Note: Descriptions are shown in the official language in which they were submitted.
CA 02236801 1998-06-02
A TIME. SLOT MANAGEMENT METHOD AND A MAIN STATION AND
SUBSTATION REALIZING SUCH A METHOD
The present invention relates to a time slot management method for
use in a dime division multiple access network as described in the preamble of
claim 1 and a main station and a substation realizing such a method as
described in the preamble of claim 8 and claim 9, respectively .
Such a time slot management method, main station and substation
are already known in the art, e.g. from the published European patent
application with publication number 0 544 975 A1. Therein, a time slot
management system is described which includes a time division multiple access
system with a main station connected to a plurality of substations by means of
a
tree like network. Substation identities are broadcasted in downstream
information packets from the main station to the substations in order to
thereby
allow each one of the substations upon detection of its own substation
identity to
transfer upstream information in upstream information packets in predefined
upstream timeslots. This time slot management system allocates the time slots
in
a flexible and dynamically way. The needed bandwidth to transmit upstream
information is requested by the substations to the main station and the
substations are informed by the main station of the allocated timeslots.
A feature of the described time slot management system is that the
downstream information packets are including a predetermined number of
blocks each of which include a header part and an information part. In order
to
reduce the overhead of the downstream information packets a first step is
realized to distribute the stream of substation identities over the header
parts of
this predetermined number of blocks and a second step is realized to performe
only one error check for this predetermined number of blocks and to distribute
the error check information bits over the header parts of the predetermined
number of blocks. In this way, the overhead of one information packet is a
reduced number of bits.
CA 02236801 1998-06-02
_2_
It has to be remarked that in order to keep a maximum transfer
capacity between the main station and the substations it is desirable to keep
the
number of bits of the overhead of the information packets as low as possible.
An object of the present invention is to provide a time slot
management method of the above known type but wherein the overhead of one
information packet is further reduced.
According to the invention, this object is achieved due to the fad that
the time slot management method of the invention includes embedding at least
part of the substation identities in a physical layer operation and
maintenance
part. Such a physical layer operation and maintenance part is a predefined
part
of one of tlhe downstream information packets which is anyway broadcasted from
the main station to the plurality of substations in order to transport
operation and
maintenance information packets related to operation and maintenance
functions.
Indeed, by embedding substation identities in unused fields of physical
layer operation and maintenance parts, which are a predefined part of one of
the downstream information packets and which are broadcasted anyway on a
predefined regular base the number of bits of the overhead of one information
packet is a reduced number of bits. This method is described in claim 1 and is
realized by the main station and the substation described in claim in 8 and
daim
9, respectively.
It has to be remarked that operation and maintenance functions for
e.g. an Asynchronous Transfer Mode layered model are described in e.g. the
book Asynchronous Transfer Mode : Solution for Broadbond ISDN written by
Martin de Prycker, and more particular in Chapter 3 : Description of ATM
according to CCITT from page 97 to page 124, published in 1991 by Ellis
Horwood Limited, ISBN 0-13-053513-3.
~4lso the scope of the Recommandation !TU-T 1.610, 03/93, previously
CCITT Recommendation described in paragraph 1.2, page 1 thereof, is to
identify
the minimum set of functions required to operate and maintain the Physical
Layer
and Asyncronous Transfer Mode ATM Layer aspects of the Broadband Integrated
CA 02236801 1998-06-02
-3-
Services Diigital Network B-ISDN user network interface. Five phases are
considered in specifying the operation and maintenance OAM functions of the
Broadband - Integrated Services Digital Network B-ISDN which are described in
more detail on page 1 and 2 of this ITU Recommendation and which are listed
hereafter
- Performance monitoring;
- Defed and failure detection;
- System protection;
- Failure or performance information;
- Fault localization.
