Error-robust multiplex process with header control field

The process for data transmission between two stations by means of data blocks each having a synchronization pattern field (SYNC), a header field (HEADER) and an information field (INFORMATION) following the header field includes providing a header control field (HEX) following the header field (HEADER) in each of the data blocks for reconstruction of the header field when a transmission of the header field contains errors.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for transmission of data blocks 
between two stations in which each of the data blocks has a 
synchronization pattern field, SYNC, signaling the beginning of the data 
block, a header field, HEADER, and an information field, INFORMATION, 
following the header field and the header field includes control 
characters for processing the following INFORMATION field. 
A process for transmission of data blocks, protocol H.22P (ITU-T Study 
Group 15, LBC 95-276, "ITU-T Recommendation H.22P") is already known in 
the art. 
In this protocol the structure, the formats of the data and the control 
field and structure for the data to be transmitted by the multiplexer, 
establish the multiplex process protocol. The multiplex protocol 
facilitates the processing of logical information, which arrives in the 
multiplex level via the adaptation level, into uniform data units. The 
protocol allows the transmission of arbitrary combinations of digital, 
audio and video data or other information under data control and provides 
a special protocol for prevention of data loss, which has a 
synchronization pattern of 31 bits in length. The HEADER field (31 to 63 
bits) and the INFORMATION field of fixed length are placed after it. The 
synchronization pattern must be detected by a correlation condition in a 
receiver so that the processing of the data blocks in the receiver can 
begin. 
In this process data loss can occur by loss of synchronization. Also an 
error-containing header field, HEADER, cannot be reconstructed. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved 
error-robust multiplex process, which reduces or eliminates the 
above-described disadvantages. 
This object, and others which will be made more apparent hereinafter, are 
attained in a process for data transmission between two stations by means 
of data blocks each having a synchronization pattern field, a header field 
and an information field, in which the synchronization pattern field 
indicates a beginning of one data block and the header field contains 
control characters for processing the information field following the 
header field. 
The process according to the invention comprises the step of providing a 
header control field, HEX, following the header for reconstructing a 
header transmitted containing an error. 
The process according to the invention has the advantage that it is 
possible to reconstruct the data in the case of an error containing 
transmission of the header field, HEADER, using the header control field, 
HEX. 
Advantageous additional features and improvements of the above-described 
inventive process are provided in the dependent claims appended 
hereinbelow. 
It is particularly advantageous when the header control field, HEX, 
includes an arbitrary number of bits, which can retain the information 
regarding possible changes in the header field, HEADER. 
This information can be a simple resetting of the count in the header 
control field, HEX, to zero. 
If one uses groups of data blocks with identical header fields and fixed 
length, the count remains constant and the indicator is reset to zero for 
a header change. 
In a preferred embodiment of the invention an incrementing or decrementing 
of a count in the header control field, HEX, is required to indicate a 
change in the type of data block. Alternatively, it is advantageous to 
increment the count within one type of data block and to set it to zero to 
indicate a change in the header field. 
In many embodiments it is advantageous if a header field change is 
indicated by decrementing the count in the header control field from a 
maximum value to zero. 
An additional type of header field change is selected by feeding back a 
signal indicating reception. The other signal is advantageously reset 
after waiting for the signal indicating reception. 
However a minimum time interval is expected prior to resetting with known 
signal on-times. 
For a reduction of errors the header field, HEADER, can be distributed over 
the types of packets. 
It is also advantageous when the header field, HEADER, and the header 
control field, HEX, are jointly advantageously protected. An advantageous 
error-protection method uses Reed-Solomon Codes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The data transmission of arbitrary data signals occurs via the 
hierarchically organized levels according to FIG. 1. The analog signals in 
part come from the individual data units to the coding level via the 
application level. After a digitalization step the contents of the logical 
channels LCN are further sent to the adaptation level of the multiplexer. 
