FR2782216A1 - Selective acknowledgement or rejection method for data frames in point-to-point communication, storing sequence numbers of frames not successfully received - Google Patents

Abstract

Sequence numbers of frames not successfully received are stored at the receiver and if loss of at least one frame is detected the sequence numbers of all consecutive lost frames are found. A selective acknowledgement is sent to the transmitting station requesting the retransmission of the lost frames based on the frames preceding and following them. Independent claims are included for a device for selectively accepting and rejecting data frames, a communication system, a communication station implementing the method of the invention and such a station included in a computer, printer, facsimile, scanner, or digital camera.

Description

The present invention relates generally to the transmission of

  point-to-point data between communication stations, and more specifically selective acknowledgment and selective rejection of data in a

  data communication system.

  In known manner, a transmitting station transmits data, for example in the form of successive data frames intended for a

receiving station.

  CCITT Recommendation X.141, Blue Book, Volume VIII, Fascicle VIII.3, IXth Plenary Meeting, November 1988, defines a procedure for the receiving station to request the transmitting station to retransmit a lost and acknowledged data frame correctly received frames, preceding the lost frame. This procedure cannot be

  used for only one lost frame at a time.

  Document EP-A-162 478 describes a method for the receiving station to request from the transmitting station the retransmission of a group of frames of consecutive lost data. This process does not allow

  to acknowledge correctly received frames.

  The present invention aims to remedy the drawbacks of the prior art, by providing a method and a device which allow a receiving station both to acknowledge correctly received frames and to request the retransmission of a group of consecutive data frames. lost. To this end, the invention proposes a method for selective acknowledgment and selective rejection of data frames, implemented in a data receiving station, each data frame being identified by a sequence number, characterized in that it comprises the steps of: - memorization of the sequence numbers of the frames received and not yet acknowledged, - verification of the loss of at least one frame, and in the event of a positive response, - search for all the consecutive numbers of frame sequence lost, and - transmission of a selective acknowledgment and selective rejection frame to the data transmitting station, which acknowledges the received frames not yet acknowledged which precedes and follows the lost frames, and which requests the retransmission of the frames lost. Correlatively, the invention proposes a device for selective acknowledgment and selective rejection of data frames, implemented in a data receiving station, each data frame being identified by a sequence number, characterized in that it comprises : - means for memorizing the sequence numbers of the frames received and not yet acknowledged, - means for verifying the loss of at least one frame, and in the event of a positive response, - means for searching for all consecutive numbers sequence of lost frames, and means for transmitting a selective acknowledgment and selective rejection frame to the data transmitting station, which acknowledges the received frames not yet acknowledged which precedes and which follow the frames

  lost, and which requests the retransmission of the lost frames.

  The present invention makes it possible to limit the number of retransmissions of data frames when one or more data frames are not

  received by the destination station.

  When one or more data frames are not received by the destination station, the present invention makes it possible to acknowledge the following frames received, without waiting for the lost frames to have been returned and correctly received, which limits the memory space required in the station

  transmitter to store the frames not yet acknowledged.

  In addition, the present invention makes it possible to emit selective rejections

recursively.

  According to a preferred characteristic, the verification step comprises the comparison of the sequence number of each frame received with a first counter whose value represents the sequence number of the next expected frame, said value being incremented by one each time that the sequence number of a received frame is equal to the value of the first counter. This characteristic makes it possible to determine whether a frame has

been lost.

  According to a preferred characteristic, the search step includes setting a second counter to the value of the last lost frame sequence number, to represent the upper bound of the consecutive lost frame numbers. This feature is useful for selective rejection of

lost frames.

  According to another preferred characteristic, the method comprises, when at least one frame has been lost, setting a third counter to the value of the sequence number of the last frame received, increased by one, to represent the upper bound sequence numbers of the frames received according to the lost frames. This feature is useful for

  selective acknowledgment of received frames.

  According to a preferred characteristic, the selective acknowledgment and selective rejection frame comprises two parameters respectively equal to the current value of the second and third counters. So the values of these

  counters are transmitted to the data transmitting station.

  According to a preferred characteristic, the method is implemented

  after each reception of data frame.

  The device includes means for implementing

previous features.

  The invention also relates to a communication station, a computer, a printer, a fax machine, a scanner, a digital photographic camera, a digital camera and a communication system, integrating means for implementing the method according to

  the invention, or integrating the device according to the invention.

