DE19832998A1 - Method for determining data packets contained in a continuous data stream - Google Patents

Abstract

The data packets (SE) have a respective cell head with a constant length l, consisting of cell-head data (CID, LI, UUI) and a terminating cell-head control sum (HEC). A test unit (PE) divides the final data l from the continuous data flow (TDM) received by a generator polynome used as a basis for calculation of the cell-head control sum (HEC) from the cell-head data (CID, LI, UUI) and when the remainder of the division is zero, a timing mark (Z) indicating the beginning of a data packet (SE) is set.

Description

Due to the increasing need for a transmission of video Information in modern communication technology, such as B. Fixed and moving images in videophone applications, or the Representation of high-resolution graphics on modern DV-An the importance of transmission and mediation increases techniques for high data transfer rates (greater than 100 Mbit / s). A well-known data transmission method for high Transmission bit rates is the so-called asynchronous transfer Mode (ATM). A data transmission based on the asynchronous Transfer mode currently enables variable transfer bit rate of up to 622 Mbit / s.

In the case of the known as Asynchronous Transfer Mode (ATM) averaging technology are used for data transport data packets fixed length, so-called ATM cells used. An ATM cell is made up of one that is used to transport an ATM cell relevant switching data containing five bytes long Cell header, the so-called "header" and a 48 byte long User data field, the so-called "payload" together. Here are only logical in the user data field of an ATM cell Connection - often in literature with "Virtual Channel" VC or ATM channel - assigned data transmitted.

In the German patent application with the file number 198 18 776.9, a method has already been proposed by which a transmission of data assigned to different logical connections in the useful data area of one or more ATM cells is made possible. For this purpose, so-called substructure elements with a variable 0 to 64 byte useful data field are defined in the user data area of one or more ATM cells according to the known ATM adaptation layer AAL type 2, by means of an address field in the cell header of the substructure element an assignment to a logical connection is made. Through the so-called ATM adaptation layer AAL - often referred to in the literature as "ATM adaptation layer" - the ATM data format is adapted - in the literature also often referred to as "ATM layer" (layer 2 ) - on the network layer (layer 3 ) according to the OSI reference model (Open Systems Interconnecti on). In addition to the subdivision of the user data area of an ATM cell into substructure elements, the first byte of the user data area of an ATM cell is defined as a pointer that points to the first substructure element arranged in the user data area, ie it is the start address of the first one User data area located cell header of a substructure element stored. Using this pointer is a restoration of synchronization between the transmitter and receiver in the event that one or more ATM cells z. B. lost by egg NEN transmission errors are possible.

Furthermore, in the patent application GR 98 P 2136 (internal file number) proposed a mediation ment of ATM cells according to the ATM adaptation layer AAL-Type2 are divided into substructure elements by an ISDN switching system by means of one arranged therein to implement time slot-oriented switching matrix module. For this, the packet-oriented ATM is converted Data format according to ATM adaptation layer AAL type 2 Time slot-oriented data format according to the TDM process (Time Division Multiplex). As part of the timeslot orien tated data format according to the TDM method stand for a Data transmission 32 user channels, which are ISDN-oriented B channels (Integrated Services Digital Network) with one Transmission bit rate of 64 kbit / s are designed to disposal.

The object of the present invention is to provide a method admit by recognizing boundaries between a individual data packets for an extraction from a continuous  Data packets inserted into the data stream becomes.

The object is achieved according to the invention with the notes paint claim 1.

For a better understanding of the packet-oriented ATM data formats according to the ATM adaptation layer AAL type 2 appears it is first necessary once again based on known principles to go into more detail.

In the case of data transmission as part of the packet-oriented ATM data format according to the ATM adaptation layer AAL type 2 there is the possibility of the user data area of an ATM cell to subdivide into substructure elements.

A substructure element according to the ATM adaptation layer AAL Type2 consists of a 3-byte cell header and one User data area of variable length (0 to 64 bytes) together. The cell header of a substructure element is subdivided again in an 8-bit channel identification CID (Chan nel identifier), a 6-bit length identifier LI (Length Indicator), a 5-bit transmitter-receiver identifier Identification UUI (User-to-User Indication) and a 5 bit long Cell header checksum HEC (Header Error Control).

