Access method.

p0001The invention relates to a method for accessing to time-division multiplex channels of a transmission system by means of tokens. Such methods and communication systems that operate according to this access method, are known.

p0002From EP-A-0115883 a method for controlling access is known on a time-division multiplex message transmission path of a transmission system, wherein in one time frame a part of time slots as the perch channels, and part of the time slots for the transmission of data packets (bursts) and the remaining time slots are used for Signalisierzwecke. The allocation of time slots between the fixed channels and burst channels is variable. The transmission system includes a plurality of stations that can simultaneously access the transmission.

p0003The access to the time slots which are intended for package-operation takes place in that the relevant station transmits on a signaling channel, a bit pattern, which plays a certain priority. Access to the time slot is replaced by the station transmitting the bit pattern with the highest priority.

p0004To access the probability for each station on the reserved time slots for burst mode, however, depends not only on the priority but also on the length of the bursts transmitted. Assuming that all the station in the central post with the same priority, but results for the stations then no predictable access times when the packet lengths vary greatly transmitted. This can lead to a component present in a station buffer overflows, and thereby lost messages, because this station was unable to access time to a time slot.

p0005It is therefore an object of the invention to provide a method for accessing a channel of a time division system used for the transmission of packets, which allows each station to access the channel within a definable access time.

p0006This object is inventively by a method in which each station before sending a packet in a signaling channel a token emits containing a priority number and a packet length number that indicates the length of the transmitted packet, and only when followed by a reception of their tokens the signaling channel transmits the packet, each station the starting point for their own mission in one of the following time frame determined from the information provided by the token contained in the Signalisierkanälen, in which it forms the sum of all packet lengths, which are specified in the token, which in itself are an objective in the time frame of one's own token space.

p0007it when the station in the case that the light emitted by their token has been overwritten by another station with higher priority, increases the priority number of their tokens to a certain level, and then their token again emits on a signaling channel. It is particularly advantageous The value of the increase corresponds expediently to the number of other stations in the transmission system. Finally, if an access is carried out on the transmission path and the packet was sent, the priority number of the relevant station is reduced again to the original value.

p0008The stations transmit invention only a token from when a free token is present in the signaling channel. If the signaling channel on the other hand a busy token available, send the stations no token from, but wait until a subsequent frame a free token is present.

p0009The free token and the busy token generated by a particular station of the transmission system, which evaluates the tokens contained in a considered frame. If the requested token by these transmission capacity less than the maximum in a frame available as a free token is transmitted in the following frame. If the sum of defined by the emitted in a given context token packet lengths greater than the length of one or more frames, so as many consecutive frames are provided with a busy token, can be fully occupied as a result of the requested packet lengths and until the following frame a free token sent.

p0010The inventive method is hereinafter explained with reference to figures shown in the examples.

p0011It shows:<dl id="dl0001"><dt>Fig. 1</dt><dd>the establishment of a timeframe for exclusive packet operation;</dd><dt>FIG. 2</dt><dd>the establishment of a time frame for package and fixed channel operation;</dd><dt>Fig. 3</dt><dd>a first example of access to the transmission and the allocation of the channels; and</dd><dt>Fig. 4</dt><dd>a second example of access to the transmission and the assignment of the channels.</dd></dl>

p0012The time frame schematically illustrated in Fig. 1 contains 1088 channels with a length of 1088 bytes. Of these are 992 channels for the transmission of information and the remaining 96 channels are signaling channels. The first 32 signaling channels form the so-called system control block SCB, where necessary for the operation of the communication system control information is transmitted. Another four channels form a so-called token control block TCB, one of which contains the first channel either a busy token or a free token. The other three channels are used as a delay compensation. The remaining 60 channels form a so-called token block TB. A token has a length of 2 bytes and consists of a priority number and a packet length PZ number PL. The packet length is specified in units of 4 bytes.

p0013FIG. 2 shows a time frame, the channels are reserved and partly provided for packet operation partly for fixed channel operation. It also includes the system control block SCB, the token control block TCB and the token block. Since the number of provided for packet-mode channels is lower than in the example of Fig. 1, only correspondingly less signaling channels, namely 28 for the token block.

