Multiplex communications – Channel assignment techniques – Using time slots
Reexamination Certificate
2001-12-05
2004-02-24
Pham, Chi (Department: 2667)
Multiplex communications
Channel assignment techniques
Using time slots
C370S468000
Reexamination Certificate
active
06697374
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to information transfer in a communication network, and specifically to a protocol for transmission over a passive optical network.
BACKGROUND OF THE INVENTION
A point-to-multi-point passive optical network (PON) operates as a communication system by broadcasting optical signals downstream from a central unit, herein termed an optical line termination (OLT), to optical network terminations (ONTs). The signals are transferred from the OLT to the ONTs via fibre optic cables and passive optical splitters, which comprise the physical fabric of the network. For upstream communication, each ONT must be able to transmit signals which are not interfered with by other ONTs. One of the methods known in the art for performing such upstream and downstream transmissions is by using time domain multiple access (TDMA), wherein each ONT is allocated a window when only it can transmit, and where the OLT also has windows for transmission to specific ONTs. Other methods for avoiding interference include transmitting signals at different wavelengths, using wavelength domain multiple access (WDMA). Combinations of TDMA and WDMA are also known in the art.
U.S. Pat. No. 5,173,899 to Ballance, whose disclosure is incorporated herein by reference, describes a method for communication in a passive optical network. An OLT transmits downstream TDM frames, the frames including information, a synchronization signal, and grants (to transmit upstream) to downstream ONTs. The ONTs transmit upstream TDM signals responsive to the grants and the synchronization signal.
U.S. Pat. No. 5,355,368 to Dore et al., whose disclosure is incorporated herein by reference, describes a method for allocating timeslots in a TDMA point-to-multi-point network. The network operates in a half-duplex manner, i.e., terminals of the network alternate between sending and receiving. The method reduces the “dead” time needed between adjacent downstream timeslots of an OLT, the dead time being the round-trip time of transmission between the OLT and an ONT in the network. The reduction is implemented by giving a first ONT an authorization to send while it is receiving information addressed to a second ONT.
U.S. Pat. No. 5,515,379 to Crisler et al., whose disclosure is incorporated herein by reference, describes a system for timeslot allocation within a communication system. A communication unit transmits a first packet of data requesting permission to transmit to a timeslot allocator. The packet contains either a request for allocation of a number of timeslots, or a request to transmit multiple packets of data. In either case, the allocator allocates contiguous time slots to the unit which the unit uses for transmission of its packets.
U.S. Pat. No. 5,528,592 to Schibler et al., whose disclosure is incorporated herein by reference, describes a method for route processing asynchronous transfer mode (ATM) cells. (A packet is comprised of a plurality of ATM cells.) The method consists of receiving, in a route cell buffer, cells corresponding to the beginning and end of a packet. A router determines routing information for the packet from these cells. The information includes a routing label determining an output port for the packet, and an identifier that determines switching paths connecting a packet source to a destination of the packet.
U.S. Pat. No. 5,838,687 to Ramfelt, whose disclosure is incorporated herein by reference, describes a slot reuse method in a Dynamic Synchronous Transfer Mode (DTM) segmented network. Access to slots is controlled by slot tokens, and writing to a slot may only be performed by a controller owning the corresponding token for that slot. A block token is used to represent a group of tokens in a single control message. The method consists of extending the DTM block token format to include parameters describing segments between source and destination nodes. Block token capacity is reserved only on segments between the nodes, and enables simultaneous transmissions in the same slot over disjointed segments of the network.
U.S. Pat. No. 5,982,780 to Bohm et al., whose disclosure is incorporated herein by reference, describes centralized and distributed management of communication resources in a DTM network. In the centralized version a server node is assigned tokens corresponding to time slots for unidirectional data flow on a communications link. The server, if it has available capacity, reserves and transfers tokens to other nodes on the link, according to requests from those nodes. In the distributed version the function of the server is spread amongst two or more nodes connected to the link.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide a method for transmitting signals in a communications network.
It is a further object of some aspects of the present invention to provide a method for transmitting TDM signals in a passive optical network (PON).
In a preferred embodiment of the present invention, an optical line termination (OLT) transmits optical signals downstream to a plurality of optical network terminations (ONTs). The OLT is coupled via a passive optical distribution fabric to the ONTs, so forming a PON. The OLT acts as a controller of the downstream signals, and also of signals sent upstream by the ONTs and received by the OLT. The downstream signals are sent in the form of frames having a constant period, and each downstream frame comprises a number of “timeslots.” Each timeslot is a fixed number of bytes, and within each frame the OLT allocates timeslots directed to each of the ONTs in a dynamic manner. Timeslot allocation for each frame is implemented according to quantities of data to be transmitted to/from each ONT, as determined by the OLT. Most preferably, the allocation of timeslots for each downstream frame is performed at substantially the same rate as the frames are transmitted. By allocating varying numbers of timeslots to each downstream frame or to an upstream window (explained below), the OLT effectively configures each frame or window in a variable TDM manner, with variable length times for each ONT which receives data.
Upstream signals are transmitted in a TDM manner from individual ONTs in windows, the windows being transmitted according to time pointers allocated by the OLT.
Both upstream and downstream signals comprise data transmitted according to one or more services, which may individually operate according to completely different protocols. Services typically include constant bit rate services and packet based services. Preferred embodiments of the present invention transfer data regardless of the type of service the data is transmitted under.
Upstream and downstream signals are transferred between the OLT and the ONTs via channels which are mapped as a one-to-one mapping from the services. The channels and channel parameters, such as bandwidth, are allocated by an operator of the PON, either at initialization or during operation of the PON. Each channel uses timeslots which are allocated according to bandwidth requirements of the channel's service.
Data for a specific channel in a downstream frame may be distributed within the frame non-contiguously. Also, data for a specific channel in an upstream window may be distributed within the window non-contiguously. Enabling a specific channel in upstream windows and downstream frames to be arranged non-contiguously significantly enhances the flexibility and efficiency of transmission of these signals, compared to systems which do not allow non-contiguous transmission.
There is therefore provided, according to a preferred embodiment of the present invention, a method for downstream communication from a central transmission point to a plurality of receiving end points by time division multiplexing of a sequence of frames, each of which is divided into multiple timeslots, the method including:
receiving at the central transmission point data for transmission to the end points, the data including at least a first quantity of f
Arol Joseph
Marmur Oren
Ron Ilan
Shraga Eyal
Blakely & Sokoloff, Taylor & Zafman
Flexlight Networks
Ly Anh-Vu H
Pham Chi
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