Patent Publication Number: US-9407394-B2

Title: Frequent flow control by replacing certain idle words with bitwise complement words

Description:
BACKGROUND 
     Many communication systems exchange one or more messages between a first node, such as the transmitting node, and a second node, such as the receiving node, in order to implement flow control. Other systems send the flow control data using one or more unique symbols transmitted during idle time, which in some cases may also be referred to as inter-packet gap. 
     There is a need for a communication system able to send frequent flow control data using signals transmitted during idle time, but without using unique symbols for conveying the flow control data. 
     BRIEF SUMMARY 
     In one embodiment, a communication link configured to implement frequent flow control without using unique symbols or designated packets, comprising: A first device configured to send to a second device, over a full-duplex communication link, high throughput packet communication of more than 1 Gb/s. The second device comprises: a buffer configured to temporarily store the high throughput packet communication, a basic idle code word sequence calculator configured to calculate a basic idle code word sequence known to the first device, an idle sequence modifier configured to produce an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words, and a transmitter configured to transmit the idle sequence to the first device during inter-packet gap; wherein each bitwise complement code word appears in the basic idle sequence, and the M bitwise complement code words are indicative of the fullness of the buffer. And the first device is configured to receive the idle sequence and determine, based on a difference between the idle sequence and the basic idle sequence, that the buffer is full or expected to get full, and thus the first device stop sending packets to the second device. 
     In another embodiment, a method for implementing a frequent flow control without using unique symbols or designated packets, comprising: Sending, from a first device to a second device over a full-duplex communication link, high throughput packet communication of more than 1 Gb/s. Temporarily storing the high throughput packet communication in a buffer of the second device. Calculating, by the second device, a basic idle code word sequence known to the first device. Producing an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words. Transmitting the idle sequence to the first device during inter-packet gap; wherein each bitwise complement code word appears in the basic idle sequence, and the M bitwise complement code words are indicative of the fullness of the buffer. Receiving the idle sequence by the first device. And determining, by the first device, based on a difference between the idle sequence and the basic idle sequence, that the buffer is full or expected to get full, and thus the first device stop sending packets to the second device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments are herein described, by way of example only, with reference to the accompanying drawings. In the drawings: 
         FIG. 1  illustrates one embodiment of a communication system; 
         FIG. 2  illustrates one embodiment of a method for encoding a frame; 
         FIG. 3  illustrates one embodiment of a method for encoding a frame having a header part and a payload part; 
         FIG. 4  illustrates one embodiment of a communication node; 
         FIG. 5  illustrates one embodiment of a method for encoding frames utilizing line-codes having different minimum Hamming distances; 
         FIG. 6  illustrates one embodiment of a communication node; 
         FIG. 7  illustrates one embodiment of a method for indicating the end of an idle sequence; 
         FIG. 8  illustrates one embodiment of a communication node; 
         FIG. 9  illustrates one embodiment of a communication system; 
         FIG. 10  illustrates one embodiment of a method for indicating the end of an idle sequence; 
         FIG. 11  illustrates one embodiment of a communication link configured to indicate a configuration change; 
         FIG. 12  illustrates one embodiment of a method for indicating a configuration change of a communication link; 
         FIG. 13  illustrates one embodiment of a method for indicating a configuration change of a communication link; 
         FIG. 14  illustrates one embodiment of a communication link that implements frequent flow control; and 
         FIG. 15  illustrates one embodiment of a method for implementing a frequent flow control. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of a communication system  100 . The communication system  100  includes a first node  102 , which sends one or more frames over a communication channel  106 , and a second node  104 , which receives the frames. The first node includes an encoder  108 , which encodes the frames, and a transmitter  110 , which transmits the frames over the communication channel. The second node  104  includes a detector  114 , which detects the frames, and a decoder  116 , which decodes the frames. The first node sends a frame  112 , which includes a first part  118  and a second part  120 . The encoder  108  encodes the first and second parts of the frame, utilizing first and second line-codes, respectively, and the decoder  116  decodes the first and second parts of the frame, utilizing the same first and second line-codes, respectively. 
     A line-code is a binary code, which encodes words of M binary symbols, referred to as input words, to words of N binary symbols, referred to as code words. The two values of the binary symbols are denoted herein as “one” (“1”) and “zero” (“0”). M is referred to as the input word length of the line-code, N is referred to as the code word length of the code, and the ratio of M divided by N is referred to as the rate R of the line-code. M is lower than N, and therefore R is less than 1. The output set of a line-code is the set of all code words that may be produced by the line-code, and is therefore a proper subset of the set of all 2N binary words of length N. 
     The first and second line-codes have input word lengths M′ and M″, respectively, binary code word lengths N′ and N″, respectively, and code rates R′ and R″, respectively. The first and second line-codes have minimum Hamming distances D′ and D″, respectively, where D″ is lower than D′. 
     In one embodiment, the “running disparity” at a certain binary symbol produced by the encoder is the difference between the number of “ones” and the number of “zeroes” produced by the encoder up to and including the certain symbol. 
     The encoder  108  maintains the running disparity over the frame  112  bounded by a predetermined value K (i.e., the absolute value of the running disparity over the frame is maintained lower than or equal to K). In one example, K is lower than N′/2. Additionally or alternatively, K may be lower than N″/2. 
     In one example, the initial value of the running disparity is zero. In another example, the initial value of the running disparity is minus one. In one example, the running disparity is reset to its initial value every predetermined number of frames, which may be unlimited. The running disparity may be calculated at the end of each code word, or at each symbol. 
     In one example, K is lower than N′/4. Additionally or alternatively, K may be lower than N″/4. In one example, K is lower than 3. In one example, K is lower than 2. The disparity of a code word is the difference between the number of “ones” and the number of “zeroes” within the word. For example, the disparity of the code word 01101100 is zero, the disparity of the code word 01111100 is two, and the disparity of the code word 01001000 is minus four. 
     In one embodiment, the first and second line-codes facilitate maintaining the running disparity bounded by selecting the disparity of the current code word based on the running disparity at the end of the previous code word. For example, the encoder may select a code word with a zero or negative disparity when the running disparity is positive, and selects a code word with a zero or positive disparity when the running disparity is negative, thereby maintaining the running disparity bounded. 
     A “paired disparity” line-code is a line-code, where each input word is encoded either to a code word with a zero disparity, or to a code word selected from a set of code words containing at least one code word with a positive disparity and at least one code word with a negative disparity. A “balanced paired disparity” line-codes is a paired disparity line-codes, where each input word is encoded either to a code word with a zero disparity, or to a code word selected from a balanced pair of code words (i.e., a first code word with a positive disparity P, and a second code word with a negative disparity minus P). 
     In one embodiment, the first and second line-codes are paired disparity line-codes, and code word selection is based on the running disparity (i.e., when the running disparity is positive, the disparity of the next code is either zero or negative, and when the running disparity is negative, the disparity of the next code word is either zero or positive). Thereby, the absolute value of the running disparity is maintained lower than or equal to P, where P is the maximum absolute value of the disparities of all code words in the output set of the line-code. In one embodiment, the first and second line-codes are balanced paired disparity line-codes, and the encoder maintains the running disparity bounded between zero and P (inclusive) by selecting a code word with negative disparity only when the running disparity is positive. Alternatively, the encoder may maintain the running disparity between zero and minus P (inclusive), by selecting a code word with positive disparity only when the running disparity is negative. In one example, P equals 2, and the running disparity is maintained between zero and 2 (inclusive). Alternatively, the running disparity is maintained between zero and minus 2 (inclusive). In one example, the initial value of the running disparity is minus one, and the running disparity is maintained between minus one and plus one. 
     In one embodiment, the encoder selects the code words of the second part of the frame based on a running disparity, which is calculated from the beginning of the first part of the frame, thereby maintaining the running disparity bounded over the entire frame. The disparity of the first code word of the second frame is selected based on the running disparity calculated at the end of the first part of the frame, although the first and second parts of the frame are encoded with different line-codes. In one example, the first and second line-codes are balanced paired disparity line-codes, both with P equals 2, and the initial value of the running disparity is set to minus one. In this example, the running disparity is maintained between minus one and one (inclusive), and the absolute value of the running disparity is accordingly maintained lower than 2. 
     In some examples, N′ and N″ are equal, and therefore the first output set of the first line-code and the second output set of the second line-code are both subsets of the set of all binary words of length N′. Following are some examples regarding the relation between the first and second output sets. In all those examples, N′ and N″ are equal. 
     In one example, the first and second output sets are mutually exclusive to each other, i.e., they do not share any common code word. In another example, the first set and second output sets, are not exclusive to each other, i.e., they share at least one common code word. 