  The device, the communication station, the computer, the printer, the fax machine, the scanner, the digital camera, the digital camera and the communication system have advantages

  analogous to those of the method previously described.

  A recording means, integrated or not in the device, possibly removable, stores a program implementing the

  method of selective acknowledgment and selective rejection of data frames.

  The characteristics and advantages of the present invention will appear more clearly on reading a preferred embodiment illustrated by the attached drawings, in which: - Figure 1 shows a communication station implementing the present invention, - the FIG. 2 represents data processing layers, - FIG. 3 represents a data frame according to the present invention, - FIG. 4 represents an example of data transmission between two communication stations, - FIGS. 5, 6 and 7 represent an algorithm for reception of an information frame by a communication station, according to the present invention, and - Figures 8, 9, 10 and 11 represent an algorithm for processing after reception of a supervision frame by a communication station communication,

according to the present invention.

  According to the embodiment chosen and shown in FIG. 1, the invention applies to a data transmission system in point-to-point mode, between two remote communication stations. Communication stations exchange data via a communication channel, the nature of which is not relevant to the present invention. This channel can be a link

  wired, or a radio channel, or an infrared channel, for example.

  A communication station SC is connected, or is integrated, to a computer terminal TI of the personal computer type. The means of communication can be produced in the form of a computer card integrated into the computer. The TI terminal can also be a printer, a fax machine, a scanner, a digital camera, a digital camera, a communication system or a file server, for example

example.

  The communication station includes a data bus 1 to which are connected: - a microcontroller 2, - a read only memory (ROM) 3, - a random access memory (RAM) 4, - an interface circuit 5 with a transmitter circuit / receiver 6 which transmits and receives data via the communication channel, - an interface circuit 7 with the application part of the computer terminal TI which processes the data received and to be transmitted via the communication station. The ROM 3 includes a program for selective acknowledgment and selective rejection of data frames according to the present invention, as

exposed in the continuation.

  The RAM 4 includes registers in which variables used by the invention are stored. These variables will be described

in the following.

  The RAM 4 also stores the data to be transmitted to a remote station, as well as the data received from a remote station. The invention relates to the data link layer of the OSI model (Open Systems Interconnection, in French Interconnection of Open Systems). This model classically defines seven layers, of which only three

are considered here.

  As shown in FIG. 2, for the communication station SC to transmit data to another station, the data link layer CL of the station SC receives data packets PDU from the network layer CR. A TD data frame is then formed, which will be described below. As will be seen, a sequence number is allocated to each data frame in order to be able to recognize and reorder the received data frames. The TD data frame is then transmitted to the physical layer CP which adds an EP header to the TD frame. This is then transmitted by the transmitting station in question to a station

  recipient, via the communication channel.

  On reception, the data link layer CL receives the data frames from the physical layer CP and restores the data packets

  in an orderly fashion at the network layer CR.

  Figure 3 shows a TD data frame as it is formed at the data link layer. The frame TD comprises a control field CC, an information field C1 containing the data packets and an error control field FCS. The FCS error control field is used on reception to detect whether the TD frame has been corrupted or

not during its transmission.

  The CC control field has several parameters. The

  first parameter is an M mode which defines the type of the data frame.

  Three types of frames exist: - information frames, called I-frames, which are used to transmit data coming from the network layer, - supervisory frames called S-frames, which are used to transmit supervision information ,

  - the combined information and supervision frames, called C-

  frames, which are used to transmit both data and supervisory information. The second parameter of the CC control field is a function F which defines the function of the supervision frames. Of course, the information and supervision frames (C- frames) are also concerned, and this

  which also applies to them.

  Three types of supervision frames exist: - a supervision frame RR, which indicates that the station is ready to receive an information frame, which acknowledges the information frames previously received, and which if necessary cancels a "receiver" condition busy "which had previously been indicated to the remote station by a

RNR supervision frame.

  - an RNR supervision frame, which indicates a "receiver busy" condition, that is to say a temporary incapacity of the station to receive information frames, when the memory is full, and which acknowledges the frames information received. When a transmitting station receives an RNR type frame, it interrupts the transmission of the information frame until the

  reception of an RR supervision frame.

