Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-06-03
2004-08-24
Vanderpuye, Kenneth (Department: 2661)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S389000
Reexamination Certificate
active
06781993
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switch and a switching method for performing a switching operation for each cell and a process for accommodating various types of data accommodated within a different network in a communications network where data is partitioned into cells being fixed-length packets, which are transferred.
2. Description of the Related Art
An ATM (Asynchronous Transfer Mode) communications method is a method for partitioning data having various speeds into fixed-length packets referred to as cells, and for transferring the cells. This method can process various types of data having diversified traffic characteristics in a unified manner in a communications network. Therefore, a communications network adopting the ATM communication method (ATM network) has been built as the infrastructure of multimedia communications.
ATM switches are arranged in such an ATM network, and relay ATM connections between users by performing a switching operation for each cell. For example, the ATM switch disclosed by the Japanese Laid-open Patent Application No. 7-307745 has the configuration shown in FIG.
1
A.
An ATM switch
1
shown in
FIG. 1A
is mainly composed of a switch unit
2
and a plurality of line units
3
(subscriber line units or trunk line units). One or a plurality of subscriber terminals
5
are connected to each of the line units
3
directly or via a transmitting device
4
, and one or a plurality of connections (ATM connections) from one or a plurality of subscribers are accommodated by each of the line units
3
.
As described above, the ATM switch
1
can accommodate a plurality of connections on a single input port (line unit). To which connection a cell belongs is identified with a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier), which are included in the header of the cell. A connection identified only with the VPI is referred to as a VP connection, while a connection identified with the VPI and the VCI is referred to as a VC connection.
When a connection is relayed by the ATM switch
1
, a VPI/VCI conversion unit
6
arranged within the line unit
3
converts the VPI/VCI of an input cell into those corresponding to an output line, and attaches tag information (TAG) to the cell by referencing a VPI/VCI conversion table
7
, as shown in FIG.
1
B. The TAG is an internal identifier used for selecting a route within the switch unit
2
.
The switch unit
2
outputs a cell to a desired route by using the TAG attached to the cell. With these operations, the ATM switch
1
can switch each of input cells to a desired line.
By the way, an ATM network can accommodate subscriber terminals other than ATM terminals. As such terminals, for example, terminals connected to an existing STM (Synchronous Transfer Mode) communications network which mainly handles telephony services can be cited.
If an STM subscriber terminal is accommodated by an ATM network, an STM-ATM converter called an IWF (InterWorking Facility) or a CLAD (Cell Assembly and Disassembly) is installed between the STM and ATM networks. The STM-ATM converter converts STM data into an ATM cell, and transfers the STM data within the ATM network.
FIG. 1C
is a block diagram showing the network configuration where an ATM network accommodates an existing STM network. In this figure, an ATM network
11
includes a plurality of ATM switches
1
, to each of which an ATM subscriber terminal
5
is connected. An STM network
12
includes a plurality of STM switches
13
, to each of which an STM subscriber terminal
14
is connected. IWFs
15
are arranged between the ATM switches
1
and the STM switches
13
.
For example, data transmitted from the STM subscriber terminal
14
in the lower left of this figure is input to the ATM network
11
via the STM switch
13
and the IWF
15
, and is transferred over a trunk line within the ATM network
11
. Then, the data reaches the STM subscriber terminal
14
in the lower right via the IWF
15
and the STM switch
13
. Or, the data may sometimes reach the ATM subscriber terminal
5
in the upper right, which is connected to the ATM network
11
.
In this case, the following two methods can be considered as a data mapping method used when STM data is converted into ATM cells (hereinafter referred to as cell processing) or ATM cells are converted into STM data (hereinafter referred to as decell processing) within the IWF
15
.
(a) A method using an AAL (ATM Adaptation Layer) type 1 (ITU-T (International Telecommunications Union-Telecommunications) Recommendation I.36.3.1)
(b) A method using an AAL type 2 (ITU-T Recommendation I.363.2)
The AAL type 1 is also referred to as an AAL1. This is a cell processing method for transmitting STM data of 47 bytes, each of which has a 125-&mgr;s speed, is transmitted as one cell. This method is suitable for the case where the cell processing is performed for data at a fixed-rate speed. Additionally, an AAL type 2 is also referred to as an AAL2. This is a cell processing method for mapping data onto short packets of a variable length, which are referred to as short cells, and for multiplexing a plurality of short cells into a single ATM cell. This method is suitable for the case where the cell processing is performed for data of variable and low-speed data.
When the IWF
15
performs the cell processing for STM data, it reduces the amount of the data by performing voice encoding (including silence suppression as occasion demands), and performs the cell processing for the encoded data with the AAL type 2. The silence suppression means that data is not transferred if it is in an unvoiced state.
Such cell processing allows the bandwidth compression of data in an ATM network. Note that, however, the bandwidth compression is implemented based on the assumption that the IWF
15
or the ATM subscriber terminal
5
, which is the destination of a connection, supports the AAL type 2. For a non-voice signal for which the bandwidth compression cannot be performed, a transfer with the AAL type 1 is more useful than that with the AAL type 2 in terms of a bandwidth.
The above described conventional communications methods, however, have the following problems.
Normally, if a transmission delay exceeds 25 ms on either of calling and called sides at the time of a voice signal transfer, an echo caused by this delay cannot be ignored and the echo must be compensated for by an echo cancellor. When STM data is transferred within an ATM network, a delay specific to the ATM network and a delay which accompanies the cell or decell processing performed by the IWF occur. Therefore, a delay time may be larger than that in an existing STM network.
FIG. 1D
is a schematic diagram showing a delay which accompanies the cell/decell processing performed by the IWF with the use of the AAL type 1. When the IWF generates a single ATM cell
22
by performing the cell processing for 47-byte STM data
21
, a cell processing delay depending on each of the bytes occurs. By way of example, a cell processing delay of approximately 6 ms (125 &mgr;s×47) occurs for the leftmost 1-byte data
21
′, while a cell processing delay is recognized to be “0” for the rightmost 1-byte data
21
″.
When the ATM cell
22
is transferred over an ATM network, a transmission line delay and a delay variation absorption time &tgr; within the ATM network are added. Normally, cells are buffered in order to prevent a loss caused by a cell conflict within the ATM network. The delay variation absorption time &tgr; is required to absorb the delay variations of cells caused by the buffering.
Furthermore, when the STM data
21
is regenerated by performing the decell processing for the ATM cell
22
by the IWF at a transfer destination, a decell processing delay occurs depending on each of the bytes. By way of example, a decell processing delay is recognized to be “0” for the data
21
′, while a decell processing delay of approximately 6 ms occurs for the data
21
″.
Accordingly, a delay accompanying the cell
Kato Tsuguo
Ono Hideaki
Takechi Ryuichi
Katten Muchin Zavis & Rosenman
Vanderpuye Kenneth
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