Patent Publication Number: US-9853721-B2

Title: Apparatus and methods for enabling recovery in optical networks

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 14/029,216, filed Sep. 17, 2013, now U.S. Pat. No. 9,344,187. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to optical networks, and more particularly to optical networks that utilize or are configured to utilize digital signal processing (DSP)-enabled optical transceivers/transponders. 
     BACKGROUND OF THE INVENTION 
     The following references are believed to represent the state of the art: 
     an article entitled “Rate-Adaptable Optics for Next Generation Long-Haul Transport Networks”, by Xiang Zhou, Lynn E. Nelson, and Peter Magill, in  IEEE Communications Magazine , March 2013, pages 41-49; 
     an article entitled “Extending Software Defined Network Principles to Include Optical Transport”, by Steven Gringeri, Nabil Bitar, and Tiejun J. Xia, in  IEEE Communications Magazine , March 2013, pages 32-40; 
     an article entitled “Spectrum-Efficient and Agile CO-OFDM Optical Transport Networks: Architecture, Design, and Operation”, by Gangxiang Shen and Moshe Zukerman, in  IEEE Communications Magazine , May 2012, pages 82-89; 
     an article entitled “Spectrum-Efficient and Scalable Elastic Optical Path Network: Architecture, Benefits, and Enabling Technologies”, by Masahiko Jinno, Hidehiko Takara, Bartlomiej Kozicki, Yukio Tsukishima, Yoshiaki Sone, and Shinji Matsuoka, in  IEEE Communications Magazine , November 2009, pages 66-73; 
     an article entitled “Advanced Optical Modulation and Multiplexing Technologies for High-Capacity OTN Based on 100 Gb/s Channel and Beyond”, by Yutaka Miyamoto and Senichi Suzuki, in  IEEE Communications Magazine , March 2010, pages S65-S72; 
     an article entitled “Higher-Order Modulation for Client Optics”, by Chris Cole, Ilya Lyubomirsky, Ali Ghiasi, and Vivek Telang, in  IEEE Communications Magazine , March 2013, pages 50-57; 
     an article entitled “IETF Work on Protection and Restoration for Optical Networks”, by David W. Griffith in  Optical Networks Magazine , July/August 2003, pages 101-106; 
     an article entitled “Optical Transceivers for 100 Gigabit Ethernet and its Transport”, by Jon Anderson and Matthew Traverso, in  IEEE Communications Magazine , March 2010, pages S35-S40; 
     an article entitled “Ultra-High-Capacity DWDM Transmission System for 100G and Beyond”, by Jianjun Yu and Xiang Zhou, in  IEEE Communications Magazine , March 2010, pages S56-S64; 
     an article entitled “Next-Generation 100 Gb/s Undersea Optical Communications”, by Yasuhiro Aoki, Yoshihisa Inada, Takaaki Ogata, Lei Xu, Shaoliang Zhang, Fatih Yaman, and Eduardo Mateo, in  IEEE Communications Magazine , February 2012, pages S50-S57; 
     an Implementation Agreement IA # OIF-PMQ-TX-01.0 entitled “Implementation Agreement for Integrated Polarization Multiplexed Quadrature Modulated Transmitters”, technical editor Martin Bouda, of the Physical and Link Layer (PLL) Working Group of the Optical Internetworking Forum (OIF), dated Mar. 12, 2010, available on the World Wide Web at www.oiforum.com/public/impagreements.html; and 
     an Implementation Agreement IA # OIF-DPC-RX-01.1 entitled “Implementation Agreement for Integrated Dual Polarization Intradyne Coherent Receivers”, maintenance editor Anthony J. Ticknor, of the Physical and Link Layer (PLL) Working Group of the Optical Internetworking Forum (OIF), dated Sep. 20, 2011 available on the World Wide Web at www.oiforum.com/public/impagreements.html. 
     SUMMARY OF THE INVENTION 
     Certain embodiments of the present invention provide apparatus and methods for enabling recovery in optical networks that utilize or are configured to utilize digital signal processing (DSP)-enabled optical transceivers/transponders. 
     The term “recovery” is used throughout the present specification and claims to denote both types of recovery, namely protection and restoration. 
     The term “optical transceiver” is used throughout the present specification and claims to include a combination of an optical transmitter and an optical receiver. The term “optical transponder” is used throughout the present specification and claims to include a device which includes an optical transmitter and an optical receiver and hence an optical transceiver. The term “optical transceiver/transponder” is used throughout the present specification and claims to include an optical transceiver which may be provided as a stand-alone optical unit, or be part of an optical transponder, or be embodied in a unit. 
     The term “DSP-enabled optical transceiver/transponder” is used throughout the present specification and claims to include an optical transceiver/transponder that includes, or is associated with, a Digital Signal Processor or Application Specific Integrated Circuit (DSP/ASIC) that is used to process electrical signals before their transmission in an optical form and to process received signals after their conversion from an optical form into an electrical form. Such signal processing by the DSP/ASIC is typically intended to perform operations such as, but not limited to, chromatic dispersion (CD) compensation after reception, and/or forward error correction (FEC) coding before transmission and FEC decoding after reception. Accordingly, the term “DSP-enabled optical transmitter” is used throughout the present specification and claims to include an optical transmitter that includes, or is associated with, a DSP/ASIC that is used to process electrical signals before their transmission in an optical form, and the term “DSP-enabled optical receiver” is used throughout the present specification and claims to include an optical receiver that includes, or is associated with, a DSP/ASIC that is used to process received signals after their conversion from an optical form into an electrical form. 
     The term “failure” is used throughout the present specification and claims in connection with an optical transceiver/transponder to include a failure which disables operation of at least one of an optical transmitter and an optical receiver of the optical transceiver/transponder or degrades operation of the at least one of an optical transmitter and an optical receiver of the optical transceiver/transponder to an unacceptable level. In connection with a working path or link the term “failure” is used throughout the present specification and claims to include a failure which affects the working path or link in such a manner that communication is disabled over the working path or link or a quality of the communication over the working path or link is at an unacceptable level. 
     The term “link” is used throughout the present specification and claims to include a communication link with a plurality of paths. 
     There is thus provided in accordance with an embodiment of the present invention apparatus for enabling an M:N recovery scheme in an optical network, wherein M and N are positive integers and 1≦M≦N, the apparatus including a set of N working DSP-enabled optical transceivers/transponders which includes at least one working DSP-enabled optical transceiver/transponder that uses a first set of transmission parameters and at least one working DSP-enabled optical transceiver/transponder that uses a second set of transmission parameters which is different from the first set of transmission parameters, and a set of M protection DSP-enabled optical transceivers/transponders operable to protect the set of N working DSP-enabled optical transceivers/transponders and including L protection DSP-enabled optical transceivers/transponders, each having a capability of using a set of adjustable transmission parameters enabling it to protect every one of the N working DSP-enabled optical transceivers/transponders, and, when M&gt;L, M-L protection DSP-enabled optical transceivers/transponders, each having a capability of protecting at least one, but not all, of the N working DSP-enabled optical transceivers/transponders, wherein L is a positive integer and 1≦L≦M. 
     In some embodiments, the transmission parameters of each of the first set of transmission parameters, the second set of transmission parameters, and each of the sets of adjustable transmission parameters usable by the L protection DSP-enabled optical transceivers/transponders include at least one of the following: a modulation scheme parameter, a forward error correction (FEC) coding overhead percent parameter, a symbol rate parameter, a parameter of a central wavelength of a channel wavelength, and a client port rate parameter. 
     In one embodiment, the modulation scheme parameter includes a parameter indicating polarization multiplexing (PM) with one of the following: quadrature phase shift keying (QPSK), 8 phase shift keying (8PSK), 8 quadrature amplitude modulation (8QAM), 16QAM, 32QAM, 64QAM, 128QAM, time-domain hybrid QPSK/8QAM, time-domain hybrid 8QAM/16QAM, and time-domain hybrid 32QAM/64QAM. 
     In one embodiment, the FEC coding overhead percent parameter includes a parameter indicating one of the following FEC coding overheads: substantially 7%, substantially 13%, substantially 20%, and substantially 25%. 
     In one embodiment, the parameter of a central wavelength of a channel wavelength includes a parameter indicating one of the following central wavelengths: a wavelength in the International Telecommunication Union (ITU) transmission C-Band, and a wavelength in the ITU transmission L-Band. 
     In one embodiment, the client port rate parameter includes a parameter indicating one of the following bit rates: substantially 100 Gigabit per second (Gb/s), substantially 200 Gb/s, substantially 400 Gb/s, substantially 1 Terabit per second (Tb/s), substantially 1600 Gb/s, and a variable bit rate. 
     In some embodiments, the M-L protection DSP-enabled optical transceivers/transponders correspond to M-L of the N working DSP-enabled optical transceivers/transponders. 
     In some embodiments, each of the at least one working DSP-enabled optical transceiver/transponder that uses the first set of transmission parameters includes one of the following: a DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters corresponding to the first set of transmission parameters, a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters of one of the L protection DSP-enabled optical transceivers/transponders and configured to use the first set of transmission parameters, and a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which does not enable it to protect every one of the N working DSP-enabled optical transceivers/transponders and is configured to use the first set of transmission parameters, and each of the at least one working DSP-enabled optical transceiver/transponder that uses the second set of transmission parameters includes one of the following: a DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters corresponding to the second set of transmission parameters, a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters of one of the L protection DSP-enabled optical transceivers/transponders and configured to use the second set of transmission parameters, and a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which does not enable it to protect every one of the N working DSP-enabled optical transceivers/transponders and is configured to use the second set of transmission parameters. 