The cell head checksum HEC - often in the literature too with block check string - is used in a so-called called cyclic block testing - common in the literature also called code-free error protection and with CRC (Cyclic Redundancy Check) abbreviated - determined and serves to check the correctness of the first 19 in the cell head nes substructure element transmitted bits.

In the cyclic block check, the binary characters of the data block to be backed up - i.e. the first 19 bits transmitted in the cell header of a substructure element - serve as coefficients of a so-called basic polynomial. For example, a data block "1000001000010001001" results in a basic polynomial "x ¹⁸ + x ¹² + x ⁷ + x ³ + 1". This basic polynomial is divided by a so-called generator polynomial "x ⁵ + x ² + 1" (standardized by ITU-T 1.362) modulo-2. The remainder remaining after the division corresponds to the cell head checksum HEC.

A major advantage of the method according to the invention is now in that a recovery from a continuous Data packets inserted without data transmission development of an additional marking the beginning of a data packet zinformation is enabled.

Advantageous developments of the invention are in the sub claims specified.

An advantage of the configuration defined in the subclaims tion of the invention is, inter alia, that by the insertion of fill cells or fill data in a sub- Structure element when converting a package-oriented Data format in a time slot-oriented data format, ei ne Transfer of compressed data (with a variable Transmission bit rate) is possible without previous decompression. Thus, in the transmission of compressed data Loss of quality avoided.

An embodiment of the invention will follow hand of the drawing explained in more detail.

Show:

Fig. 1 is a structural diagram for the schematic representation of the conversion of the ATM packet-oriented data format in the time-slot-oriented data format TDM accelerator as a first conversion mode;

Fig. 2 is a structural diagram schematically illustrating the conversion of the ATM packet-oriented data format in the time-slot-oriented data format TDM accelerator as a second conversion mode;

Fig. 3 is a structural diagram for the schematic representation of a TDM channel with associated time stamps.

Fig. 1 shows a schematic representation of an Encrypt development of packet-oriented ATM data format according to the ATM Adaptation Layer AAL-Type 2 (ATM Adaptation Layer) in the time data format according oriented slit the TDM (Time Division Multiplex) according to a first conversion mode of a conversion unit UE. Data transmission in the context of the packet-oriented ATM data format takes place via ATM cells ATM-Z1, ATM-Z2. An ATM cell ATM-Z1, ATM-Z2 is composed of a five-byte long cell header H containing the switching data relevant for the transport of an ATM cell ATM-Z1, ATM-Z2 and a 48-byte user data field. In the case of data transmission in the context of the packet-oriented ATM data format in accordance with the ATM adaptation layer AAL type 2, it is possible to subdivide the useful data area of an ATM cell ATM-Z1, ATM-Z2 into substructure elements SE.

A substructure element SE according to the ATM adaptation layer AAL-Type2 consists of a 3-byte cell header and egg nem user data area I of variable length (0 to 64 bytes) together men. The cell header of a substructure element SE is below in turn is an 8-bit channel identification CID (Channel Identifier), a 6-bit length identifier ornament LI (Length Indicator), a 5-bit transmitter emp catcher identification UUI (User-to-User Indication) and a 5 bit long cell header checksum HEC (Header Error Con trol).

By subdividing an ATM connection using Substructure elements SE into individual, independent of each other Data streams, as in the example of the ATM cells ATM-Z1, ATM-Z2 in  the figure shown can be within an ATM connection based on the 8-bit channel identification CID several (256) logical connections are addressed, all with the same ATM address - consisting of a VPI value (Virtual Path Identifier) and a VCI value (Virtual Channel Identifier) - can be addressed. Furthermore there is Possibility of a substructure element SE for a transmission of signaling assigned to the logical connections Define information.

For example, there are four different sub in the figure Structural elements SE shown based on differ cher channel identifications CID in cell header 0 ,. . ., 3 the sub structure elements SE are defined. Through the 6 bit long Length identification LI in the cell header of a substructure Elements SE can have a user data field I of variable length (0 to 26 bytes) can be defined so that for the different logical connections a data transmission with variable Transmission bit rate is realizable.

