CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of application Ser. No. 11/703,080, filed on Feb. 7, 2007, incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Today&#39;s work and play environments in the home and/or office are replete with numerous and various devices and appliances, hereinafter referred to generically as appliances, such as home entertainment systems, play stations, computers and surveillance equipment that people use for entertainment and work. The appliances actively interface and communicate with their users via video and/or audio displays to provide entertainment and/or to present information that they generate or receive. Behind the communication interface with human users, the appliances carry on a continuous chatter of digital communication among themselves, sending and receiving control, information and entertainment data that enable them to maintain the data processing and presentations they provide to the users 
         [0003]    Various digital communication systems and networks supported by different physical infrastructures and standards for their operation have been developed to provide the communication needs and demands of the appliances and the users who use them. The physical infrastructures may comprise coaxial cables, twisted pair cables, phone lines, and/or power lines and/or may comprise wireless communication channels. However, new appliances and configurations of appliances are constantly offering users new features and services that generally require digital communication networks having increased bandwidth. The continual flow of new features and services and demand for increased bandwidth have generated a “patch work” of different communication systems and networks and standards for managing the systems and networks aimed at satisfying the demand. 
         [0004]    For example, the development of flat panel LCD digital displays stimulated the development of digital video interfacing (DVI) utilizing Transition Minimized Differential Signaling (TMDS) technology for high-speed transmission of uncompressed serial digital data. Different communication standards defining communication systems incorporating add on features and improvements for transmitting high definition multimedia content such as high definition video, digital music, digital photos and DVD entertainment from a Source of such content, e.g. a DVD or Set Top Box (STB), to a Sink of the content, such as a TV, material followed. Among the standards are High Definition Multimedia Interface (HDMI), Unified Display Interface (UDI) and DisplayPort. 
         [0005]    Whereas the various multimedia communication standards and systems provide relatively high data transfer rates, they are generally configured to provide simplex data transmission of multimedia content at relatively high bit error rates (BER) from a Source of multimedia content to a Sink of the content over relatively short distances. The standards and systems generally require special dedicated connectors and cable assemblies and their transmission is generally limited to a reach of less than about 15 meters. Duplex and half duplex transmission of control data between a Source and a Sink that the standards require is performed at relatively low bandwidth of up to 1 Mbps and is generally supported by a dedicated channel separate from the channel over which the simplex multimedia data is transmitted. 
         [0006]    The noted multimedia standards and systems do not support high quality relatively low BER full duplex data transmission that is generally required by communication networks that support information exchange between various appliances, e.g. various computer platforms, in a household or office environment. For example, uncompressed high definition video may be streamed from Source to Sink by these systems at data rates of about a gigabit per second (Gbps) for each of the red green and blue (RGB) color channels used to provide video. At these rates, substantial noise and in particular echo and near end cross talk (NEXT) may be generated that interferes with data transmission and to provide information exchange at an acceptable BER, the exchanged information must generally be protected by a relatively large coding overhead of redundant bits. The increased overhead substantially increases complexity of coding and decoding software and hardware required to process the information and thereby the cost of the systems if they are configured to support full duplex communication. 
         [0007]    Full duplex information exchange between appliances at the home and office that are characterized by relatively low BER are typically provided by networks such Ethernet 100Base-T, WIFI, MOCA HOMEPNA or HOMEPLUG AV. The various Ethernet networks provide duplex information exchange over twisted pair wires at bandwidths that correspond to ratings of the twisted pairs. WIFI provides information exchange over wireless channels. MOCA provides communication over coaxial cable. HOMEPNA and HOMEPLUG AV support in-home networking over copper phone lines and power lines respectively that exist in the home. 
         [0008]    The various information exchange networks do not in general provide sufficient bandwidth to support transmission of uncompressed multimedia content, which typically requires data transmission rates of about 1 Gbps for each of the R, G and B color channels of a video display. A latest Ethernet standard 10Gbase-T, while having sufficient bandwidth for supporting transmission of uncompressed high definition multimedia content, is not configured to do so. It does not provide multimedia clock regeneration or support for Hot Plug Detection (HPD), Consumer Electronics Control (CEC) or Display Data Channel (DDC). 10Gbase-T networks are designed to cope with echo, NEXT and alien cross talk (ANEXT) and comprise terminal equipment that implements relatively complex echo/cross talk cancellers and support processing relatively large error correction overheads. As a result, 10Gbase-T terminal equipment and networks are relatively complicated and expensive. 
         [0009]    Since none of the various multimedia communication systems and information exchange communication networks provide both satisfactory uncompressed multimedia transmission and information exchange, home and office environments that provide both are equipped with a complex of different hardware and software systems. 
       SUMMARY 
       [0010]    An aspect of some embodiments of the invention relates to providing a single flexible format for transmitting different types of data over a same physical communication network. In accordance with an embodiment of the invention, data is transmitted over the network in packets of waveforms and the uniform flexible format comprises a uniform format for the packet, hereinafter referred to as a “U-Pac”, for encapsulating data for transmission over the physical communication network. The different types of data optionally comprise information exchange data such as Ethernet data and uncompressed multimedia audiovisual data such as HDMI 1.3, DVI 1.0, UDI and/or DisplayPort data. 
