Note: Descriptions are shown in the official language in which they were submitted.
CA 02302708 2000-03-07
GR 97 P 2191 - 1 -
Description
Method for transmitting user data which can be
allocated to different applications
The invention relates to a method for
transmitting user data which can be allocated to
different applications, between an A end and a B end of
a transmission path, an ATM connection being
established by signaling in a signaling phase before
the transmission of data between the A and the B end,
data which are allocated to the individual applications
"' being transmitted in each case within an ATM adaptation
layer frame containing a plurality of ATM cells, in the
substructures containing ATM cells, and it being
possible to pass on the user data of the substructures
in different ways depending in each case on receiver-
end application allocations.
In this context, the interface which permits
access to the ATM network, the ATM adaptation layer,
which is also referred to as ATM adaptation layer or
AAL, is of prime importance.
The AAL is the interface between ATM and the
higher protocol layers. It conceals the ATM-specific
properties of the transmission from the higher layers
and adapts the ATM layer (bidirectionally) to them. For
this purpose, the data of the higher layers are packed,
together with the protocol information of the AAL
layer, into the information fields of the ATM cells and
are transmitted as user information, also referred to
as payload. Because the AAL is responsible for the
adaptation of the services of higher
CA 02302708 2000-03-07
GR 97 P 2191 - 2
layers to ATM, it does not itself play any role in the
network during transmission. The AAL is for the user ~ s
benefit. It established the connection between the
subscriber and network.
In increasing numbers of applications, driver
programs are being used at the subscriber end instead
of specific processors, said drive programs using
powerful processors, which are present in any case, in
computers. The intention of this was also to implement
the adaptation of the data to be transmitted to the
conditions of the network. This has given rise,
inter alia, to the fact that the AAL-5 which was
p~' specified according to the international standard
ITU-T I.362 and was provided initially only for data
transmission and for the transmission of signaling data
is also used in the voice domain.
In order to be able to fulfill the requirements
which different services make of the AAL layer, said
layer is divided into sublayers, so-called AAL
sublayers, with respectively different functions.
According to the international standard
ITU-T I.362, a further subdivision is expressly
possible. Currently, the functions are defined as
follows:
Segmentation and reassembly (SAR)
- The data which are to be transmitted are adapted to
the ATM structure by appropriate segmentation into a
variable which is matched to the available
information field of the ATM cell
- 30 - The information content of the information fi
ld
i
e
s
s
recovered from ATM cells for the higher layers, the
Convergence Sublayers (CS)
CA 02302708 2000-03-07
GR 97 P 2191 - 3 -
- The data are adapted to the requirements of the
respective services by making available the service-
specific properties of the AAL
Because AAL-specific protocol elements have to
be inserted into the datastream by the higher layers in
order to implement the properties required by the
services, the function CS influences the method of
operation of the SAR.
The requirements which individual services make
of the transmission can be grouped together in classes,
so-called Classes of Services. The requirements are
given in ITU-T I.362.
For the categorization into services classes,
the following criteria are to be taken into account in
particular:
Is a "timing relation" necessary or not between the
source and destination;
Is the bit rate constant or variable; and
Is a connection-oriented service or a connectionless
service provided.
AAL-5 has, in comparison with other AALs, the
advantage of a lower protocol overhead. In addition,
AAL-5 provides better possibilities for detecting cell
losses over the entire information content when using
CRC mechanisms. In the case of voice transmission, AAL-
5 fulfills most service requirements, namely
synchronization of source and destination (timing
relation between source and destination), constant bit
rate CBR, variable bit rate VBR, connection-oriented
CA 02302708 2000-03-07
GR 97 P 2191 - 4 _
service, connection pipes service, low proportion of
protocol in comparison with the user data (payload) of
the ATM cells (protocol overhead), favorable behavior
in terms of "binary alignment". Only the delay which is
caused by the cell filling time when compressed voice
data are transmitted is unfavorable. As long as
uncompressed ISDN data are being transmitted, the cell
filling times do not play any particular role. However,
in the field of mobile radio, data compression is
necessary in order to be able to use the restricted
radio frequency bandwidth in an optimum way.