  - an SREJ supervision frame, called a selective rejection frame. Conventionally, an SREJ supervision frame requests the retransmission of a frame, and acknowledges the information frames received before the frame to

retransmit.

  The present invention increases the functionalities of the supervision frame SREJ, so as to be able to request the retransmission of a block of frames instead of a single frame, and to be able to acknowledge the frames which are

  received before and after the block of lost frames.

  For this, the control field CC of a supervision frame SREJ comprises four parameters NS, NR, NL and NA. NS and NR parameters are known from CCITT recommendation X.141, blue book, volume

  VIII, booklet VIII.3, IXth plenary assembly, November 1988.

  The NS parameter is the transmission sequence number assigned to the frame by the transmitting station. This number is assigned at the data link layer to a frame received from the network layer. The sequence number is used to determine the order in which the data must be restored to the network layer of the station receiving the data. If a frame is transmitted several times, its sequence number is the same for

all transmissions.

  The NR parameter is the reception sequence number. The sequence number in reception is equal to the sequence number in

  transmission of the next information frame expected by the receiving station.

  The NL parameter according to the invention is a sequence number used for selective retransmission and selective rejection, as described in the

after.

  The parameter NA according to the invention is the acknowledgment number

  selective used for a selective acknowledgment which will be described below.

  An information frame includes the NS and NR parameters, a supervision frame includes the NR, NL and NA parameters, and a frame

  information and supervision comprises the four parameters.

  According to the present invention, when a station sends an SREJ frame to another station, this means that the latter must retransmit the frames whose sequence number in transmission is between the values of the parameters NR and NL, and that the frames whose the sequence number in transmission is strictly between NL and NA, as well as the frames whose

  number is less than NR-1, are acknowledged.

  The value of the parameters NR, NL and NA is determined respectively by the value of three state variables VR, VL and VA which are

  managed by each communication station.

  The state variable VR is equal to the sequence number in transmission

  of the next information frame that the station in question is to receive.

  The recovery state variable VL is equal to the sequence number in transmission minus one of the first frame which was received by

  the station considered after the loss of a frame or a block of frames.

  The selective acknowledgment state variable VA is equal to the sequence number in transmission of the next frame to be received by the station considered and of which it is the first transmission. When no frame has been lost previously, the value of the variable VA is equal to that of the

variable VR.

  With reference to FIG. 4, an example of data transmission requires the sending of an SREJ supervision frame to perform a rejection

  selective and a selective acknowledgment according to the invention.

  We consider here two communication stations A and B which communicate in point-to-point mode. Station A transmits data to station B. We consider a one-way communication here, but the invention applies equally well to a two-way communication. In

  in all cases, only one communication is considered at a given time.

  Each communication station capable of transmitting data includes an FE transmission window and an FA anticipation window

stored in RAM 4.

  The FE transmission window contains the list of sequence numbers of the frames which have been transmitted to the remote station and for which the

  transmitting station has not yet received an acknowledgment.

  The frames whose sequence number in transmission NS is contained in the transmission window FE of station A are stored in the RAM of station A. A frame is erased from RAM when the station

  transmitter A receives the acknowledgment corresponding to this frame.

  The FE transmission window contains a maximum of TFE frame sequence numbers. When it is full, no other frame can more

  be transmitted to a remote station.

  The anticipation window FA also contains frame sequence numbers and is defined by two terminals IFA and SFA. The lower limit IFA is equal to the sequence number of the oldest frame awaiting acknowledgment. The upper bound SFA is equal to the number of

  sequence of the most recent frame that was sent.

  All the frames whose sequence number is between IFA and SFA have been transmitted to the data link layer of receiving station B but have not yet been transmitted to the network layer of receiving station B. The window d anticipation contains maximum TFA numbers of

  frame sequence. The value of TFA is at least equal to the value of TFE.

  Having a larger anticipation window than the transmission window makes it possible to have more memory size for processing the reception of new data. The windows FE and FA make it possible to know at all times the difference between the number of frames transmitted by the network layer of the transmitting station A and the number of frames actually received by the network layer of the receiving station B. As soon as a frame is acknowledged, its sequence number is removed from the FE transmission window and it is deleted from the RAM. However, the sequence number of this frame is not removed from the anticipation window

  FA only if its frame sequence number is equal to the lower bound IFA.

  In fact, the FA anticipation window contains sequence numbers which

are necessarily successive.

