D.M. Castagnozzi

A wideband all-optical WDM network

We describe some of the results of the Advanced Research Projects Agency (ARPA) sponsored Consortium on Wideband All-Optical Networks in developing architectures, technology components, and applications for the realization of scaleable, wideband, and transparent optical wavelength-division multiplexing (WDM) networks. Our architecture addresses all-optical transport over the wide, metropolitan, and local areas. It utilizes wavelength partitioning, routing, and active multiwavelength cross-connect switches to achieve a network that is scaleable in the number of users, data rates, and geographic span. The network supports two services which can be point-to-multipoint or multipoint-to-multipoint simplex or duplex connections. The A service is a transparent physically circuit-switched service and the B-service is a scheduled time-slotted circuit which is transparent within its time slots. We have developed a 20-channel local and metropolitan area WDM testbed deployed in the Boston area, now undergoing characterization and experimental applications.

High-data-rate error correcting coding

Applications such as high resolution image transmission and the aggregation of multiple bit streams have increased the interest in very high speed (Gbit/s) data communications for space applications. The ability to perform error correction on a data link can yield significant improvements in channel efficiency and can mitigate the effects of various bit error rate (BER) impairments afflicting many communications systems. A codec integrated circuit (IC) has been constructed which is capable of supporting up to 2 Gbit/s of data throughput. The device has been demonstrated in two optical communications systems, a 1 Gbit/s binary optically preamplified OOK system at 1.5 µm and an 800 Mbit/s FSK MOPA system at 0.98 µm. At a BER of 10-9, this coding provided a 3.7 dB sensitivity improvement for the OOK system, and a 6.2 dB improvement with the FSK system. The measured sensitivity of the coded OOK system was 37 photons/bit, which is better than could be realized with an ideal uncoded system. The viability of applying error correcting coding to higher data rate systems is discussed and a method of utilizing these ICs to provide four Gbit/s operation is shown.

Autonomous Timing Determination in a Time-Slotted WDM All-Optical Network

Introduction: There are several network services provided by the 20 wavelength all-optical network (AON) testbed deployed in the Boston metropolitan area[ 11 by the Consortium on Wideband AllOptical Networks[2]. One is a scheduled time and wavelength division multiplexed service which supports multiple simultaneous and independently optically routed connections for users with data rates variable from -10 Mbps to 1.244 Gbps. A key feature of this T D W M service is the time-slot synchronization required for the efficient use of network resources, especially when data is transmitted in short high-rate bursts or time-slots. We demonstrate autonomous transmitter/receiver synchronization over a metropolitan area testbed. This allows signals from widely distributed sources to mesh without collision on common a wavelength at a common fiber, router or other time-shared network resource. TDWWDM Service Description: In the TDM/WDM communication service (which we call the AON B-service) transmitters and receivers are assigned time slots lasting 1.953 ps into which data is encoded at a channel rate of 1.244 Gbps. 128 time slots are defined for each 250 ps frame. Each time slot is carried on one of 20 optical wavelengths each of which may be independently routed or broadcast to different destinations via the hierarchical AON architecture (Fig. 1). Users are scheduled sets of time slots and wavelengths for their transmission and connection needs. Fig. 2 illustrates the mapping of user sessions across time slots and channel frequencies [3]. For each wavelength, time slots not scheduled for one user are available to other users. Fast-tuning transmitters may be assigned one time-slot/wavelength schedule and fast-tuning receivers[4] another, depending on the ne:eds of their users. The timing problem arises when time slots originating in widely separated nodes of the network (e.g. the optical terminals in Fig. 1) must mesh without collision on a common wavelength at (a common network resource such as a fiber, star coupler, router, switch or other component. For high network efficiency, scheduled signals must never collide at a given time on a given wavelength in a given resource. In addition, only insignificant amounts of dead or guard time must be allotted to separate successive scheduled transmissions on a given wavelength and resource. Because transmitters and receivers have distinct schedules, receivers also must be able to identify the slot number of the received data in order to tune at the right time to the appropriate frequency and to correctly deliver data to the user channels. Autonomous Timing in the AON: The time-slotted WDM B-Service of the AON testbed derives its timing from a 1.244 GHz master oscillator which is injiected into the network at Level-1 and propagated downward to the L-0 hubs and the OTs via an out of band optical control channel. The 1.244 GHz signal is recovered at the optical terminals (OTs) and usedl to generate a 4 kHz (250 ps) frame reference. Since the frame reference for all OTs are derived from a common master oscillator they are frequency-locked but possess an arbitrary phase offset determined by the initial state of the counters used to generate the frame clock from the 1.244 GHz master and the differences in propagation delay. Absolute phases at are established when a B-service connection is first made. Determination of clock phase relies on detection of a valid-data transmission window in the received signal stream and the recognition of valid 8B/10B data code-words within that window (]Fig. 3). This takes place autonomously at startup by the optical terminals and requires no test-equipment. The receiver timing offset (delay) is determined relative to existing metropolitan-area (L1) traffic by :sliding in time the received valid data window (which is slightly longer than the user data payload) to center the position of the received data payload within the window. The edges of the valid data window are sensed by FIFO status indicators in

