Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02288291 1999-10-22
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Description
A method for switching transmission devices to
an equivalent circuit for the bidirectional
transmission of ATM cells.
The invention relates to a method according to
the preamble of patent claim 1.
Such a method has already been suggested in the
German patent application DE 19646016.6.
This known method relates to transmission
devices of the asynchronous transfer mode (ATM). In
said mode, transmission devices for bidirectional
transmission of ATM cells are provided, in which
devices two switching devices which function as
terminals are connected to one another via a plurality
of service links and only one standby link. The two
terminals each contain a plurality of monitoring
devices for detecting transmission faults. A switching
device which can be controlled by the monitoring device
connects a reception device into a first switched state
with the service link and in a second switched state
with the standby link.
Control information is exchanged between the
control devices of the two terminals. The switching
device is controlled in each case by the local
monitoring device as a function of local control
criteria which are contained in the control information
received from the opposite station. Furthermore,
according to this disclosure it is proposed, if
appropriate, to feed special data (EXTRA TRAFFIC) to
the receiving switching device during the time in which
the standby link remains unused.
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2
Description
However, this disclosure does not mention what
type the special data are.
The publication KAI Y ENG et al ~~Memory and
Channel-Sharing Techniques for Congestion Control in ATM
Networks", Networking: Foundation for the Future, San
Francisco, 28.03.93 Vol. 1 No. Conf 12, pages 266-273, IEEE"
discloses a method for standby switching transmission
devices for the bidirectional transmission of ATM cells with
at least two switching centers. Arranged between these two
switching centers are in each case a plurality of service
links and a standby link. ATM cells are fed to the
respectively received [sic] switching center on the
plurality of service links, in which case in the event of
standby operation on one of the service links the ATM cells
which are transmitted on it are transmitted on the standby
link. If appropriate, special data can also be transmitted
on the standby link. However, how these special data are
configured is not mentioned here.
The invention is based on the object of indicating
a way in which special data can be transmitted efficiently
via a plurality of network nodes.
In accordance with one aspect, there is provided a
method for switching transmission devices to an equivalent
circuit for bidirectional transmission of ATM cells, the
method comprising the steps of: (a) providing at least two
switching centers which each terminate a transmission
section formed from a plurality of service links; (b)
feeding information in the ATM cells to a respective
receiving switching center on the plurality of service
links; (c) providing a
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2a
standby link arranged between the at least two switching
centers; (d) transmitting in a standby operation the ATM
cells, which are transmitted during fault-free operation on
the plurality of service links, on the standby link in the
event of a fault on one of the plurality of service links;
(e) transmitting special data on the standby link during
fault-free operation; and (f) configuring the special data
as traffic data having an assigned priority such that
transmission of the special data is not interrupted in a
case of standby operation, wherein in case of standby
operation the total of guaranteed bandwidths does not exceed
the capacity of the standby link.
An advantage of the invention is, in particular,
that just one standby link is provided and it is assigned to a
plurality of service links. The ATM cells of the faulty
service link are transmitted on this standby link. In the
case of fault-free operation, if appropriate, special data are
transmitted on this standby link. Here, the special data are
configured as traffic data with which, if appropriate, a
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minimum bit rate is guaranteed. This entails the advantage
of a dynamic relief of the service links.
In accordance with this invention, there is
provided a method for switching transmission devices to an
equivalent circuit for the bidirectional transmission of ATM
cells, having at least two switching centers (W, E) which
each terminate a transmission section formed from a
plurality of service links (WE1. . . WEn) and feed the
information in ATM cells to the respective receiving
switching center (W, E) on the plurality of service links
(WEl...WEn), and having a standby link (PE) which is arranged
between the two switching centers (W, E) and on which the
ATM cells which are transmitted on the service links
(WE1...WEn) are transmitted in the event of a fault on one of
said service links and on which special data are transmitted
during fault-free operation, characterized in that the
special data are configured as traffic data (ABR, UBR) with
which, if required by the definition of the traffic class of
the traffic, a minimum bit rate is guaranteed, the
transmission of the special data not being interrupted in
the case of standby operation.
