RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent applications having Ser. Nos. ______, and ______ entitled “Interconnection Techniques” and “Cable Interconnection Techniques,” both filed Mar. 9, 2009, inventors Ganga and Mellitz and incorporates by reference the contents of those applications in their entirety. 
     
    
     FIELD  
       [0002]    The subject matter disclosed herein relates generally to techniques to transmit signals. 
       RELATED ART  
       [0003]    The Institute of Electrical and Electronics Engineers, Inc. (IEEE) has defined numerous networking standards. For example, 10 GBASE-CX4 defines signal transmission at 10 Gbps using four lanes over copper cabling. Similarly, 10 GBASE-KX4 defines signal transmission at 10 Gbps using four lanes over backplane whereas 10 GBASE-KR defines signal transmission at 10 Gbps over a single lane over backplane. 
         [0004]    The evolving IEEE 802.3ba draft 1.1 (2008) standard defines 40 Gbps operation over copper cable. This evolving standard is also known as 40 GBASE-CR4. Copper coaxial cables are characterized by frequency dependent loss and exhibit loss strongly dependent on channel length. Various vendors have proposed manners to transmit at 40 Gbps over copper cable. For example, one implementation involves use of fixed transmit equalization in compliance with SFP+ as described in IEEE Std. 802.3ae-2002 and IEEE Std. 802.3aq-2006. In this implementation, the transmitter circuit is coupled to a hot pluggable optical cable using a fixed length printed circuit board trace. Fixed transmit equalization involves establishing a desired eye opening at a transmitter side. An eye diagram represents transitions of a signal. The more open an eye diagram, the less likelihood the signal will be misread. However, because copper cable length varies, this scheme leads to varying voltage levels and eye patterns at the receiver. Different eye openings at the receiver can lead to errors in reading received data. 
         [0005]    Another approach to reproducing received signals includes use of an analog filter at the receiver (e.g., a continuous-time linear equalizer (CTLE)). However, this approach provides suboptimal eye opening characteristics at the receiver. Yet another approach involves use of an analog-to-digital converter (A/D) and digital signal processor at the receiver. However, this approach uses high power and is complicated to implement. 
         [0006]    It is desirable to develop techniques to provide acceptable received signal performance over copper cable at speeds over 10 Gbps such as at 40 Gbps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]    Embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the drawings and in which like reference numerals refer to similar elements. 
           [0008]      FIG. 1  depicts a system, in accordance with an embodiment. 
           [0009]      FIG. 2  depicts an example of a transmitter equalizer defined by  FIG. 72-11  of IEEE Std 802.3ap-2007. 
       
    
    
     DETAILED DESCRIPTION  
       [0010]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in one or more embodiments. 
         [0011]    Various authors have expressed that use of multiple cables to increase the rate of data transmission may incur cross talk among signals on the cables and lead to unacceptable performance. For example, slide  10  of Cole, “Nx10G Electrical I/O Issues,” IEEE 802.3 Higher Speed Study Group (November 2007) indicates that cross talk from adjacent conductors, traces, and connectors may lead to unacceptable performance. However, embodiments described herein may exhibit acceptable bit error rates when IEEE 802.3ap (2007) is applied across multiple adjacent conductors. 
         [0012]      FIG. 1  depicts a system, in accordance with an embodiment. System  100  includes transmitters  110 -A and  110 -B and respective complementary receivers  150 -A and  150 -B. Transmitter  110 -A and receiver  150 -A are part of the same remote device whereas transmitter  110 -B and receiver  150 -B are part of the same local device. A receiver may request to adjust equalizer settings of each transmitter from which the receiver receives signals. For example, receiver  150 -A may request to adjust equalizer settings of transmitter  110 -B. Transmitter  110 -A and receiver  150 -A operate in a similar manner as that of transmitter  110 -B and receiver  150 -B. 
