C.L. Lu

CORD: contention resolution by delay lines

The implementation of optical packet-switched networks requires that the problems of resource contention, signalling and local and global synchronization be resolved. A possible optical solution to resource contention is based on the use of switching matrices suitably connected with optical delay lines. Signalling could be dealt with using subcarrier multiplexing of packet headers. Synchronization could take advantage of clock tone multiplexing techniques, digital processing for ultra-fast clock recovery, and new distributed techniques for global packet-slot alignment. To explore the practical feasibility and effectiveness of these key techniques, a consortium was formed among the University of Massachusetts, Stanford University, and GTE Laboratories. The consortium, funded by ARPA, has three main goals: investigating networking issues involved in optical contention resolution (University of Massachusetts), constructing an experimental contention-resolution optical (CRO) device (GTE Laboratories), and building a packet-switched optical network prototype employing a CRO and novel signaling/synchronization techniques (Stanford University). This paper describes the details of the project and provides an overview of the main results obtained so far.

Dynamic range requirements for microcellular personal communication systems using analog fiber-optic links

Fiber infrastructures in future personal communication systems (PCSs) must minimize remote antenna size and cost, and facilitate system maintenance and upgradeability. These goals can be met by a centralized PCS infrastructure using analog fiber-optic links. It is essential that the relationship between optical-link quality in terms of spurious-free dynamic range (SFDR) and PCS quality of service be accurately quantified so that optical device and other infrastructure design requirements can be determined. This paper presents a comprehensive wireless/optical simulation model which combines wireless system characteristics (such as fading, cochannel interference, diversity, and power control) with the noise and nonlinearities of fiber-optic links. Results of the simulation indicate that representative SFDR requirements for fiber infrastructures in PCS systems are in the 72-83 dB/spl middot/Hz/sup 2/3/ range. The impact of varying environmental characteristics as quantified by distance loss and shadowing variance is between 7-10 dB. A larger distance loss or lower shadowing variance result in lower SFDR requirements. The required automatic gain control (AGC) accuracy decreases as the SFDR increases. These results indicate that either distributed feedback (DFB) or Fabry-Perot (FP) semiconductor laser diodes can be used in the implementation of PCS infrastructures.

CORD-a WDMA optical network: subcarrier-based signaling and control scheme

CORD (COntention Resolution using Delay lines) is a 2.5-Gb/s//spl lambda/ wavelength-division multiple-access (WDMA) packet-switched network experiment. We experimentally demonstrated the simultaneous transmission of a 2.5-Gb/s packet-switched payload data and an 80-Mb/s control channel using the multichannel subcarrier multiplexing (MSCM) signaling scheme with 3 and 3.5 GHz subcarriers. We optimized the control channel modulation depth at the transmitter to 0.4, and the splitting ratio to a 90/10 payload data/control channel split at the receiver, to obtain the optimal receiver sensitivity of -12.9 dBm.<<ETX>>

Improving the dynamic range of a coherent AM analog optical link using a cascaded linearized modulator

An AM coherent analog optical link is experimentally demonstrated to achieve the highest spurious-free dynamic range reported to date for a coherent system by employing a linearized cascaded electro-optic modulator. The link gives a spurious-free dynamic range of 115 dB/spl middot/Hz/sup 2/3/, with a 34-dB reduction in third-order intermodulation distortion over conventional Mach-Zehnder modulators. With the linearized modulator, our experimental results are just 3 to 6 dB away from the fundamental limit of this link; the remaining penalty is mainly due to RF components.<<ETX>>

CORD-a WDM optical network: control mechanism using subcarrier multiplexing and novel synchronization solutions

Stanford University is implementing and investigating CORD, a 2.488 Gb/s//spl lambda/ WDM ATM packet-switched network experiment. CORD performs optical contention resolution at the receiver using optical switches and delay lines. CORD uses the multichannel subcarrier multiplexing (MSCM) technique to transmit the control channel and the payload data on the same wavelength and an ultra-fast clock recovery technique to synchronize the received control packets in every slot. We have experimentally shown that CORD incurs an MSCM penalty of 2.2 dB. In addition, we have demonstrated that the clock recovery scheme performs clock acquisition within 4 preamble bits with a penalty of 1 dB.

CORD : Optical packet-switched network testbed

Abstract We are implementing and investigating CORD, a 2.5 Gbit/s/λ WDM packet-switched network testbed. CORD features: (i) contention resolution optics (CRO), which consist of optical switches and delay lines to perform all-optical packet contention resolution; (ii) multichannel subcarrier multiplexed (MSCM) signaling to transmit an 80 Mbit/s control channel and 2.5 Gbit/s payload data on the same wavelength; (iii) networkwide distributed slot synchronization technique with maximum slot jitter of ± 6.5 ns; (iv) novel ultrafast signaling synchronization using delay-line phase alignment for data recovery within 4 bits; and (v) pilot-tone payload data synchronization for clock recovery within 16 ns. In this article, we describe the CORD network testbed and discuss the performance and scaling potential of the technologies developed for CORD.