     In one example, the first output set is a subset of a second output set, M′ is lower than M″, and R′ is lower than R″. Accordingly, the first and second line-codes provide a trade-off between error resilience and bandwidth efficiency: the first line-code features better error resilience (D′ is higher), while the second line-code features better bandwidth efficiency (R′ is lower). In this example, the first line code may be utilized when higher error resilience is desired, thereby gaining the higher bandwidth efficiency of the second code whenever lower error resilience can be tolerated. 
     In one example, the first output set is not a subset of a second output set, i.e. the first output set includes at least one code word exclusive to the second output set. 
     The 8b/10b is a known family of balanced paired disparity line-codes with M=8, N=10, and P=2. In one example, the first output set is a subset of the output set of an 8b/10b line-code. Additionally or alternatively, the second output set may be a subset of the output set of an 8b/10b line-code. 
     In one embodiment, the encoder  108  maintains over the frame transition density equal to or better than a predetermined transition density. The predetermined transition density may be at least one transition within any sequence of Z consecutive symbols. Z may equal 6, e.g., when the first and second output sets are subsets of the output set of the data and control words of an 8b/10b line-code. Z may also be lower than 6, e.g., when the first and second output sets are subsets of the output set of the data words of an 8b/10b line-code. 
     In one embodiment, the encoder  108  maintains over the frame spectral uniformity equal to or better than a predetermined spectral uniformity. The encoder may receive words that are already selected to produce the required spectral uniformity. Additionally or alternatively, the encoder may randomize the stream of input words, thereby maintaining the spectral uniformity of the stream of encoded words. 
     In one example, the communication channel  106  includes an optical fiber. Additionally or alternatively, the communication channel may include a conductive wire, a wireless channel, and/or any other suitable communication channel. 
     The second part of different frames may be encoded utilizing different line-codes, e.g. encoding the second part of some frames utilizing the first line-code, and the second part of other frames utilizing the second line-code. The line-code may be selected based on conditions of the communication channel, such as received signal level, received noise level, signal to noise ratio, symbol error rate, and/or any other suitable channel condition. For example, the first line-code may be selected only when its higher error resilience capability is required due to the channel condition (e.g. the symbol error rate is higher than some accepted value). In this example, the system may benefit from the higher bandwidth efficiency of the second line-code whenever allowed by the channel condition. 
     Additionally or alternatively, the line-code utilized for encoding the second part of the frame may be selected based on the type of the data contained therein. For example, the second part of the frame may be encoded utilizing the first line-code only when its higher error resilience capability is required due to the type of the data (e.g. the data is highly sensitive to errors). In one example, the line-code is selected based on both data type and channel condition. 
     In one example, the first part of the frame may include a header of the frame, and the second part of the frame may include a payload of the frame. The first part of the frame may include an indication of the line-code utilized for encoding the second part of the frame. Accordingly, the first part of the frame may contain an indication that the second part of the frame is encoded utilizing the second line-code. The decoder  116  may select an appropriate line-code for decoding the second part of the frame based on the indication included in the first part of the frame. 
     In one embodiment, the encoder  108  produces an idle sequence  122 , which resides between the frame  112  and a following frame  124 . The idle sequence includes code words of length N_idle, which may optionally be equal to N′ or to N″. In one example, the second node  104  cannot predict the starting point of the following frame  124 , because the length of the idle sequence  122  is unknown. In this example, the detector  114  detects the idle sequence  122 , and the decoder  116  identifies the end thereof, thereby determining the starting point of the following frame  124 . 
     In one embodiment, the encoder  108  maintains the absolute value of the running disparity over the idle sequence  122  lower than or equal to K. K may optionally be lower than N_idle/2 or lower than N_idle/4. 
     In one embodiment, the idle sequence is composed of code words of a fourth line-code. The idle sequence may be produced by encoding a pseudo-random sequence of binary symbols utilizing the fourth line-code. The encoder  108  may select the disparity of the current code word based on the running disparity at the end of the previous code word, as explained above regarding the first and second line-codes. 
     In one embodiment, the encoder  108  maintains the absolute running disparity over the frame and the idle sequence lower than or equal to K. The encoder  108  may select the disparity of the first code word of the idle sequence based on the running disparity calculated at the end of the frame, although the frame and the idle sequence are encoded with different line codes. In one example, the first, second, and fourth line-codes are balanced paired disparity line-codes with P equals two, the initial value of the running disparity is set to minus one, and the absolute value of the running disparity is maintained lower than 2, although the frame and the idle sequence may be encoded with different line codes. 
     In one embodiment, the encoder  108  maintains the transition density over the idle sequence equal to or better than the predetermined transition density of at least one transition within any sequence of Z consecutive symbols. In one example, Z equals. In another example, Z is lower than 6. 
     In one embodiment, the encoder  108  maintains the spectral uniformity over the idle sequence equal to or better than the predetermined spectral uniformity. The spectral uniformity of the idle sequence may result from the distribution of the output set of the fourth line-code, and/or from randomizing the input of the fourth line-code. 
     In one embodiment, the encoder  108  maintains the running disparity, from the beginning of the frame  112  to the end of the following frame  124 , bounded by K (i.e., maintaining the absolute value of the running disparity lower than or equal to K). The running disparity may be maintained bounded over a sequence of the frame  112 , the following frame  124 , and the idle sequence  122 . Additionally or alternatively, the running disparity may be maintained bounded over a sequence of the frame  112  and the following frame  124 , with no intermediate idle sequence. 
     In one embodiment, the following frame is encoded utilizing one or more line-codes, and the encoder  108  selects the code words of the following frame based on a running disparity, which is calculated from the beginning of the frame, thereby maintaining the running disparity bounded by K over the stream of the code words of the frame and the following frame. The disparity of the first code word of the following frame is selected based on the running disparity calculated at the end of the frame, or at the end of the idle sequence, as applicable. 
     In one embodiment, the following frame  124  includes a first part  126  of the following frame, and a second part  128  of the following frame, and encoder  108  encodes the first and second parts of the following frame utilizing the first line-code and a third line-code, respectively. The third line-code has input word length M″′, binary code word lengths N′″, and code rate R′″. The third line-code has a minimum Hamming distance D′″, where D′″ is lower than D″. The encoder  108  maintains the running disparity from the beginning of the frame to the end of the following frame bounded by K (i.e., the absolute value of the running disparity is maintained lower than or equal to K). In one example, the disparity of the first code word of the second part of the following frame is selected based on the running disparity calculated at the end of the first part of the second frame, although the first and the second parts of the following frame are encoded with different line codes. K may be lower than N′″/2 or lower than N′″/4, and N″′ may be equal to N′. 
     When N″′ is equal to N′, the first output set of the first line-code and the third output set of the third line-code are both subsets of the set of all binary words of length N′. In one example, where N″′ is equal to N′, the third output set of the third line-code is not a subset of the first output set of the first line-code (i.e., the third output set includes at least one code word exclusive to a first output set). 
       FIG. 2  illustrates one embodiment of a method for encoding a frame. The method may be performed by a first communication node, such as the first node  102  in  FIG. 1 , and the frame may be the frame  112  in  FIG. 1 . In addition, the method may be performed by any other communication node, or by any other suitable device. The method includes at least the following steps: In step  202 , maintaining the running disparity over the frame bounded by a predetermined value K (i.e., the absolute value of the running disparity over the frame is maintained lower than or equal to K). In step  204 , encoding the first part of the frame utilizing a first line-code. And in step  206 , encoding the second part of the frame utilizing a second line-code. The first and second line-codes have code word lengths N′ and N″, respectively, and minimum Hamming distances D′ and D″, respectively, where D″ is lower than D′. In one example, K is lower than N′/2. Additionally or alternatively, K may be lower than N″/2. 
     In one example, K is lower than N′/4. Additionally or alternatively, K may be lower than N″/4. In one example, K is lower than 3. In one example, K is lower than 2. 
     In some examples, N′ and N″ are equal. Following are some examples regarding the relation between the first and second output sets. In all those examples, N′ and N″ are equal. In one example, the first and second output sets are mutually exclusive to each other, i.e., they do not share any common code word. In another example, the first set and second output sets, are not exclusive to each other, i.e., they share at least one common code word. In one example, the first output set is a subset of a second output set. In one example, the first output set is not a subset of a second output set, i.e. the first output set includes at least one code word exclusive to the second output set. 
     In one embodiment, the first output set is a subset of the output set of an 8b/10b line-code. Additionally or alternatively, the second output set may be a subset of the output set of an 8b/10b line-code. 