Common operation and maintenance OAM cell fields for point-to-
point connc:etions are described in paragraph 7, ATM Layer OAM Cell Format,
pages 17-18 of this ITU-T 1.610 Recommendation and the specific fields for
each
type of OANI cells are described in paragraph 7.2 Specific Fields for Fault
Management Cell, pages 18 to 21 of this recomma»dation. As it can be seen
from the description of these fields the OAM cell indeed includes unused
Operation and Maintenance cell information fields. By inserting , following
the
invention, in a format similar to such above format but for point-to-
multipoint
connections., at least part of the substation identities in unused fields of
the
operation and maintenance cells less substation identities i.e. less bits are
inserted in irhe overhead of the infarmation packets.
An important advantage of the time slot management method
following the invention is that in the event when all substation identities
are
inserted in physical layer and operation and maintenance parts the time slot
management method is more flexible towards an increase of the number of
substations. Indeed, with an increase of the number of substations more
substation iidentities are required whereby the number of bits of a substation
identity also increases. In the event when e.g. substation identities are
distributed
over the overhead of downstream information packets, an increase of the word
length of the substation identities has an impact on the overhead of the
information packets and eventually on the structure of the downstream frame
CA 02236801 1998-06-02
-4-
format. However, in the event when all substation identities are inserted in
physical la;~er and operation and maintenance parts, an increase of the word
length of the substation identities has only an impact on the organization of
the
fields of the physical layer and operation and maintenance parts.
It has to be remarked that the downstream information packets and
the upstream information packets may be carried as e.g. a continuous stream of
cells in a cell based format with a cell based physical layer. Physical layer
operation and maintenance cells are used for the conveyance of the physical
layer operaition and maintenance information and are inserted in the
continuous
stream of information packets with a predefined insertion rote. Embedding at
least part of the substation identities in such physical layer operation and
maintenance cells, shortly PLOAM cells, is a possible implementation of the
method of the invention.
It has to be remarked that a substation identity is not necessarily a
manufacturing number assigned during manufacturing following a predefined
unique and programmed manufacturing serial number. It can also be a ranging
grant which is used during a ranging process whereby this ranging grant is
generated by a main station in order to initiate the ranging process. Two
conditions for a substation to read on such a ranging grant is that the
substation
has not yet received an identification number from the main station and that
its
manufacturiing serial number matches a mask given by the central station. If
both conditions are valid the substation is allowed to react on such a ranging
grant. A feature of the method according to the present invention is that a
substation identity embedded in a physical layer operation and maintenance
cell
is a ranging grant. This is described in claim 2.
Once a substation has received an identification number from the
main station and the ranging process is completed, the substation is allowed
to
react on a data grant from the main station which includes his identification
number and is allowed to send data to the main station. A time slot
management method according to the invention wherein the substation identity
is
a data grant is described in claim 3.
CA 02236801 1998-06-02
- 5 -
A time slot management method following the invention used in an
optical communication network is described in claim 4.
Yet an important advantage of the management method following the
invention becomes clear in the event when a downstream information packet is
an asynchronous transfer mode cell i.e. an ATM cell and in the event when all
substation identities are embedded in physical layer operation and maintenance
cells and no total overhead is required. Indeed, for such a format the
downstream frame format of the downstream information packets lines up with
the requirements of the International Telecommunication Union ITU-T
Recommendation 1.432, 03/93, Integrated Services Digital Network ISDN User
Network Interfaces / 8roadband Integrated services Digital Network B-ISDN User
Network Interface - Physical Layer Specification. This standard describes more
in
detail on page 8 the interface structure of the physical layer for a cell
based
interface consisting of a continuous stream of ATM cells, each containing 53
octets and with a maximum spacing between successive physical layer cells of
26
ATM layer cells. Such a physical layer cell can either be an idle cell or a
physical
layer operation and maintenance cell depending on the operation and
maintenance requirements. A time slot management method following the
invention whereof the downstream information packets are organized following
an asynchronous transfer mode cell ATM is described in claim 5.