         [0011]    An aspect of some embodiments of the invention, relates to providing resistance to noise for data transmitted over the same physical network responsive to the type of data transmitted and/or a desired degree of protection against noise. Different degrees of resistance to noise are provided by modulating different types of data that warrant different degrees of protection against noise onto waveform sets having different numbers of waveforms. Optionally, the waveform sets having different numbers of waveforms are chosen from waveform sets comprising a largest set having a largest number of waveforms and subsets of the largest set. 
         [0012]    According to an aspect of some embodiments of the invention, in a data stream comprising data modulated onto a sequence of waveforms, choice of a waveform onto which given data is modulated is independent of choices of waveforms onto which other data in the data stream is modulated. For example, in some embodiments of the invention, in a data stream comprising data modulated onto a sequence of waveforms, temporally adjacent waveforms are, optionally, chosen from different waveform sets. If data in the data stream is modulated onto and transmitted in waveform packets, such as a U-Pac, a same waveform packet may contain waveforms chosen from different waveform sets. 
         [0013]    By way of example, let a waveform set comprising i waveforms chosen from M waveforms comprised in a largest set of waveforms be represented by S(M,i), where the largest set of waveforms is represented by S(M,M) and i satisfies a relation 0&lt;i≦M. Then, in accordance with an embodiment of the invention, if a first type of data is modulated onto a waveform set S(M,i), a second type of data warranting higher resistance to noise than the first type of data is modulated for transmission onto a waveform set S(M,j) where j&lt;i. If data is encapsulated in U-Pacs, a same U-Pac optionally comprises encapsulated data modulated onto different symbol sets to provide different data in the U-Pac with different levels of protections against noise. Optionally, the different symbol sets are chosen from a set {S(M,i): 0&lt;i≦M} of symbol sets. 
         [0014]    It is noted that choosing waveform sets from a same largest waveform set can be advantageous in simplifying implementation of a communication network that transmits data in accordance with an embodiment of the invention. For example, if all waveforms onto which the data is modulated are chosen from a same largest waveform set, different slicers do not have to be used to slice waveforms from different waveform sets and substantially identical slicers may be used in all receivers in the network. The slicers optionally slice and generate error directions for all received waveforms assuming that the waveforms are waveforms in the largest waveform set, independent of the actual waveform set to which a given waveform belongs. In accordance with an embodiment of the invention, demodulation of a given received waveform to a correct waveform of a waveform set to which the given waveform belongs is performed using information carried in the data indicating the waveform set to which the given waveform belongs. 
         [0015]    Hereinafter, a waveform onto which data is modulated for transmission is also referred to as a “symbol” and a set of waveforms onto which data is modulated is also referred to as a “symbol set”. Modulating data onto waveform sets comprising different numbers of waveforms in accordance with an embodiment of the invention is referred to as dynamic waveform modulation (DWM). For convenience alphabetic and/or alphanumeric designations used to represent symbol sets may also be used in referencing symbols belonging to the symbol set. For example, the alphabetic symbol set designation S(M,i) may also be used in referencing a symbol belonging to the symbol set S(M,i). Similarly, a designation of a symbol may be used to designate a symbol set to which the symbol belongs. 
         [0016]    Different types of waveforms, such as for example, frequency shift, phase shift or amplitude shift keyed waveforms, may be used in the practice of embodiments of the invention. Optionally, a largest waveform set S(M,M) comprises an M-ary pulse-modulation symbol set having M=2 k  different waveforms. In some embodiments of the invention, the pulse-modulation waveforms are pulse amplitude modulation (PAM) symbols. Optionally, the symbols are pulse-position modulation (PPM) symbols. Optionally, the symbols are pulse-duration modulation (PDM) symbols. For convenience of presentation, it is assumed hereinafter that symbols are PAM symbols, a largest set of PAM symbols is represented by P(M,M) and a subset of P(M,M) comprising i PAM symbols is represented by P(M,i). 
         [0017]    In some embodiments of the invention, the symbols onto which data is modulated for transmission are transmitted over a twisted pair (TP) cable optionally comprising four pairs of twisted wires. Optionally, each symbol is a four-dimensional symbol comprising four substantially simultaneously transmitted “one dimensional” symbols, each transmitted over a different twisted pair of the TP cable. 
         [0018]    Let, a four-dimensional symbol in accordance with an embodiment of the invention be represented by “S4D”. In accordance with an embodiment of the invention, each of the one dimensional symbols that make up a four dimensional symbol S4D is chosen from a same set of one-dimensional symbols S(M,i). The index i is smaller for types of data warranting higher noise resistance than for types of data warranting lower noise protection. Let a four dimensional symbol comprising one dimensional symbols S(M,i) (i.e. symbols chosen from the S(M,i) symbol set) be represented by “S4D-S(M,i)”. A four dimensional symbol comprising one dimensional PAM symbols P(M)i is represented by “S4D-P(M,i)”. 
