The present invention relates to data networks, employing WDM multiplexing, and incorporating wavelength addressing, and in particular, to a laser power grid, for operation with these networks.
Communication traffic is steadily increasing, both in size and in complexity. Leading service providers for Internet Protocol (IP), for example, report a 300 percent growth per year in Internet traffic, while traditional voice traffic has grown at a about 13 percent. [Cisco documentation, of Sep. 28 05:50:55 PDT 2002, www.cisco.com/univercd/cc/td/doc/product/mels/cm1500/dwdm/dwdm_fns.htm]. In response to this explosive growth in bandwidth demand, long-haul service providers are moving away from Time division multiplexing (TDM) based systems, which were optimized for voice but now prove to be inefficient, to wavelength division multiplexing (WDM).
TDM was invented as a way of maximizing the amount of voice traffic over a medium by multiplexing, so that more than one telephone call could be put on a single link. In essence, TDM increases the capacity of a transmission link by slicing time into smaller intervals, so that the bits from multiple input sources can be carried on the link, thus, increasing the number of bits transmitted per second.
Using TDM, data may be transmitted at 10 Gbps and recent advances have resulted in speeds of 40 Gbps. Yet, the electronic circuitry that makes this possible is complex and costly. Furthermore, technical issues, such as chromatic dispersion and nonlinear effects that can affect waveform quality, may restrict the applicability of TDM at these rates.
An alternative to TDM, wavelength division multiplexing (WDM) now reigns as the leading technologies for transmitting high volume of data traffic over long distances. It is based on the inherent advantage of photons as data carriers, which is that photons of different wavelengths do not normally interact, thus enabling the transmission of many channels of data, in parallel, in a single fiber, with photons of a different wavelengths acting as channels. WDM uses single-mode fiber to carry multiple light waves of differing frequencies.
In a sense, multiplexing by WDM is analogous to radio broadcasting on different wavelengths. Each channel is transmitted at a different frequency, and one can select a frequency, as if by using a tuner.
Thus, in WDM, many wavelengths are combined onto a single fiber, so as to simultaneously multiplex signals of 2.5 to 40 Gbps each over a strand of fiber. In this manner, the effective capacity of existing fiber infrastructure can be increased by a factor of up to 100.
When using WDM, each of the wavelengths is launched into the fiber, at the transmitting end, and the signals are demultiplexed by an optical wavelength demultiplexer, at the receiving end. As with TDM, the resulting capacity is an aggregate of the input signals. But an important difference between TDM and WDM is that WDM carries each input signal independently of the others. In other words, each wavelength channel has its own dedicated bandwidth; all signals arrive independently, rather than being broken up and carried in time slots.
Dense wavelength division multiplexing (DWDM) is different form WDM mainly in degree. DWDM spaces the wavelengths more closely than WDM; in consequence, it has a greater overall capacity. DWDM systems with 100 channels per fiber each carrying 10 Gb/sec are commercially available, enabling data traffic at a rate of approximately 1 Terabit per second in a single fiber. The spacing limit is not precisely known yet, and has probably not been reached. The state of art of commercial systems is 50 GHz.
Similarly, coarse wavelength division multiplexing (CWDM) relates to spacing the wavelengths sparsely, thus reducing the cost of the systems.
The capability of two optical signals of different wavelengths to occupy the same fiber, at the same time, makes WDM point-to-point segments extraordinarily powerful. The challenge is how to harness this capability to supply the huge bandwidth that flows in high performance computing systems that are predominantly a complex mesh of parallel interconnections. In particular it is desirable to combine the inherent advantages of the packet/burst-switching paradigm with the vast data transfer capability of the optical fiber.
An added feature when using WDM is the possibility of using the channel wavelength as an address vehicle for delivering the transmissions to their destinations. This idea is termed wavelength addressing.
Wavelength addressing relates to assigning every processing element (PE), or node, in a data network a wavelength, as a receiving address. The processing element, or node, may be a chip, a board, a cabinet, or even a routing switch. A schematic illustration of a PE adapted for wavelength addressing is provided in FIG. 1A.
As seen in FIG. 1A, a PE 10 includes a processing unit 30, a transmitter 20 and a wavelength addressed receiver 90[λ0]. Processing unit 30 communicates to transmitter 20 an information defined by Ei; λi, wherein Ei relates to an electronic data to be transmitted, and λi is the address of the receiving PE, at a specific instant. It will be appreciated that the information defined by Ei; λi relates to a specific transmittal. A moment later, processing unit 30 may communicate to transmitter 20 an information defined by En; λn.
Transmitter 20 communicates the wavelength designation λi to a tunable laser 40 which produces a light propagation of wavelength λi. Additionally, transmitter 20 communicates the electronic data Ei to a modulator 60, which modulates the light propagation of wavelength λi so as to produce a data packet 50i;λi containing electronic information Ei and addressed to a PE whose receiving address is λi.
When information is received, wavelength addressed receiver 90[λ0] communicates the received data, for example, 59n;λ0 to processing unit 30.
FIG. 1B illustrates a data network 150 having a plurality of PEs of the type described in FIG. 1A, denoted, PE 12, PE 14, PE 15, PE 16, and PE 18, having processing units 32, 34, 35, 36, and 38, respectively, transmitters 22, 24, 25, 26, and 28, respectively, tunable lasers, 42, 44, 45, 46, and 48, respectively, modulators 62, 64, 65, 66, and 68, respectively, and wavelength addressed receivers 92[λ5], 94[λ6], 95[λ4], 96[λ8], and 98[λ2], respectively, the wavelength addressed receivers communicating their received data to processing units 32, 34, 35, 36, and 38, respectively.
Accordingly, a plurality of data packets, 52i; λ2, 54i; λ4, 55i; λ5, 56i; λ6, and 58i; λ8, each issuing from its respective PE, are coupled to an optical coupler 70 and transmitted, by WDM.
An optical-wavelength demultiplexer 80 decouples the data packets by wavelengths, and each is routed to its wavelength address.
It will be appreciated that by coupling the data packets to a single fiber, each PE needs to communicate only with optical coupler 70 and optical-wavelength demultiplexer 80, rather than with all the other PEs.
Additionally, the use of wavelength addressing reduces the electronic information that needs to be contained in the data packet.
The architecture of FIGS. 1A and 1B relies heavily on the performance and price of the tunable lasers; in fact, a major incentive for the development of tunable lasers is their application for wavelength multiplexing and addressing. Yet, tunable lasers are expensive, cumbersome, and slow, and their use limits scalability.
Rubin et al., (S. Rubin, E., Buimovich, G. Ingber and D. Sadot, “Implementation of an Ultra-Fast Widely-Tunable Burst-Mode 10 Gbps Transceiver,” Electronic Letters vol. 38 No. 23 pp. 1462-1463, November 2002) describe a tunable 10 Gbps transceiver, for optical burst-switching applications. The burst-mode receiver has fast tuning of less then 50 nano-seconds between 80 channels over the entire C-band, together with fast locking, of less than 100 nanosecond.
Faster tunable lasers are under development; however, their projected performance and price may not justify their deployment in computer networks with multitude of nodes, in particular for board-to-board and chip-to-chip communication.
As a consequence, the range of applications implemented by tunable lasers is limited to networks in which the price of each node is high, and the length of the burst is comparatively long.
There is thus a widely recognized need for, and it would be highly advantageous to have WDM systems which implement wavelength addressing, devoid of the above limitations.