Declarations of Independence
And allowing for a mix of protocols isn't as complicated as it sounds. That's because most metro DWDMs are protocol and bit-rate independent (Sycamore's is the exception). Customers plug their equipment into a single fiber interface; the device recognizes the bit rate and service coming in and notifies the management console. The console checks its database to ensure the customer has paid for that amount of bandwidth, and then maps the service onto the appropriate wavelength.
Then again, not all gear lives up to its independent billing. Ciena and Nortel, for example, give customers a choice of two fiber interfaces, one for handling lower-speed services like OC3, OC12, fast Ethernet, and Escon, the other for OC48, gigabit Ethernet, and Fibre Channel. Upgrading between speeds requires cards to be swapped out.
And while Osicom claims to support services of all speeds on the same physical port, the vendor also sells additional modules that work with specific protocols. That's because its universal interface can't recognize which service is being carried, so it's unable to manage and bill for it. Similarly, Ericsson's universal interface handles all services but can only manage and bill for a small number of them.
Give It Up
There's plenty to be gained by switching from one technology to another, but carriers may also wonder what they'll lose by dropping Sonet in favor of DWDM. Fact is, most DWDM vendors are still struggling to replicate the path restoration, management, and time-slot assignment capabilities available in Sonet ADMs.
Start with the ring configuration. In Sonet, rings can be organized according to two Bellcore standards: UPSR (unidirectional path-switched ring), or BLSR (bidirectional line-switched ring). UPSRs enforce a rigid 1:1 redundancy throughout the ring; BLSRs allow some of the redundant capacity inherent in Sonet to be reused.
But most DWDM vendors can't offer BLSR support. That would be a tricky engineering feat, according to Weingarten. In fact, Sycamore is the only vendor to have pulled it off. [We need to ask this of New Access]
As for protection switching, DWDM actually beats Sonet in one way: In the event of a fiber cut, most devices can reroute to the backup fiber in 25 milliseconds or less, whereas Sonet ADMs can take as long as 50 ms. But problems crop up when Sonet and DWDM are used in conjunction. DWDM's own protection switching capabilities have to be turned off to avoid contention between the two fail-over schemes. All vendors let users turn off protection switching-except for Sycamore. It says its fail-over rate is just 3 ms, which means it can switch over to the protect line before Sonet even realizes it.
Generally speaking, Sonet also beats DWDM on time-slot assignment. From a management console, carriers can program a Sonet ADM to assign a time-slot from one customer to another; an internal cross-connect simply switches the circuit over. But with DWDM, there's no way to switch one wavelength assignment from one customer to another without physically moving interface cards between slots in the chassis. The exception is Nortel's Optera, which contains an electrical cross-connect to reprovision wavelengths between customers.
Wavelength assignment is vital because it's one of the requirements for just-in-time provisioning. "Let's say Amazon.com calls its service provider and says it's having a book sale for a week," explains Weingarten. "They might need an extra circuit to handle the increased traffic load." Wavelengths could be reassigned specifically to handle that temporary increased load.
Management capabilities are next on the checklist. Vendors furnish their devices with management software that can talk to various OSSs (operation support systems). Most DWDMs communicate with the OSS through various legacy Bellcore standards like TMN (telecommunications management network) and NMA (network management application). Also, Nortel, Osicom, and Sycamore support Corba (common object request broker architecture) programming tools, which allow DWDMs to be programmed to communicate with OSSs. Generally, incumbent carriers are sticking with the Bellcore specs, while competitive entrants prefer to write Corba code-or even rely on SNMP from the data networking world. "CLECs want to manage everything like one big WAN," says Osicom's Mackey, adding that SNMP lets them do that.
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