         [0019]    In some embodiments of the invention, the TP cable is a Cat5e or Cat6 unshielded TP (UTP) cable comprising unshielded twisted pairs. Optionally, the TP cable is up to about 100 m in length and four-dimensional symbols S4D are transmitted over the TP cable at a rate of 250 Msym/s (mega-symbols per second) per TP. Assuming that each one dimensional symbol in a S4D symbol is a PAM(16,16) symbol (i.e. each PAM symbol is a four bit symbol) each TP transmits data at a rate of about 1 Gbps (a gigabit per second) for a total bandwidth of about 4 Gbps. In some embodiments of the invention, the TP cable is up to about 70 m in length and S4D symbols are transmitted over the cable at a rate of 500 Msym/s per TP for a total bandwidth of about 8 Gbps. 
         [0020]    An aspect of some embodiments of the invention relates to providing a method of reducing noise generated in a communication channel over which both uncompressed multimedia data is transmitted from a Source to a Sink in simplex mode and over which data is exchanged between the Source and the Sink in full duplex mode. 
         [0021]    The inventors have noted that for a communication channel transmitting uncompressed multimedia data in simplex and information data in duplex between a Source and Sink, relatively large amounts of noise at the Sink are due to Echo and Near End Cross Talk (NEXT). In accordance with an embodiment of the invention, to reduce Echo and NEXT, information data from the Sink to the Source is transmitted at a lower symbol rate than a symbol rate at which the multimedia data is transmitted from the Source to the Sink. Optionally, the Source to Sink multimedia symbol rate is greater than about 10 times the Source to Sink information symbol rate. Optionally, the multimedia symbol rate is greater than 20 times the Source to Sink information symbol rate. Optionally, the multimedia symbol rate is greater than 30 times the Source to Sink information symbol rate. Optionally, the multimedia symbol rate about 40 times the Source to Sink information symbol rate. A network configured to transmit from Sink to Source in accordance with an embodiment of the invention can substantially reduce NEXT and Echo at the sink and as a result may enable relatively inexpensive terminal equipment to be used in the network. 
         [0022]    There is therefore provided in accordance with an embodiment of the invention, a method of transmitting a data stream over a communication channel, the method comprising: providing symbol sets having different numbers of symbols; modulating data in the data stream that warrant different degrees of protection against noise onto symbols from symbol sets having different numbers of symbols, wherein which symbol set given data in the stream is modulated onto is independent of symbol sets onto which other data in the data stream is modulated onto; and transmitting the symbols. 
         [0023]    Optionally, the symbol sets are chosen from symbol sets comprising a largest set having a largest number of symbols and subsets of the largest set. Additionally or alternatively, the method comprises configuring symbols into packets. Optionally, configuring symbols into packets comprises configuring symbols from symbol sets having different numbers of symbols into a same packet. 
         [0024]    In some embodiments of the invention, temporally adjacent symbols are chosen from symbol sets having different numbers of symbols. 
         [0025]    In some embodiments of the invention, each symbol is a multidimensional symbol comprising a plurality of symbols that are substantially simultaneously transmitted over the channel. Optionally, the multidimensional symbol is a four dimensional symbol comprising four symbols chosen from a same set of symbols. 
         [0026]    In some embodiments of the invention, the symbols comprise frequency shift symbols. In some embodiments of the invention, he symbols comprise phase shift symbols. In some embodiments of the invention, the symbols comprise amplitude shift symbols. In some embodiments of the invention, the symbols comprise pulse-modulation symbols. In some embodiments of the invention, the symbols comprise pulse-position modulation (PPM) symbols. In some embodiments of the invention, the symbols comprise pulse-duration modulation (PDM) symbols. In some embodiments of the invention, the symbols comprise pulse amplitude modulation (PAM) symbols. 
         [0027]    In some embodiments of the invention, the largest set comprises 16 symbols. 
         [0028]    In some embodiments of the invention the method comprises transmitting the symbols at a transmission rate up to about 250 Msym/sec. In some embodiments of the invention the method comprises transmitting the symbols at a transmission rate up to about 500 Msym/sec. 
         [0029]    In some embodiments of the invention the method comprises transmitting data comprises transmitting in simplex mode. Optionally, transmitting data comprises transmitting in full duplex mode over the channel. Optionally, transmitting in full duplex mode comprises transmitting data at different rates in different directions over the channel. 
         [0030]    There is further provided in accordance with an embodiment of the invention, a method of transmitting data over a communication channel, the method comprising: transmitting data in simplex mode over the communication channel; and transmitting data in full duplex mode at different rates in different directions over the channel. 
         [0031]    Alternatively or additionally, transmitting data in full duplex mode comprises transmitting data at a first rate in a direction in which the simplex data is transmitted and at a second rate lower than the first rate in a direction opposite the simplex direction. 
         [0032]    In some embodiments of the invention, the data comprises Transition Minimized Differential Signaling (TMDS) data. In some embodiments of the invention, the data comprises DisplayPort data. In some embodiments of the invention, the data comprises uncompressed data. In some embodiments of the invention, the data comprises multimedia data. Optionally, the multimedia data comprises high definition multimedia data. 
         [0033]    In some embodiments of the invention, the method comprises transmitting data in full duplex mode over the channel. In some embodiments of the invention, the data comprises Ethernet data. 
         [0034]    In some embodiments of the invention, the channel comprises a twisted pair cable. Optionally, the twisted pair cable comprises four twisted pairs. Additionally or alternatively, the twisted pairs are unshielded. In some embodiments of the invention, the twisted pairs are shielded. In some embodiments of the invention, the cable is a Cat5e cable. In some embodiments of the invention, the cable is a Cat6 cable. 
