Note: Descriptions are shown in the official language in which they were submitted.
20107~6
METHOD AND APPARATUS FOR ACCESSING A DISTRIBUTED COMMUNICATIONS
NFTWORK
BACKG~OUND OF THE INVENTION
1. Field of the Invention
05 The present invention relates to a communications network
for transmitting frames containing data or signalling information,
comprising a set of links connecting data transmission terminals
and providing the network with an arbitrary topology, each link
carrying one or several signalling and data channel(s). The
invention more specifically relates to a protocol for transmission
access onto the network that optimizes its exploitation while
avoiding collisions between terminals.
2. Prior Art
It is known that several data transmission terminals
~5 connected to a single network can sometimes seek to transmitsimultaneously onto the latter. Various procedures have been
devised to solve this problem.
A first known procedure consists in using a bus access
protocol of the CSMA/CD type (Carrier Sense Multiple
Access/Collision Detection~. According to this protocol, the
terminal seeking to transmit first ensures that the bus is free by
detecting its rest state. But, because of the propagation time in
the network, collisions can nevertheless occur between data
transmitted virtually simultaneously from terminals for which the
network appeared free at the local level. A procedure makes it
possible, in these events, to ro-initialize the transmissions
after a time delay that varies according to the sender9 so
avoiding a new collision.
This procedure is well suited to local networks in which
the data are transmitted in the form of asynchronous frames.
On the other hand, it does not allow transmission of
synchronous data, and in particular speech transmission on the
same network. Moreover, when the length of the latter increases,
the propagation times become large, risks of collision are
multiplied and transmission efficiency decreases. Thus, in a
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network that is theoretically capable of a flow rate of 10
Mbits/s, the maximum flow rate in practice is only 5 Mbits/s.
Accordingly, the length of the network is ùsually limited to a few
kilometers. Likewise, in cases where the number of messages to
05 be sent begins to increase, the collision probability rises, so
reducing the systems efficiency.
A second known procedure consists in having a token
circulating in a network configured as a loop. The *oken
authorizes transmission to the transmission terminal that
intercepts it.
This allows the coexistence, on a same network, of data
transmissions in the form of both asynchronous packets and
synchronous speech data, possibly with a dynamic allocation of
time channels of each frame in a packet mode (data) or a circuit
mode (speech). -
Performance is little affected by the propagation times -
thus making it possible to manage very extensive networks.
However, in the case of very long networks, it is necessary to
provide costly connection interfaces between the network and the
data transmission terminals.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
method and apparatus for accessing a distributed communications
network linking several synchronous or asynchronous data
transmission terminals and avoiding the above-mentioned drawbacks
of networks employing a CSMA/CD type or a token type protocol.
In particular, the invention has for object a process and
apparatus for accessing a network, offering the possibility of
simultaneously transmitting asynchronous data, packets or
signalling information, and synchronous data over a large distance
without calling for costly connection interfaces at the level of
each terminal.
This object is attained by means of a method for
accessing a distributed communications network of the type
comprising stations having data transmission terminals connected
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thPreto and which are joined by links according to an arbitrary
topology, said links conveying at least one token, or access
token, which, when intercepted by a station, authorizes said
station to grant its terminals access for transmission on a link
05 corresponding to said token and to which said station is
connected, wherein said links define at least one virtual loop
forming at least one communications channel passing through all
said stations and conveying frames that carry at least one token,
signalling information, and data; wherein said terminals connected
to a station having taken a said token for access to a said
communications channel can gain access to said channel by a common
local bus that is specific to said station and into which right of
access by said terminals is controlled by a contention system;
wherein said station having intercepted a token for transmission
access to a communications channel in response to a request for
access from one or several terminals connected to said station,
retransmits said access token on said communications channel
after the said one or several said requesting terminal(s) has/have
successively transmitted its/their respective message(s) via said
local bus.
Here, the term contention system to control right
access onto the local bus is taken to mean a protocol of the
CSMA/CA (Access/Collision/Avoidance), CSMA/CR (Carrier Sense
Multiple Access/Collision Resolution) or CSMA/CD (Carrier Sense
Multiple Access/Collision Detection) type, or a similar protocol.