It should further be noticed that the term "including", used in the
claims, should not be interpreted as being limitative to the means listed
thereafter. Thus, the scope of the expression "a device including means A and
B" should not be limited to devices consisting only of components A and B. It
means that with respect to the present invention, the only relevant components
of
the device are A and B.
Similarly, it is to be noted that the term "coupled", also used in the
claims, should not be interpreted as being limitative to direct connections
only.
Thus, the scope of the expression "a device A coupled to a device B" should
not
be limited to devices or systems wherein an output of device A is directly
connected to an input of device B. It means that there exists a path between
an
CA 02236801 1998-06-02
-6-
output of A and an input of B which may be a path including other devices or
means.
'The above and other objects and features of the invention will become
more apparent and the invention itself will be best understood by referring to
the
following description of an embodiment taken in conjunction with the
accompanying drawings wherein
IFig. 1 shows a block scheme of an embodiment of a time division
multiple access network wherein the method of the invention is used;
IFig 2 shows a downstream frame format and an upstream frame
format used by the time division multiple access network of Fig. 1.
(Referring to Fig. 1 a time slot management method used in a time
division multiple access network will be described. First, the working of the
time
division multiple access network will be explained by means of a functional
description of the blocks shown in Fig. 1. Based on this description,
implementation of the functional blocks in Figure 1 will be obvious to a
person
skilled in the art and will therefor not be described in detail. In addition,
the
principle working of the time slot management method following the invention
will be described in further detail.
'the time division multiple access network includes a main station MS
and a plurality of substations S1, S2, S3, ..., S15, Sl 6. The main station MS
is
coupled to each substation S1, S2, S3, ..., S15, S16 via the cascade
connection
of a common transmission link Lc and an individual user link L1, L2, L3, ...,
L15,
L16.
'The time division multiple access network is an optical network
transporting asynchronous transfer mode ATM cells over optical fibers from the
main station MS to the substations S1, S2, S3, ..., S15, S16. The goal is to
have
a network which is transparent for ATM cells and which has an optimized
throughput with minimal constraints on the optical components. Optical passive
splitters which are not shown in the figure in order not to overload the
figure are
used in order to reach the substations S1, S2, S3, ..., S15, S16.
CA 02236801 1998-06-02
_ 7
The time division multiple access network broadcasts substation
identities TEA1, TEA12, TEA16, TEA3, TEA7, ... in downstream information
packets form the mains station MS to the plurality of substations S1, S2, S3,
...,
S15, S16. Upon detection of its own identity a substation is allowed to
transfer a
predetermined amount of upstream information packets in predetermined
upstream timeslots to the main station MS. For example : upon detection of
substation S3 of its own identity TEA3, substation S3 is allowed to send to
the
main station upstream information packets in predetermined timeslots.
The main station MS includes a packet formatting module PFM,
inserting ~reans INS and queuing means Q. The queuing means Q is coupled
to the inserting means INS which is included following this embodiment in the
packet formatting module PFM.
Each substation, whereof only substation S3 is shown in detail in order
not to overload the figure, includes detecting means DET.
The functions of each functional block of above will be described in
the following paragraphs.
The time slot management network allocates the time slots in a flexible
and dynamically way. Indeed the upstream transfer capacity of the time
division
multiple access network is shared amongst the substations S1, S2, S3, ...,
S15,
S16 based on their needed and requested upstream bandwidth to transmit
upstream information. This needed upstream bandwidth is requested by the
substations S1, S2, S3, ..., S15, S16 to the main station MS. The requested
bandwidth is translated by the main station MS in a predetermined number of
allocated timeslots. This is realized by creating according the requested
bandwidth of the substations S1, S2, S3, ..., S15, S16 a stream of transmit
enable addresses which are called in this application substation identities
TEA1,
TEA12, TE:A16, TEA3, TEA7, ... . It has to be remarked that the detailed
working
of this allocation is described in the cited patent application but goes
beyond the
scope of this invention . The aim is the use of the stream of substation
identities
TEA1, TEA12, TEAI 6, TEA3, TEA7, ... to inform the substations S1, S2, S3,
...,
S15, S16 of the allocated timeslots. Following this embodiment the stream of
CA 02236801 1998-06-02
_ $ _
substation identities TEA1, TEA12, TEA16, TEA3, TEA7, ... is provided to the
inserting means INS by the queuing means Q.