     In one embodiment, the at least one working DSP-enabled optical transceiver/transponder that uses the first set of transmission parameters includes a substantially 100 Gb/s DSP-enabled optical transceiver/transponder. 
     In some embodiments, the apparatus is within a network element (NE) of the optical network. In other embodiments, the apparatus is external to and operatively associated with an NE of the optical network. 
     There is also provided in accordance with a further embodiment of the present invention a method of enabling an M:N recovery scheme in an optical network, wherein M and N are positive integers and 1≦M≦N, the method including allocating a set of M protection DSP-enabled optical transceivers/transponders for protecting a set of N working DSP-enabled optical transceivers/transponders which includes at least one working DSP-enabled optical transceiver/transponder that uses a first set of transmission parameters and at least one working DSP-enabled optical transceiver/transponder that uses a second set of transmission parameters which is different from the first set of transmission parameters, wherein the set of M protection DSP-enabled optical transceivers/transponders includes L protection DSP-enabled optical transceivers/transponders, each having a capability of using a set of adjustable transmission parameters enabling it to protect every one of the N working DSP-enabled optical transceivers/transponders, and, when M&gt;L, M-L protection DSP-enabled optical transceivers/transponders, each having a capability of protecting at least one, but not all, of the N working DSP-enabled optical transceivers/transponders, wherein L is a positive integer and 1≦L≦M. 
     In some embodiments, the allocating includes allocating the set of M protection DSP-enabled optical transceivers/transponders via a control plane of the optical network. 
     In some embodiments, the allocating includes allocating the set of M protection DSP-enabled optical transceivers/transponders in response to a determination that the set of N working DSP-enabled optical transceivers/transponders includes working DSP-enabled optical transceivers/transponders that use different sets of transmission parameters. 
     In some embodiments, the method further includes allocating the set of N working DSP-enabled optical transceivers/transponders prior to allocating the set of M protection DSP-enabled optical transceivers/transponders, wherein the allocating the set of M protection DSP-enabled optical transceivers/transponders includes allocating the set of M protection DSP-enabled optical transceivers/transponders in response to allocation of the set of N working DSP-enabled optical transceivers/transponders. 
     There is also provided in accordance with another embodiment of the present invention a method of using a first DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters in an optical network, the method including using the first DSP-enabled optical transceiver/transponder in a protection mode of operation as a protection DSP-enabled optical transceiver/transponder to protect one of a plurality of working DSP-enabled optical transceivers/transponders, and using the first DSP-enabled optical transceiver/transponder in an association mode of operation together with a second DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters to jointly overcome a change in at least one transmission parameter over a working path associated with the second DSP-enabled optical transceiver/transponder. 
     In some embodiments, the method further includes determining an operation mode of the first DSP-enabled optical transceiver/transponder as one of the protection mode of operation and the association mode of operation prior to the using, and the using includes using the first DSP-enabled optical transceiver/transponder in the determined one of the protection mode of operation and the association mode of operation. 
     In further embodiments, the determining includes determining the operation mode of the first DSP-enabled optical transceiver/transponder via a control plane of the optical network. 
     In some embodiments, the using the first DSP-enabled optical transceiver/transponder in the protection mode of operation includes detecting a failure in the one of a plurality of working DSP-enabled optical transceivers/transponders, configuring adjustable transmission parameters of the first DSP-enabled optical transceiver/transponder to correspond to transmission parameters of the one of a plurality of working DSP-enabled optical transceivers/transponders, and using the first DSP-enabled optical transceiver/transponder for communication instead of the one of a plurality of working DSP-enabled optical transceivers/transponders. 
     In some embodiments, the using the first DSP-enabled optical transceiver/transponder in the association mode of operation includes configuring adjustable transmission parameters of the first and second DSP-enabled optical transceivers/transponders to enable the first and second DSP-enabled optical transceivers/transponders to jointly overcome the change in at least one transmission parameter over the working path associated with the second DSP-enabled optical transceiver/transponder, and using the first DSP-enabled optical transceiver/transponder together with the second DSP-enabled optical transceiver/transponder for communication over a link including the working path associated with the second DSP-enabled optical transceiver/transponder and an additional working path associated with the first DSP-enabled optical transceiver/transponder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
         FIG. 1  is a simplified block diagram illustration of apparatus for enabling an M:N recovery scheme, the apparatus being constructed and operative in accordance with an embodiment of the present invention; 
         FIG. 2  is a simplified block diagram illustration of a DSP-enabled optical transmitter in the apparatus of  FIG. 1 , the DSP-enabled optical transmitter having a capability of using a set of adjustable transmission parameters; 
         FIG. 3  is a simplified block diagram illustration of a DSP-enabled optical receiver that uses coherent detection, the DSP-enabled optical receiver being comprised in the apparatus of  FIG. 1  and having a capability of using a set of adjustable transmission parameters; 
         FIG. 4  is a simplified block diagram illustration of an optical network utilizing the apparatus of  FIG. 1 , the optical network being constructed and operative in accordance with an embodiment of the present invention; 
         FIG. 5  is a simplified flowchart illustration of a method of enabling an M:N recovery scheme in an optical network of the type of the optical network of  FIG. 4 , in accordance with an embodiment of the present invention; 
         FIG. 6  is a simplified flowchart illustration of a method of using a DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters in an optical network of the type of the optical network of  FIG. 4 , in accordance with an embodiment of the present invention; and 
         FIG. 7  is a simplified flowchart illustration of a method of protecting a working DSP-enabled optical transceiver/transponder in an optical network of the type of the optical network of  FIG. 4 , in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to  FIG. 1 , which is a simplified block diagram illustration of apparatus for enabling an M:N recovery scheme, the apparatus being constructed and operative in accordance with an embodiment of the present invention. 
     The apparatus of  FIG. 1 , which is generally designated  100 , includes a set of N working digital signal processing (DSP)-enabled optical transceivers/transponders  110  and a set of M protection DSP-enabled optical transceivers/transponders  120 , wherein M and N are positive integers and 1≦M≦N. The set of M protection optical transceivers/transponders  120  is operable to protect the set of N working optical transceivers/transponders  110 . 
     In the context of recovery schemes a separation is sometimes made, for example in the above-mentioned article entitled “IETF Work on Protection and Restoration for Optical Networks” of David W. Griffith, between a 1:N recovery scheme and an M:N recovery scheme in which M&gt;1. However, throughout the present specification and claims the term “M:N recovery scheme” is referred to as covering both a 1:N recovery scheme and an M:N recovery scheme with M&gt;1. 
     Each working optical transceiver/transponder  110  includes a working optical transmitter  130  and a working optical receiver  140 . Each protection optical transceiver/transponder  120  includes a protection optical transmitter  150  and a protection optical receiver  160 . 
     By way of a non-limiting example, in  FIG. 1  the apparatus  100  is depicted in an architecture in which each optical transmitter  130  and paired optical receiver  140  of each working optical transceiver/transponder  110  are arranged in a back-to-back arrangement, and similarly each optical transmitter  150  and paired optical receiver  160  of each protection optical transceiver/transponder  120  are arranged in a back-to-back arrangement. It is, however, appreciated that the apparatus  100  may be arranged in any other suitable architecture, such as, by way of a non-limiting example, an alternative architecture (not shown) in which the optical transmitters  130  and  150  are grouped together in one group and the optical receivers  140  and  160  are grouped together in another group, and the group of the optical transmitters  130  and  150  is separate from the group of the optical receivers  140  and  160 . In such alternative architecture, the group of the optical transmitters  130  and  150  may, for example, be arranged as an array of tightly packaged optical transmitters, and the group of the optical receivers  140  and  160  may, for example, be arranged as an array of tightly packaged optical receivers. 
     The optical transmitters  130  and  150  are operable to transmit optical signals to a multiplexer (MUX) or a switching/routing unit (both not shown), and the optical receivers  140  and  160  are operable to receive optical signals from a demultiplexer (DeMUX) (not shown) or the switching/routing unit. By way of a non-limiting example, the MUX may be combined with the optical transmitters  130  and  150  in one or more units (not shown) which may form part of or be associated with an optical network, such as an optical network as described below with reference to  FIG. 4 , and the DeMUX may be combined with the optical receivers  140  and  160  in one or more units (not shown) which may also form part of or be associated with the optical network. The switching/routing unit may also typically form part of the optical network and be comprised in a network element (not shown) in the optical network. 
     In accordance with an embodiment of the present invention the set of N working DSP-enabled optical transceivers/transponders  110  includes at least one working DSP-enabled optical transceiver/transponder  110  that uses a first set of transmission parameters and at least one working DSP-enabled optical transceiver/transponder  110  that uses a second set of transmission parameters which is different from the first set of transmission parameters, and the set of M protection DSP-enabled optical transceivers/transponders  120  includes L protection DSP-enabled optical transceivers/transponders  120 , each having a capability of using a set of adjustable transmission parameters enabling it to protect every one of the N working DSP-enabled optical transceivers/transponders  110 , and, when M&gt;L, M-L protection DSP-enabled optical transceivers/transponders  120 , each having a capability of protecting at least one, but not all, of the N working DSP-enabled optical transceivers/transponders  110 , wherein L is a positive integer and 1≦L≦M. 