Therefore, the invention combines a token system for
granting a communications channel to a station wishing to
transmit, and a ~CSMA type or similar procedure to solve local
contention problems at the level of each station.
By virtue of this combination, the proposed solution
combines the advantages of both proceduras while avoiding each of
their drawbacks. The network can be very extensive and the
connection cost per data transmision terminal can be relatively
low. Furthermore, the possibility of conveying synchronous data in
sddition to asynchronous data frames makes it possible to h3ve
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non-speech data transmission terminals coexisting with speech
processing terminals.
In each station, except the one having intercepted a
communications channel access token, the signalling information
05 and/or data circulating in that channel are fed into the local bus
for resd access by the terminals connected to the station.
But, in the station having intercepted the communications
channel access token, the loop forming the latter is open for
retransmission of data on the channel. Loop closure in that
channel is carried out in response to reception of a second token,
or looping token, which is transmitted on the communications
channel when the station has finished transmitting on that channel.
The looping token can be transmitted by the station
having just finished transmitting, immediately after retransmitting
the access token. Accordingly, if the access token is intercepted
by a new station informed of a request for transmission access on
the communications channel, the looping token will be ahead when
it reaches the station that has just finished transmission, after
having gone round the loop forming the communications channel.
Alternatively, the looping token can be transmitted by
station located downstream of the one tha~ has just finished
transmitting, e.g. by the new station having intercepted the access
token. The links joining the stations together can be made to
define several virtual loops forming several communication channels
passing through all stations and each having a transmission access
token. The virtual loops forming the different channels may have
different topologies while passing through all stations.
The transmission mode may be asynchronous, e.y. ATM
(Asynchroncus Transfer Mode) or any other type, or synchronous,
e.g. conveying PCM (Pulse Code Modulation) type frames, or of the
FDDI type (Fiber Distributed Digital Interface). As an example,
each link can csrry four bi-directional PCM synchronous channels,
each at 2,048 kb/s, conveying synchronization information and data
The access token for a channel need not necessarily be
conveyed on that channel. For instance, the access tokens for
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different channels could be transmitted on a single channel or
distriSuted over a limited number of channels.
Likewise, the signalling data relative to the different
channels can be transmitted over a single chsnnel or over a limited
05 number of channels.
When the signalling information and data are transmittsd
on respective channels, the signalling channel conveys a third
token which, when intercepted by a station, grants the latter
transmission access to the signalling channel, the third token
being distinct from the access token conveyed by the data channel.
Finally, still in the case where the links between
stations support a plurality of communication channels, a mixing
can be carried out at each station between the communications
channels and a number of channel~ carried by a local bus.
The flow rates on the, or each, communications channel
and on the local buses can be different. In that case, a change
in modulation speed is performed between the, or each,
communications channel passing via a station and the local bus of
the latter.
Likewise, a change of modulation speed can be performed
in a station between the data transmission terminals and the local
bus, in the form of time multiplexing or demultiplexing going from
the terminals' transmission outputs to the local bus and 8 time
demultiplexing or multiplexing going from the local bus to the
terminals' receiving inputs.
The invention also relates to an apparatus for
implementing the above process. `
BRIEF DESCRIPTIûN OF THE DRAWINGS
Other features and advantages of the invention shall be
more clearly understood from reading the following description of
the preferred embodiments, given as a non-limiting example, with
reference to the appended drawings in which:
- Figure 1 is a general diagram of 3 communications
network access according to the present invention;
- Figure 2 is a general disgram of a station inserted
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20~0716
into the network of figure 1;
- Figure 3 depicts an embodiment of a PCM interface
circuit forming part of the station shown in figure 2; and
- Figure 4 illustrates an embodiment of a terminal
05 interface circuit forming part of the station shown in figure 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The network, as shown in figure 1, comprises a
transmission medium consisting of links in the form of
bi-directional PCM channels that interconnect stations S1, S2, ~..,
S(n-1), Sn, S(n+1), ..., Sp. References PCM(n-1)-n and pcmn(n+1)
designate the PCM channels respectively connecting station S(n-1)
to station Sn, and station Sn to station S(n+1).