Advantageous developments of the invention are
specified in the subclaims.
The invention is explained in more detail below
with reference to an exemplary embodiment.
Figure 1 shows the method as claimed in the invention
for the bidirectional transmission of
ATM cells in a l:n structure,
Figure 2 shows a specific refinement of the [lacuna]
according to the invention
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Figure 3 shows a further specific refinement of the
method according to the invention in a 1+1
structure.
Figure 4 shows the priorities used, in accordance with
which the switching to an equivalent circuit
is carried out.
Fig. 1 shows two nodes of an ATM network which
are each embodied as a switching center W, E. In the
present exemplary embodiment it is assumed that these
switching centers are cross connect switching centers.
The use of switching centers of this design does not,
however, signify any limitation of the invention; other
switching centers can also be used. Fig. 1a shows the
transmission of ATM cells from the switching center W
to the switching center E, whereas Fig. lb discloses
the reverse direction of this link.
The switching centers W, E are connected to one
another by means of service links WE1...WEn (WORKING
ENTITY) and just one standby link PE (PROTECTION
ENTITY). In addition, switch devices So...Sn (BRIDGE)
are shown, via which the incoming ATM cells and the
associated service links WE1...WEn are transmitted to
the switching center E. The ATM cells are transmitted
according to an asynchronous transfer mode and each
have a header part and an information part. The header
part is used to hold connection information while the
information part can be used to hold user information.
The connection information contained in the header part
is in the form of logic information and is usually
embodied as a virtual path number VPI or virtual
channel number VCI.
In addition, Fig. 1 shows selection devices SN
whose function consists in feeding the ATM cells
transmitted on the service links WE1...WEn to the output
of the switching
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center E. According to the present exemplary
embodiment, the selection devices SN are embodied as a
ATM switching matrix. The ATM switching matrix SN is
contained both in the switching center W and the
switching center E.
In addition, monitoring devices UEo...LTEn
(PROTECTION DOMAIN SINK, PROTECTION DOMAIN SOURCE),
which monitor the state or the quality of the ATM cells
transmitted on the service links WE1..WEn, are shown in
both switching centers W, E. For example, the ATM cells
of the link with the number 1 WT1 are provided, before
they are transmitted to the switching center E on the
service link WE1, with control information in the
monitoring device LTE1 of the switching center W, which
information is extracted from the receiving switching
center E, and checked, by the monitoring device L1E1.
With reference to this control information it ~is then
possible to determine whether or not the ATM cell is
being transmitted correctly. In particular, it is
possible to determine a total failure (SIGNAL FAIL FOR
WORKING ENTITY) of one of the service links WE1...WEn
here. Likewise, degradation of the transmission quality
(SIGNAL DEGRADE) can be determined, but also using
known methods.
The monitoring devices LTE1...UEn terminate the
service links WE1...WEn at both ends. Further monitoring
devices IlEo are arranged at both ends of the standby
link PE. The latter is intended to serve, in the case
of a fault, as a transmission link for the service link
WEx which is taken out of operation. In addition,
equivalent circuit switching protocols ES are
transmitted in this way so that the intactness of the
standby link has top priority.
In addition, central control devices ZST are
arranged in each of the switching centers W, E. Said
control devices ZST each contain priority tables PG,
PL. The priority tables
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PL are local priority tables in which the state and
priority of the local switching center is stored. The
priority tables PG are global priority tables in which
there is the state and priority of the local, but also
remaining, switching center. The introduction of the
priorities ensures that when a plurality of equivalent
circuit requests occur at the same time it is defined
which service link will be switched to an equivalent
circuit. Likewise, the equivalent circuit requests are
prioritized in the priority tables. Thus, there is, for
example, a high priority request by a user. Since this
equivalent circuit request is assigned a high priority,
it is thus controlled with preference. An equivalent
circuit request which is controlled by one of the
service links is thus rejected. The individual
priorities are shown in Fig. 4.