     In one embodiment, the method illustrated in  FIG. 2  further includes maintaining over the frame transition density equal to or better than a predetermined transition density, which may be at least one transition within any sequence of Z consecutive symbols. In one example, Z equals 6. In one example, Z is lower than 6. 
     In one embodiment, the method further includes maintaining over the frame spectral uniformity equal to or better than a predetermined spectral uniformity. 
     In one embodiment, the method optionally includes an additional step of transmitting the frame over a communication channel. The step of transmitting the frame may be performed by a transmitter, such as the transmitter  110  in  FIG. 1 , or by any other transmitter, or by any other suitable device. 
     The method may further include an optional step of detecting the frame and a step of decoding thereof, which may be performed by a second communication node, such as the second node  104  in  FIG. 1 , or by any other communication node. 
     In one example, the first part of the frame may include the header of the frame, and the second part of the frame may include the payload of the frame. In one example, the second part of different frames may be encoded utilizing different line-codes. The line-code may be may be selected based on the type of the data carried by the certain frame, or by the state of the channel during the transmission of the certain frame, or by some combination thereof, or by any other suitable parameter. The first part of the certain frame may include an indication of the line-code utilized for encoding the second part of the certain frame, in order to facilitate utilization of an appropriate line-code for decoding the second part of the certain frame. Accordingly, the first part of the frame may contain an indication that the second part of the frame is encoded utilizing the second line-code. 
     In one embodiment, the method illustrated in  FIG. 2  may further include an optional step of producing an idle sequence, which may be performed by an encoder, such as the encoder  108  in  FIG. 1 , or by any other suitable encoder. The idle sequence may be the idle sequence  122  in  FIG. 1 . The idle sequence includes code words of length N_idle, which may optionally be equal to N′ or to N″. 
     Additionally, the method may include an optional step of transmitting the idle sequence over the communication channel, which may be performed by a transmitter, such as the transmitter  110  in  FIG. 1 , or by any other transmitter, or by any other suitable device. The method may optionally further include a step of detecting the idle sequence, and a step of identifying the end of the idle sequence. The step of detecting the idle sequence may be performed by a detector, such as the detector  114  in  FIG. 1 , or by any other detector, or by any other suitable device; and the step of identifying the end of the idle sequence may be performed by a decoder, such as the decoder  116  in  FIG. 1 , or by any other decoder, or by any other suitable device. In one example, identifying of the end of the idle sequence facilitates determining the beginning of the following frame. 
     In one embodiment, the step of producing an idle sequence further includes maintaining the absolute running disparity over the idle sequence lower than the predetermined value K. In one example, K is lower than N_idle/2. In one example, K is lower than N_idle/4. In one embodiment, the idle sequence is produced of code words of a fourth line-code, e.g., by encoding a pseudo-random sequence of binary symbols utilizing the fourth line-code. In one embodiment, the method illustrated in  FIG. 2  further includes maintaining the absolute running disparity over the frame and the idle sequence lower than K. In one embodiment, the step of producing an idle sequence further includes maintaining the transition density over the idle sequence equal to or better than the predetermined transition density. In one embodiment, the step of producing an idle sequence further includes maintaining the spectral uniformity over the idle sequence equal to or better than the predetermined spectral uniformity. 
     In one embodiment, the method illustrated in  FIG. 2  includes an optional step of encoding a following frame, which may be performed by an encoder, such as the encoder  108  in  FIG. 1 , or by any other encoder, or by any other suitable device. In addition, the following frame may be the following frame  124  in  FIG. 1 . In one example, the idle sequence  122  resides between the frame  112  and the following frame  124 , and the step of encoding a following frame is accordingly performed after the step of producing an idle sequence. Additionally or alternatively, the following frame may adjacent to the frame, without an intermediate idle sequence, and the step of encoding a following frame may accordingly be performed directly after encoding the first frame, without performing the intermediate step of producing an idle sequence. 
     In one embodiment, the method include an optional step of transmitting the following frame over the communication channel, which may be performed by a transmitter, such as the transmitter  110  in  FIG. 1 , or by any other transmitter, or by any other suitable device. Additionally, the method may include an optional step of detecting the following frame and an optional step of decoding thereof. The step of detecting the following frame may be performed by a detector, such as the detector  114  in  FIG. 1 , or by any other detector, or by any other suitable device; and the step of decoding the following frame may be performed by a decoder, such as the decoder  116  in  FIG. 1 , or by any other decoder, or by any other suitable device. 
     In one embodiment, the method further includes maintaining the running disparity, from the beginning of the frame to the end of the following frame, bounded by K (i.e., maintaining the absolute value of the running disparity lower than or equal to K). The running disparity is maintained bounded over a sequence including the frame and the following frame, with or without intermediate idle sequence. 
     In one embodiment, the following frame includes first and second parts of the following frame, and the step of encoding the following frame includes a step of encoding the first part of the following frame utilizing the first line-code, and a step of encoding the second part of the following frame utilizing a third line-code. The third line-code has binary code word lengths N′ and a minimum Hamming distance D′, where D′″ is lower than D″. The method may further include maintaining the running disparity from the beginning of the frame to the end of the following frame bounded by K (i.e., maintaining the absolute value of the running disparity lower than or equal to K). In one example, K is lower than N″′/2. In one example, K is lower than N′/4. 
     N″′ may be equal to N′. In one example, in which N′ and N″′ are equal, the third output set of the third line-code is not a subset of the first output set of the first line-code (i.e., the third output set includes at least one code word exclusive to a first output set). 
       FIG. 3  illustrates one embodiment of a method for encoding a frame having a header part and a payload part. The method illustrated in  FIG. 3  includes at least the following steps: In step  302 , encoding the header part utilizing a first code having a minimal Hamming distance D 1 . And in step  304 , encoding the payload part utilizing a second code having a minimal Hamming distance D 2  higher than D 1 . 
     In one embodiment, the method further includes a step of transmitting the frame over a binary channel. In one embodiment, the first and second codes are first and second line-codes having binary code word lengths N1 and N2, respectively, and the method further includes maintaining the running disparity over the frame bounded by K (i.e., maintaining the absolute value of the running disparity lower than or equal to K). In one example, K is lower than N1/2. Additionally or alternatively, in one example, K is lower than N2/2. In one example, N2 equals N1. 
       FIG. 4  illustrates one embodiment of a communication node  400 . The communication node  400  may be the first node  102  of  FIG. 1 . The communication node  400  includes an encoder  402  encoding frames utilizing two or more line-codes, and a transmitter  404  transmitting the frames over a communication channel  406 . The frames are received by a second communication node  408 . In one embodiment, each frame includes a header and a payload. 
     In one embodiment, encoder  402  encodes the payloads of different frames utilizing different line-codes selected from the set of two or more line-codes, while the headers of the frames are encoded utilizing the same line-code, referred to as the first line-code. In one embodiment, a payload of a certain frame is encoded utilizing a line-code selected based on the type of the data within the payload of the certain frame. In one embodiment, the header of the certain frame includes an indication of the line-code utilized for encoding the payload of the certain frame, thereby facilitating the second communication node  408  to decode the payload of the certain frame utilizing the respective line-code. 
     In one example, the set of two or more line-codes include the first line-code and a second line-code, having input word lengths M′ and M″, respectively, binary code word lengths N′ and N″, respectively, and code rates R′ and R″, respectively. The first and second line-codes have minimum Hamming distances D′ and D″, respectively, where D″ is lower than D′. 
     In one example, the encoder  402  encodes a first frame  410 , which includes a first header  412  and a first payload  414 , and a second frames  416 , which includes a second header  418  and a second payload  420 . The encoder  402  encodes the first and second header utilizing the first line-code, and the first and second payloads utilizing the first and second line-codes, respectively. 
     In one example, the first line-code is selected for encoding the first payload based on a first data type of a first data included in the first payload, and the second line-code is selected for encoding the second payload based on a second data type of a second data included in the second payload. 
     In one example, the selection of a line-code for encoding the payload of a certain frame may be based only on the respective data type of the data included therein. In another example, line-code selection may be further based on other applicable criteria, such as a criterion based on the condition of the communication channel, or any other suitable criterion. The applicable criteria may be based on channel condition such as signal to noise ratio, symbol error rate, and/or any other suitable channel condition. However, in both examples the selection is affected by the respective data type. 
     In one example, the communication channel  406  is characterized by first and second channel conditions, which are respectively associated with the transmission of the first and second frames. In this example, the first and second line-codes may be selected for encoding the first and second payloads, respectively, although the differences between the first and second channel conditions are not enough for implying selection of different line-codes. 
