Abstract:
A technique for communicating a change in bit rates that dependent transponders should each transfer. In one implementation, an upstream transponder outputs a specific frequency communications signal to a downstream transponders. The downstream transponder reacts to receipt of the specific bit rate by requesting new bit rate window information. The upstream transponder communicates such new bit rate window. Thereafter both the upstream and downstream transponders transfer payloads of signals having bit rates within the programmed bit rate windows.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from U.S. Provisional Patent Application Ser. No. 60/196,207 filed Apr. 11, 2000, which is incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention is directed to a transponder and more specifically to a transponder programmable to vary the transferred input signal bit rate. 
   RELATED ART 
   In communications systems, frequently it is desirable to control which signals can enter a distinct network or location. Devices known as transponders are used in such scenarios. A transponder is a device that receives an input signal, regenerates it, and then retransmits that signal. See for example, U.S. Pat. No. 6,075,634 to Casper et al. For example,  FIG. 1  depicts an example in which transponder  110  couples network  120  to user  150 , where user  150  is for example, a corporate office&#39;s telecommunications equipment. 
   One manner in which a transponder controls which signals it transfers is based on bit rate. Some transponders are programmable as to the signal bit rate transferred. For example, an optical transponder should be able to forward SONET traffic at 155, 622, and 2488 Mbs, as well as gigabit ethernet traffic at 1250 Mbs. The well-known “3R” type of transponder transfers a signal having a bit rate within a small range of bit rates. In order to use a transponder, such as the 3R transponder, a mechanism is required to set the bit rate or narrow band of bit rates of signals that the transponder transfers (so called “bit rate window”). Further, some technique is required to determine the correct bit rate window. Because transponders are often located far from a central control site, it is important to provide a technique for performing such bit rate window changes remotely. 
   In the example of  FIG. 1 , transponder  110  serves as an interface between user  150  and network  120 . If the equipment of user  150  is changed so to either increase or decrease the bit rate at which user  150  receives signals from transponder  110 , then the bit rate window of transponder  110  should be changed to match the appropriate signal bit rate transferred to user  150 . 
   One approach is to configure the bit rate window of transponder  110  at installation. If the input signal&#39;s bit rate is subsequently changed, however, a field technician must be dispatched to the transponder location to readjust the bit rate. Manually adjusting bit rates is time consuming and expensive. 
   If the transponder can be controlled through a secondary communication link, such as the “optical service channel” on a wave division multiplexed (WDM) ring, then this channel can be used to remotely provision the bit rate window. Note that this communication typically occurs “out-of-band”, i.e., on a link dedicated to communications and not carrying data. In certain scenarios, however, such as tree-architecture deployments, an optical service channel is impractical due to the point-to-multipoint nature of the architecture. Moreover, the additional optoelectronics required to implement a separate channel add significant cost to the transponder unit. 
   Thus what is needed is a method and apparatus that eliminates the need for labor intensive efforts and/or dedicated communications channel to establish or change a bit rate window. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention includes a method of selecting signals to transfer between a head transponder and tail transponder, the method including the acts of: (a) tuning an applied bit rate window of a head transponder to a predetermined frequency; (b) receiving a request at the head transponder to change the bit rate window; (c) at the head transponder, generating a communications signal having a bit rate of the predetermined frequency; (d) at the head transponder, inserting a command into the communications signal, where the command states to establish a new bit rate window; (e) at the head transponder, transmitting the communications signal to a downstream element; and (f) at the head transponder, transferring payload of incoming signals having a bit rate within the bit rate window. 
   This embodiment can further include the acts of: (g) receiving the communications signal at the tail transponder; (h) detecting a change in input bit rate at the tail transponder; (i) at the tail transponder, generating a response signal having a similar bit rate as the communications signal; (j) at the tail transponder, inserting a command into the response signal indicating receipt of a command to set a new bit rate window; and (k) at the tail transponder, transferring to the head transponder a payload of signals having a bit rate within the bit rate window. 
   Various embodiments of the present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts an example in which transponder  110  couples network  120  to user  150 . 
       FIG. 2  depicts in block diagram form an exemplary environment in which an embodiment of the present invention can be used. 
       FIG. 3A  depicts in block diagram form an embodiment of transponder  200 . 
       FIG. 3B  depicts in block diagram form an embodiment of transponder  202 . 
