PROCEDE DE SURVEILLANCE DE LA QUALITE DE TRANSMISSION DE BITS LORS DE LA TRANSMISSION ORIENTEE PAQUETS

METHOD FOR MONITORING BIT TRANSMISSION QUALITY IN PACKET-ORIENTED TRANSMISSION

Abrégé français

Pour surveiller la qualité de transmission de bits lors d'une commutation radio, telle qu'une commutation de paquets, chaque paquet est doté d'une somme de vérification. Cette somme de vérification permet de détecter côté réception si une erreur s'est produite pendant l'opération de transmission. En revanche, elle ne permet pas de déterminer le nombre d'erreurs survenu sur les bits. Selon l'invention on y remédie en transférant à une commutation radio des méthodes de calcul parité par entrelacement de bits connus dans la commutation orientée paquets.

Abrégé anglais

The bit transmission quality of a connectionless switching operation such as
an IP-packet switching operation is monitored by providing each packet with a
checksum. This checksum only helps to determine, at the receiving end, whether
or not an error has occurred during the transmission process. It is not
possible to determine the number of bit errors that have occurred using this
procedure. According to the invention, these problems are solved by applying
the BIP calculation method used in connection-oriented switching to
connectionless switching.

Revendications

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claims
1. A method for monitoring the bit transmission
quality in packet-oriented transmission,
comprising
a transmitting facility (R A) which supplies
information in packets with variable length to a
receiving facility (R B) via a transmission section
(UA) as determined by connectionless switching,
characterized
in that, in the transmitting facility (R A), a
check information item, which is stored in a
control packet (PM), is formed bit by bit over the
transmitted packets within a predeterminable time
interval in accordance with an algorithm,
in that the control packets (PM) are quasi-
periodically inserted into the stream of packets
output by the transmitting facility (R A),
in that, when a control packet (PM) arrives in the
receiving facility (R B), a check information item
is also formed bit by bit over the packets
received in the time interval in accordance with
the same algorithm,
in that an estimate is made with respect to the
bit errors which have occurred during the
transmission process in accordance with a logic
operation between this check information item and
the check information item transmitted in the
control packet (PM).
2. The method as claimed in claim 1, characterized in
that the estimation is made by dividing the number
of bit errors which have occurred at a minimum
(n B) by the number (n R) of the transmitted bits.
3. The method as claimed in claim 1, 2, characterized
in that the number of transmitted bits (n R) is
measured at the receiving end.

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4. The method as claimed in claim 1, 2, characterized
in that the number of transmitted bits (n R) is
estimated at the receiving end by assuming a
periodic insertion of the control packets (PM).
5. The method as claimed in claims 1 to 4,
characterized in that the time interval is defined
by the sending out of two successive control
packets (PM).
6. The method as claimed in claims 1 to 5,
characterized in that the validity of the control
packet (PM) is decided at the receiving end by
analysis of the sequence number.
7. The method as claimed in one of the preceding
claims, characterized in that the packets transmit
information in accordance with an Internet
protocol (IP packets).
8. The method as claimed in one of the preceding
claims, characterized in that the logic operation
is a differentiation.
9. The method as claimed in one of the preceding
claims, characterized in that the check
information is provided in accordance with a BIP
(bit interleaved parity) calculation.
10. The method as claimed in one of the preceding
claims, characterized in that transmitting (R A)
and receiving facility (R B) are constructed as
switching nodes.

-9-
11. The method as claimed in one of claims 1 to 9,
characterized in that transmitting (R A) and
receiving facility (R B) are constructed as
regenerative repeaters.

Description

GR 99 P 1980
Description
CA 02375819 2001-11-30
Method for monitoring the bit transmission quality in
packet-oriented transmission
The invention relates to a method according to the
preamble of claim 1.
In contemporary information processing systems, the
information is transmitted in SDH (synchronous digital
hierarchy) frames. The SDH transmission is a
connection-oriented switching system. ATM cells or IP
(Internet protocol) cells can be inserted, for example,
into the SDH frames. These must be inserted into the
SDH frames at the transmitting end and removed from
them again at the receiving end. In the prior art, the
IP cells are incorporated in ATM cells and these are
transmitted in the SDH frames. In this arrangement, the
packet assembly and disassembly processes must be
performed twice in each case.
To monitor the bit transmission quality, checksum
calculations are periodically performed. over all data
transmitted between two reference points in the case of
SDH frames. For example, two switching nodes can act as
reference points between which a transmission section
is arranged. During this process, the bi.t error rate on
the transmission section is quantified. The checksum
calculations are performed independently of one another
both in the transmitting and in the receiving switching
node. During this process, a parity check is made via,
e.g. every eighth bit in a cyclic timing pattern. At
the transmitting end, the result found is entered in a
bit interleaved parity (BIP) check field and supplied
to the receiving switching node in the SDH frame. In
the SDH frame, there are three different BIP fields
which are in each case used in the path overhead,
multiplex section overhead and regenerator section
overhead and have different sizes. The sizes