     In one embodiment, the encoder  402  maintains the absolute value of the running disparity, form the beginning of the first frame to the end of the second frame, lower than or equal to a predetermined value K. In one example, K is lower than N′/2. Additionally or alternatively, in one example K is lower than N″/2. 
     In one example, K is lower than N′/4. Additionally or alternatively, K may be lower than N″/4. In one example, K is lower than 3. In one example, K is lower than 2. 
     In some examples, N′ and N″ are equal. Following are some examples regarding the relation between the first and second output sets. In all those examples, N′ and N″ are equal. In one example, the first and second output sets are mutually exclusive to each other, i.e., they do not share any common code word. In another example, the first set and second output sets, are not exclusive to each other, i.e., they share at least one common code word. In one example, the first output set is a subset of a second output set. In one example, the first output set is not a subset of a second output set. 
     In one embodiment, the first output set is a subset of the output set of an 8b/10b line-code. Additionally or alternatively, the second output set may be a subset of the output set of an 8b/10b line-code. 
     In one embodiment, the encoder  402  maintains, from the beginning of the frame to the end of the second frame, transition density equal to or better than a predetermined transition density, which is at least one transition within any sequence of Z consecutive symbols. In one example, Z equals 6. In one example, Z is lower than 6. In one embodiment, the encoder  402  maintains, from the beginning of the frame to the end of the second frame, spectral uniformity equal to or better than a predetermined spectral uniformity. 
     In one example, the communication channel  406  includes an optical fiber. Additionally or alternatively, the communication channel may include a conductive wire, a wireless channel, and/or any other suitable communication channel. In one example, the first header includes an indication that the first payload is encoded utilizing the first line-code, and the second header includes an indication that the second payload is encoded utilizing the second line-code. 
     In one embodiment, the encoder  402  produces an idle sequence  422 , which resides between the first frame  410  and a second frame  416 , and includes code words of length N_idle. In one example, N_idle may be equal to N′. Additionally or alternatively, N_idle may be equal to N″. 
     In one embodiment, the encoder  402  maintains the absolute value of the running disparity, over the first frame, the idle sequence, and the second frame, lower than or equal to K. K may optionally be lower than N_idle/2, or lower than N_idle/4. 
     In one embodiment, the encoder  402  maintains the transition density, over the first frame, the idle sequence, and the second frame, equal to or better than the predetermined transition density. In one embodiment, the encoder  420  maintains the spectral uniformity over the first frame, the idle sequence, and the second frame, equal to or better than the predetermined spectral uniformity. 
     In one example, the set of two or more line-codes further includes a third line-code having input word length M″′, binary code word length N″′, code rate R″′, and minimum Hamming distance D″′, which is different from D″. 
     In one example, the encoder  402  encodes a third frame  424  including a third header  426 , and third payload  428 , and the third payload  428  includes a third data  430  having a third data type, and a fourth data  432  having a fourth data type. Encoder  402  encodes the third header  426 , the third data  430 , and the forth data  432 , utilizing the first, second and third line-codes, respectively, which are selected for encoding the third data  430  and the fourth data  432 , respectively, based on a third and fourth data types, respectively. 
     In one example, D′″ is lower than D″, and R′″ is higher than R″. In this example the fourth data features higher bandwidth efficiency (R″′&gt;R″) but lower error resilience (D′″&gt;D″) than the third data. In another example the third line-code coincides with the first line-code, and D″′ is equal to D′. 
       FIG. 5  illustrates one embodiment of a method for encoding frames utilizing line-codes having different minimum Hamming distances. The method illustrated in  FIG. 5  involves encoding at least first and second frames, utilizing at least first and second line-codes, and transmitting the frames over a communication channel. The method may be performed by a communication node such as the communication node  400  in  FIG. 4 , or by other communication node, or by any other suitable device. In one example, the first and second frames include first and second headers, respectively, and first and second payloads, respectively. The first and second frames may be the first and second frames  410  and  416  in  FIG. 4 . The frames may be received by a second communication node, such as the second communication node  408  in  FIG. 4 , or any other suitable device. The first and second line-codes have binary code word lengths N′ and N″, respectively, and minimum Hamming distances D′ and D″, respectively, where D″ is lower than D′. 
     In one embodiment, the method illustrated in  FIG. 5  includes at least the following steps: In a step  502 , maintaining, from the beginning of the first frame to the end of a second frame, the absolute value of running disparity lower than or equal to a predetermined value K. In one example, K is lower than N′/2. Additionally or alternatively, K may be lower than N″/2. In step  504 , encoding the first header utilizing the first line-code. In step  505 , selecting the first line-code for encoding the first payload based on a first data type of a first data included in the first payload. In step  506 , encoding the first payload utilizing the first line-code. In step  508 , encoding the second header utilizing the first line-code. In step  509 , selecting the second line-code for encoding the second payload based on a second data type of a second data included in the second payload. In step  510 , encoding the second payload utilizing the second line-code. And in step  512 , transmitting the first and second frames over the communication channel. 
     In one example, the communication channel is characterized by first and second channel conditions, which are respectively associated with the transmission of the first and second frames. In this example, the first and second line-codes are selected for encoding the first and second payloads, respectively, although the differences between the first and second channel conditions are not enough for implying selection of different line-codes. 
     In one example, K is lower than N′/4. Additionally or alternatively, K may be lower than N″/4. K may optionally be is lower than 3, or lower than 2. In some examples, N′ and N″ are equal. Following are some examples regarding the relation between the first and second output sets. In all those examples, N′ and N″ are equal. In one example, the first and second output sets are mutually exclusive to each other, i.e., they do not share any common code word. In another example, the first set and second output sets, are not exclusive to each other, i.e., they share at least one common code word. In one example, the first output set is a subset of a second output set. In one example, the first output set is not a subset of a second output set. In one example, the first output set is a subset of the output set of an 8b/10b line-code. Additionally or alternatively, the second output set may be a subset of the output set of an 8b/10b line-code. 
     In one embodiment, the method illustrated in  FIG. 5  further includes maintaining, from the beginning of the frame to the end of the second frame, transition density equal to or better than a predetermined transition density, which is at least one transition within any sequence of Z consecutive symbols. In one example, Z equals 6. In one example, Z is lower than 6. 
     In one embodiment, the method further includes maintaining, from the beginning of the frame to the end of the second frame, spectral uniformity equal to or better than a predetermined spectral uniformity. 
     In one example, the communication channel includes an optical fiber. Additionally or alternatively, the communication channel may include a conductive wire, a wireless channel, and/or any other suitable communication channel. 
     In one example, the first header includes an indication that the first payload is encoded utilizing the first line-code, and the second header includes an indication that the second payload is encoded utilizing the second line-code. 
     The method illustrated in  FIG. 5  may further include an optional step of producing an idle sequence, which may be performed by an encoder, such as the encoder  402  in  FIG. 4 , or by any other suitable encoder. The idle sequence may be the idle sequence  422  in  FIG. 4 . The idle sequence includes code words of length N_idle, which may optionally be equal to N′ or to N″. 
     In one embodiment, the method further involves maintaining the absolute value of the running disparity, over the first frame, the idle sequence, and the second frame, lower than or equal to K. K may optionally be lower than N_idle/2, or lower than N_idle/4. 
     In one embodiment, the method further involves maintaining the transition density, over the first frame, the idle sequence, and the second frame, equal to or better than the predetermined transition density. 
     In one embodiment, the method further involves maintaining the spectral uniformity over the first frame, the idle sequence, and the second frame, equal to or better than the predetermined spectral uniformity. 
     In one example, the set of two or more line-codes further includes a third line-code having input word length M′″, binary code word length N′″, code rate R″′, and minimum Hamming distance D″′, which is different from D″. 
     In one example, the method illustrated in  FIG. 5  may further include an optional step of encoding a third frame including a third header and third payload. The step of encoding the third frame may be performed by an encoder, such as the encoder  402  in  FIG. 4 , or by any other encoder, or by any other suitable device. In addition, the third frame may be the third frame  428  in  FIG. 4 . The third payload includes a third data and a fourth data, having third and fourth data types, respectively. The third header is encoded utilizing the first line-code, and the third and fourth data are encoded utilizing the second and third line-codes, respectively, which are selected based on a third and fourth data types, respectively. In one example, D′″ is lower than D″. In another example the third line-code coincides with the first line-code, and D′″ is equal to D′. 
       FIG. 6  illustrates one embodiment of a communication node  600 . The communication node  600  may be the first node  102  in  FIG. 1 , or the communication node  400  in  FIG. 4 , or any other suitable communication node. The communication node  600  includes at least an encoder  602  and an idle sequence modifier  604 . The communication node  600  may further include a transmitter  606 . 
