POINT-TO-MULTIPOINT PASSIVE OPTICAL NETWORK THAT UTILIZES VARIABLE-LENGTH PACKETS AND VARIABLE-LENGTH UPSTREAM TINE SLOTS

Abstract: A point-to-multipoint passive optical network transmits downstream data from an optical line terminal (OLT) to multiple optical network units (ONUs) in variable-length packets and upstream data from the ONUs to the OLT in variable-length packets utilizing time division multiplexing with variable-length time slots to avoid transmission collisions. In an embodiment, the system further includes a time slot controller in communication with the OLT and the ONUs for changing the length of the ONU-specific variable-length time slots in response to upstream traffic demand from the ONUs. In a further embodiment, the time slot controller includes logic for increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU, the first ONU being one of the ONUs. In a further embodiment, the time slot controller includes logic for decreasing the length of a second ONU-specific time slot in response to the increase in the length of the first ONU-specific time slot. In an embodiment, the variable-length downstream packets and the variable-length upstream packets are formatted according to IEEE 802.3.

Description: POINT-TO-MULTIPOINT PASSIVE OPTICAL NETWORK THAT UTILIZES VARIABLE-LENGTH PACKETS AND VARIABLE-LENGTH UPSTREAM TIME SLOTS FIELD OF THE INVENTION The invention relates generally to broadband optical communications networks, and more particularly to point-to-multipoint passive optical networks. BACKGROUND OF THE INVENTION The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. One broadband network architecture that improves access to end users is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splitters and couplers to passively distribute optical signals between the OLT and the remote ONUs. Figs. 1A and 1B represent the downstream and upstream flow of network traffic between an OLT 102 and three ONUs 104 in a point-to- multipoint PON. Although only three ONUs are depicted, more than three ONUs may be included in a point-to-multipoint PON. Referring to Fig. 1A, downstream traffic containing ONU-specific information blocks is transmitted from the OLT. The downstream traffic is optically split by a passive optical splitter 112 into three separate signals that each carries all of the ONU- specific information blocks. Each ONU reads the information blocks that are intended for the ONU and discards the information blocks that are intended for the other ONUs. For example, ONU-1 receives information blocks 1, 2, and 3, however it only delivers information block 1 to end user 1. Likewise, ONU-2 delivers information block 2 to end user 2 and ONU-3 delivers information block 3 to end user 3. Referring to Fig. 1B, upstream traffic is managed utilizing time division multiplexing, in which transmission time slots are dedicated to the ONUs. The time slots are synchronized so that upstream information blocks from the ONUs do not interfere with each other once the information blocks are coupled onto the common fiber 110, often referred to as the trunk. For example, ONU-1 transmits information block 1 in a first time slot, ONU-2 transmits information block 2 in a second non-overlapping time slot, and ONU-3 transmits information block 3 in a third non-overlapping time slot. As shown in Fig. 1 B, all of the information blocks travel on the trunk in non-overlapping time slots. Because point-to-multipoint PONs are intended to deliver integrated voice, data, and video services, existing point-to-multipoint PONs have been designed around the ATM data link protocol, which was designed with quality of service (QoS) features that enable integrated voice, data, and video delivery over a single communications channel. As is well known in the field of packet-switched communications, the ATM protocol transmits information in fixed-length 53 byte cells (48 bytes of payload and 5 bytes of overhead). In an ATM-based point-to-multipoint PON, fixed-length ATM cells are used to transmit information in both the downstream and upstream directions. For example, as disclosed in U. S. Pat. No. 5,978,374, each time slot in the upstream traffic flow is filled with a single fixed-length ATM cell and a fixed- length traffic control field. Although the ATM protocol utilizes fixed-length 53-byte cells, ATM networks are often required to carry traffic that is formatted according to the widely used Internet protocol (IP). The Internet protocol calls for data to be segmented into variable-length datagrams of up to 65,535 bytes. In order for an ATM-based point-to-multipoint PON to carry IP traffic, the IP datagrams must be broken into 48 byte segments and a 5 byte header must be added. Breaking all incoming IP datagrams into 48 byte segments and adding a 5 byte header creates a large quantity of overhead that consumes valuable bandwidth in a point-to-multipoint