STAR-TRACK OPTOELECTRONIC PACKET SWITCHES

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

w x STAR-TRACK 15 is another hybrid switch architecture. It is based on a two-phase contention resolution algorithm. It also supports multicasting. As shown in Figure 11.5, the switch is composed of two internal networks: an optical star transport network, and an electronic control track surrounding the star network. The optical transport network has fixed-wavelength optical transmitters at the input port side, and wavelength-tunable optical receivers at the output port side. There is a unique wavelength associated with each input port. Input and output ports are connected through an optical star coupler. Output port conflicts are resolved by the ring reservation technique w x 16 . The electronic control network that implements the ring reservation technique is the major track linking input ports, output ports, and a token generator sequentially. Cells arriving at the input ports are stored in the input buffers after optical-to-electronic conversion. There are two control phases in a cell transmission cycle. In the first phase, input ports write their output port requests into the tokens circulating in the control network. In the second Ž . Fig. 11.5 STAR-TRACK architecture basic single track . OPTOELECTRONIC PACKET SWITCHES 287 phase, the output ports read the tokens and tune their receivers to the appropriate input port wavelengths. Then, the cell transmission starts over the star transport network. The transmission and control cycles are over- lapped in time in order to increase throughput. Since each input has a unique wavelength and there is input᎐output port pair scheduling prior to transmis- sion, cells are transmitted simultaneously without causing contention. This architecture allows multicasting. However, the throughput of the switch may degrade as the number of multicasting connections increases, due to output port collisions in the first phase. It is shown that this problem can Ž be alleviated by call splitting i.e., allowing a multicast call to be completed in . multiple cell slots . This architecture can support different priority levels for the cell by adding minor tracks into the control network. However, in that case, the token should recirculate among the input ports more than once, depending on the number of priority levels. This will increase the length of the write phase and result in longer cell processing time. The main drawback of the switch is the sequential processing of the token by input and output ports. As a result, the time taken for the token to travel through the entire ring increases as the size of the switch increases. In the case of multiple priority levels, the recirculation period for the token be- comes even longer. Here, HOL blocking is another factor that degrades throughput.

11.2.3 Cisneros and Brackett’s Architecture