Wave division multiplexing based optical switch

An optical switch utilizing WDM or DWDM techniques for use in both WAN and LAN environments. Each input to the switch is assigned a separate wavelength via a tunable transmitter. The output of the transmitters are input to a star coupler which combines all the optical signals into a single optical output signal. This signal is input to an optical demultiplexor which functions to split the incoming optical signal into a plurality of separate wavelengths with each wavelength steered to a particular output port. The output of each port, corresponding to a particular wavelength, is then converted into an electrical signal by a receiver. Unicast, broadcast and multicast calls are supported. A plurality of unicast connections can be established simultaneously by assigning each tunable transmitter a different wavelength such that all wavelengths are mutually exclusive with each other. In a broadcast connection, the source node transmits and all output ports receive the optical signal on the particular wavelength assigned to that port. The rest of the input ports are placed in an idle state. The output of each port is input to a multiplexor along with an output of the optical demux dedicated to broadcast traffic. A controller switches the multiplexors to output the broadcast signal such that all receivers output the same signal. Multicast traffic is handled similarly except that only selected multiplexors are switched. The remaining multiplexors carry unicast traffic as normal. As a result, the output ports of the members of the multicast group all output the same signal.

FIELD OF THE INVENTION 
The present invention relates generally to data communications networks and 
more particularly relates to an optical switching matrix capable of 
handling unicast, broadcast and multicast data traffic. 
BACKGROUND OF THE INVENTION 
More and more reliance is being placed on data communication networks to 
carry increasing amounts of data. In a data communications network, data 
is transmitted from end to end in groups of bits which are called packets, 
frames, cells, messages, etc. depending on the type of data communication 
network. For example, Ethernet networks transport frames, X.25 and TCP/IP 
networks transport packets and ATM networks transport cells. Regardless of 
what the data unit is called, each data unit is defined as part of the 
complete message that the higher level software application desires to 
send from one a source to a destination. Alternatively, the application 
may wish to send the data unit to multiple destinations. 
Asynchronous Transfer Mode 
ATM originated as a telecommunication concept defined by the Comite 
Consulatif International Telegraphique et Telephonique (CCITT), now known 
as the ITU, and the American National Standards Institute (ANSI) for 
carrying user traffic on any User to Network Interface (UNI) and to 
facilitate multimedia networking between high speed devices at 
multi-megabit data rates. ATM is a method for transferring network 
traffic, including voice, video and data, at high speed. Using this 
connection oriented switched networking technology centered around a 
switch, a great number of virtual connections can be supported by multiple 
applications through the same physical connection. The switching 
technology enables bandwidth to be dedicated for each application, 
overcoming the problems that exist in a shared media networking 
technology, like Ethernet, Token Ring and Fiber Distributed Data Interface 
(FDDI). ATM allows different types of physical layer technology to share 
the same higher layer--the ATM layer. 
ATM uses very short, fixed length packets called cells. The first five 
bytes, called the header, of each cell contain the information necessary 
to deliver the cell to its destination. The cell header also provides the 
network with the ability to implement congestion control and traffic 
management mechanisms. The fixed length cells offer smaller and more 
predictable switching delays as cell switching is less complex than 
variable length packet switching and can be accomplished in hardware for 
many cells in parallel. The cell format also allows for multi-protocol 
transmissions. Since ATM is protocol transparent, the various protocols 
can be transported at the same time. With ATM, phone, fax, video, data and 
other information can be transported simultaneously. 
ATM is a connection oriented transport service. To access the ATM network, 
a station requests a virtual circuit between itself and other end 
stations, using the signaling protocol to the ATM switch. ATM provides the 
User Network Interface (UNI) which is typically used to interconnect an 
ATM user with an ATM switch that is managed as part of the same network. 
Current LAN Topology 
Using ATM network technology as an example, the current topology of high 
performance ATM local area networks (LANs) includes ATM core switches at 
the backbone and an edge device having an ATM downlink to the one or more 
core switches. When a connection is established between two edge devices, 
the traffic must pass through the ATM switches in the core. Therefore, in 
order to support all potential connections between all edge devices, the 
ATM switches at the core need to be non blocking. Non blocking ATM 
switches are difficult to develop and thus are much more expensive. 