Distributed slot synchronization (DSS): a network-wide slot synchronization technique for packet-switched optical networks

Distributed slot synchronization (DSS) is a network-wide packet synchronization technique which coordinates node transmissions so that packets arrive aligned to one another at a reference point in the network, independent of propagation delays. DSS was developed for use in the contention resolution with delay-lines (CORD) project, a DARPA-funded 2.5 Gb/s//spl lambda/, wavelength division multiplexer (WDM) optical packet-switched network testbed. In this implementation, it was experimentally demonstrated that the DSS system, operating with 80 MHz control logic, achieves a packet arrival jitter of less than 13 ns with 12 km node spacings. DSS was also shown to be robust against noise and node failure or fiber breaks. The technique is data rate and format independent and can be used in other star, extended ring, or tree-and-branch network architectures for metropolitan area network (MAN) and access applications.

Ultra-fast Clock Recovery And Subcarrier-based Signaling Technique For Optical Packet Switched Networks

Stanford University, GTE Laboratories, and the University of Massachusetts at Amherst are members of an ARPA consortium that is experimentally investigating a method for optical contention resolution utilizing optical switches and delay lines. Our CORD* testbed is a two-node, WDM, all-optical, packet switched network with a passive star topology. Each node transmits: (a) 2.488 Gbps payload data at baseband; (b) 16 bit headers at 80 Mbps on a subcarrier frequency, multiplexed on a unique wavelength; and (c) a 2.488 GHz clock pilot tone. Payload data is transmitted in fixed length, ATM-compatible packets (53 bytes) and the headers contain the destination node address. Both the payload data and header are transmitted in a 250 ns slot. Slots are synchronized among all nodes; this is obtained using a conventional PLL to lock onto a reference signal (called PING) generated by a master node. This method is scalable to an arbitrary number of nodes. Our signaling technique is a special case of multiple subcarrier signaling (MSS), which has been proposed as a means to control a WDM network [ I ] , [2]. In our implementation, each node contains one header detector and one payload data receiver. Nodes transmit their headers at a subcarrier frequency unique to the transmitting node, allowing a single photodetector at the header detector to simultaneously receive headers from all nodes. When a header with the receiving nodes destination address arrives, the corresponding wavelength is selected for the payload data receiver. Optical switches and delay lines allow data packet contentions to be resolved in the optical domain at the receiver [3]. The short packet slot size (250 ns) and high data rate (2.488 Gbps) require ultra-fast clock recovery at the payload data receiver. We have solved this problem by explicitly transmitting the payload data clock tone (2.488 GHz). The clock tone, baseband data, and header subcarrier are all combined in the microwave domain before being transmitted over a single optical carrier so that only one laser is required per node. Similarly to the payload data receiver, the header detector must recover the header clock within a few bits. Conventional PLL techniques are not nearly fast enough, requiring 10,000 to 100,000 bits I:O complete clock acquisition. Serial over-sampling techniques are also impractical because the recoveiy logic circuits would have to run at extremely high speeds, at least 320 MHz for the 80 Mbps header channel bit rate. We have developed a novel technique, Delay-line Phase Alignment (DPA), to reliably recover the header channel bit stream with digital circuitry niiming at the bit rate, 80 MHz. With DPA, a multi-tap delay line and selector are used to align the received bit stream with the local clock as shown in Figure 1. A preamble is transmitted at the beginning of each header. The DPA module monitors the bit transitions during the preamble and determines the optimum sampling tap to be used for the header bits. Substantial signal processing, including bounce suppression and transition position averaging. is performed by the

Novel distributed slot synchronization technique for optical WDM packet networks

Recently, a number of projects aimed at the implementation of optical packet-switched network prototypes have been announced. A common and critical element of these networks is the need to achieve time-slot synchronization over the whole network. Stanford University is currently implementing CORD, a two-node optical wavelength-division multiplexing (WDM) packet-switched network testbed with a star topology, featuring all-optical receiver contention resolution by means of optical switches and delay lines. This paper describes a flexible, distributed digital slot synchronization technique for CORD which is robust and scalable.

CORD-A WDM optical network: design and experiment of fast data synchronization by pilot-tone transport

CORD is a 2.5-Gb/s//spl lambda/ wavelength division multiplexing (WDM) packet switched network experiment. We have experimentally demonstrated a pilot-tone transport for the high-speed data synchronization in CORD. The optical power penalty is minimized to about 1 dB by equalizing the frequency response of the payload data channel and optimizing the pilot-tone modulation depth. The main advantages of the pilot-tone transport are: 1) a simple transceiver structure; and 2) a fast clock recovery time (within 40 b, 16 ns).