         [0013]    For 40 Gbps operation, the transmitter-receiver pairs can be replicated four times for receipt of signals on four lanes. For 100 Gbps operation, the transmitter-receiver pairs can be replicated ten times for receipt of signals on ten lanes. Signals from transmitter-receiver pair channel can be transmitted independently so that each lane operates independently. The number of pairs can be increased to achieve any higher multiple of the basic signaling rate. Also, the basic signaling rate may be higher or lower than that specified by 10 GBASE-KR. 
         [0014]    Electrical transmit and receive specifications of transmitter-receiver pairs may be based on clause 72.7.1 of IEEE 802.3ba draft 1.1 (2008). Combining signals from multiple lanes can be accomplished at a logic level as defined in IEEE 802.3ba draft 1.1 (2008). 
         [0015]    Receiver  150 -B includes clock and data recovery logic  152 , remote transmit (TX) adaptation engine  154 , control channel decoder  156 , and transmit control  158 . Clock and data recovery logic  152  may recover a clock from a signal received from transmitter  110 -A and manipulate the data (e.g., through filtering or adaptive or fixed equalization) to recover the data which was transmitted by transmitter  110 -A. Clock and data recovery logic  152  may reproduce a data signal from a signal received from transmitter  110 -A and generate an error signal. 
         [0016]    Control channel decoder  156  is a decoder which deciphers the answers (e.g., UPDATED, NOT UPDATED, MIN, OR MAX) received from remote transmitter  110 -A generated in response to requests from transmitter  110 -B to adjust equalizer settings of remote transmitter  110 -A. Table  72 - 5 ,  FIG. 72-6 , and clauses 72.6.10.2.4, 72.6.10.2.5, 72.7.1.11 in IEEE Std 802.3ap provide examples of some answers from remote transmitter  110 -A. Control channel decoder  156  transfers the answers to remote TX adaptation engine  154 . 
         [0017]    Control channel decoder  156  may also decipher the requests from a remote device to change the settings of transmitter equalizer  114  of transmitter  110 -B (e.g., increment a tap gain). Control channel decoder  156  may pass the requests to TX control block  158 . TX control block  158  may produce answers to requests from a remote device. TX control block  158  may insert the answers into the control channel for transmission to the remote device by transferring requests to control channel encoder  112 . 
         [0018]    Control channel encoder  112  inserts requests and answers into the control channel, in order to send them to the remote partner. Control channel decoder  156  may try to change the settings of transmit equalizer  114  per requests from a remote device, if possible. 
         [0019]    Transmit equalizer  114  may include a 3-tap transmit FIR driving the analog front-end, as described in 802.3ap (2007). For example, an embodiment of transmit equalizer is depicted in  FIG. 2 . 
         [0020]    Some implementations of transmit equalizers may have a bank of N predefined (or arbitrary) transmit equalizers of any form (e.g., Continuous-Time Linear Equalizers (CTLE), finite impulse response filter, and Digital Infinite Impulse Response (IIR) filter), and apply a protocol which scans, while handshaking, between all the equalizer settings, and chooses the best equalizer in the remote transmitter for the receiver. 
         [0021]    Referring again to receiver  150 -B, remote transmit (TX) adaptation engine  154  may determine how to adapt the equalizer of remote transmitter  110 -A based in part on the error signal and the received data from clock and data recovery  152  and an answer from control channel decoder  156 . Remote TX adaptation engine  154  passes the adaptation requirements to control channel encoder  112  of transmitter  110 -B. Control channel encoder  112  inserts these requests into the control channel to send them to receiver  150 -A for transfer to transmitter  110 -A. 
         [0022]    In one embodiment, a receiver  150 -A adjusts the equalization applied by a complementary transmitter  110 -A using a control channel described with regard to the 10 GBASE-KR PMD control function as defined in clause 72.6.10 of IEEE Std 802.3ap-2007. Of note, 10 GBASE-KR is defined in IEEE Std 802.3ap-2007 for backplane but not cable. Establishing equalizer settings may take place after auto-negotiation (described later) successfully completes in order for the receiver to know the number of transmit-receive pairs. 
