Method and Apparatus for Conforming the Speed of a Called Terminal Adapter to the Speed of a Calling Terminal Adapter

This invention relates to data transmission and more particularly to equipments which are designed to be connected to an Integrated Services Digital Network (ISDN).

ISDN is expected to be a rapidly expanding branch in the data communicating field as it allows the possibility of high-rate communications in the near future, particularly for sending voice, data etc.

FIG. 1 shows an example of ISDN connections. A calling terminal adapter 20 allows the connection of a Data Terminating Equipment (DTE) 10 to I.S.D.N. telecommunication network 50.

In the considered example, a calling terminal adapter 20, assumed to be a telecommunication equipment located within the United States of America, is designed to be connected to the well-known AT&T ACCUNET network. With this assumption, Terminal adapter 20 is capable of handling 56 Kbps links and utilizes the well known I.S.D.N numbering plan. A second Terminal Adapter 60 is also connected to the network 50 and is assumed to be an ISDN terminal adapter capable of handling 64 Kbps. At last, a third terminal adapter 30 allows the connection of a DTE 40 to the network.

Therefore, it appears that when one call is received by one terminal Adapter, e.g. adapter 30, the latter needs to recognize the rate and accordingly adapts its own speed to that used by the calling party. Indeed, assuming that adapter 20 is the calling one, the called adapter 30 needs to be adapted to a 56 Kbps rate. Conversely, in the case where the calling adapter is adapter 60, the called adapter needs to switch its operating speed to a rate of 64 Kbps.

In the prior art, the switching between the two operating rates was performed manually by the user prior to the call or a user was compelled to buy a series of two distinctive equipments, each one being dedicated to one particular speed.

Therefore, a need has appeared in the telecommunication field for an ISDN equipment which is capable of automatically recognizing and adapting its own speed to either 56 Kbps and 64 Kbps.

The problem to be solved by the present invention is to provide an ISDN terminal adapter which is capable of recognizing the rate used by a calling adapter, either 56 Kbps or 64 Kbps, and adapts its own speed to this rate.

This problem is solved by the method according to the present invention for automatically adapting and configuring the speed of a terminal adapter (TA) being called to the rate 56 Kbps or 64 Kbps which is used by a calling adapter. Referring both to FIG. 2 and to the message flow representation in FIG. 1, after the sending of the CONNECT message to the ISDN network in accordance with CCITT Q.931 Recommendations, the called TA is initialized to a rate of 64 Kbps and then, continuously transmits (204) an alignment pattern ALL.sub.-- ONES while starting a first timing process (T1). This first timing process will cause the called TA to switch to a 56 Kbps speed if the 64 Kbps validation process does not succeed at the end of the first period T1. The method involves the step of checking (206) the reception of a ALL.sub.-- ZEROS pattern coming from said calling adapter (20) before the end of said first predetermined period (T1). If this case, a 64 Kbps validation process is performed which comprises the checking whether a ALL.sub.-- ONES pattern is actually received within a second period (T2) of a second timing process. Depending on the success of this checking, the validation of the 64 Kbps rate will be confirmed.

On the other hand, if the ALL.sub.-- ZEROS pattern is not received at the end of the first period T1, the called TA initiates a 56 Kbps rate validation process which is based on the continuous detection of a so-called 56 Kbps pattern in the data flow. The 56 Kbps pattern is assumed to be detected on the occurrence of a sequence of n bytes, with each byte having its first bit being set to a ONE.

Preferably, the 64 Kbps validation process involves the checking (210) of the continuous receiving of said ALL.sub.-- ZEROS pattern during said first period (T1) as long as the ALL.sub.-- ONES pattern is not yet received, and at the end of said first period (T1) validating (209, 213) the 64 Kbps rate configuration. This avoids the failing of the rate determination process in some circumstances involving long distance calls where the transmission of the ALL.sub.-- ONES pattern to the calling TA may require a great deal of time which therefore might delay the receiving of the ALL.sub.-- ONES pattern in the called adapter.

In a more preferred embodiment of the invention, the 56 Kbps rate validation process comprises the steps of starting a third timing process based on a third period T3 and checking the detection of said 56 Kbps pattern within this third period T3. If this 56 Kbps pattern cannot be detected at the end of this third period T3, the rate configuration is switched back to the 64 Kbps rate configuration. However, in the contrary case, a fourth period T4 is used in order to check whether the 56 Kbps pattern remains present in the data flow for at least the fourth period T4 before the final validation of the 56 Kbps is performed.

FIG. 1 shows an example of ISDN connection.

FIGS. 2 and 3 are flow charts illustrating the basic steps which are involved in the method according to the present invention.

The process which is used in accordance with the present invention for determining which speed is used by a calling adapter, is initiated during the establishment of the link between the two terminal adapters, and more accurately during the signalling phase prior to any communication of data. It should be noticed that such a signalling phase is well-known to the skilled man and will not be described with further details. The reader will find more information in the appropriate CCITT Recommendations, and particularly in the Fascicule VIII.2, Rec. X.30.

Basically, the detection of the right rate which is used by the calling adapter is achieved by an appropriate use of a combination of timing processes which will be performed in the OSI layer 1 procedures. Therefore, this allows the possibility to carry out the desired speed detection even in the case where the particular network to which is connected the Terminal Adapter being considered does not provide any information element concerning the speed.