For a conversion of the packet-oriented data format according to of the ATM adaptation layer AAL type 2 on the time slot oriented data format according to the TDM method substructure defined for the transmission of user data Element SE of an ATM cell ATM-Z1, ATM-Z2 a TDM channel K0,. . ., K3 of the time slot-oriented data format according to assigned to the TDM process. An assignment of a substructure tur elements SE to a TDM channel K0,. . ., K3 takes place in a signal that precedes the transmission of useful data development phase. For data transmission within the time slot-oriented data format according to the TDM process there are generally 32 user channels, which are ISDN-oriented B channels with a constant transmission bit rate of each 64 kbit / s are available.

As part of the conversion of the packet-oriented data format according to the ATM adaptation layer AAL type 2 on the time  slot-oriented data format according to the TDM process must additionally an adjustment of the, by the size and the one meet of substructure elements SE resulting - even tuell variable - transmission bit rate of the packet-oriented Data format to the constant transmission bit rate of 64 kBit / s of the time slot-oriented data format. This becomes part of the first conversion mode of a conversion tion unit UE by inserting so-called filler cells FZ variable length in the continuous TDM data stream enough.

In the conversion unit UE are a package orien tated data transmission path received and in ATM cells ATM-Z1, ATM-Z2 unpacked substructure elements SE. Then, for the implementation, by size and the arrival of the substructure elements SE result the - possibly variable - transmission bit rate to the con constant transmission bit rate of 64 kbit / s of the time slot oriented data format so-called filler cells FZ to the the substructure elements SE containing user data adds. The so-called fill cell header FZH Length of a filling cell FZ determined. The length of a fill cell FZ is chosen so that the total transmission bit rate a substructure element SE and a filling cell FZ Integer multiple of 64 kbit / s results. Is the over carry bit rate of a substructure element SE greater than 64 kbit / s - i.e. greater than the transmission bit rate of a TDM Channel K1,. . ., K4 - are those in a substructure element SE transmitted user data on several TDM channels K1,. . ., K4 on divided.

Finally, this data (substructure elements SE and FZ cells together) one in the signaling phase agreed TDM channel K0,. . ., K1 a time slot-oriented assigned and forwarded telt.  

Fig. 2 shows a schematic representation of an Encrypt development of packet-oriented ATM data format according to the ATM Adaptation Layer AAL-Type 2 (ATM Adaptation Layer) in the time data format according oriented slit the TDM (Time Division Multiplex) according to a second conversion mode of the conversion unit UE.

In contrast to the first conversion mode, the second Conversion mode no separate filling cells FZ in the cont Nuclear TDM data stream inserted. An adaptation of the by the size and arrival of the substructure elements SE resulting at the conversion unit UE - possibly variable - transmission bit rate of the packet-oriented data formats according to the ATM adaptation layer AAL type 2 on the constant transmission bit rate of 64 kbit / s of the time slot oriented data format according to the TDM process by filling the substructure elements SE with filler data FD, so that the total transmission bit rate of a substructure Elementes SE - consisting of user data and filler data FD Integer multiple of 64 kbit / s results.

Fig. 3 shows a schematic representation of a TDM channel TDM with substructure elements SE packaged therein. The octet-oriented TDM data stream of the TDM channel passes through a test unit PE, in which a test - hereinafter referred to as a cell head test - of the received data is carried out for a specific bit sequence which characterizes the start of a substructure element SE. This cell head check is carried out using the cell head checksum HEC transmitted in the cell head SEH of a substructure element SE.

As already described, the binary characters of a data block to be secured - in this case the first 19 bits transmitted in the cell header SEH of a substructure element SE - are used as coefficients of a so-called basic polynomial as part of a cyclic block check. This basic polynomial is modified by a so-called generator polynomial "x ⁵ + x ² + 1" (standardized by ITU-T I.362) Modulo-2 di. The remainder remaining after the division corresponds to the cell head checksum HEC.

For the cell head test, the last three of the Test unit PE received bytes in a shift register - often also called FIFO (First In First Out) memory - cached and by the generator polynomial modulo-2 divided. If the remainder of the division is zero, see above the 3 buffered bytes are the Characteristic bit sequence sought and one at the beginning of the Zellkopf's synchronous time stamp Z is set. Subsequently the byte first stored in the shift register is taken from the Shift registers deleted and a new one - the last one on the Test unit PE received - bytes for a new divisi on written in the shift register.

