CA2132854A1 - Process for allotting transmission time slots in a passive optical network - Google Patents
Process for allotting transmission time slots in a passive optical networkInfo
- Publication number
- CA2132854A1 CA2132854A1 CA002132854A CA2132854A CA2132854A1 CA 2132854 A1 CA2132854 A1 CA 2132854A1 CA 002132854 A CA002132854 A CA 002132854A CA 2132854 A CA2132854 A CA 2132854A CA 2132854 A1 CA2132854 A1 CA 2132854A1
- Authority
- CA
- Canada
- Prior art keywords
- message
- counter
- cells
- subscriber station
- counter reading
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04Q—SELECTING
- H04Q11/00—Selecting arrangements for multiplex systems
- H04Q11/04—Selecting arrangements for multiplex systems for time-division multiplexing
- H04Q11/0428—Integrated services digital network, i.e. systems for transmission of different types of digitised signals, e.g. speech, data, telecentral, television signals
- H04Q11/0478—Provisions for broadband connections
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/27—Arrangements for networking
- H04B10/272—Star-type networks or tree-type networks
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J3/00—Time-division multiplex systems
- H04J3/16—Time-division multiplex systems in which the time allocation to individual channels within a transmission cycle is variable, e.g. to accommodate varying complexity of signals, to vary number of channels transmitted
- H04J3/1694—Allocation of channels in TDM/TDMA networks, e.g. distributed multiplexers
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/08—Time-division multiplex systems
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L12/00—Data switching networks
- H04L12/54—Store-and-forward switching systems
- H04L12/56—Packet switching systems
- H04L12/5601—Transfer mode dependent, e.g. ATM
- H04L2012/5603—Access techniques
- H04L2012/5604—Medium of transmission, e.g. fibre, cable, radio
- H04L2012/5605—Fibre
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L12/00—Data switching networks
- H04L12/54—Store-and-forward switching systems
- H04L12/56—Packet switching systems
- H04L12/5601—Transfer mode dependent, e.g. ATM
- H04L2012/5603—Access techniques
- H04L2012/5609—Topology
- H04L2012/561—Star, e.g. cross-connect, concentrator, subscriber group equipment, remote electronics
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L12/00—Data switching networks
- H04L12/54—Store-and-forward switching systems
- H04L12/56—Packet switching systems
- H04L12/5601—Transfer mode dependent, e.g. ATM
- H04L2012/5672—Multiplexing, e.g. coding, scrambling
- H04L2012/5674—Synchronisation, timing recovery or alignment
Landscapes
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Computing Systems (AREA)
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Data Exchanges In Wide-Area Networks (AREA)
- Small-Scale Networks (AREA)
- Optical Communication System (AREA)
Abstract
Abstract In a passive optical network, the message cells output by a plurality of subscriber terminals are sup-plied to a single connection of a head end of a switching system; in this arrangement each subscriber terminal transmits a number of successive message cells grouped into a data block, which is preceded by a sequence of calibrating characters, only following a transmit author-ization which is determined depending on its defined message cell rate and is output by the head end.
Description
~1328S~
i HI~ ~.lE~!:
;~T TRANSLAT10~ ~ ~
,.
Process for allotting transmis~ion time ~lot3 in a passive optical network :. -Current endeavor3 to equip subscribers of com~munication systems with a subscriber line that supports S both narrowbant and broadband services have led to the development of passive optical networks in which a plurality of subscriber terminals are connected via optical fibers and passive couplers to a single connec-tion of a switching system. An asynchronous transmission method is provided for transmitting data in said passive optical networks in which data are transmitted in the payload section of message cells transmitted aB required.
In a passive optical network the fiber-optic links - between the individual ubscriber terminals and the connection of the switching 3ystem must be calibrated to an identical transit time; in addition each subscriber terminal must transmit a series of calibrating characters each time before transmitting mes3age cells, with the aid of which a receiving device is adjusted to the amplitude and the phase of the current incoming message cell in the connection of the switching system. Both the calibration of the fiber-optic links and the transmission of the calibrating characters require transmission capacity which i8 not available for the tran~mission of user information in the form of message cells.
It i9 possible in thi~ case to increase the transport bit rate in the passive optical network in comparison with the transport bit rate in the switching sy~tem with which the passive optical network is exchang~
ing data, which consequently makes transmission capacity aYailable for the caIi~ration and adjustment operations in the psssive optical network.
` ~132854 With this approach, the outlay for the increa~ed transport bit rate in the passive optical network i8 clearly disadvantageous.
Another approach i8 described in EP-A-O 337 619 and in Globecom 1990, pp. 206..... 211 "A 3-ISDN Local Distribution System Based on a Passive Optical Network".
With this approach, each me~sage cell transmitted by a subscriber terminal via the passive optical network in the direction of the connection of the switching system is preceded by a sequence of calibrating characters.
Assumlng firstly an identical transport bit rate in thP
passive optical network and in the switching sy~tem, and secondly a utilization of the transmission capacity u3ed by the synchronous frame in the switching sy~tem, each sequence of calibrating characters has a length of 2 bytes for example. In this case, it is clearly dis-advantagsous that the amplitude adjustment requires an increased adjustment outlay for such short calibrating characters.