With respect to FIGS. 2 and 3, there is described the succession of steps which are performed for achieved the rate detection. The process is initiated in step 201 on the occurrence of a CONNECT message sent by the called adapter 30. Then an appropriate software routine or primitive, called SYNCHRO REQUEST, which is performed within terminal adapter 30 is called, step 202. It should be understood that the calling process of the latter routine will result in the execution of the technical steps which will be described hereinafter with reference to FIGS. 2 and 3. Then step 203, terminal adapter 30 initializes its own registers and its own operating speed to a 64 Kbps. Then, step 204 and in accordance with the CCITT X.30 Recommendations, the terminal adapter 30 performs the continuous sending of the alignment pattern ALL.sub.-- ONES on the B-Channel. Simultaneously, with the beginning of the latter sending of the ALL.sub.-- ONES pattern, the terminal adapter 30 starts an internal timing process in step 205. As it will appear hereinafter, this timing process will be used for switching the rate detection process to a 56 Kbps when no positive result occurs after a predetermined period T1.

Then, step 206, a test is performed to determine whether the ALL.sub.-- ZERO pattern is received as expected within the above mentioned predetermined period T1. If the ALL.sub.-- ZERO pattern is actually received by the TA 30 on his allocated B-channel, then the process proceeds to step 207 where the terminal adapter starts a second internal timing process. This timing will be used for confirming that the received pattern is actually a ALL.sub.-- ZERO pattern. In the reverse case, the process proceeds to step 302 where the rate detection process is switched to a 56 Kbps as will appear below. Step 207 and as mentioned above, the terminal adapter 30 starts a second predetermined timing process T2 which will be used for confirming the receiving of ALL.sub.-- ONES coming from the Terminal Adapter 20 on the allocated B-channel which, in accordance to the above mentioned CCITT X.30 recommendations, switches to the transmission of a ALL.sub.-- ONES pattern as soon as it receives the ALL.sub.-- ONE pattern which was generated in the called terminal adapter (30) in step 204. From this instant adapter 20 continuously transmits the ALL.sub.-- ONES pattern. Then, step 208, a test is performed to determine whether the terminal adapter 30 has received the first ONES elements of the continuous ALL.sub.-- ONES synchronization pattern generated by the terminal adapter 20 within the above mentioned predetermined period T2. If this case, the process proceeds to step 213 where the 64 Kbps synchronization is confirmed. In the reverse case, however, where the ALL.sub. -- ONES pattern is still expected, a test is performed in step 209 to determine whether the above mentioned T1 period is elapsed, in which case the 64 Kbps synchronization process is confirmed. It should be noticed that this case generally occurs for international telecommunications which requires a non-negligible delay before the called terminal adapter 30 receives the ALL.sub.-- ONES synchronization pattern coming from the calling party. If the timing period T1 has not yet elapsed, a test is performed in step 210 to determine whether the ALL.sub.-- ZEROS pattern is received within the predetermined period T2. If the latter ALL.sub.-- ZEROS is received, the process stops the second internal timing process T2, in step 212, and then proceeds to step 207 again for the initiation of a new confirmation process. In the reverse case, i.e. if the ALL.sub.-- ZEROS is not received during the period T2, the process proceeds to step 206 again. This loop is used for eliminating the occurrence of patterns which might be interpreted by the terminal adapter 30 as being the expected ALL.sub.-- ZEROS pattern and which are not confirmed.

Now, it will be described the process which is performed after the test 206 in the case where the first predetermined period T1 is elapsed and the expected ALL.sub.-- ZEROS pattern has not been received. As mentioned above, the process proceeds to step 302 where the terminal adapter 30 is initialized in a 56 Kbps. Then, step 303, a third timing process based on a period T3 is started. As it will appear below, this third period is used for determining the maximum time which will be allowed for confirming the 56 Kbps configuration. In step 304, an analysis is performed on the data received on the allocated B-channel to ascertain that the received frame is structured on the basis of a 56 Kbps. To achieve this, the received frame is checked on every first bit of each byte composing the frame. If the frame appears to be containing a number of n bytes, each one having a first one being set to a one, the latter frame will be considered by the terminal adapter 30 has comprising a 56 Kbps pattern. Therefore, in step 304, a test is performed to determine whether the above mentioned 56 Kbps pattern is received during the third period T3. In this case, the process proceeds to step 306 where an additional confirmation will be performed in order to ascertain that the 56 Kbps pattern is still received during a fourth period T4. If this is true, then the 56 Kbps assumption can be confirmed and the corresponding initialization is validated, step 307.

If the 56 Kbps pattern has not been received after the elapse of the T3 period, the process proceeds to step 311 where the terminal adapter 30 is switched back to a 64 Kbps configuration.

Therefore, it should be noticed that the method in accordance with the present invention processes the 56 Kbps pattern in order to distinguish the actual transmission rate which is effective. This results in the following advantage: no further information, particularly provided by the higher layers is needed to provide the rate adaptation as the lower physical layer is sufficient to achieve this result.

In the preferred embodiment of the invention, the timing T1, T2, T3 and T4 are respectively set on a period of 3 seconds; 80 milliseconds; 2.5 seconds and 1 second approximately.