In addition to the above disadvantage, the resulting network may be limited 
in bandwidth. When attempting to establish large numbers of connection 
from the edge device, there may be a need for faster downlink data rates. 
Depending on the number of connections and the throughput required for 
each connection, the downlink capacity might not be sufficient to meet the 
needs of the users. 
An additional disadvantage is the amount of physical wiring required to 
implement such a network. In practice, each edge device must be connected 
to the ATM core via physical wires (i.e., cables). When considering a 
typical office building there may be many wires installed in parallel. A 
separate cable from each edge device on each floor must be run down to the 
ATM core farm that typically is located in the basement. Wherever the 
switch core farm or server farm is located, cables must be run from the 
switch core farm to each edge device. The total length of the required 
cabling can be relatively very high and thus have an associated very high 
cost. 
The cost may be even higher depending on the type and length of cabling 
used. For example, in ATM networks, it is common to run high speed fiber 
optic cable from the ATM switch core to all the edge devices in the 
network. Data rates may range from OC-3 155 Mbps to OC-12 622 Mbps on the 
optical fiber, for example. Note that each optical fiber used in the 
network carries only a single communication channel using a single 
wavelength of light. If it is desired to maintain several communications 
channels at one time, more than one optical fiber is required. Using prior 
art transmission techniques, each communication channel requires a 
separate optical fiber. 
Today, most legacy local area networks utilize ATM technology in 
combination with and mainly based on Switched Ethernet or Token Ring 
network topologies. The existing switching technology enables each user on 
the network to have their own dedicated bandwidth, e.g., 10 Mbps or 100 
Mbps, for their networked software applications. Each user is given 
network connectivity to the local switched hub, e.g., 100 Mbps for a Fast 
Ethernet network interface card (NIC). In typically office building 
environments, each floor is provided with one or more switched hubs that 
users are directly connected to. If the switched hub has 16 10 Mbps ports 
than it may potentially be forced to handle 1,600 Mbps data rate from all 
the connected users. 
Currently available conventional technology, using electrical processing, 
forces the switched hub to analyze every bit of information and to 
determine its destination. Even in the event though most of the data is 
not switched between the local ports on the switch but rather is passed up 
to higher levels of switching, all the information still must be analyzed 
by the switched hub. This bottleneck for data that is not switched locally 
leads to high data rates within the switch. The high internal data rates 
result in a more complicated design in terms of both hardware and 
software, thus increasing the cost of the switch. 
A networking strategy commonly used today is to use an all Ethernet network 
comprising a plurality of switches (switching hubs) connected to a network 
backbone. Each floor in the enterprise has one or more switched hubs 
connected to end users. Each switch comprises a port interface section, 
switch section and an interface that is typically at a higher speed that 
the port interfaces. A plurality of ports connects the end users to the 
switch. 
Each switch on each floor is connected via a dedicated physical cable to 
the network backbone. The network backbone comprises one or more switches 
connected in some arrangement. In addition, the switches or other network 
equipment from one or more other buildings may be connected to this 
network. An example of a suitable workstation Ethernet switch is the 
LinkSwitch 2700 manufactured by 3Com Corporation, Santa Clara, Calif. 
Each end user on the network is connected to a port in one of the switches 
at a rate of either 10 or 100 Mbps. The link between each switch and the 
network backbone may be over fiber optic cable at Fast Ethernet or 1 Gbps 
data rates, for example. Alternatively, the downlinks from each of the 
switched hubs to the network backbone can be a protocol other then 
Ethernet such as ATM, FDDI, etc. For example, the interface portion may 
comprise an ATM interface, FDDI interface, etc. If a protocol other then 
Ethernet, e.g., ATM, is used on the downlinks from the switched to the 
network backbone, than some form of local area network emulation (LANE) 
must be used to provide connectivity Ethernet between end users. 
In many cases, the protocol in use on the downlinks will differ from the 
protocol used on the connections to the end users, e.g., 10 Mbps to the 
end users and ATM on the downlinks. It is important to note, however, that 
regardless of the protocol used on the downlinks, a separate cable 
(optical fiber or copper) is required from each switched hub to the 
network backbone. 