The invention 301ves the problem of stating a proce3s for allotting for a passive optical network which avoids the aforesaid disadvantages.
~he problem iR solved by a proce~s for allotting transmission time slots for a transmission system having a plurality of subscriber ~tations which are connected via optical fibers and passive coupler~ to a head end common to all subscriber stations, wherein - the data supplied to a subscriber station or output by one are transmitted in message cells in the course of a virtual connection in accordance with an asynchron-OU8 transmission method, - each message cell has a payload section of constant length for accommodating the data and a message cell ~ 2 1 ~ 4 header of constant length preceding the payload ~ection, - a message cell rate i~ specified for each virtual connection, - the counter reading of a counter provided on the head end side for eac~ virtual connection is incremented regularly depending on the specified mes~age cell rate, - a status character is set for a counter whose counter reading has exceeded a predetermined counter reading, - the counters are interrogated for the presence of set status characters, - a subscriber station who~e associated counter has a set status character during the interrogation receives a transmlt authorization for the largest possible number of message cells corresponding to the respect-ive counter reading, the counter reading of Raid counter being decremented by an amount corresponding to said number and the status character being reset once the reading falls below the predetermined counter reading, and - a ~ubscriber station that has received a transmit authorization transmits a sequence of calibrating characters and directly following that a number of message cells corresponding to the respective transmit authorization to the head end.
In addition to the po~ibility of defining a generous number of calibrating characters, the process according to the inventionl in which the passive optical network can be operated at the 3ame transport bit rate a~
the ~witching system connected to the passive optical n~twork, allows the length of messaye cells grouped into data blocks to be adjusted to the currently svailable transmission capacity. As the load increases, the time interval between two transmit authorizations will namely be extended for a respective sub~criber terminal, which, particularly in the case of connections having a high bit .~ . : - i , ~1328~
.
rate, leads to the grouping of a plurality of message cells into a data block preceded by only one sequence of calibrating characters, with the result that the number of calibrating characters per mes~age cell is reduced;
connections having a low bit rate, which may be time-critical, in this case form data blocks with only one message cell in each case up to high loading values.
According to one particular refinement of the invention, the largest possible number of message cells grouped into a data block i8 limited by a defined maximum value.
On the one hand this measure lLmits the occur-rence of message cells of a connection in large numbers, and on the other hand it l;mits the repetition interval lS of the counter interrogation.
According to a further embodiment of the inven-tion, once a cyclical interrogation of all other sub-scriber terminals for which a connection is established has found no set status character, a subscriber terminal will be issued a transmit authorization for an additional message cell without the counter reading of the associ-ated counter being decremented.
This measure permits the transmis~ion of sub-scriber terminal-specific information to the head end beyond the defined message cell rate.
According to a further refinement of the inven-tion, a subscriber terminal fully utilizes the number of message cells indicated in the transmit authorization by transmitting empty aRll~ if necessary-This measure avoids idle times on the fiber ~nd facilitates regular activation of the subscriber ter-minals even if the traffic is light.
The invention will now be explained as an ..
21328~4 exemplary embodiment to the extent required for compre-hension thereof with reference to figures, in which:
Fig. 1 show3 the ba~ic representation of a passive optical network, and Fig. 2 and Fig. 3 show a flowchart in each case.
Fig. 1 shows a transmis~ion system that is known per se, for example from British Telecom Technological Journal, Vol. 7, No. 2, April 1989, pp. 151-160, in which a plurality of subscriber tations with their subscriber terminals NTl...NT4 are connected via optical fibers and passive couplers SPl...SP4 to a head end LT of a switch-ing system.
Data having a binary data structure are tran~-mitted in message cell~ in the switching system and the transmission system in accordance with an asynchronous transmission method. The mes age cells are assumed in each case to have a payload section having a length of 48 bytes in the exemplary embodiment and a message cell header having a length of 5 bytes in the exemplary embodiment preceding the payload section. The message cell header contains the identification of the connection to which the respective message cell belongs. In the exemplary embodiment the transport bit rate is assumed to be 155.52 Mbit/s and the user information bit rate is assumed to be 149.76 Mbit/s on the fiber-optic links.
The message cells transmitted from the head end via the optical fibers and passive couplers to the subscriber terminals having a fir~t wavelength reach all the subscriber terminals, a respective sub~criber ter-minal only evaluating the me~sage cells indicated as belonging to the connection on the basis of their 21328~
identification in the message cell header.
Message cells to be transmitted from the sub-scriber terminals to the head end are transmitted via the optical fi~ers and the passive couplers using the 80-called wavelength-division multiplex method on a second wavelength. Message cells transmitted from various ~ubscriber terminals to th head end must on the one hand not overlap one another in terms of time, but on the other hand they mu~t follow one another without an appreciable interval. To avoid overlapping, it is pro-vided that a Rubscriber terminal only transmits message cells in the direction of the head end when it has received a transmit authorization from the head end (polling); in thiq case according to the invention in each case as many successive mes~age cells are trans-mitted as are indicated in the current transmit authori7-ation. In the case where no number of message ~ells corresponding to the transmit authorization is ready in a subscriber terminal, the number of message cells according to the transmit authorization can be fully utilized by transmitting empty cells. An empty cell is understood to refer to a message cell that does not contain any data in its data ~ection and is not forwarded to the switching system.