The data reaches the multiplex level as MUX-SDU (Service Data Units) still 
in the separate channels. These levels combine the plurality of channels 
from the different data sources into a single channel and provide MUX-PDUs 
(Protocol Data Units). These data packets are filled with signals from the 
different sources according to the protocol of the invention. 
FIG. 2 shows the sequence of the control and data fields in one such 
MUX-PDU. The first field SYNC contains a synchronization word or pattern 
of variable length, which contains a bit sequence to be detected, e.g. of 
31 bits. The synchronization pattern is placed at the beginning of each 
data block in the synchronization pattern field, SYNC. For example, a 
Barker sequence or Williard sequence can be used as the synchronization 
pattern. 
In the header field, HEADER, a transmission scheme for the information 
block following the RET field is provided. One example of one such 
transmission scheme is described in the Protocol H.233 (ITU-T Study Group 
15). One such HEADER field has, e.g., 4 bits. All 16 states which the 
HEADER can be described with the 4 bits are tabulated in a table. If one, 
for example, only transmits audio signals, a certain bit sequence is set 
up, the information block for audio signals and video signals is divided 
and another bit sequence is sent. 
The information field, INFORMATION, follows the other fields. It is 
structured according to the control devices set up in the HEADER field for 
the different data sources. The information field is filled with data 
according to the multiplex scheme provided in the HEADER field until the 
packet length n is reached. 
A connection must be made as the first step for a data transmission. 
Moreover the length n of the data block is set up with the help of a 
control protocol. The length n is set up for the receiver and transmitter, 
also at a later point in time. Furthermore the control protocol must act 
during the transmission and make a comparison. 
Since the structural length n of the data blocks can be maintained constant 
by a time interval established in the control protocol, the following 
synchronization strategy may be used: to begin the transmission the 
receiver seeks the synchronization pattern. The synchronization pattern 
must only select at the suitable positions along the length n and must be 
tested. It is of advantage to use a shorter length for n at the beginning 
of the transmission (connection made with the help of a control protocol) 
during making the connection and at the synchronization start. A time 
point is established at which the length n is changed with the help of the 
control protocol. 
For detection of the SYNC on the receiver end of the data transmission in 
the demultiplexer a minimum number of bits is defined, which must agree 
between a pattern in the data flow and the synchronization pattern set up 
by the control protocol. If this minimum correlation condition is 
attained(correlation condition), the synchronization pattern has been 
found. In case a synchronization pattern has been found, an error 
detection process takes place for the HEADER. A successful structuring of 
the synchronization only occurs in the receiver, when an error-free HEADER 
was found at the synchronization pattern. An error detection can be a 
parity check in the simplest case, however it is advantageously performed 
with a CRC code. If an error is discovered during transmission of the 
data, the synchronization process is continued with the search for the 
next synchronization pattern. In this example the SYNC and the associated 
error-free HEADER must be found once in order to bring about the 
start-synchronization. 
When the start-synchronization process is performed successfully, the next 
synchronization pattern is sought at times after a complete loop of length 
n. At the same time a counter is incremented, when the synchronization 
pattern does not fulfill the correlation condition and the HEADER cannot 
be detected error-free. When the counter has exceeded a certain limiting 
value G2 (a whole number value, which is established by the control 
protocol), the synchronization must be considered as lost and according to 
the above-described scheme must be synchronized. Typically the 
synchronization is considered lost after four attempts and a new 
start-synchronization follows. 
If the synchronization is successfully completed during reception, the 
processing of the header field, HEADER; the header control field, HEX; and 
the information field, INFORMATION begins. 
The header control field, HEX, includes z bits, which are used as a count 
(0 to 2.sup.z-1). First the count is set to 0. If a multiplex packet is 
transmitted with the same header field, HEADER, as the previous packet, 
the count in the header field HEX is incremented. If the count already has 
reached the value 2.sup.z-1, the counter state is retained. If the header 
field changes, count is again set to 0. If the header field, HEADER, 
contains errors, the associated multiplex packet is not further processed. 