     In some embodiments, the M-L protection DSP-enabled optical transceivers/transponders  120  correspond to M-L of the N working DSP-enabled optical transceivers/transponders  110 . 
     Each of the L protection DSP-enabled optical transceivers/transponders  120  may, for example, include a DSP-enabled optical transceiver/transponder with an optical transmitter as described below with reference to  FIG. 2  and an optical receiver as described below with reference to  FIG. 3 , or a less-encompassing DSP-enabled optical transceiver/transponder with a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver as described below. 
     Each of the at least one working DSP-enabled optical transceiver/transponder  110  that uses the first set of transmission parameters may include one of the following: a DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters corresponding to the first set of transmission parameters; a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters of one of the L protection DSP-enabled optical transceivers/transponders  120  and configured to use the first set of transmission parameters; and a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which does not enable it to protect every one of the N working DSP-enabled optical transceivers/transponders  110  and is configured to use the first set of transmission parameters. Each of the at least one working DSP-enabled optical transceiver/transponder  110  that uses the second set of transmission parameters may include one of the following: a DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters corresponding to the second set of transmission parameters; a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters of one of the L protection DSP-enabled optical transceivers/transponders  120  and configured to use the second set of transmission parameters; and a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which does not enable it to protect every one of the N working DSP-enabled optical transceivers/transponders  110  and is configured to use the second set of transmission parameters. 
     Each DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters, irrespective of whether the set of fixed transmission parameters corresponds to the first set of transmission parameters or to the second set of transmission parameters, may be a conventional DSP-enabled universal polarization-multiplexed quadrature amplitude modulation (QAM) optical transceiver/transponder that uses coherent detection. Such a conventional DSP-enabled universal optical transceiver/transponder may, for example, include DSP-enabled universal polarization-multiplexed QAM transmitter and receiver as described and shown in the above-mentioned article entitled “Rate-Adaptable Optics for Next Generation Long-Haul Transport Networks” of Xiang Zhou, Lynn E. Nelson, and Peter Magill. 
     Each DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters of one of the L protection DSP-enabled optical transceivers/transponders  120 , irrespective of whether it is configured to use the first set of transmission parameters, the second set of transmission parameters, or any other set of transmission parameters within adjustability ranges/values covered by the set of adjustable transmission parameters of the one of the L protection DSP-enabled optical transceivers/transponders  120 , may be similar in structure and functionality to the one of the L protection DSP-enabled optical transceivers/transponders  120  and may similarly include an optical transmitter as described below with reference to  FIG. 2  and an optical receiver as described below with reference to  FIG. 3 , or a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver as described below. 
     Each DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which does not enable it to protect every one of the N working DSP-enabled optical transceivers/transponders  110 , irrespective of whether it is configured to use the first set of transmission parameters or the second set of transmission parameters, may include a less-adjustable version of the optical transmitter of  FIG. 2  and a less-adjustable version of the optical receiver of  FIG. 3  as described below. 
     In one embodiment, the at least one working DSP-enabled optical transceiver/transponder  110  that uses the first set of transmission parameters includes a substantially 100 Gigabit per second (Gb/s) DSP-enabled optical transceiver/transponder. 
     In some embodiments, the transmission parameters of each of the first set of transmission parameters, the second set of transmission parameters, and each of the sets of adjustable transmission parameters usable by the L protection DSP-enabled optical transceivers/transponders  120  include at least one of the following: a modulation scheme parameter; a forward error correction (FEC) coding overhead percent parameter; a symbol rate parameter; a parameter of a central wavelength of a channel wavelength; and a client port rate parameter. 
     In one embodiment, the modulation scheme parameter includes a parameter indicating polarization multiplexing (PM) with one of the following: quadrature phase shift keying (QPSK); 8 phase shift keying (8PSK); 8QAM; 16QAM; 32QAM; 64QAM; 128QAM; time-domain hybrid QPSK/8QAM; time-domain hybrid 8QAM/16QAM; and time-domain hybrid 32QAM/64QAM. 
     In one embodiment, the FEC coding overhead percent parameter includes a parameter indicating one of the following FEC coding overheads: substantially 7%; substantially 13%; substantially 20%; and substantially 25%. 
     Throughout the present specification and claims, the term “substantially”, when used in conjunction with a specified percentage value, refers to the specified percentage value or to approximately the specified percentage value. For example, the term “substantially 7%” refers to 7% or approximately 7%. 
     In one embodiment, the parameter of a central wavelength of a channel wavelength includes a parameter indicating one of the following central wavelengths: a wavelength in the International Telecommunication Union (ITU) transmission C-Band; and a wavelength in the ITU transmission L-Band. 
     In one embodiment, the client port rate parameter includes a parameter indicating one of the following bit rates: substantially 100 Gb/s; substantially 200 Gb/s; substantially 400 Gb/s; substantially 1 Terabit per second (Tb/s); substantially 1600 Gb/s; and a variable bit rate. 
     Throughout the present specification and claims, the term “substantially”, when used in conjunction with a specified bit-rate, refers to the specified bit rate or to approximately the specified bit rate. Thus, the term “substantially 100 Gb/s” refers to a bit rate of 100 Gb/s or approximately 100 Gb/s, the term “substantially 400 Gb/s” refers to a bit rate of 400 Gb/s or approximately 400 Gb/s, the term “substantially 1000 Gb/s” or “substantially 1 Tb/s” refers to a bit rate of 1000 Gb/s (1 Tb/s) or approximately 1 Tb/s, and so forth. For example, which is not meant to be limiting, the bit rate of substantially 100 Gb/s may be 103.125 Gb/s (due to 66B/64B block coding) or 112 Gb/s (due to both 66B/64B coding and standard FEC), which are both greater than 100 Gb/s. Further for example, which is not meant to be limiting, the bit rate of substantially 400 Gb/s may be four times the bit rate of substantially 100 Gb/s, and in a case where the bit rate of substantially 100 Gb/s is greater than 100 Gb/s the bit rate of substantially 400 Gb/s is greater than 400 Gb/s. 
     Non-limiting examples of symbol rates that may be indicated by the symbol rate parameter include symbol rates between 10 and 100 Giga-Baud (GBaud). 
     The first and second sets of transmission parameters may differ in values of one or more of the parameters and/or in indications indicated by one or more of the parameters and/or in number of the parameters. A non-limiting example of a value difference between the first set of transmission parameters and the second set of transmission parameters is when the symbol rate in the first set of transmission parameters is different from the symbol rate in the second set of transmission parameters. A non-limiting example of an indication difference between the first set of transmission parameters and the second set of transmission parameters is when the modulation scheme parameter in the first set of transmission parameters indicates a selection of 8PSK and the modulation scheme parameter in the second set of transmission parameters indicates a selection of hybrid QPSK/8QAM. A non-limiting example of a difference in number of the parameters is when the at least one working DSP-enabled optical transceiver/transponder  110  that uses the first set of transmission parameters does not use FEC whereas the at least one working DSP-enabled optical transceiver/transponder  110  that uses the second set of transmission parameters uses FEC, and hence the second set of transmission parameters includes the FEC coding overhead percent parameter whereas the first set of transmission parameters does not include the FEC coding overhead percent parameter. 
     It is appreciated that enablement of each of the L protection DSP-enabled optical transceivers/transponders  120  to protect every one of the N working DSP-enabled optical transceivers/transponders  110  is typically obtained when all sets of transmission parameters actually used by the N working DSP-enabled optical transceivers/transponders  110  are within adjustability ranges/values covered by each set of adjustable transmission parameters of each of the L protection DSP-enabled optical transceivers/transponders  120 . For example, which is not meant to be limiting, if the N working DSP-enabled optical transceivers/transponders  110  use sets of transmission parameters which, in total, only differ in two parameters, one being, for example, the modulation scheme parameter and the other being the parameter of a central wavelength of a channel wavelength, and the modulation scheme parameter can take one of two indicated selections, for example QPSK and 16QAM, and the parameter of a central wavelength of a channel wavelength can take different central wavelengths of channel wavelengths in the ITU transmission C-Band, then each of the L protection DSP-enabled optical transceivers/transponders  120  is enabled to protect every one of the N working DSP-enabled optical transceivers/transponders  110 , for example, when each set of adjustable transmission parameters usable by each of the L protection DSP-enabled optical transceivers/transponders  120  uses the transmission parameters that are common to all the sets of transmission parameters of the N working DSP-enabled optical transceivers/transponders  110  and also covers a possibility of selecting any one modulation scheme of a plurality of modulation schemes which include at least QPSK and 16QAM and a possibility of selecting any one central wavelength of a channel wavelength within the entire ITU transmission C-Band. 
     In one embodiment, the adjustability ranges/values of some or all of the sets of adjustable transmission parameters usable by the L protection DSP-enabled optical transceivers/transponders  120  may be identical. 