In general, the transit times between the indi-vidual subscriber terminals and the head end are differ-ent, which can be attributed to different path distances and temperature fluctuation~ of the individual optical fibers. To compensate for these differences in transit times, a delay device is provided for each subscriber terminal; the subscriber terminal with the longest transit time determine~ the minimum transit time for all the ~ubscriber terminals here. A calibration to identical transit times between the subscriber , ~1328~4 ' terminals and the head end en~ures a virtually phase-locXed arrival o~ the message cells at the head end. The calibration of a sub~criber term;nal-~pecifir delay device, which may be arranged in the subscriber terminal, is performed during the connection establishment and is repeated at regular time~intervals of, for example, one second.
The fiber-optic links between the individual subscriber terminals and the head end usually have different attenuations, so that the binary characters of message cell8 output by different subscriber terminals can have different amplitudes. A receiving device pro-vided on the head end side requires a certain amount of time to adjust to the amplitude of a currently incoming message cell, and the outlay for the adjustment is higher, the less time available for the adjustment. To avoid the corruption of user information, it is provided that a ~ubscriber term;nal transmits a defined number of, for example, 64 bits calibrating characters each time before transmitting message cells, which are directly followed by the me~sage cells to be transmitted. In addition to the aforementioned adjustment to the ampli-tude of a currently incoming message cell, the calibrat-ing characters permi~ a fine adjustment to the phase position of the currently incoming message cells.
~ he time required for calibration of the sub-scriber terminal-specific delay devices and the trans-mission of the calibrating characters is not available for the transmis~ion of user information in the form of message cells; during a transmission of calibrating characters of predetermined length with each message cell, a substantial part of the transmission capacity not available for the tran~mission of user information is ., . ~ ' ~ : , .~.
~;1328~4 used. According to the invention, a plurality of me~sage cells output by a subscriber terminal can be grouped into a data block, each data block being preceded by calibrat-ing characters of constant length, so that the proportion of calibrating characters to the transmission capacity used is reduced in relation to the user information. A
minimum number of one message cell per data block can nevertheless be provided, in particular for connections having a low data rate of, for example, 64 kbitls. ~he delay cau3ed by the collection of message cells to form a data group is less noticeable, the higher the message cell rate output by a subscriber terminal.
A counter is provided for each subscriber ter-minal, the counter reading of which is incremented or decremented depending on two procedures described below.
The counters are preferably arranged in the head end of the switching system.
~ ig. 2 shows a flowchart, according to which the counter reading of all counters is incremented during an incrementation interval by a respective amcunt correspon-ding to the transmission bit rate specified for the associated subscriber terminal. The amount by which the counter reading of a respective counter is increment~d i~
assumed to be a multiple of a largest common sub-channel contained in all the possible transmission bit rates. In the exemplary embodiment, a largest common sub-channel is assumed to have a transmi~sion bit rate of 64 k~it/~.
~ ccording to the top action field INCn in Fig. 2, a respective counter is incremented by the amount corre-sponding to its multiple of sub-channels. According to 2132~4 g the following decision field FW, a check is made as to whether the coun~er reading of the counter currently in question has exceeded a predetermined level FW. The exceeding of the level FW indicates that at least one message cell is probably ready for transmission at ths associated subscriber terminal. If the level FW is exceeded (Y~ a status character is set for the respect-ive counter in accordance with the action field CFS. The setting of the status character can be produced by the overflow of a binary counter with predetermlned bit width. In the exemplary embodiment, the setting of a status character is assumed to be performed when the amount, which can be represented by 8 bits at most, is exceeded. In the case where the level FW for the counter currently in question is not exceeded (N) in the decision field FW, or a status character was set for the counter in question in accordance with the action field CFS, the procedure continues with the decision field LC. In the decision field LC it is checked whether all the counters of ~ubscriber terminals which have just established connections during a current incrementation interval have been incremented. If at least one ~ounter is still to be incremented (N), the procedure continues with the action field I~Cn. If all the counters have been incremented (Y), the procedure continues with the action field TW. In the action field TW, the end of the current incrementa-tion interval is awaited in order to begin a new incre mentation interval then with the action field I~Cn. In the ex~mplary embodiment an incrementation interval is assumed to have the tran~mission duration of 3,S45 bits.
The flowchart illustrated in Fig. 3 brings about the issuing of transmit authorizations for a respective number of message cells for the individual subscriber ter~;nals. By se~ting status characters in the flowchart ~1328~4 - 1 o according to Fig. 2, the interrogation as to whether at lea~t one message cell i8 probably ready for transmission at a respective sub~criber terminal i8 reduced to the interrogation o~ said status character. According to the flowchart in Fig. 3, all subscriber terminals for which a connection i~ currently established are interrogated for the presence of set status characters. According to the action field SHn, the procedure moves on from the last counter interrogated to the next counter. According to the decision field CFG, the counter currently in question is interrogated for a set ~tatus character. If a status character i8 set tY) for the counter in ques-tion, according to the action field SB a transmit author-ization for the associated subscriber terminal is pro-vided. The transmit authorization indicates here themaximum number of message cells which are to be expected depending on the cumulative counter reading of the respective counter in the as~ociated subscriber terminal.