       FIG. 4  depicts, in block diagram form, framing device  230  that is in accordance with an embodiment of the present invention. 
       FIG. 5  depicts an exemplary overhead bit format for communicating to the downstream network element. 
       FIG. 6  is a flow diagram of exemplary operation between transponders  200  and  202 . 
   

   Note that use of the same reference numbers in different figures indicates the same or like elements. 
   DETAILED DESCRIPTION 
     FIG. 2  depicts in block diagram form an exemplary environment in which an embodiment of the present invention can be used. Network  120  is capable of transmitting signals of varying protocols and formats (e.g., SONET, SDH, passive optical network and/or gigabit Ethernet). User  100  is coupled to network  120  using an embodiment of the present invention in transponder  200 , which is described in more detail below. User  150  is coupled, for example, to network  120  by use of a transponder  202 , which is a duplicate of transponder  200 . In this example, users  100  and  150  represent entities that pay for use of network  120 . The network  120  is capable of transmitting signals from user  100  to a variety of users, such as users  170  and  180  through respective transponders  165  and  175 . 
   In this example, transponders  200  and  202  are respectively referred to as head and tail transponders. The head transponder transmits signals and instructions downstream to the tail transponder through network  120 . 
   As an example, transponder  200  transfers signal  105  from user  100  to network  120 . Transponder  202  transfers signal  105  from network  120  to user  150 . Similarly, transponder  202  transfers signal  107  from user  150  to network  120 . Transponder  200  transfers signal  107  from network  120  to user  100 . Signals  105  and  107  can be encoded in any protocol (e.g., SONET, SDH, passive optical network and/or gigabit Ethernet). Herein, signal  105  refers to the signal  105  as transmitted from user  100  or generated by transponder  200  and any modifications made to signal  105  during processing or transport through transponders  200  and  202 . Similarly, signal  107  refers to the signal  107  as transmitted from user  150  or generated by transponder  202  and any modifications made to signal  107  during processing or transport through transponders  200  and  202 . 
   In this configuration, transponder  200  can be programmed externally by a central control device  160  as to the format and bit rate of signal  105  that transponder  200  transfers from user  100  to network  120 . The central control device  160  may also instruct transponder  200  to insert various commands into the signal  105  or otherwise modify the signal  105 . In accordance with an embodiment of the present invention, transponder  200  communicates with tail transponder  202  using the instructions inserted into signal  105 . In this example, in accordance with commands inserted into overhead fields of signal  105  from transponder  200 , transponder  202  selectively transfers and modifies signal  105  from network  120  to user  150 . The use of commands inserted into overhead fields of signal  105  reduces the need for deployment of field technicians to change the bit rate window of tail transponder  202 . An example interoperation between transponder  200  and tail transponder  202  is described with respect to  FIGS. 3A to 6 . 
   Transponder Block Diagram 
     FIG. 3A  depicts in block diagram form an embodiment of transponder  200 . Transponder  200  includes central processing unit (CPU)  205 , receiver  210 - 1 , clock and data recovery device (CDR)  215 - 1 , switch  220 - 1 , MUX  219 - 1 , clock  217 - 1 , framing device  230 , switch  220 - 2 , and transmitter  225 - 1 . Receiver  210 - 2 , CDR  215 - 2 , and transmitter  225 - 2  are used when the transponder  200  processes signal  107  and thus operates in duplex mode, i.e., bi-directional signal transfer and processing, of both signals  105  and  107 . 
   Signal  105  from user  100  is routed through transponder  200  through receiver  210 - 1 , CDR  215 - 1 , switch  220 - 1 , MUX  219 - 1 , framing device  230 , switch  220 - 2 , and transmitter  225 - 1 . Similarly, signal  107  from user  150  is routed through transponder  200  through receiver  210 - 2 , CDR  215 - 2 , switch  220 - 2 , framing device  230 , switch  220 - 1 , and transmitter  225 - 2 . 
     FIG. 3B  depicts in block diagram form an embodiment of transponder  202 , which is similar to transponder  200 . In this example, when transponder  200  and transponder  202  communicate, signal  105  is received by receiver  210 - 2  of transponder  202  as opposed to receiver  210 - 1 . For the sake of clarity, description shall be provided of transponder  200  only and such description applies to transponder  202  unless otherwise stated. 
