Patent Publication Number: US-RE40918-E

Title: Method and device for transmitting data frames

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This is a Reissue of application Ser. No.  09 / 184 , 254  filed Jan.  27 ,  1999  now U.S. Pat. No.  6 , 546 , 025 , which was a based on PCT/FR 98 / 00352 , filed Feb.  23 ,  1998 . The entire disclosures of the prior applications, are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     
         1 . Field of the Invention  
      
     
     The present invention concerns data transmission techniques, in particular data transmission techniques in which the transmitted data is structured in blocks known as frames comprising payload data and data referred to herein as auxiliary data enabling frame synchronization of a receiver to a transmitter. 
       2 . Description of the Related Art  
     Auxiliary data of the above kind generally includes a synchronization pattern (flag) formed of a predetermined sequence of bits usually placed at the head of the frame. 
     Because it is necessary to guard against the payload data imitating the synchronization pattern, auxiliary data of the above kind usually further comprises synchronization bits intended to prevent such imitation interleaved with the payload data: if the synchronization pattern is a sequence of eight bits at 0, for example, bits at 1 are inserted every seven bits. 
     However, inserting auxiliary data of the above kind is clearly to the detriment of the payload data, which therefore raises a problem of frame efficiency, especially if it is required to increase the transmitted payload without increasing the bit rate. 
     SUMMARY OF THE INVENTION  
     To this end, the present invention consists in method of transmitting data frames including payload data and auxiliary data enabling frame synchronization of a receiver to a transmitter, said method being essentially characterized in that:
         it has two transmission modes:
           a first mode used on initialization or in the event of loss of frame synchronization by a receiver referred to as a first receiver whereby a transmitter referred to as a first transmitter sends to said first receiver frames including sufficient auxiliary data to enable said first receiver to acquire said frame synchronization in a reliable fashion, said auxiliary data including a synchronization pattern,   a second mode used otherwise whereby said first transmitter sends frames including insufficient auxiliary data to enable said first receiver to acquire said frame synchronization in a reliable fashion but, by virtue of knowledge of said frame synchronization acquired previously, in said first mode, enabling loss of frame synchronization to be determined,   
           loss of frame synchronization by said first receiver is signalled to said first transmitter by a second transmitter associated with said first receiver in a common transmission equipment changing from said second transmission mode to said first transmission mode and detection of said change of transmission mode by a second receiver associated with said first transmitter in the same transmission equipment, and   said second transmitter transmits modified frames in said second mode including no payload data sequence imitating the synchronization pattern of the first mode, also referred to as illegal sequences, which are replaced by substitution data enabling the receiver to insert said illegal sequences into the received payload data.       