     In another embodiment, the adjustability ranges/values of some or all of the sets of adjustable transmission parameters usable by the L protection DSP-enabled optical transceivers/transponders  120  may be different. For example, in the above-mentioned example in which each set of adjustable transmission parameters usable by each of the L protection DSP-enabled optical transceivers/transponders  120  covers a possibility of selecting any one modulation scheme of a plurality of modulation schemes which include at least QPSK and 16QAM, the plurality of modulation schemes may include for some sets of adjustable transmission parameters usable by the L protection DSP-enabled optical transceivers/transponders  120  two additional modulation schemes and for other sets of adjustable transmission parameters usable by the L protection DSP-enabled optical transceivers/transponders  120  three additional modulation schemes. It is appreciated that such adjustability difference does not affect the enablement of each of the L protection DSP-enabled optical transceivers/transponders  120  to protect every one of the N working DSP-enabled optical transceivers/transponders  110 . 
     The set of N working DSP-enabled optical transceivers/transponders  110  may also include one or more DSP-enabled optical transceivers/transponders  110  that use sets of transmission parameters other than the first and second sets of transmission parameters, wherein such other sets of transmission parameters are within adjustability ranges/values covered by the sets of adjustable transmission parameters of the L protection optical transceivers/transponders  120 , and each of the L protection optical transceivers/transponders  120  may also be used to protect every one of the DSP-enabled optical transceivers/transponders  110  that uses such other sets of transmission parameters. For example, which is not meant to be limiting, the set of N working DSP-enabled optical transceivers/transponders  110  may also include one DSP-enabled optical transceiver/transponder  110  that uses a third set of transmission parameters that is within adjustability ranges/values covered by the sets of adjustable transmission parameters of the L protection optical transceivers/transponders  120 , and each of the L protection optical transceivers/transponders  120  may also be used to protect the one DSP-enabled optical transceiver/transponder  110  that uses the third set of transmission parameters. 
     In operation, in accordance with an embodiment of the present invention, the set of M protection DSP-enabled optical transceivers/transponders  120  is allocated for protecting the set of N working DSP-enabled optical transceivers/transponders  110 . Allocation of the set of M protection DSP-enabled optical transceivers/transponders  120  may be performed when it is determined that the set of N working DSP-enabled optical transceivers/transponders  110  includes working DSP-enabled optical transceivers/transponders  110  that use different sets of transmission parameters, or upon installation or allocation of the entire set of N working DSP-enabled optical transceivers/transponders  110 , or upon installation or allocation of working DSP-enabled optical transceivers/transponders  110  that use different sets of transmission parameters. It is appreciated that a determination that the set of N working DSP-enabled optical transceivers/transponders  110  includes working DSP-enabled optical transceivers/transponders  110  that use different sets of transmission parameters, allocation of the set of M protection DSP-enabled optical transceivers/transponders  120  and the set of N working DSP-enabled optical transceivers/transponders  110 , and a determination of which of the M protection DSP-enabled optical transceivers/transponders  120  to use for protecting a failed working DSP-enabled optical transceiver/transponder  110  may, for example, be performed by a network management system (not shown) which may be comprised in or associated with the optical network or by a network element (not shown) in the optical network. 
     In one embodiment, the set of N working DSP-enabled optical transceivers/transponders  110  is allocated prior to allocating the set of M protection DSP-enabled optical transceivers/transponders  120  and allocation of the set of M protection DSP-enabled optical transceivers/transponders  120  is performed in response to allocation of the set of N working DSP-enabled optical transceivers/transponders  110 . 
     In one embodiment, the set of M protection DSP-enabled optical transceivers/transponders  120  is allocated via a control plane (not shown) of the optical network. 
     The allocation of the set of M protection DSP-enabled optical transceivers/transponders  120  for protecting the set of N working DSP-enabled optical transceivers/transponders  110  enables an M:N recovery scheme in which M of the N working DSP-enabled optical transceivers/transponders  110  may be simultaneously protected by the M protection DSP-enabled optical transceivers/transponders  120 , or less than M of the N working DSP-enabled optical transceivers/transponders  110  may be simultaneously protected by a respective number of protection DSP-enabled optical transceivers/transponders  120 . In a case where M=1, that is the M:N recovery scheme is a 1:N recovery scheme, the set of M protection DSP-enabled optical transceivers/transponders  120  reduces to a single protection DSP-enabled optical transceiver/transponder  120  having a capability of using a set of adjustable transmission parameters enabling it to protect every one of the N working DSP-enabled optical transceivers/transponders  110 . 
     When a failure is detected in any one working DSP-enabled optical transceiver/transponder  110 , such as one working optical transceiver/transponder  110  that uses the first set of transmission parameters or one working optical transceiver/transponder  110  that uses the second set of transmission parameters, one of the M protection DSP-enabled optical transceivers/transponders  120  is allocated for protecting the failed working DSP-enabled optical transceiver/transponder  110 . The failure may, for example, be detected by the network management system or by the network element, and the one of the M protection DSP-enabled optical transceivers/transponders  120  may be allocated by the network management system or by the network element, for example, via the control plane. 
     A selection of the one of the M protection DSP-enabled optical transceivers/transponders  120  to be allocated for protecting the failed working DSP-enabled optical transceiver/transponder  110  may be performed according to various criteria, typically including an availability criterion which seeks to select a suitable one of the M protection DSP-enabled optical transceivers/transponders  120  from those of the M protection DSP-enabled optical transceivers/transponders  120  that are not already used for protecting other working DSP-enabled optical transceivers/transponders  110 . The selected and allocated protection DSP-enabled optical transceiver/transponder  120  may be one of the L protection DSP-enabled optical transceivers/transponders  120 , or, only in a case where M&gt;L, either one of the L protection DSP-enabled optical transceivers/transponders  120  or one of the M-L protection DSP-enabled optical transceivers/transponders  120 . 
     When the selected and allocated protection DSP-enabled optical transceiver/transponder  120  is one of the L protection DSP-enabled optical transceivers/transponders  120 , the adjustable transmission parameters of the allocated one of the L protection DSP-enabled optical transceivers/transponders  120  are configured, automatically or in response to an instruction or an input by a network operator (not shown), to correspond to transmission parameters of the failed working DSP-enabled optical transceiver/transponder  110 . Thus, if, for example, the failed working DSP-enabled optical transceiver/transponder  110  is a working optical transceiver/transponder  110  that uses the first set of transmission parameters, the adjustable transmission parameters of the allocated one of the L protection DSP-enabled optical transceivers/transponders  120  are configured to correspond to the first set of transmission parameters, and if, for example, the failed working DSP-enabled optical transceiver/transponder  110  is a working optical transceiver/transponder  110  that uses the second set of transmission parameters, the adjustable transmission parameters of the allocated one of the L protection DSP-enabled optical transceivers/transponders  120  are configured to correspond to the second set of transmission parameters. 
     After the adjustable transmission parameters of the allocated one of the L protection DSP-enabled optical transceivers/transponders  120  are configured to correspond to the transmission parameters of the failed working DSP-enabled optical transceiver/transponder  110 , the allocated one of the L protection DSP-enabled optical transceivers/transponders  120  is used for communication instead of the failed working DSP-enabled optical transceiver/transponder  110 . 
     In the case where M&gt;L and the selected and allocated protection DSP-enabled optical transceiver/transponder  120  is not one of the L protection DSP-enabled optical transceivers/transponders  120 , a determination is made which of the M-L protection DSP-enabled optical transceivers/transponders  120  is to be selected and allocated. If the failed working DSP-enabled optical transceiver/transponder  110  corresponds to an available one of the M-L protection DSP-enabled optical transceivers/transponders  120 , the corresponding available one of the M-L protection DSP-enabled optical transceivers/transponders  120  may be selected and allocated and then used for communication instead of the failed working DSP-enabled optical transceiver/transponder  110 . Thus, for example, if the failed working DSP-enabled optical transceiver/transponder  110  is a working optical transceiver/transponder  110  that uses the first set of transmission parameters and the M-L protection DSP-enabled optical transceivers/transponders  120  include an available protection DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters corresponding to the first set of transmission parameters, the available protection DSP-enabled optical transceiver/transponder that uses the set of fixed transmission parameters corresponding to the first set of transmission parameters may be selected and allocated and then used for communication instead of the failed working DSP-enabled optical transceiver/transponder  110 . 
     If the failed working DSP-enabled optical transceiver/transponder  110  does not correspond to one of the M-L protection DSP-enabled optical transceivers/transponders  120 , another one of the M-L protection DSP-enabled optical transceivers/transponders  120 , such as an available DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which enables it to protect the failed working DSP-enabled optical transceiver/transponder  110  but does not enable it to protect every other one of the N working DSP-enabled optical transceivers/transponders  110 , may be selected and allocated. The set of adjustable transmission parameters of the allocated protection DSP-enabled optical transceiver/transponder  120  is then configured to correspond to the transmission parameters of the failed working DSP-enabled optical transceiver/transponder  110 , and the allocated protection DSP-enabled optical transceiver/transponder  120  is then used for communication instead of the failed working DSP-enabled optical transceiver/transponder  110 . 
     Operations similar to those mentioned above in connection with and after detection of a failure in one working DSP-enabled optical transceiver/transponder  110  may be performed when failures are detected in additional (up to M-1) working optical transceivers/transponders  110  to result in using additional (up to M-1) protection optical transceivers/transponders  120  instead of the additional working optical transceivers/transponders  110 , respectively. 