Physical layer operation and maintenance cells, shortly PLOAM cells,
are also provided to the inserting means INS. Besides another important
function which is described in a following paragraph, the inserting means INS
inserts the PLOAM cells in the downstream information packets. This means that
such a PLOAM cell is in fact a predefined part included in one of the
downstream
information packets.
The content and the functions of such a PLOAM cell is described in the
introductory part of this application. In order to support the downstream
capacity
provided at the main station MS and transmitted from the main station MS to
the
substations S1, S2, S3, ..., S15, S16 it is desirable to insert a minimum
number
of PLOAM cells. Indeed, in this way a minimum overhead is added. However,
in order to make the interface of the network of this embodiment compliant to
the ITU-T Recommendation 1.432 the maximum spacing between successive
physical layer cells is 26 ATM layer cells i.e. after 26 continuous ATM layer
cells
have been transmitted from the main station MS to the substations S1, S2, S3,
...,
S15, S16 a physical layer cell is inserted by the packet formatting module PFM
in
the downstream information packets in order to adapt the transfer capability
to
the interface rate.
Referring to figure 2 the downstream frame format and the upstream
frame format used by the time division multiple access network of Figure 1 is
shown. As it can be seen on figure 2, after 26 ATM cells a PLOAM cell is
inserted.
The time slot management method of the invention is introduced
thereafter i.e. inserting at least part of the substation identities TEA1,
TEA12,
TEA16, TEA3, TEA7, ... in the PLOAM cells. This function is realized by the
inserting means INS. In the described preferred embodiment all substation
identities TEA1, TEA12, TEA16, TEA3, TEA7, ... are inserted in the PLOAM
cells.
In this way the requirements of the ITU-T Recommendation (.432 are still
CA 02236801 1998-06-02
-9-
supported and the overhead of a downstream information packet becomes
superfluous.
This preferred embodiment also ensures that the boundaries of the
downstream frame format are in accordance with the boundaries of the
upstream format. In this way it is easier to calculate the delay ranging i.e.
the
time needed for an information packet to travel from the main station MS to a
particular substation e.g. substation S3 and back to the main station MS. By
this
way a symmetrical interface is realized and no stuffing bytes have to be
inserted
in order to align the downstream bitrate with the upstream bitrate. The
accordance of the boundaries can be seen on Figure 2.
It has to be remarked that by choosing the number of upstream cells
in an upstream frame format as a multiple of the number of bytes of a
downstream cell which is 53 bytes for one ATM cell the boundaries of the
frames
are aligned and the lookout of the upstream and downstream frame format can
easily be determined. A direct relation between the number of downstream cells
in a downstream frame format and the number of overhead bytes of one
upstream cell is established. This means that for any length of an overhead of
an upstream cell the number of downstream cells is a whole number. The above
assumptions and relations are given by the following formulae's whereby
d : number of downstream cells in a downstream frame format;
Cd : number of bytes of a downstream cell;
Hd : number of bytes of the overhead of a downstream cell;
a : number of upstream cells in an upstream frame format;
C" : number of bytes of an upstream cell;
H" : number of bytes of the overhead of an upstream cell;
m : multiple of number of upstream cells in an upstream frame format
to the number of bytes of a downstream cell which is 53 bytes;
Assumptions
CA 02236801 1998-06-02
- 10-
1. Downstream cell format and upstream cell format are ATM cells
whereby =~ length ATM cell = 53 bytes = Cd = C"
2.Hd=0
3. a = m * number of bytes of downstream cell = m * 53
Conclusions
In the event when the number of downstream bytes of the
downstream frame equals the number of upstream bytes of an upstream frame
(Hd+Cd)*d=(H"+C")*u
(0+53)*d=(H" +53)*m*53
d=(H"+53)*m
Since it is more simple to integrate the involved functionality's in the
design of ~an application specific integrated circuits when the frame formats
are
shorter it is preferred in this embodiment to equalize the number of upstream
cells to the number of bytes of a downstream cell
a = m * number of bytes of downstream cell = m * 53 with
m= 1
u=53
d=H"+53
When an upstream overhead of 1 byte is needed the number of cells
of the downstream frame format equals 54 whereby all parameters of the
downstream and upstream frame format are defined. In this particular
embodiment the number of bytes of the overhead of an upstream cell is indeed
chosen to be three.