     The encoder  602  encodes a first frame  608 , a basic idle sequence  610 , and a second frame  612 . The first frame  608 , the basic idle sequence  610 , and the second frame  612  include code words, and the length of the idle sequence (measured in words) is denoted by X. The code words may include binary symbols, i.e. symbols belonging to an alphabet consisting of two values. Alternatively, the symbols may belong to a non-binary alphabet, i.e. an alphabet consisting of more than two values. 
     The idle sequence modifier  604  modifies the basic idle sequence  610  into an idle sequence  614 , by replacing M certain code words out of the X code words of the basic idle sequence with M alternative code words. The M alternative code words are not unique, i.e., each alternative code word belongs to the same output set as the code words of the idle sequence. Therefore, in one example, each alternative code word is equal to at least one code word of the idle sequence. 
     In one example, the first frame  608 , the idle sequence  614 , and the second frame  612  are transmitted by the transmitter  606  over a communication channel  620 , and received by a second communication node  622 , which includes a detector  624  and a decoder  626 . The detector produces a detected first frame  627 , a detected idle sequence  628 , and a detected second frame  629 , also referred to as the received firsts frame, the received idle sequence, and the received second frame. The code words of the detected first frame, the detected idle sequence, and the detected second frame may include one or more erroneous detected symbols, i.e. detected symbols that are different from the respective transmitted symbols. The erroneous symbols are referred to as channel errors. 
     In one example, the second communication node  622  does not know in advance the length X of the idle sequence, and therefore the decoder  624  does not know in advance the starting point of the detected second frame  629 . Furthermore, the decoder  624  is unable to determine the starting point of the detected second frame  629  by inspecting the detected idle sequence  628  per se, since the idle sequence by itself does not indicate its end. 
     The code words of the basic idle sequence  610  are known in advance to the second communication node  622 . For example, the encoder  602  may produce the basic idle sequence utilizing a certain algorithm (e.g. a pseudo random symbol generator) and certain one or more parameters thereof (e.g. initial state of the generator), and the second communication node may produce an identical replica of the basic idle sequence by utilizing the same certain algorithm and the same certain one or more parameters thereof. 
     The decoder  624  compares the detected idle sequence  628  with a replica of the basic idle sequence  610 , and determines the differences between respective code words of the two sequences, which are referred to as the detected differences. In case of no channel errors, the detected differences are identical to the differences between the code words of the basic idle sequence  610  and the respective code words of the idle sequence  614 , the latter differences being accordingly referred to as the error-free differences. As explained above, the error-free differences include X-M zero words, and M non-zero words. 
     A sub-sequence of the error-free differences, which includes the M non-zero words, is referred to as the synchronization sequence. The synchronization sequence is located at a predetermined distance from the end of the idle sequence  614 , and the end of the detected idle sequence  628  may be determined by determining the presence of the synchronization sequence. 
     In one example, M equals one, and the synchronization sequence is a single non-zero word located at a predetermined distance from the end of the idle sequence, for example at the end of the idle sequence. In another example, M is higher than one, and the M non-zero code words are located at predetermined distances from the end of the idle sequence. The M non-zero code words may be consecutive, or not consecutive. 
     The idle sequence modifier  604  determines the value of each alternative code word to be different from the value of the respective certain code word within the basic idle sequence  610 , thereby resulting in the M non-zero words of the synchronization sequence. In one example, the idle sequence modifier  604  determines the value of each alternative code word based only on the value of the respective basic code word appearing in the basic idle sequence  610 , thereby resulting in a deterministic synchronization sequence. In another example, the idle sequence modifier  604  may determine the value of each alternative code word based also on other considerations, thereby resulting in different synchronization sequences. The other consideration may be, for example, line-code related considerations, such as running disparity. 
     The detected sequence of differences may include channel errors. However, in one example, the synchronization sequence facilitates detection thereof also in presence of channel errors, as long as the number of channel errors does not exceed a predetermined threshold. 
     The difference between the idle sequence and the basic idle sequence may be measured using Hamming distance. When the number of alternative code words is one, the decoder decides between to hypotheses: a first hypothesis that the received word represents an original code word of the basic idle sequence, and a second hypothesis that the received word represents an alternative code word. In this example, T is equal to the integer part of (D_idle 1)/2, where D_idle is the Hamming distance between the two hypotheses, i.e., the distance between the alternative code word and the respective code word in the basic idle sequence. 
     In one example, D_idle is equal to or higher than 3. In one example, the code words of the idle sequence have code word length N_idle, and D_idle is higher than or equal to N_idle/2. In one example, at least N_idle 1 symbols, out of the N_idle symbols of the alternative code word, are different from the respective symbols of the certain code word. In this example, D_idle is higher than or equal to N_idle 1. In one example, all N_idle symbols of the alternative code word are different from the respective symbols of the certain code word. In this example, D_idle equals N_idle. 
     In one example, the communication channel  606  includes an optical fiber. Additionally or alternatively, the communication channel may include a conductive wire, a wireless channel, and/or any other suitable communication channel. 
     In one embodiment, the encoder  602  maintains over the idle sequence  614  an absolute running disparity lower than or equal to K, which is lower than N_idle/2. In one example, K is lower than N_idle/4. In one example, K is lower than 3. In one example, K is lower than 2. 
     In one example, the first output set of the first line-code is a subset of the output set of all code words produced by an 8b/10b line-code. In this example, each code word of the basic idle sequence is included within an output set of all code words produced by an 8b/10b line-code. In one example, the alternative code words are also included within the first output set, and therefore each code word of the idle sequence is included within the output set output set of all code words produced by an 8b/10b line-code. In one example, the first line-code is an 8b/10b line-code. 
       FIG. 7  illustrates one embodiment of a method for indicating the end of an idle sequence. The method illustrated in  FIG. 7  may be performed by the first communication node  600  in  FIG. 6 . In addition, the method may be performed by any other communication node, or by any other suitable device. The method includes at least the following steps: 
     In step  702 , encoding a first frame. Step  702  may be performed by the encoder  602  in  FIG. 6 , or by any other suitable encoder. Additionally, the first frame may be the first frame  608  in  FIG. 6 . 
     In step  704 , encoding a basic idle sequence, which includes code words. Step  704  may be performed by the encoder  602  in  FIG. 6 , or by any other suitable encoder. Additionally, the basic idle sequence may be the basic idle sequence  610  in  FIG. 6 . 
     In step  706 , producing an idle sequence by replacing certain M code words of the idle sequence with M alternative code words. Step  706  may be performed by the idle sequence modifier  604  in  FIG. 6 , or by any other suitable device. Additionally, the idle sequence may be the idle sequence  614  in  FIG. 6 . The M alternative code words belong to the same output set as the code words of the idle sequence. 
     And in step  710 , encoding a second frame. Step  710  may be performed by the encoder  602  in  FIG. 6 , or by any other suitable encoder. Additionally, the second frame may be the second frame  612  in  FIG. 6 . 
     The method illustrated in  FIG. 7  may further includes an optional step of transmitting the first frame, the idle sequence, and the second frame over a communication channel, which may be performed by the transmitter  608  in  FIG. 6 , or by any other suitable transmitter. Furthermore, the communication channel may be the communication channel  620  in  FIG. 6 , or any other suitable communication channel. The first frame, the idle sequence, and the second frame may be detected and decoded by a second communication node. The code words of the detected idle sequence may include one or more channel errors. 
     In one example, the second communication node is unable to determine the starting point of the detected second frame by inspection the detected idle sequence per se, because the second communication node does not know in advance the length of the idle sequence, and the idle sequence by itself does not indicate its end. 
     In one embodiment, the code words of the basic idle sequence are known in advance to the second communication node, which compares the received idle sequence with a replica of the basic idle sequence. Based on the sequence of differences between the received idle sequence and replica of the basic idle sequence, the second communication node determines the end of the detected idle sequence, thereby determining the starting point of the detected second frame. Furthermore, the second communication node should be able to determine the end of the detected idle sequence correctly, as long as the number of channel errors does not exceed a predetermined threshold. 
     In one example, the difference between the idle sequence and the basic idle sequence is measured using Hamming distance. When the idle sequence include a single alternative code word, T is equal to the integer part of (D_idle 1)/2, where D_idle is the Hamming distance between the alternative code word and the respective code word in the basic idle sequence. In this example, the second communication node is able to determine the end of the detected idle as long as the number of channel errors within the detected idle sequence is lower than D_idle/2. 
     In one example, D is equal to or higher than 3. In one example, the code words of the idle sequence have code word length N_idle, and D is higher than or equal to N_idle/2. In one example, D_idle is higher than or equal to N_idle-1. In one example, D_idle equals N_idle. 
