fig2 shows a communication system 10 according to the present invention . communication system 10 allows for the transfer of data from one site 12 to another site 12a through a public switched telephone line 16 . site 12 , which is essentially similar to site 12a , is shown with a computer system 20 , a data communication subsystem 22 , a d + v ( data + voice ) modem 24 and a telephone handset 26 . d + v modem 24 is coupled to subsystem 22 via a communications cable 28 , and to handset 26 via a twisted pair 30 . d + v modem 24 is also connected to line 16 . station 12a includes a computer system 20a , a data communications subsystem 22a , a d + v modem 24a , and a handset 26a . it should be apparent that other variations are possible . for example , d + v modem 24 can be integral to subsystem 22 and / or system 20 , as would be the case if system 20 is a lap - top computer and d + v modem 24 is an internal telephone / modem . also , handset 26 need not be as depicted , but could be a headset or other microphone and / or earphone combination . for other applications , handset 26 is interchangeable with other devices which communicate real - time data . if d + v modem 24 is internal to subsystem 22 , then cable 28 might be replaced by an internal bus with similar functionality . in operation , computer system 20 transfers data between itself and system 20a over line 16 , while voice communication occurs between handset 26 and handset 26a , all using only line 16 . in many applications , computer systems 20 , 20a and subsystems 22 , 22a cannot be easily modified . with the d + v modem , such systems do not need to be modified , because the interface over cable 28 is transparent to voice traffic . this is possible since most modern communications subsystems are configured for handshaking with a modem which allows a modem to slow down the passage of data from the communications subsystem to the modem when the modem has difficulty keeping up . the effect of voice transmission on data transmission at cable 28 is only to create an apparent slowdown in the d + v modem &# 39 ; s data transmission rate , and only when voice data is being transferred . of course , in some embodiments , d + v modem 24 accepts configuration commands over cable 28 which are not found in a standard data communications interface . as shown in fig2 d + v modem 24 also accepts other data sources , which carry either non - real - time data 32 or real - time data 34 . each of these sources of data is multiplexed onto line 16 as explained below in connection with fig4 . fig3 is a more detailed block diagram of one site , 12 , of communication system 10 . site 12 is shown with d + v modem 24 in greater detail , an image codec ( analog coder / decoder ) 40 and a ccd camera 41 as a typical source and destination of non - real - time data , and a midi instrument 42 as a typical source and destination of real - time data . d + v modem 24 is shown including a voice codec 50 which codes / decodes voice signals including compression / decompression , an hdlc ( high - level data link control ) framer / deframer 52 ( hdlc controller , for short ), which communicates with a modem module 54 using hdlc frames such as frame 56 , and an analog front end ( afe ) 58 , which is coupled to modem module 54 . an output of afe 58 is coupled to a data access arrangement ( daa ) 60 , which is in turn coupled to line 16 . frame storage 62 is also provided for buffering frames where data is framed faster than the capacity of telephone line 16 . except where noted otherwise , hdlc controller 52 operates per the hdlc protocol as defined in the iso document numbered iso - 3309 , &# 34 ; data communication -- high - level data link control procedure -- frame structure &# 34 ;. voice codec 50 is coupled to handset 26 via twisted pair 30 , and is also coupled to hdlc controller 52 to provide digitized compressed voice data to hdlc controller 52 . a data port of hdlc controller 52 is coupled to subsystem 22 ( see fig2 ) to receive data from computer system 20 . this coupling , in one embodiment , is a standard rs - 232 connection , while in another embodiment , it is a parallel data bus . hdlc controller 52 is also shown coupled to image codec 40 and midi instrument 42 using those devices &# 39 ; standard interfaces . in a video game application , voice might be obtained from a pilot headset / microphone or a speakerphone . hdlc controller 52 frames each of the data streams input thereto into frames , while multiplexing the frames at its output to modem module 54 . hdlc controller 52 also de - packetizes received frames and directs them to the proper outputs . because frames are only sent when data is presented to hdlc controller 52 , the channel bandwidth provided by line 16 is used efficiently . because hdlc controller 52 gives frames for real - time data priority over frames for non - real - time data , the real - time data is received in real - time , with the non - real - time data filling the gaps in the real - time data streams . of course , if the real - time data does not fit into the channel bandwidth , such as uncompressed , full - motion video , it cannot be sent in real time . hdlc controller 52 is also capable of separating ( de - multiplexing ) frames received from a remote site through the use of a frame address at the link layer , as shown in fig4 . fig4 is a chart showing several sources of data which are transferrable over a telephone line . for each of the data types , an exemplary application is shown , although other applications are possible . as indicated , all the various data types use the hdlc protocol at the link network layer ( of the standardized seven layer network model ). as fig4 illustrates , different error correcting schemes are used for real - time data and non - real - time data . real - time data may be error - corrected using forward error correction ( fec ), while non - real - time data may be corrected using the v . 42 standard &# 39 ; s protocols , wherein a block received in error results in the receiving modem requesting re - transmission . although voice uses fec , some voice frames might have too many errors and thus not be recoverable . if the error is not recoverable , then the receiving modem replaces the signal which would have resulted from the frame with silence or the previous voice frame to avoid crackling and pops due to noise . the v . 42 standard is defined in ccitt recommendation v . 42 , &# 34 ; error correcting procedures for dces using asynchronous to synchronous conversion &# 34 ;. fig5 is a schematic diagram of an hdlc frame , as used with the present invention . an hdlc frame comprises the fields shown in table 1 . the addresses that may be assigned in a typical implementation for several data stream types are shown in table 2 , below . as should be apparent , other assignments of values for data stream addresses are possible so long as the sender and the receiver of a data stream agree on the meaning of the addresses . table 1______________________________________fields of an hdlc frame . field contents______________________________________start flag 01111110address identifies the data stream type ( see table 2 ). control the v . 