     Accordingly, in the event of initialization or of loss of synchronization, the first transmission mode is used to enable the receiver to acquire frame synchronization in a reliable manner and other than in either of the above two cases the second transmission mode is used so that the payload transported can be increased, the system reverting to the first mode as soon as loss of synchronization is detected in the second mode. 
     Furthermore, any problem due to a risk of erroneous detection of this change from the second mode to the first mode itself due to a risk of imitation of the synchronization pattern of the first mode by the payload data transmitted in the second mode is avoided by the transmission of said modified frames in the second mode. 
     The present invention also consists in a corresponding transmission device. 
     The present invention also consists in a modified frame of the above kind. 
     The present invention is applicable to frames like V110 frames for transporting data at bit rates that are not sub-multiples of 64 kbit/s, for example those obtained for the data transmission services offered by the GSM (Global System for Mobile Communications) network, in 64 kbit/s channels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Other aims and features of the present invention will become apparent on reading the following description of embodiments of the invention given with reference to the accompanying,/drawings, in which: 
         FIG. 1  is a diagram illustrating one example of a frame transmitted in said first mode, 
         FIG. 2  is a diagram illustrating a first example of a frame transmitted in said second mode, 
         FIG. 3  is a diagram illustrating a second example of a frame transmitted in said second mode, 
         FIG. 4  is a block diagram explaining the principle of transmission using said first and second transmission modes, for simplicity in the case of a unidirectional link, 
         FIG. 5  is a block diagram illustrating a transmission device using said first and second transmission modes in the case of a bidirectional link, 
         FIG. 6  is a diagram illustrating one example of a payload data sequence to be transmitted used to explain the principle of forming said modified frames, 
         FIGS. 7 ,  8  and  9  are diagrams respectively illustrating first, second and third examples of a modified frame enabling the transmission of a payload data sequence of the above kind, 
         FIG. 10  is a diagram illustrating one example of the content of substitution sequences transmitted in a modified frame of the above kind, 
         FIG. 11  is a diagram illustrating a method of producing modified frames in accordance with said first example or in accordance with said second example from an incoming payload data stream, 
         FIG. 12  is a diagram illustrating a method of producing modified frames in accordance with said third example from an incoming payload data stream. 
         FIG. 13  is a diagram illustrating a method of extracting payload data. 
         FIG. 14  is a diagram illustrating a method of extracting a payload data stream from modified frames receiving in accordance with said second example or in accordance with said third example. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION  
     In the example of a frame transmitted in said first mode illustrated in  FIG. 1 , said auxiliary data includes a synchronization pattern referred to as the first synchronization pattern F 1  formed of a predetermined sequence of bits which here are at the head of the frame and of synchronization bits f 1 , f 2 , . . . , fn, interleaved with the payload data D 1 , D 2 , . . . , Dn and intended to prevent imitation of the synchronization pattern by the payload data. 
     In the example of a frame transmitted in said second mode illustrated in  FIG. 2 , said auxiliary data includes only one synchronization pattern referred to as the second synchronization pattern F 2  which has a length different than (in particular less than) that of the first synchronization pattern F 1 . 
     In the example of a frame transmitted in said second mode illustrated in  FIG. 3 , said auxiliary data includes only error correcting code data C obtained by applying a block code error correcting code to the payload data to be transmitted inside the same block or frame. 
     Note that in this last example said auxiliary data further protects the payload data transmitted in this way against transmission errors. 
     Examples of frames other than those illustrated by  FIGS. 1 through 3  are naturally possible; also, frames transmitted in said second mode can be obtained using, as the auxiliary data, both a synchronization pattern and error correcting code data, possibly shorter in length than those respectively used in the embodiments illustrated by  FIGS. 2 and 3 . 
     In the various examples, the auxiliary data of frames transmitted in said first mode enables the receiver to acquire frame synchronization in a reliable fashion and the auxiliary data of frames transmitted in said second mode is insufficient to enable the receiver to acquire such synchronization in a reliable fashion. Nevertheless, by virtue of a knowledge of said frame synchronization acquired beforehand, in said first mode, it enables loss of frame synchronization to be detected. 
     Furthermore, to speed up the return to frame synchronization of the receiver in the event of loss of such synchronization, the frames transmitted in said first mode can include bits having the same value as the synchronization bits instead of payload data. 
     The device shown in  FIG. 4  is used to transfer data frames between a transmitter  1  (TR) and a receiver  2  (RR). 
     The transmitter  1  includes means  3  for sending frames in said first mode, means  4  for sending frames in said second mode (MSM2) and sending mode selector means  5  (MSM) for selecting one or other of the means  3  or  4 , as appropriate, in the present instance to feed incoming payload data “d” to one or other of the aforesaid means, as appropriate. 
     In addition to conventional means not referred to here because they are not directly relevant to the invention for restoring the transmitted payload data, the receiver  2  includes means  6  for detecting frame synchronization (FSDM) in the first mode, means  7  for detecting loss of frame synchronization (LSFM) in the second mode and receive mode selector means  8  for selecting one or other of the aforesaid means, as appropriate. 
     The means  6  for detecting frame synchronization in the first mode use techniques known in themselves to observe the received data through a sliding window and to determine if the synchronization pattern F 1  has been found or not for each position of the sliding window. 
     The means  7  for detecting loss of frame synchronization in the second mode include simplified frame synchronization detector means  9  (SFSDM) activated by a frame clock  10  (FC) locked onto a position depending on knowledge previously acquired of said frame synchronization, this prior knowledge being in this instance supplied by the first mode frame synchronization detector means  6 . The simplified frame synchronization detector means  9  are simplified in the sense that, unlike the frame synchronization detector means  6 , they do not observe the received data through a sliding window but instead through a single window the position of which is therefore determined by said previously acquired knowledge of frame synchronization. 
     For example, in the case of the second transmission mode shown in  FIG. 2 , the simplified synchronization detector means  9  include means for determining to what extent the sequence of data selected by means of said single window corresponds to the synchronization pattern F 2 . 
     In the case of the second transmission mode shown in  FIG. 3  the simplified synchronization means  9  include means for determining to what degree the data obtained by applying the same block code as on transmission to the payload data forming a received frame delimited in this way by virtue of the position of said observation window corresponds to the error correcting code data produced by the transmitter. 
     The send mode selector means  5  shown in  FIG. 4  receive a signal S 1  that can indicate initialization of the transmission procedure and a signal S 2  that can indicate loss of frame synchronization by the receiver. The selector means  5  select the means  3  for sending frames in said first mode if the signal S 1  indicates initialization or if the signal S 2  indicates loss of frame synchronization by the receiver or the means  4  for sending frames in said second mode otherwise. 
     The receive mode selector means  8  receive the signal S 1  that can indicate initialization of the transmission procedure and the signal S 2  that can indicate detection of loss of synchronization, the latter coming in this instance directly from the second mode frame synchronization loss detector means  7 . The selector means  8  select the first mode frame synchronization detector means  6  if the signal S 1  indicates initialization or if the signal S 2  indicates loss of synchronization or the loss of synchronization detector means  7  otherwise. 
     Accordingly, the first transmission mode is used to enable the receiver to acquire frame synchronization in a reliable fashion in the event of initialization or of loss or synchronization and the second transmission mode is used otherwise, so enabling the payload transported to be increased, the system returning to the first mode as soon as loss of synchronization is detected in the second mode. 
     The  FIG. 5  diagram shows how the loss of synchronization information carried by the signal S 2  is fed from the receiver to the transmitter in the case of bidirectional transmission, schematically represented in the form of two unidirectional links  11  and  12 . For simplicity, items common to FIG.  4  and to  FIG. 5  have the same reference symbols. Like the link from  FIG. 4 , the link  11  connects a transmitter  1 , referred to in this context as the first transmitter, of transmission equipment  13  (TE) to a receiver  2 , referred to here as the first receiver, of transmission equipment  14 . Likewise the link  2  connects a transmitter  15 , here called the second transmitter, of the transmission equipment  14  to a receiver  16 , here called the second receiver, of the transmission equipment  13 . 
     To feed the loss of synchronization information carried by the signal S 2  from the receiver  2  to the transmitter  1  the second transmitter  15  includes means  17  (SM1) for sending frames in said first mode, means  18  (SM2) for sending frames in said second mode and send mode selector means  19  (SMSM) for selecting one or other of the aforementioned means, as appropriate, here for applying incoming payload data “d” to one or other of said means, as appropriate. 
     In addition to means not specifically shown for restoring the transmitted payload data, the second receiver  16  includes means  20  for detecting a change in the mode of transmitting frames (DTFM) sent by the transmitter  15  from the second mode to the first mode. 
     The signal S 2  fed to the transmitter  1  comes from the means  20  for detecting a change in the mode of transmitting frames sent by the transmitter  15 , the means  17  for sending frames in said first mode being selected in said transmitter  15  if the signal S 2  from the receiver  2  indicates detection of loss of synchronization. 
     The means  20  for detecting a change in the mode of transmission (DCTM) include, operating in parallel:
         first mode frame synchronization detector means  201  (FSDM1) employing techniques known in themselves to observe the data received through a sliding window and to determine, for each position of said sliding window, whether the first mode synchronization pattern has been found or not, and   second mode frame synchronization loss detector means  202  (FSDM2) which can include, like the means  7 , simplified frame synchronization detector means activated by a frame clock locked onto a position depending on a knowledge of said frame synchronization previously acquired in the first mode.       