         [0035]    In some embodiments of the invention, simplex and full duplex data are transmitted over a same twisted pair. 
         [0036]    There is further provided in accordance with an embodiment of the invention, a communication system comprising: a communication channel; and first and second transceivers that communicate with each other over the channel in simplex and in full duplex mode; wherein data in full duplex mode is transmitted at different rates in different directions over the channel. 
         [0037]    Optionally, data in full duplex mode is transmitted at a first rate in a direction in which the simplex data is transmitted and at a second rate lower than the first rate in a direction opposite the simplex direction. Additionally or alternatively, the first transceiver transmits data in a data stream to the second transceiver and modulates data in the data stream that warrant different degrees of protection against noise onto symbols from symbol sets having different numbers of symbols. Optionally, the symbol sets are chosen from symbol sets comprising a largest set having a largest number of symbols and subsets of the largest set. Optionally, the first transceiver configures symbols into packets. Optionally, the first transceiver packets symbols from symbol sets having different numbers of symbols into a same packet. In some embodiments of the invention, the first transceiver packets symbols from symbol sets having different numbers of symbols temporally adjacent each other. 
         [0038]    In some embodiments of the invention, the data comprises Transition Minimized Differential Signaling (TMDS) data. In some embodiments of the invention, the data comprises DisplayPort data. In some embodiments of the invention, the data comprises uncompressed data. In some embodiments of the invention, the data comprises multimedia data. Optionally, the multimedia data comprises high definition multimedia data. In some embodiments of the invention, the data comprises Ethernet data. 
         [0039]    In some embodiments of the invention, the channel comprises a twisted pair cable. Optionally, the twisted pair cable comprises four twisted pairs. Additionally or alternatively, the twisted pairs are unshielded. In some embodiments of the invention, the twisted pairs are shielded. In some embodiments of the invention, the cable is a Cat5e cable. In some embodiments of the invention, the cable is a Cat6 cable. In some embodiments of the invention, simplex and full duplex data are transmitted over a same twisted pair. 
         [0040]    There is further provided in accordance with an embodiment of the invention, a transceiver comprising: a receiver for receiving data transmitted over a communication channel; a transmitter for transmitting data over the communication channel; and circuitry for modulating data for transmission over the channel by the transmitter that modulates data that warrant different degrees of protection against noise onto symbols from symbol sets having different numbers of symbols. Optionally, the symbol sets are chosen from symbol sets comprising a largest set having a largest number of symbols and subsets of the largest set. Additionally or alternatively, the circuitry configures symbols into packets. In some embodiments of the invention, the circuitry packets symbols from symbol sets having different numbers of symbols into a same packet. In some embodiments of the invention, the circuitry packets symbols from symbol sets having different numbers of symbols temporally adjacent each other. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0041]    Examples illustrative of embodiments of the invention are described below with reference to figures attached hereto. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0042]      FIG. 1A  schematically illustrates transmitting uncompressed multimedia data and Ethernet data over a channel comprising a UTP cable, in accordance with an embodiment of the invention; 
           [0043]      FIG. 1B  schematically shows a U-Pac comprising symbols, in accordance with an embodiment of the invention. 
           [0044]      FIG. 2  schematically illustrates encapsulating Ethernet data in a U-Pac of the form shown in  FIG. 1B , in accordance with an embodiment of the invention; and 
           [0045]      FIG. 3  schematically illustrates encoding multimedia Transition Minimized Differential Signaling (TMDS) data in a U-Pac of the form shown in  FIG. 1B , in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0046]      FIG. 1  schematically shows first and second transceivers  21  and  31 , hereinafter referred to as DWM transceivers  21  and  31 , communicating using dynamic waveform modulation (DWM) to package data in U-Pacs, in accordance with an embodiment of the invention. 
         [0047]    By way of example, DWM transceiver  21  is coupled to the Ethernet and at least one Source  51  of high definition uncompressed audiovisual (AV) multimedia data from which DWM transceiver receives data to transmit to a Sink  52  via DWM transceiver  31 . Optionally, the DWM transceivers  21  and  31  transmit data to each other over an unshielded twisted pair (UTP) cable  40 . Optionally, UTP cable  40  is a Cat5e or Cat6 cable comprising four twisted pairs, TP 41 , TP 42 , TP 43  and TP 44 . The multimedia data is assumed to be Transition Minimized Differential Signaling Audio Visual (TMDS-AV) data. 
         [0048]    DWM transceiver  21  optionally comprises signal processing and control circuitry  24  for encoding/decoding and modulating/demodulating data it receives and for each twisted pair TP 41 , TP 42 , TP 43  and TP 44  of UTP cable  40 , a transmitter, receiver and hybrid circuit. The various transmitters, receivers and hybrid circuits are schematically represented by a transmitter  25 , a receiver  26  and a hybrid circuit  27  respectively. Optionally, an RJ45 patch  46  mounted to a wall plate  47  couples hybrid circuit  27  to each twisted pair TP 41 , TP 42 , TP 43  and TP 44 . 