This commonly used network topology has several disadvantages. One 
disadvantage is that depending on the length and type of cabling used, the 
cost could end up being quite high. In addition, depending on the number 
of switches used in the network, the number of individual fiber optic 
cables could be very high. Another disadvantage is that the bandwidth 
available from each floor to the network backbone is limited. For example 
if fast Ethernet 100 Mbps is used, that the maximum bandwidth available to 
the switch is 100 Mbps, no more. 
Also, another disadvantage is that the only type of connections possible 
using such a network topology are point to point connections. Multicast 
(MC) connections are possible but they are not simple or trivial to 
implement. Multicast connections require large amounts of overhead to 
implement whereby each call must be routed through the network backbone. 
Multicast connections also require special call setup procedures that can 
be potentially draining on system resources if the number of connections 
is large. 
Another disadvantage is that the network backbone must be used to establish 
many of the connections. The connections that must be routed through the 
backbone include any connection between two different switches. 
Wave Division Multiplexing 
Wave division multiplexing (WDM) technology enables the simultaneous 
transmission of multiple data channel connections on the same physical 
optical fiber. This is achieved by utilizing several different wavelengths 
on the same optical fiber at the same time. The WDM transmission network 
comprises a plurality of optical transmitters, a wave division 
multiplexor, optical transmitter, optical fiber transmission line, optical 
receiver, wave division demultiplexor and a plurality of optical 
receivers. 
Using this type of network, several data sources can be sent simultaneously 
into the WDM mux whereby each data source uses a different wavelength. The 
optical WDM mux functions to combine the different wavelengths into one 
optical transmission light beam. This optical light beam is transmitted 
onto the optical fiber using the optical transmitter. The fiber carries 
all the connections simultaneously. The optical light beam reaches the 
optical receiver that outputs the light beam to the WDM demux. The WDM 
demux functions to split the optical light beam into the different 
wavelengths that were originally sent. The different wavelength outputs of 
the WDM demux are input to the individual receivers that convert the light 
energy into electrical signals. 
Currently, the major use of WDM technology is in Wide Area Network (WAN) 
applications. The majority of WANs already has a large installed base of 
optical fiber. The optical fiber installed in WANs typically carry very 
high data rate traffic on the order of many gigabits per second. In 
addition, the demand for bandwidth and capacity is growing at an explosive 
rate. Today's WAN installations are being pushed to capacity in order to 
satisfy the demand for increasing levels of bandwidth. 
Two different techniques can be used to transmit data at higher rates: (1) 
adding additional optical fibers or (2) to increase the rate of data at 
the edge devices on either end of the optical fiber. Both of these 
solutions are very costly: installing additional fiber optic cable is very 
costly and developing faster end equipment is difficult and expensive. 
Currently available WDM technology, however, is a viable alternative to 
installing new fiber optic cable or upgrading the equipment on either end 
of the fiber. Using conventional WDM technology, several `slow` 
conventional end devices can be connected to a WDM mux whereby several 
slower data sources are combined onto the same fiber and transmitted to 
the other end. At the far end of the fiber optic cable, the operation is 
reversed, i.e., the optical signal is optically WDM demuxed. Thus, WDM 
technology can be used as a bandwidth concentrator. 
In a conventional switch, the assignment of a wavelength to an input data 
stream defines a specific path between an input port and an output port. 
In a broadcast connection, the data must be forwarded to all the ports. 
This means the transmitters must transmit on all wavelengths 
simultaneously, which is currently difficult to achieve even for a limited 
number of wavelengths. Further, in the case of a multicast connection, the 
problem is even more complicated. The transmitter port must transmit to a 
select group of ports wherein the members of the group are constantly 
changing. In the optical domain, this translates to sending multiple 
wavelengths simultaneously whereby the wavelengths are changing in random 
fashion, which is very difficult and impractical to achieve. 
SUMMARY OF THE INVENTION 
Throughout this document the term wave division multiplexing (WDM) denotes 
using a single optical fiber to transmit several communications channels 
simultaneously whereby each channel transmits data utilizing a different 
wavelength of light. The term dense wavelength division multiplexing 
(DWDM) denotes WDM that utilizes several wavelengths of light that are 
relatively close to one another. 