On each link, there can be e.g. four PCM channels having
a data rate of 2,048 kb/s" They each support a time multiplex
capable of transmitting, within frames, asynchronous data
(packets), synchronous data (digitized speech) and signalling
information.
A communications channel for the data and signalling
information passing through all stations is set up by successively
employing the PCM channels in one propagation direction, with tha
return to the initial station for looping obtained by using the
opposite propagation direction along the same PCM channels or
another path. In the present example, since a link between two
stations consists of four bidirectional PCM channels, it is
possible to define four looped communication channels that pass
through all stations without necessarily having the same topology.
~ Each of the stationsicomprises a local bus BL1, BL2, ....
BLn, ..., BLp to which several data transmission terminals or
speech processing terminals are connected. Thus, terminals T1~
T1-2, .... , T1-n1 are connected to bus BL1, terminals T2-1, T2-2,
..., T2-n2 are connected to bus BL2, terminals Tn-1, Tn-2, ....
Tn-nn are connected to bus BLn and terminals Tp-1, Tp-2, ... , Tp-np `
are connected to bus BLp.
To each communications channels configured as a loop
going ~hrough all the stations there is associated a first token,
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20~07~6
or access token, that circulates concomitantly with the conveyed
data and/or signalling information. The access token is conveyed
by the communications channel to which it is associated. It is
also possible to convey four tokens on a same communications
05 channel and, if needs be, on at least one other for redundancy, or
to distribute these tokens over two channels.
Only the station that intercepts the access token
associated to a given communications channel is authorized to
transmit on the latter. The station retains the access token all
lû the while it transmits data or signa~ling information on the
corresponding communications channel. At the end of transmission,
it frees the access token which then propagates along the
communications channel in the same direction and at the same speed
as the data or the signalling information travelling along that
communications channel.
The data transmission terminals connected to a same local
bus can trans~it on that bus according to a contention system of
the CSMA/CA, GSMA/CR, CSMA/CD type or similar, in order to prevent
collisions thereon, or to accommodate for them. Each message
transmitted from a terminal on the local bus is thus available at
the input of all terminals connected to this same local bus. It
may be noted that the use of a protocol allowing collisions, such
as CSMA/CD is not too troublesome since the number of potential
transmitters on the bus is limited to the number of terminals
attached to the station having intercepted the access token, and
the probability of virtually simultaneous transmission is low,
owing to the short propagation time of the bus. It may also be
noted that the ChSA/CR protocol can here be used at high
modulation speed since the contention only occurs on the local bus
having a limited length, and since moreover the length of the
links between stations does not come into play, which is not the
case in a simple bus network structure where it is necessary to
wait till a bit has propagated from one end of the bus to the
other before transmitting the following bit.
Each local bus comprises a time multiplex of four
8 ~0~
bi-directional PCM channels, as for the link between the stations.
Advantageously, the local bus is enabled for writing on one or
several PCM channel(s) only during the time when the station
possesses the access token(s) associated to the communications
05 channel~s) corresponding to the PCM channel(s) of the local bus.
In this way, for a given communications channel, when the
associated access token has not been intercepted by a station, the
corresponding PCM channel of the local bus is disabled for writing
onto the bus by the station.
As soon as a terminal wishes to transmit data or
signalling information, it sends a transmission request signal to
the station to which it belongs. The station intercepts the first
passing access token and write enables the local bus for the PCM
channel of the bus corresponding to the communications channel
associated to the intercepted access token. The requesting
terminal can then transmit its message e.g. data in the form of
frames, over that PCM channel of the local bus according to a
chosen contention system: CSMA/CA, CSMA/CR, CSMA/CD, or similar.
The data sent by the terminal are transmitted over the
communications channel corresponding to the intercepted access
token. In each of the other stations, these data are fed into the
the local bus for read access by the various terminals attached
thereto. Thus, the same data are supplied to all the connected
terminals of the network without requiring intermediate storage.
The communications channel is opened by interrupting the
retransmission of the PCM channel data entering the transmitting
station in the opposite direction to the data in order to prevent
that data from being transmitted a second time by that station.