p0014Each station of the transmission system is a certain token space and assigned a certain priority number. The assignment of priority numbers and the token places is such that, different priority numbers are awarded to those stations assigned the same token space. The stations assigned priority numbers may be the same, unless the stations different token places have been allocated. The token of a station rightful priority depends on the priority number and the station associated token space.

p0015A particular station of the transmission system the halogenated busy and free tokens in response to the sum of all the packets to be transmitted. A free token in a frame N indicates the stations that can be sent in the following token block of the frame N token. A busy token indicates that no token block is present in the considered frame, but all channels are occupied for transmitting information. The particular station counts all existing under N packet length numbers together. The sum of all packet length numbers gives the number of frames in which they must send out a busy token. If the sum of all packet lengths less than 992 bytes, it sends the following frame N + 1 from a free token.

p0016Want a station to transmit a message, it looks for a free token in the token-control block. After recognizing a free token she writes a priority of their number and the packet length of the message to be transmitted in the assigned token space. Should other stations be assigned the same token place, a token can be on this token place already are. Possesses the token the considered station a higher priority number than the token of the other station, so the observed station overwrites the other token. Otherwise, it waits for the next time frame. Then controls the station looked at whether your token has been overridden by a third station. Will she find her token in the token block again, it gains access to the transmission path.

p0017If a station has not allowed access to the transmission path because another station has overridden their token, increasing its priority number by a certain amount and sends its new token in one of the following frame. The amount of the increase is in the simplest case is the number of stations that have been assigned the same token space. By the amount in question and because of the assignment of a particular token place each station can calculate their access time deterministic. After gaining access, the priority number is reset to its original value.

p0018Has received on the transmission path a station access, you can send your package in one of the next time frame. The starting point for this release determines the station from those specified in the token block packet lengths of packets originating from stations with a date prior to their own token space.

p0019A packet consists of a header, the actual information and Datensicherungsbits. The header begins with a start word and contains the recipient and the sender as well as the length of the present in the packet information.

p0020Fig. 3A shows a time frame N in FIG. 1, the token control block a free token and the token block on the fourth, fifth, eleventh and thirtieth token space includes a token. The station with the token number 4 has the priority number 10 and the packet length specified number 74th This station has thus 74 x to transfer 4 bytes. The total number of in this example, information to be transferred is 656 bytes. Since the sum of all pacts lengths does not exceed the available in the next frame N + 1992 bytes, a free token is sent and a token block provided in the next frame in the token control block again. As shown in FIG. 3b, the transmission of the first packet starts on the first channel according to the token block. The other packages directly adjoin the previous packages. The rest of the frame N + 1 is not occupied.

p0021FIG. 4a shows just like also FIG. 3a shows an example of a time frame N, which contains a free token in the token control block and four Token on four places token in the token block. However, here the sum of all packet lengths with 1800 bytes is greater than the transmitted in a frame 992 bytes. Therefore, the frame N + 1, according to the system control block on a busy token. The frame N + 1 does not contain a token block. Directly after the busy token the first packet to which seamlessly connects the second packet starts (Fig. 4b). The part of the second package, which is part N + 1 is no more room is transmitted in frame N +. 2 At the second packet to the third and fourth packet follows. Since the frame is N + 2 not fully occupied, it includes a free token and a token block (Fig. 4c). Since + 2 no token is included in the token-block of the frame N, in frame N + 3 (Fig. 4d) is in general not transmit information.

p0022Upon receiving each station the recipient address contained in the header of the packets with their own address compares. If they match the addresses the station stores the packet in a receive memory. The length of the packet it removes the header and can thereby determine the end of the packet and the beginning of the next packet carefully.

p0023The distribution of the time slots to fixed channels, and for the transmission of packets used channels is controlled depending on the transmission capacity needed in each case. Corresponding control information is transmitted in the system control block. In this case also the time slots of a time frame may be divided into a plurality of so-called sub-frames, which are each included their own control block token, token block and a certain number of time slots for transmitting information and assigned to different groups of stations. This reservation of transmission capacity for individual groups of stations is possible even with a heavy load of the transmission path.