Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
1998-11-19
2002-11-26
Nguyen, Chau (Department: 2663)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C370S395100
Reexamination Certificate
active
06487176
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to a measuring method and a measuring device for data communications networks which use asynchronous transfer mode (ATM).
RELATED TECHNOLOGY
ATM transfer systems are known, which are used to transmit synchronous digital signal strings, existing, for example, as digitized voice, digitized music, digitized images, or as a binary series of numbers, that have been split up into eight-bit-width data words and combined into packets; data packets of this kind are the payload of ATM cells. The AAL
1
format (ATM Adaptation Layer
1
) is a customary cell format for transmitting synchronous data over ATM systems at a constant bit rate (CBR=constant bit rate). An AAL
1
cell has 53 data words of eight bit length, i.e. 53 octets or bytes, of which five data words form the cell header, and another data word is used for numbering the data packet. 47 data words thus remain for the payload of the cell.
On the transmitting side, synchronous digital data strings are able to be easily converted into ATM cells. Since it is the transmission system, nowadays predominantly a fiber optic system of the so-called “synchronous digital hierarchy” (SDH) including the most frequently occurring STM
1
-interface, which predefines the transmitting bit rate of 155.52 Mbit/s, quite a few data systems employing the most frequently used synchronous transmission rate of 2.048 Mbit/s are able to be inserted into such an ATM transmission system. The individual 2.048 Mbit/s systems are distinguished by their individual cell header contents and can be reassigned later to the correct receiver.
Given a less than fully utilized transmission capacity over a 155.52 Mbit/s transmission route, it is possible to insert about 72 ATM cells of other users or idle cells into the gaps between two 2.048 Mbit/s payload cells of one specific data stream. Splitting the synchronous data stream into eight-bit words, as required in ATM cell formation, reduces the rate of the word clock or block clock in the processing and retransmission of signals at the parallel port to one eighth of 155.52 Mbit/s, resulting in 19.44 Mbit/s and facilitating the use of more highly integrated switch elements.
Greater difficulties have to be overcome on the receiving side to retrieve the original digital data string from the ATM cells of a specific data stream, because it is necessary to recover the synchronous data timing or clock. Clock recovery circuits require reference oscillators, whose frequency can be variably tuned within certain limits by the incoming line signal. VCOs (voltage controlled oscillators) are used to perform the variable tuning.
As long as the synchronous digital data strings continue to be transmitted at 2.048 Mbit/s, the receiver's clock recovery circuit, equipped with an appropriate VCO, is properly set up for reconverting the incoming ATM cells. However, as soon as a lower or higher transmission rate is needed, for example in world-wide data communication traffic based on a customary, yet different, U.S. or Japanese standard, the variety of measuring methods is restricted by the availability of reference oscillators. Assuming it takes about eight weeks to acquire VCO modules to retrofit an ATM measuring instrument, a measurement could be delayed by this period of time, in the event of an urgently needed conversion.
An even greater problem is posed by payload measuring procedures when working with VBR data strings (VBR=variable bit rate), thus when the data that arise require a variable transmission rate. This is the case when working with MPEG encoded digitized television pictures, where the required transmission rate is a function of the changes in the differential-encoded picture content over time. Also the transmission of voice over ATM systems raises this measuring problem, because digitized voice transmission can occur with a transmission rate substantially lower than 2.048 Mbit/s.
Therefore, in the case of VBR data strings, the measuring procedure does well enough with ATM cell-oriented measuring methods; cell losses are registered using cell header data and mostly output as cell loss rates. However, ATM operating companies are obligated to transmit their customers'data and are asked about the quality of their networks with respect to these data. This customer data are inserted in the payload portion of the ATM cells. Therefore, a method for measuring the payload data is required, even when it is only a question of representing the time distribution of ATM cell losses in detail, cell for cell, to facilitate application of suitable error correction methods.
SUMMARY OF THE INVENTION
An object of the invention is to devise a measuring method and a measuring device for the asynchronous transfer mode used in telecommunications networks which functions without recovering the original synchronous data timing. This permits flexibility within the transmission rate of the synchronous customer data and that the measuring process also works with the variable bit rate of the original, synchronous digital data strings.
The present invention provides a measuring method for data communications networks which makes use of asynchronous transfer mode (ATM) for continuous payload measurements, particularly at a 19.44 Mbit/s parallel port of ATM transmission devices where ATM cells are applied. The cells in this case are composed of 53 eight-bit width data words, and whose 47 useful signal words (payload) originate from originally synchronous digital data strings, whose frequency (rate of occurrence) is determined by the bit rate of the originally synchronous digital data, and which permit the cell-by-cell reading out of the payload data. The payload data are fed consecutively and at a higher rate as burst data to a data test receiver, which is able to ascertain the correct or corrupted receipt of the transmitting-side data. The parallel data (DA
0
through DA
7
) arrive together with a 19.44 Mbit/s block clock pulse (
7
) and a cell starting pulse (
10
) in an ATM cell evaluation circuit (
2
); in the evaluation circuit, the 47 payload words are extracted from the altogether 53 words of each valid ATM cell and are fed to data inputs (d
0
through d
7
) of a memory (
3
). With the aid of a timed write signal (
11
), which is fed by the ATM cell evaluation circuit (
2
) to the memory input for the write signal (
11
), the 47 payload words are then read in one after another into the memory (
3
). The memory (
3
) indicates at which instant the cell contents are available and uses an empty flag signal (
13
) as a filling level indicator to start a clocked burst data generator (
6
), which is also timed by the block clock pulse (
12
). The generator uses the block clock pulse to produce different pulse bursts on separate lines (for example 14, 15, 16 and 19). With the aid of the first clock-pulse burst, the 47 payload words are read out one after another out of memory (
3
) and fed to the parallel inputs of a parallel-to-serial converter (
4
). This parallel-to-serial converter is supplied at its input (
16
) for shift/load signals with a specific number (in this case
47
) of load pulses and, at its clock-pulse input (
15
), with clock pulses (in this case with eight times 47), which were produced as second and third pulse bursts. At a serial output (
17
), the parallel-to-serial converter relays, in the rhythm of the clock pulses, the payload data of an ATM cell to burst-data output (
5
), at a serial output (
18
) of the output (
5
).
Additional features or refinements of the present method include that the write signal (
11
) is clocked using the block clock pulse (
7
). The clocked burst data generator (
6
) is either likewise supplied with the block clock pulse (
12
) or, in an alternative embodiment, with a higher-rate block clock pulse via the clock-pulse supplier (
20
), with whose aid the clocked burst data generator (
6
) produces a specific number, in particular three different pulse bursts of the same duration. With the aid of the first clock-p
Abelson Ronald
Deutsche Telekom AG
Nguyen Chau
LandOfFree
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