 by the same SONET network without losing efficiency in the data transport: thus, it is not exact to say that GFP allows convergence over IP. We can say that GFP is much more flexible: it allows convergence on TDM maintaining, if needed, different packet modes for different services. 3.3.2.2 Virtual Concatenation Virtual concatenation (VCAT) is a technique that allows SONET channels to be multi- plexed together in arbitrary arrangements. This permits custom-sized SONET pipes to be created that are any multiple of the basic rates. VCAT is valid for STS-1 rates as well as for VT rates. Networks Fundamentals and Present Architectures network IP virtual IP routing element network SONET layer Ring - 1 Ring-: 2 WDM transmission FIGURE 3.51 Representative topology of a network sustaining over a SONET multiring physical layer both an IP virtual network and a storage area network (SAN) using GFP encapsulation. All the intelligence to handle VCAT is located at the endpoints of the connections, SO each SONET channel may be routed independently through the network without requir- ing any knowledge of the VCAT. In this manner, virtually concatenated channels may be deployed on the existing SONET/SDH network with a simple endpoint upgrade. All the equipment currently in the center of the network need not be aware of the VCAT. In contrast, arbitrary contiguous concatenation that is used in POS architectures also allows for custom-sized pipes to be created, but requires that the concatenated pipe be treated as single entity through the network. This requirement makes deployment of this service virtually impossible over the legacy SONET networks without a significant upgrade. For example, using VCAT, the SONET transport links may be "right-sized" for Ethernet transport. In effect, the SONET pipe size may be any multiple of Mbit/s for high-order VCAT (STS-1), or 1.6Mbit/s (VT1.5)/2.176 Mbit/s (VT2) for low-order VCAT. The matching between virtually concatenated SONET channels and Ethernet frames is shown in Table 3.4 besides the v