[ 0019 ] fig1 is a block diagram of an optical line terminal ( olt ) 2 which is the basic element of the present embodiment . the olt 2 has an input / output line interface 4 which is connected to an external fiber facility and transmits / receives an optical signal having n optical wavelengths , for example 32 wavelengths , on a single optical fiber which is multiplexed / demultiplexed by a multiplexer / demultiplexer 6 , which outputs demultiplexed wavelengths λ 1 - λn on individual optical fibers . the respective wavelengths λ 1 - λn are sent either to a peer olt via a pass - through port or to client equipment via a transponder and a local port . the client equipment includes sonet equipment , add / drop multiplexers , cross - connect switches , internet protocol ( ip ) routers , asynchronous transfer mode switches ( atm ) and the like . as employed herein an optical signal is generally intended to encompass wavelengths in the range of approximately 300 nanometers to approximately 2000 nanometers ( uv to far ir ). this range of wavelengths can be accommodated by the preferred type of optical conductor ( a fiber optic ), which typically operates in the range of approximately 800 nanometers to approximately 1600 nanometers . consider λ 1 which is provided to a 1 × 2 switch 8 which is controlled by a control signal , having at least n states , from a controller 10 . the controller 10 responds to a command , from a management system ( not shown ), at a terminal 12 to provide the control signal at a terminal 14 and then to control terminal 16 of switch 8 to position the switch 8 in a first or second position . when in the first position , λ 1 is provided to a transponder 18 which transmits λ 1 to a client apparatus 20 via a local port 19 . when in the second position λ 1 is provided to a pass - through port 22 to a corresponding pass - through port in a peer olt 24 . the control signal is also provided to output terminal 15 , and then to control terminal 16 of a corresponding switch 8 in peer olt 24 to route λ 1 to the corresponding multiplexer / demultiplexer 6 . if it is desired to send λ 1 to both client apparatus 20 and peer olt 24 , an optical splitter can be used in place of the switch 8 . switch 26 selects λ 1 coming from the opposite direction in response to a control signal at terminal 28 from controller 10 to position switch 26 in a first or second position . when in the first position , λ 1 is received from client 20 via local port 19 and transponder 18 , and when in the second position λ 1 is received from peer olt 24 via pass - through port 22 , and then is provided to multiplexer / demultiplex 6 to be multiplexed with the other received wavelengths λ 2 - λn . a wavelength can be directly passed - through to a peer olt rather than being sent to a client apparatus . for example , λ 2 is directly sent to , and received from , peer olt 30 via pass - through port 32 . a 1 × n switch can be used to send / receive a wavelength to / from one of n − 1 peer olts or a client apparatus . for example , 1 × n switch 34 under control of a control signal , having at least n states , provided to terminal 36 from controller 10 sends λ 3 to either peer olt 38 via pass - through port 40 , or peer olt 42 via pass - through port 44 , or peer olt 46 via pass - through port 48 or client apparatus 50 via transponder 52 and local port 53 . reception of λ 3 in the opposite direction is controlled by n × 1 switch 54 under control of a control signal provided to terminal 56 from controller 10 , and than is provided to multiplexer / demultiplexer 6 to be multiplexed with the other received wavelengths . as discussed above , a wavelength can be passed - through to a peer olt via a pass - through port or can be optically switched to a client apparatus via a local port . λn is shown as being directly passed through to , or received from , peer olt 60 via pass - through port 62 . [ 0026 ] fig2 is a flow chart of the steps performed by the controller 10 of fig1 to control the 1 × 2 switches 8 and 26 , and the 1 × n switches 34 and 54 to route the respective wavelengths λ 1 - λn . in step s 101 the controller 10 waits for a command from a management system such as a computer ( not shown ). at step s 102 a determination is made as to whether or not the command is a switch control signal to either pass - through the wavelength via a pass - through port to a peer olt or drop / add the wavelength locally at / from a client apparatus via a transponder and a local port . if the answer is no , the command is handled by another interface ( not shown ) at step s 103 . if the answer is yes , a signal is sent to switch a ( for example switch 8 or 34 ) to move switch a to transmit position x ( the selected position ) at step s 104 , and at s 105 a signal is sent to switch b ( for example switch 26 or 54 ) to move switch b to receive position x ( the selected position ). at step 106 the control signal at terminal 15 of controller 10 is sent to the peer olt to set its switches a and b in a corresponding manner . a loop - back is then made to step s 101 to wait for the next command . in the multiplexer / demultiplexer 6 of fig1 wavelengths on a single optical fiber received at line interface 4 are demultiplexed into 32 individual wavelengths λ 1 - λ 32 . however , according to another aspect of the invention the 32 wavelengths can be demultiplexed into bands , for example four bands of 8 wavelengths each , by a first multiplexer , and the resultant four bands can be processed by the olt . according to another aspect of the invention at least one of the four bands of wavelengths can be demultiplexed by a second