Since every 26 downstream ATM cells a physical layer cell has to be
inserted it can be calculated that 2 cells of the 54 downstream cells have to
be a
PLOAM cell. Figure 2 shows 54 ATM structured cells whereof 2 PLOAM cells and
52 ATM information cells.
CA 02236801 1998-06-02
Since an upstream frame formot includes 53 cells and since for each
upstream cell a substation identity e.g. TEA3 is required in order to transfer
an
upstream information packet in an upstream timeslot i.e. such an upstream cell
from a substation e.g: S3 to the main station MS it can also be calculated
that in
these two PLOAM cells 53 substation identities TEA1, TEA12, TEA16, TEA3, TEA7,
... have to be inserted. Following this embodiment 27 substation identities
ore
inserted in the first PLOAM cell of a downstream frame format and 27
substation
identities whereof one idle substation identity are inserted in the second
PLOAM
cell of a downstream frame format. This is realized by the inserting means INS
of the main station MS.
An example of the allocation of operation and maintenance functions
is given on page 10 of the above mentioned Recommendation 1.432. Since this is
a recommendation for point to point connections and a recommendation for
point to multipoint connections is not yet available but is expected to be
processed in the future it is also expected to have some reserved fields
available
to insert substation identities.
Like already mentioned above a substation e.g. S3 has to detect its
own identity in a received PLOAM cell in order to be allowed to transfer an
upstream information packet. This is realized by the detecting means DET. In
order not to overload figure 1 only for substation S3 the detecting means are
shown DET(TEA3).
Although the principle working of the invention has become clear by
the above description of the functionality's of each functional block included
in
the mains station MS and the substations S1, S2, S3, ..., S15, S16 the
consecutive steps of the method of the invention will be repeated here
shortly.
A stream of substation identities TEA1, TEA12, TEA16, TEA3, TEA7, ...
and consecutive PLOAM cells are provided to the inserting means INS.
T'he inserting means INS inserts the first 27 substation identities of this
stream whereof the identity of substation S3 i.e. TEA3 at predefined fields in
the
first coming PLOAM cell. It has to be remarked that in Figure 1 the PLOAM cell
after insertion of the substation identities is shown as PLOAM'. The PLOAM
cell
CA 02236801 1998-06-02
-12-
is packed by the packet formatting module PFM into the downstream frame
format and distributed to the plurality of substations. Substation S3 receives
the
PLOAM cell which is provided to the detecting means DET(TEA3). The detecting
means DET(TEA3) of substation S3 detects its own identity TEAS in the PLOAM
cell and knows that it is allowed to transfer an upstream information packet
in a
predefined upstream timeslot.
It should be noted that although the above described network of the
chosen embodiment is an asynchronous transfer mode ATM network the
application of the present invention is not restricted to the field of ATM.
Small
modifications, evident to a person skilled in the art may be applied to the
above
described embodiment to adapt it to be integrated in other time division
multiple
access networks wherein physical layer operation and maintenance parts are
predefined in downstream information packets.
While the principles of the invention have been described above in
connection with specific apparatus, it is to be clearly understood that this
description is made only by way of example and not as a limitation on the
scope
of the invention, as defined in the appended claims.