         [0049]    DWM transceiver  31  comprises signal processing and control circuitry  34  for encoding/decoding and modulating/demodulating data it receives and for each twisted pair TP 41 , TP 42 , TP 43  and TP 44  of UTP cable  40 , a transmitter, receiver and hybrid circuit. The various transmitters, receivers and hybrid circuits are schematically represented by a transmitter  35 , a receiver  36  and a hybrid circuit  37  respectively. Optionally, an RJ45 patch  46  mounted to a wall plate  47  couples hybrid circuit  37  to each twisted pair TP 41 , TP 42 , TP 43  and TP 44 . 
         [0050]    DWM transceiver  21  receives Ethernet data from the Ethernet and TMDS-AV data and control data from Source  51  and its processing and control circuitry  24  encodes and modulates the data optionally onto symbol (i.e. waveform) sets S4D-S(M,i) of different size responsive to a degree of resistance to noise that is desired for the data. Circuitry  24  then controls transmitter  25  to transmit the symbols in packets, U-Pacs, having a same format. In accordance with an embodiment of the invention, the index i is smaller for data warranting higher noise resistance than for data warranting lower noise protection. 
         [0051]    DWM transceiver  31  receives the U-Pacs and its processing and control circuitry  34  demodulates and decodes the symbols they comprise to un-packetize the data they contain and transmits the data to Sink  52 . Whereas TMDS-AV data is transmitted in simplex mode from Source  51  to Sink  52 , Ethernet and/or control data transmitted from the Sink to the Source in general requires that the Sink provide a response to the Source and Ethernet and control data is transmitted in full duplex between the Source and Sink, in accordance with an embodiment of the invention. Large block arrows  61  schematically represent simplex multimedia TMDS-AV data and double arrowhead block arrows  62  represent full duplex Ethernet and/or control data. 
         [0052]    In responding to Ethernet and control data received from Source  51 , Sink  52  optionally packetizes its response in U-Pacs using symbol sets S4D-S(M,i) for transmission similarly to the way in which Source  51  packetizes its data for transmission. However, the inventors have noted that transmission of TMDS data and full duplex Ethernet data over a same channel, such as the twisted pair (TP), hybrid terminated channel shown in  FIG. 1A , at high transmission rates required by TMDS data can generate substantial amounts of noise in the channel. In particular, echo and near end cross talk (NEXT) generated by near end transmitters make substantial contributions to the noise. Both the near end transmitter Echo and Next can exhibit exponential growth with frequency of signal transmission over a TP channel similar to that shown in  FIG. 1A . For example, for a 50 m Cat6 cable, noise can increase by as much as about 30 dB for an increase in frequency of transmission from about 10 MHz to about 300 MHz. The inventors have further noted that full duplex transmission of Ethernet does not in general require a same data transmission rate as transmission of simplex TMDS data. Therefore, in some embodiments of the invention, whereas transceiver  21  is configured to transmit both TMDS data and full duplex Ethernet and/or control data, at a relatively high transmission rate required by TMDS data, transceiver  31  is configured to transmit Ethernet and/or control data to transceiver  21  at a substantially lower transmission rate. The lower transmission rate at which transceiver  31  transmits data can substantially reduce echo and NEXT at transceiver  31 . 
         [0053]    In some embodiments of the invention, DWM transceiver  21  transmits TMDS and Ethernet and/or control data to transceiver  31  at a transmission rate greater than about 10 times the transmission rate at which DWM transceiver  31  transmits Ethernet and/or control data to transceiver  21 . Optionally, DWM transceiver  21  transmits at a transmission rate 20 times greater than DWM transceiver  31 . Optionally, DWM transceiver  21  transmits at a transmission rate 30 times greater than DWM transceiver  31 . Optionally, DWM transceiver  21  transmits at a transmission rate 40 times greater than DWM transceiver  31 . For example, in some embodiments of the invention, DWM transceiver  21  transmits TMDS and Ethernet and/or control data to DWM transceiver  31  at 250 Msym/sec or 500 Msym/sec and DWM transceiver  31  transmits Ethernet and/or control data to transceiver  21  at 12.5 Msym/sec. 
         [0054]    It is noted that the relatively low transmission rates at which DWM transceiver  31  transmits data to DWM transceiver  21  can generate significant base line wander (BLW) due to transformers that optionally couple each transceiver to TP cable  40 . To reduce possible BLW, DWM transceiver  31  optionally transmits data using “DC-balanced” waveform sets, i.e. DC-balanced symbol sets. Whereas, any suitable DC-balanced waveform set and methods of transmitting such waveform sets known in the art may be used by DWM transceiver  31  to transmit data, optionally DWM transceiver  31  is configured to transmit data using S4D-P(M,i) symbols. 
         [0055]    Let a DC-balanced symbol set in accordance with an embodiment of the invention be represented by “±S4D-P(M,i)”. In accordance with an embodiment of the invention, the set comprises positive and negative voltage level symbols S4D-P(M,i) and for each positive voltage level symbol, the set comprises a “mirror image” negative voltage level symbol having a same magnitude as the positive level symbol. Mirror image symbols represent identical data and are selectively transmitted by DWM transceiver  31  so that BLW generated by positive symbols is substantially neutralized by transmission of negative mirror image symbols. In accordance with an embodiment of the invention, DWM transceiver  31  uses symbols from a data set ±S4D-P(M,i) having a smaller value of i for data warranting higher noise resistance. 