         [0023]    In some embodiments, twin axial cables can be used as a medium between complementary transmitter and receiver pair. A twin axial cable may include two coaxial cables, with one coaxial cable for transmit direction and another coaxial cable for receive direction. For 40 Gbps signal transmission, eight coaxial cables are used, namely four for the transmit direction and four for the receive direction. However, other types of cable types may be used provided the specifications of clauses 85.10 and 85.11 of IEEE 802.3ba draft 1.1 (2008) are met. Cables with metal conductors other than copper can be used (e.g., any alloy such as but not limited to silver or platinum). Optical single and multimode cables can be used. Optical cables can be used in active cable assemblies. Active means there are electrical to optical converters in the cable assembly at each end. An equalization setting may be applied for each length of cable between complementary transmitter and receiver pairs. 
         [0024]    Various embodiments of system are capable of transmitting signals at least at 40 Gbps in compliance with 40 GBASE-CR4. 
         [0025]      FIG. 2  depicts an example of a transmitter equalizer defined by  FIG. 72-11  of IEEE Std 802.3ap-2007. The feed forward equalizer (FFE) structure is described at clauses 72.7.1.10 and 72.7.1.11 of IEEE Std 802.3ap-2007. In this example, UI represents a clock cycle, C(1) represents a gain applied to a bit two clock cycles ago, C(0) represents a gain of a bit one clock cycle ago, and C(−1) represents a gain of the current bit. An output from the transmitter equalizer is a sum of weighted bits. For example, receiver  150 -B may control the gains C(−1), C(0), and C(1). For example, remote transmit adaptation engine  154  ( FIG. 1 ) may request to adjust the gains of the transmitter equalizer. 
         [0026]    linear optical modules, standard IEEE 802.3aq (LRM) defines how to perform equalization at receiver to combat inter-symbol interference at receiver. Equalizer settings at linear optical modules of a transmitter may be adjusted to adjust an eye at the receiver side using techniques described herein. 
         [0027]    Adaptive transmit equalization enables enhanced performance compared to fixed equalization. For example, margins for noise (including crosstalk) and jitter may be better for adaptive transmit equalization than those of fixed equalization. Adaptive transmit equalization may permit a broader range of cable lengths and supported tolerances. In addition, adaptive transmit equalization may provide a simplified receiver design as compared to a system with a fixed transmitter equalizer and any form of adaptive receiver equalizer, e.g., equalization in the digital domain after analog to digital conversion, thereby resulting in reduced power consumption. In addition, adaptive transmit equalization may save power in the transmitter by using power back-off, e.g., for short channels. As part of the equalization, the signal power at the receiver is implicitly known and the signal power at the receiver can be used to request the transmitter to reduce its power. 
         [0028]    Embodiments of the present invention may be provided, for example, as a computer program product which may include one or more machine-readable media having stored thereon machine-executable instructions that, when executed by one or more machines such as a computer, network of computers, or other electronic devices, may result in the one or more machines carrying out operations in accordance with embodiments of the present invention. A machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs (Compact Disc-Read Only Memories), and magneto-optical disks, ROMs (Read Only Memories), RAMs (Random Access Memories), EPROMs (Erasable Programmable Read Only Memories), EEPROMs (Electrically Erasable Programmable Read Only Memories), magnetic or optical cards, flash memory, or other type of media/machine-readable medium suitable for storing machine-executable instructions. 
         [0029]    The drawings and the forgoing description gave examples of the present invention. Although depicted as a number of disparate functional items, those skilled in the art will appreciate that one or more of such elements may well be combined into single functional elements. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of the present invention, however, is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of the invention is at least as broad as given by the following claims.
Techniques are described to adaptively adjust the equalizer settings of each transmitter in a transmitter-receiver pair. The transmitter-receiver pair can be used at least with implementations that comply with 40GBASE-CR4 or 100GBASE-CR10. For implementations that comply with 40GBASE-CR4, equalizer settings of four transmitters may be independently established.