   Receiver  210 - 1  is coupled to receive signal  105  from user  100 . CDR  215 - 1  is coupled to receive signals from receiver  210 - 1 . Similarly, receiver  210 - 2  is coupled to receive signal  107  from user  150  and CDR  215 - 2  is coupled to receive signals from receiver  210 - 2 . Switch  220 - 1  transfers (a) signals from CDR  215 - 1  to MUX  219 - 1  and (b) signals from framing device  230  to transmitter  225 - 2 . Switch  220 - 2  transfers (x) signals from CDR  215 - 2  to framing device  230  and (y) signals from framing device  230  to transmitter  225 - 1 . 
   For the sake of clarity, unidirectional operation is described with respect to signal  105  and receiver  210 - 1 , CDR  215 - 1 , switch  220 - 1 , and transmitter  225 - 1 . For bi-directional (duplex) operation, the operations of receiver  210 - 2 , CDR  215 - 2 , switch  220 - 2 , and transmitter  225 - 2  on signal  107  are similar to those of respective receiver  210 - 1 , CDR  215 - 1 , and transmitter  225 - 1 . 
   CPU  205  coordinates the operations of the elements of the transponder  200 . Using link  235 , CPU  205  communicates with, for example, a central control device  160  or field operator external to transponder  200  that directs the operations of the transponder  200 . An exemplary implementation of CPU  205  is any microprocessor in the Motorola MPC 860 family. 
   Conventional receiver  210 - 1  of transponder  200  is coupled to receive signals  105  from an external device or network such as user  100 . In some embodiments, signals  105  are optical signals and receiver  210 - 1  converts the signals  105  into electrical equivalents. Receiver is capable of converting optical signals encoded in a variety of protocols (e.g., SONET, SDH, passive optical network and/or gigabit Ethernet) at a wide range of data rates, including OC-48. An exemplary implementation of receiver  210 - 1  is the SDM 7128 available from Sumitomo. Receiver transfers the signal  105  to CDR  215 - 1 . 
   Conventional CDR  215 - 1  controls whether signal  105  is transferred to switch  220 - 1 . CDR  215 - 1  transfers signals within a programmable bit rate window. The CDR  215 - 1  provides a clock and data recovery system over a wide range data rates including OC-48. A suitable implementation of CDR  215 - 1  is the VSC 8123 available from Vitesse Semiconductor Corporation. U.S. Pat. No. 6,178,213 to McCormack et al., which is hereby incorporated by reference in its entirety, describes a suitable implementation of CDR  215 - 1 . The bit rate window of CDR  215 - 1  is controlled by the CPU  205 . CDR  215 - 1  selectively transfers signal  105  to switch  220 - 1  in accordance with the programmed bit rate window. 
   In some embodiments, the transponder  200  can be configured to adaptively set any bit rate window independent of the format and protocol of the signal. In some embodiments, a device external to the transponder  200 , such as for example, a central controller device  160  or local field technician, sets bit rate window of the transponder  200 . For example, a central controller device  160  can transmit to CPU  205  an instruction of the appropriate bit rate window and format of the signal  105  (e.g., SONET, SDH, or gigabit Ethernet) that transponder  200  will transfer. 
   In this embodiment, switch  220 - 1  transfers (a) signal  105  from CDR  215 - 1  to MUX  219 - 1  and (b) signal  107  from framing device  230  to transmitter  225 - 2 . Switch  220 - 1  is any conventional signal routing device that has the bit rate transfer capacity to bi-directionally transfer signals without significant delay. A suitable implementation of switch  220 - 1  is the S3054 available from Applied Micro Circuits Corporation (AMCC). Switch  220 - 1  can also be capable of loopback operation, e.g., transferring signals from CDR  215 - 1  to transmitter  225 - 2 . 
   MUX  219 - 1  is any conventional multiplexer and is coupled to receive (a) the signal  105  from switch  220 - 1  and (b) a clock signal from clock  217 - 1 . Clock  217 - 1  generates a clock signal having a frequency set to be a distinct communications channel between transponders  200  and  202 . The output of MUX  219 - 1  is coupled to framing device  230  and is selected by CPU  205 . Signal  105  refers to either the clock signal or the signal  105  from user  100  unless otherwise stated. In this embodiment, MUX  219 - 1  transfers the signal  105  from switch  220 - 1  to framing device  230  except when transponder  200  attempts to establish the bit rate window of both transponders  200  and  202 . 