It can be provided here that the maximum number is li~;ted by a defined maximum value. According to the action field D~Cn, the counter reading of the counter currsntly in question is decrementsd by the amount corresponding to the number of message cells provided for transmission. If, for example, the counter reading of the counter just interrogated has a current counter reading of 768 and if a counter reading of 256 corresponds to one message c011 ready for transmission, then a transmit authorization is provided for three message cells and the counter reading is decremented to 219. According to the action field CFR, the status character of the counter currently in question is reset once the counter reading falls below the predetermined counter rsading. According to the action field WAIT, there is a wait until the transmission duration for the transmi~sion of the number of message cells corresponding to the directly preceding , : ,:: , ~ -~1328~,,4 transmit authorization has elapsed.
If, however, the interrogation in the decision field CFG had indicated that no C~tatus character is set (N) for the counter in question, then the procedure S continues with the decision field ALT. In the decision field ALT, starting~ from the counter who~e associated subscriber terminal transmitted a message cell last, it is checked whether no mess~ge cells are ready for trans-mission in all the other subscriber terminals for which connections are currently e~tablished. If all the other subscriber terminals have not yet been interrogated (N), then the procedure continues with the action field S~n, in which the procedure moves on to the next counter. Said counter is interrogated for the presence of a ~et status character. Even if only one counter has a set status character during this cyclical interrogation of the status characters, then the procedure continues as described further above for the decision field CFG in the ca~e of (Y).
It is now to be as~umed in accordance with the decision field ALT that no message cells are ready for transmission (Y) in all the other subscriber terminals for which connections are established; the procedure then continues with the action field SEB in which the tran~mit authorization is provided for an extra message cell. An extra message cell refers to a messaqe cell transmitted without the counter reading for the subscriber terminal that receives permission to transmit the extra message cell being decremented. From the action field SE~, the procedure continues further with the action field WAIT, which has already been discussed further above. From the action field WAIT, the procedure continues with the decision field ADJ. In the decision field ADJ a check is ~ 2;L328~
made as to whether a calibration of the as~ociated delay device is required for the sub~criber terminal for which a transmit authorization has ju~t been provided. The requirement for calibr~tion may be constituted by a time criterion, which in the exemplary embodiment i5 as~umed to comprise a perio~ of one seeond. In the ca~e where calibration is required (Y), the procedure continues with the action field ESW in which there is a wait for the duration of the period for calibrating the respective delay device.
In the case where no calibration is required (N) according to the decision field ADJ, or where the waiting time according to the action field ESW has expired, the procedure continues with the action field SEND. According to the action field SEND, the respective subscriber terminal is issued a transmit authorization for a corre-sponding number of message cells. A subscriber terminal that has received a tran~mit authorization reacts by transmitting the calibrating characters, which are followed directly by a number of message cells in accord-ance with the transmit authorization. If there are not as many message cell~ ready for transmission in a subscriber terminal as are indicated in the transmit authorization, then the respective sub~criber terminal can fully utilize the transmit authorization by transmitting a number of empty cells. Empty cells are mes~age cells which contain no information in their data ~ection and are not for-warded to the switching system.
During the calibration of a delay line, as already mentioned, no user information is transmitted from the individual subscriber terminals to the head end.
If a FIF0 memory is provided in the head end which is maintained at an increased filling level with message ::;:
. - :'~
~. : :
~ ' ~
~ ~1328~4 cells transmitted by the subscriber terminals, then the calibration of delay devices of the individual subscriber terminals can remain u~noticeable to the Qwitching system if message cells are regularly read out of the FIFO
memory and the filling level i~ thus reduced during the calibration; on average the counters have increa~ed counter readings i~mediately after a calibration oper-ation, which leads to the transmission of longer data blocks and hence to a briefly increased data rate, with the result that the FIFO memory reaches an increased filling level again. A continuous ~tream of data which is delayed by the time required for calibration is produced for the switching .Qystem.
i HI~ ~.lE~!:
;~T TRANSLAT10~ ~ ~
,.
Process for allotting transmis~ion time ~lot3 in a passive optical network :. -Current endeavor3 to equip subscribers of com~munication systems with a subscriber line that supports S both narrowbant and broadband services have led to the development of passive optical networks in which a plurality of subscriber terminals are connected via optical fibers and passive couplers to a single connec-tion of a switching system. An asynchronous transmission method is provided for transmitting data in said passive optical networks in which data are transmitted in the payload section of message cells transmitted aB required.
In a passive optical network the fiber-optic links - between the individual ubscriber terminals and the connection of the switching 3ystem must be calibrated to an identical transit time; in addition each subscriber terminal must transmit a series of calibrating characters each time before transmitting mes3age cells, with the aid of which a receiving device is adjusted to the amplitude and the phase of the current incoming message cell in the connection of the switching system. Both the calibration of the fiber-optic links and the transmission of the calibrating characters require transmission capacity which i8 not available for the tran~mission of user information in the form of message cells.
It i9 possible in thi~ case to increase the transport bit rate in the passive optical network in comparison with the transport bit rate in the switching sy~tem with which the passive optical network is exchang~
ing data, which consequently makes transmission capacity aYailable for the caIi~ration and adjustment operations in the psssive optical network.
