IP-over-DWDM in core networks: Interfaces pose complex problems
The situation is completely different in the high-speed network cores where you need ultrafast failure detection and robust and fast rerouting around failed links or nodes. If you want to implement these features in a (potentially) multivendor environment, you have to enforce strict standard compliance. This is where the IP-over-DWDM story gets complex.
Let's start from the outside, where the routers (including layer 3 switches) usually offer these connectivity options:
- Gigabit Ethernet (GE)
- 10 GE
- Packet-over-SONET (POS) interfaces with speeds ranging from optical carrier levels of OC-3/STM-1 (155.52 Mbps) to OC-192/STM-64 (10 Gbps).
Most of these interfaces cannot be patched directly into a DWDM core owing to lack of G.709 (a standardized method for transparent transport of services over optical wavelengths in DWDM systems, also known as the Optical Transport Hierarchy, OTH, standard) framing. A transponder card is needed in the DWDM shelf to encapsulate the raw data (payload) received from the client devices (routers or switches) into a G.709 Optical Transport Unit (OTU).
Not surprisingly, some vendors want you to spend less money on DWDM transponders and invest more in high-end routers. They offer you various IP-over-DWDM schemes using 10 Gigabit Ethernet or 10 Gigabit (OTU2) or 40 Gigabit (OTU3) Packet-over-SONET/SDH interfaces. The POS interfaces should not present a significant challenge; after all, they're nicely integrated into the Optical Transport Network (OTN) framework defined by G.709. You just have to make sure of the following:
- The router's interface can insert the payload into the G.709 OTU;
- The router's hardware supports G.709 FEC and EFEC;
- The router's software recognizes the performance and alarm data carried in the G.709 header.
Interfaces for 10GE are trickier. The 10GE standard (IEEE 802.3ae) defines two line rates: WAN PHY (9.95328 Gbps), which matches the OC-192/STM-64 line rate and LAN PHY (10.3125 Gbps), which results in 10 Gbps throughput. The addition of the G.709 wrapper around the LAN PHY-encoded data results in an overclocked signal at 11.0975 Gbps. This signal passes through the (passive) optical part of the DWDM network just fine, but might not be recognized correctly by a third-party optical-electric-optical (OEO) regenerator.
IP over DWDM and end-to-end management systems
Last but not least, you need an end-to-end management system that simplifies network provisioning and monitoring. Ideally, the elements in the end-to-end solution should be able to communicate and establish on-demand paths across the network based on true bandwidth needs.
The ultimate network management solution in IP-over-optical environments is (Generalized Multiprotocol Label Switching (GMPLS), the signaling protocol that unites packet switching, TDM, wavelength switching and fiber switching (optical cross connects). Vendors already deliver GMPLS-enabled equipment (for example, Juniper's M-series routers and Cisco's CRS-1).
But there's a long way to go before you'll be able to deploy a seamless multivendor solution and provision it automatically through GMPLS. In the meantime, you're left with a patchwork similar to the early days of ATM networks: proprietary network management systems, customized integration solutions and manual configuration.
Ivan Pepelnjak is a 25-year veteran of the networking industry
This article originally appeared on SearchTelecom.com