This characteristic bit sequence is repeated with everyone Cell head SEH of a substructure element SE. There is also the possibility that the characteristic bit sequence in the cont Nuclear data stream due to a random bit constellation lation occurs anywhere. To such due bit constellations when recognizing the Beginning of a substructure element SE - i.e. the cell header SEH of a substructure element SE - to be able to suppress is an additional test in addition to the cell head test - hereinafter referred to as distance check - carried out.

Occur as in the case of a conversion of the package-oriented ATM data format in the time slot-oriented TDM data format according to the second conversion mode of conversion UE only substructure elements SE with the same length - the The length of a substructure element SE is filled with Fill data FD to the required constant transmission bit rate of a TDM channel adjusted - so, in the context of the Distance check only checks whether the characteristics bit sequence occurs at the same distance. Kick the charak  Teristic bit sequence three times in a row in the same Ab stood up, the recognition is considered synchronous. Kick one characteristic bit sequence in one of the spacing interval deviating distance, the detection becomes asynchronous. The recognition is after three times the characteri tical bit sequence in sync again at the same distance. With Help of such a substructure element in the cell header SEH Length identification LI transmitted via SE can average user data are separated from the filler data FD.

Occur as in the case of a conversion of the package-oriented ATM data format in the time slot-oriented TDM data format according to the first conversion mode of the conversion unit UE substructure elements SE with different lengths - the lengths of the respective substructure element SE and the Lengths of the respective filling cells FZ can vary be on, the distance check is carried out under consideration adjustment of the SEH of a substructure element SE transmitted length identification LI. Kick the character bit sequence three times in a row in the Length identification LI predetermined distance, so applies detection as synchronous. Occurs a characteristic bit in one do not follow the length identification LI given distance, the detection becomes asynchronous. The Recognition is after three times the characteristic cal bit sequence in the by the length identification LI given distance back in sync.

Claims (8)

1. Method for the detection of data packets (SE) transmitted successively in a continuous data stream (TDM),
wherein the data packets (SE) each have an m-bit long cell header (SEH), which is composed of an n-bit long cell header data field (CID, LI, UUI) and a p-bit long cell header checksum (HEC),
wherein the last m bits received from the continuous data stream (TDM) are checked by a checking unit (PE) for a predeterminable correlation between the first n bits and the subsequent p bits, and
in cases in which this correlation is recognized, synchronization information (Z) which marks the beginning of a data packet (SE) and is assigned to the last m bits is generated. 2. The method according to claim 1, characterized, that for a subsequent review one of the last m bits q-bit received first at the test unit (PE) Group separated from the m bits and one last at the test unit (PE) received q-bit group is newly attached. 3. The method according to any one of the preceding claims, characterized, that for a check of the last m bits that come from the kon continuous data stream (TDM) at the test unit (PE) were received by these m bits, creating the Cell head checksum (HEC) from the cell head data (CID, LI, UUI) underlying generator polynomial are divided, and in cases where the remainder of the division is zero, the syn Chronization information (Z) is generated.   4. The method according to claim 3, characterized in
that the last m bits received from the continuous data stream (TDM) are temporarily stored in a shift register, and
that for a subsequent division, the q-bit group received from the stored last m bits first received at the test unit (PE) are shifted out of the shift register and the last received q-bit group at the test unit (PE) are reinserted . 5. The method according to claim 2 or 4, characterized, that in cases where the length of the cell head (SEH) is a is an integer multiple of 8 bits, the q-bit group is one Has a length of 8 bits. 6. The method according to any one of the preceding claims, characterized, that in cases where the data packets (SE) are constant Length, the last synchronization information generated mation (Z) is only considered valid if the last ten x synchronization information (Z) at the same time Chen distance were generated. 7. The method according to any one of the preceding claims 1 to 5, characterized, that in cases where the data packets (SE) have a variable, by a length identification (LI) in the cell header data have a predetermined length, the last generated sync sationsinformation (Z) is only evaluated as valid, if the last x synchronization information (Z) in the specified by the respective length identifications (LI) time intervals were generated.   8. The method according to claim 6 or 7, characterized, that as the last x synchronization information (Z) the last 3 synchronization information (Z) are provided.