Note: Descriptions are shown in the official language in which they were submitted.
CA 02338293 2001-O1-19
1
METHOD FOR SWITCHING DATA RECEIVED VIA A PACKET-
ORIENTED DATA TRANSMISSION LINK
The significance of transmission and switching techniques for high data
transmission rates (above 100 Mbit/s) is increasing due to the increasing need
for a
transmission of video information in modern communications technology such as,
for
example, still and moving images in picture telephony applications or the
presentation
of high-resolution graphics at modern data processing systems. A known dat
transmission method for high transmission bit rates is what is referred to as
the
asynchronous transfer mode (ATM). A data transmission on the basis of the
asynchronous transfer mode currently enables a variable transmission bit rate
of up to
622 MbitJs.
In the transmission technique known as asynchronous transfer mode
(ATM), data packets having a fixed length, what are referred to as ATM cells,
are
used for the data transport. An ATM cell is composed of a cell header that is
five
bytes long and contains the switching data relevant for the transport of an
ATM cell
and of a 48 byte long payload field. Only data allocated to one logical
connection -
frequently referred to as virtual channel VC or ATM channel in the literature -
are
thereby transmitted in the payload field of an ATM cell.
US Published Application US-A-5784371 discloses a communication
2 0 network formed of a plurality of communication systems that are connected
to one
another via an ATM network. The communication systems respectively comprise a
timeslot-oriented switching network module for a connection of timeslot-
oriented
terminal devices to the respective communication system, whereby a
bidirectional
switching of data to be exchanged bet [...] device and the packet-oriented ATM
2 5 network ensues with the timeslot-oriented switching network modules.
The German Patent Application bearing the serial number 198 187 76.9
has already disclosed a method that enables a transmission of data belonging
to
different logical connections in the payload region of one or, respectively,
several
ATM cells. To this end, what are referred to as sub-structure elements having
a
3 o variable payload field 0 through 64 bytes long is defined in the payload
field of an
CA 02338293 2001-O1-19
2
ATM cell, said sub-structure elements being capable of being respectively
allocated to
a logical connection via an address field in the cell header of the sub-
structure
element. Due to the 8-bit long address field in the cell header of a sub-
structure
element, a maximum of 2g = 256 different logical connections can be addressed.
Additionally, at least one sub-structure element is reserved for a
transmission of
signalling information allocated to the logical connections.
The article by Mauger, R., et al., "ATM Adaptation Layer Switching" ISS,
World Telecommunications Congress (International Switching Symposium), Ca,
Toronto, Pinnacle group, pages 207-214, XP000720525, discloses an arrangement
for
a switching of data received via a timeslot-oriented data transmission link
and a
packet-oriented data transmission link. The arrangement thereby comprises both
a
timeslot-oriented switching network module as well as a packet-oriented
switching
network module. A switching of data received via the packet-oriented data
transmission link and to be forwarded via a packet-oriented data transmission
link as
well thereby ensues with the packet-oriented switching network module.
An object of the present invention is to specify an alternative method with
which a switching of data that are received via a packet-oriented data
transmission
link and are to be forwarded is enabled.
Proceeding from the features of the preamble of patent claim l, this object
2 0 is inventively achieved by the characterizing features thereof.
A critical advantage of the inventive method is then comprised'therein that
a switching of data allocated to different logical connections and transmitted
in one
or, respectively, several data cells can ensue via a traditional timeslot-
oriented
switching network module. A development of a switching network module designed
2 5 for the present packet-oriented data format and a signalling adapted
thereto are thus
not necessary.
Advantageous developments of the invention are recited in the subclaims.
One advantage of developments of the invention defined in the subclaims
is comprised, among other things, therein that the insertion of filler cells
or,
3 o respectively, of filler data into a sub-structure element during the
conversion of a
packet-oriented data format into a timeslot-oriented data format makes a
switching of
CA 02338293 2001-O1-19
2a
compressed data possible without preceding decompression. A quality loss in
the
switching of compressed data is thus avoided.
An exemplary embodiment of the invention is explained in greater detail
below on the basis of the drawing.
Thereby shown are:
Figure 1 a structogram of the schematic illustration of the critical function
units
participating in the inventive method;
Figure 2 a structogram of the schematic illustration of the conversion of a
packet-
1 o oriented data format into a timeslot-oriented data format according to a
first operating mode of a conversion unit;
Figure 3 a structogram of the schematic illustration of the conversion of the
packet-
oriented data format into the timeslot-oriented data format according to a
second operating mode of the conversion unit.
Figure 1 shows a schematic illustration of a communication system PBX.