Compression factors of 10 lead to a situation in which
the filling times for an ATM cell rise to up to
60 msec . For this reason it is appropriate in the case
of mobile radio and, if appropriate, also for other
low-bit-rate applications such as ATM, to transmit more
than one channel multiplexed over one VCI. For this
purpose, a suitable substructure must be defined which
can be embedded in ATM adaptation layers, such as AAL
5.
For this purpose, at the AAL level, an integral
multiple of ATM cells should be selected as the frame,
for example on the basis of AAL-5. In this context,
AAL-5 would permit the protection systems contained in
it to be used.
From the definition of AAL-5 it is possible to
infer that a substructure can easily also be selected
to be larger than the actual cell format and that it
must be able to overlap the boundaries between two (or
more) cells.
The fact that data are compressed in the case
of mobile radio, and there is therefore no longer a
constant datastream, gives rises to the need for the
possibility
CA 02302708 2000-03-07
GR 97 P 2191 _ 5 _
to be able to define in the substructure both structure
elements of different magnitudes and structure elements
whose magnitude changes over time.
In addition, it should be possible to be able
to select the structure elements so that they all have
the same magnitude in order to be able to support CBR.
It should also be possible to transmit elements to
support the synchronization at the same time.
It would also be desirable to be able to
transmit datastreams with different addressees w;t-h;r
one AAL frame.
For this reason, additional properties of the
structure elements would have to be known at the
transmitter end and receiver end, said properties being
namely the magnitude of the AAL frame, the length of a
structure element, the number of the structure elements
in a virtual channel, the allocation of individual
structure elements to addresses, an information item
relating to synchronicity or asynchronicity and an
information item relating to the constancy of the bit
rate, that is to say CBR/VBR. If this information is
transmitted in a known way within AAL frames, the
protocol overhead is greatly increased.
The object of the present invention is to
specify a method for transmitting user data which can
,~~.
be allocated to different applications, between an A
end and a B end of a transmission path, an ATM
connection being established by signaling in a
signaling phase before the transmission of data between
the A and the B end, data which are allocated to the
individual applications being transmitted in each case
within an ATM adaptation layer frame containing a
plurality of ATM cells, in the substructures
CA 02302708 2000-03-07
GR 97 P 2191 - 6 -
containing ATM cells, without the protocol overhead
being greatly increased.
This is achieved according to the invention in
that the A-end and the B-end allocations of the
substructures of an ATM adaptation layer frame are
defined by signaling in the signaling phase.
This means that the address information of the
individual substructures is exchanged within the scope
of the signaling operations. The protocol overhead is
on average increased only very slightly as a
consequence of the fact that the address information
only needs to be transmitted within the scope of the
r"""' connection setup, and further information only has to
be transmitted to a limited degree.
In addition, additional information relating to
the format of the substructures can be defined by
signaling in the signaling phase.
If, for example in one development of the
invention, the number of ATM cells which an ATM
adaptation layer frame contains is defined by signaling
in the signaling phase, this information does not need
to be transmitted either when there is an existing
connection.
For example, in one refinement of a method
according to the invention it is possible to define, by
'~ si nalin in the si nalin
g g g g phase, whether or not the
individual substructures are of equal magnitude.
CA 02302708 2000-03-07
GR 97 P 2191 - 7
In the method according to the invention, the
start of the first substructure is preferably defined
within a frame by the start of the frame.
When there are substructure elements of
different magnitudes, in particularly favorable
refinements of a method according to the invention the
first element of each substructure element indicates
the length of the substructure element to which it
belongs, and thus indicates when the next substructure
begins.