` ~132854 With this approach, the outlay for the increa~ed transport bit rate in the passive optical network i8 clearly disadvantageous.
Another approach i8 described in EP-A-O 337 619 and in Globecom 1990, pp. 206..... 211 "A 3-ISDN Local Distribution System Based on a Passive Optical Network".
With this approach, each me~sage cell transmitted by a subscriber terminal via the passive optical network in the direction of the connection of the switching system is preceded by a sequence of calibrating characters.
Assumlng firstly an identical transport bit rate in thP
passive optical network and in the switching sy~tem, and secondly a utilization of the transmission capacity u3ed by the synchronous frame in the switching sy~tem, each sequence of calibrating characters has a length of 2 bytes for example. In this case, it is clearly dis-advantagsous that the amplitude adjustment requires an increased adjustment outlay for such short calibrating characters.
The invention 301ves the problem of stating a proce3s for allotting for a passive optical network which avoids the aforesaid disadvantages.
~he problem iR solved by a proce~s for allotting transmission time slots for a transmission system having a plurality of subscriber ~tations which are connected via optical fibers and passive coupler~ to a head end common to all subscriber stations, wherein - the data supplied to a subscriber station or output by one are transmitted in message cells in the course of a virtual connection in accordance with an asynchron-OU8 transmission method, - each message cell has a payload section of constant length for accommodating the data and a message cell ~ 2 1 ~ 4 header of constant length preceding the payload ~ection, - a message cell rate i~ specified for each virtual connection, - the counter reading of a counter provided on the head end side for eac~ virtual connection is incremented regularly depending on the specified mes~age cell rate, - a status character is set for a counter whose counter reading has exceeded a predetermined counter reading, - the counters are interrogated for the presence of set status characters, - a subscriber station who~e associated counter has a set status character during the interrogation receives a transmlt authorization for the largest possible number of message cells corresponding to the respect-ive counter reading, the counter reading of Raid counter being decremented by an amount corresponding to said number and the status character being reset once the reading falls below the predetermined counter reading, and - a ~ubscriber station that has received a transmit authorization transmits a sequence of calibrating characters and directly following that a number of message cells corresponding to the respective transmit authorization to the head end.
In addition to the po~ibility of defining a generous number of calibrating characters, the process according to the inventionl in which the passive optical network can be operated at the 3ame transport bit rate a~
the ~witching system connected to the passive optical n~twork, allows the length of messaye cells grouped into data blocks to be adjusted to the currently svailable transmission capacity. As the load increases, the time interval between two transmit authorizations will namely be extended for a respective sub~criber terminal, which, particularly in the case of connections having a high bit .~ . : - i , ~1328~
.
rate, leads to the grouping of a plurality of message cells into a data block preceded by only one sequence of calibrating characters, with the result that the number of calibrating characters per mes~age cell is reduced;
connections having a low bit rate, which may be time-critical, in this case form data blocks with only one message cell in each case up to high loading values.
According to one particular refinement of the invention, the largest possible number of message cells grouped into a data block i8 limited by a defined maximum value.
On the one hand this measure lLmits the occur-rence of message cells of a connection in large numbers, and on the other hand it l;mits the repetition interval lS of the counter interrogation.
According to a further embodiment of the inven-tion, once a cyclical interrogation of all other sub-scriber terminals for which a connection is established has found no set status character, a subscriber terminal will be issued a transmit authorization for an additional message cell without the counter reading of the associ-ated counter being decremented.
This measure permits the transmis~ion of sub-scriber terminal-specific information to the head end beyond the defined message cell rate.
According to a further refinement of the inven-tion, a subscriber terminal fully utilizes the number of message cells indicated in the transmit authorization by transmitting empty aRll~ if necessary-This measure avoids idle times on the fiber ~nd facilitates regular activation of the subscriber ter-minals even if the traffic is light.
The invention will now be explained as an ..
21328~4 exemplary embodiment to the extent required for compre-hension thereof with reference to figures, in which:
Fig. 1 show3 the ba~ic representation of a passive optical network, and Fig. 2 and Fig. 3 show a flowchart in each case.
Fig. 1 shows a transmis~ion system that is known per se, for example from British Telecom Technological Journal, Vol. 7, No. 2, April 1989, pp. 151-160, in which a plurality of subscriber tations with their subscriber terminals NTl...NT4 are connected via optical fibers and passive couplers SPl...SP4 to a head end LT of a switch-ing system.
Data having a binary data structure are tran~-mitted in message cell~ in the switching system and the transmission system in accordance with an asynchronous transmission method. The mes age cells are assumed in each case to have a payload section having a length of 48 bytes in the exemplary embodiment and a message cell header having a length of 5 bytes in the exemplary embodiment preceding the payload section. The message cell header contains the identification of the connection to which the respective message cell belongs. In the exemplary embodiment the transport bit rate is assumed to be 155.52 Mbit/s and the user information bit rate is assumed to be 149.76 Mbit/s on the fiber-optic links.
The message cells transmitted from the head end via the optical fibers and passive couplers to the subscriber terminals having a fir~t wavelength reach all the subscriber terminals, a respective sub~criber ter-minal only evaluating the me~sage cells indicated as belonging to the connection on the basis of their 21328~
identification in the message cell header.