multiplexer / demultiplexer into its individual wave lengths such that the olt can process the individual wavelengths of the at least one band and the remaining ones of the four bands . [ 0029 ] fig3 is a block diagram illustrating a modular olt 200 having two stages of multiplexing / demultiplexing . the operation of the olt 200 is described with respect to the demultiplexing operation ; however , it is to be understood that the multiplexing is merely the reverse operation . it is to be noted that the 1 × 2 switches and 1 × n switches shown in fig1 are not included in fig3 in order to simplify the drawing . however , it is to be understood that in practice such switches may be utilized in the practice of the invention . the olt terminal 200 has an input / output line interface 202 which is connected to an external fiber facility and receives on a single optical fiber n , for example 32 , wavelengths which are demultiplexed by a multiplexer / demultiplexer 204 , which is situated on a first modular card , into m , for example 4 , bands of 8 wavelengths each . the first band 206 ( λ 1 - λ 8 ) is demultiplexed into its 8 individual wavelengths by a multiplexer / demultiplexer 208 , which is situated on a second modular card , with each such wavelength being provided to a pass - through port ( p ) or a local port ( l ) via transponder ( t ). each of the pass - through ports ( p ) is situated on a different modular card , and each of the transponder ( t ) and its associated local port ( p ) are situated together on yet another modular card . although direct connections are shown , as discussed above the respective wavelengths may be selectively switched to either of a local port ( l ) via transponder ( t ), or a pass - through port ( p ) as described with respect to fig1 . the second band 210 ( λ 9 - λ 16 ) is provided directly to a pass - through port ( p ), and the third band 212 ( λ 17 - λ 24 ) is provided directly to a pass - through port ( p ). the fourth band 214 ( λ 25 - λ 32 ) is demultiplexed into its 8 individual wavelengths by a multiplexer / demultiplexer 216 , which is situated on a modular card 217 , with each such wavelength being provided to a pass - through port ( p ) or a local port ( l ) via a transponder ( t ). again , switching may be used to select a connection to either p or t . [ 0032 ] fig4 is a simplified schematic diagram representative of the olt 2 shown in fig1 or the olt 200 of fig3 . however , it is to be noted that for simplicity only 16 wavelengths are utilized . the olt 300 interfaces and operates in a bidirectional manner as discussed in detail with respect to fig1 and 3 . the line interface 302 is adapted for wavelength division multiplexed ( wdm ) optical communication signals of the highest relative order , in this example 16 wavelengths λ 1 - λ 16 , corresponding to the n optical wavelengths on a single optical fiber which are applied to input / output line interfaces 4 and 202 of olt 2 ( fig1 ) and olt 200 ( fig3 ), respectively . the pass - through interface connected to the lines wl 1 - 4 , wl 5 - 8 , wl 9 - 12 and wl 13 - 16 corresponds to the respective pass - through ports , and the local - interface connected to the lines labeled 16 local ports correspond to the local ports connected to the respective transponders , where wavelengths from or to client equipment are added or dropped . [ 0033 ] fig5 illustrates two olts 300 a and 300 b as shown in fig4 connected in a back - to - back relationship by way of their respective all - optical pass - through interfaces . thus , it is seen that the connection results in an optical add / drop multiplexer ( oadm ) functionality without requiring intermediate electro - optical conversion ( oeo ) of the communicated optical signals . as discussed above , the add / drop feature is achieved at the 16 local ports of each olt , where channels ( wavelengths ) can be added or dropped by a manual configuration , or via add / drop switching , as controlled by switches 8 and 26 of fig1 to achieve a switchable add / drop multiplexer . the pass - through may be accomplished using single conductors and / or ribbon connectors that pass multiple individual channels ( wavelengths ) in one cable . the pass - through connections between olts 300 a and 300 b is preferably made using ribbon connectors / cables . [ 0035 ] fig6 illustrates three separate in - service wdm point - to - point optical communication systems a , b and c which are not initially interconnected . wdm system a includes optical nodes 400 and 402 which are optically connected via their respective line interfaces , with at least optical node 402 being an olt . wdm system b includes optical nodes 404 and 406 which are optically connected via their respective line interfaces , with at least optical node 404 being an olt . wdm system c includes optical nodes 408 and 410 which are optically connected via their respective line interfaces , with at least optical node 408 being an olt . as discussed above , the three separate wdm systems are not initially interconnected . however , any two of the three wdm systems , or all three of the wdm systems , may be interconnected by connecting respective olts of the separate wdm system back - to - back at respective pass - through ports as shown in fig5 without disrupting service . for example , wdm system a may be connected to wdm system b by directly optically connecting pass - through ports of the olt of node 402 to pass - through ports of the olt of node 404 via optical fibers 416 and 418 . wdm system a may also be connected to wdm system c by directly optically connecting pass - through optical ports of the olt