   In this embodiment, clock signal from clock  217 - 1  is an “in-band” control signal, i.e., it is transmitted using a link dedicated to communications. Advantageously, use of an “in-band” clock signal as a communications channel avoids the added cost of an optical service channel, which requires a separate wavelength (and hence additional wavelength filters), plus separate transmitters and receivers. 
     FIG. 4  depicts, in block diagram form, framing device  230  that is in accordance with an embodiment of the present invention. In some embodiments, framing device  230  supports embedded payload data and signal performance assessment for at least SONET/SDH and 8B10B encoded data. Framing device  230  includes controller  418 , FEC decoder  405 - 1 , performance monitor (PM)  410 - 1 , gate  412 - 1 , FEC encoder  415 - 1 , FEC decoder  405 - 2 , performance monitor (PM)  410 - 2 , gate  412 - 2 , and FEC encoder  415 - 2 . For transponder  200 , FEC decoder  405 - 1 , performance monitor (PM)  410 - 1 , gate  412 - 1 , and FEC encoder  415 - 1  are used to process signal  105  whereas FEC decoder  405 - 2 , performance monitor (PM)  410 - 2 , gate  412 - 2 , and FEC encoder  415 - 2  are used to process signal  107 . For transponder  202 , FEC decoder  405 - 1 , performance monitor (PM)  410 - 1 , gate  412 - 1 , and FEC encoder  415 - 1  are used to process signal  107  whereas FEC decoder  405 - 2 , performance monitor (PM)  410 - 2 , gate  412 - 2 , and FEC encoder  415 - 2  are used to process signal  105 . The operations of FEC decoder  405 - 1 , performance monitor (PM)  410 - 1 , gate  412 - 1 , and FEC encoder  415 - 1  are similar to those of respective FEC decoder  405 - 2 , performance monitor (PM)  410 - 2 , gate  412 - 2 , and FEC encoder  415 - 2 . For the sake of clarity, the operations of FEC decoder  405 - 1 , performance monitor (PM)  410 - 1 , gate  412 - 1 , and FEC encoder  415 - 1  are described herein. 
   Framing device  230  can be implemented as software executed by CPU and/or firmware. Framing device  230  is capable of processing bi-directional bit streams (e.g., signals  105  and  107 ) without substantial delay. FEC decoder  405 - 1  is coupled to receive signal  105  from MUX  219 - 1 . FEC decoder  405 - 1  outputs signal  105  to PM  410 - 1  and provides the overhead fields from signal  105  to processor  420  of controller  418 . PM  410 - 1  outputs signal  105  to gate  412 - 1  and provides payload data performance information concerning signal  105  to processor  420  (which will be sent to CPU  205 ). Gate  412 - 1  transfers the overhead fields of signal  105  to FEC encoder  415 - 1  but selectively transfers the data payload of signal  105  to FEC encoder  415 - 1 . Controller  418  controls whether gate  412 - 1  transfers the data payload of signal  105  to FEC encoder  415 - 1 . FEC encoder  415 - 1  provides commands embedded in signal  105  to processor  420  and provides signal  105  to switch  220 - 2 . FEC decoder  405 - 1 , PM  410 - 1  and FEC encoder  415 - 1  can be implemented using a single AMCC S3062 available from Applied Micro Circuits Corporation. 
   In one embodiment, if signal  105  includes forward error correction (FEC) encoding, FEC decoder  405 - 1  applies Reed-Solomon FEC decoding in compliance with ITU standard G.709. For a description of Reed-Solomon FEC, see ITU G.709 and G.975, which is hereby incorporated by reference in its entirety. FEC decoder  405 - 1  provides the FEC decoded signal  105  to PM  410 - 1  and provides overhead fields (e.g., optical overhead channel (OCH)) from the signal  105  to controller  418 . 
   PM  410 - 1  performs optical channel payload data protocol performance monitoring on signal  105  in accordance with the relevant protocol of signal  105 . Exemplary protocols include BellCore&#39;s GR.253 standard relating to SONET or 8B10B encoding. PM  410 - 1  provides signal  105  to gate  412 - 1  and provides optical channel payload data performance results to controller  418 . 