  When the anticipation window is full, station A can no longer transmit other frames to station B. On the other hand, for reception, each communication station B includes a reception window FR which contains the list of sequence numbers of the frames which have been received correctly and which have not yet been transmitted to the network layer. The reception window contains at

  maximum TFR frame sequence numbers.

  The operation of stations A and B is generally described

  by the algorithms of Figures 5 to 11.

  Figures 5, 6 and 7 show the algorithm for receiving an information frame. This algorithm is stored in memory 3 of each communication station, and can be stored in whole or in part in any information storage means capable of cooperating with the microcontroller 2. This storage means can be read by a computer or by a microprocessor. This storage means is integrated or not to the device, and can be removable. For example, it can include a magnetic tape, a floppy disk, a smart card or a CD-ROM (compact disk with fixed memory).

  The algorithm of FIG. 5 comprises steps El00 to E104.

  The step E100 is a test to determine whether a valid information frame, that is to say not erroneous, is received. This frame contains the parameters NS and NR. When the response to the test is positive, step E100 is followed by step E101. The processing of erroneous frames is also carried out,

  conventional manner and will not be described here.

  In step E101, the sequence number NS of the frame is written in the reception window FR. It should be noted that if the reception window is full, the receiving station B sends an RNR type supervision frame to

  transmitting station A, to tell it not to send any more frames.

  The next step E102 is a test to determine whether the value of the

  sequence number NS is equal to the current value of the state variable VR.

  This test makes it possible to check whether the information frame received is that expected by the station considered. It should be noted that the state variables VA, VL and VR are initialized to zero when the system is started up or during a specific reset operation. The sequence number of the first frame

transmitted is zero.

  When the response is positive, step E102 is followed by step E103, in which the frame reception procedure is called. This

  procedure will be described with reference to figure 6.

  When the answer is negative in step E102, this step is followed by step E104, in which the error processing procedure of

  sequence is called. This procedure will be described with reference to Figure 7.

  Figure 6 describes the processing performed upon receipt of a

  frame, in the form of an algorithm comprising steps E201 to E210.

  This processing follows a positive response in step E102, that is to say

  that the frame received is the frame expected by the station considered.

  Step E201 is the transmission to the network layer of station B of the data contained in the information field C1 of the frame being processed. The frame sequence number NS is then deleted from the

reception window FR.

  The next step E202 is a test to determine if the state variable

  VR is equal to the state variable VA.

  When the answer is positive, it means that no frame has been lost previously. Step E202 is then followed by step E203 in which the state variable VA is incremented by one to remain equal to the

  variable VR, which will be incremented later.

  When the answer is negative in step E202, this means that at least one frame has been lost. This step is followed by step E204 in which the state variable VR is incremented by one. The value of the state variable VR represents the number of the next frame expected by the station

  considered. Step E203 is also followed by step E204.

  Step E204 is followed by step E205 which is a test for determining whether a sequence number equal to the variable VR is stored in the reception window FR, which makes it possible to check whether the frame having an equal sequence number to the state variable VR has already been received. If the answer is positive, step E205 is followed by step E201 previously described. If the answer is negative in step E205, this step is followed by

  step E206 which is a test to determine whether the reception window is empty.

  When this window is empty, it means that there is no more frame waiting

  and the reception procedure is finished.

  When the reception window is not empty, step E206 is followed by step E207 in which the recovery state variable VL is initialized to the current value of the state variable VR. Indeed, we are here in the case where there is at least one frame in the reception window FR whose sequence number is different from the current value of VR, which means that at least one frame has been lost. You must then request a selective rejection

  for the lost frame (s), as explained below.

  The next step E208 is a test to determine whether the sequence number equal to the recovery state variable VL increased by one unit is present in the reception window FR, that is to say whether the frame having the sequence number following the value of the state variable VL has been correctly received. If the answer is negative, this step is followed by step E209 in which the value of the state variable VL is incremented by one. Step E209 is followed by step E208. The value of the state variable VL is thus incremented up to the largest value of the lost frame sequence number. Steps E208 and E209 thus make it possible to determine the number of missing frames which will have to be retransmitted by station A. If the response to step E208 is positive, step E208 is followed by step E210 to which a selective rejection is sent to the data transmitting station for the frames whose sequence number is between the values of the state variables VR and VL, as well as an acknowledgment of the frames whose numbers are strictly less than VR and strictly between VL and go. For this, station B sends to station A a SREJ type supervision frame with the parameter NR equal to the variable VR, the parameter NL equal to the variable VL and the parameter NA equal to the variable VA. If station B has an information frame to transmit to station A at the same time, it sends

a combined frame (C-frame).