PON. In addition to the increased bandwidth consumed by the ATM header, the process of converting IP datagrams into ATM cells is time consuming and the specialized hardware adds additional cost to the OLT and ONUs. Another data link protocol that has been incorporated into a point-to- multipoint PON is the IEEE 802.3 protocol (commonly referred to as ethernet). Ethernet carries payload data (such as IP datagrams) in variable-length packets of up to 1 ,518 bytes. Although the ethernet protocol data units are described as "packets," the protocol data units are also commonly referred to as "frames." Using variable-length packets of up to 1 ,518 bytes in a point-to- multipoint PON can greatly reduce the overhead of IP traffic when compared to the overhead of an ATM-based point-to-multipoint PON. In addition to the advantage of reduced overhead, ethernet network components are relatively affordable. When multiple stations in an ethernet network share a common physical transmission channel, the ethernet protocol utilizes a carrier sense multiple access/ collision detection protocol (CSMA/CD) as a media access control mechanism to avoid collisions between transmitted traffic. CSMA/CD is an efficient media access control protocol that does not require multiple stations to be synchronized. Applying CSMA/CD in an ethernet network requires that the minimum length of a packet must be longer than the maximum round-trip propagation time of the network in order to avoid collisions that cannot be detected by all of the stations on the network. That is, the maximum distance of separation between users in a multi-station ethernet network is limited by the collision domain. For example, in an ethernet network operating at 1 Gb/s, the maximum separation between stations is limited by CSMA/CD to approximately 200 meters. For point-to- multipoint PONs to be commercially feasible, the OLT and ONUs need to be able to be separated by more than the maximum distance allowed by CSMA/CD. In addition to the collision domain limitation, ethernet networks that rely on CSMA/CD are non-deterministic. That is, QoS guarantees cannot be made for traffic between the OLT and the ONUs. In view of the limitations of ATM-based point-to-multipoint PONs and ethernet-based point-to-multipoint PONs that utilize CSMA/CD, what is needed is a point-to-multipoint PON that utilizes variable-length packets and that increases the maximum allowable separation between the OLT and the ONUs. SUMMARY OF THE INVENTION A system and method for point-to-multipoint communications involves a PON in which downstream data is transmitted from an OLT to multiple ONUs in variable-length packets and in which upstream data is transmitted from the ONUs to the OLT in variable-length packets utilizing time division multiplexing with variable-length time slots to avoid transmission collisions. Utilizing variable-length packets instead of fixed-length ATM cells to transmit data such as IP data reduces the transmission overhead when compared to an ATM- based point-to-multipoint PON. Utilizing time division multiplexing to avoid upstream transmission collisions removes the distance limitations of shared media networks that utilize CSMA CD as a media access control protocol and utilizing variable-length time slots for upstream data transmissions allows for flexible provisioning of the available upstream transmission bandwidth between the ONUs. An embodiment of a point-to-multipoint optical communications system includes an OLT and a plurality of ONUs connected to the OLT by a passive optical network in which downstream data is transmitted from the OLT to the ONUs over the PON and upstream data is transmitted from the ONUs to the OLT over the PON. The OLT transmits downstream data over the passive optical network in variable-length downstream packets. The ONUs transmit upstream data over the passive optical network within ONU-specific variable- length time slots utilizing time division multiplexing, wherein the ONU-specific variable-length time slots are filled with multiple variable-length upstream packets. An embodiment of the system further includes a time slot controller in communication with the OLT and the ONUs for changing the length of the ONU-specific variable-length time slots in response to upstream traffic demand from the ONUs. In a further embodiment, the time slot controller includes logic for increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU, the first ONU being one of the ONUs. In a further embodiment, the time slot controller includes logic for decreasing the length of a second ONU-specific time slot to accommodate for the increase in the length of the first ONU-specific time slot. In an embodiment, the system includes a time division multiplexing (TDM) controller within the OLT for generating super frames which are sent downstream to the ONUs to synchronize the upstream data