   In this embodiment, controller  418  performs frame synchronization on signal  105  and counts the bit rate of signal  105 .  FIG. 4  depicts a detailed diagram of controller  418 . As depicted, controller  418  includes clock counter  425  and overhead (OH) and alarm processor  420 . Controller  418  is coupled to receive: (a) OCH OH bits from FEC decoder  405 - 1 , (b) optical channel payload data protocol performance results from PM  410 - 1 , and (c) commands embedded in signal  105  from FEC encoder  415 - 1 . 
   Overhead and alarm processor  420  is coupled to receive the signal  105  from PM  410 - 1 . Processor  420  performs frame synchronization to determine the start of each frame of the signal  105  in accordance with the relevant signal protocol. For example, when signal  105  is transmitted using the optical transport network (OTN) architecture defined in ITU recommendation G.872 or SONET, framing device  230  identifies each “frame” within the signal including the overhead fields. 
   In addition, processor  420  identifies any commands embedded within the overhead fields or other information carrying fields of signal  105 . As described in more detail below, overhead fields of signal  105  may include commands directed to transponder  202 . An exemplary command is for transponder  202  to set a new bit rate window. When transponder  202  acts as a tail transponder, processor  420  embeds an acknowledgement of receiving a command to establish a new bit rate window signal into the overhead fields of signal  107  and transmits such signal to transponder  200 . Processor  420  forwards embedded commands or messages to CPU  205  for processing. 
   Clock counter  425  is coupled to receive signal  105  from switch  220 - 1 .  FIG. 4  depicts, as an example, a configuration whereby a demultipexer  435  is coupled to receive the signal  105  from switch  220 - 1 . Demultiplexer  435  is used to decrease the bit rate of signal  105  and thus allows for the use of a clock counter  425  that does not count as fast as the bit rate of signal  105 . For example, the demultiplexer  435  may divide the data rate by 16 and output the slower data rate to clock counter. Clock counter  425  counts the bit rate of signal  105 . In some embodiments, the clock counter  425  counts the bit rate every second although the period in which bit rates are measured can be varied. 
   The clock counter  425  outputs the bit rate of the signal  105  to CPU  205 . CPU  205  determines whether the correct bit rate window is being applied by CDR  215 - 1 . If the correct bit rate window is being applied by CDR  215 - 1 , then CPU  205  commands gate  412 - 1  to transfer the data payload portion of signal  105  to FEC encoder  415 - 1 . Gate  412 - 1  is any conventional signal gating element controlled by a signal from an external device. In this embodiment of the present invention, the controller  418  determines if CDR  215 - 1  transfers the correct bit rate because exemplary implementations of the CDR, such as the VSC 8123 from Vitesse Semiconductor Corporation, do not accurately determine the bit rate of incoming signals. 
   In some embodiments of the present invention, the framing device  230  utilizes the overhead fields of signal  105  to communicate to downstream network elements, such as transponder  202 , to transfer a different signal bit rate. In this embodiment,  FIG. 5  depicts an exemplary overhead bit format for communicating to the downstream network element. As shown in  FIG. 5 , the transponder  200  transmits (a) the new bit rate window that transponder  202  is to transfer and/or modify (shown as item  505 ), (b) the format of the signal (e.g., SONET, SDH, or gigabit Ethernet) (shown as item  510 ), (c) the identification of the tail transponder  202  that is to obey the command (shown as item  515 ), and (d) at least one message or instruction (e.g., (i) set new bit rate window or (ii) acknowledge receipt of instruction to set new bit rate window) (show as item  520 ). The bit length of each field (a), (b), (c), and (d) is arbitrary and are represented respectively as w-1, w+x−1, w+x+y−1, and w+x+y+z−1 bits. For example, when the signal  105  is encoded using OTN as defined in ITU G.872, unused bytes of the optical channel section layer (OCH) can be used to transmit the information of  FIG. 5 . In this embodiment, processor  420  commands FEC encoder  415 - 1  to insert the information described with respect to  FIG. 5  and generated by processor  420  into the overhead fields of signal  105 . 
