Today, many carriers are investigating the best way to migrate their legacy SDH/SONET metro-aggregation networks to packet-optical transport networks (P-OTN) that can scale to higher bandwidths, deliver lower cost per bit transported and provide the same resiliency as the legacy networks.
The traditional access points for aggregates of user traffic into SDH/SONET metro ring networks have been the T1 (1.5 Mbit/s) and E1 (2 Mbit/s) interfaces. However, these are quickly being replaced by Gigabit Ethernet (GbE), which provides lower cost and much higher bandwidth.
This article will describe one important step on the way to packet-optical transport — namely how the new and important International Telecommunications Union Standardization Sector (ITU-T) OTN protocol can be used to provide connectivity in WDM rings at the GbE level in a manner equivalent to E1/T1 connectivity in SDH/SONET networks.
A brief overview of OTN
The ITU-T G.709 standard "Interfaces for the Optical Transport Network (OTN)" was originally developed to provide a common transport mechanism in the optical layer to enable interworking between different DWDM systems. With development starting in 2001, the G.709 standard was primarily optimized for transport of SDH/SONET, which was the dominant technology of the day, and not for Ethernet. However, as Ethernet connectivity has grown in the metro, the ITU-T is now working on adaptation to G.709 that provides more efficient transport mechanism for Ethernet within OTN.
An important aspect of OTN is the ability to provide complete transparency for client signals, and for the frequency of the line signal (important for the synchronization of radio cells in mobile networks). Transparency means that the complete client signal is transported untouched by OTN, in contrast to IP/MPLS-routing, where each router may rewrite the header of the packets in the data flow. Furthermore, any client signal is transported at its own clock rate and phase in contrast to SDH/SONET, where the complete network is synchronised to a common clock.
So far OTN has mostly been used for connecting complete wavelengths. However, OTN can provide connectivity at the sub-wavelength level via the optical data unit (ODU) container. For example, a 10 Gbit/s optical wavelength may carry four ODU1 containers each at 2.5 Gbit/s. In April 2009, ITU-T approved amendment three to the G.709 standard (sec. 17.7.1) which defines a new ODU container — ODU0 — which is exactly half the size of ODU1 and can carry a GbE stream transparently.
Protection switching in SDH/SONET
The left side of figure 1 shows a typical SDH/SONET ring with add-drop multiplexers providing access connections for T1/E1 interfaces carrying aggregated user traffic. Inside the ring, connectivity between T1/E1 access points is obtained by configuring virtual containers (VCs) inside SDH/SONET as fixed-bandwidth channels of VT-1.5 (1.544 kbit/s) or VC-12 (2.048 Mbit/s). Note that even if the underlying payload is packet-based, there is no forwarding based on packet headers (like MAC addresses or MPLS labels) taking place. The VC-channel is simply a continuous transmission of bits at a constant bit rate. When there is no user traffic, the VC-channel transmits idle bits.
Another important point is that a protection VC-channel can be pre-configured (as indicated by the dashed line) in the opposite direction around the ring than the working VC-channel. In case of a failure in the network, such as a fibre cut, the traffic will automatically be switched from the working VC-channel to the protection VC-channel in less than 50 ms, thereby restoring customer connections extremely efficiently and quickly.
This works as follows: at each end of the VC-channel (the VT-1.5/VC-12 termination points), user traffic is broadcast on both the working and protection VC-channel at all times, while in the receiving direction there is a simple switch that selects traffic from either the working (normal operation) or protection channel (in a failure situation). The VC-channel has overhead bytes that carry information about signal integrity and quality of the channel. An automatic switching event from working to protection VC-channel is typically triggered by a loss-of-signal (LOS) or degraded signal defect.
Protection switching in OTN
The right side of figure 1 shows a WDM ring with OTN as the transport mechanism. The access points for aggregated user traffic are now GbE and not T1/E1. Also, instead of a VC-1.5/VC-12 channel, an ODU0 channel is configured to carry the GbE payload transparently between the two connection points. As in the SDH/SONET case there is no forwarding based on packet labels. Furthermore, a protection ODU0 channel can be configured and set up for automatic protection switching in less than 50 ms. Switching decisions can be based on signal integrity information in the ODU0 overhead bytes.
Figure 2 is a detailed view of what an OTN ADM on a linecard may look like using, for instance, the TPACK TPO124 OTN mapper device. From the right, a wavelength containing eight ODU0 containers is transmitted and received in both directions (east and west) of the WDM ring. In this example, seven of the ODU0 containers are being forwarded straight through the node, while one ODU0 container is terminated, and its GbE payload forwarded on the client port. This add-drop functionality is made possible through a simple built-in ODU crossconnect in the TPO124 device.
Figure 2 also shows the protection mechanism. In the transmit direction, the ODU0 with GbE payload is being broadcast on two ODU0 channels — one going out on the east fibre and one going out on the west. In the receive direction, the payload is received from either the ODU0 channel on the east (normal operation) or the ODU0 channel on the west fibre (failure situation).
In conclusion, we have described how connectivity at the GbE level through a WDM ring with OTN can be obtained in a manner that is analogous to the way carriers configure T1/E1 connectivity through an SDH/SONET ring. Furthermore, such GbE connections can be protected, with protection switching in under 50 ms matching the protection switching capabilities of SDH/SONET rings. The two key enablers for this are the newly standardized ODU0 for mapping GbE into OTN, and a new generation of OTN mapper components like the TPACK TPO124.