ûnce the transmitting terminal has finiæhed its
transmission, the PCM channel of the corresponding local bus
becomes available to any other terminal attached to the same
station and having addressed a transmit request signal. When all
the local transmit requests addressed to the station have been
satisfied, or when the maximum allocated time for keeping an
access token is exceeded, the token is freed, making the
201~
corresponding communications channel available. At the same time,
a second token is transmitted by the station immediately behind
the data just transmitted. This second token, or looping token,
follows these data up to the transmitting station. The latter is
05 thus informed that all the data it has transmitted have arrived at
their destination and that it can close the communication channel
at its level.
Alternatively, the looping token is transmitted by a
station located downstream of the transmitting station, e.g. by
the new station intercepting the access token that has been freed,
just before this new station transmits on the corresponding
communications channel. Any other solution resulting in the
looping token being received at the initial transmitting station
before the arrival of data originating from a downstream station
is of course possible.
The looping token can be transmitted on the corresponding
communications channel or on any other channel, e.g. a channel
conveying the signalling information in so fsr as that channel does
not have a path exceeding that of the data.
By providing several bidirectional PCM channels in each
link between the stations - four channels in the present example -
it is possible, as already shown, to exploit four relatively
independent communications channels in the form of loops.
Thus, should there occur a collision between seYeral
terminals on a local bus, one of the terminals having lost the
contention will be able to retransmit if an access token on another
communications channel appears. The same applies for terminals
attached to differènt stations: the fact that a station has
intercepted a communications channel access token does not block
transmissions for the other stations, along the other
communications channels. This therefore provides a decrease in the
average wait time. It is also possible to customize the
communications channels, e.g. by having some that will only allow
a token to be kept for a limited time, so as to minimize the
maximum wait time.
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By configuring the four communications channels so that
they have different topologies, it is also possible, in the case of
point-to-point transmissions, to select the communications channel
access token for which the path to the target is the shortest, so
05 minimizing token utilization time.
Also, the availability of several access tokens reduces
the risk of a total failure of the network in ca~e of the loss of
a token. Finally, it is possible to specialize the communications
channels~ so that each deals with a limited number of types of
transmission: packets or circuits, and to authorize connection to
a communications channel only to those terminals corresponding to
a type of transmission for which the channel is destined. In the
same general concept, the signalling information relative to the
four communications channels can be conveyed on a single channel
as well as possibly on at least one other channel for redundancy,
rather than passing the signalling information for a respective
channel on each channel.
As already explained, the data and or signalling
information conveyed by the four communications channels are fed
into the local bus of each station ~o be made accessible to
their data transmission terminals. The local bus is, in this case,
a high-speed bus supporting four time multiplexed bidirectional
PCM channels and, in the present case, operating at a speed of
4 x 2,048 kbit/s. The bits of the four communications channels are
interlaced on the local bus, and a mixing may be carried out by
the station between the four communications channels and the four
PCM channels of the local bus.
iHowaver, should ~the components providing the interface
between the terminals and the local bus not be capable of operating
at 4 x 2,048 kb/s and working on 1 out of every 4 bits, the
following alternatives will be required:
- while keeping the high-speed local bus, providing 3
multiplexer between the transmission outputs of the terminals and
the local bus, and a demultiplexer between the local bus and the
retransmission inputs of the terminals; or
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11
- physically separating the local bus into several
elementary bidirectional buses e.g. four 2,048 kb/s bidirectional
buses, and placing a multiplexer between the elementary buses
forminy the local bus and the link supporting the communications
05 channels and passing through the station, and a demultiplexer
between the link passing through the station and the elementary
buses forming the local bus.
There shall now be given a more detailed description of a
station Sn in the context of the invention when applied to an
Integrated Service Private Branch eXchange (ISPBX) having a
decentralized architecture.
