ATM switch

Details ATM switch ATM switch

1999-03-03

2003-05-27

Kizou, Hassan (Department: 2662)

Multiplex communications

Pathfinding or routing

Through a circuit switch

C370S395100, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06570874

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an ATM switch, and, more particularly, to technologies in which a plurality of ATM switches are interconnected to realize a large-scale ATM switch.
2. Description of the Related Art
It is common to configure a large-scale ATM switch by interconnecting basic switches.
FIG. 28
is a block diagram of an ATM switch in which basic switches are interconnected.
Conventionally, as shown in
FIG. 28
, output ports of basic switches #
11
-#
1
N at a front stage are connected to input ports of basic switches #
21
-#
2
N at a back stage in a mesh manner. In other words, the number of the output ports of the basic switches #
11
-#
1
N is the same as the number of the input ports of the basic switches #
21
-#
2
N, such that the output ports are in a one-to-one correspondence with the input ports, and each of the basic switches #
11
-#
1
N selects an output port for a cell which is entered into one of the basic switches #
11
-#
1
N so that a basic switch among the basic switches #
21
-#
2
N is decided for the cell to be sent out to.
For example, in the case of using basic switches of 8 inputs by 8 outputs (8×8), an ATM switch of 64 inputs by 64 outputs as a whole can be realized by interconnecting 8 basic switches #
11
-#
18
in the first stage and 8 basic switches #
21
-#
28
in the back stage.
Regarding the switch interconnection, a technology using a barrel shifter and a wavelength-multiplexing-optical-fiber is illustrated in N. Yamanaka, S. Yasukawa, E. Oki and T. Kawamura, “OPTIMA:Tb/s ATM Switching System Architecture: Based on Highly Statistical Optical WDM Interconnection,” Proc. IEEE ISS'97, System Architecture, 1997 and S. Yasukawa, N. Yamanaka, “640 Gb/s Ultra-High-Speed Optical interconnection System using Wide-Channel-Spacing Wavelength Division Multiplexing,” Proc.IEEE BSS'97, p.p.101-105, December 1997.
FIG. 29
is a block diagram showing an ATM switch in which basic switches are interconnected in a multistage manner. In the ATM switch, the basic switches are interconnected via WDM (wavelength division multiplexing) links by using a barrel shifter which switches signals in the WDM links according to wavelength, and, in the basic switches of the front stage, destinations of signals are determined by selecting wavelengths by which the signals are sent. Each basic switch in the back stage is connected to every basic switch in the front stage by logical links of different wavelengths. According to the above-mentioned configuration, a number of physical links between the basic switches can be eliminated.
In the following, the barrel shifter will be described with reference to FIG.
30
.
FIG. 30
shows how optical signals are switched within the barrel shifter.
FIG. 30
shows an example in which the barrel shifter has 2 input lines I
0
, I
1
and 4 output lines O
0
-O
3
. Here, optical signals of wavelengths &lgr;
0
-&lgr;
3
are transmitted on each of the input lines I
0
, I
1
, and optical signals of wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
on the input lines I
0
are respectively sent to the output lines O
0
, O
1
, O
2
, O
3
. Further, optical signals of wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
on the input line I
1
are respectively sent to the output lines O
1
, O
2
, O
3
, O
0
. Therefore, for example, a wavelength which goes to the output line O
1
is &lgr;
1
among the wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
which are transmitted on the input line I
0
, and a wavelength which goes to the output line O
1
is &lgr;
0
among the wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
which are transmitted on the input line I
1
.
The barrel shifter is known to those skilled in the art, so only a brief description on the barrel shifter will be given in the following. On the description below, H. Takahashi, et al., “Wavelength MUX and DMUX by using Arrayed Waveguide Grating,” IEICE, PST-91-48, pp.41-46 can be refereed to.
The barrel shifter is one of optical devices called “Arrayed Waveguide Grating (AWG).” The conceptual diagram of the AWG is shown in FIG.
31
. Generally, the AWG is made as a wavelength multiplexer-and-demultiplexer, and is an integrated on-board circuit which includes input/output waveguides and slab waveguides which act as collimators/light-gathering lenses.
As shown in
FIG. 31
, the AWG includes a plurality of different-length waveguides which are arranged at regular intervals. Similar to a diffraction grating, phase differences between the waveguides cause dispersion. Therefore, a wavelength-multiplexed signal from the input waveguide is demultiplexed, and the demultiplexed signals are extracted from different output waveguides. If the AWG is used in the reverse direction, the AWG acts as a wavelength multiplexer. Because the slab waveguide has the shape of a sector which has a center of curvature at the endpoint of the waveguide and the axis of the waveguide points to the center of curvature, the slab waveguide has a light-gathering function in a manner similar to a concave mirror. Generally, taper waveguides are inserted between the waveguides and the slab waveguide in order to decrease connection losses.
A wavelength interval &Dgr;&lgr;, which is one of the most important parameters in the wavelength multiplexer-and-demultiplexer by using the AWG, is represented as follows:
&Dgr;&lgr;=&Dgr;
X
/(
f·m

x
·d
)  (1)
m
=(
n
c
·&Dgr;L
)/&lgr;
0
  (2)
wherein d is a pitch of the grating of the AWG, &Dgr;L is a difference of length between the waveguides, f is a focal length (a radius of curvature) of the slab waveguides, &Dgr;X is an interval between the waveguides, n
x
is an effective refractive index of the slab waveguide, the denominator of the right side of the equation (1), (f·m

x
·d), is a linear dispersion which is a proportionally constant of a relationship between the wavelength and the light-gathering position, n
c
is an effective refractive index of the waveguide, &lgr;
0
is a central wavelength of the AWG and can be obtained from the central output waveguide, and m is a diffraction order indicating the number of wavelengths which represents the amount of the phase difference between the neighboring waveguides. The larger m is, the larger the linear dispersion is. Therefore, signals of many wavelengths the intervals of which are small can be multiplexed-and-demultiplexed as m increases. In other words, the larger m is, the higher the wavelength resolution of the AWG is. Regarding an ordinary diffraction grating, it is necessary to decrease the size of the pitch in order to increase the resolution. Therefore, the resolution is limited by the technology of the pitch making. But, regarding the AWG, a high resolution can be easily realized by increasing the diffraction order by increasing the length of the waveguide. This is the main difference between the AWG and the ordinary diffraction grating.
As shown in the equation (2), a plurality of central wavelengths may exist in the AWG because the m can take any number. For example, in the case of a design in which &Dgr;L=126 &mgr;m and n
c
=1.45, if m=118, &lgr;
0
=1548.3 nm, and if m=119, &lgr;
0
=1535.3 nm, and a plurality of optical signals which include 1548.3-nm and 1535.3-nm signals are output from the central output port. Here, the bandwidth which can be used without overlapping is 13 nm, and, in the case of a 0.8-nm wavelength-interval WDM (wavelength division multiplexing) technology, the biggest number of wavelengths is “16”. As mentioned above, the larger m is, the higher the resolution is, but the narrower the bandwidth which can be used without overlapping is. Therefore, the m needs to be set carefully.
The barrel shifter used here is the AWG which utilizes a characteristic that same wavelength signals per bandwidth which is usable without overlapping are cyclically output as shown in FIG.
32
.
FIG. 3

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