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The fiber length L must be proportional to the
slot duration S, which corresponds to the maximum
data unit length, to confine the total signals. The
physical implementation of the buffer takes the
maximum data unit length into account but remains
independent of each data unit length.
An array of FGBs (Srivastava et al., 2008) is
inserted at the virtual memory output path and a
second array is inserted between the delay loop and
the amplification loop. As the buffer can enclose K
signals at the same time, each array constitutes a
sequence of K reflection bands (RB): the array at the
output path constitutes the first sequence of K RB
named RB
k1
, k
∈
[0, K[ and the array between both
loops constitutes the second sequence of K RB
named RB
k2
, k
∈
[0, K[. As shown in Figure 2, the
RB
k1
and RB
k2
are overlapped and the K bands
consisting of the union of RB
k1
and RB
k2
are
disjointed one of the other.
Figure 2: Organization of the reflection bands sequences.
Signals arriving on the input path pass to the
delay loop throw the combiner. At this moment,
theirs wavelengths are in the band B
1
, it means that
each k
th
signal has as wavelength λ
k1
(∈ RB
k1
∩
RB
k2
), which corresponds to overlapping zone
between RB
k1
and RB
k2
. Having these wavelengths,
signals remain enclosed in the delay loop until theirs
wavelengths are converted. Signals pass throw the
optical fiber which induces a delay equal to
Δ
T each
revolution. Then, they pass throw the wavelength
converter block. The signal number k leaves the
converter block with wavelength equal to: λ
k1
if it
has to make another turn in the delay loop, λ
k2
(
∈
RB
k1
-(RB
k1
∩
RB
k2
)) if it has to be amplified and λ
k0
(
∈
RB
k2
-(RB
k1
∩
RB
k2
)) if it has to exit the buffering
block. Then, signals reach the first port of the first
circulator and they are led to its second port. The
signals pass throw the second FBGs array which
constitutes the reflection bands RB
k2
, k
∈
[0, K[. Two
cases may occur: signals with wavelengths out of the
RB
k2
, that means signals with wavelengths equal to
λ
k2
, are transmitted to the second circulator; but
signals with wavelengths inside the RB
k2
, that means
signals with wavelengths equal to λ
k1
or λ
k0
, are
reflected to the first circulator. The transmitted
signals arrive at the second port of the second
circulator and are led to its third port. So these
signals enter to the second loop where they are
amplified. Passing from the first port to the second
port of the second circulator, these signals leave the
amplification loop, cross the reflection bands RB
k2
and are led from the second port to the third port of
the first circulator. Also the reflected signals are led
from the second port to the third port of the first
circulator. Signals arriving at the first port of the
third circulator are led to its second port. Then, they
pass throw the first FBGs array which constitutes the
reflection bands RB
k1
, k
∈
[0, K[. As happen with the
first reflection bands, two cases may occur: signals
with wavelengths out of the RB
k1
, that means signals
with wavelengths equal to λ
k0
, are transmitted on the
virtual memory output path; but signals with
wavelengths inside the RB
k1
, meaning signals with
wavelengths equal to λ
k1
or λ
k2
, k
∈
[0, K[, are
reflected to the second port of the third circulator.
Transmitted to the third port of the third circulator,
the reflected signals reenter to the delay loop and are
passed by the combiner to the optical fiber. Signals
remain enclosed in the delay loop until the
wavelength converter block shifts theirs wavelengths
to: (a) λ
k2
and so they are passed to the amplification
loop, or (b) λ
k0
to be exited from the memory.
2.2 Wavelength Converter Block
A wavelength converter block is inserted inside the
delay loop. This block is used to convert the signals
wavelengths independently one of each other. In
fact, several signals can be enclosed in the virtual
memory at the same time (at most, K signals);
signals having to be amplified must have theirs
wavelengths converted to λ
k2
(k
∈
[0, K[); signals
which were just amplified, during the previous turn,
must have theirs wavelengths reconverted to λ
k1
(k
∈
[0, K[); signals having to exit the virtual memory
to the switching block must have theirs wavelengths
converted to λ
k0
(k
∈
[0, K[) and signals having to
turn again in the delay loop must have theirs
wavelengths maintained to λ
k1
(no conversions are
needed).
As shown in Figure 3, the wavelength converter
block consists in a set of K tunable wavelength
converter (Wang et al., 2006) managed by a memory
control unit. When signals turning in the delay loop
arrive at the wavelength converter block, a
demultiplexer separates them on K paths. On each
path an optical sensor is used to detect the arrival
moment of each signal. These sensors inform the
memory control unit of the arrival moment of
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