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We propose a modular scalable long-haul architecture supporting variable-bandwidth channels with bit rates from 100 Gbit/s to beyond 1 Tbit/s. Colorless transceivers have two operating modes. One transceiver can transmit/receive a conventional narrow-band channel occupying a 30-GHz bandwidth and conveying 100 Gbit/s, or a set of M transceivers can cooperate to transmit/receive a wideband channel occupying an M 50-GHz bandwidth and conveying roughly 160 Gbit/s per 50 GHz (assuming polarization-multiplexed quaternary phase-shift keying). A colorless wavelength-selective switch supports two modes of add/drop (or (de)multiplexing) operation. It can add/drop narrow-band channels (each from/to one port) with minimal loss, or can add/drop wideband channels (each from/to M ports) without spectral gaps, with an additional loss not exceeding 1/3 (-4.8 dB), independent of M. Transceivers can use either single-carrier modulation or orthogonal frequency-division multiplexing (OFDM). We analyze and simulate OFDM-based systems to determine key design requirements, especially for synchronization of cooperating transceivers. A representative design achieves 1520-km reach with 1.4-dB margin in a dispersion-managed network using only erbium-doped fiber amplifiers, improving average spectral efficiency from about 2 to nearly 3 bits/s/Hz.