PONs, in both ATM- and Ethernet-specific versions, are finding favor as a low-cost means to fan out broadband service from optical network units in fiber-to-the-curb or fiber-to-the-building architectures. The passive networks compete with VDSL, cable- based hybrid fiber/coax and other access methods that utilize neighborhood-based optical terminals.

While a few semiconductor vendors, including Infineon Technologies, have offered semicustom PON solutions, most PON OEMs have elected to develop full-custom access systems. Motorola appears to be the first to shift PON design to the application-specific standard-product model.

Motorola chose broadband PON technology based on the old ATM PON as its first PON architecture, though chip sets for Ethernet and Gigabit PONs have not been ruled out. This was not due merely to Motorola's past experience with ATM, said Niket Jindal, marketing manager for the company's Semicustom Products Sector. It also arose from the fact that both NTT in Japan and Verizon Communications, leading a group of local incumbent carriers in the United States, are committed to broadband PON as a means of bringing broadband services to a wider audience.

Motorola's MC92701 uses a Utopia interface to connect directly to a PowerPC processor. On the physical-layer side, the device has an embedded clock-and-data recovery (CDR) unit so that it can link directly to an optical module. Two separate 16-bit data buffer interfaces link to off-chip SDRAM for upstream and downstream data flows. The CDR block has an integrated analog phase-locked loop for a reference clock input.

The PON physical-layer block on the chip, tightly linked to the CDR block, supports the ITU Dynamic Bandwidth Assignment (DBA) spec, G.983.4. That standard provides quality-of-service mechanisms specific to passive optical access methods. The physical-layer block can support symmetric 155-Mbit flows or asymmetric flows of 622-Mbit/155-Mbit services.

The ATM transmission convergence block on the chip supports all DBA specs and can handle up to 32 independent connections per Utopia port, or 64 total, Motorola said. DBA support in ATM and physical-layer portions of the chip requires support for burst-mode services, which Jindal called one of the most difficult features of broadband PON design.

While PON is gaining some traction in small, municipal U.S. markets and in Chinese regional markets, Jindal said Motorola realizes the two biggest near-term opportunities are among larger carriers, particularly NTT in Japan, and among local incumbent carriers in the United States. Emerging PON markets will be addressed cautiously, he said.

"In some areas of the world PON is an established market, but in many other places it's a speculative market," Jindal said.

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