Optical packet switch

An interconnect fabric is constructed from a plurality of fixed wavelength transmitters which are used to transmit arriving data packets through a star coupler, and a plurality of tunable receivers which tune to whatever frequency necessary to receive the desired data from the star coupler. A control network, constructed from a plurality of fixed wavelength receivers and a plurality of tunable transmitters, determines what frequencies the tunable receivers should tune to, and sends a signal to effectuate such tuning.

TECHNICAL FIELD 
This invention relates generally to the field of packet switching, and more 
particularly, to an improved optical packet switch. 
DESCRIPTION OF THE PRIOR ART 
Recently, it has been proposed to construct growable packet switches from a 
relatively large memoryless interconnect fabric and a plurality of smaller 
packet switching modules. See for example, U.S. Pat. Nos. 4,955,016 and 
4,955,017, issued to Eng et al., both of which are incorporated herein by 
reference. A high level block diagram of such an arrangement is shown in 
FIG. 1. 
In the Eng arrangement, a memoryless interconnect fabric 101 reads the 
address in each arriving data packet and maps that address to a particular 
group of interconnect fabric outputs, where each group of interconnect 
fabric outputs are coupled to the inputs of a separate one of relatively 
small packet switches 102-104. Simultaneously arriving data packets 
comprising the same address are not mapped to the same interconnect fabric 
output, but rather, are mapped to the same group of interconnect fabric 
outputs and are each then transmitted to a different interconnect fabric 
output for conveyance to a packet switch. If the number of simultaneously 
arriving data packets with the same address is no greater than the group 
size, all packets are transmitted to a separate interconnect fabric output 
and are thereby routed through the appropriate one of packet switches 
102-104 to the proper output 111-116. However, if the number of 
simultaneously arriving packets with the same address is greater than the 
group size, excess packets are simply discarded or if necessary, stored 
until a later time slot. By correctly adjusting the group size, the 
probability of lost packets can be made extremely small. The entire 
arrangement, including the interconnect fabric 101 and the plurality of 
small packet switches 102-104, functions as one large packet switch. The 
details of how to select the group size as a function of a particular 
system's requirements are set forth in the previously incorporated United 
States patents. 
The Eng arrangement of FIG. 1 provides a technique whereby a packet switch 
may be grown as large as an expanding network may require by simply adding 
more groups of outputs to the interconnect fabric, and then connecting a 
relatively small packet switch to each of the new groups of outputs. The 
drawback with the arrangement is that it requires a rather complex 
scheduling algorithm to route the packets through the interconnect fabric 
to the output packet switches. Due to the advantages that the Eng 
arrangement provides, it is desirable to provide a simple and easily 
implementable technique for routing the packets through the interconnect 
fabric so that the entire arrangement of FIG. 1 can be advantageously 
utilized. 
SUMMARY OF THE INVENTION 
The above problem is overcome and a technical advance achieved in 
accordance with the present invention which relates to a novel 
interconnect fabric constructed from both a tunable receiver/fixed 
transmitter network and a tunable transmitter/fixed receiver network. Each 
of a plurality of incoming data streams is used to modulate a fixed 
frequency transmitter, where each fixed frequency transmitter transmits 
data on a separate frequency. The outputs of the fixed frequency 
transmitters are all combined by a star coupler. Each of the groups of 
interconnect fabric outputs is equipped with a plurality of tunable 
receivers, where each tunable receiver is connected to a separate output 
of the star coupler. Depending upon which frequency each tunable receiver 
tunes to, it will receive packets from a particular fixed frequency 
transmitter. Receivers at each interconnect output group can be tuned to 
select a predetermined number of the input packets for transmission out of 
the interconnect fabric. Thus, after mapping the address in each arriving 
packet to a group of interconnect outputs, the packets are each 
transmitted to a separate output of the group by simply tuning the tunable 
receivers at the output group accordingly. 
The determination of which tunable receivers should be tuned to which 
frequencies is done by transmitting requests from each transmitter to a 
control network. The control network includes means for receiving the 
requests and for causing the tunable receivers in each group of outputs to 
retune in order to receive one or more packets destined for that group. If 
the number of packets arriving during a time slot is greater than the 
number of outputs in the group, the excess packets are either discarded or 
stored until a subsequent time slot. 
