Switch arrangement

Details Switch arrangement Switch arrangement

2000-01-10

2004-05-11

Sam, Phirin (Department: 2661)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S386000, C370S389000, C370S395300, C370S417000, C370S412000, C370S429000

Reexamination Certificate

active

06735203

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to switching implemented in a packet-switched telecommunications network. More particularly, the invention relates to a switch arrangement used in such a network, specifically an ATM network.
BACKGROUND OF THE INVENTION
To aid the understanding of the following description, some terms to be used later will first be defined.
A switching fabric or switching matrix (both terms are used) is comprised of a plurality of switching elements which are either identical or dissimilar and are interconnected according to a given topology. In the (English-language) literature of the art, such a switching matrix may also be called a “switching network”, since the switching elements form a network having the given topology. Hence, a switching matrix is considered to have a defined form when its switching elements and their interconnections are known.
A switching fabric is composed of switching elements by connecting a number of switching elements into a network comprising switching elements in parallel and in succession. Parallel switching elements make up one switching stage. Switching elements in successive switching stages are interconnected by internal links in accordance with the above topology.
The term switch is used to denote the entity configured about a switching matrix. Hence, a switch can denote any means employed for signal switching in a telecommunications network. In the present context, a switch is a packet switch as the invention is related to switching in a packet-switched telecommunications network, particularly an ATM network. A switch is also sometimes termed a switching system.
ATM (Asynchronous Transfer Mode) is a connection-oriented packet-switching technique, which has been selected by the international organization for telecommunications standardization, ITU-T, as the target transfer mode solution for implementing a broadband multimedia network (B-ISDN). In an ATM network, the problems of conventional packet-switched networks (such as X.25 networks) are overcome by transmitting short packets of a constant length (53 bytes) called cells. Each cell comprises a 48-byte payload portion and a 5-byte header. The header comprises, along with other data, address information on the basis of which the cell is routed in an ATM network. Further discussion of an ATM network herein will be omitted as non-essential subject to the understanding of the invention. When required, a closer description of this topic can be found in international standards and textbooks of the art.
Switches in a conventional TDM network (Time Division Multiplexing, also called by the name STM, Synchronous Transfer Mode) cannot be directly implemented to handle the switching in an ATM network. Neither are the switching solutions developed for conventional packet networks usually suitable as switches for an ATM network. The selection of an optimum ATM switching architecture is namely influenced not only by the fixed cell size and the limited functionality of the cell header but also by the statistical behaviour of the cell stream and the fact that an ATM switch must operate at a very high rate (currently typically about 150 . . . 600 Mbit/s).
FIG. 1
shows schematically an ATM switch seen from the outside. The switch has n input lines I
1
. . . I
n
and m output lines O
1
. . . O
m
. A cell stream CS arrives over each input line to the ATM switch
11
. The header of an individual cell in the cell stream is denoted by reference HD. In the ATM switch, the cells are switched from the input line I
i
to the output line O
1
, and simultaneously the value of the cell header is translated from an incoming value to an outgoing value. For this purpose, the switch includes a translation table
12
by means of which said translation is made. It is to be seen from the table that, for example, all the cells received over line I
1
and having a header with a value X are switched onto output port O
1
whereby their header is simultaneously given the value K. Cells present on different input lines may have headers of equal value; for example, cells received at input line I
n
with the same header value X are also switched onto output line O
1
, but their header is given the value J on the output line.
A prior art header translation method, which will be used in describing the present invention, is to translate the header in steps by using unique connection identifiers internal to the switch, such as the identifier ICI on the input side and the identifier ECI on the output side. In that case, the header translation chain will be VPI
in
/VCI
in
→ICI→ECI→VPI
out
/VCI
out
. The advantage of such a procedure is that the identifiers ICI and ECI can be direct memory addresses by means of which the connection-related data can be rapidly accessed.
Hence, the main tasks of a switch are: transfer of cells (packets) from the input line to the desired output line, and header translation. Occasionally, however, as is also evident from the figure, two cells may be simultaneously contending for access onto the same output line. For this purpose, the switch must have buffering capacity to avoid the necessity of discarding cells in such a situation. Hence, the third main task of a switch is to provide buffering. The manner in which these three main tasks are performed and in which part of the switch the implementation is handled distinguishes different switching solutions from one another.
ATM switching fabrics, on the other hand, can be subdivided into two classes depending on whether the switching elements used in the fabric are buffered or unbuffered.
In a fabric using unbuffered switching elements, routing of cells through the fabric operates at the cell level in such a way that a route is separately selected for each cell irrespective of which virtual channel the cell belongs to. In simplified rendition, proceeding of cells through the fabric may be thought of as comprising two steps. In the first step, the cells are sent from the input ports through the fabric, and in the second step either the switching elements or the output ports give an indication to the input port which cells were successful in traversing the fabric. The latter step must be performed, since if the cells possibly attempt to use the same internal link of the fabric, all simultaneous cells except one must be discarded since the switching elements have no buffers. The problem with such a switching fabric is that the operation of all its elements must be mutually synchronized, i.e., all elements and input and out-put ports must be in the above-described steps at the same time. Since cell transmission is very rapid and in practice even several further steps may be necessary besides the two described above, synchronization cannot be achieved merely by means of handshaking signals between the elements, but all elements must be synchronized from a common clock source, and it is difficult to distribute the clock signal to all elements if the switching fabric is very broad and it has been necessary to divide it among several plug-in units. It may also be noted that even a switch provided with non-buffered switching elements must have buffering capacity either in the input or in the output ports of the fabric to avoid the need of discarding cells contending simultaneously for the same output line of the switch.
A switching fabric comprised of buffered switching elements does not have the above synchronizing requirement. The selection of the route of the cells through the switching fabric can be virtual channel-related. For this purpose, however, a record of the load on the internal links of the switching fabric must usually be kept. A buffered switching fabric is usually blocking at the connection level, as to construct it to be non-blocking usually requires so much extra capacity that this is no longer economically feasible. In a blocking fabric, the selection of the route for the connection is a highly critical factor when it is attempted to reduce the blocking.
On account of the foregoing, many manuf

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