally sized to match available SONET/SDH pipe capacity. VCAT is applied at the end-points of the connections, which permits each channel used to be independently transmitted through a legacy transport network. Data is typically encapsulated using GFP. The ITU standard for VCAT is G.707.

GFP Generic Framing Procedure
GFP is a traffic adaptation protocol, providing convergence between packet-switched and transmission networks. GFP elegantly maps packet-based protocols such as Ethernet, Fibre Channel, FICON, ESCON, and various forms of digital video into SONET/SDH, typically using VCAT to provide right-sized pipes for data services. Compared to older framing schemes such as PoS (Packet over SONET/SDH), ATM (Asynchronous Transfer Mode) and 8B/10B encoding, GFP offers significantly reduced latency and improved bandwidth utilization. GFP offers two different mapping modes, GFP-T and GFP-F. The ITU standard defining GFP is G.7041.

LCAS Link Capacity Adjustment Scheme
LCAS allows carriers using next-gen SONET/SDH networks to provide bandwidth-on-demand to users such as Ethernet private lines clients. This dynamic bandwidth control can also be used by carriers to provide protection/survivability when network problems arise. LCAS works with VCAT to dynamically allocate bandwidth by reconfiguring VCAT groups in real-time. LCAS is a bidirectional signaling protocol between network elements that continually monitors a link, ensuring that changes in network capacity do not impair a client's traffic transmission. The ITU standard defining LCAS is G.7042