Control and interfaces for an all-optical network testbed

We describe some of the control and interface techniques employed in the construction of an all-optical network testbed fielded in the Boston metropolitan area. The network was developed by a consortium of AT&T Bell Laboratories, Digital Equipment Corporation, and the Massachusetts Institute of Technology under a grant from ARPA. The network testbed contains local, and metropolitan area nodes that support optical broadcast and routing modes. Electronic access is provided through optical terminals that support multiple services having data rates between 10 Mbps/user and 10 Gbps/user. Applications include video, support for ATM traffic, and telemedicine applications.

Description of all-optical network test bed and applications

We describe an all-optical network testbed deployed in the Boston metropolitan area, and some of the experimental applications running over the network. The network was developed by a consortium of AT&T Bell Laboratories, Digital Equipment Corporation, and Massachusetts Institute of Technology under a grant from ARPA. The network is an optical WDM system organized as a hierarchy consisting of local, metropolitan, and wide area nodes that support optical broadcast and routing modes. Frequencies are shared and reused to enhance network scalability. Electronic access is provided through optical terminals that support multiple services having data rates between 10 Mbps/user and 10 Gbps/user. Novel components used to implement the network include fast-tuning 1.5 micrometers distributed Bragg reflector lasers, passive wavelength routers, and broadband optical frequency converters. An overlay control network implemented at 1.3 micrometers allows reliable out-of-band control and standardized network management of all network nodes. We have created interfaces between the AON and commercially available electronic circuit-switched and packet-switched networks. We will report on network applications that can dynamically allocate optical bandwidth between electronic packet-switches based on the offered load presented by users, without requiring interfaces between users and the AON control system. We will also describe video and telemedicine applications running over the network. We have demonstrated an audio/video codec that is directly interfaced to the optical network, and is capable of transmitting high-rate digitized video signals for broadcast or videoconferencing applications. We have also demonstrated a state-of-the-art radiological workstation that uses the AON to transport 2000 X 2000 X 16 bit images from a remote image server.

Demonstration of multigigabit/per second services over a 20-channel WDM wavelength-routed all-optical metropolitan-area network

An experimental all-optical, wavelength-routed network testbed has been constructed in the Boston metropolitan area. The network has 20 optical channels, space by 50 GHz and provides dedicated circuit-switched wide-band service at user defined modulation formats and rates up to 10 Gbps, and time-slotted WDM services for medium and low-rate users. We are now characterizing the deployed network which spans over 87 km interconnecting four all-optical local-area networks in Littleton, Lexington, and Cambridge Massachusetts. We discuss wavelength sharing and reuse, local broadcast, routing, multi-cast and multi-hop connections at 1.244, 2.488, and 10 Gbps. We present the system design and the performance (e.g. BER and cross-talk) of local-broadcast, metropolitan-area-routed and broadcast transmission modes.

Forward Error Correction In A I Gbit/s/channel Wavelength-division-multiplexed System

Forward error correction (FEC) can be used to improve the performance of an optical communications system in a number of different ways. FEC can reduce the impact of crosstalk between channels in a wavelength-division-multiplexed (WDM) network, so that more channels may utilize a given bandwidth. For a single channel, FEC may be used by the system designer to increase link distances, reduce transmitter power requirements, or tolerate intersymbol interference. In either application the performance improvement achieved with FEC can overcome component limitations, particularly at high data rates. Most previous high data rate coding work has concentrated on theory or coding a single TDMA slot or tributary [ 1,2] because of the difficulty of implementing standard decoders at high rates. Recent encodeddecoder implementations (codecs) support Gb/s data rates, but provide little sensitivity improvement or coding gain, and use low rate codes that substantially increase the required channel bandwidth over that for uncoded data [3]. We have implemented hard decision threshold decoding [4] of a rate 4/5, constraint length 332 convolutional code on a single VLSI bipolar silicon IC dissipating -5W. The chip accepts serial data rates up to 1 Gb/s, and up to 2 Gb/s in parallel mode, while requiring only 25% bandwidth expansion.