     Accordingly, if either of the following two conditions is satisfied: loss of synchronization in the second mode (detected by the means  202 ), or re-acquisition of synchronization in the first mode (detected by the means  201 ), as indicated by the means  203  (L2R1), the means  20  detect a change of mode. 
     Note that even if, for simplicity, the foregoing description refers only to the units needed to transmit data “d” from the transmission equipment  13  to the transmission equipment  14 , the transmission principle is of course symmetrical and applies equally for transmission of data “d” from the transmission equipment  14  to the transmission equipment  13 . 
     Problems arising from the risk of erroneous detection of this change from the second mode to the first mode due to the risk of imitation of the first mode synchronization pattern by the payload data transmitted in the second mode are avoided, as will now be explained with reference to the subsequent figures of the accompanying drawings, in which the frames concerned are second mode frames unless otherwise specified. 
     As shown in  FIG. 6 , a sequence of payload data to be transmitted can always be broken down into sequences, referred to herein as authorized sequences, that do not themselves contain any sequence imitating the synchronization pattern and sequences, referred to herein as illegal sequences, imitating the synchronization pattern.  FIG. 6  shows a sequence of payload data of the above kind divided into three authorized sequences D 1 , D 2 , D 3 , and into two illegal sequences X 1 , X 2 , for example, said payload data sequence here including, in this order, the sequences D 1 , X 1 , D 2 , X 2 , D 3 . 
     As shown in  FIGS. 7  to  9  the modified frame in accordance with the invention enabling a payload data sequence of the above kind to be transmitted does not include any illegal sequence such as sequence X 1  or X 2  but to the contrary includes so-called substitution data enabling the illegal sequences to be put back into the received payload data at the receiver. 
     The substitution data advantageously comprises data of two types:
         data referred to herein as operating data, enabling equipment receiving these modified frames to determine to what degree the transmitted payload data sequence differs from the payload data sequence to be transmitted, that is to say the degree to which illegal sequences are present in the sequence of payload data to be transmitted, and   data referred to herein as address data, enabling equipment receiving these modified frames to determine more precisely the location or locations where an illegal sequence must be inserted into the received payload data.       