     In one embodiment, the method further includes maintaining over the idle sequence an absolute running disparity lower than or equal to K, which is lower than N_idle/2. 
     K may optionally be lower than N_idle/4, lower than 3, or lower than 2. 
     In one example, each code word of the basic idle sequence is included within an output set consisting of all code words produced by an 8b/10b line code. In one example, each code word of the idle sequence is included within the output set output set of all code words produced by an 8b/10b line code. 
     In one embodiment, the step  704  of encoding the basic idle sequence further includes maintaining over the basic idle sequence transition density equal to or better than a predetermined transition density. In one example, the predetermined transition density is at least one transition within any sequence of 6 consecutive symbols. In one embodiment, the step  706  of producing the idle sequence further includes maintaining over the idle sequence  614  transition density equal to or better than the predetermined transition density. 
     In one embodiment, the step  704  of encoding the basic idle sequence further includes maintaining over the frame spectral uniformity equal to or better than a predetermined spectral uniformity. 
       FIG. 8  illustrates one embodiment of a communication node  800 . The communication node  800  may be the first node  102  in  FIG. 1 , the communication node  400  in  FIG. 4 , or any other suitable communication node. The communication node  800  includes at least an encoder  802  and an idle sequence modifier  804 . In one embodiment, the communication node  600  further includes a transmitter  806 . 
     In one example, the encoder  802  encodes a first frame  808 , a basic idle sequence  810 , and a second frame  812 . The first frame  808 , the basic idle sequence  810 , and the second frame  812  include symbols. In one example, the symbols are binary symbols, i.e. symbols belonging to an alphabet consisting of two values. Alternatively, the symbols may belong to a non-binary alphabet, i.e. an alphabet consisting of more than two values. 
     In one embodiment, the idle sequence modifier  804  modifies the basic idle sequence  810  into an idle sequence  814 , by replacing M certain symbols of the basic idle sequence with M alternative symbols. The M alternative symbols belong to the same alphabet as the symbols of the idle sequence. Therefore, in one example, each alternative symbol is equal to at least one symbol of the idle sequence. 
     In one example, the first frame  808 , the idle sequence  814 , and the second frame  812  are transmitted by the transmitter  806  over a communication channel  820 , and received by a second communication node  822 , which includes a detector  824  and a decoder  826 . The detector produces a detected first frame  827 , a detected idle sequence  828 , and a detected second frame  829 , also referred to as the received first frame, the received idle sequence, and the received second frame, respectively. In one example, the detected first frame, the detected idle sequence, and the detected second frame may include one or more channel errors. 
     In one example, the decoder  824  is unable to determine the starting point of the detected second frame  829  by inspection the detected idle sequence  828  per se, because the second communication node  822  does not know in advance the length of the idle sequence, and the idle sequence by itself does not include an indication of its end. 
     In one embodiment, the code words of the basic idle sequence are known in advance to the second communication node  822 , and the decoder  824  compares the received idle sequence with a replica of the basic idle sequence. The sequence of differences between the received idle sequence and the basic idle sequence is referred to as the detected sequence of differences. The differences may be represented in binary symbols, where zero indicates no difference between the two symbols being compared. The decoder  824  correlates the detected sequence of differences with a synchronization sequence, which represents the difference between the basic idle sequence and the idle sequence. Based on the correlation, the second communication node determines the end of the detected idle sequence, thereby determining the starting point of the detected second frame. 
     In one example, the synchronization sequence is a Barker sequence of length 13. The weight (i.e. number of non-zero symbols) of this synchronization sequence is 9, and its maximum side-lobe is one. Therefore, in this example, T is equal to 3. 
     In one embodiment, the difference between the received idle sequence and the basic idle sequence is determined by a detector configured to identify a synchronization sequence. In one example, the difference is a single symbol and deterministic, therefore the detector compares the symbol with the basic idle sequence. In another example, the difference is not deterministic, and the operation of the encoder is reconstructed in the receiver, which checks whether the hypothesis that the idle sequence ends at a certain symbol is correct, or the hypothesis that the idle sequence does not end at the certain symbol is correct. 
       FIG. 9  illustrates one embodiment of a communication system  900 . The communication system  900  may be the communication system  100  in  FIG. 1 , or any other suitable communication system. The communication system  900  includes at least a first node  902  and a second node  904 . In embodiment, the first node  902  includes an encoder  906  and a transmitter  908 . In one example, the first node  902  communicates with the second node  904  over a communication channel  910 . The second node may include a detector  912  and a decoder  914 . 
     In one example, the encoder  906  encodes a first frame  916 , an idle sequence  918 , and a second frame  920 , utilizing one or more line-codes, and the transmitter  908  transmits them over the communication channel  910 . The first frame  916 , idle sequence  918 , and second frame  920  may be the frame  112 , idle sequence  122 , and following frame  124 , respectively, in  FIG. 1 . The first frame, idle sequence, and second frame may be received (detected) by the detector  912  and decoded by the decoder  914 , and the received first frame  922 , received idle sequence  924 , and received second frame  926  may include channel errors. In one example, the length X of the idle sequence is not known in advance to the second node  914 . 
     In one example, the idle sequence  918  is based on a basic idle  930  having X code words, which is obtained utilizing a first line-code having a binary code word length N_idle. The idle sequence differs from the basic idle sequence in M out of the X code words. In one embodiment, the encoder  906  determines X minus M code words of the idle sequence to be equal to the corresponding X minus M code words of a basic idle sequence. The encoder  906  further determines the remaining M code words of the idle sequence to be alternative code words, which are different from the corresponding M code words of the idle sequence. In one example, the M alternative code words are not unique, i.e., each alternative code word belongs to the same output set as the code words of the idle sequence. Therefore, in one example, each alternative code word is equal to at least one code word of the basic idle sequence. In one example, each alternative code word belongs to the same output set as the code words of the second frame. Therefore, in one example, each alternative code word is equal to at least one code word of the second frame. 
     In one example, M equals one, and the single alternative code word is located at a predetermined distance from the end of the idle sequence. In one example, the single alternative code word is located at the end of the idle sequence. In another example, M is higher than one. In one example, the M alternative code words are located at predetermined distances from the end of the idle sequence. The M alternative code words may be consecutive, or not consecutive. 
     In one embodiment, the encoder  906  maintains over the idle sequence absolute value of running disparity lower than or equal to K. In one example K is lower than N_idle/2. 
     In one example, the first frame  916 , idle sequence  918 , and second frame  920  are transmitted by the transmitter  908  over the communication channel  910 , and received by the second node  904 . The detector  912  of the second node produces a detected first frame  622 , a detected idle sequence  624 , and a detected second frame  626 , also referred to as the received firsts frame, the received idle sequence, and the received second frame. The code words of the detected first frame, the detected idle sequence, and the detected second frame may include one or more channel errors. 
     In one example, the second node  904  does not know in advance the length X of the idle sequence  918 , and therefore the decoder  614  does not know in advance the starting point of the detected second frame  626 . Furthermore, the decoder  614  is unable to determine the starting point of the detected second frame  626  by inspection the detected idle sequence  624  per se, since the idle sequence by itself does not indicates its end. 
     In one embodiment, the code words of the basic idle sequence are known in advance to the second node  904 . In one embodiment, the decoder  914  compares the detected idle sequence  624  with a replica of the basic idle sequence, thereby producing a detected sequence of differences, which is the sequence of differences between the code words of the detected idle sequence  624  and the respective code words of the basic idle sequence. Based on the detected sequence of differences, the decoder  914  determines the end of the detected idle sequence  624 , thereby determining the starting point of the detected second frame  626 . Furthermore, the decoder  914  should be able to determine the end of the detected idle sequence  624  correctly, as long as the number of channel errors within the detected idle sequence  624  does not exceed a predetermined threshold. 
     In one example, the encoder  906  obtains the basic idle sequence  930  by encoding the output of a pseudorandom bit generator. In one example, the pseudorandom bit generator starts at a predetermined state. The decoder  914  may produce a replica of the basic idle sequence by utilizing the same pseudo random bit generator, starting at the same predetermined state. 
     In one example, the second frame  920  includes a header and a payload. In one example, the encoder encodes the header utilizing a second line-code, and the first output set of all words of the first line-code and the second output set of all code words produced by the second line-code are mutually exclusive to each other. 
     In one example, the encoder  906  encodes the payload utilizing a third line-code, and the third output set of all code words produced by the third line-code and the first output set have at least one common code word. 
     In one example, the encoder  906  encodes the payload utilizing the first line-code. 
