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Unformatted text preview: 396 Client Layers of the Optical Layer mix of ODU1s and ODU2s can be multiplexed into an ODU3. OTN also supports virtual concatenation. Here, we will limit the discussion to the OTN frame of an ODU2 carrying four ODU1s. OTU2 frames are organized into multiframes of size four, where each multiframe carries the frames of four ODU1s: ODU1, ODU1, ODU1, and ODU1. The payloads of the ODU1s are byte interleaved into the payload of a ODU2 frame. The OPU overhead of an OTU2 frame will carry information in its PSI byte about the multiplexed signals. The payload type indicates multiplexed ODU signals. Bytes 2 through 17 of the PSI indicate the type of signals being multiplexed, for example, whether the signals are ODU1, ODU2, or ODU3, and their position in the payload. 6.3 Generic Framing Procedure Generic Framing Procedure (GFP) is a common method to adapt diverse packet pro- tocols at the link layer to be transported over SONET/SDH or the Optical Transport Network (OTN) as shown in Figure 6.12. It facilitates interoperability of equipment of different vendors. The packet protocols can have variable-length packets, such as Ethernet, IP over PPP, Gigabit Ethernet, and Resilient Packet Ring (RPR); or have fixed-length packets, such as Fibre Channel. For clients with variable-length packets, GFP can be combined with SONET/SDH Virtual Concatenation (VCAT) and Link Capacity Adjustment Scheme (LCAS) to provide packet links. As discussed in Section 6.1.2, with VCAT, a packet link can be configured to have the right amount of bandwidth for the client traffic, and with LCAS, the link bandwidth can be adjusted to meet variations in the packet traffic load. For clients with fixed-length packets, a GFP link can resemble a constant bit rate link, leading to low latencies.constant bit rate link, leading to low latencies....
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- Spring '09