42 specification specifies the use of this field for data . for voice , this field is optional . data a portion of the data stream which has been framedfcs frame checksum ( calculated per the hdlc standard ) end flag 01111110 ( could be combined with start flag of next frame ) ______________________________________ table 2______________________________________data stream type address______________________________________voice 32video 31sound 35image 33data blocks 37control 34graphics 38text 36______________________________________ the control field contains one or two bytes , per the v . 42 specification . the fcs field is calculated by performing a check sum on the address , control and data fields . since each hdlc frame contains an address indicating its data stream type , and each frame contains only data from one type of data stream , hdlc controller 52 can easily separate received frames for each data stream multiplexed onto line 16 . and since frames for the various data types can be freely intermixed , the full bandwidth of the channel is dynamically allocated to the various data streams . if data from all data streams is presented to hdlc controller 52 faster than the channel can carry the data , some of the data is framed and bufferred into frame storage 62 . alternately , where provided , hdlc controller 52 will send control signals back to the source of a non - real - time data stream to indicate that modem 24 is temporarily busy ( e . g ., sending an xoff byte or a cts off signal back to subsystem 22 over an rs - 232 line which uses software handshaking to hold up data at the sending end ). when real - time data arrives fast enough to occupy the entire channel , the non - real - time data streams are held up or bufferred , so that the real - time data arrives at the remote site in real time . the d + v modem at the remote site can be configured to ignore any unrecoverable packets which contain voice data and replace the voice data with voice data which would translate to silence or average the erred gap . this way , a listener is not subjected to noises and pops which might occur if errors in the voice frames were present in the voice signal presented to the listener . remote video game data is real - time data which presents particular problems . in a typical video game application , a player interacts with his or her own game system , such as the game systems made by sega , nintendo , atari , etc ., or such as a computer game running on a personal computer . in an interactive video game , more than one player is involved . typically , each player interacts with their own game system as in the non - interactive game , but other players can take actions which affect that player &# 39 ; s game . thus , an interactive video game accepts local input as well as remote input . if the players are geographically separated and no other communication link is provided , then the game systems are coupled by a telephone line . for more than two players , a conference call arrangement can be used . where a player wants to be in voice contact with the other players and only has one telephone line available , the present invention is used to provide the voice connection while transferring remote video game data between game systems . remote video game data is extremely time sensitive , but is relative low bandwidth . for example , suppose each player manipulates a joystick to cause a player &# 39 ; s character in a video game to move , and the game is interactive because each player sees each player &# 39 ; s character and its movements . this is made possible by the transfer of remote video game data between game systems , the remote video game data representing the joystick movements of each player . if the remote joystick movement information is delayed too much , some players will be seeing out - of - date views of the video game . for this reason , remote video game data is extremely time - sensitive , and once presented to a local modem such as modem 24 shown in fig2 it must be quickly transported to the remote modem and to the remote game system . fortunately , the bandwidth required for this data is very low , and this low bandwidth requirement can be used to great advantage . for example , if 5 bytes of data each refresh cycle of the video game display is sufficient to communicate the remote joystick movements , a bandwidth of only 300 bytes / sec is needed if the refresh cycle is 60 hz . fig6 is a schematic diagram of a modified hdlc frame used to transmit video game control data or other low delay data . the video game control data packets are of a pre - agreed size , such as n bytes , where each modem agrees on the value of n , so that no bandwidth need be used to signal the end of a video game control data packet . if regular hdlc frames were used , either the remote video game data would need to be accumulated into a frame , which causes an unacceptable delay , or each n - byte packet would be transmitted as a separate packet , which would also cause a delay , since the transmission of the data must wait at least until any packet transmission in progress when the video game data is presented to the modem has completed . the use of an entire frame for each n - byte packet is also very wasteful of bandwidth for low values of n , such as n = 4 or n = 5 , since the other fields of the frame might be larger than the data field . as shown in fig6 the modified hdlc frame contains the same fields as an hdlc frame , except that the address , control , and fcs fields are optionally removed . the n - byte packet is simply placed between voice packets with an escape code . the escape code is known as an &# 34 ; abort flag &# 34 ;, and is inserted in between the voice packet being transmitted by the local modem when the video game data is received by the local modem . that way , the remote video game data need not be delayed until the end of the frame . the address and control fields are not needed in this application , since all frames are addresses as voice packets . the fcs flag is eliminated , because otherwise the insertion of the n - byte data packet would cause a check sum error . the elimination of these various fields leads to more efficient use of bandwidth . the insertion of the n - byte packet is detectable at the remote modem , since the abort flag is not a bit sequence which would occur within a voice packet . the end of the frame is still detectable because the end flag is also not a bit sequence which would occur in a voice packet . if the value of n is fixed ahead of time , then the bytes in the n - byte packet can take on any possible bit sequence without ambiguity . the above description is illustrative and not restrictive . many variations of the invention will become apparent to those of skill in the art upon review of this disclosure . the scope of the invention should , therefore , be determined not with reference to the above description , but instead should be determined with reference to the appended claims along with their full scope of equivalents .