Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1997-06-05
2002-02-19
Nguyen, Chau T. (Department: 2739)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S395430, C370S413000
Reexamination Certificate
active
06349097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multicasting in switching apparatus for use, for example, in asynchronous transfer mode (ATM) networks. In particular the present invention relates to data units for use in such apparatus to implement multicasting.
2. Description of the Prior Art
FIG. 19
of the accompanying drawings shows parts of switching apparatus previously considered for use in an ATM network. The switching apparatus
1
includes sixty-four data units
2
0
to
2
63
, each data unit having an input portion
4
0
to
4
63
and an output portion
6
0
to
6
63
. For the sake of clarity only the input and output portions of the data units
2
0
,
2
1
,
2
2
and
2
63
are shown in FIG.
19
. In
FIG. 19
, the output portion
6
of each data unit
2
is shown separately from the input portion
4
of the unit concerned, so as to clarify the operation of the apparatus, but it will be appreciated that both the input and output portions
4
and
6
form part of the same data unit
2
.
Each data unit
2
is connected to one or more data input/output ports DP
0
to DP
127
of the switching apparatus; for example, as shown in
FIG. 19
, each data unit is connected to a pair of data ports DP.
The data ports DP of the
FIG. 19
apparatus are connected to respective ATM communications lines, which typically each provide a large number of separate channels (virtual channels). Each ATM virtual channel carries data in the form of fixed-length cells.
The apparatus
1
further comprises a cross-connect switching unit
8
having as many input ports IP
0
to IP
63
and as many output ports OP
0
to OP
63
as there are data units
2
. Each input portion
4
is connected to one of the input ports IP and each of the output portions
6
is connected to one of the output ports OP.
The cross-connect switching unit
8
is controllable selectively to provide connections between its input ports and its output ports. Up to 64 simultaneous connections, each between one of its input ports and one of its output ports, can be provided. For example, if data is received by the apparatus
1
at data port DP
2
(the “source” data port) that is to be routed to data port DP
4
(the “destination” data port), the cross-connect switching unit
8
is configured to provide a connection between the input portion
4
1
of the data unit
2
1
(the “source data unit” having the source data port DP
2
connected to it) to the output portion
6
2
of the data unit
2
2
(the “destination data unit”) which has the destination data port DP
4
connected to it. Thus, the source data unit
2
1
and the destination data unit
2
2
form a data-unit pair and are allocated a data transfer path within the apparatus for use in passing data from the source data unit of the pair to the destination data unit of the pair. At the same time, up to 63 other such data-unit pairs can be allocated respective data transfer paths by the switching unit
8
to enable data received at source data ports connected with the respective source data units of those pairs also to be routed through the switch to the respective destination data units of the pairs, those destination data units being connected with the relevant destination data ports.
Since it is possible for two (or more) source data ports to wish to communicate at the same time to the same destination data port, it is normal to make provision for some buffering of the data at some point within the switching apparatus, either within the data units
2
or in the cross-connect switching unit
8
, or possibly in both. This buffering holds up the data of one of the two contending source data ports pending transfer to the intended destination data port of the data received at the other of those two contending source data ports.
Of the virtual channels connected to the data ports, some channels are so-called “unicast” channels, the cells of which are to be switched by the apparatus to just a single other virtual channel. However, other channels connected to the data ports may be so-called “multicast” channels, the cells of which are to be switched by the apparatus to more than one other channel. Thus, to deal with cells received from such a multicast channel it is usually necessary to transfer the cells from the source data port to a plurality of destination data ports.
There are a number of ways in which provision can be made for such multicasting. For example, as described in more detail in our copending United Kingdom application no. 9617110.3, it is possible to operate the data units in “multicasting groups”, each group having a source data unit which outputs one or more cells to all of the destination data units of the group at the same time. In order to avoid contention at the output ports of the switching unit, it will generally only be possible for one or a limited number of such multicasting groups to be formed at the same time. Thus, in this method the data units have to be allocated multicasting opportunities individually or in small groups in turn, for example at predetermined time intervals.
However, this multicasting method remains relatively inefficient, since it is complex to control and arrange, for example from the software point of view, and, being prone to cell contention problems, may lead to cell loss unless significant buffering is provided in the apparatus.
In addition, just as with unicast channels, multicast channels may be used to carry different types of data traffic. For example, some multicast channels may be used to carry low-priority computer data files, whereas other multicast channels may be used to carry high-priority constant-bit-rate (CBR) traffic such as voice and video. All ATM channels, including multicast channels, must meet the quality of service (QoS) requirements specified at the time the channel concerned is set up. In particular, cell delays and cell delay variations (CDV) must be within agreed limits commensurate with the specified QoS requirements. There may be unicast and multicast channels, passing through the switching apparatus to the same destination data port, that have the same priority (QoS requirements). The allocation of multicasting opportunities at predetermined time intervals can result in the multicast channels receiving priority in the switching apparatus so that a multicast cell destined for a particular data port and received after a unicast cell for that same destination data port is nonetheless transferred to the destination data port before the unicast cell, even though the two cells have the same priority.
In effect, therefore, the cell sequence integrity for different virtual channels of the same priority cannot be guaranteed. By tending to accord higher priority to multicast channels in the switching apparatus, cell delays for unicast channels can build up elsewhere in the network which must be strenuously avoided as cell delays and cell delay variation have serious degrading effects on real-time traffic such as voice and video traffic. Voice traffic, for example, may be truncated and unwanted signals and undesirable sound effects may be introduced. Video traffic is even more susceptible to such delay/phase variations which manifest themselves as dropped picture elements, colour changes and other visual anomalies.
It may also be desired to make the switching apparatus “self-routing” so that data passes along a suitable route through the various components of the apparatus without every component having to have its routing of the data controlled directly by the switching controller, thereby reducing the control burden on the switching controller. The need to perform multicasting complicates design of the switching apparatus components and accordingly it is desirable to provide a multicasting method for use in such apparatus having self-routing components, in particular a self-routing switching unit, without over-complicating the designs of those components.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a data unit, for r
Fujitsu Limited
Nguyen Chau T.
Nguyen Phuongchau Ba
Staas & Halsey , LLP
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