   Referring to FEC encoder  415 - 1  of framing device  230 , FEC encoder  415 - 1  is coupled to gate  412 - 1  to receive (a) the data payload of signal  105 , if the gate  412 - 1  is instructed so by CPU  205 , and (b) the overhead fields of signal  105 . If CPU  205  does not instruct gate  412 - 1  to transmit the data payload of signal  105 , then dummy information are transferred as the data payload of signal  105 . FEC encoder  415 - 1  performs conventional FEC encoding. FEC encoder  415 - 1  provides signal  105  to switch  220 - 2 . 
   Framing device  230  can be used to process bi-directional signals  105  and  107  without significant delays. For example, to process signal  107  from user  150 , the FEC decoder  405 - 2 , PM  410 - 2 , gate  412 - 2 , and FEC encoder  415 - 2  are used and operate similarly as respective FEC decoder  405 - 1 , PM  410 - 1 , gate  412 - 1 , and FEC encoder  415 - 1 . As depicted, a demultiplexer  440  can be used to couple signal  107  from switch  220 - 2  to clock counter  425 . Demultiplexer  440  is used to decrease the bit rate of signal  107  and thus allows for the use of a clock counter  425  that does not count as fast as the bit rate of signal. For example, the demultiplexer  440  may divide the data rate by 16 and output the slower data rate to clock counter. 
   Switch  220 - 2  is a duplicate of switch  220 - 1 . Switch  220 - 2  transfers (a) signal  107  from CDR  215 - 2  to framing device  230  and (b) signal  105  from framing device  230  to transmitter  225 - 1 . 
   Transmitter  225 - 1  converts the electrical signal  105  into optical form. An exemplary embodiment of transmitter  225 - 1  is the C48 DWDM transmitter available from Lucent Technologies. In transponder  200 , transmitter  225 - 1  outputs the signal  105  to network  120 . In transponder  202 , transmitter  225 - 1  outputs the signal  107  to network  120 . 
   Example Operation 
     FIG. 6  depicts a flow diagram of a suitable interoperation between transponders  200  and  202  in accordance with an embodiment of the present invention. The example of  FIG. 6  is used to establish or re-establish a bit rate window or other communication between head transponder  200  and tail transponder  202 . In  FIG. 6 , actions having associated numbers within the 600&#39;s are operations by head transponder  200  whereas actions numbered in the 700&#39;s are operations by tail transponder  202 . In this example, actions  610 ,  720 ,  730 ,  640 ,  650 ,  760 , and  670  are a sequence. 
   In action  610 , head transponder  200  outputs a signal having a bit rate that is recognized by the target tail transponder  202  as an attempt to establish a communications link. The clock signal is set at any bit rate identifiable by transponders  200  and  202  as a communications link. Hereafter “communications signal” means the signal having a bit rate that is recognized by the target tail transponder  202  as a communications link. For example, action  610  can include the following sub actions: (a) CPU  205  commands MUX  219 - 1  to output the clock signal from clock  217 - 1 ; (b) transmitter  225 - 1  transmits the clock signal (i.e., communications signal) as signal  105  to tail transponder  202 . 
   In action  720 , tail transponder  202  determines that it is receiving a signal having a bit rate outside of its applied bit rate window. For example, this action  720  can include the following sub actions: (a) receiver  210 - 2  of tail transponder  202  receives the communications signal from head transponder  200 ; (b) CDR  215 - 2  of tail transponder  202  recognizes that the signal received by receiver  210 - 1  is not of a bit rate within a programmed bit rate window; and (c) CDR  215 - 2  of tail transponder  202  communicates the failure to CPU  205 . 
   In action  730 , tail transponder  202  adjusts the applied bit rate window to receive the communications signal. For example, this action  730  can include the following sub actions: (a) in response to the communicated bit rate failure of action  720 , CPU  205  commands CDR  215 - 2  to change its programmed bit rate window to that which includes the bit rate of the communications signal; (b) CDR  215 - 2  changes its programmed bit rate window in response to the command from CPU  205 ; (c) CPU  205  commands MUX  219 - 1  to output the clock signal from clock  217 - 1  as signal  107 . Subaction  730 - c  is used to establish a communications/response signal to head transponder  200 . 
   In action  640 , head transponder  200  receives a request from an external person or device to change its applied bit rate window. For example, this action  640  can include the following sub actions: (a) CPU  205  receives a request from central controller  160  to change its applied bit rate window; (b) CPU  205  instructs CDR  215 - 1  to change its bit rate window to that specified by central controller  160 ; and (c) CDR  215 - 1  changes its bit rate window to that specified by CPU  205 . 