         [0056]    Optionally, the S4D-S(M,i) symbol sets are four dimensional PAM symbols sets S4D-P(M,i) and with each component one-dimension PAM(M,i) symbol of the four dimensional S4D-P(M,i) symbol transmitted substantially simultaneously over a different one of TP 41 , TP 42 , TP 43  and TP 44 . For convenience of presentation, it is assumed that the one dimensional PAM symbols are PAM(16,i) symbols and that i=2 k  where k is equal to a number of bits of information represented by a PAM(16,i) and satisfies a condition 1≦k≦4. The index i has a value equal to 16 for the largest PAM set and assumes values 8, 4 or 2 for subsets of the largest set with symbols in subsets having smaller i and therefore smaller numbers of symbols being easier to distinguish one from the other and having improved symbol error rate (SER). For values of i equal to 16, 8, 4 and 2 each PAM symbol respectively contains 4, 3, 2 and 1 bit of information. Each symbol of the corresponding four-dimensional S4D-P(M,i) symbol sets contains 16, 12, 8 and 4 bits of information. 
         [0057]    It is noted that for each decrease in k by 1, an amplitude difference between symbols in a symbol set S4D-P(M,i) is doubled, making it is easier to differentiate between symbols transmitted between transceiver  21  and  31  and improving the signal to mean squared error (MSE) ratio of slicers in the transceivers used in determining which symbols is received by the transceivers. Signal to MSE ratio (MSER) is defined by an expression MSER=(10*log 10 ((d/2) 2 /E(e 2 )) where d is a minimal distance between slicer decision levels and E(e 2 ) is a mean of the squared slicer error signal at the decision levels. As a result, for each decrease in k by 1, d doubles and the MSER improves by 6 dB. Assuming Additive White Gaussian Noise (AWGN), in the channel coupling transceivers  21  and  31 , for a given level of noise in the channel, improvement in MSER by 6 dB, substantially improves a symbol error rate (SER) in symbols transmitted between the transceivers. For example assuming a SER of 10 −5  a 6 dB improvement in MSE improves the SER to 10 −17  and SERs of 10 −7  and 10 −9  are improved to 10 −25 , and 10 −32  respectively. The inventors have determined that the improvement in SER provided by reducing i by 1 provides about a same improvement in SER for a AWGN channel as is provided by encoding data in accordance with a Reed-Solomon (RS) code having an error correction capability of up to 3 data symbols. 
         [0058]      FIG. 1B  schematically shows a U-Pac  100  comprising S4D-P(16,i) symbols in accordance with an embodiment of the invention. U-Pac  100  comprises a header section  101 , a payload section  102  and a tail section  103 . Payload section  102  comprises a plurality of symbols  110  that encode “payload” data to be delivered from one to the other of Source  51  and Sink  52  ( FIG. 1A ). The data in the payload section of U-Pac  100  is encoded and modulated onto S4D-P(16,i) symbols having index i which depends upon a level of protection against noise with which it is desired to protect the data. In accordance with an embodiment of the invention, different parts of payload  102  may have different values of i and thereby different levels of SER and anti-noise protection. Header  101  and tail  103  comprise management data used for processing information comprised in the packet and data in the header and tail is encoded and modulated onto S4D-P(16,4) symbols each representing 8 bits to provide the data with relatively low SER. Optionally, the header comprises two symbols, a type symbol  112  and a Stream ID symbol  113 . Type symbol  112  is optionally configured to characterize up to 64 different types, examples of which are discussed below, of U-Pacs. Stream ID comprises data that identifies Source  51  and Sink  52 . Tail  103  optionally comprises a CRC-8 symbol  114  and an idle symbol  115  that marks the end of U-Pac  100 . 
         [0059]    By way of example,  FIG. 2  shows a flow chart  200  schematically illustrating encapsulating Ethernet data into a U-Pac  100 , in accordance with an embodiment of the invention. 
         [0060]    In a process step  201  a stream of Ethernet data to be transmitted to Sink  52  is received by first transceiver  21 . A block  202  of eight (8) Ethernet octets in the stream of Ethernet data is schematically shown to the right of process step  201 . In a process step  203  a data/control bit shown shaded is added to the eight (8) Ethernet octets shown in block  202  to a form a “64 B/65 B” code block  204  of sixty-five (65) bits. 
         [0061]    In a step  205  twelve sixty-five (65) bit code blocks  204  are aggregated to form a payload data section  207  that will become with further processing a payload section of U-Pac  100 . A header data section  206  and tail data section  208  are added to the payload section to form an aggregate code block  210 . Header data section  206  optionally comprises a “Type” octet that defines the type of data in the aggregate and U-Pac  100  as Ethernet data and a Stream ID octet. Tail  208  optionally comprises a CRC-8 octet. Aggregate code block  210  comprises 768 Ethernet payload bits and 36 control bits (header and tale bits plus the control bit added in step  203 ) for a total of 804 bits. Optionally, the data in aggregate block  210  is scrambled in a step  212  to provide a scrambled aggregate data block S 210  having header, payload and tail sections S 206 , S 207  and S 208  respectively. In a step  214 , the data in scrambled, aggregate code block S 210  is mapped onto a set of S4D-P(16,i) symbols and a symbol is added to tail S 208  to generate header  101 , payload  102  and tail  103  of a U-Pac  100 . 