     In one example, the encoder  906  encodes the payload utilizing a third line-code, and each one of the code words of the idle sequence belongs to a third output set of all code words produced by the third line-code. 
     In one example, the difference between the idle sequence and the basic idle sequence is measured using Hamming distance. When the idle sequence include a single alternative code word, the decoder  914  is able to determine the end of the detected idle sequence  924  as long as the number of channel errors within the detected idle sequence is lower than D_idle/2, where D_idle is the Hamming distance between the alternative code word and the respective code word in the basic idle sequence. 
     In one example, D_idle is equal to or higher than 3. In one example, D_idle is higher than or equal to N_idle/2. In one example, D_idle is higher than or equal to N_idle 1. In one example, D_idle equals N_idle. 
     In one example, the communication channel  910  includes an optical fiber. Additionally or alternatively, the communication channel may include a conductive wire, a wireless channel, and/or any other suitable communication channel. 
     In one example, the encoder  906  maintains the absolute value of the running disparity, from a beginning of a first frame to an end of the second frame, lower than or equal to K. 
     In one example, K is lower than N_idle/4. In one example, K is lower than 3. In one example, K is lower than 2. 
     In one example, the first output set of the first line-code is a subset of the output set of all code words of an 8b/10b line-code. In this example, each code word of the basic idle sequence is included within an output set of all code words produced by an 8b/10b line-code. In one example, the alternative code words are also included within the first output set, and therefore each code word of the idle sequence is included within the output set output set of all code words produced by an 8b/10b line-code. In one example, the first line-code is an 8b/10b line-code. 
       FIG. 10  illustrates one embodiment of a method for indicating the end of an idle sequence. The method illustrated in  FIG. 10  may be performed by the first communication node  900  in  FIG. 9 . In addition, the method may be performed by any other communication node, or by any other suitable device. The method involves encoding and transmitting a first frame, a second frame, and an idle sequence residing between the first frame and the second frame. The first frame, idle sequence, and second frame include code words. The code words of the idle sequence have binary code word length N_idle. The method includes at least the following steps: In step  1002 , maintaining, from the beginning of the first frame to the end of the second frame, the absolute value of running disparity lower than or equal to K. In one example, K is lower than N_idle/2. In step  1004 , encoding the first frame. In step  1006 , encoding a basic idle sequence utilizing a first line-code having a binary code word length N_idle. In step  1008 , producing an idle sequence by replacing M code words of the basic idle sequence with M alternative code words. In one example, each one of the M alternative code words is equal to at least one code word of the basic idle sequence. In step  1010 , encoding the second frame. In step  1012 , transmitting the first frame, the idle sequence, and the second frame over a communication channel. And in step  1014 , receiving the second frame by a second communication node. In one example, the second communication node is unable to determine a starting point of the second frame based only on the idle sequence and the second frame, but is able to determine the starting point of the second frame based on the difference between the basic idle sequence and the idle sequence. In one example, the difference between the basic idle sequence and the idle sequence facilitates determine the starting point of the second frame even in presence of one or more channel errors. 
     In one example, the step  1006  of encoding the basic idle sequence includes encoding the output of a pseudorandom bit generator, which may start at a predetermined state. 
     In one example, the header of the second frame is encoded utilizing a second line-code, where the first and second output sets of all code words of the first and second line-codes, respectively, are mutually exclusive to each other. 
     In one example, the payload of the second frame is encoded utilizing a third line-code, where the first and third output sets of all code words of the first and third line-codes, respectively, have at least one common code word. 
     In one example, the payload of the second frame is encoded utilizing the first line-code. 
     In one example, payload of the second frame is encoded utilizing a third line-code, where each one of the code words of the idle sequence belongs to a third output set of all code words of the third line-code. 
     In one example, the difference between the idle sequence and the basic idle sequence is measured using Hamming distance. When the idle sequence includes a single alternative code word, the second communication node is able to determine the end of the idle sequence as long as the number of channel errors within the idle sequence is lower than D_idle/2, where D_idle is the Hamming distance between the alternative code word and the respective code word in the basic idle sequence. 
     In one example, D_idle is equal to or higher than 3. In one example, D_idle is higher than or equal to N_idle/2. In one example, D_idle is higher than or equal to N_idle 1. In one example, D_idle equals N_idle. 
     In one example, K is lower than N_idle/4. K may optionally be lower than 3, or lower than 2. 
     In one example, the first output set of the first line-code is a subset of the output set of all code words produced by an 8b/10b line-code. In this example, each code word of the basic idle sequence is included within an output set of all code words produced by an 8b/10b line-code. In one example, the alternative code words are also included within the first output set, and therefore each code word of the idle sequence is included within the output set output set of all code words produced by an 8b/10b line-code. In one example, the first line-code is an 8b/10b line-code. 
       FIG. 11  illustrates one embodiment of a communication link configured to indicate a configuration change by replacing certain idle code words with bitwise complement code words. Transmitter  650  includes an encoder configured to encode a first frame  658 , a basic idle sequence  660 , and a second frame  662 , wherein the first frame, the basic idle sequence, and the second frame include code words. The transmitter  650  further includes an idle sequence modifier  654  configured to produce an idle sequence  664  by replacing certain M code words of the basic idle sequence  660  with M bitwise complement code words, wherein each bitwise complement code word appears in the basic idle sequence  660 . Bitwise complement, also known as bitwise NOT, applies logical negation on each bit, forming the ones&#39; complement of a given binary value. For unsigned integers, the bitwise complement of a number is the mirror reflection of the number across essentially the half-way point of the unsigned integer&#39;s range. 
     Receiver  672  receives from the transmitter  650 , over link  670 , the first frame  658 , the idle sequence  664 , and the second frame  662 . The receiver  672  acquires the basic idle sequence  660  before it receives the idle sequence  664 , for example, during the setup phase of the communication link, as part of a handshaking processes between the transmitter  650  the receiver  672 , and/or from a preprogrammed memory. Then the receiver  672  identifies a change in configuration of the communication link based on a difference between the idle sequence and the basic idle sequence. Optionally, the receiver may also identify the starting point of the second frame based on the difference between the idle sequence and the basic idle sequence. Optionally, the receiver may utilize different subsets of the difference between the idle sequence and the basic idle sequence in order to identify the change in configuration of the communication link and in order to identify the starting point of the second frame. Additionally or alternatively, the M bitwise complement code words may be located at M predetermined locations relative to the end of the idle sequence. In one example, the receiver  672  includes a detector  674  configured to detect the first frame  677  and the second frame  679 . The receiver  672  may also include a decoder  676  as known in the art. 
     The receiver  672  may identify different types of configuration changes based on the difference between the idle sequence and the basic idle sequence, such as a modulation change, a change in number of active communication channels between the transmitter and the receiver, and/or a change in power consumption of the communication link. In one embodiment, the code words are symbols, and the communication link utilizes 8b/10b encoding. 
       FIG. 12  illustrates one embodiment of a method for indicating a configuration change of a communication link by replacing certain idle code words with bitwise complement code words. The method includes the following steps: In step  250 , encoding a first frame, a basic idle sequence, and a second frame. Wherein the first frame, the basic idle sequence, and the second frame include code words. In step  252 , producing, by a transmitter, an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words. Wherein each bitwise complement code word appears in the basic idle sequence. In step  254 , receiving, by a receiver, the first frame, the idle sequence, and the second frame. Wherein the basic idle sequence is known to the receiver. And in step  256 , identifying, by the receiver, a change in configuration of the communication link based on a difference between the idle sequence and the basic idle sequence. 
       FIG. 13  illustrates one embodiment of a method for indicating a configuration change of a communication link by replacing certain idle code words with bitwise complement code words. The method includes the following steps: In step  350 , encoding a first frame, a basic idle sequence, and a second frame. Wherein the first frame, the basic idle sequence, and the second frame include code words. In step  352 , producing, by a transmitter, an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words. Wherein each bitwise complement code word appears in the basic idle sequence. In step  354 , receiving, by a receiver, the first frame, the idle sequence, and the second frame. Wherein the basic idle sequence is known to the receiver. In step  356 , calculating, by the receiver, a difference between the idle sequence and the basic idle sequence. In step  358 , utilizing the difference to identify at least a non-zero subset of the M code words that were replaced with the bitwise complement code words. And in step  360 , identifying a change in configuration of the communication link based on the non-zero subset of the M code words. 
     The identifying of the change may be achieved in various ways. Optionally, the identifying of the change is based on positions of the non-zero subset of the M code words relative to the starting point of the second frame. Optionally, the identifying of the change is based on a pattern generated by the non-zero subset of the M code words. Optionally, the identifying of the change is based on the number of the non-zero subset of the M code words. 