The type of environment suitable for application of the present invention 
is any data communications network such as found on college campuses or 
other large enterprises. Many companies that currently implement data 
networks with backbones using switched Ethernet and/or ATM technology can 
benefit from the features of the present invention. The optical switching 
apparatus of the present invention, in combination with wave division 
multiplexing, provides a novel solution to the problems of the prior art 
as described hereinabove. 
The present invention utilizes WDM or DWDM technology to construct an 
optical switch suitable for use in both WAN and LAN environments. 
Devices are connected to the optical switch via a physical interface 
module. Each input to the switch is assigned a separate wavelength via a 
tunable electrical to optical transmitter. The output of the transmitters 
are input to a star coupler which combines all the optical signals into a 
single optical output signal. This signal, in turn, is input to an optical 
demultiplexor which functions to split the incoming optical signal into a 
plurality of separate wavelengths with each wavelength steered to a 
particular output port. The output of each port, corresponding to a 
particular wavelength, is then converted into an electrical signal by an 
optical to electrical receiver. This first embodiment of the switch 
supports unicast connects. A controller configures the tunable 
transmitters to a particular wavelength in accordance with the desired 
output port for that input. 
A plurality of unicast connections can be established simultaneously by 
assigning each tunable transmitter a different wavelength such that all 
wavelengths are mutually exclusive with each other. No two transmitters 
are tuned to the same wavelength at the same time. This prevents the 
unicast connections from overlapping with each other in the switch. 
In a unicast connection, only the two end nodes transmit or receive the 
optical signals on the particular wavelength assigned to the connection. 
In a second embodiment of the optical switch, broadcast and multicast 
connections are handled. In a broadcast connection, the source node 
transmits and all output ports receive the optical signal on the 
particular wavelength assigned to that broadcast connection. One port 
transmits data while the rest of the input ports are placed in an idle 
state. The output of each port is input to a multiplexor along with an 
output of the optical demux dedicated to broadcast traffic, i.e., 
wavelength. A controller switches the multiplexors to output the broadcast 
signal such that all receivers output the same signal. 
Multicast traffic is handled similarly except that rather than switch all 
the multiplexors to the dedicated multicast wavelength, only selected 
multiplexors are switched. The remaining multiplexors carry unicast 
traffic as normal. As a result, the output ports of the members of the 
multicast group all output the same wavelength, i.e., traffic. 
There is provided in accordance with the present invention an optical 
switching matrix apparatus having N input ports and N output ports 
comprising N tunable transmitters adapted to convert an electrical input 
signal into an optical output signal, an N to 1 star coupler adapted to 
receive the N optical outputs of the tunable transmitters, the star 
coupled operative to generate a single optical output incorporating the N 
signals input thereto, a 1 to N demultiplexor adapted to receive the 
single optical output of the star coupler, the demultiplexor operative to 
generate N outputs each output having a unique dedicated wavelength 
associated therewith, N receivers adapted to receive the N output signals 
from the demultiplexor, the receivers operative to convert an optical 
input signal to an electrical output signal, a controller adapted to 
configure the N tunable transmitters such that each transmitter is tuned 
to a unique wavelength corresponding to a destination output port and 
wherein N is a positive integer. 
The star coupler comprises a N to 1 dense wave division multiplexing (DWDM) 
star coupler and the demultiplexor comprises a 1 to N dense wave division 
multiplexing (DWDM) demultiplexor. 