  Note that in the absence of a lost frame, the procedure

  Acknowledgment is conventional and will not be described here.

  FIG. 7 represents the processing carried out when a station receives a frame whose sequence number is not equal to the current value of the reception state variable VR, that is to say that the frame received n is not there

  frame expected by the station considered.

  This processing is represented in the form of an algorithm comprising steps E301 to E308 and follows a negative response in the step

E102.

  Step E301 is a test to determine whether the number of frames

  contained in the reception window FR is strictly greater than 1.

  If the answer is negative, this means that the reception window FR contains only the frame being processed, and that the frames whose sequence number is between the state variable VR and the number of the

  frame being processed decreased by one have been lost.

  Step E301 is then followed by step E302 in which the state variable VL is set to the value of the sequence number NS of the frame received, reduced by one. The state variable VL will be used to request the retransmission of frames whose sequence number in transmission is between VR and NS-1. In the next step E303, the state variable VA is set to the value of the sequence number NS of the frame received, increased by one, to be equal to the sequence number of the next expected frame, and of which it is the first transmission. The state variable VA will be used for the selective acknowledgment of

the frame which has just been received.

  The next step E304 is the sending of a selective rejection, that is to say of a supervision frame SREJ with the parameter NR equal to the state variable VR, the parameter NL equal to the variable VL and the parameter NA equal to variable VA, for requesting the retransmission of the frames whose sequence number is between NR and NL, and for selectively acknowledging the frames whose sequence number is strictly between NL and NA, and the frames of which

  the number is strictly less than NR.

  When in step E301 the reception window contains more than one frame sequence number, this step is followed by step E305 which is a test to determine whether the value of the sequence number NS of the frame being processing is strictly greater than the value of the state variable

  VR and less than or equal to that of the state variable VL.

  If the response is positive, it means that the frame received has been retransmitted following a previous selective rejection message. However, the frame received is not the first frame in the expected frame block. Step E305 is then followed by step E302 to redo a selective rejection. The present invention thus makes it possible to issue selective rejections recursively. In the event that a group of frames is lost, a retransmission of these frames is requested. If a subgroup of these frames is lost again during

  retransmission, it is possible to request the retransmission of this sub-

  group. If the answer is negative, it means that the frame received has been transmitted for the first time and that it is not one of the frames for which selective rejection has been requested. In this case, the selective acknowledgment state variable VA is updated taking account of the reception of this new frame. Step E305 is then followed by step E306 in which it

  is tested if the value of the parameter NS is equal to that of the state variable VA.

  If the answer is positive, then step E306 is followed by step E307 in which the state variable VA is incremented by one, to designate the sequence number of the next expected frame and for

  which a selective rejection is not requested.

  Step E307 is followed by step E308 in which it is tested if a sequence number equal to the current value of the state variable VA is stored in the reception window FR, that is to say if a frame with a

  sequence number equal to state variable VA has been received.

  If the answer to step E308 is negative, the procedure is terminated.

  If the response to step E308 is positive, this step is followed by step E307 already described. The value of the state variable VA is thus

  incremented to the value of the next frame to be received.

  In step E306, if the value of the parameter NS is not equal to that of the state variable VA, this means that the frame received is not a frame

  expected. There is no need to change the value of the VA state variable.

The procedure is then finished.

  Figures 8 to 11 show the processing performed by a communication station when it receives an SREJ supervision frame with the parameters NR, NL and NA. The station in question has previously sent data frames to another station and is receiving the supervision frame

SREJ in return.

  This processing is carried out in the form of an algorithm stored in memory 3 of each communication station, and can be stored in whole or in part in any information storage means capable of cooperating with the microcontroller 2. This storage means can be read by a computer or a microprocessor. This storage means is integrated or not to the device, and can be removable. For example, it may include a magnetic strip, a floppy disk, a smart card or a CD-ROM (disc

compact with frozen memory).