A later correction process is possible with the help of the count. If a 
valid header field, HEADER, with associated header control field HEX is 
present in a following packet, the previous header field can be inferred 
with the aid of the count in the header control field, HEX ("Backwards 
detection"). This situation can be illustrated by example. Three received 
multiplex packets are considered. Packet 2 may be received with an 
error-containing header and is thus temporarily retained in memory. If the 
count HEX 3 in packet 3 is larger or equal to 1, thus the previously 
supplied packet 2 may be processed with the header field from packet 3. If 
the count HEX 3 is in contrast 0, thus the header field for packet 3 is 
not identical with the error-containing header field for packet 2. Thus it 
is better to process the packet 2 in temporary memory with the header from 
packet 1. 
Thus it is of course uncertain whether the correct header field was used, 
however usually it is advantageous to process the data in the packet with 
an incorrect header rather than not process the multiplex packet. 
Additional embodiments are described in the following paragraphs. 
An additional use of the header control field (HEX) presupposes that a 
minimum number of successive multiplex packets with the same header field 
are assembled for transmission in the transmitter, e.g. with the aid of a 
control protocol, before the header field changes. Thus the process can be 
made more error-robust. Accordingly then a common group of m packets would 
then be provided (with identical header field, HEADER). For example in the 
three data packets with HEX 3=0 and m&gt;1, it would be clear that the HEADER 
2 must be identical with HEADER 1. In case of a common HEADER group one 
could modify the ways of counting in the header control field also in the 
following way: 
a) the count in the header control field, HEX, for the header field, 
HEADER, is identical with the count in the same group of data blocks with 
equal header fields. Then at the beginning of a new group the count 
according to the above-described scheme or rules is incremented and/or set 
to 0; and 
b) in each group the header control field HEX is set equal to 0 in the 
first packet or data block and subsequently incremented for every other 
data block in the group. 
The further transmission to the decoder of the data sources is delayed by 
the intervening storing of a packet or data group with an error-containing 
header field (HEADER). For certain data sources this delay should be 
avoided. Thus these data sources should be transmitted in separate 
multiplex fields which are not defined by the header field, HEADER. 
"Forward signaling" is also another possible use for the header control 
field, HEX. How often the actual header field, HEADER, is used in the 
other data blocks is determined in the transmitter. This count can be 
determined from the actual buffer state of the data sources, statistical 
considerations or other strategies. After determination of the current 
header fields with an associated count in the header control field HEX 
error-containing data groups in the receiver can be corrected in case they 
are found to be in the value range located in the header control field 
HEX. In the receiver then the correct header field for the 
error-containing data block can be immediately and directly determined. 
This results in no delay. The further in advance that the header field 
contents can be determined, the better the process works. 
The following counting method is given as an example: first it is 
determined how often the actual header field HEADER appears or is used 
uninterruptedly in the following multiplex packets or data groups. This 
value m can, e.g., be determined from the buffer state of the data source 
(alternatively an arbitrarily chosen count is conceivable). The m is 
assigned to the count in the header control field, HEX. If m goes beyond 
the value range of the count, the count is set to a maximum value of 
2.sup.Z -1. A change of the header field is possible after at least 
2.sup.z data blocks or packets. Should a header field change take place, 
the count is decremented in each packet or data block until the count 
reaches 0. In the following packet or data block the header field may be 
changed. The count is now reset or set anew to m and/or 2.sup.z -1. The 
value of m is eventually newly determined. 
It is significant and also possible to combine both the rearward detection 
and forward signaling methods as needed. The use of the count in the 
header control field, HEX, is possible both with variable and also with 
constant multiplex length. 