The central control devices ZST of the
switching centers W, E exchange information in a
standby protocol ES. This protocol is transmitted on
the standby link PE and obtained from the associated
monitoring device LTEo of the respective receiving
switching center, and fed to the relevant central
control device ZST. In addition, measures in the
central control device ZST ensure that, in the event
of a fault, the switching devices So...Sn are controlled
in a corresponding way.
Information K2 is stored in the protocol ES.
This is information relating to the instantaneous
states of the switching devices. Information K1 is also
stored. This is information relating to the generated
equivalent circuit request. The protocol is exchanged
between the two switching centers whenever the
equivalent circuit request is generated. In one
specific refinement of the invention it is provided
that the
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protocol ES be transmitted cyclically between the two
switching centers.
The way in which the method according to the
invention will be carried out will be explained in more
detail below with reference to Fig. 1. Fig. la shows
the transmission of the ATM cells from the switching
center W to the switching center E via the service
links WE1...WEn. The associated opposite direction
(bidirectional transmission) is explained in Fig. lb.
In the present exemplary embodiment it is then
initially assumed that the service links WE1...WEn are
still intact and the incoming ATM cells are
transmitting correctly.
According to Fig. la, the ATM cells are fed to
the switching center W. The ATM cells are associated
here with a plurality of links WT1...WTn. The individual
links are distinguished with reference to the logic
operation number VPI entered in the header part of the
ATM cells.
The switching devices S1...Sn of the switching
center W are switched, in this (still intact) operating
situation, in such a way that the ATM cells are fed
directly to the monitoring devices IJE1...LTEn. In the
latter devices, the ATM cells are provided with the
control information already referred to and fed on the
respective service link WE1...WEn to the monitoring
devices LTE1...LTEn of the receiving switching center E.
The control information which is also carried is
checked there and, if appropriate, a fault situation is
determined. If the transmission has taken place
correctly, the ATM cells are fed to the ATM switching
matrix SN. Here, the logic operation information VPI is
evaluated and the ATM cell is passed on to the ATM
network via the respective output of the switching
matrix SN in accordance with this evaluation.
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The standby link PE can remain unused during
this time. However, if appropriate, special data (EXTRA
TRAFFIC) can also be fed to the switching center E
during this time. The switching device So of the
switching center W therefore assumes the positions 1 or
3. The special data are also transmitted in ATM cells.
The monitoring device t7Eo of the of the switching
center W feeds control information to the ATM cells in
the same way as has already been described for the case
of the on the service links WE1...WEn. The link is also
monitored.
It is assumed below that the service link WE2
has failed. This is determined by the monitoring device
LTE2, assigned to the latter, of the receiving switching
center E. The equivalent circuit request K1 is then
transmitted to the respective central control device
ZST and stored there in the local priority~table PL and
the global priority table PG.
In accordance with the priorities stored in the
global priority table PG it is then determined whether
requests with still higher priorities are present.
These could be, for example, the user's switchover
request (FORCED SWITCH FOR WORKING ENTITY) already
mentioned. Even if other fault situations, such as on
the service link WE1, for example, occur
simultaneously, the equivalent circuit of this service
link would be handled with preference since a higher
priority is assigned to this service link. In this
case, a request with higher priority is handled first.
The priorities stored in the local and global priority
tables PL, PG are shown in Fig. 4.
If there are no requests present with higher
priority, the switching device Sz of the switching
center E is placed in the remaining operating state, as
shown in Fig. lb. The equivalent circuit switching
. protocol ES is then
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fed to the switching center W on the standby link PE.
The information K1 and K2 already mentioned is
contained in this equivalent circuit switching
protocol. The central feature is that the local
priority logic defines the configuration of the
information K1, and the global priority logic defines
the position of the switching device So.
The equivalent circuit switching protocol ES is
then transferred from the monitoring device LIEo of the
switching center E and fed to the central control
device ZST of the switching center W. If there are no
further requests with higher priority present in the
global priority table PG here either, the switching
device S2 is also driven and set here in a
corresponding way. In addition, the switching device Sp
of the switching center W is also switched over. The
new status of the two switching devices So, S2 is
acknowledged to the switching center E and updated in
the global priority table PG there. The ATM cells of
the link WT2 are thus fed to the switching center E on
the standby link PE.