  The processing of FIG. 8 is represented in the form of a

  algorithm comprising steps E400 to E403.

  Step E400 is the reception of an SREJ supervision frame. The

  SREJ supervision frame includes the NR, NL and NA parameters.

  The next step E401 is the acknowledgment of the frames whose sequence number is less than or equal to NR-1. These frames are erased from the RAM 4 and their sequence numbers are erased from the windows.

  FE transmission and FA anticipation. This step is detailed below.

  The next step E402 is an acknowledgment of the frames whose sequence number is strictly between NL and NA. These frames are erased from the RAM 4 and their sequence numbers are erased from

  FE transmission windows. This step is detailed below.

  The next step E403 is the selective retransmission of the frames whose sequence number is between NR and NL. This step is

detailed below.

  FIG. 9 represents the processing carried out to acknowledge the frames whose sequence number is less than or equal to NR-1. This treatment

comprises steps E501 to E504.

  Step E501 is a test to determine whether the value of NR-1 is between the lower and upper limits IFA and SFA of the anticipation window FA. If the answer is negative, it means that there is no

  frame to be acknowledged and the procedure is finished.

  If the response to step E501 is positive, this step is followed by step E502 in which: - the acknowledgment wait delay associated with the frame whose sequence number is equal to IFA is interrupted, - the frame whose sequence number equal to IFA is deleted from the RAM 4, - the sequence number equal to IFA is deleted from the window

  FE transmission and FA anticipation window.

  Step E502 is followed by step E503 which is a test to determine if the value of NR-1 is equal to the lower limit IFA. If the answer is positive, it means that there is no more frame to acknowledge and the

procedure is finished.

  If the answer is negative, it means that at least one frame remains to be acknowledged. Step E503 is then followed by step E504 at which the lower limit IFA of the anticipation window FA is incremented by one

  unit. Step E504 is followed by step E502 previously described.

  FIG. 10 represents the processing carried out to selectively acknowledge the frames whose sequence number is between NL + 1

  and NA-1. This processing includes steps E601 to E603.

  Step E601 is the initialization of a working variable VT to the value NL + 1. The following step E602 comprises the following operations - the frame whose sequence number is equal to VT is erased from the RAM 4, - the sequence number equal to VT is erased from the transmission window FE,

  - the working variable VT is incremented by one.

  The next step E603 is a test to determine whether the working variable VT is equal to the value of NA. If the answer is negative, step E603

  is followed by step E602 previously described.

  If the answer at step E603 is positive, this means that there is no longer

  of frame to be acknowledged selectively and this procedure is finished.

  FIG. 11 represents the processing carried out for selectively retransmitting the frames whose sequence number is between NR and

  NL. This processing includes steps E701 to E704.

  In step E701, the acknowledgment wait timers respectively associated with the frames whose sequence numbers are

  between NR and NL are interrupted.

  In the next step E702, a working variable VT1 is initialized to the value NR. The next step E703 is the transmission to the communication channel of the frame whose sequence number is equal to VT1. The

  working variable VT1 is incremented by one.

  The next step E704 is a test to determine whether the working variable VT1 is less than the value NL. If the answer is positive, this means that there is still a frame to be transmitted, then step E704 is followed by step

  E703. Otherwise, the processing is finished.

  Referring again to Figure 4, a particular example is

now developed.

  At time t0, station A transmits to station B the information frame whose sequence number is equal to 0. The sequence number 0 is then stored in the transmission window FE and in the window

FA anticipation.

  The information frame contains the parameter NS equal to zero.

  Frame number 0 is correctly received by station B at time tl and sequence number 0 is stored in the reception window FR of station B. In station B, the reception state variable VR is equal to the

value of NS, ie zero.

  Frame number 0 can be transmitted to the upper layer. The state variables VR, VA and VL are then incremented by one and the sequence number 0 is deleted in the reception window FR of station B. At time t2, station A transmits the frame of information whose sequence number is equal to 1. The sequence number 1 is then

  stored in the FE transmission window and in the FA anticipation window.

  Frame number 1 is not received by station B. At time t3, station A transmits the information frame whose sequence number is equal to 2. The sequence number 2 is then

  stored in the FE transmission window and in the FA anticipation window.

  Frame number 2 is not received by station B. At time t4, station A transmits the information frame whose sequence number is equal to 3. The sequence number 3 is then

  stored in the FE transmission window and in the FA anticipation window.

