HYPASS OPTOELECTRONIC PACKET SWITCHES

OPTICAL PACKET SWITCHES 284 Fig. 11.3 Architecture of the ATM wavelength routing system. This switch cannot perform multicast functions, because of the fixed-wave- length filters at the output ports. Furthermore, if there is more than one cell destined to the same output port, an arbitration mechanism is necessary in order to assign the incoming cells with the same wavelength to different fiber delay lines. Such a requirement increases the control complexity. In order to meet the cell loss requirement, more fiber delay lines are necessary. More- over, the electronic controller always has to monitor the status of fiber delay lines to preserve the cell sequence.

11.2 OPTOELECTRONIC PACKET SWITCHES

For the optoelectronic packet switches, optical switching networks are used for interconnection and transmission between electronic input and output modules. Logical control, contention resolution, and packet storage are handled electronically.

11.2.1 HYPASS

w x HYPASS 14 in Figure 11.4 is an optoelectronic hybrid cell switch in which electronic components are used for memory and logic functions, and optical components are used for routing and transporting data. In this figure, bold continuous lines represent optical paths, bold dashed lines represent serial data paths, dotted lines are tuning current paths, and thin continuous lines are control signal paths. The switch is composed of two networks: the transport network and the control network. The architecture is based on the broadcast-and-select approach in both of the networks. There is a unique OPTOELECTRONIC PACKET SWITCHES 285 Ž . Fig. 11.4 Global diagram of the HYPASS implementation. 䊚 1988 IEEE. optical wavelength associated with each of the output ports.As shown in Figure 11.4, the transport network has tunable-wavelength laser transmitters at the input side, fixed-wavelength receivers at the output side, and an N = N star coupler, which transfers the incoming data from inputs to outputs. In order to transfer control information from output ports to the input ports, a similar network is used. When a cell arrives at an input port of the switch, first it is converted from optical to electronic and its destination address is obtained. Then the cell is temporarily stored in the corresponding input buffer. The tunable-wave- length laser transmitter of the corresponding input port is tuned to the Ž wavelength of the requested output port. When a request-to-send signal or . poll is received from the corresponding output port via the control network, the cell is transmitted through the transport network. The acknowledgment representing successful delivery of the cell is also transmitted through the control network. If there are multiple cells for the same output port, contention occurs. Power threshold detection or multiple bit detection on the cell preamble could be used to detect collision. The cells which do not get acknowledgments in a slot time are kept to retry later. In order to resolve contention and provide successful transmission of cells, the tree-polling w x algorithm, which is explained in 14 , is employed in the selection of inputs in the following cell slots. The cells that reach the output ports successfully are stored in the elastic buffers and transmitted over the optical fiber trunks after the necessary electrical-to-optical conversion. OPTICAL PACKET SWITCHES 286 The HYPASS architecture has advantages due to its parallel structure. However, since a slot time is based on the length of the polling step, transmission of a cell, and receipt of the acknowledgment, the time overhead for the electronic control and optical tuning operations are the factors limiting its capacity. The switch does not have multicasting capability, due to the use of fixed wavelength receivers at the output ports.

11.2.2 STAR-TRACK