     In the example shown said address data A 1  and A 2  is contained in substitution sequences S 1  and S 2  which are respectively substituted for the illegal sequences X 1  and X 2  (also referred to as substituted sequences). Thus the substitution sequence S 1  contains an address A 1  which in the example shown enables an equipment receiving such modified frames to insert the illegal sequence X 1  between the received sequences corresponding to the authorized sequences (non-substituted sequences) D 1  and D 2  and the substitution sequence S 1  therefore contains an address A 2  enabling the receiver to insert the illegal sequence X 2  between the received sequences corresponding to the authorized (non-substituted) sequences D 2  and D 3 . 
     By way of example, said operating data can include:
         first operating data S 0  ( FIGS. 7 through 9 ) transmitted at a particular location after the synchronization pattern concerned, i.e. pattern F 2  (for example immediately after that synchronization pattern), and indicating if the transmitted payload data sequence differs or not from the payload data sequence to be transmitted, that is to say whether the transmitted payload data sequence contains substituted sequences or not,   second operating data S 10  and S 20  ( FIGS. 7 through 9 ) advantageously transmitted in said substitution sequences S 1  and S 2  and indicating if each of said substitution sequences is the last of the transmitted frame or not.       

     In another example, not specifically illustrated, said operating data could also include the number (possibly 0) of substituted sequences contained in the transmitted payload data sequence. 
       FIG. 10  is a diagram illustrating one example of the content of a substitution sequence. 
     In this example a substitution sequence S includes:
         said second operating data S 10 , for example, consisting for example of a bit at 1 or 0 according to whether the substitution sequence concerned is the last in the transmitted frame or not,   address data A 1  or A 2  consisting, for example, of a binary number on “n” bits indicating the rank of a bit in a sequence of 2 n  payload bits to be transmitted,   data P protecting the substitution sequences against transmission errors (the protection data consisting of a parity bit, for example), and   locking data V to prevent an illegal sequence being reproduced by inserting a substitution sequence into the frame transmitted (the locking data comprising a bit at 1, for example, in the example considered here of an illegal sequence made up only of bits at 0).       