     Without limiting the generality of the foregoing,  FIG. 1  depicts a non-limiting example in which: (1) M is much greater than one and N is greater than M; (2) each of the N working DSP-enabled optical transceivers/transponders  110  is a DSP-enabled optical transceiver/transponder that uses a set of fixed transmission parameters; (3) two of the N working optical transceivers/transponders  110  are optical transceivers/transponders that use the second set of transmission parameters and are also referred to using a sub-designation “2”, that is, each such working DSP-enabled optical transceiver/transponder  110  is designated “ 110 - 2 ”, and the optical transmitter  130  and optical receiver  140  thereof are designated “ 130 - 2 ” and “ 140 - 2 ”, respectively; (4) the other N−2 working optical transceivers/transponders  110  are optical transceivers/transponders that use the first set of transmission parameters and are also referred to using a sub-designation “1”, that is, each such working DSP-enabled optical transceiver/transponder  110  is designated “ 110 - 1 ”, and the optical transmitter  130  and optical receiver  140  thereof are designated “ 130 - 1 ” and “ 140 - 1 ”, respectively; (5) L=2 and the L protection DSP-enabled optical transceivers/transponders  120  are also referred to using a sub-designation “ADJ”, that is, each such protection DSP-enabled optical transceiver/transponder  120  is designated “ 120 -ADJ”, and the optical transmitter  150  and optical receiver  160  thereof are designated “ 150 -ADJ” and “ 160 -ADJ”, respectively; and (6) the M-2 protection DSP-enabled optical transceivers/transponders  120  correspond to and may be used to protect M-2 of the N working DSP-enabled optical transceivers/transponders  110 - 1  and are also referred to using the sub-designation “ 1 ”, that is, each such protection DSP-enabled optical transceiver/transponder  120  is designated “ 120 - 1 ”, and the optical transmitter  150  and optical receiver  160  thereof are designated “ 150 - 1 ” and “ 160 - 1 ”, respectively. 
     In the example of  FIG. 1  the M protection optical transceivers/transponders  120  may protect and replace any combination of up to M working optical transceivers/transponders  110  because up to M-2 of the M protection optical transceivers/transponders  120 - 1  may protect and replace up to M-2 of the working optical transceivers/transponders  110 - 1  and the two protection optical transceivers/transponders  120 -ADJ may protect and replace any one of the following combinations of working optical transceivers/transponders: a combination including any two working optical transceivers/transponders  110 - 1 ; a combination including the two working optical transceivers/transponders  110 - 2 ; and a combination including one working optical transceiver/transponder  110 - 1  and one of the two working optical transceivers/transponders  110 - 2 . 
     Reference is now additionally made to  FIG. 2 , which is a simplified block diagram illustration of a DSP-enabled optical transmitter  200  in the apparatus  100  of  FIG. 1 , the DSP-enabled optical transmitter  200  having a capability of using a set of adjustable transmission parameters. 
     The DSP-enabled optical transmitter  200  may form the optical transmitter part of every one of the following in the apparatus  100 : every one of the L protection DSP-enabled optical transceivers/transponders  120 ; and every one working DSP-enabled optical transceiver/transponder  110  having a capability of using the set of adjustable transmission parameters of any one of the L protection DSP-enabled optical transceivers/transponders  120 , irrespective of any specific set of transmission parameters, such as the first set of transmission parameters or the second set of transmission parameters, which it is configured to use. In the non-limiting example of  FIG. 1 , each optical transmitter  150 -ADJ may be implemented by one DSP-enabled optical transmitter  200 . 
     The DSP-enabled optical transmitter  200  includes a transmitter DSP/ASIC (Tx DSP/ASIC)  205  and a transmitter (Tx) opto-electronic front end  210 . The DSP-enabled optical transmitter  200  is operative to convert a binary signal (B.S.), fed into the Tx DSP/ASIC  205 , into an optical signal and to transmit the optical signal to the MUX or the switching/routing unit mentioned above with reference to  FIG. 1 . 
     The Tx DSP/ASIC  205  includes a FEC encoding unit  215 , a mapping unit  220 , and a digital spectral shaping (DSS) unit  225 . The FEC encoding unit  215  includes a module  230  which includes a plurality of FEC encoders, and switches/selectors  235  and  240  which enable selection of one of the plurality of FEC encoders. The module  230  may also include an optional NO FEC route, a selection of which by the switches/selectors  235  and  240  skips performance of FEC encoding. Alternatively, the module  230  may be bypassed via a route external to the FEC encoding unit  215  for skipping FEC encoding. By way of a non-limiting example, the module  230  in  FIG. 2  includes two FEC encoders referred to as FEC ENC.  1  and FEC ENC.  2 , and the NO FEC route. It is, however, appreciated that the module  230  may include other FEC encoders and/or more than two FEC encoders. 
     The mapping unit  220  includes a module  245  which includes a plurality of QAM mappers, and switches/selectors  250  and  255  which enable selection of one of the plurality of QAM mappers. By way of a non-limiting example, the module  245  in  FIG. 2  includes QPSK, 8QAM, 16QAM and 64QAM mappers. It is, however, appreciated that the module  245  may include other mappers and/or more than four mappers. 
     The Tx opto-electronic front end  210  includes a tunable laser (TL)  260 , a beam splitter  265 , two digital-to-analog converters (DACs)  270  for X-polarization inphase (I) and quadrature (Q) signal components, two DACs  275  for Y-polarization I and Q signal components, a quadrature modulator  280  for the X-polarization signal components and a quadrature modulator  285  for the Y-polarization signal components, a 90° polarization rotator (PR)  290  associated with the quadrature modulator  285 , and a polarization beam combiner (PBC)  295 . 
     The TL  260  is operative to emit a continuous wave (CW) laser beam towards the beam splitter  265 . The beam splitter  265  is operative to split a received CW laser beam into two beams and to provide one of the two beams to the quadrature modulator  280  and the other beam to the quadrature modulator  285 . 
     Each of the quadrature modulators  280  and  285  typically includes two nested Mach-Zehnder modulators (MZMs) with their ends coupled. Each of the DACs  270  is operatively associated with the DSS  225  of the Tx DSP/ASIC  205  and with the quadrature modulator  280 , and each of the DACs  275  is operatively associated with the DSS  225  and with the quadrature modulator  285 . The MZMs in the quadrature modulator  280  are driven by the DACs  270  so as to enable the quadrature modulator  280  to perform quadrature modulation of signals provided thereto, and the MZMs in the quadrature modulator  285  are driven by the DACs  275  so as to enable the quadrature modulator  285  to perform quadrature modulation of signals provided thereto. The PR  290  and the PBC  295  together form a polarization multiplexer which is operative to perform polarization multiplexing of signals provided thereto from the quadrature modulators  280  and  285 . 
     In operation, a binary signal that is fed into the Tx DSP/ASIC  205  either bypasses the FEC encoding unit  215  or undergoes FEC encoding in the FEC encoding unit  215  according to a FEC coding selection. The FEC-coded binary signal outputted from the FEC encoding unit  215 , or the binary signal bypassing the FEC encoding unit  215 , is fed into the mapping unit  220  where it is mapped into a selected QAM mapping to produce a mapped signal. The mapped signal then undergoes digital spectral shaping in the DSS  225 , which outputs I and Q signal components for X-polarization (X-I and X-Q components) to the DACs  270  for driving the MZMs included in the quadrature modulator  280  and I and Q signal components for Y-polarization (Y-I and Y-Q components) to the DACs  275  for driving the MZMs included in the quadrature modulator  285 . 
     The quadrature modulators  280  and  285  also receive and distribute to the MZMs included therein a CW laser beam emitted by the TL  260  and split by the beam splitter  265 . The TL  260  is tuned by a tuning signal (T.S.) to emit the CW laser beam at a selected central wavelength of a channel wavelength. 
     The outputs of the MZMs included in the quadrature modulator  280  are combined to produce an X-polarization IQ signal, and the outputs of the MZMs included in the quadrature modulator  285  are combined to produce an IQ signal for Y-polarization. The IQ signal for Y-polarization is then polarized by the PR  290  to produce a Y-polarization IQ signal in a polarization which is orthogonal to a polarization of the X-polarization IQ signal, and the X-polarization IQ signal and the Y-polarization IQ signal with their polarizations orthogonal to each other are fed into the PBC  295  for combination thereby. The PBC  295  outputs a polarization multiplexed signal which is provided to the MUX or the switching/routing unit. 
     Reference is now additionally made to  FIG. 3 , which is a simplified block diagram illustration of a DSP-enabled optical receiver  300  that uses coherent detection, the DSP-enabled optical receiver  300  being comprised in the apparatus  100  of  FIG. 1  and having a capability of using a set of adjustable transmission parameters. 
     The DSP-enabled optical receiver  300  may form the optical receiver part of every one of the following in the apparatus  100 : every one of the L protection DSP-enabled optical transceivers/transponders  120 ; and every one working DSP-enabled optical transceiver/transponder  110  having a capability of using the set of adjustable transmission parameters of any one of the L protection DSP-enabled optical transceivers/transponders  120 , irrespective of any specific set of transmission parameters, such as the first set of transmission parameters or the second set of transmission parameters, which it is configured to use. In the non-limiting example of  FIG. 1 , each optical receiver  160 -ADJ may be implemented by one DSP-enabled optical receiver  300 . 