   In action  650 , head transponder  200  embeds information into the communications signal concerning the change of bit rate window, set by an external person or device. For example, this action  650  can include the following sub actions:
         (a) the framing device  230  embeds into the signal  105  the information described with respect to  FIG. 5  earlier to instruct tail transponder  202  that the applied bit rate window will change; and (b) the transmitter  225 - 1  transmits such signal  105  to the transponder  202  using network  120 .       

   In action  760 , tail transponder  202  acknowledges receipt of the new bit rate window. For example, this action  760  can include the following sub actions: (a) CPU  205  commands processor  420  to embed an acknowledgement of new bit rate window signal into the signal  107 ; (b) processor  420  provides the acknowledgement of new bit rate window message to FEC encoder  415 - 1  for embedding into the overhead fields of signal  107 ; and (c) transmitter  225 - 2  transmits signal  107  having the acknowledgement to transponder  200  through network  120 . 
   In action  670 , head transponder  200  waits for receipt of an acknowledgement signal from tail transponder  202 . For example, this action  670  can include the following sub actions: (a) processor  420  transfers the acknowledge message from tail transponder  202  to CPU  205 ; (b) CPU  205  reads the acknowledgement message. If head transponder  200  does not receive an acknowledge signal from tail transponder  202 , this action  670  repeats. 
   For head transponder  200 , actions  680  and  690  follow. For tail transponder  202 , actions  770  and  780  follow. Actions  680  and  690  are similar to respective actions  770  and  780 . In actions  680  and  770 , respective transponders  200  and  202  apply the new bit rate window on respective incoming signals  105  and  107  from respective users  100  and  150 . For example, this action  680  can include the following sub actions within transponder  200 : (a) CPU  205  commands CDR  215 - 1  to change its bit rate window; (b) CDR  215 - 1  applies the new bit rate window and transfers signals  105  from user  100  having a bit rate within the bit rate window through switch  220 - 1 , MUX  219 - 1 , FEC decoder  405 - 1  and PM  410 - 1  to gate  412 - 1 ; (c) clock counter  425  counts the bit rate of signal  105  and transfers the bit rate to CPU  205 ; (d) CPU  205  verifies that the bit rate of signal  105  is within the programmed bit rate window; (e) if the bit rate of signal  105  is within the programmed bit rate window, CPU  205  commands gate  412 - 1  to transfer the payload of signal  105  to FEC encoder  415 - 1 ; and (f) transmitter  225 - 1  transfers the signal  105  to tail transponder  202 . If the bit rate of signal  105  is not within the programmed bit rate window, then the transponder  200  does not transmit the payload of signal  105  but rather dummy frames. Note that previously, until action  680 - e , gate  412 - 1  does not transfer the payload of any input data but rather empty (dummy) frames are transmitted to transponder  202 . For transponder  202 , action  770  is similar to that of action  680 . 
   In actions  690  and  780 , respective transponders  200  and  202  verify that the bit rate of respective signals  105  and  107  are within the programmed bit rate windows. For example, this action  690  can include the following sub actions: (a) CPU  205  verifies that the bit rate of signal  105  is within the programmed bit rate window; (b) if the bit rate of signal  105  is within the programmed bit rate window, CPU  205  commands gate  412 - 1  to transfer the payload of signal  105  to FEC encoder  415 - 1 ; and (c) transmitter  225 - 1  transfers the signal  105  to tail transponder  202 . If the bit rate of signal  105  is not within the programmed bit rate window, then the transponder  200  does not transmit the payload of signal  105  but rather dummy frames. For transponder  202 , action  780  is similar to that of action  690 . 
   Subsequently, transponders  200  and  202  can communicate using overhead fields of signals  105  and  107  to verify that the correct bit rate windows are being applied by both transponders  200  and  202 . For example, (a) using overhead fields of signal  105  and the fields described with respect to  FIG. 5 , transponder  200  can query transponder  202  whether transponder  202  is successfully applying a bit rate window and (b) transponder  202  responds using overhead fields of signal  107  and the fields described with respect to  FIG. 5 . Transponder  202  can similarly communicate with transponder  200  to determine that transponder  200  is successfully applying a bit rate window. 
   Modifications 
   The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.