A PCM interface circuit 10 (figure 2) is connected to the
PCM(n-1)-n and PCMn-(n+1) channels and is joined to a local PCM
bus to form an interface between the external PCM channels and the
PCM channels of the local bus. The data transmission terminals and
optionally the speech processors served by the station Sn are
locally connected to the latter through interface links S (as
defined by note I430 of the CCITT). The terminals at interface S
are connected to circuits 30 of that interface S, which control the
latter and are themselves connected to the local bus BLn. Also9
the PCM interface circuit 10 receives transmit request signals TR~
coming from the terminals via the circuits 30 of the interfaces,
when the terminals wish to transmit, and produces by
ready-to-transmit signals RTT applied to the circuits 30 of the
interface S to authorize transmission from the requesting
terminals according to a CSMA/CA, CSMA/CR or CSMA/CD procecdure.
Exchanges between the circuits 30 of interface S and the PCM
interface circuit 10 are through the local bus BLn.
Each link to the interface S joining the terminals to the
station Sn is capable of conveying speech, data and signalling
information between the corresponding terminal and the station in
both directions.
As an example, there shall be considered the transmission
of signalling data. The latter are transmitted in the form of
normalized High-level Data Link Control (HDLC).
20~716
12
In the overall frame, or superframe, conveyed by a
communications channel, time interval N 16 or TI16, is reserved
for conveying signalling frames between stations. Likewise, TI16
of one of the PCM channels of the local bus is reserved for
05 exchanging signalling information between the circuits 30 of
interface S and circuit 10 of interface PCM.
The signalling channel conveys signalling information
used to manage the channels in a circuit mode in the communications
channels and to provids the signalling associated to each packet.
lû The latter can also be conveyed in the data channels at
predetermined positions or according to specific codes.
When a terminal wishes to transmit a signalling frame,
the associated interface circuit S sends a TRQ signal to the PCM
interface circuit 10. The latter intercepts the first access token
coming from the PCM communications channel as soon as it arrives
and sends an RTT signsl intended for all the circuits 30 of
interface S. Correspondingly, the TI16 connection, which is
normally established between the PCM(n-1)-n and PCMn-(n~
channels, is temporarily interrupted in the propagation direction
of the intercepted token, and replaced by TI16 of the concerned
PCM channel of the local bus. The interface circuit S
corresponding to the requesting terminal can then transmit its
signalling frame which shall be transmitted throughout the
communication~ channel to be made present on all the local buses
of all stations connected to that communications channel.
If several circuits 30 of the interface S belonging to a
same station request to transmit, a CSMA/CA, CSMA/CR or CSMA/CD
procedure is established on T116 of the PCM channel on ~he local
bus to avoid collison, or accommodate for them without undue
3û waste of time, and to transmit the signalling frames in succession.
When there are no more requests to be satisfied, the PCM
interface circuit 10 frees the access token, sending it to the
external PCM communications channel and at the same time sends a
looping token which re-establishes the TI16 connection between the
PCM(n-1)-n and PCMn-(n+1) sections in the propogation directiùn
13 2010716
when it returns to the transmitting circuit 10.
An embodiment of the PCM interface circuit is shown on
figure 3.
A processing unit 11 e.g. based on the 80188
û5 microprocessor from Intel Corp.of the USA controls the different
successive phases of the procedure carried out by the active
components of circuit 10 which are connected to the bus 12 of the
microprocessor unit 11.
The PCM interface circuit is adapted to the external PCM
channels PCM(n-1)-n and PCMn-(n+1) by means of two respective
circuits 13, 14. The latter are connected to a switching matrix 15
via respective circuits 16, 17 that serve to manage the PCM frames
and adapt them to the switching matrix. The latter can be
controlled by the processing unit 11 to carry out all possible
connections between the different TI's of the external PCM
communications channel and the PCM channel of the local bus to
which the switching matrix is also connected. A circuit 18 is
provided for the exchange of signalling frames between TI1~ of the
PCM channel of the local bus and the processing unit 11.
Circuits 13 to 18 can e.g. be produced using components
from Siemens of West Germany. Accordingly, the adaptor circuits 13
and 14 are of the "IPAT-PEB 2235" type, the PCM frame management
circuits are of the "ACFA PEB 2035" type, the switching matrix of
"PEB 2045" type and circuit 18 of the "3SAB 82520 HSCC" type.
When circuit 18 intends to transmit signalling data, it
produces a TRQ signal to a gate 19 providing a logical OR
function. The gate also receives TRa signals from the interface
circuits S upon reqùest of the terminals connected to the station.
