Note: Descriptions are shown in the official language in which they were submitted.
~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
-- 2 --
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
- 3 -
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
-- 5 --
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~
-- 6 --
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 '
- 7 -
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
-- 8 --
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~
- 12 -
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
- 13 -
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.