     The DSP-enabled optical receiver  300  includes a receiver DSP/ASIC (Rx DSP/ASIC)  305  and a receiver (Rx) opto-electronic front end  310 . The DSP-enabled optical receiver  300  is operative to receive an optical signal from the DeMUX or the switching/routing unit mentioned above with reference to  FIG. 1  via the Rx opto-electronic front end  310  and to convert the received optical signal into a binary signal. 
     The Rx DSP/ASIC  305  includes two FEC decoding units  315 , two de-mapping units  320 , a 2×2 adaptive equalizer and phase recovery unit  325 , and two chromatic dispersion (CD) compensation units  330 . Each FEC decoding unit  315  includes a module  335  which includes a plurality of FEC decoders, and switches/selectors  340  and  345  which enable selection of one of the plurality of FEC decoders. Each module  335  may also include an optional NO FEC route, a selection of which by the respective switches/selectors  340  and  345  skips performance of FEC decoding. Alternatively, each module  335  may be bypassed via a route external to the respective FEC decoding unit  315  for skipping FEC decoding. Each FEC decoding unit  315  typically matches the FEC encoding unit  215  of the DSP-enabled optical transmitter  200 , and hence each of the modules  335  includes two FEC decoders referred to as FEC DEC.  1  and FEC DEC.  2 , and also the NO FEC route. In a case where the FEC encoding unit  215  includes other FEC encoders and/or more than two FEC encoders, each of the modules  335  typically includes FEC decoders matching actual FEC encoders in the FEC encoding unit  215 . 
     Each de-mapping unit  320  includes a module  350  which includes a plurality of QAM de-mappers, and switches/selectors  355  and  360  which enable QAM selection of a decided one of the plurality of QAM de-mappers. Each de-mapping unit  320  typically matches the mapping unit  220  of the DSP-enabled optical transmitter  200 , and hence each of the modules  350  includes QPSK, 8QAM, 16QAM and 64QAM de-mappers. In a case where the mapping unit  220  includes other de-mappers and/or more than four de-mappers, each of the modules  350  typically includes de-mappers matching actual mappers in the mapping unit  220 . 
     The Rx opto-electronic front end  310  includes a tunable local oscillator (TLO) laser  365 , a beam splitter  370 , a polarization beam splitter (PBS)  375 , two 90° hybrid mixers  380  with differential output, eight photo-detectors (PDs)  385  comprised of four sets of balanced PDs, four linear amplifiers  390  with differential output, and four analog-to-digital converters (ADCs)  395 . Each of the hybrid mixers  380  is operative to receive optical signals arriving from the DeMUX or the switching/routing unit and split by the PBS  375 , and a CW laser beam emitted by the TLO laser  365  and split by the beam splitter  370 . The TLO laser  365  is tuned by a tuning signal (T.S.) to emit the CW laser beam at a selected central wavelength of a channel wavelength. 
     A first one of the hybrid mixers  380  is operatively associated with four of the PDs  385 , and a second one of the hybrid mixers  380  is operatively associated with the other four of the PDs  385 . The four PDs  385  associated with the first one of the hybrid mixers  380  are operatively associated with two of the amplifiers  390 , which are operatively associated with two of the ADCs  395 , respectively, and the four PDs  385  associated with the second one of the hybrid mixers  380  are operatively associated with the other two of the amplifiers  390 , which are operatively associated with the other two of the ADCs  395 , respectively. The PDs  385  are operative to detect optical signals received thereat and to convert the optical signals into electrical signals, and to provide the electrical signals to the amplifiers  390 . The amplifiers  390  are operative to amplify electrical signals received thereat and to provide amplified signals to the ADCs  395  for analog-to-digital conversion thereby. 
     In operation, an incoming optical signal arriving at the DSP-enabled optical receiver  300  from the DeMUX or the switching/routing unit is split by the PBS  375  into two components X and Y with orthogonal polarizations, which are provided to the hybrid mixers  380 . The hybrid mixers  380  also receive a CW laser beam emitted by the TLO laser  365  and split by the beam splitter  370 . The first one of the hybrid mixers  380  coherently mixes the polarized optical signal received thereat with the split CW laser beam received thereat and separates the I and Q components of the received optical field in the X-polarization, and the second one of the hybrid mixers  380  coherently mixes the polarized optical signal received thereat with the split CW laser beam received thereat and separates the I and Q components of the received optical field in the Y-polarization. 
     The PDs  385  associated with the first one of the hybrid mixers  380  detect the I and Q components of the received optical field in the X-polarization and output analog electrical signals to the two amplifiers  390  associated therewith, which in turn amplify the analog electrical signals and provide amplified analog electrical signals to the ADCs  395  associated therewith for digitization thereby. The PDs  385  associated with the second one of the hybrid mixers  380  detect the I and Q components of the received optical field in the Y-polarization and output analog electrical signals to the two amplifiers  390  associated therewith, which in turn amplify the analog electrical signals and provide amplified analog electrical signals to the ADCs  395  associated therewith for digitization thereby. 
     After digitization by the ADCs  395 , digital signals associated with the X-polarization are provided to one of the CD compensation units  330  and digital signals associated with the Y-polarization are provided to the other one of the CD compensation units  330 . The CD compensation units  330  perform fiber CD compensation on the digital signals received thereat and provide CD compensated signals to the 2×2 adaptive equalizer and phase recovery unit  325 . The 2×2 adaptive equalizer and phase recovery unit  325  performs adaptive equalization, including automatic polarization tracking and polarization mode dispersion (PMD) and residual CD compensation, and carrier phase recovery. 
     After adaptive equalization and carrier phase recovery, the digital signals associated with the X-polarization are provided to a first one of the two de-mapping units  320  where the digital signals associated with the X-polarization are de-mapped, according to a QAM de-mapping decision, to produce a first de-mapped signal, and the digital signals associated with the Y-polarization are provided to a second one of the two de-mapping units  320  where the digital signals associated with the Y-polarization are de-mapped, according to the QAM de-mapping decision, to produce a second de-mapped signal. The first de-mapped signal then either bypasses FEC decoding and exits the DSP-enabled optical receiver  300  or undergoes FEC decoding in a first one of the two FEC decoding units  315  according to a FEC coding selection and then exits the DSP-enabled optical receiver  300 . Similarly, the second de-mapped signal either bypasses FEC decoding and exits the DSP-enabled optical receiver  300  or undergoes FEC decoding in a second one of the two FEC decoding units  315  according to the FEC coding selection and then exits the DSP-enabled optical receiver  300 . 
     Every DSP-enabled optical transceiver/transponder that includes the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  has performance adjustability capabilities which may be expressed in terms of parameter selection capabilities and are determined by the following: the module  230  in the DSP-enabled optical transmitter  200  and the matching modules  335  in the DSP-enabled optical receiver  300 ; the module  245  in the DSP-enabled optical transmitter  200  and the matching modules  350  in the DSP-enabled optical receiver  300 ; and the TL  260  in the DSP-enabled optical transmitter  200  and the matching TLO laser  365  in the DSP-enabled optical receiver  300 . For example, a QAM mapping selection enabled by the modules  245  and  350 , alone or in combination with a FEC coding selection enabled by the modules  230  and  335 , may be used to adjust a symbol rate and a net bit rate and hence a client port rate, and such selections may be expressed in terms of a selection of a value for the symbol rate parameter and a selection of a value for the client port rate parameter. In another example, a tuning selection of the TL  260  and TLO laser  365  may be used to adjust a central wavelength of a channel wavelength, and such selection may be expressed in terms of a selection of a value for the parameter of a central wavelength of a channel wavelength. 
     The parameter selection capabilities of every DSP-enabled optical transceiver/transponder that includes the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  may be represented by a two-dimensional table including, for example, columns for the following five parameters: the modulation scheme parameter; the FEC coding overhead percent parameter; the symbol rate parameter; the parameter of a central wavelength of a channel wavelength; and the client port rate parameter, and rows with values from groups of values or from ranges of values for each of the five parameters. It is appreciated that the two-dimensional table may, by way of a non-limiting example, be implemented as a look-up table (LUT). 
     It is, however, appreciated that the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300 , and every DSP-enabled optical transceiver/transponder comprising them may alternatively have configurations with less than the above-mentioned performance adjustability capabilities. The less performance adjustability capabilities refers to less parameters to select from and/or to smaller groups of parameter values or smaller ranges of parameter values to select from. For simplicity of description, and without limiting the generality of the foregoing, a configuration of the DSP-enabled optical transmitter  200  having less than the above-mentioned performance adjustability capabilities is referred to as a less-encompassing DSP-enabled optical transmitter, a configuration of the DSP-enabled optical receiver  300  having less than the above-mentioned performance adjustability capabilities is referred to as a less-encompassing DSP-enabled optical receiver, and a configuration of a DSP-enabled optical transceiver/transponder comprising a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver is referred to as a less-encompassing DSP-enabled optical transceiver/transponder. 