     In the example illustrated by  FIG. 7  the combination of the operating data and the address data (i.e. the combination of the first operating data S 0  and the substitution sequences S 1 , S 2  in turn including second operating data S 10  and S 20  and address data A 1  and A 2 ) is transmitted before the payload data. 
     This example is also more particularly suited to the situation in which the time-delay after which the payload bits are extracted from a frame of the above kind at the receiver may not be fixed, the time-delay in this example essentially being variable and depending on the number of sequences present in the frame, which can be unacceptable in some applications, for example the application previously mentioned to transmission within infrastructures of a mobile radio network such as the GSM network. 
     This is avoided in the example illustrated by FIG.  8 . 
     To be more precise, in that example, the first substitution sequence S 1  is transmitted at a particular location after the first operating data S 0  in turn transmitted at a particular location after the synchronization pattern F 2  (for example, the first substitution sequence S 1  is transmitted immediately after the first operating data S 0  which is transmitted immediately after the synchronization pattern F 2 ) and the second substitution sequence S 2  is transmitted instead of the first substituted sequence X 1  (or, more generally, the nth substitution sequence would be transmitted instead of (n−1)th substituted sequence). 
     The examples illustrated by  FIGS. 7 and 8  are more particularly suited to the situation in which there is no constraint in respect of the time-delay after which such frames can be produced in this way from an incoming payload data stream, the time-delay in these examples being equal to the total time to analyze an incoming payload data sequence, which is necessary if said operating data is to be obtained, and which can be prohibitive in some applications and in particular in the previously mentioned application to transmission of data within infrastructures of a mobile radio network such as the GSM network in particular. 
     The example illustrated by  FIG. 9  can satisfy a constraint of the above kind, however. In this example said operating data is inverted relatively to that shown in  FIGS. 7 and 8 , in the sense that:
         the first operating data S 0  indicating if the transmitted payload data sequence differs or does not differ from the payload data sequence to be transmitted is not inserted after the synchronization pattern of the frame concerned, but at the end of the frame, i.e. before the synchronization pattern of the next frame, and   the second operating data S 10  and S 20  no longer indicates if the corresponding substitution sequence S 1  and S 2  is the last from the start of the frame or not, but instead whether it is the last from the end of the frame or not.       

     Note that a frame of the above kind then cannot be processed by the receiver before it has been received completely, but that this does not introduce any significant time-delay, unlike the analysis time that would otherwise have been required at the transmitter. 
     Furthermore, it is necessary to begin analyzing said sequence of payload data to be transmitted before it is possible to produce a corresponding frame, whence a short time-delay in the transmission of such frames, as shown in FIG.  12 . 
     Also, in the example shown, the second (i.e. here the last) substitution sequence S 2  is transmitted at a particular location before the first operating data S 0  which is transmitted at a particular location before the synchronization pattern of the next frame (for example the last substitution sequence is transmitted immediately before the first operating data S 0  which is transmitted immediately before the synchronization pattern F 2  of the next frame), and the first substitution sequence S 1  is transmitted instead of the second (i.e. here the last) substituted sequence X 2  (or more generally the (n−1)th substitution sequence would be transmitted in place of the nth substituted sequence). 
       FIG. 11  is a diagram illustrating a method of producing modified frames of the above kind that is equally valid for the  FIG. 7  example and for the  FIG. 8  example. 
     The method includes the following steps:
         calculating substitution data from incoming payload data, and   sending substitution data calculated in this way and authorized sequences in the required order.       

       FIG. 12  shows an algorithm illustrating one method of producing modified frames of the above kind in the example illustrated by  FIG. 9  from an incoming payload data stream. 
     The method includes the following steps for producing the payload of each frame to be transmitted:
         initializing to a value equal to zero a variable P corresponding to a current address,   initializing a variable A to a value corresponding to an impossible address value, for example equal to −1,   waiting to receive 2M bits of the incoming stream (where M denotes the number of bits in the synchronization pattern) in an input buffer register,   detecting if the first M bits received stored in the input buffer register correspond to an illegal sequence X:   if an illegal sequence is detected:
           if the variable A is equal to said impossible value:
               removing said M first bits received from the input buffer register,   reinitializing the variable A to the value of the variable P,   incrementing the variable P by a value equal to M,   detecting if the current address corresponds to an end of frame:
                   if an end of frame is detected:    if A is not equal to said impossible value; sending the substitution sequence S(A) corresponding to an illegal sequence to be inserted, at the receiver, at the address A, and then sending first operating data S 0  indicating the presence of substitution sequencers) in the frame (indicated by “send modification indication”), and returning to the beginning of the algorithm,    if A is equal to said impossible value, sending first operating data S 0  indicating absence of substitution sequence(s) in the frame (indicated by “send non-modification indication”) and then returning to the beginning of the algorithm,   if no end of frame is detected:    returning to the illegal sequence detecting step,   
                   
               if A is not equal to said impossible value: sending the substitution sequence S(A),   
           if no illegal sequence is detected:
           removing the first bit received from the buffer register,   sending that bit,   incrementing the variable P by 1,   returning to the end of frame detection step.   
               