of node 402 to pass - through ports of the olt of node 408 via optical fibers 420 and 422 . thus , an all optical path is provided from optical node 400 of wdm system a to optical node 406 of wdm system b , and likewise an all optical path is provided from optical node 400 of wdm system a to optical node 410 of wdm system c , resulting in a merger of wdm systems a , b and c without disrupting service . at the back - side of the respective optical nodes , lines with a box are indicative of local ports ( l ) to which client equipment is normally connected . [ 0037 ] fig7 illustrates three separate in - service wdm network optical communication systems d , e and f which are not initially interconnected . wdm system d includes optical nodes 500 and 502 which are optically connected via their respective line interfaces through an optical network 503 , with at least optical node 502 being an olt . wdm system e includes optical nodes 504 and 506 which are optically connected via their respective line interfaces through an optical network 507 , with at least optical node 504 being an olt . wdm system f includes optical nodes 508 and 510 which are optically connected via their respective line interfaces through an optical network 511 , with at least optical node 508 being an olt . as discussed above , the three separate wdm optical networks are not initially interconnected . however , any two of the three wdm optical networks , or all three of the wdm optical networks may be interconnected by connecting respective olts of the separate wdm optical networks back - to - back at respective pass - through ports as shown in fig5 without disrupting service . for example , wdm optical network d may be connected to wdm optical network e by directly optically connecting pass - through ports of the olt of node 502 to pass - through ports of the olt of node 504 via optical fibers 516 and 518 . wdm system d may also be connected to wdm optical network f by directly optically connecting pass - through optical ports of the olt of node 502 to pass - through ports of the olt of node 508 via optical fibers 520 and 522 . thus , an all optical path is provided from optical node 500 of wdm optical network d to optical node 506 of wdm optical network e , and likewise an all optical path is provided from optical node 500 of wdm optical network d to optical node 510 of wdm optical network f , resulting in a merger of wdm network optical communication systems d , e and f without disrupting service . at the back - side of the respective optical nodes , lines with a box are indicative of local ports ( l ) to which client equipment is normally connected . [ 0039 ] fig8 illustrates how olts can be connected in more complex ways to achieve greater functionality , such as , for example , limited cross - connection capabilities . specifically , olt 600 and olt 602 are connected back - to - back to form a first oadm , olt 604 and olt 606 are connected back - to - back to form a second oadm , olt 600 and olt 606 are connected back - to - back to form a third oadm and olt 602 and olt 604 are connected back - to - back to form a fourth oadm . olt 600 , olt 602 and olt 604 each have add / drop switching capability , whereas olt 606 has no switching capability . the arrangement shown in fig7 illustrates how a group of olts in an office , which may be part of separate wdm networks , can be coupled to form different oadms on an individual channel or per band basis . wavelengths 1 , 2 , 3 and 4 ( channels 1 , 2 , 3 and 4 ) are connected between pass - through optical ports of olt 600 and olt 602 via optical fiber 603 and are also connected between pass - through optical ports of olt 604 and olt 606 via optical fiber 607 . wavelengths 5 , 6 , 7 and 8 ( channels 5 , 6 , 7 and 8 ) are connected between pass - through optical ports of olt 600 and olt 606 via optical fiber 608 and are also connected between pass - through optical ports of olt 602 and olt 604 via optical fiber 609 . wavelengths 9 , 10 , 11 and 12 ( channels 9 , 10 , 11 and 12 ) can be separated into individual channels that are connected between local ports of the respective olts . for example , channel 9 is directly connected between a local port of olt 600 and a local port of olt 602 via optical fiber 610 , and channel 10 is directly connected between a local port of olt 600 and a local port of olt 606 via optical fiber 612 . to simplify the drawing , no connections are shown for wavelengths 11 and 12 ; however , they may be connected in a like manner . the local ports may also be connected to client equipment as discussed above . it is to be noted that the connection configuration of fig8 does not constitute a plain patch - panel form of connectivity , insofar as it allows for switching of channels without manual reconfigurations . in summary , the methods and apparatus of the present invention allow upgrading of a wavelength division multiplexed optical communication system including a pair of olts that reside in the same office or facility and are part of separate wdm networks ( whether point - to - point links or more advanced networks ) to form an oadm . such upgrade is accomplished without service disruption to the network by appropriate connection of the olts through the pass - through interfaces . although certain embodiments of the invention have been described and illustrated herein , it will be readily apparent to those of ordinary skill in the art that a number of modifications and substitutions can be made to the preferred example methods and apparatus disclosed and described herein without departing from the true spirit and scope of the invention .