     When the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  are in a configuration forming a specific less-encompassing DSP-enabled optical transmitter and a specific less-encompassing DSP-enabled optical receiver, respectively, each of the L protection DSP-enabled optical transceivers/transponders  120  and each working DSP-enabled optical transceiver/transponder  110  that includes a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters of one of the L protection DSP-enabled optical transceivers/transponders  120  may typically include a less-encompassing DSP-enabled optical transceiver/transponder including the specific less-encompassing DSP-enabled optical transmitter and the specific less-encompassing DSP-enabled optical receiver. 
     A non-limiting example of a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver is a configuration of the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  in which a fixed FEC code is used. In such configuration there is no FEC adjustability capability, that is, FEC encoding/decoding selection is not enabled, and the FEC encoding unit  215  of the transmitter  200  reduces to a single FEC encoder in the less-encompassing DSP-enabled optical transmitter, and each of the two FEC decoding units  315  of the receiver  300  reduces to a single FEC decoder in the less-encompassing DSP-enabled optical receiver. 
     Another non-limiting example of a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver is a configuration of the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  in which a fixed QAM mapping/de-mapping is used. In such configuration there is no QAM mapping adjustability capability, that is, QAM mapping/de-mapping selection is not enabled and the mapping unit  220  of the transmitter  200  reduces to a single QAM mapper in the less-encompassing DSP-enabled optical transmitter, and each of the two de-mapping units  320  of the receiver  300  reduces to a single QAM de-mapper in the less-encompassing DSP-enabled optical receiver. 
     Still another non-limiting example of a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver is a configuration of the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  in which there are less QAM mapping/de-mapping possibilities to select from, namely there is a smaller variety of available QAM mappers/de-mappers to select from. For example, which is not meant to be limiting, the mapping unit  220  of the less-encompassing DSP-enabled optical transmitter may include only three QAM mappers from which selection can be made instead of the four QAM mappers in the mapping unit  220  in the transmitter  200 , and each of the two de-mapping units  320  of the less-encompassing DSP-enabled optical receiver may include only three QAM de-mappers from which selection can be made instead of the four QAM de-mappers in each of the de-mapping units  320  in the receiver  300 . 
     The two-dimensional table for the less-encompassing DSP-enabled optical transmitter and the less-encompassing DSP-enabled optical receiver has a smaller number of columns and/or a smaller number of rows than the two-dimensional table for the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300 . A smaller number of columns reflect a smaller number of parameters to select from, and a smaller number of rows reflect a smaller group of selectable values and/or a smaller range of selectable values for one or more of the parameters. 
     The configurations of the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  which lead to the less-encompassing DSP-enabled optical transmitter and the less-encompassing DSP-enabled optical receiver typically result from hardware and/or software differences between the Tx DSP/ASIC  205  and/or the TL  260  in the DSP-enabled optical transmitter  200  and the Tx DSP/ASIC  205  and/or the TL  260  in the less-encompassing DSP-enabled optical transmitter, respectively, and between the Rx DSP/ASIC  305  and/or the TLO laser  365  in the DSP-enabled optical receiver  300  and the Rx DSP/ASIC  305  and/or the TLO laser  365  in the less-encompassing DSP-enabled optical receiver, respectively. By way of example, a less-encompassing DSP-enabled optical transmitter which is a configuration of the DSP-enabled optical transmitter  200  in which QAM mapping is not selectable differs from the DSP-enabled optical transmitter  200  in that a single QAM mapper replaces the mapping unit  220  and such difference may result from a hardware and/or software difference between the Tx DSP/ASIC  205  in the DSP-enabled optical transmitter  200  and the Tx DSP/ASIC  205  in the less-encompassing DSP-enabled optical transmitter, and a corresponding less-encompassing DSP-enabled optical receiver which is a configuration of the DSP-enabled optical receiver  300  in which QAM de-mapping is not selectable differs from the DSP-enabled optical receiver  300  in that two single QAM de-mappers respectively replace the two de-mapping units  320  and such difference may result from a hardware and/or software difference between the Rx DSP/ASIC  305  in the DSP-enabled optical receiver  300  and the Rx DSP/ASIC  305  in the less-encompassing DSP-enabled optical receiver. 
     By way of another example, a less-encompassing DSP-enabled optical transmitter which is a configuration of the DSP-enabled optical transmitter  200  in which FEC encoding is not selectable differs from the DSP-enabled optical transmitter  200  in that a single FEC encoder replaces the FEC encoding unit  215  and such difference may result from a hardware and/or software difference between the Tx DSP/ASIC  205  in the DSP-enabled optical transmitter  200  and the Tx DSP/ASIC  205  in the less-encompassing DSP-enabled optical transmitter, and a corresponding less-encompassing DSP-enabled optical receiver which is a configuration of the DSP-enabled optical receiver  300  in which FEC decoding is not selectable differs from the DSP-enabled optical receiver  300  in that two single FEC decoders respectively replace the two FEC decoding units  315  and such difference may result from a hardware and/or software difference between the Rx DSP/ASIC  305  in the DSP-enabled optical receiver  300  and the Rx DSP/ASIC  305  in the less-encompassing DSP-enabled optical receiver. 
     By way of still another example, a less-encompassing DSP-enabled optical transmitter which is a configuration of the DSP-enabled optical transmitter  200  in which a central wavelength of a channel wavelength is selectable from a smaller wavelength range than in the DSP-enabled optical transmitter  200  may differ from the DSP-enabled optical transmitter  200  in type of TL  260  used and hence in hardware, and a corresponding less-encompassing DSP-enabled optical receiver which is a configuration of the DSP-enabled optical receiver  300  in which a central wavelength of a channel wavelength is selectable from a smaller wavelength range than in the DSP-enabled optical receiver  300  may differ from the DSP-enabled optical receiver  300  in type of TLO laser  365  used and hence in hardware. 
     In respect of a DSP-enabled optical transceiver/transponder having a capability of using a set of adjustable transmission parameters which does not enable it to protect every one of the N working DSP-enabled optical transceivers/transponders  110 , as mentioned above, such DSP-enabled optical transceiver/transponder, irrespective of whether it is configured to use the first set of transmission parameters or the second set of transmission parameters, may include a less-adjustable version of the DSP-enabled optical transmitter  200  and a corresponding less-adjustable version of the DSP-enabled optical receiver  300 . 
     The less-adjustable version of the DSP-enabled optical transmitter  200  and the less-adjustable version of the DSP-enabled optical receiver  300  may be similar in structure and functionality to the less-encompassing DSP-enabled optical transmitter and the less-encompassing DSP-enabled optical receiver, respectively. However, since a less-encompassing DSP-enabled optical transceiver/transponder with a less-encompassing DSP-enabled optical transmitter and a less-encompassing DSP-enabled optical receiver may form any one of the L protection DSP-enabled optical transceivers/transponders  120  and since, as mentioned above with reference to  FIG. 1 , the enablement of each of the L protection DSP-enabled optical transceivers/transponders  120  to protect every one of the N working DSP-enabled optical transceivers/transponders  110  is typically obtained when all sets of transmission parameters actually used by the N working DSP-enabled optical transceivers/transponders  110  are within adjustability ranges/values covered by each set of adjustable transmission parameters of each of the L protection DSP-enabled optical transceivers/transponders  120 , performance adjustability capabilities of the less-encompassing DSP-enabled optical transmitter are greater than and typically encompass performance adjustability capabilities of the less-adjustable version of the DSP-enabled optical transmitter  200  and performance adjustability capabilities of the less-encompassing DSP-enabled optical receiver are greater than and typically encompass performance adjustability capabilities of the less-adjustable version of the DSP-enabled optical receiver  300 . 
     For example, if, as shown by way of a non-limiting example in  FIGS. 2 and 3 , the performance adjustability capabilities of the DSP-enabled optical transmitter  200  and the DSP-enabled optical receiver  300  enable four QAM mapping/de-mapping possibilities to select from, which include QPSK, 8QAM, 16QAM and 64QAM, the less-encompassing DSP-enabled optical transmitter and the less-encompassing DSP-enabled optical receiver may have only three QAM mapping/de-mapping possibilities to select from, such as QPSK, 8QAM and 16QAM, and the less-adjustable version of the DSP-enabled optical transmitter  200  and the less-adjustable version of the DSP-enabled optical receiver  300  may have only two QAM mapping/de-mapping possibilities to select from, such as QPSK and 8QAM. 
     Reference is now additionally made to  FIG. 4 , which is a simplified block diagram illustration of an optical network utilizing the apparatus  100  of  FIG. 1 , the optical network being constructed and operative in accordance with an embodiment of the present invention. 
     The optical network of  FIG. 4 , which is generally designated  400 , includes a network management system (NMS)  410 , a plurality of network elements (NEs)  420  that communicate with one another via links  430 , and a control plane  440 . Each link  430  may, by way of a non-limiting example, include both working paths and protection paths. 
     Each NE  420  includes one apparatus  100  for communication with another NE  420 , and optionally additional apparatuses  100  for communication with additional NEs  420 . Alternatively, for each NE  420  the one apparatus  100  as well as the optional additional apparatuses  100  may be external to the NE  420  and operatively associated therewith. In cases where any ones of the apparatuses  100  are external to any ones of the NEs  420 , the external ones of the apparatuses  100  may operate under instructions from the associated NEs  420  or from the NMS  410 . 