In one embodiment, the control network is implemented with a tunable 
transmitter/fixed receiver star coupler network. Specifically, the control 
network includes a plurality of fixed receivers, each at a different 
wavelength and associated with a separate group of outputs of the 
interconnect fabric, a plurality of tunable transmitters, one associated 
with each input to the interconnect fabric, and a star coupler 
therebetween. In operation, the tunable transmitters each read the packet 
header from the incoming packet of their associated interconnect input, 
determine which group of interconnect fabric outputs to route the packet 
to, and then tune to the frequency of the fixed receiver associated with 
that group of outputs. The fixed receiver then selects a predetermined 
number of packets, no greater than the group size, to transmit to that 
group of interconnect outputs and causes the tunable receivers in that 
group of interconnect outputs to tune to the proper frequencies for 
receiving the selected packets from the fixed frequency transmitters.

DETAILED DESCRIPTION 
FIG. 1 shows a high level block diagram of a growable packet switch 
architecture. In operation, a plurality of packets arrive concurrently, 
during predetermined time slots, on inputs 105-110 of interconnect fabric 
101. Interconnect fabric 101 then examines the address in each packet 
header and routes up to four packets to each of output packet switches 
102-104. If more than four packets are destined for the same output packet 
switch, the excess packets are simply dropped, the probability of such an 
occurrence being acceptably small. In order that the growable switch 
architecture operate properly, there must be a technique employed by 
interconnect fabric 101 to examine the address in the arriving packets and 
determine which packets should be routed to the interconnect outputs, and 
which packets, if any, should be dropped. 
FIG. 2 shows a conceptual block diagram of an interconnect fabric that may 
be employed in the growable switch architecture of FIG. 1. The 
interconnect fabric 240 is intended to be used in a 64.times.64 packet 
switch, and is, therefore, shown with more inputs and outputs than 
interconnect fabric 101 of FIG. 1. Exemplary interconnect fabric 240 is an 
optical implementation of the invention, however, it should be understood 
that the invention is not limited to such an implementation and that it 
may be built using any type of transmitters and receivers. 
The interconnect outputs 218-225 are divided into an exemplary eight groups 
of twenty six exemplary receivers each. The tunable optical receivers are 
in each group are labelled 210-213 and 214-217, respectively and only four 
of the twenty six are shown for purposes of clarity. 
In operation, a plurality of data packets arrive concurrently, in 
predetermined time slots, on inputs 234-239 and are each used to modulate 
a separate one of lasers 201-206 as shown in FIG. 2. Each of lasers 
201-206 is arranged to transmit the incoming data on a separate optical 
wavelength to star network 207, where all of the optical signals are 
combined and distributed. Thus, each of star coupler outputs 226-233 
contains the same Wavelength Division Multiplexed (WDM) signal; i.e., the 
sum of all of the signals transmitted from lasers 201-206. 
It can be appreciated from the above that each of tunable receivers 210-213 
and 214-217 can receive any of the packets that arrived in a particular 
time slot by simply tuning to the proper frequency. Thus, up to twenty six 
concurrently arriving packets may be received by group 208 by simply 
tuning the twenty six tunable receivers 210-213 to the proper 26 
wavelengths; i.e., the wavelengths transmitted by the twenty six lasers 
201-206 on which the desired twenty six packets arrived. Thus, in each 
time slot, the packets arrive at the 64 inputs 234-239, are modulated each 
at a different frequency for transmission through the star network 207, 
and each group of tunable receivers up to twenty six of such packets by 
tuning to the proper frequency. 
The one remaining problem for interconnect fabric 240 is the control of the 
returning. Specifically, it should be appreciated from the above that in 
each time slot, each group of retunable optical receivers must retune in 
order to receive the packets that are destined for that particular group 
in that particular time slot. The addresses included in the incoming 
packets determine which group of outputs each packet is destined for and, 
thus, there exists the need for a technique to read the incoming addresses 
from the arriving packets in each time slot and, based thereon, cause the 
tunable receivers to retune to the proper frequencies so that each group 
of outputs will receive all packets destained for it up to a maximum of 
twenty six. An exemplary implementation of a solution to this control 
problem is described below. 
FIG. 3 shows a conceptual block diagram of a control network which can be 
used to tune all of the tunable receivers 210-217. The arrangement of FIG. 