In another embodiment an especially reliable determination of the state of 
the header fields in each data block or packet is possible by coordination 
between the transmitter and receiver. The header control field, HEX, 
includes two bits (B1 and B2). The bit B1 is set to signal a header field 
change while the bit B2 is set to signal "answer to a desired header field 
change signal from the receiver side". In the following a possible scheme 
for use of the bits B1 and B2 is described: 
At the start both bits are set to 0. Next based on that the header field 
does not change. If the transmitter would like a new header field the bit 
B1 is set to 1. The receiver detects this in an error-free transmission of 
the desired change and sets the bit B2 to 1. As soon as the transmitter 
contains these signals, the header field, HEADER, can be changed. When the 
answer does not arrive because of a transmission failure, the transmitter 
retransmits the signal for a desired change in the header field after a 
predetermined waiting time interval. The transmitter does not change the 
header field before it detects that B2=1. The receiver then sets B2 to 1, 
when it immediately receives B1=1. In other cases then B2 is set equal to 
0. For the resetting of B1 to 0, the following possibilities are 
conceivable: 
B1 is again set equal to 0 after the header field change signaling. A 
resetting of B1 to 1 may occur first, when the answer B2=1 is received, or 
when a minimum time interval expires since the header field change 
signaling event. B1 is set to 1 until the return signal B2=1 is received. 
For both methods it is convenient to determine the running time which is 
necessary in order to provide a multiplex packet for reception and the 
reverse. With this knowledge the data packet or data block in which the 
data change should be performed can be determined in the receiver, in the 
event that the answer signal B2 would be received error-free by the 
transmitter. 
For all embodiments it is possible to use error-protection methods. Thus 
the header control field, HEX, can be protected with the aid of an 
error-detecting and subsequently an error-correcting code. It is of course 
also possible to regard the header field HEADER and the header control 
field HEX as a data block which contains a common error protection. 
If one would like to make a packet-overlapping distribution of the header 
fields, HEADER, (Interleaving), it is presupposed that the header field 
may only be changed after m packets (m packets following each other 
successively with identical header fields corresponding to a "group"). 
With the establishment of a number of multiplex packets with constant 
header field, it is possible to distribute the header field over the enter 
group. In order to avoid delay at the receiver it is significant that the 
header field in a transmitted data block within a group contain the 
multiplex scheme of the following group. At the beginning of the 
transmission of the data an empty group without information must be first 
transmitted to provide the header field for the following group which does 
contain information. 
For the protection of the header fields (HEADER) distributed over the m 
data blocks, different error correction methods are possible, e.g., with 
the help of Reed-Solomon Codes. According to the Reed-Solomon Coding one 
obtains 2.sup.k -1 symbols, which are made up of k bits. The distribution 
of the header fields, HEADER, along with the error protection along the m 
packets should occur symbolically. Should more symbols be present then 
packets, it is convenient to place a larger number of symbols in the front 
portion of the group. So that reception among other things is possible a 
decoding of the Reed-Solomon codes takes place before all the symbols are 
known. The packet-overlapping interleaving allows an improved error 
correctability of the header field, since the effect of bundling errors is 
reduced. 
For all embodiments described all multiplex protocols provided with the 
header fields, HEADER, profit from the use of the header field to be 
transmitted at least m-times in succession. In principle the determination 
of this count m means nothing other than the expansion of the multiplex 
packet length when a fixed length is used; without that there is a 
disadvantageous delay. 
If in spite of all other error strategies no possiblity exists to determine 
the header field (HEADER) associated with a packet, the final error-free 
header field received should be used. 
The disclosure of German Patent Application 1 96 14 739.5 of Apr. 15, 1996 
is incorporated here by reference. This German Patent Application 
describes the same invention as described hereinabove and claimed in the 
claims appended hereinbelow and forms the basis for a claim of priority 
under 35 U.S.C. 119. 
While the invention has been illustrated and described as embodied in an 
error-robust multiplex process with header control field, it is not 
intended to be limited to the details shown, since various modifications 
and changes may be made without departing in any way from the spirit of 
the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention. 
What is claimed is new and is set forth in the following appended claims: