text: facsimile transmission in presence of long propagation delays is a challenging issue due to the strict time - outs governed by the command / response sequences as defined in itu - t t . 30 . a typical command / response sequence for a two page facsimile call is illustrated in fig1 . as seen from fig1 different phases of the facsimile transmission have different time - outs and the worst case time - out is 2 . 55 seconds . systems that provide wireless access to its customers typically experience delays of several hundred milliseconds between subscriber and base stations , and the delay is even longer when access is via geostationary satellite links , i . e ., a wireless link 10 . such systems have to provide acceptable facsimile quality of service in call scenarios which should include international calls to and from the public switched telephone network ( pstn ) 12 users where pstn 12 itself may have an intelsat satellite link which uses digital circuit multiplication equipment ( dcme ) conforming to itu - t g . 766 . this delay , in addition to the delay that the facsimile machine itself introduces to formulate a response , may potentially exceed the 2 . 55 seconds time - out . the conceptual block diagram of fig2 shows the base station 20 including a modem bank 22 which interfaces with the pstn 12 . a facsimile machine 24 interfaces with the pstn 12 of the base station and via a conventional 2 - wire analog telephone interface . the facsimile service can be enhanced with the implementation of a pair of intelligent facsimile relays ( ifrs ) 14 , 16 that are physically located in the wireless network ( one at subscriber terminal and one at base station 20 ), successful facsimile call completion rate is expected to increase dramatically . on the subscriber end of the wireless link 10 , the subscriber terminal 18 is connected to the ifr 16 of the described embodiment , which may be coupled either to a subscriber end modem bank 28 which also is interfaced to a facsimile machine 26 via a 2 - wire analog telephone interface , or alternatively the ifr 16 may be connected to a personal computer such as laptop computer 30 via a v . 24 telecommunications interface or the like . as illustrated , combined propagation delays relating to the wireless link 10 , which may include terrestrial or satellite communications , typically result in one - way delay between 300 and 800 milliseconds , additionally the pstn 12 typically includes a one - way delay between 50 and 600 milliseconds . thus , it can be appreciated that while typical delays may be on the order of 1 . 5 seconds , such delays may potentially exceed the 2 . 55 second time - out associated with typical dcme equipment conforming to itu - t standards . fig2 also illustrates the access methods and a range of typical delays as seen in wireless access systems . the upper limits of wireless access delay in fig2 is applicable when the access is a geostationary mobile satellite system that uses hundreds of milliseconds of interleaving to accommodate possible fading . the upper limit of pstn 12 delay is typical of some intelsat links with dcme equipments conforming to g . 766 . it is noted that for subscriber - to - subscriber calls , the total one - way delays can be even longer than what is illustrated in fig2 . one of the deleterious consequences of long delays in facsimile transmission is the signal collision effects on the 2 - wire link between a pstn user and the nearest central office and / or between the wireless user and the ifrs 14 , 16 nearest to it . this is illustrated in fig3 where a retransmitted digital identification signal ( dis ) collides with digital command signal ( dcs ). the primary goal of the proposed solution is to complete the pre - message ( phase b as defined in itu - t t . 30 ) and post - message ( phase d as defined in itu - t t . 30 ) hand - shaking of the t . 30 successfully , by having the ifr monitor , store , and regenerate the t . 30 handshake messages and responses . the ifrs 14 , 16 are described in terms of the specific functionality of the apparatus to solve a specific problem . although most of the description is centered around the ifr located in the base station ( referred to as network_ifr ), most of the techniques are equally applicable to the ifr located at subscriber side ( referred to as subscriber_ifr ). it is however not necessary that all techniques described be implemented on subscriber_ifr . one extreme case where collision techniques need not be implemented in subscriber_ifr is when the wireless channel and the facsimile connection to the wireless subscriber are completely 4 - wire and entities controlling these 4 - wire links also operate in full - duplex mode . furthermore , if it is known that the wireless channel is either half - duplex or it is controlled by an entity operating in half - duplex manner , it is possible for network_ifr to use exactly the same techniques on wireless channel side to avoid collision as it would use to avoid collision on the pstn link . as seen in fig3 the retransmitted dis signal collides with dcs ( the response to first dis ) in the 2 - wire link of the pstn user . this will result in loss of dis and dcs . furthermore , all further attempts to retransmit command will result in collision because of the synchronous nature in which commands and responses are generated and the fixed delay between the two facsimile machines . hence the call terminates with a disconnect ( dcn ) message from the calling facsimile terminal during the pre - message phase ( phase b ) of the facsimile call . accordingly , the ifrs 14 , 16 monitor t . 30 and make intelligent decisions as to whether the response to a command should be forwarded to the other end . the objective is to avoid collision at the 2 - wire link and successfully complete phase b and phase d portions of the call , as defined in itu - t t . 30 . collision avoidance techniques that do not need a collision detector have been well published in gsm 03 . 46 and itu - t x . 38 . here the fax adaptor ( fa in gsm 03 . 46 ) or fpad of itu - t x . 38 blindly blocks or “ ignores ” a retransmitted command which will prevent the retransmitted command to go and collide with a response to the command at the far end . however , collisions may still occur at the near end if the time of transmission of the retransmitted command coincides with arrival of response . to solve this problem , gsm 03 . 46 uses a supervision timer which essentially is ail estimate of a safe period in which a response can be relayed to the command sending entity without colliding with a retransmitted command . the timer is started upon arrival of a command at the fa . if the response is received by the fa after the supervision timer has expired , then the response is buffered until the arrival of a retransmitted command and then relayed to the command sending entity . this collision avoidance technique is illustrated in fig4 . here collision is avoided on the pstn side which has a half - duplex modem based on supervision timer . if it is known that the wireless channel ( which , in general , is a full - duplex channel ) is controlled by an entity that operates in a half - duplex manner , then it is obvious that the technique of using supervision timer of 03 . 46 can be applied towards wireless channel to avoid simultaneous transmission and reception on wireless link . this obvious extension of gsm 03 . 46 collision avoidance concept towards a wireless link is illustrated in fig5 . one such wireless channel that ignores data arriving on the wireless channel when it is receiving data from its near - end fax machine is the gsm transparent fax channel . the obvious extension of gsm 03 . 46 collision avoidance technique to the satellite side is applicable for such channels . if the delays in the systems are long such that a response is always provided to a command - sending entity after it has retransmitted a command and if second retransmitted command is always blocked , then the protocol becomes extremely sensitive to errors on the channel . if the first command or first response is lost , then this would lead to a call disconnect . as will be discussed later some techniques described in itu - t x . 38 to overcome the situation still have problems . the invention describes techniques to improve the robustness of the protocol when collision avoidance techniques described in gsm 03 . 46 and itt - t x . 38 are used in wireless environments . the invention is described in terms of the specific functionality of the ifr to solve a specific problem . although most of the description is centered around the ifr located in the base station ( referred to as network_ifr ), most of the techniques are equally applicable to the ifr located at subscriber side ( referred to as subscriber_ifr ). applicability does not imply that the solutions have to be implemented on both sides of the connection . an obvious instance where the techniques mentioned in this invention need not be applied is when the subscriber side does not have an analog 2 - wire link since the major objective is to avoid collision on such links . as indicated above , collision avoidance may be achieved in many practical situations , e . g ., where true half - duplex modem implementations are used with the ifr . this may be accomplished without collision detection through use of supervision timers in a manner similar to that described in gsm 03 . 46 . in situations where a response arrives at a network_ifr after expiry of supervision timer , the response is only relayed to the command - sending entity after receipt of a retransmitted command from the command - sending entity . furthermore , if the response is valid ( has a valid fcs ) this may be stored for possible future use . in any case , after the network_ifr receives a valid response , the network_ifr is in full control for that command - response handshake . therefore any retransmitted command that is received after a “ valid response ” is blocked by network_ifr . however , for wireless channels that are prone to channel errors , it may so happen that either the first command itself is received incorrectly and / or the response to that command is not received even after some time . in such a case , a retransmitted command must not be blocked as described above . the itu - t recommendation x . 38 describes retransmitting the previous command if no response is received until a t4 timer or if response to previous command is in error . the technique described in itu - t x . 38 has the potential danger of a call disconnect if the first command itself was received in error , in which case all subsequent retransmitted commands will be in error . according to an alternate embodiment , the following rules shall be implemented in network_ifr : 1 - a ) the network_ifr “ buffers ” the previous command if and only if the command had a valid fcs ; 1 - b ) the network_ifr “ relays ” the retransmitted command if previous command was received in error ( with invalid fcs ); and 1 - c ) the network_ifr “ replaces ” the retransmitted command by a buffered command and transmits to the other end if no response was received until the arrival of retransmitted command or tax , whichever is greater . tax is a timer that is started after the last byte of the previous command has been transmitted out of the ifr . fig6 illustrates the scenario where the first command is buffered according to rule 1 - a . this command gets corrupted after it is transmitted by ifr , which results in a lack of response even when a retransmitted command arrives at ifr . as shown in fig6 rule 1 - c is applied for the retransmitted command . this prevents a call disconnect . the blocking of a retransmitted command after receiving a valid response is applicable for all command / response sequences , except for dcs - tcf / ftt sequence . the network_ifr shall not block a retransmitted command ( dcs - tcf ) if it receives a ftt as a response ( even if it has a valid fcs ) to the first dcs - tcf . instead , the ifr shall simply forward the retransmitted command in accordance with rules 1 - a , 1 - b , and 1 - c and ftt is blocked . if ftt is received in response to second dcs - tcf , or , if the ftt for the first dcs - tcf is received from the receiving facsimile machine after the network - ifr has received the second dcs - tcf , then ftt is relayed to the transmitter and a manufactured dcs - tcf ( as will be described next in this embodiment ) will be transmitted to the receiving facsimile machine . there are some fax machines that do not respond to the first dis command and deliberately wait for the retransmitted command . a similar situation exists in some systems which depend on detection of the v . 21 fsk signal ( dis being the first such signal in the t . 30 protocol ) to switch from the default “ voice - mode ” operation to facsimile mode of operation . in either case the first dis is rendered useless . in a long delay wireless environment where the second dis may get corrupted or when the dcs - tcf response to the second dis gets corrupted , there is a risk of a call disconnect . for the case when the second dis gets corrupted , the receiving fax machine receives no response even after the receiving fax machine transmits the third dis . for the case when the dcs - tcf gets corrupted , the receiving fax machine receives a response , but is invalid . both situations can lead to a potential disconnect since the fax machine has already transmitted the command three times . the network_ifr , upon receipt of the third dis , may “ manufacture ” a dcs with a valid fcs , followed by 1 . 5 seconds of tcf that is intentionally and heavily corrupted . for example , rather than transmitting 1 . 5 seconds of zeroes , the network_ifr may transmit all ones . in any case , the objective is to provoke a ftt from the receiving fax machine that will keep the connection alive . lack of such a procedure may result in a disconnect ( dcn ) from the receiving fax machine . fig7 illustrates the scenario where the first dis is ignored and second dis is lost ( corrupted ) on the wireless channel resulting in the dcs - tcf arriving at the receiving fax machine after t 4 timer has expired , leading to a possible dcn . fig8 illustrates the method proposed to avoid a potential dcn . a similar situation exists for the mps , eop , and eom commands in post message phase . some fax machines ignore the first mps / eop / eom since they are busy preparing for the next page . in this case , if the second retransmitted command is corrupted or if the response to this command is corrupted , the protocol then has to depend entirely on the third retransmitted command . in a long delay environment this can lead to a disconnect since the command transmitting entity has already transmitted three times . in such situations , the network_ifr shall manufacture a retrain negative ( rtn ) and transmit it to the command sending entity immediately after receiving the third retransmitted command . any response received to third retransmitted command is ignored . this will trigger a new dcs - tcf from the transmitting fax machine and keep the connection alive . after a receiving analog fax machine has sent out confirmation to receive ( cfr ) it must see training followed by facsimile message in order to proceed successfully through the later stages of the fax call . this training must arrive before the itu - t t . 30 t 2 timer (= 6 +/− 1 seconds ) at the end of cfr expires . however , due to long delays and particularly due to the collision avoidance mechanisms implemented in the ifr , the cfr reaches the other transmitting fax machine four to five seconds after it was transmitted . it is noted that if the cfr were to wait at ifr for a dcs - tcf sequence to end before getting forward to the transmitting machine , then the wait time itself can be greater than 3 . 5 seconds . this along with about 1 . 5 seconds of one way delay between transmitting and recieving fax machine will result in cfr reaching the transmitting fax machine after 4 . 5 seconds . a response to this ( which is page data ) will usually arrive after the expiry of lower limit of itu - t t . 30 t 2 timer , leading to a possible disconnect . this is illustrated in fig9 . also illustrated in fig9 are solutions to alleviate this problem as described in published technical specifications such as gsm 03 . 46 and itu - t x . 38 which essentially comprises of “ fill data ” being artificially generated towards receiving fax machine . in both cases , the objective is to send dummy fill data until an eol is actually received from the transmitting fax machine . the main problem with this approach is that if the image data from transmitting data machine does not arrive in time at the ifr before the maximum fill - time ( as specified in itu - t t . 4 ) of 5 seconds after start of generating of fill data , then the receiving fax machine can behave unpredictably . it is also important to note that although gsm 03 . 46 and itu - t x . 38 describes generating fill data towards the fax machines to the near end fax machine , this concept may be extended to the far end fax machine when only one ifr is engaged in collision avoidance and time - out avoidance mechanisms . the same is true where the fill techniques described below are applicable both on the wireless channel and on the wireline channel . accordingly , the connection may be kept alive as follows : 1 ) a series of coded lines ( ultimately shown on fax page as blank white lines ), each terminated with zero fill and eol pattern , is generated at the ifr and sent towards the receiving machine . each of these lines containing fill will actually show up at the top of the first fax page as white space . 2 ) the use of the extra zero fill within each of the coded white lines forces their transmission time to be prolonged . if each line were coded with only enough fill to meet the minimum scan length duration , the lines would be transmitted more quickly and a larger than necessary amount of white space would be prefaced to the image in order to defeat the t 2 timer . 