3 includes a plurality of table lookup means 311-313 and 304-306, a 
plurality of tunable lasers 308-310, an optical star coupler 307, and a 
plurality of fixed wavelength receivers 301-303. It should be noted that 
for purposes of clarity, all of the components required to implement the 
control network for the interconnect fabric 240 are not shown. 
Specifically, there would actually be eight fixed wavelength optical 
receivers, one corresponding to each group of outputs, and eight table 
lookup means 304-306, one for each group of outputs. Moreover, there would 
actually be sixty four input table lookup means 311-313, and sixty four 
tunable lasers 308-310, one for each input of the interconnect fabric. In 
general, the preferred technique for practicing the invention is to make 
the number of interconnect inputs match the number of input table lookup 
means and the number of fixed wavelength optical receivers match the 
number of groups of outputs. 
In operation, packets that arrive concurrently at inputs 234-239 include an 
address field which is routed to input table lookup means 311-313. Each 
input table lookup means 311-313 is associated with a separate input 
234-239, and the address in the data packet arriving on that input is 
routed to the associated input table lookup means. Each input table lookup 
means then determines which output group the arriving packet is destined 
for. After making this determination, each input table lookup means then 
determines which fixed optical receiver corresponds to that group of 
interconnect outputs, and what the fixed wavelength of that fixed optical 
receiver is. Moreover, each input table lookup means will then send a 
signal to its associated one of tunable lasers 308-310 and cause the 
tunable laser to tune to the frequency of the fixed receiver associated 
with the fixed wavelength receiver of the desired output group. 
From the above it can be appreciated that in each predetermined time slot 
during which packets arrive, each tunable laser will transmit a message to 
the fixed wavelength optical receiver associated with the group of outputs 
for which the packets are destined. Conversely, each of fixed wavelength 
optical receivers will receive a plurality of requests from the various 
tunable lasers. The fixed wavelength optical receivers 301-303 then 
transmit the request to the table lookup means 304-306. Each of table 
lookup means 304-306 then selects a maximum of twenty six requests to 
accept, while discarding/ignoring the remaining requests if any. Finally, 
the table lookup means 304-306 each send signals on their respective 
outputs to their associated tunable receivers 210-217 in order to instruct 
the tunable receivers what frequencies to tune to. 
One other implementation detail must be resolved. Specifically, it is 
highly likely that several of the inputs 235-239 of FIG. 2 will receive 
packets destined for the same group of outputs. Consequently, several of 
the tunable lasers of FIG. 3 will transmit on the same wavelength during 
the same time slot, resulting in collisions at star coupler 307 of FIG. 3. 
The basic problem, therefore, is to provide a technique of conveying 
several information signals at the same wavelength through star coupler 
307. This apparent problem, however, can be overcome via time division 
multiplexing (TDM), subcarrier multiplexing, or through a variety of other 
means which are well known in the art. 
One optional improvement may be realized by implementing star network 207 
as a plurality of star couplers, rather than as a single star coupler. 
FIG. 4 shows an expanded view of star network 207 of FIG. 2. As can be 
seen from FIG. 4, half of the inputs 235-239 would be routed to star 
coupler 402, while the remaining half would be routed to star coupler 401. 
Each tunable receiver would then be coupled to two outputs, one from each 
of the star couplers 401-402 as shown in FIG. 4. Furthermore, each tunable 
receiver is preceded by a 2:1 switch, 403-406, which can route the outputs 
of either star coupler 401 or star coupler 402 to the tunable receiver. 
Assuming there were N tunable transmitters 201-206, the first N/2 would 
utilize one set of N/2 frequencies, and the second N/2 transmitters would 
use the same set of N/2 frequencies. In each time slot, the two star 
couplers could operate independently, with no chance of data from one star 
coupler colliding with data from the other. The arrangement provides the 
advantage of wavelength reuse, thereby eliminating the need for optical 
receivers which can receive signals over a wide spectrum of wavelengths. 
It should also be noted that the concept can be readily extended to any 
number of star couplers. 
While the above description shows the most preferred embodiment, it should 
be understood that the invention is not limited thereto, and that other 
modifications may be readily constructed from those of ordinary skill in 
the art. For example, either heterodyne or homodyne receivers may be 
employed, and transmission media other than optical may be used. Various 
group sizes and numbers of groups may be selected, and the table lookup 
means may be implemented using any of a variety of well known techniques.