Message cells to be transmitted from the sub-scriber terminals to the head end are transmitted via the optical fi~ers and the passive couplers using the 80-called wavelength-division multiplex method on a second wavelength. Message cells transmitted from various ~ubscriber terminals to th head end must on the one hand not overlap one another in terms of time, but on the other hand they mu~t follow one another without an appreciable interval. To avoid overlapping, it is pro-vided that a Rubscriber terminal only transmits message cells in the direction of the head end when it has received a transmit authorization from the head end (polling); in thiq case according to the invention in each case as many successive mes~age cells are trans-mitted as are indicated in the current transmit authori7-ation. In the case where no number of message ~ells corresponding to the transmit authorization is ready in a subscriber terminal, the number of message cells according to the transmit authorization can be fully utilized by transmitting empty cells. An empty cell is understood to refer to a message cell that does not contain any data in its data ~ection and is not forwarded to the switching system.
In general, the transit times between the indi-vidual subscriber terminals and the head end are differ-ent, which can be attributed to different path distances and temperature fluctuation~ of the individual optical fibers. To compensate for these differences in transit times, a delay device is provided for each subscriber terminal; the subscriber terminal with the longest transit time determine~ the minimum transit time for all the ~ubscriber terminals here. A calibration to identical transit times between the subscriber , ~1328~4 ' terminals and the head end en~ures a virtually phase-locXed arrival o~ the message cells at the head end. The calibration of a sub~criber term;nal-~pecifir delay device, which may be arranged in the subscriber terminal, is performed during the connection establishment and is repeated at regular time~intervals of, for example, one second.
The fiber-optic links between the individual subscriber terminals and the head end usually have different attenuations, so that the binary characters of message cell8 output by different subscriber terminals can have different amplitudes. A receiving device pro-vided on the head end side requires a certain amount of time to adjust to the amplitude of a currently incoming message cell, and the outlay for the adjustment is higher, the less time available for the adjustment. To avoid the corruption of user information, it is provided that a ~ubscriber term;nal transmits a defined number of, for example, 64 bits calibrating characters each time before transmitting message cells, which are directly followed by the me~sage cells to be transmitted. In addition to the aforementioned adjustment to the ampli-tude of a currently incoming message cell, the calibrat-ing characters permi~ a fine adjustment to the phase position of the currently incoming message cells.
~ he time required for calibration of the sub-scriber terminal-specific delay devices and the trans-mission of the calibrating characters is not available for the transmis~ion of user information in the form of message cells; during a transmission of calibrating characters of predetermined length with each message cell, a substantial part of the transmission capacity not available for the tran~mission of user information is ., . ~ ' ~ : , .~.
~;1328~4 used. According to the invention, a plurality of me~sage cells output by a subscriber terminal can be grouped into a data block, each data block being preceded by calibrat-ing characters of constant length, so that the proportion of calibrating characters to the transmission capacity used is reduced in relation to the user information. A
minimum number of one message cell per data block can nevertheless be provided, in particular for connections having a low data rate of, for example, 64 kbitls. ~he delay cau3ed by the collection of message cells to form a data group is less noticeable, the higher the message cell rate output by a subscriber terminal.
A counter is provided for each subscriber ter-minal, the counter reading of which is incremented or decremented depending on two procedures described below.
The counters are preferably arranged in the head end of the switching system.
~ ig. 2 shows a flowchart, according to which the counter reading of all counters is incremented during an incrementation interval by a respective amcunt correspon-ding to the transmission bit rate specified for the associated subscriber terminal. The amount by which the counter reading of a respective counter is increment~d i~
assumed to be a multiple of a largest common sub-channel contained in all the possible transmission bit rates. In the exemplary embodiment, a largest common sub-channel is assumed to have a transmi~sion bit rate of 64 k~it/~.
~ ccording to the top action field INCn in Fig. 2, a respective counter is incremented by the amount corre-sponding to its multiple of sub-channels. According to 2132~4 g the following decision field FW, a check is made as to whether the coun~er reading of the counter currently in question has exceeded a predetermined level FW. The exceeding of the level FW indicates that at least one message cell is probably ready for transmission at ths associated subscriber terminal. If the level FW is exceeded (Y~ a status character is set for the respect-ive counter in accordance with the action field CFS. The setting of the status character can be produced by the overflow of a binary counter with predetermlned bit width. In the exemplary embodiment, the setting of a status character is assumed to be performed when the amount, which can be represented by 8 bits at most, is exceeded. In the case where the level FW for the counter currently in question is not exceeded (N) in the decision field FW, or a status character was set for the counter in question in accordance with the action field CFS, the procedure continues with the decision field LC. In the decision field LC it is checked whether all the counters of ~ubscriber terminals which have just established connections during a current incrementation interval have been incremented. If at least one ~ounter is still to be incremented (N), the procedure continues with the action field I~Cn. If all the counters have been incremented (Y), the procedure continues with the action field TW. In the action field TW, the end of the current incrementa-tion interval is awaited in order to begin a new incre mentation interval then with the action field I~Cn. In the ex~mplary embodiment an incrementation interval is assumed to have the tran~mission duration of 3,S45 bits.