     Thus a method of producing frames in accordance with either  FIG. 11  or  FIG. 12  is used in the means  18  from  FIG. 5  for sending frames in said second mode. 
     The present invention also consists in a device for sending modified frames of the above kind, the device including means for implementing the various steps of the sending processes described. 
     As implementing such means will not represent any particular problem to the skilled person, they will not be described in more detail than stating their function. Generally, a device of the above kind includes:
         means for calculating substitution data from incoming payload data, and   means for sending payload data containing no illegal sequences and substitution data calculated as above for forming said frame.       

       FIG. 13  is a diagram illustrating a method of extracting payload data from received frames in the case of the  FIG. 7  example. 
     The method includes the steps of:
         analyzing substitution data, and   inserting illegal sequences into the received payload data in the manner determined by analyzing said substitution data.       

       FIG. 14  is a diagram illustrating a method of extracting a payload data stream from received frames valid for the example illustrated by  FIG. 8 and , subject to modification indicated below, the example illustrated by FIG.  9 . 
     The method includes the following steps, each time a synchronization pattern is detected:
         initializing a variable “a” corresponding to a current address,   analyzing first operating data S 0  in accordance with the detected synchronization pattern and indicating if the transmitted payload data sequence differs from the payload data sequence to be transmitted or not:
           if the payload data sequence transmitted does not differ from the payload data sequence to be transmitted, extracting said transmitted payload data to form the outgoing payload data stream,   if the payload data sequence transmitted differs from the payload data sequence to be transmitted:
               analyzing the first substitution sequence S 1  in accordance with the first operating data S 0  to determine the address A 1  at which to insert the first illegal sequence, i.e. in the present context also the address of the second substitution sequence S 2 , and to determine from the operating data S 10  contained in said substitution sequence S 1  whether that substitution sequence is the last of the frame or not,   comparing the current address “a” and the address A 1 :
                   while “a” is less than A 1 , extracting the payload data bit at the current address to form said outgoing payload data stream and then correspondingly incrementing the current address “a” and returning to the step of comparing the current address “a” and the address A 1 ,   if “a” is equal to A 1 , inserting an illegal sequence to form said outgoing payload data stream and then correspondingly incrementing the current address “a”,   
                   in parallel with the above, if the substitution sequence S 1  is not the last one of the frame, analyzing the second substitution sequence S 2  at the address A 1  to determine the address A 2  at which to insert the second illegal sequence (i.e. also the address of any substitution sequence S 3 ) and for determining, from the operating data S 20  contained in the substitution sequence S 2 , whether that substitution sequence is the last of the frame or not,
                   while “a” is less than A 2 , extracting the payload data bit at the current address to form said outgoing payload data stream and then correspondingly incrementing the current address “a” and returning to the step of comparing the current address “a” and the address A 2 ,   if “a” is equal to A 2 , inserting an illegal sequence to form said outgoing payload data stream and then correspondingly incrementing the current address “a”,   
                   and so on, up to the last sequence Sd of the received frame symbolized in  FIG. 9  by a loop in which an index i is varied until Si becomes equal to Sd, in which case:
                   while “a” is less than address Ap of the last payload bit of the incoming frame, extracting the payload data bit at the current address to form said outgoing payload data stream and then correspondingly incrementing the current address, and   if “a” is equal to Ap returning to the start of the algorithm.   
                   
               
               

     The same process holds good for received frames in the case of the example illustrated by  FIG. 9  provided that the synchronization pattern to be detected is considered to be that of the next frame and provided that the order in which the substitution sequences are analyzed is reversed, as explained in connection with FIG.  9 . 
     A method of extracting payload data in accordance with either  FIG. 13  or  FIG. 14  is therefore used in the  FIG. 5  process to extract payload data in the second receiver  16 . 
     The present invention also consists in a device for receiving modified frames of the above kind, the device including means for implementing the various steps of the receive processes described. 
     As implementing such means will not represent any particular problem to the skilled person, they will not be described in more detail than stating their function. Generally, a device of the above kind includes:
         means for analyzing substitution data of incoming frames, and   means for inserting illegal sequences into the payload data extracted from said incoming frames at locations obtained by analyzing said substitution data to form said outgoing payload data streams.       

     Further, an exemplary embodiment of the present invention can be written as computer programs. Codes and code segments for accomplishing the exemplary embodiment of the present invention can be constructed by programmers skilled in the art to which the present invention pertains. Also, the computer programs are stored in a computer readable media and are read by a computer to be executed.