         [0062]    In accordance with an embodiment of the invention, data in header section S 206  and tail section S 208  of scrambled aggregate S 210  is mapped to S4D-P(16,4) symbols, each of which represents 8 bits of data, to provide the control data with a relatively low SER. Header  101  and tail  103  have two (2) S4D-P(16,4) symbols each. The Ethernet data in payload section S 207  is optionally mapped to S4D-P(16,8) symbols in payload  102 , each of which symbols represents 12 bits of data, so that payload  102  has sixty-five (65) S4D-P(16,8) symbols. U-Pac  100  therefore comprises a total of sixty-nine (69) S4D-P(16,i) symbols and comprises 768 bits of Ethernet payload data. Assuming the Ethernet data received by DWM transceiver  21  is 100 Mbps Ethernet, to support the data transmission rate the transceiver transmits an Ethernet U-Pac  100  of 69 S4D-P(16,i) symbols to Sink  52  via DWM transceiver  31 , in accordance with an embodiment of the invention, every 7.68 μs for a transmission rate of about 9 Mega-symbols of Ethernet data per second (Msym/sec). Optionally, Sink  52  ( FIG. 1A ) responds to the Ethernet information it receives at a same rate, and transmits back to transceiver  21  via transceiver  31  about 9 Mega-symbols of Ethernet data per second (Msym/sec). Symbol transmission between DWM transceivers  21  and  31  in accordance with an embodiment of the invention, therefore operates in a full duplex mode that supports 100 Mbps full duplex Ethernet transmission. 
         [0063]      FIG. 3  shows a flow chart  300  that schematically illustrates encapsulating TMDS-AV data from a TMDS-AV data stream used for generating an audiovisual display into U-Pacs, in accordance with an embodiment of the invention. 
         [0064]    A stream of TMDS-AV data comprises three different types of data transmitted during periods, hereinafter referred to as “TMDS periods” or “T-periods”, having fixed duration “T”. During each TMDS period one of the three different types of data is transmitted for each of three TMDS channels. The types of data are video data (“V” data), control data (“C” data) and data-island packet data (“I” data). During video data periods, also referred to as a “V periods”, each TMDS channel carries pixel color data encoded in 8 bits, for a total of 24 bits of video data per period. During data island TMDS periods, also referred to as “I periods”, the TMDS channels carry audio data, which may comprise for example audio samples acquired at 192 kHz for each of 8 audio channels and information frames, “infoframes”, comprising data that characterizes audio and video data in the TMDS-AV stream. During an I period each TMDS channel carries 4 bits of data so that the three TDMS channels carry a total of 12 bits of data during the I period. During control data TMDS periods, also referred to as “C periods”, the TMDS channels carry inter alia horizontal and vertical synchronization data. Each TDMS channel typically carries 2 bits of control data during a C period for a total of 6 bits of control data during the period. Sequences of different types of TMDS periods in the TDMS-AV stream are generally separated from each other by “guard bands” that are 2 TDMS periods, “G periods”, long. 
         [0065]    In a process step  302  in  FIG. 3  DWM transceiver  21  receives a TMDS-AV data stream to be transmitted to Sink  52  via DWM transceiver  31 . A data block  304  of data in the TMDS-AV stream encoding a single horizontal line of video data and accompanying audio data is schematically shown being received by the transceiver. The data is assumed, by way of example, to be used to generate a progressive video display that is refreshed at 60 Hz and comprises 1080 active and 45 blank horizontal lines, each having 1920 24 bit pixels and 280 blank pixels. The video display is assumed accompanied by eight 8 audio channels sampled at 192 kHz to provide 8 level samples. Data block  304  therefore is 2200 TMDS periods long, of which 1920 periods are video data periods, i.e. V periods, during each of which 24 bits of pixel data are transmitted and 280 TMDS periods are “blank” TMDS periods. In  FIG. 3  a TMDS T-period is generically denoted by its duration “T”. Of the 280 blank T-periods a first 96 T-periods comprise control, C periods, or guard band, G periods each carrying 6 bits of data, a middle 96 periods comprise data island, I periods each carrying 12 bits encoding audio data and a last 88 periods comprise control C or G periods. 
         [0066]    In accordance with an embodiment of the invention, in a process block  306  each 16 T-periods of TMDS data in data block  304  are encoded into S4D-P(16,i) symbols, and a header, hereinafter a “sub-packet header”, added to the symbols to form a sub-packet. The sub-packet header optionally comprises a S4D-P(16,4) symbol that characterizes the data in the sub-packet. For example, the sub-packet header optionally distinguishes between a sub-packet comprising only control data from a sub-packet comprising control and guard data or a sub-packet comprising only data island data. Sub-packets  310  generated in process step  306  from data in data block  304  are schematically shown in a data block  308  and are labeled with a letter or letters indicating the type of data they contain. Sub-packets  310  labeled “C”, “I”, or “V” comprise only control, data island or video pixel data respectively. “Mixed” sub-packets comprising more than one type of data are labeled by the letters of each of the data types they contain. For example, sub-packets  310  in block  308  that contain both control (C) data and guard (G) data are labeled by both G and C. 