The output of the OR gate is acknowledged by the
processing unit 11 to control the interception of the access token
at its following passage, in response to the appearance of a TRQ
signal. The token can e.g. be in the form of a bit transmitted in
the time interval N O (TIO) on the external PCM channel.
Once the access token is intercepted, the processing
unit 11 sets high the contents of a register 20 whose output is
Z0~07~6
14
connected to the input of a gate 21 providing an AND funcion. The
passage through ~he gate 21 is authorized when TI16 of the PCM
channel of the local bus is enabled. This authorization to transmit
at the output of register 20, after passage through gate 21,
05 constitutes the ready-to-transmit signal RTT applied to circuits
18. The RTT signal is also applied in parallel to the circuits
3û of interface S. At the same time, the processing unit 11
sends a command to the switching matrix 15 to cut off locally the
transmission of TI16 of the external communications channel, in
the propa~ation direction, so as to prevent looping of the
signalling information that would otherwise have gone round the
loop formed by that external channel a first time.
Once all the TRQ requests have been satisfied, the drop
in logic state of output of gate 19 commands, via the processing
unit 11 the retransmission of the access token for the external
PCM communications channel and the transmission of the looping
token the return of which serves to signal to the processing unit
11 that it must instruct the switching matrix 15 to locally close
the previously interrupted TI16. The looping token can also be in
the form of a bit transmitted in TIO.
An embodiment of a circuit 30 of the interface S is shown
in figure 4.
The illustrated circuit 3û allows connection of several
terminals (three in the example shown) onto interfaces S connected
to interface lines 31, 32, 33. Each of the latter serves to adapt
the logic signals of a respective circuit 34, 35, 36 used for the
local processing of that interface and connected to a bus of a
local processing unit controlled by a microprocessor 38.
The interface processing circuits 34, 35, 36 are also
connected to a circuit serving to provide connection of the B
channels to the PCM channel of the local bus BLn to which it is
connected. It is known that the interface according to the S
protocol is characterized by two channels termed B channels at 64
kb/s occupying 2 x 8 bits in the frame, a signalling channel, or D
channel, at 16 kb/s occupying two bits in the frame, the
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Z0107~6
re-reading of the signalling bus tCSMA/CR procedure) at 16 kb/s
occupying two bits and the parity control occupying four bits. In
the present example, the focus is on the signalling. The two
signalling bits are read cyclically in successive frames to be
05 configured as a HDLC format message. The signalling frames,
exchanged on the D channels of the different terminals, are
transmitted over a bus 37 of the control unit 38 and by a circuit
connected to the PCM channel of the local bus flow in TI16 of
that local PCM channel according to the above described procedure.
To that end, the circuit 4û sends a TRQ signal to gate 19 of the
PCM interface circuit in response to a transmission request coming
from a channel and interpreted by the control unit 38 and the
circuit 41 receiving the RTT signal.
The circuits 34, 35, 36 can e.g. comprise ~ISAC-S PE~
~085" type components from Siemens men~ioned above, while circuits
39 and 40 can e.g. respectively consist of "PBC-PEB 2050~ and
~SAB-82520 HSCC" components from the same company.
Naturally, the above described procedures for conveying
signalling information can be used to transmit data within a frame.
In the case of data transmission, time intervals different to TI16
can be used. It is also possible to increase the flow rate by
making the data link support several B channsls having an access
circuit 30.
The above example of channels carried by syncronous PCM
frames is not limiting. In particular it is possible to use
synchronous frames such as FDDI or others as well as asynchronous
frames.
The physical nature of the transmission medium can
alternatively include a twisted wire pair, coaxial cable, optical
fibers, etc. ... The coupling between the external communications
channel and the local bus can be achieved by means of e.g. coaxial
couplers, controllable optical couplers, etc. ...
In each station, the local bus, having a limited size,
can be made using a different technology and can operate at speeds
higher than the link between stations. This will in particular
201;0716
16
allow local exchanges between terminals connected to a station in
addition to the exchange of traffic with the external PCM channels.
05
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