There is also provided in accordance with the present invention an optical 
switching matrix apparatus having N input ports, N output ports and M 
broadcast/multicast channels comprising N tunable transmitters adapted to 
convert an electrical input signal into an optical output signal, an N to 
1 star coupler adapted to receive the N optical outputs of the tunable 
transmitters, the star coupled operative to generate a single optical 
output incorporating the N signals input thereto, a 1 to N+M demultiplexor 
adapted to receive the single optical output of the star coupler, the 
demultiplexor operative to generate N+M outputs each output having a 
unique dedicated wavelength associated therewith, N multiplexors coupled 
to the demultiplexor, each multiplexor adapted to receive one of N 
channels output of the demultiplexor and M dedicated broadcast/multicast 
channels, each multiplexor adapted to be configured to output one of N 
channels input thereto or one of the M dedicated broadcast/multicast 
channels, N receivers adapted to receive the N output signals from the N 
multiplexors, the receivers operative to convert an optical input signal 
to an electrical output signal, a controller adapted to configure the N 
tunable transmitters such that each transmitter is tuned to a unique 
wavelength corresponding to a destination output port or a 
broadcast/multicast group, the controller operative to configure any of 
the N input ports for unicast, broadcast or multicast transmission and 
wherein N and M are positive integers. 
The multiplexors comprise M+1 inputs whereby a first input is coupled to 
one of N channels output of the demultiplexor and the remaining M inputs 
are coupled to the M broadcast/multicast channels output of the 
demultiplexor. The star coupler comprises a N to 1 dense wave division 
multiplexing (DWDM) star coupler and the demultiplexor comprises a 1 to 
N+M dense wave division multiplexing (DWDM) demultiplexor.

DETAILED DESCRIPTION OF THE INVENTION 
Notation Used Throughout 
The following notation is used throughout this document. 
______________________________________ 
Term Definition 
______________________________________ 
ANSI American National Standards Institute 
ATM Asynchronous Transfer Mode 
CCITT Comite Consulatif International Telegraphique et Telephonique 
DWDM Dense Wavelength Division Multiplexing 
FDDI Fiber Distributed Data Interface 
IP Internet Protocol 
ITU International Telecommunications Union 
LAN Local Area Network 
LANE LAN Emulation 
MC Multicast 
NIC Network Interface Card 
OC Optical Carrier 
TCP Transmission Control Protocol 
UNI User to Network Interface 
WAN Wide Area Network 
WDM Wavelength Division Multiplexing 
______________________________________ 
General Description 
The present invention is an optical switch that is capable of handling 
unicast, broadcast and multicast connections. The optical switch enables 
data to be switched in the optical domain as opposed to conventional 
switching in the electrical domain. This permits a very high data capacity 
and data rate. In accordance with the invention, any number of broadcast 
and/or multicast channels can be implemented in the switch. The switch is 
based on well-known WDM technology. The electrical inputs to the switching 
matrix are converted into optical signals on pre-assigned wavelengths. 
Each wavelength is dedicated to a specific optical output port. The sum of 
all the optical signals is broadcast towards each destination port. A WDM 
demux filters the signals such that each output only receives an optical 
signal having a specific wavelength. Unicast traffic is directed from a 
specific input port to a specific output port. Broadcast traffic is 
broadcast to all the output ports and multicast traffic is directed to a 
selected group of output ports. 
A block diagram illustrating a first embodiment of an optical switching 
core constructed in accordance with the present invention capable of 
handling unicast traffic. The optical switching core, generally referenced 
10, comprises a plurality of tunable transmitters 12 labeled tunable Tx #1 
through tunable Tx #N. An electrical signal 14 is input to each tunable 
Tx. The tunable transmitters 12 function to convert the electrical input 
signal to an optical output signal 16. The wavelength of each transmitter 
is set by the controller 28 via control bus 34. The controller functions 
to set the wavelengths such that no two transmitters are set to the same 
wavelengths simultaneously. 
Each optical transmitter 12 functions to convert electrical signals into 
optical signals. The enabling and disabling of each optical transmitter 12 
can optionally be controlled by the controller 28. The Optical Transmitter 
Module part number NYW-40 ITU Tunable Channel Plan Laser, manufactured by 
Altitun AB, Kista, Sweden, is suitable for use with the present invention. 
Note that each transmitter is tuned so as to generate an optical signal 
having a specific wavelength. 
The optical signal generated by each transmitter 12 is input to a N:1 star 
coupler (WDM mux) 18 via optical fibers 16. Each of the N optical signals 
output of the plurality of transmitters 12 are input to one of the inputs 
ports of the N to 1 star coupler or WDM mux 18. The WDM mux functions to 
combine the N optical input channels into a single egress optical output 
channel. Each of the N optical input signals, each having a different 
wavelength, i.e., .lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.N, are 
combined into a single optical signal. A suitable 16 to 1 optical WDM mux 
is the 16 channel narrow band Dense WDM mux model WD15016-M2 manufactured 
by JDS Fitel, Inc., Ontario, Canada. Optical channel spacings on the order 
of 200 GHz can be achieved with this dense WDM mux device. 