The flowchart illustrated in Fig. 3 brings about the issuing of transmit authorizations for a respective number of message cells for the individual subscriber ter~;nals. By se~ting status characters in the flowchart ~1328~4 - 1 o according to Fig. 2, the interrogation as to whether at lea~t one message cell i8 probably ready for transmission at a respective sub~criber terminal i8 reduced to the interrogation o~ said status character. According to the flowchart in Fig. 3, all subscriber terminals for which a connection i~ currently established are interrogated for the presence of set status characters. According to the action field SHn, the procedure moves on from the last counter interrogated to the next counter. According to the decision field CFG, the counter currently in question is interrogated for a set ~tatus character. If a status character i8 set tY) for the counter in ques-tion, according to the action field SB a transmit author-ization for the associated subscriber terminal is pro-vided. The transmit authorization indicates here themaximum number of message cells which are to be expected depending on the cumulative counter reading of the respective counter in the as~ociated subscriber terminal.
It can be provided here that the maximum number is li~;ted by a defined maximum value. According to the action field D~Cn, the counter reading of the counter currsntly in question is decrementsd by the amount corresponding to the number of message cells provided for transmission. If, for example, the counter reading of the counter just interrogated has a current counter reading of 768 and if a counter reading of 256 corresponds to one message c011 ready for transmission, then a transmit authorization is provided for three message cells and the counter reading is decremented to 219. According to the action field CFR, the status character of the counter currently in question is reset once the counter reading falls below the predetermined counter rsading. According to the action field WAIT, there is a wait until the transmission duration for the transmi~sion of the number of message cells corresponding to the directly preceding , : ,:: , ~ -~1328~,,4 transmit authorization has elapsed.
If, however, the interrogation in the decision field CFG had indicated that no C~tatus character is set (N) for the counter in question, then the procedure S continues with the decision field ALT. In the decision field ALT, starting~ from the counter who~e associated subscriber terminal transmitted a message cell last, it is checked whether no mess~ge cells are ready for trans-mission in all the other subscriber terminals for which connections are currently e~tablished. If all the other subscriber terminals have not yet been interrogated (N), then the procedure continues with the action field S~n, in which the procedure moves on to the next counter. Said counter is interrogated for the presence of a ~et status character. Even if only one counter has a set status character during this cyclical interrogation of the status characters, then the procedure continues as described further above for the decision field CFG in the ca~e of (Y).
It is now to be as~umed in accordance with the decision field ALT that no message cells are ready for transmission (Y) in all the other subscriber terminals for which connections are established; the procedure then continues with the action field SEB in which the tran~mit authorization is provided for an extra message cell. An extra message cell refers to a messaqe cell transmitted without the counter reading for the subscriber terminal that receives permission to transmit the extra message cell being decremented. From the action field SE~, the procedure continues further with the action field WAIT, which has already been discussed further above. From the action field WAIT, the procedure continues with the decision field ADJ. In the decision field ADJ a check is ~ 2;L328~
made as to whether a calibration of the as~ociated delay device is required for the sub~criber terminal for which a transmit authorization has ju~t been provided. The requirement for calibr~tion may be constituted by a time criterion, which in the exemplary embodiment i5 as~umed to comprise a perio~ of one seeond. In the ca~e where calibration is required (Y), the procedure continues with the action field ESW in which there is a wait for the duration of the period for calibrating the respective delay device.
In the case where no calibration is required (N) according to the decision field ADJ, or where the waiting time according to the action field ESW has expired, the procedure continues with the action field SEND. According to the action field SEND, the respective subscriber terminal is issued a transmit authorization for a corre-sponding number of message cells. A subscriber terminal that has received a tran~mit authorization reacts by transmitting the calibrating characters, which are followed directly by a number of message cells in accord-ance with the transmit authorization. If there are not as many message cell~ ready for transmission in a subscriber terminal as are indicated in the transmit authorization, then the respective sub~criber terminal can fully utilize the transmit authorization by transmitting a number of empty cells. Empty cells are mes~age cells which contain no information in their data ~ection and are not for-warded to the switching system.
During the calibration of a delay line, as already mentioned, no user information is transmitted from the individual subscriber terminals to the head end.
If a FIF0 memory is provided in the head end which is maintained at an increased filling level with message ::;:
. - :'~
~. : :
~ ' ~
~ ~1328~4 cells transmitted by the subscriber terminals, then the calibration of delay devices of the individual subscriber terminals can remain u~noticeable to the Qwitching system if message cells are regularly read out of the FIFO
memory and the filling level i~ thus reduced during the calibration; on average the counters have increa~ed counter readings i~mediately after a calibration oper-ation, which leads to the transmission of longer data blocks and hence to a briefly increased data rate, with the result that the FIFO memory reaches an increased filling level again. A continuous ~tream of data which is delayed by the time required for calibration is produced for the switching .Qystem.