         [0067]    In accordance with an embodiment of the invention, V data, is encoded into S4D-P(16,16) symbols, and I data, C data and G data are encoded into S4D-P(16,8) symbols. Since each T-period of V data comprises 24 bits of data, each V period is encoded to 1.5 S4D-P(16,16) symbols. Similarly, each I data period comprises 12 bits of data and is encoded to 1 S4D-P(16,8) symbol and each C data or G period comprises 6 bits of data and is encoded to 0.5 S4D-P(16,8) symbols. With the addition of the sub-packet header comprising one S4D-P(16,4) symbol, each type of V data sub-packet  310  comprises 25 S4D-P(16,16) symbols, each I data sub-packet comprises 17 S4D-P(16,8) symbols and each C or CG sub-packet  310  comprises 9 S4D-P(16,8) symbols. The number of S4D-P(16,i) symbols in each type of sub-packet  310  in  FIG. 3  is shown in parentheses for at least one of the type of sub-packet below the sub-packet. 
         [0068]    In accordance with an embodiment of the invention, V data is always encapsulated in sub-packets  310  comprising only V data in addition to the sub-packet header. Therefore, if data in a sequence of 16 T-periods that is to be encapsulated in a sub-packet includes T-periods having data other than V data followed by T-periods having V data, the sub-packet is a “shortened sub-packet” generated only from data in the non-V data T-periods and includes data in less than 16 T-periods. V data in the remaining T-periods are encapsulated in a next sub-packet. A shortened sub-packet  310  comprising data from only 8 T-periods is distinguished by a reference numeral  312 . 
         [0069]    In a process step  314 , sub-packets  310  are encapsulated in U-Pacs  320  having a configuration shown for U-Pac  100  ( FIG. 1B ). U-Pacs  320  encapsulating sub-packets  310  generated in process step  314  are shown in a U-Pac data block  330 . In accordance with an embodiment of the invention, each U-Pac  320  typically comprises, 4 sub-packets  310  (data from 64 T-periods of data block  304 ), a U-Pac header  321  comprising two S4D-P(16,4) symbols and a U-Pac tail  322  comprising two S4D-P(16,4) symbols. Similar to the case of sub-packets  310 , optionally, a U-Pac  320  does not “mix” sub-packets  310  having V data with sub-packets  310  comprising other than V data. In accordance with an embodiment of the invention, a sub-packet  310  comprising V data is encapsulated in a same U-Pac  320  only with other sub-packets  310  comprising V data. As a result, to satisfy the non-mixing constraint, a U-Pac  310  such as for example a U-Pac  320  distinguished in  FIG. 3  by a reference numeral  324 , may comprise less than 4 sub-packets  310 . 
         [0070]    A total of 35 U-Pacs  320 , labeled U-Pac  1 -U-Pac  35 , generated as described above are required to encapsulate all the TMDS data comprised in data block  304  that defines a single 1080 p 24 bpp+8 L-PCM audio sampled at 192 KHz. Of the 35 U-Pacs  320 , 30 U-Pacs comprise pixel defining D data and 5 U-Pacs comprise control and/or audio data. Since different sub-packets  310  may comprise different numbers of S4D-P(16,i) symbols and as noted above, different U-Pacs  320  may comprise different numbers of sub-packets, different U-Pacs  320  may comprise different numbers of S4D-P(16,i) symbols. A number of S4D-P(16,i) symbols in each U-Pac  320  in U-Pac data block  330  is shown for each U-Pac  320 . A total of 3346 S4D-P(16,i) symbols are used to encapsulate the TMDS data for data block  304 . 
         [0071]    In order to support the refresh rate of 60 Hz, DWM transceiver  21  must transmit 60×1125 lines of 3346 S4D-P(16,i) symbols to DWM transceiver  31  every second for a transmission rate of about 226 Msym/sec. In an embodiment of the invention, DWM transceivers  21  and  31  operate at transmission rates of 250 Msym/sec, which readily supports the bandwidth required for 226 Msym/sec simplex transmission of TMDS data plus 9 Msym/sec full duplex Ethernet transmission noted above between the transceivers. In an embodiment of the invention, the transceivers operate at transmission rates of 500 Msym/sec which supports simultaneous transmission of two TMDS streams plus full duplex Ethernet. 
         [0072]    In the description and claims of the application, each of the words “comprise” “include” and “have”, and forms thereof, are not necessarily limited to members in a list with which the words may be associated. 
         [0073]    The invention has been described using various detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments may comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described and embodiments of the invention comprising different combinations of features noted in the described embodiments will occur to persons with skill in the art. It is intended that the scope of the invention be limited only by the claims and that the claims be interpreted to include all such variations and combinations.
A method of transmitting a data stream over a communication channel, the method comprising: providing symbol sets having different numbers of symbols; modulating data in the data stream that warrant different degrees of protection against noise onto symbols from symbol sets having different numbers of symbols, wherein which symbol set given data in the stream is modulated onto is independent of symbol sets onto which other data in the data stream is modulated onto; and transmitting the symbols.