The output signal is subsequently input to a 1:N WDM demultiplexor (demux) 
22 via optical fiber 20. The demux 22 functions to demultiplex the N 
wavelengths into N individual output ports whereby each output port is 
dedicated to a particular wavelength. The optical signal input to the 
demux 22 is split into N different optical signals each having a different 
wavelength, i.e., .lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.N. A 
suitable 1 to 16 optical WDM demux is the 16 channel narrow band Dense WDM 
demux model WD15016-D2 manufactured by JDS Fitel, Inc., Ontario, Canada. 
Optical channel spacings on the order of 200 GHz can be achieved with this 
dense WDM demux device. 
Each of the optical output signals from the demux 22 is input via optical 
fibers 24 to N optical receivers 26 labeled Rx #1 through Rx #N. Each 
receiver 26 functions to convert an optical input signal to an electrical 
output signal 30. Optionally, a control signal 35 output from the 
controller 28 to each of the optical receivers determines which of the 
channels in the plurality of optical receivers are enabled and which are 
disabled, to save power, etc. The electrical signals output from the 
optical receivers 26 constitute the electrical output signals of the 
switching matrix 10. A suitable optical receiver module that can be used 
to construct a multichannel optical receiver is the PGR 5025 Optical 
Receiver Module manufactured by Ericsson Components AB, Kista-Stockholm, 
Sweden. Each of the receive channels is tuned to receive an optical signal 
on a specific wavelength. 
The switching matrix 10 is suitable for handling unicast connection. At any 
one time, the switching matrix the tunable transmitters are configured to 
connect each input port to a different output port. During each cycle of 
the switch, i.e., each cell time or other cycle time, the transmitters 12 
are programmed to a particular wavelength, the wavelength determining the 
destination output port. 
The present invention also comprises a second embodiment that enables the 
establishment of broadcast and multicast connections. A block diagram 
illustrating a second embodiment of an optical switching core constructed 
in accordance with the present invention capable of handling unicast 
traffic and includes a single broadcast/multicast channel is shown in FIG. 
2. 
The switching matrix, generally referenced 40, is constructed similarly to 
the matrix of FIG. 1 with the exception being the addition of a plurality 
of 2 to 1 muxes. In particular, the switching matrix 40 comprises a 
plurality of tunable transmitters 44 labeled tunable Tx #1 through tunable 
Tx #N. An electrical signal 42 is input to each tunable Tx. The tunable 
transmitters 44 function to convert the electrical input signal to an 
optical output signal 46. The wavelength of each transmitter is set by the 
controller 64 via control bus 48. The controller functions to set the 
wavelengths such that no two transmitters are set to the same wavelengths 
simultaneously. The operation and construction of optical transmitters 44 
is similar to that of optical transmitters 12 (FIG. 1). 
The outputs of the tunable transmitters 44 is input to an N to 1 star 
coupler (WDM mux) 50. The star coupler functions similarly to that of star 
coupler 18 (FIG. 1). The output of the star coupler is input to a 1:N+1 
WDM demux 54 via optical fiber 52. In accordance with the invention, the 
demux 54 comprises an additional output channel having a dedicated unique 
wavelength. The first N channels correspond to the N input ports and the 
N+1.sup.th channel corresponds to the broadcast/multicast channel. 
The optical output signal from channels 1 through N are input to 2 to 1 
optical multiplexors 58 via optical fibers 56. Each mux 58 comprises two 
input ports, A and B, and an output port. The A input port of each mux 58 
is coupled to one of output ports 1 through N of the WDM demux 54. The B 
input port of each mux 58 is coupled to the output of channel N+1 of the 
demux 54. In accordance with an input control signal, each 2 to 1 mux 58 
outputs either the optical signal output from channels 1 through N or the 
output from channel N+1, i.e., the broadcast/multicast channel. A suitable 
2 to 1 switch that can be used to construct the switch matrix 40 is the 
SL, SR or SW series of Fiber Optic Switch Modules manufactured by JDS 
Fitel, Inc., Ontario, Canada. 
