Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2000-07-07
2004-01-13
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S071000, C398S058000, C398S135000
Reexamination Certificate
active
06678475
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of transmitting a concatenated signal consisting of at least two component signals over an optical network comprising at least three network elements.
In digital networks, signals are generally transmitted from a transmitting unit to one or more receiving units in the form of data packets. Signals having a greater data volume than that of one packet are distributed among different packets, which are transmitted in a temporal sequence and are reassembled in the receiving unit. Continuous signals, such as analog telephone signals or television signals, are divided into component signals, digitized, and transmitted in the form of consecutive packets. If time-division multiplexing is used, signals from different sources can be transmitted over the network together, also simultaneously to different receiving units.
In SDH networks (SDH=Synchronous Digital Hierarchy), so-called VC-4 containers, among others, are used for the component signals. Two or more concatenated VC-4 containers then contain the overall signal.
For the transmission of SDH signals, WDM networks (WDM=wavelength-division multiplex) are increasingly being used. A WDM network is an optical network that makes at least two wavelengths available for the transmission of signals. One method of transmitting a broadband signal in at least two logically concatenated VC-4 containers involves transmitting the individual VC-4 containers, i.e., the individual component signals, in a temporal sequence and in interleaved form using only one wavelength, as is already known from the SDH standards for electric SDH signals. A further signal consisting of two or more VC-4 containers can be transmitted simultaneously on another wavelength.
If the individual component signals of a signal are transmitted in a temporal sequence, i.e., contiguously in time, this is referred to in agreed standards as “contiguous concatenation”.
Another method of transmitting a signal consisting of at least two VC-4 containers involves introducing a logical concatenation i.e., transmitting the individual containers simultaneously with individual overheads and possibly using two or more different wavelengths. This is referred to as “virtual concatenation”. By virtual concatenation, the bit rate can be adapted to customer requirements in smaller steps. The containers can be distributed arbitrarily via an optical multiplexer, for example. Optimum use can be made of the available transmission capacity. The assignment of the component signals to the wavelengths is made in the transmitting facility at the transmitting location and is maintained over the entire transmission link up to the specified receiving location. A disadvantage of virtual concatenation is, however, that the different propagation velocities for different wavelengths may result in undesirably and sometimes inadmissibly great differential delays for the individual component signals. On long-distance links, e.g., on international links, the differential delays between the component signals may be so great, e.g., 70 &mgr;s, that the order of the received component signals no longer corresponds with the order of the transmitted component signals.
One way of avoiding this problem is not to use virtual concatenation or to restrict it to short distances. Another approach uses buffers in relay stations. Each buffer causes additional differential delays and requires an additional control facility for detecting the component signals and restoring the correct sequence.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method of or apparatus for transmitting a concatenated signal which does not have the above disadvantages.
This object is attained by a method of transmitting a concatenated signal comprising at least two component signals over an optical network of at least three network elements, characterized in that the at least two component signals are transmitted on at least two different wavelengths, and the component signals are assigned to the wavelengths for each section between two network elements in such a way as to minimize differential delays between the component signals for a predetermined receiving location.
The object is further attained by a network element of an optical network for transmitting concatenated signals, characterized in that the network element comprises a receiving unit for receiving at least two component signals of a concatenated signal on at least two wavelengths, a measuring unit is provided for measuring differential delays between the component signals of the received, concatenated signal, and a transmitting unit is provided for transmitting the received, concatenated signal, the transmitting unit being adapted to change the assignment of the component signals to the wavelengths before transmission if at least one measured differential delay exceeds a predetermined threshold.
The object of the invention is further attained by a network comprising at least three network elements and a network management connected to all network elements, characterized in that each of the network elements is adapted to assign different wavelengths to different component signals of a concatenated signal to be transmitted, and the network management specifies the individual assignments for each of the network elements in such a way that differential delays of the component signals of the concatenated signal are minimized for a predetermined receiving location.
The transmission of the concatenated signal takes place with an assignment of the component signals to the wavelengths that changes during transmission. In this way, an average propagation velocity is formed for the individual component signals. This means that each component signal, viewed over the entire transmission link, is delayed by approximately the same amount, so that the differential delays between individual component signals are minimized or even compensated for.
To accomplish this, the transmission link is divided into sections, for example. If the differential delay between the component signals varies so widely as to exceed a predetermined value, the assignment of the component signals to the wavelengths will be changed on particular sections by assigning the component signal having the least delay to that wavelength which has the lowest velocity of propagation. Thus, component signals transmitted fast on the first sections are transmitted at reduced speed on the subsequent sections. Component signals transmitted slowly on the first sections are transmitted on the subsequent sections on a wavelength that has a higher velocity of propagation. Viewed over the entire transmission link, changes in the assignments of the component signals to the wavelengths can be made several times. For example, a component signal is transmitted on the first three sections on a wavelength with a high propagation velocity, on the fourth and fifth sections on a wavelength with a low propagation velocity, on the sixth section on a wavelength with a very high propagation velocity, and on the seventh, eighth, and ninth sections on a wavelength with a low propagation velocity.
In this way, differential delays are minimized or, ideally, compensated for, particularly for long transmission links.
In addition, delays and differential delays can be registered and analyzed in a central network management. The network management can also perform the assignment control function.
Through the distribution of individual signals to two or more wavelengths, blocking can be avoided.
The individual VC-4s of the virtual concatenation can be distributed over two or more wavelengths. If there is not enough capacity available on one wavelength, the individual VC-4s can be distributed to two or more wavelengths in an appropriate and cost-effective manner. If two or more wavelengths are used, the individual wavelengths can be utilized up to 100% before any further wavelengths are used, so that no unused capacities remain.
Concatenated transmission capacity c
Turban Karl-Albert
Weis Bernd X.
Alcatel
Negash Kinfe-Michael
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