  Frame number 3 is correctly received at time t5 by station B. The sequence number 3 is stored in the reception window of station B. The sequence number is compared with the current value of the state variable VR. Since there is no equality, there is an error in the

reception sequence.

  The recovery status variable VL takes the value of the number of

  sequence of the last frame received (frame number 3) reduced by one.

  This means that the frames whose sequence number is between VR = 1 and VL = 2 have been lost. Station B has thus detected that the frames

  number 1 and 2 have not been received, and frame number 3 has been received.

  The state variable VA then takes the value of the sequence number of the last correctly received frame, increased by one, ie 4. This means that, apart from frames number 1 and 2 which will have to be retransmitted, the

  next frame expected by station B has sequence number 4.

  At time t6, station B transmits a selective rejection supervision frame SREJ containing the parameters NR, NL, NA respectively equal to the

  current values of the state variables VR, VL and VA.

  At time t7, station A receives the frame SREJ.

  All frames whose sequence number is strictly less than NR are acknowledged. As a result, the frame of sequence number 0 is deleted from the RAM 4 and the sequence number 0

  is deleted from the FE transmission and FA anticipation windows.

* Frames whose sequence number is strictly greater than NL and strictly less than NA are acknowledged. The frame of sequence number equal to 3 can thus be erased from the RAM 4, the sequence number equal to 3 is erased from the transmission window FE, but not from the

FA anticipation window.

  In addition, the frames whose sequence number is between NR and NL must be retransmitted to station B. At time t8, station A retransmits the frame of number of

  sequence equal to 1. This information frame contains the parameter NS equal to 1.

  At time t9, this frame is correctly received by station B and the sequence number 1 is stored in the reception window FR of the

  station B. The transmission sequence number is equal to the state variable VR.

  The frame of sequence number equal to 1 can be transmitted to the upper layer and its sequence number is deleted from the reception window FR of station B. The reception state variable VR is incremented by one, and is

here equal to 2.

  As the frame of sequence number equal to two has not been received, the state variables VL and VA are not modified and keep their

current values.

  At time t10, station A retransmits the number frame

  sequence equal to 2. The information frame contains the parameter NS equal to 2.

  At time t11, this frame is correctly received by station B and the sequence number 2 is stored in the reception window FR of the

  station B. The transmission sequence number is equal to the state variable VR.

  The frame of sequence number equal to 2 can be transmitted to the upper layer and its sequence number is deleted from the reception window FR of station B. The reception state variable VR is incremented by one and is equal to 3. As the frame of sequence number equal to 3 has already been received, and the sequence number 3 is in the reception window, this frame can be transmitted to the upper layer and its sequence number

  is deleted from the reception window. It is then empty.

  The reception state variable VR is incremented by one and is equal to 4. As there is no more error in the reception of the frames, the values of the state variables VL and VA are set to the value of the VR variable,

here 4.

  At time t12, the station A transmits the information frame whose sequence number is equal to 4. The sequence number 4 is then

  stored in the FE transmission window and in the FA anticipation window.

  Frame number 4 is correctly received at time t13 by station B. The sequence number 4 is stored in the reception window FR of station B. The transmission sequence number 4 is equal to the value of the variable VR. Frame number 4 is then transmitted to the upper layer. The

  State variables VR, VL and VA are incremented by one.

  Sequence number 4 is deleted from the reception window

FR.

  At time t14, station B transmits an RR type supervision frame. This supervision frame contains the parameter NR equal to the value of the state variable VR. The supervision frame makes it possible to acknowledge all the frames

  whose sequence number is strictly less than NR.

  At time t1 5, station A receives the supervision frame RR. All frames whose sequence number is strictly less than 5 are acknowledged. The sequence numbers 1, 2 and 4 are deleted from the FE transmission window and the sequence numbers 1, 2, 3 and 4 are deleted from the

FA anticipation window.

  Frames 1, 2 and 4 are deleted from the RAM.

  The invention therefore makes it possible to retransmit only the frames which have been lost. In addition, frame number 3 is erased from the RAM of station A as soon as the acknowledgment has been received, without waiting for the frames

  number 1 and 2 have been correctly received.

  Of course, the present invention is in no way limited to the embodiments described and shown, but includes, on the contrary,

  any variant within the reach of the skilled person.

