TECHNOLOGY TRENDS
ROADM stages, however—in the metro, 10-20 ROADM stages are common—the lightpath’s direction can at least be switched, but its wavelength remains fixed. As a result, changing the signal’s start and end points and hence network configura- tion occurs infrequently because manual intervention is needed, says Wellbrock. An engineer is thus required to visit both ends of the link to change the lightpath’s direction and wavelength during a network restoration event, for example, or when modifying traffic as the network evolves. Despite the limited flexibility, ROADMs
benefit operators’ networks. Wavelengths can remain in the optical domain, passing through intermediate locations without using transponders and saving on costly optical/electrical conversions. ROADMs also replace the previous arrangement of fixed optical add/drop multiplexers, exter- nal optical patch panels and cabling. But operators want more from their
ROADMs. “In today’s networks, wave- lengths are established and are rarely changed,” says Brandon Collins, CTO of the consumer and commercial optical products division at JDS Uniphase, a WSS maker. “But the reality is that the network is always changing, always growing, always being maintained and there are always failures [like fibre cuts].” When provisioning a wavelength, the
most efficient route is sought, says AT&T’s King. But six months later this can change, given traffic growth, such that an opera- tor will want to move the wavelength to another route. “Should you not be able to move a wavelength deployed on one route onto another more efficiently? Heck, yes,” says King. “We want to move around those wavelengths just like we move around channels or customer VPN circuits in today’s world.” WSS makers are thus developing new
ROADMS that allow signals’ direction and wavelengths to be changed. Known as colourless and directionless, they will help enable automatic wavelength provisioning and circumvent manual servicing. Automatic network restoration will also
benefit from such ROADMs. Currently, an operator must decide when it can send engineers to fix a fault. If the fault can be
July/August 2010
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Standards bearing: advancing control plane technology
Another key component for agile optical networks is the control plane software. “Many of the networks today have some of the hardware components to make them agile but lack the software,” says Andrew Schmitt, directing analyst, optical, at Infonetics Research. The network can be segmented into the data plane, used to transport traffic, the control plane that uses routing and signalling protocols to set up connections between nodes, and the management plane that oversees the control plane. “What is deployed mostly today is a Sonet/SDH control plane,” says Tom Rarick, principal engineer, transport, at Tellabs. “This is to manage Sonet/SDH ring or mesh networks, using standalone cross-connects or partnered with ROADMs, with the switching primarily done electrically.” Three industry bodies are involved in advancing control plane technology. The Internet Engineering Task Force (IETF) is standardising Generalized Multiprotocol Label Switching (GMPLS) while the ITU is developing control plane requirements and architecture dubbed Automatically Switched Optical Networks (ASON). The third body, the Optical Internetworking Forum (OIF), oversees the implementation efforts. “Using a control plane with all-optical is a challenge,” says Hans-Martin Foisel, OIF president. “The control plane has to have a very simplified knowledge of the optical parameters.” The photonic layer is highly analogue and there are numerous optical parameters that can be taken into account. Any protocol needs to streamline the process such that simple rules can be used for operators to decide whether a route can be completed or whether signal regeneration is needed. The IETF is working on wavelength switched optical networks (WSON), the all-optical component of GMPLS, to enable such simplified rules within a single network domain. “What is completely out of scope is routing transparently between vendors,” says Foisel. There is still much work to be done before management and control planes exploit the IP, electrical and optical network layers. “GMPLS/ASON are still years out and some operators may never deploy them,” says Schmitt at Infonetics. But Kline at Ovum highlights Huawei and Alcatel- Lucent as keen promoters of control-plane-enabled networking technologies: “Huawei has 250 ASON applications with over 80 carriers, and 30-plus OTN WDM ASON applications.”
fixed in two hours and meets the service level agreement, engineers will be sent. But if it is a remote site and will take days to rectify, a restoration activity will be initiated. “This would try and reroute at the physical layer and colourless, direc- tionless ROADMs would allow you to do that automatically [in conjunction with a control plane],” says Wellbrock. “The ability to deploy an all-ROADM
mesh network and remotely control it, to build what we need as we need it, and reconfigure it when needed, is a tremen- dously powerful vision,” says King. By combining electrical and optical
switching, operators will be able to contin- ually optimise their networks, claims Berthold at Ciena: “They can devolve their networks to the lowest cost and most power-efficient solution.” Ciena is adding colourless/directionless
ROADMs to its 3.6 terabit-per-second 5430 electrical switch. “When you start growing traffic from a low level you need electrical switches in many places in order to effi- ciently fill wavelengths,” says Berthold. “But as traffic grows there is more oppor- tunity to bypass intermediate nodes with an optical path.” By tying the ROADM
with the electric switch, traffic can be regroomed and electrical paths set up on-demand to continually optimise the network. To achieve this Ciena will need to tie two control planes together, at the electrical and optical level. “We also need to develop a total network optimisation suite,” says Berthold. Other challenges remain, such as
handling customer application rates at 1-, 10- and 40-gigabits-per-seconds (Gbps) on 100 Gbps-based infrastructure. This will use the Optical Transport Network (OTN) protocol, which will require electrical switch and control plane support. Interoperability between vendors’ equip- ment will also need to be demonstrated. Verizon expects colourless, direction-
less ROADMs to be available in 2012, but the vision of a dynamic optical network will take longer as these challenges are addressed and the control plane work progresses. There are also business case challenges. “We really are at the cusp of dramatic changes in the way transport is built and architected,” says King at AT&T. “Making [agile optical mesh networking] happen: big challenge; but big challenge, big reward.” n
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