3 ) each line being coded as white space , beginning with a makeup code corresponding to the number of white pixels that are in a line of the negotiated scan line length ( typically 1728 pixels or 215 mm ) followed by a terminating code of 0 white pixels . each blank line is intended to occupy approximately 2 seconds and therefore approximately 19 , 169 fill zeroes are transmitted followed by end of line ( eol ) pattern ( e . g ., eleven zeroes followed by a 1 ). the above is illustrated in fig1 where the ifr artificially generates an eol labeled as ifr_eol . one objective of the preferred embodiment is to hold the receiving fax machine with as few of these dummy lines as necessary and relay the actual image data as soon as is practicable after its arrival at the ifr . to begin relay of the actual image data , one might perform the following : ( 1 ) stop putting fill , ( 2 ) put out an eol sequence to terminate the current blank line , and ( 3 ) continue the symbol transmission with the very first image symbols recovered at the ifr . however , it is noted that the actual image symbols into the ifr may or may not be the actual image symbols sent by the transmitting fax machine . if the very first symbols to be recovered after the training sequence are erroneous symbols , perhaps due to the characteristics of the recovering modem rather than the actual symbols sent from the transmitting fax equipment , the fact that they may be relayed to the receiving fax after a valid eol may cause undesirable results with some receiving fax equipment . inserting an eol into the zero fill and following it with the first image data received will cause a line error at the receiving fax machine in the case where the first image data to arrive are not the beginning of a valid line code sequence terminated with an eol . since the fax machine is likely to ignore any data that precedes the first valid eol , this invalid data may not normally cause any problem . however , allowing such bad or corrupted data to be presented to a fax machine after that machine has seen a valid eol on that page , may cause the receiving machine to consider the line bad . to avoid this undesirable side effect of posing zero fill and eol in front of what may or may not be valid image data and thereby forcing the receiver to deal with a bad image line , the present embodiment inserts an eol into the fill section of the current dummy line only when the data available from the sending fax machine contains its first valid eol . once the eol has been inserted to terminate the fill , the transmission continues with the received image data that immediately follows the first valid eol received at the ifr . this is illustrated in fig1 . while the “ fill ” technique above and in fig1 are described in response to a cfr , the same technique is applicable to mps / mcf stage of the t . 30 protocol to avoid time - out related problems . as discussed above , fill data - may be generated towards the receiving fax machine after reception of cfr ( in response to dcs - tcf ) or mcf ( in response to mps ) responses . there are occasions where a ftt is received instead of cfr or when mcf is received in response to eop or eom . in both these cases , the t 2 timer can expire for the same reason as previously described leading to a disconnect . in such cases , rather than “ fill ” pattern with eol , the ifr shall generate a short sequence ( about 500 ms in duration ) of high level data link control ( hdlc ) flags and discontinue transmission for a period of greater than 200 ms . this , according to itu - t t . 30 should reset the t 2 timer thereby allowing an additional 5 to 7 seconds for the next command to reach the receiving fax machine . as shown in fig1 , the network_ifr shall store the responses such as cfr and mcf in a buffer before forwarding to the other end . if the forwarded response cfr or mcf gets corrupted in the wireless or wireline channel , the network_ifr may receive a retransmitted dcs - tcf or mps while it is transmitting fill data towards the receiving fax machine as defined above . in this case , the network_ifr shall not forward the retransmitted dcs - tcf or mps to the other end . instead , it should simply ignore those retransmitted commands and autonomously regenerate the response to the retransmitted command from the buffer where it has been previously stored . the network_ifr shall continue with the transmission of the fill data without any interruption . as shown in fig1 , the fax machine may send out a command repeat ( crp ) frame as a response if it receives a corrupted hdlc frame , to request the sending entity to retransmit the previous command or response frame . so , if the network_ifr receives crp frame as a response to any of the transmitted command / response frame , it shall regenerate the valid previous command / response frame that was stored in the buffer without forwarding the crp frame to the other end . if a valid command has not previously been stored in the buffer , the network_ifr should forward the crp frame to the other end . while operating at 2400 bps , some of the fax machines that do not have adequate memory to store the pages may disconnect if they receive rtn message that requests the sending fax machine to retransmit the previous page after exchanging the training sequence again . to let the call successfully complete at the fallback rate , it is recommended that the network_ifr shall modify the rtn message type to that of rtp before forwarding to the other end . it should be appreciated that a wide range of changes and modifications may be made to the preferred embodiments for improving facsimile call success rates in long delay environments as described herein . thus , it is intended that the foregoing detailed description be regarded as illustrative rather than limiting , and that the following claims , including all equivalents , are intended to define the scope of the invention .