The communication system PBX comprises subscriber or, respectively, network
line/trunk modules - a line/trunk module ABG is shown by way of example - for
the
connection of communication terminal devices or, respectively, for a
connection to a
CA 02338293 2001-O1-19
3
communication network - for example, an ISDN-oriented communication network,
an
analog communication network, a radio communication network or an ATM-based
communication network.
Further, the communication system PBX contains a timeslot-oriented
switching network module KN comprising a plurality of bidirectional, time-
division
multiplex-oriented switching terminals KA, whereby the time-division multiplex-
oriented switching terminals KA are fashioned as PCM terminals (pulse code
modulation), also referred to as PCM highways, speech highways or SZM
terminals.
Given an internal data transmission of the communication system, a PCM highway
generally comprises 32 payload channels that are fashioned as ISDN-oriented B-
channels (integrated services digital network) with a respective transmission
bit rate
of 64 kbit/s.
A line unit AE and a conversion unit UE are arranged on the line/trunk
module ABG. The communication system PBX is connected to an ATM-based
communication network ATM-KN via a network interface NA of the line unit AE,
said ATM-based communication network ATM-KN being composed of a plurality of
communication systems connected to one another. A first and a second
communication terminal device KE-A, KE-B are connected to the ATM-based
communication network ATM KN. The line unit AE is connected to a
bidirectional,
2 0 packet-oriented terminal SK of the conversion unit UE via a bidirectional,
packet-
oriented terminal SK. [sic)
The conversion unit UE, further, is connected to a switching terminal KA
of the timeslot-oriented switching network module KN via a bidirectional, time
division multiplex-oriented switching terminal KA. [sic) Via further switching
2 5 terminals KA (not shown), the timeslot-oriented switching network module
KN is
respectively connected to a bidirectional time-division multiplex-oriented
terminal SK
of further subscriber or, respectively, line/trunk modules (not shown)
arranged in the
communication system PBX.
A bidirectional conversion between the packet-oriented data format of a
3 o connecting line PO-VL between the conversion unit UE and the line unit AE
and the
timeslot-oriented data format of a connecting line ZO-VL between the
conversion unit
CA 02338293 2001-O1-19
4
UE and the timeslot-oriented switching network module KN ensues with the
conversion unit UE according to two different operating modes of said
conversion
unit UE that are described in greater detail below.
Further, a control unit STE comprising a plurality of control terminals Sl,
S2 is arranged in the communication system PBX. The control unit STE is
connected
to a control input SE of the timeslot-oriented switching network module KN via
a
control terminal S2, and is connected to a control input SE of the line/trunk
module
ABG via a control terminal Sl. the control unit STE is connected to control
inputs of
further subscriber or, respectively, line/trunk modules arranged in the
communication
system PBX via further control terminals (not shown). A communication of
signalling information between the control unit STE and the timeslot-oriented
switching network module KN or, respectively, the line/trunk module ABG
thereby
ensues according to the HDLC data format (high level data link control).
Figure 2 shows a schematic illustration of a conversion of the packet-
oriented ATM data format according to the ATM adaption layer AAL type 2 into
the
timeslot-oriented data format according to the TDM method (time-division
multiplex)
in a first operating mode of the conversion unit UE. A data transmission in
the
framework of the packet-oriented ATM data format ensues via ATM cells ATM-Z1,
ATM-Z2. An ATM cell ATM-Zl, ATM-Z2 is composed of a five byte long cell
2 0 header H containing the switching data relevant for the transport of an
ATM cell
ATM-ZI, ATM-Z2 and of a 48 byte long payload field.
In a data transmission in the framework of the packet-oriented ATM data
format according to the ATM adaption layer AAL type 2, there is the
possibility of
subdividing the payload area of an ATM cell ATM-Z1, ATM-Z2 into sub-structure
2 5 elements SE. The adaptation of the ATM data format - also frequently
referred to as
"ATM layer" (layer 2) in the literature - to the switching layer (layer 3)
according to
the OSI reference model (open systems interconnection) thereby ensues with
what is
referred to as the ATM adaption layer AAL.
A sub-structure element SE according to the ATM adaption layer AAL
3 o type 2 is composed of a 3 byte long cell header and of a variable-length
payload area I
(0 through 64 bytes). The cell header of a sub-structure element SE is
subdivided into
CA 02338293 2001-O1-19
an 8 bit long channel identifier CID, a 6 bit long length indicator LI, a 5
bit long
transmitter-receiver indication UUI (user-to-user indication) and a 5 bit long
cell
header checksum HEC (header error control).
