In the early days of the telecom crash, the metropolitan area was perceived as the next front for intensive hardware buildout, since city-area bandwidth needed to catch up with upgraded long-haul links.

Today, metropolitan-area networks are regaining attention, as the realm where Layer 2 and 3 protocols are aggregated and translated over a fiber network. What's more, Ethernet has found its place among existing asynchronous transfer mode, Sonet and circuit-switched protocols.

Fibre Channel and Escon protocols, traditionally used in storage-area networks for strict enterprise storage duties, are being tasked with spanning metro regions as corporations turn to distributed storage strategies. SAN protocols are meeting fiber MANs in a variety of forms: Fibre Channel over Sonet, Fibre Channel over Ethernet, iSCSI over any MAN link and Fibre Channel carried directly over dense wave-division multiplexing.

DWDM and its shorter-haul cousin, coarse WDM, are finding a widened variety of tasks as protocols meet and converge. Traditionally, CWDM is used at the edge of the enterprise, while DWDM is used in linking Sonet rings or in long-haul fiber links. Now, both can be used in any number of applications and topologies.

Effective means of aggregating the traffic types are arriving just in time. After Verizon Communications and AT&T announced some of the first new long-haul contracts in early 2004, several local-area carriers, including SBC Communications, Time Warner Telecom and BellSouth, are expected to announce requests for proposals this spring for new metro fiber networks. This week's In Focus articles look at the way traffic aggregation will be handled in such networks.

Applied Microcircuits Corp. and Agere Systems Inc. examine two Sonet technologies: the Generic Framing Protocol and virtual concatenation. Optinel Systems makes the case for melding DWDM and Gigabit Ethernet. Adva Optical Networking Inc. describes a two-stage DWDM filter architecture that allows for "wavelength banding" to aggregate traffic. And Mahi Networks Inc. describes a "metro core aggregation system," a platform in the carrier central office that can efficiently aggregate different traffic types.