For unicast transmission, all the muxes 58 are configured by the controller 
64 via control bus 66 to couple their respective A inputs to the output. 
For broadcast transmission, the controller 64 configures each mux 58 to 
couple its respective B input, i.e., the broadcast/multicast wavelength, 
to the output. While the switch is configured for broadcast, no unicast 
traffic can flow for that cycle. 
The optical output of each mux 58 is input to an optical receiver 62 via 
optical fibers 60. The optical receiver 68 are constructed and operate 
similarly to the receivers 26 of FIG. 1. The receivers output an 
electrical signal 68 and constitute the output destination of the 
switching matrix. 
Thus, for broadcast transmission, one of the input ports is given 
permission to broadcast to all the output ports. This is achieved by 
setting the port's corresponding tunable transmitter 44 to the 
broadcast/multicast wavelength. In addition, the controller must configure 
the plurality of 2 to 1 muxes 58 to couple the data at the B input to the 
output. This permits a single input port to transmit to all the output 
ports while the remainder of the transmit ports are in the idle state. The 
transmitting port sends the data on a dedicated broadcast/multicast 
wavelength that is not used by any other output port. The signal having 
this wavelength is filtered by the WDM demux 54 and will appear at the 
output channel N+1. The 2 to 1 muxes are switched so as to couple the 
broadcast signal to their outputs. When the broadcast is finished, the 
muxes are switched back to their A input ports. 
Multicast transmissions as handled in a similar manner with the difference 
being that not all the 2 to 1 muxes 58 are switched to their B inputs. 
Only the muxes 58 corresponding to members of the multicast group are 
switched to connect the broadcast/multicast signal to the output. The 
remaining muxes are left in the unicast mode. The controller 64 functions 
to configure the muxes in accordance with the destination ports making up 
the multicast group. 
In operation, the controller 64 configures the tunable transmitter 44 that 
is associated with the input port wishing to send multicast data. That 
transmitter is configured to the wavelength corresponding to the N+1 
broadcast/multicast channel on the WDM demux 54. In addition, the 2 to 1 
muxes 58 that correspond to the members of the multicast group are 
configured to connect their B inputs to the output. The remaining muxes 
are left in unicast mode, i.e., the A input is connected to the output. 
It is important to point out that although the switching matrix comprises N 
input ports and N output ports, the matrix utilizes N+1 unique 
wavelengths. The additional wavelength being used for the 
broadcast/multicast channel. 
The switching matrix 40 shown in FIG. 2 provides a single 
broadcast/multicast channel. Switching matrices with additional 
broadcast/multicast channels can be constructed by adding additional 
wavelength channels to the WDM demux combined with using multiplexors 
having a larger number of inputs. 
In general, a switching matrix comprising N unicast channels and M 
broadcast/multicast channels requires N tunable transmitters wherein each 
transmitter is tunable over N+M wavelengths. In addition, a 1 to N+M WDM 
demux is required in addition to N (M+1) to 1 muxes. 
For example, an eight port switching matrix comprising two 
broadcast/multicast channels, i.e., N equals 8 and M equals 2, requires 8 
tunable transmitters wherein each transmitter is tunable over 10 
wavelengths. A 1 to 10 WDM demux is required along with eight 3 to 1 
muxes. 
A block diagram illustrating a third embodiment of an optical switching 
core constructed in accordance with the present invention capable of 
handling unicast traffic and includes two broadcast/multicast channels is 
shown in FIG. 3. 
The switching matrix, generally referenced 70, is constructed similarly to 
the matrix of FIG. 2 with the exception being the 3 to 1 muxes in place of 
2 to 1 muxes. In particular, the switching matrix 70 comprises a plurality 
of tunable transmitters 74 labeled tunable Tx #1 through tunable Tx #N. 
