Multiplex communications – Communication techniques for information carried in plural... – Byte assembly and formatting
Reexamination Certificate
1999-01-21
2002-02-12
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Byte assembly and formatting
C370S442000
Reexamination Certificate
active
06347096
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the structuring of digital data, transmitted in both directions, on a passive optical network (PON) inside a broad band PON access system, operating in the user-exchange direction, having Time Division Multiple Access (TDMA), in order to enable a particularly effective transfer of digital data flows, relevant to different service types.
TECHNICAL BACKGROUND
Generally speaking, the different types of services can be classified in two classes:
broad-band services transferred according to standard asynchronous mechanisms (53 byte ATM packages, called cells)
narrow-band services carried according to synchronous methods (SDH)
The access network is commonly identified with one or more connection interfaces (of the V type) at the exchange and by different kinds of interfaces (of the S type) for users' terminals, generally connected to equipment placed close to the users.
Equipment used to the above mentioned purpose in the access network and forming the so-called PON access system, are represented by a line termination (Optical Une Termination, or OLT), generally installed or located close to a switching exchange and by a plurality of network units (Optical Network Unit or ONU), typically placed close to the users, connected to the OLT through a passive optical network according to a scheme as shown in FIG.
1
.
The passive optical network forming the so-called access network infrastructure, has a typical tree structure, that is point-multipoint, where the root is connected to the OLT and terminal branches to each ONU, and consisting of lengths of optical fibers interconnected with passive optical components, called power splitter/combiners, enabling to split over the outputs or recombine optical signals presenting at their inputs.
The invention finds its general application in telecommunication digital networks and more in particular in the frame of digital networks for broad band integrated services, that is at high bit rate, based on the use of the ATM carrier method and briefly indicated B-ISDN.
In particular, the invention concerns a structuring and transport method, through PON broad band access systems, of transport containers, defined in the frame of Synchronous Digital Hierarchy (SDH) transmission systems, to carry digital data relevant to telephone connections or narrow band integrated services.
For the transport of digital multimedia services in the B-ISDN network, it is specified the use of the package transfer and switching format ATM (Asynchronous Transfer Mode) whose data flows crossing the network, are divided into information blocks having limited and constant width, each one carried by a 53-byte package or cell, out of which 48 bytes representing the so-called payload of each cell, destined to the transfer of the above mentioned data blocks, while the first five bytes, globally represented as a first cell portion called header, identify the information flow the cell belongs to (in other words, the destination address of data carried in the cell payload portion).
To minimize the costs of the PON access system and optimize the performance as for jitter (synchronism phase noise), it is propitious that the bit rate in the downstream direction, or exchange-to-user, is equal to the bit rate of the transmission systems of the Synchronous Digital Hierarchy (SDH) and therefore equal to a multiple of 155.52 MHz.
To assure the effective transfer of new multimedia services, such as for instance interactive television, whose characteristics require a capacity or digital band in the direction exchange-to-user (downstream direction)—where the audio video digital signal flows—considerably higher than the opposite direction (upstream). In this opposite direction in fact only user controls have to be carried. According to the above multimedia application, PON access systems shall have a higher transfer capacity in downstream direction than the opposite one.
The transfer capacity in downstream direction shall assure the support of digital flows relevant to multimedia services for a number of users enabling to make costs of the opto-electronic components necessarily employed for the optical transmission on the PON network acceptable: said number is identified in 500 users at least.
To adequately satisfy 500 users at least, it results that on the basis of typical service and traffic profiles, the net capacity in the downstream direction shall be equal at least to the capacity of level 4 of the SDH hierarchy, equal to 599.04 Mb/s.
In the upstream direction, net capacity can be lower, but equal at least to one quarter of the downstream one, in order to assure the transfer of symmetric flows relevant to narrow band services and, in the future, the transfer of bidirectional multimedia communications.
Additional requirements to be met result from the characteristics of the TDMA protocol used in the PON system: here, the same optical signal emitted by the OLT termination, reaches through the PON all ONU network units, while in the opposite direction, optical signals emitted by each ONU join on the PON to form a unique optical signal received by the OLT.
Using the TDMA protocol, the transmission performed by each unit of the ONU network is allowed only in well defined time intervals in order to avoid that, on a same moment, signals coming from two or more different ONUs can overlap (and therefore cancel each other).
The enabling transmission control, or permit, in each time interval called slot, is generated and transmitted by the OLT to all ONUs: the structure of the digital signal transmitted by the OLT shall then be able to carry said “permit” in each slot of the downstream transmission structure.
In the upstream direction (ONU to OLT) the signal received by the OLT consists of a sequence of signals, coinciding with each slot, coming from different ONUs. The signal corresponding to a given slot, called “burst” can have width differing from the previous one and from the subsequent one, as well as its bit phase can be different, just for the fact that each burst, possibly generated by a ONU different from the one generating the previous burst or from that generating the subsequent burst, follows a different path along the PON: the delay compensation mechanism, called ranging, succeeds in causing that each burst falls with a given accuracy inside the slot, but said accuracy is in the range of some bit only. To assure the phase continuity at bit level between a burst and the subsequent one is impossible for the bit period considered for a broad band PON access system, where the bit period is not higher than 5 nanoseconds.
Due to the above, a phase discontinuity determines between a burst and the other one. Therefore, the OLT receiver shall be able to correctly reconstruct each burst, amplifying it in an adequate manner and using the correct bit sampling phase (or bit clock phase) for each burst. These two operations, recovery of the amplitude and of the sampling phase require a given time equal to tens of bits between a burst and the other one, due to the discontinuity between two bursts: the upstream transmission structure for each slot shall be such that the field, called payload, destined to the transfer of the useful data is preceded by an adequate number of bits, generally called “preamble”, to enable the correct performance of the two operations mentioned above to which an additional field, called Guard, has to be put before, during which, the transmission from any ONU is inhibited to prevent that the end of transmission of a given ONU overlaps to the starting of the transmission of another ONU, due to the non accuracy of timing of the different ONU.
An additional requirement comes from the needs of the telecommunication network operators, when they want to use broad band PON access systems in the frame of the cabling of residential user areas or small business areas.
To minimize operation and maintenance costs it is essential in this phase, sharing of the new infrastructure and the new broad band access system by new multimedi
Castoldi Pierluigi
D'Ascoli Lucio
Podesta Angelo
Profumo Alberto
Serio Franco
Italtel SPA
Nguyen Chau
Nguyen Phuongchau Ba
LandOfFree
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