The selection device SN of the switching center
E is embodied as an ATM switching matrix. The ATM cells
transmitted on the standby link PE are fed to this
switching matrix. Here, the logic path number VPI is
obtained from the cell header and evaluated and routed
through the switching matrix. The driving of switching
devices is thus omitted in this case.
Since these links are a bidirectional link, it
is also necessary to take measures to ensure the
transmission of the ATM cells in the reverse direction.
This is carried out according to Fig. 1b in the same
way as has just been described for the transmission of
the ATM cells from the switching center W to the
switching center E.
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In the exemplary embodiment just described, a
l:n structure was assumed. This means that there is
just one standby link available for n service links. It
is therefore a special case if n=1. In this case, a 1:1
structure is therefore used. The corresponding
conditions are shown in Fig. 2.
The selection device is also embodied as an ATM
switching money in this case so that switching through
is carried out in accordance with the VPI number. The
switching centers according to Fig. 2 also contain
central control devices (not shown) with local and
global priority tables.
A further refinement of the invention is shown
in Fig. 3. This is a 1+1 structure. This structure is
obtained from the l:n structure in that the switching
devices S are set permanently and can no longer be
controlled by means of the central control devices ZST.
In this way, the ATM cells are transmitted both on the
service link WE and on the standby link PE, even in a
fault-free operating situation. The selection device SN
is not embodied here as an ATM switching matrix but
rather as a switching device. The equivalent circuit
switching protocol ES assumes a simpler form in this
case. The information K2 describes the state of the
selection device here. Whenever, in the case of the l:n
structure, the switching devices Sp...Sn have been
controlled, in the case of the 1+1 structure the
selection device SN is controlled instead.
As already mentioned at the beginning, there is
provision for the standby link PE to remain unused
during the time in which there is undisrupted
operation. However, if appropriate, special data (EXTRA
TRAFFIC) can also be fed to the switching device E
during this time. The dynamics of the system are thus
increased. ABR (Available Bit Rate) traffic data or UBR
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(Unspecified Bit Rate) traffic data are used here as
the special data. They are defined, for example, in the
ATM forum "Traffic Management Specificationp, Version
4Ø
According to the above, ABR/UBR traffic is
understood in particular to be specifically defined
classes of traffic. The ABR traffic class is defined
here in such a way that the transmission of the
associated traffic data is performed under the
guarantee of a minimum bandwidth (Minimum Cell Rate,
MCR). The remaining bandwidth is, if appropriate,
regulated depending on the usage factor of the nodes
(quality of servive). In the case of the UBR traffic
class, no guarantee whatsoever is made for the
transmission quality (quality of servive). Likewise, no
lower bandwidth is guaranteed. However, the
transmission of data traffic of a general type is thus
perfectly tolerable. Both traffic classes are also
assigned a priority.
The transmission of the ABR/UBR traffic data
which is controlled by means of the standby link PE in
the case of fault-free operation does not necessarily
have to be interrupted in the case of standby
operation. The reason for this is that the traffic data
which is configured in this way have a lower priority
than the traffic data which are switched to standby
circuits, and are thus automatically moved on depending
on their priority. However, in this case, the ABR/UBR
traffic which may be present on the service link
WE1...WEn which is not available are no longer treated
with higher priority than the ABR/UBR traffic on the
standby link PE. Here, in the case of standby
operation, it must be ensured that the total of the
guaranteed bandwidths including the guaranteed minimum
bandwidth does not exceed the capacity of the standby
link.
The transmission of the ABR/UBR traffic data on
the standby link PE does not necessarily have to take
place between the switching devices W, E. Instead, the
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transmission could also merely occur over one section
of the standby link PE. Such signal
injection/extraction could then be performed by means
of by means of further switching devices which are
designed, for example, as cross-connect switching
devices and which are arranged between the switching
devices W, E. In addition, the bandwidth of the ABR/UBR
traffic data on the standby link PE may be distributed
as desired amongst a large number of other virtual
paths/channels (route freedom).