Claims (21)

  1. Method for selective acknowledgment and selective rejection of data frames, implemented in a data receiving station, each data frame (TD) being identified by a sequence number, characterized in that it comprises the steps of: - memorization (E101) of the sequence numbers (NS) of the frames received and not yet acknowledged, - verification (E102) of the loss of at least one frame, and in the event of a positive response, search (E208, E209, E301) of all consecutive sequence numbers of lost frames, and - transmission (E210, E304) of a selective acknowledgment and selective rejection frame bound for the data transmitting station, which acknowledges the received frames not yet acknowledged which precedes and follows the lost frames, and which requests the retransmission of lost frames.   2. Method according to claim 1, characterized in that the verification step comprises the comparison of the sequence number (NS) of each frame received with a first counter (VR) whose value represents the sequence number of the next frame expected, said value being incremented by one each time the sequence number of a   frame received is equal to the value of the first counter.   3. Method according to claim 1 or 2, characterized in that the search step comprises setting (E209, E301) a second counter (VL) to the value of the last number of lost frame sequence sequence, for   represent the upper bound of consecutive numbers of lost frames.   4. Method according to any one of claims 1 to 3,   characterized in that it comprises, when at least one frame has been lost, setting (E303) a third counter (VA) to the value of the sequence number of the last frame received, increased by one, to represent the upper bound of the sequence numbers of the frames received according to the lost frames.   5. Method according to claims 3 and 4, characterized in that the   selective acknowledgment and selective rejection frame (SREJ) has two parameters (NL, NA) respectively equal to the current value of the second and third counters.   6. Method according to any one of claims 1 to 3,   characterized in that it is implemented after each reception of data frame. 7. Device for selective acknowledgment and selective rejection of data frames (TD), implemented in a data receiving station (SC), each data frame being identified by a sequence number, characterized in that it comprises: - means (4) for storing the sequence numbers (NS) of the frames received and not yet acknowledged, - means (2) for verifying the loss of at least one frame, and in the event of a positive response, - means (2) for searching for all the consecutive numbers of sequence of lost frames, and - means (2, 6) for sending a frame (SREJ) of selective acknowledgment and selective rejection to the data sending station, which acknowledges the received frames not yet acknowledged which precedes and follows the lost frames, and which requests   retransmission of lost frames.   8. Device according to claim 7, characterized in that the verification means are adapted to compare the sequence number (NS) of each frame received with a first counter (VR) whose value represents the sequence number of the next expected frame, said value being incremented by one each time the number of   sequence of a received frame is equal to the value of the first counter.   9. Device according to claim 7 or 8, characterized in that the search means are adapted to put a second counter (VL) at the value of the last lost frame sequence number, to represent the   upper bound of consecutive numbers of lost frames.   10. Device according to any one of claims 6 to 9,   characterized in that it is suitable, when at least one frame has been lost, to set a third counter (VA) at the value of the sequence number of the last frame received, increased by one, to represent the upper bound sequence numbers of the frames received according to the lost frames.   11. Device according to claims 9 and 10, characterized in that it   is suitable for transmitting a selective acknowledgment and selective rejection frame which comprises two parameters (NL, NA) respectively equal to the current value   second and third counters.   12. Device according to any one of claims 7 to 11,   characterized in that the storage, verification, search and transmission means are incorporated in: - a microcontroller (2), a read-only memory (3) comprising a program for selective acknowledgment and selective rejection of frames data, and - a random access memory (4) comprising registers adapted to   save variables modified during the execution of said program.   13. Communication station characterized in that it comprises means suitable for implementing the method according to any one of claims 1 to 6.   14. Communication station characterized in that it comprises the   device according to any one of claims 7 to 12.   15. Computer characterized in that it includes a   communication according to claim 13 or 14.   16. Printer characterized in that it includes a   communication according to claim 13 or 14.   17. Fax machine characterized in that it comprises a station   communication according to claim 13 or 14.   18. Scanner characterized in that it comprises a station for   communication according to claim 13 or 14.   19. Communication system comprising a plurality of communication stations according to claim 13 or 14, and adapted to exchange data. 20. Digital camera characterized in that it   comprises a communication station according to claim 13 or 14.   21. Digital camera characterized in that it comprises a   communication station according to claim 13 or 14.