As a result of the subdivision of an ATM connection with the assistance of
5 sub-structure elements SE into mutually independent data streams, as shown
in the
Figure with reference to the example of the ATM cells ATM-Z1, ATM-Z2, up to 28
-
256 different logical connections can be addressed within an ATM connection of
the
basis of the 8 bit long channel identifier CID, all of these logical
connections being
addressed with the same ATM address - composed of a VPI value (virtual path
identifier) and of a VCI value (virtual channel identifier). In addition,
there is the
possibility of defining a sub-structure element SE for a transmission of
signalling
information allocated to the logical connections. For a transmission of
payload data
allocated to the logical connections, one sub-structure element SE can be
defined for
every currently required logical connection, so that the transmission capacity
can be
exactly matched to the current need.
For example, four different sub-structure elements SE are shown in the
Figure that are defined on the basis of different channel identifier CID in
the cell
header - referred to below as sub-structure element header 0, 1, 2, 3 - of the
sub-
structure elements SE. A payload field I of variable length (0 through 26
bytes) can be
2 o defined by the 6 bit long length indicator LI in the cell header, so that
a data
transmission with variable transmission bit rate can be realized for the
different
logical connections.
For a conversion of the packet-oriented data format according to the ATM
adaption layer AAL type 2 onto the timeslot-oriented data format according to
the
2 5 TDM method, a TDM channel K0, ..., K3 of the timeslot-oriented data format
according to the TDM method is allocated to each element SE of an ATM cell ATM-
Z1, ATM-Z2 defined for a transmission of payload data. An allocation of a sub-
structure element SE to a TDM channel K0, ..., K3 thereby ensues in a
signalling
phase preceding the payload transmission. 32 payload channels, which are
configured
3 o as ISDN-oriented B-channels with a constant transmission bit rate of
respectively 64
CA 02338293 2001-O1-19
kbit/s, are generally available for a data transmission in the framework of
the timeslot-
oriented data format according to the TDM method.
In the framework of the conversion of the packet-oriented data format
according to the ATM adaption layer AAL type 2 onto the timeslot-oriented data
format according to the TDM method, an adaptation of the - potentially
variable -
transmission bit rate of the packet-oriented data format deriving due to the
size and
the arrival of sub-structure elements SE onto the constant transmission bit
rate of 64
kbit/s of the timeslot-oriented data format must additionally ensue. This is
achieved
in the scope of the first operating mode of the conversion unit UE by an
insertion of
1 o what are referred to as filler cells FZ of variable length into the
continuous TDM data
stream.
The sub-structure element SE received via the packet-oriented connecting
line PO-VL and packed in ATM cells ATM-Z1, ATM-Z2 must be unpacked in the
conversion unit UE. For the conversion of the - potentially variable -
transmission
bit rate deriving due to the size and the arrival of the sub-structure
elements SE onto
the constant transmission bit rate of 64 kbit/s of the timeslot-oriented data
format,
what are referred to as filler cells FZ are subsequently attached to the sub-
structure
elements SE containing the payload data. The length of a filler cell FZ is
defined by
what is referred to as a filler cell header FZH. the length of a filler cell
FZ is thereby
2 o selected such that the overall transmission bit rate of a sub-structure
element SE and
of a filler cell FZ yields a whole multiple of 64 kbit/s. When the
transmission bit rate
of a sub-structure element SE is higher than 64 kbit/s - i.e. higher than the
transmission bit rate of a TDM channel Kl, ..., K4 - the payload data
communicated
in a sub-structure element SE are divided onto a plurality of TDM channels K1,
...,
2 5 K4.
In conclusion, these data (sub-structure elements SE and filler cells FX
together) are allocated to a TDM channel K0, ... Kl of the timeslot-oriented
connecting line ZO-VL declared in the signalling phase and are transmitted via
this to
the timeslot-oriented switching network module KN.
3 0 The signalling information communicated from the conversion unit UE to
the control unit STE of the communication system PBX in the framework of the
CA 02338293 2001-O1-19
7
signalling phase are converted in the control unit STE into switching-oriented
control
data for the timeslot-oriented switching network module KN. A switching of the
data
(sub-structure elements SE and filler cells FZ together) received via the
respective
TDM channels K0, ..., K3 of the timeslot-oriented connecting line ZO-VL ensues
in
the timeslot-oriented switching network module KN on the basis of the
switching-
oriented control data, i.e. an allocation of a TDM channel of an input line of
the
timeslot-oriented switching network module KN onto a TDM channel of an output
line of the timeslot-oriented switching network module KN.