       FIG. 14  illustrates one embodiment of a communication link that implements frequent flow control without using unique symbols or designated packets, by replacing certain idle words with bitwise complement words. The communication link includes at least a first device  140 , a full-duplex communication link  142 , and a second device  144 . In one embodiment, the first device  140  sends to a second device  144 , over the full-duplex communication link  142 , high throughput packet communication of more than 1 Gb/s. The second device  144  includes at least a buffer  146  for temporarily storing the high throughput packet communication, a basic idle code word sequence calculator  148  for calculating a basic idle code word sequence known to the first device  140 , an idle sequence modifier  150  for producing an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words, and a transmitter  152  configured to transmit the idle sequence to the first device  140  during inter-packet gap. Wherein each bitwise complement code word appears in the basic idle sequence and the M bitwise complement code words are indicative of the fullness of the buffer. The first device receives the idle sequence and determines at least one of the following, based on a difference between the idle sequence and the basic idle sequence: (i) that the buffer is full or expected to get full and thus the first device stops sending packets to the second device, (ii) that the rate is proper and thus the first device continues sending packets at essentially the same rate to the second device, (iii) that the buffer is almost empty and thus the first device can increase the rate of sending packets to the second device. 
     In one embodiment, the first device  140  receives the idle sequence and determines, based on the difference between the idle sequence and the basic idle sequence, that the buffer  146  of the second device  144  is full or expected to get full, and thus the first device stops sending packets to the second device. Sometime later, the second device sends a second idle sequence to the first device during a following inter-packet gap, and the first device determines, based on a difference between the second idle sequence and the basic idle sequence, that the buffer is not full and thus the first device resumes sending packets to the second device. Sometime later, the second device sends a third idle sequence to the first device during a third inter-packet gap, and the first device determines, based on a difference between the third idle sequence and the basic idle sequence, that the buffer is not expected to get full in the current rate, and thus the first device increases its rate of sending packets to the second device. Sometime later, the second device sends a fourth idle sequence to the first device during a fourth inter-packet gap, and the first device determines, based on a difference between the fourth idle sequence and the basic idle sequence, that the buffer is expected to get full in the current rate, and thus the first device reduces its rate of sending packets to the second device to a non-zero rate. 
     In one embodiment, the first device  140  identifies starting point of a frame based on the difference between the second idle sequence and the basic idle sequence. Optionally, the M bitwise complement code words are located at M predetermined locations relative to the end of the idle sequence. 
     In one embodiment, the first device  140  receives the idle sequence and determines, based on the difference between the idle sequence and the basic idle sequence, that the buffer  146  of the second device  144  is full or expected to get full, and thus the first device stop sending packets to the second device. Sometime later, the second device sends a second idle sequence to the first device during a following inter-packet gap, and the first device determines, based on a difference between the second idle sequence and the basic idle sequence, that the buffer is not full and thus the first device resumes sending packets to the second device. Sometime later, the second device sends a third idle sequence to the first device during a third inter-packet gap. The second device is further configured to encode in the third idle sequence to be indicative of the space available in the buffer, and the first device is further configured to determine, based on a difference between the third idle sequence and the basic idle sequence, the amount of data to be transmitted to the second device based on the indication of the space available in the buffer. Alternatively, the second device receives a low bandwidth signal, and then sends a third idle sequence to the first device during a third inter-packet gap. Then the first device extracts the low bandwidth signal from a difference between the third idle sequence and the basic idle sequence. 
     Optionally, the full-duplex communication link  142  is an asymmetric full-duplex communication link, and the maximum throughput from the first device  140  to the second device  144  is at least ten times higher than the maximum throughput from the second device  144  to the first device  140 . Optionally, the code words are symbols, and the communication link utilizes 8b/10b encoding. 
       FIG. 15  illustrates one embodiment of a method for implementing a frequent flow control without using unique symbols or designated packets. The method includes the following steps: In step  153 , sending, from a first device to a second device over a full-duplex communication link, high throughput packet communication of more than 1 Gb/s. In step  154 , temporarily storing the high throughput packet communication in a buffer of the second device. In step  155 , calculating, by the second device, a basic idle code word sequence known to the first device. In step  156 , producing an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words. In step  157 , transmitting the idle sequence to the first device during inter-packet gap. Wherein each bitwise complement code word appears in the basic idle sequence, and the M bitwise complement code words are indicative of the fullness of the buffer. In step  158 , receiving the idle sequence by the first device. And in step  159 , determining, by the first device, based on a difference between the idle sequence and the basic idle sequence, that the buffer is full or expected to get full, and thus the first device stop sending packets to the second device. 
     In one embodiment, the second device sends a second idle sequence to the first device during a second inter-packet gap, and the first device determines, based on a difference between the second idle sequence and the basic idle sequence, that the buffer is not full and thus the first device resumes sending packets to the second device. Sometime later, the second device sends a third idle sequence to the first device during a third inter-packet gap, and the first device determines, based on a difference between the third idle sequence and the basic idle sequence, that the buffer is not expected to get full in the current rate, and thus the first device increases its rate of sending packets to the second device. Sometime later, the second device sends a fourth idle sequence to the first device during a fourth inter-packet gap, and the first device determines, based on a difference between the fourth idle sequence and the basic idle sequence, that the buffer is expected to get full in the current rate, and thus the first device reduces its rate of sending packets to the second device to a non-zero rate. 
     In an alternative embodiment, the second device sends a second idle sequence to the first device during a second inter-packet gap, and the first device determines, based on a difference between the second idle sequence and the basic idle sequence, that the buffer is not full and thus the first device resumes sending packets to the second device. Sometime later, the second device sends a third idle sequence to the first device during a third inter-packet gap, and the first device determines, based on a difference between the third idle sequence and the basic idle sequence, an amount of data to be transmitted to the second device. 
     In still alternative embodiment, the second device sends a second idle sequence to the first device during a second inter-packet gap, and the first device determines, based on a difference between the second idle sequence and the basic idle sequence, that the buffer is not full and thus the first device resumes sending packets to the second device. Sometime later, the second device receives a low bandwidth signal, and then sends a third idle sequence to the first device during a third inter-packet gap. The first device extracts the low bandwidth signal from a difference between the third idle sequence and the basic idle sequence. 
     In one embodiment, the first device further identifies the starting point of a frame based on the difference between the second idle sequence and the basic idle sequence. Optionally, the M bitwise complement code words are located at M predetermined locations relative to the end of the idle sequence. 
     Herein, a predetermined value, such as a predetermined confidence level or a predetermined threshold, is a fixed value and/or a value determined any time before performing a calculation that compares a certain value with the predetermined value. A value is also considered to be a predetermined value when the logic, used to determine whether a threshold that utilizes the value is reached, is known before start of performing computations to determine whether the threshold is reached. 
     In this description, references to “one embodiment” mean that the feature being referred to may be included in at least one embodiment of the invention. Moreover, separate references to “one embodiment” or “some embodiments” in this description do not necessarily refer to the same embodiment. Additionally, references to “one embodiment” and “another embodiment” may not necessarily refer to different embodiments, but may be terms used, at times, to illustrate different aspects of an embodiment. 
     The embodiments of the invention may include any variety of combinations and/or integrations of the features of the embodiments described herein. Although some embodiments may depict serial operations, the embodiments may perform certain operations in parallel and/or in different orders from those depicted. Moreover, the use of repeated reference numerals and/or letters in the text and/or drawings is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. The embodiments are not limited in their applications to the details of the order or sequence of steps of operation of methods, or to details of implementation of devices, set in the description, drawings, or examples. Moreover, individual blocks illustrated in the figures may be functional in nature and therefore may not necessarily correspond to discrete hardware elements. 
     While the methods disclosed herein have been described and shown with reference to particular steps performed in a particular order, it is understood that these steps may be combined, sub-divided, and/or reordered to form an equivalent method without departing from the teachings of the embodiments. Accordingly, unless specifically indicated herein, the order and grouping of the steps is not a limitation of the embodiments. Furthermore, methods and mechanisms of the embodiments will sometimes be described in singular form for clarity. However, some embodiments may include multiple iterations of a method or multiple instantiations of a mechanism unless noted otherwise. For example, when a processor is disclosed in one embodiment, the scope of the embodiment is intended to also cover the use of multiple processors. Certain features of the embodiments, which may have been, for clarity, described in the context of separate embodiments, may also be provided in various combinations in a single embodiment. Conversely, various features of the embodiments, which may have been, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. Embodiments described in conjunction with specific examples are presented by way of example, and not limitation. Moreover, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the embodiments. Accordingly, this disclosure is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims and their equivalents.