Claims (11)
1. A process for allotting transmission time slots for a transmission system having a plurality of sub-scriber stations which are connected via optical fibers and passive couplers to a head end common to all sub-scriber stations, wherein - the data supplied to a subscriber station or output by one are transmitted in message cells in the course of a virtual connection in accordance with an asynchron-ous transmission method, - each message cell has a payload section of constant length for accommodating the data and a message cell header of constant length preceding the payload section, - a message cell rate is specified for each virtual connection, - the counter reading of a counter provided on the head end side for each virtual connection is incremented regularly depending on the specified message cell rate, - a status character is set for a counter whose counter reading has exceeded a predetermined counter reading, - the counters are interrogated for the presence of set status characters, - a subscriber station whose associated counter has a set status character during the interrogation receives a transmit authorization for the largest possible number of message cells corresponding to the respect-ive counter reading, the counter reading of said counter being decremented by an amount corresponding to said number and the status character being reset once the reading falls below the predetermined counter reading, and - a subscriber station that has received a transmit authorization transmits a sequence of calibrating characters and directly following that a number of message cells corresponding to the respective transmit authorization to the head end.
2. The process as claimed in claim 1, wherein the largest possible number of message cells is limited by a defined maximum value.
3. The process as claimed in claim 1 or 2, wherein the counters are interrogated cyclically.
4. The process as claimed in claim 3, wherein no set status character was found, starting from the counter for whose subscriber station a transmit authorization was issued last, given a single cyclical interrogation of all other counters, a transmit authorization is issued for an additional message cell for said subscriber station without the associated counter reading being decremented accordingly.
5. The process as claimed in one of the preceding claims, wherein the predetermined counter reading corre-sponds to the time interval between two successive message cells at the respective message cell rate.
6. The process as claimed in one of the preceding claims, wherein a subscriber terminal-specific delay device is calibrated for each subscriber station at regular time intervals comprising a multiple of the transmission duration of a message cell.
7. The process as claimed in claim 6, wherein the delay device is calibrated at regular time intervals of the order of one second.
8. The process as claimed in one of the preceding claims, wherein a subscriber station in which a number of message cells filled with data corresponding to the transmit authorization is not ready transmits a number of empty cells calculated from the difference between the number according to the transmit authorization and the number of message cells filled with data.
9. The process as claimed in one of the preceding claims, wherein the transmission system forwards message cells to a switching system that connects through message cells at the same transport bit rate in accordance with an asynchronous transmission method.
10. The process as claimed in claim 9, wherein the sum of the specified message cell rates is less than the maximum possible sum of all such message cell rates.
11. The process as claimed in one of the preceding claims, wherein the incrementing and the decrementing of the counter readings of the individual counters is effected by two mutually independent procedures.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP92105237A EP0565739A1 (en) | 1992-03-26 | 1992-03-26 | Procedure for the allocation of timeslots in a passive optical network |
| EP92105237.9 | 1992-03-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2132854A1 true CA2132854A1 (en) | 1993-09-27 |
Family
ID=8209474
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002132854A Abandoned CA2132854A1 (en) | 1992-03-26 | 1993-03-09 | Process for allotting transmission time slots in a passive optical network |
Country Status (7)
| Country | Link |
|---|---|
| EP (2) | EP0565739A1 (en) |
| JP (1) | JPH07505032A (en) |
| AT (1) | ATE139389T1 (en) |
| CA (1) | CA2132854A1 (en) |
| DE (1) | DE59302930D1 (en) |
| HU (1) | HUT68131A (en) |
| WO (1) | WO1993019540A1 (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE4405461C1 (en) * | 1994-02-21 | 1994-10-06 | Siemens Ag | Calibration of TDMA systems including a passive optical network |
| FR2725093B1 (en) * | 1994-09-23 | 1996-12-27 | Cit Alcatel | TIME-TO-MULTI-POINT POINT-TO-MULTI-POINT TRANSMISSION NETWORK |
| DE69534445T2 (en) * | 1995-04-28 | 2006-04-27 | Alcatel | Method for TDMA management, central station, subscriber station and network for carrying out the method |
| GB2359228A (en) * | 1998-11-17 | 2001-08-15 | Schlumberger Technology Corp | Transmitting information over a communication link |
| US6980519B1 (en) * | 2000-07-14 | 2005-12-27 | Lucent Technologies Inc. | Multi-table based grant generator for improved granularity in an ATM-PON |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB8807050D0 (en) * | 1988-03-24 | 1988-04-27 | British Telecomm | Communication system |
-
1992
- 1992-03-26 EP EP92105237A patent/EP0565739A1/en not_active Withdrawn
-
1993
- 1993-03-09 HU HU9402752A patent/HUT68131A/en unknown
- 1993-03-09 AT AT93905174T patent/ATE139389T1/en active
- 1993-03-09 WO PCT/DE1993/000218 patent/WO1993019540A1/en active IP Right Grant
- 1993-03-09 DE DE59302930T patent/DE59302930D1/en not_active Expired - Fee Related
- 1993-03-09 EP EP93905174A patent/EP0632942B1/en not_active Expired - Lifetime
- 1993-03-09 JP JP5516158A patent/JPH07505032A/en active Pending
- 1993-03-09 CA CA002132854A patent/CA2132854A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| EP0565739A1 (en) | 1993-10-20 |
| HUT68131A (en) | 1995-05-29 |
| EP0632942B1 (en) | 1996-06-12 |
| ATE139389T1 (en) | 1996-06-15 |
| JPH07505032A (en) | 1995-06-01 |
| DE59302930D1 (en) | 1996-07-18 |
| EP0632942A1 (en) | 1995-01-11 |
| WO1993019540A1 (en) | 1993-09-30 |
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