When the payload data to be communicated are to be transmitted anew via
the ATM-based communication network ATM-KN to a receiver, the data (sub-
structure elements SE and filler cells FZ together) are transmitted from the
timeslot-
oriented switching network module KN to the conversion unit UE, wherein the
filler
cells FZ are removed from the TDM data stream, so that the data stream then
only
comprises sub-structure elements SE containing payload data. The sub-structure
elements SE to be transmitted are packed in ATM cells ATM-Z1, ATM-Z2 in the
conversion unit UE and are communicated via the ATM-based communication
network ATM-KN to the addressed recipient. When the data are to be transmitted
to,
for example, an internal communication terminal device (not shown), then these
are
transmitted directly to a subscriber line module (not shown) via which the
addressed
2 0 communication terminal device is connected to the communication system
PBX.
Figure 3 shows a schematic illustration of a conversion of the packet-
oriented ATM data format according to the ATM adaption layer AAL type 2 into
the
timeslot-oriented data format according to the TDM method (time division
multiplex)
in a second operating mode of the conversion unit UE.
2 5 In contrast to the first operating mode of the conversion unit UE, no
separate filler cells FZ are inserted into the continuous TDM data stream in
the second
operating mode. An adaptation of the - potentially variable - transmission bit
rate of
the packet-oriented data format to the constant transmission bit rate of 64
kbit/s of the
timeslot-oriented data format ensues by filling the sub-structure elements SE
with
3 0 filler data FD, so that the overall transmission bit rate of a sub-
structure element SE
(payload data and filler data FD together) yields a whole multiple of 64 bits.
This,
CA 02338293 2001-O1-19
however, assumes that each TDM channel K0, ..., K3 additionally has an
information
about the length of the sub-structure elements SE that is transmitted and
supplemented
with filler data FD allocated to it such that a separation of the payload data
to be
transmitted from the filler data FD is enabled with the assistance of this
information.
When, proceeding from the first communication terminal device KE-A,
data are to be communicated to the second communication terminal device KE-B,
the
first communication terminal device KE-A sends the necessary signalling
information
to the communication system PBX in the framework of a signalling phase
preceding
the payload transmission, sending these information via a defined sub-
structure
element SE of a first ATM channel V-A, which is frequently abbreviated as VC
(virtual channel ) in the literature. The transmitted signalling information
are
unpacked in the conversion unit UE, converted into the HDLC data format and
communicated to the control unit STE.
On the basis of the communicated signalling information, a TDM channel
- for example, the TDM channel 17 - of the timeslot-oriented connecting line
ZO-VL
is allocated to the sub-structure elements Se of the first ATM channel V-A
that are
defined for the transmission of the payload data from the first communication
terminal device KE-A to the communication system PBX. Further, the
communicated
signalling information are converted into switching-oriented control data for
the
2 0 timeslot-oriented switching network module KN. The switching-oriented
control data
define which input TDM channel - for example, the TDM channel 17 of the
timeslot-
oriented connecting line ZO-VL - is connected to which output TDM channel of
the
timeslot-oriented switching network module KN - for example, the TDM channel
23
of the timeslot-oriented connecting line ZO-VL.
2 5 Subsequently, the frist communication terminal device KE-A packs
payload data to be transmitted into sub-structure elements SE that are in turn
packed
in ATM cells ATM-Zl, ATM-Z2 and subsequently communicated via the first ATM
channel V-A to the communication system PBX. The sub-structure elements SE are
unpacked from the ATM cells ATM-Z1, ATM-Z2 in the conversion unit UE. In a
3 o next step, the transmission bit rate deriving due to the size and the
arnval of the sub-
structure elements SE is matched to the constant transmission bit rate of 64
kbit/s by
CA 02338293 2001-O1-19
9
inserting filler cells FZ according to the first operating mode of the
conversion unit
UE.
The data - composed of sub-structure elements SE and filler cells FZ - are
subsequently forwarded via the TDM channel 17 of the timeslot-oriented
connecting
line ZO-VL to the timeslot-oriented switching network module KN. The data are
switched onto the TDM channel 23 of the timeslot-oriented connecting line ZO-
VL
by the timeslot-oriented switching network module KN and are sent back to the
conversion unit UE. The filler cells FZ are removed from the continuous data
stream
in the conversion unit UE, so that the data stream is not composed only of sub-
1 o structure elements SE containing payload data. These sub-structure
elements SE are
subsequently packed into ATM cells ATM-Z1, ATM-Z2 and transmitted to the
second communication terminal device KE-B via a second ATM channel V-B.