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of the path and multiplex section BIPs are dependent on
the size of the SDH frame and extend from 2 to n*24
bits. The regenerator section BIP always has 8 bits. At
the receiving end, the transmitted check field BIP is
analyzed and the bit error rate on the transmission
section is quantified as determined by a comparison of
this analysis with a separate checksum calculation.
The advantage of this method can be seen in that the
number of bit errors which have occurred is determined.
Thus, for example, bit errors which are close together
(e. g. bits 5, 7) can also be determined (burst errors).
However, the circumstance that the equipment needed for
SDH frames is very expensive and complex presents
problems.
In contrast to connection-oriented switching, no
permanent connection is established between the
transmitting facility and the receiving facility in
connectionless switching. IP packet switching can be
used as an example of connectionless switching. To
monitor the bit transmission quality, each packet is
provided with a checksum. Using this checksum, bit
errors can also be determined at the receiving end.
However, the checksum only allows the detection of
whether a fault has occurred during the transmission
process, or not. As a consequence, the relevant IP
packet is discarded at the receiving end. This
procedure does not allow the number of errors which
have occurred to be determined as is the case, for
example, with the BIP calculation method.
The invention is based on the object of demonstrating
an approach of how the bit transmission quality can be
monitored more efficiently, even in the case of
connectionless switching.

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On the basis of the preamble of claim 1, the invention
is , achieved by the features specified in the
characterizing clause.
The advantageous factor of the invention is, in
particular, that the BIP calculation method known with
connection-oriented switching is transferred to
connectionless packet switching. This is associated
with the advantage that a more detailed error pattern
is presented by determining the number of bit errors
which have occurred in the system. Furthermore, it is
possible to detect these bit errors section by section
whereas the checksum method used in the prior art only
allows an end-to-end statement.
Advantageous further developments of the invention are
provided in the subclaims,
In the text which follows, the invention is explained
in greater detail with reference to an exemplary
embodiment, in which:
Figure 1 shows 2 switching nodes which terminate a
transmission section,
Figure 2 shows a flowchart of the method according to
the invention.
Figure 1 shows a configuration in which the method
according to the invention is run. Accordingly, two
switching nodes RA, R$ are shown which terminate a
transmission section UA. The switching nodes RA, RB are
to be constructed, for example, as IP routers.
Furthermore, IP packets are to be conducted by WDM
(wave division multiplexing) via the transmission
section UA. According to the present exemplary
embodiment, it is assumed that

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the transmitting switching node is the IP router RA and
the, receiving switching node is the IP router RB. As
can be seen in figure 1, the transmitting IP router RA
transmits IP packets of different lengths via the
transmission section UA to the receiving IP router RB.
The transmission section UA is constructed to be
bidirectional but only the transmission from the
transmitting IP router RA to the receiving IP router RB
is shown for clarity.
In the text which follows, the invention is explained
in greater detail by means of the flowchart shown in
figure 2.
In the transmitting IP router RA, the BIP calculations
are performed over a number of transmitted bits within
a time interval. The higher-level packet. structure does
not play a role in this case, i.e. the checksum
calculation is performed over the bits of a number of
packets. The time interval is predetermined by two
successive control packets PM. The result determined is
then stored in a check field BIP together with a
sequence number in a control packet PM which is quasi-
periodically inserted in the stream of outgoing IP
packets. The quasi-periodic insertion is necessary
since the IP packets have a variable length and their
transmission cannot be interrupted. To this extent, it
is not possible to insert the control packets PM
periodically. According to the invention, it is then
provided to reset a counting device on insertion of a
control packet PM by the IP router RA. If this counting
device has reached a predeterminable threshold value,
the control packet PM is inserted into the IP packet
stream as next packet immediately after the current IP
packet was sent out. In the interval needed by the
counting device for reaching the threshold value, the
bits are counted and the BIP checksum calculation is
performed (time interval).

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If the receiving IP router RB determines that a control
packet PM has arrived, its content is compared with its
own

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BIP calculation performed since the last time a control
packet was received. Furthermore, the sequence number
which is also transmitted is verified, i.e. it must be
higher by 1 than the one transmitted in the last
control packet PM. If this is so, the difference
between the result of the locally determined BIP
calculation and the BIP check field is formed. Then the
number nH of bit errors which have occurred is
determined in accordance with the result. Since bit
errors can average out during the checksum calculation,
the value nH is a lower limit to the number of errors
which have occurred.
Using the value nH determined in this manner, an
estimation pE of the instantaneous bit error rate is
performed. This is:
pE = nB/nR where nR = number of received bits
In this process, the quantity nR can be counted
precisely at the receiving end, e.g. by means of ASIC
chips. As an alternative, however, an estimate is
completely adequate. Accordingly, it is then assumed
that constant times would elapse between 2 control
packets PM. This is equivalent to saying that the
control packets PM are inserted periodically into the
outgoing stream of IP packets. Thus, the same number of
bits is transmitted with the same transmission rate.
According to the present exemplary embodiment, it had
been assumed that the facilities RA, R$ are constructed
as switching nodes between which the method according
to the invention is used. However, the invention is not
restricted to such a configuration. Thus, it can also
be used on individual transmission sections, bounded by
regenerative repeaters, of a point-to-point connection
between two switching nodes. In this arrangement, the
transmission sections formed by regenerative repeaters

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form the entire link section between the transmitting
and receiving

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facility. In this case, each regenerative repeater can
carry out the BIP calculation or check and can also
write the result of its own calculation back into the
forwarded control packet PM. This makes it possible to
estimate the bit error rate section by section. The
section-by-section bit error rate estimates stored in
the individual regenerative repeaters can then be read
out, for example, via X.25 protocols.

Dessin représentatif