     The NEs  420  may be located at nodes (not shown) of the network  400  and may, for example, include routers and reconfigurable optical add-drop multiplexers (ROADMs). One or more of the NEs  420  may each include the MUX and DeMUX or the switching/routing unit mentioned above with reference to  FIG. 1 . Each MUX and DeMUX in an NE  420  may be comprised in and/or associated with the one apparatus  100  of the NE  420 . 
     For simplicity of depiction and description, and without limiting the generality of the foregoing, only two of the NEs  420  in  FIG. 4 , which are designated NE-A and NE-B, are depicted as including one apparatus  100  each, but it is appreciated that NE-A and NE-B may also include additional apparatuses  100 , and each of the other NEs  420  may include one apparatus  100  or a plurality thereof. Each of NE-A and NE-B uses the set of N working DSP-enabled optical transceivers/transponders  110  in the apparatus  100  included therein for communication with each other over working paths in the link  430  associated with NE-A and NE-B, and the set of M protection DSP-enabled optical transceivers/transponders  120  in the apparatus  100  included therein for communication protection over protection paths in the link  430  associated with NE-A and NE-B. 
     The NMS  410  is shown in  FIG. 4  and referred to below as a unit which is separate from the NEs  420 , but in some embodiments, the NMS  410  may form one of the NEs  420  or be comprised in one of the NEs  420 . The NMS  410  is operatively associated with the NEs  420  via the control plane  440  and is operative to control the NEs  420 , typically via the control plane  440  using, for example, routing and signaling control modules (not shown). The control plane  440  may further be used in responding to requests, selections and control instructions generated by the NMS  410  and/or by one or more of the NEs  420 . 
     In operation, the NEs  420  may communicate with one another via the links  430  under control of and management by the NMS  410 . The NMS  410  may also apply a recovery scheme for use in the optical network  400 , and, in accordance with an embodiment of the present invention, each NE  420 , alone or in combination with the NMS  410 , operates the apparatus  100  and the optional additional apparatuses  100  included therein or associated therewith, and the apparatuses  100  of the NEs  420  are operated as described above with reference to  FIG. 1 . When operated as described above with reference to  FIG. 1 , the N working DSP-enabled optical transceivers/transponders  110  of each apparatus  100  operate as working transceivers/transponders and the M protection DSP-enabled optical transceivers/transponders  120  of each apparatus  100  operate as protection transceivers/transponders. 
     In accordance with another embodiment of the present invention, a first DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters, which may be any one of the L protection DSP-enabled optical transceivers/transponders  120  in one apparatus  100  of one NE  420  in the network  400 , may be used in any one of two modes of operation. One of the two modes of operation is a protection mode of operation and the other mode of operation is an association mode of operation. 
     In the protection mode of operation, the first DSP-enabled optical transceiver/transponder is used as a protection DSP-enabled optical transceiver/transponder as described above with reference to  FIG. 1  to protect one of a plurality of working DSP-enabled optical transceivers/transponders. The one of a plurality of working DSP-enabled optical transceivers/transponders may, for example, be one of the N working DSP-enabled optical transceivers/transponders  110  in the one apparatus  100  of the one NE  420  in the network  400 . 
     In the association mode of operation, the first DSP-enabled optical transceiver/transponder is used together with a second DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters to jointly overcome a change in at least one transmission parameter over a working path associated with the second DSP-enabled optical transceiver/transponder. The second DSP-enabled optical transceiver/transponder may, for example, be another one of the N working DSP-enabled optical transceivers/transponders  110  in the one apparatus  100  of the one NE  420  in the network  400 , and the working path associated with the second DSP-enabled optical transceiver/transponder may be a working path associated with the another one of the N working DSP-enabled optical transceivers/transponders  110 . 
     A non-limiting example of the change in at least one transmission parameter over the working path associated with the second DSP-enabled optical transceiver/transponder is a change in client port rate. The change in client port rate may, for example, occur in a case where a variable rate client interface communicating with the second DSP-enabled optical transceiver/transponder at a desired bit rate experiences at some point a failure which is only partly resolved by using, instead of the variable rate client interface, a fixed rate client interface which is capable of communicating with the second DSP-enabled optical transceiver/transponder at only half the desired bit rate. In such a case, the first DSP-enabled optical transceiver/transponder may be operated in the association mode of operation to join the second DSP-enabled optical transceiver/transponder and communicate at half the desired bit rate with another fixed rate client interface over an additional working path so that communication using the first DSP-enabled optical transceiver/transponder together with the second DSP-enabled optical transceiver/transponder reaches the desired bit rate over a link comprising both the working path and the additional working path. 
     In some embodiments, an operation mode of the first DSP-enabled optical transceiver/transponder is determined as one of the protection mode of operation and the association mode of operation prior to using the first DSP-enabled optical transceiver/transponder. The first DSP-enabled optical transceiver/transponder is then used in the determined one of the protection mode of operation and the association mode of operation. In one embodiment, the operation mode of the first DSP-enabled optical transceiver/transponder is determined via the control plane  440 . 
     In some embodiments, using the first DSP-enabled optical transceiver/transponder in the protection mode of operation includes detecting a failure in the one of a plurality of working DSP-enabled optical transceivers/transponders, configuring adjustable transmission parameters of the first DSP-enabled optical transceiver/transponder to correspond to transmission parameters of the one of a plurality of working DSP-enabled optical transceivers/transponders, and using the first DSP-enabled optical transceiver/transponder for communication instead of the one of a plurality of working DSP-enabled optical transceivers/transponders. 
     In some embodiments, using the first DSP-enabled optical transceiver/transponder in the association mode of operation includes configuring adjustable transmission parameters of the first and second DSP-enabled optical transceivers/transponders to enable the first and second DSP-enabled optical transceivers/transponders to jointly overcome the change in at least one transmission parameter over the working path associated with the second DSP-enabled optical transceiver/transponder, and using the first DSP-enabled optical transceiver/transponder together with the second DSP-enabled optical transceiver/transponder for communication over a link comprising the working path associated with the second DSP-enabled optical transceiver/transponder and an additional working path associated with the first DSP-enabled optical transceiver/transponder. 
     Reference is now made to  FIG. 5 , which is a simplified flowchart illustration of a method of enabling an M:N recovery scheme in an optical network of the type of the optical network  400  of  FIG. 4 , in accordance with an embodiment of the present invention. 
     The M:N recovery scheme is enabled in the optical network by allocating a set of M protection DSP-enabled optical transceivers/transponders for protecting a set of N working DSP-enabled optical transceivers/transponders (step  500 ), wherein M and N are positive integers and 1≦M≦N. The set of N working DSP-enabled optical transceivers/transponders includes at least one working DSP-enabled optical transceiver/transponder that uses a first set of transmission parameters and at least one working DSP-enabled optical transceiver/transponder that uses a second set of transmission parameters which is different from the first set of transmission parameters. The set of M protection DSP-enabled optical transceivers/transponders includes L protection DSP-enabled optical transceivers/transponders, each having a capability of using a set of adjustable transmission parameters enabling it to protect every one of the N working DSP-enabled optical transceivers/transponders, and, when M&gt;L, M-L protection DSP-enabled optical transceivers/transponders, each having a capability of protecting at least one, but not all, of the N working DSP-enabled optical transceivers/transponders, wherein L is a positive integer and 1≦L≦M. 
     Reference is now made to  FIG. 6 , which is a simplified flowchart illustration of a method of using a DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters in an optical network of the type of the optical network  400  of  FIG. 4 , in accordance with an embodiment of the present invention. 
     A determination is made whether a first DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters is to be used in the optical network in a protection mode of operation or in an association mode of operation (step  600 ). 
     If it is determined that the first DSP-enabled optical transceiver/transponder is to be used in the protection mode of operation, the first DSP-enabled optical transceiver/transponder is used as a protection DSP-enabled optical transceiver/transponder to protect one of a plurality of working DSP-enabled optical transceivers/transponders (step  610 ). 
     If it is determined that the first DSP-enabled optical transceiver/transponder is to be used in the association mode of operation, the first DSP-enabled optical transceiver/transponder is used together with a second DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters to jointly overcome a change in at least one transmission parameter over a working path associated with the second DSP-enabled optical transceiver/transponder (step  620 ). 
     Reference is now made to  FIG. 7 , which is a simplified flowchart illustration of a method of protecting a working DSP-enabled optical transceiver/transponder in an optical network of the type of the optical network  400  of  FIG. 4 , in accordance with an embodiment of the present invention. 
     A failure in the working DSP-enabled optical transceiver/transponder is detected (step  700 ). The failure may, for example, be detected via a control plane of the optical network. 
     In response to detection of the failure, a protection DSP-enabled optical transceiver/transponder having a capability of using adjustable transmission parameters enabling it to protect the failed working DSP-enabled optical transceiver/transponder is allocated for protecting the failed working DSP-enabled optical transceiver/transponder (step  710 ). The adjustable transmission parameters of the allocated protection DSP-enabled optical transceiver/transponder are configured to correspond to transmission parameters of the failed working DSP-enabled optical transceiver/transponder (step  720 ), and the allocated protection DSP-enabled optical transceiver/transponder is used for communication instead of the failed working DSP-enabled optical transceiver/transponder (step  730 ). 
     It is appreciated that various features of the invention which are, for clarity, described in the contexts of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable subcombination. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the invention is defined by the appended claims and equivalents thereof.