When there are substructures of different
magnitudes, the length of one substructure element is
""' preferably defined by the value range 1 of a length
indicator field.
When 1 - 0 there is no longer a substructure
element present. The maximum length which a
substructure element can assume is thus defined by
lmax - 1. If the length indicator field is eight bits,
that' is to say one octet, long, up to 256 octets can be
numbered sequentially. In this way, an AAL-5 frame can
consist of up to a maximum of 5 ATM cells when there
are flexible structure elements.
The ATM adaptation layer frame can preferably
correspond to the AAL-5 frame according to the ATM
form.
A connection between the A end and the B end
can be bidirectional in terms of the substructures of
an ATM adaptation layer frame.
CA 02302708 2000-03-07
GR 97 P 2191 - g _
One particularly favorable refinement of a
method according to the invention provides that the
length of the substructure is shortened by the part
which is provided for user data if no network data are
to be transmitted when substructures of different
magnitudes are used within one substructure.
A substructure can, if necessary, also extend
over the user data area of two adj acent ATM cells , and
thus include the header information area of an ATM
cell.
The invention is summarized below with
y>.
reference to an example.
'" An integral number of ATM cells is used as the
structure of an AAL-5 frame. The number of cells per
structure is to be negotiated during the signaling
within the scope of the connection setup. When the
structure elements are of flexible length, the length
of one frame should not exceed 5, as is explained
further below.
Whether a fixed or flexible format is to be
selected is agreed through signaling. The same applies
to the number of structure elements, i.e.
substructures, which are to be transmitted in one
frame.
Substructures can overlap from one ATM cell
into the next ATM cell within one AAL-5 frame.
The allocation of individual substructures to
addresses is agreed by signaling. An additional loading
on the
CA 02302708 2000-03-07
GR 97 P 2191 - g -
substructures by the transportation of addressing data
is thus avoided.
So that structure elements of variable length
can be linked to one another, each element contains as
a first octet a pointer which points to the start of
the following element. This pointer has the length of
one octet, with the result that up to 256 octets can be
numbered sequentially. In this way, one AAL-5 frame can
comprise up to a maximum of 5 cells when the structure
elements are flexible. This restriction is also useful
because pointing which has been possibly lost can be
recovered immediately at the start of the next AAL-5
~""'~ frame .
The pointing can be dispensed with in the case
of structure elements of fixed length. The selected
length must be negotiated here either within the scope
of the signaling or be defined by administration. The
length of the structure elements must always be defined
as integral multiples of octets. Specific processor or
bus properties can be taken into account here.
The method described is characterized by the
fact that the individual structure elements do not need
to be accompanied by any address information.
Synchronization measures which are necessary due to
frame loss can be supported on AAL-5 mechanisms. This
does not require any additional agreements relating to
the formats used. The identification of the last cell
of a frame is sufficient with AAL-5.
CA 02302708 2000-03-07
GR 97 P 2191 - 10 -
Both substructures of flexible length and
permanently set substructures can be used.
Within one frame, information which is
chronologically correlated along one path, such as
coherent audio and video signals which are transmitted
in different channels, can always be sure to be
transmitted in synchronism.
New formats can be defined and can then be
negotiated by signaling, without having to intervene in
existing processes.
In order to clarify the conversion of
information from a higher layer into substructures of
'"'" an AAL-5 frame, the figure shows an AAL-5 frame of the
second layer L2:AAL-5, composed of four ATM cells, each
with a header H and a payload element which is not
identified individually. Data of a higher layer
L3:PDU1, L3:PDU2 and L3:PDU3 are converted into this
AAL-5 frame.
The conversion of the information L3:PDU1 gives
rise here to an equidistant substructure, the
conversion of the information L3:PDU2 gives rise to a
variable substructuring, the individual substructures
being greater than cell formats and the conversion of
the information L3:PDU3 gives rise to a variable
substructuring, the individual substructures being
"~ smaller than cell formats.