Electrical signals 72 are input to tunable transmitters 74. The tunable 
transmitters 74 function to convert the electrical input signal to an 
optical output signal 76. The wavelength of each transmitter is set by the 
controller 96 via control bus 98. The controller functions to set the 
wavelengths such that no two transmitters are set to the same wavelengths 
simultaneously. The operation and construction of optical transmitters 74 
is similar to that of optical transmitters 44 (FIG. 2). 
The outputs of the tunable transmitters 74 is input to an N to 1 star 
coupler (WDM mux) 78. The star coupler functions similarly to that of star 
coupler 50 (FIG. 2). The output of the star coupler is input to a 1:N+2 
WDM demux 82 via optical fiber 80. In accordance with the invention, the 
demux 82 comprises two additional output channels having dedicated unique 
wavelengths. The first N channels correspond to the N input ports and the 
N+1 and N+2 channel correspond to the two broadcast/multicast channels. 
The optical output signal from channels 1 through N are input to 3 to 1 
optical multiplexors 86 via optical fibers 84. Each mux 86 comprises three 
input ports, A, B, C and an output port. The A input port of each mux 86 
is coupled to one of output ports 1 through N of the WDM demux 82. The B 
input port of each mux 86 is coupled to the output of channel N+1 of the 
demux 82. The C input port of each mux 86 is coupled to the output of 
channel N+2 of the demux 82. In accordance with an input control signal, 
each 3 to 1 mux 86 outputs either the optical signal output from channels 
1 through N, the output from channel N+1 or the output of channel N+2, 
i.e., the two broadcast/multicast channels. A suitable 3 to 1 switch that 
can be used is the SL, SR or SW series of Fiber Optic Switch Modules 
manufactured by JDS Fitel, Inc., Ontario, Canada. 
For unicast transmission, all the muxes 86 are configured by the controller 
96 via control bus 94 to couple their respective A inputs to the output. 
For broadcast transmission, the controller 96 configures each mux 86 to 
couple either its respective B or C input, i.e., the broadcast/multicast 
wavelengths, to the output. While the switch is configured for broadcast, 
no unicast traffic can flow for that cycle for all output ports. 
The optical output of each mux 86 is input to an optical receiver 90 via 
optical fibers 88. The optical receivers 90 are constructed and operate 
similarly to the receivers 26 of FIG. 1. The receivers output an 
electrical signal 92 and constitute the output destination of the 
switching matrix. 
Thus, for broadcast transmission, one of the input ports is given 
permission to broadcast to all the output ports. This is achieved by 
setting the port's corresponding tunable transmitter 74 to one of the 
broadcast/multicast wavelengths. In addition, the controller must 
configure the plurality of 3 to 1 muxes 86 to couple the data at either 
the B or C input to the output. This permits a single input port to 
transmit to all the output ports while the remainder of the transmitting 
ports are in the idle state. The transmitting port sends the data on a 
dedicated broadcast/multicast wavelength that is not used by any other 
output port. The signal having this wavelength is filtered by the WDM 
demux 86 and will appear at either output channel N+1 or N+2. The 3 to 1 
muxes are switched so as to couple one of the broadcast signals to the 
output. When the broadcast is finished, the muxes are switched back to 
their A input ports. 
Multicast transmissions as handled in a similar manner with the difference 
being that not all the 3 to 1 muxes 86 are switched to their B or C 
inputs. Only the muxes 86 corresponding to members of a multicast group 
are switched to connect the broadeast/multicast signal to the output. The 
remaining muxes are left in the unicast mode. The controller 96 functions 
to configure the muxes in accordance with the destination ports making up 
the multicast group. 
In this third embodiment, two multicast channels can operate 
simultaneously. The transmitter corresponding to a first multicast group 
is assigned and configured with wavelength N+1. The transmitter 
corresponding to a second multicast group is assigned and configured to 
wavelength N+2. Similarly, the 3 to 1 muxes 86 corresponding to the first 
multicast group are set by the controller 96 to connect their B inputs to 
the output. The 3 to 1 muxes 86 corresponding to the second multicast 
group are set to connect their C inputs to the output. The remaining muxes 
are left in unicast mode, i.e., the A input is connected to their output. 
While the invention has been described with respect to a limited number of 
embodiments, it will be appreciated that many variations, modifications 
and other applications of the invention may be made.