transmissions. In an further embodiment, the time slot controller generates time slot tables in response to traffic demand data and the ONUs begin using a new time slot table in response to receiving a super frame. In an embodiment the variable-length downstream packets are formatted according to IEEE 802.3. In an embodiment, the variable-length downstream packets include IP datagrams, and in another embodiment, the lengths of the variable-length downstream packets are related to the lengths of the IP datagrams. In another embodiment the variable-length upstream packets are formatted according to IEEE 802.3. In an embodiment, the variable-length upstream packets include IP datagrams, and in another embodiment, the lengths of the variable-length upstream packets are related to the lengths of the IP datagrams. A method for exchanging information between an OLT and multiple ONUs in a point-to-multipoint PON includes transmitting downstream data from the OLT to the ONUs in variable-length downstream packets and transmitting upstream data from the ONUs to the OLT in ONU-specific variable-length time slots utilizing time division multiplexing to avoid transmission collisions, wherein the ONU-specific variable-length time slots are filled with variable-length upstream packets. An embodiment of the method further includes a step of changing the length of said ONU-specific variable-length time slots in response to upstream traffic demand from said ONUs. A further embodiment of the method includes a step of increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU, the first ONU being one of said ONUs. A further embodiment of the method includes a step of decreasing the length of a second ONU-specific time slot to accommodate for the increase in the length of the first ONU-specific time slot. Another embodiment of the method includes a step of decreasing the length of a first ONU-specific variable-length time slot in response to a decrease in upstream traffic demand from a first ONU, the first ONU being one of the multiple ONUs. In an embodiment, the variable-length downstream and upstream packets are formatted in accordance with the IEEE 802.3 protocol. In an embodiment, the variable-length downstream and upstream packets include a header and a payload and the length of the variable-length packets is related to the length of an IP datagram that is included in the payload of the variable- length packets. An embodiment includes inserting downstream IP datagrams into the variable-length downstream packets and inserting upstream IP datagrams into the variable-length upstream packets. In an embodiment, the variable-length downstream and upstream packets are formatted in accordance with the IEEE 802.3 protocol. In an embodiment, the step of transmitting downstream data includes transmitting downstream synchronization markers at constant time intervals. In an embodiment, the ONU-specific variable-length time slots are filled with multiple variable-length packets. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 A depicts the downstream flow of traffic from an OLT to multiple ONUs in a point-to-multipoint PON. Fig. 1 B depicts the upstream flow of traffic from multiple ONUs to an OLT in a point-to-multipoint PON. Fig. 2 depicts a point-to-multipoint PON with a tree topology. Fig. 3 depicts a functional block diagram of an OLT for transmitting variable-length packets downstream, in accordance with an embodiment of the invention. Fig. 4 depicts an example of downstream traffic that is transmitted from an OLT to multiple ONUs utilizing variable-length packets, in accordance with an embodiment of the invention. Fig. 5 depicts a functional block diagram of an ONU for transmitting variable-length packets upstream with time division multiplexing, in accordance with an embodiment of the invention. Fig. 6 depicts an example of upstream traffic including variable-length packets that are time division multiplexed in order to avoid collisions, in accordance with an embodiment of the invention. Figs. 7A - 7C depict three different examples of time slot allocations for upstream traffic in accordance with an embodiment of the invention. Fig. 8 is a process flow diagram of he time slot control technique in accordance with an embodiment of the invention. Fig. 9 represents the timing of upstream traffic in terms of super frames, upstream frames, and time slots relative to the steps from the process flow diagram of Fig. 8. Fig. 10 is a process flow diagram of a method for exchanging information between the OLT and multiple ONUs in accordance with an embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION A system and method for point-to-multipoint communications involves a PON in which downstream data is transmitted from an OLT to multiple ONUs in variable-length packets and in which upstream data is transmitted from the ONUs to the OLT in variable-length packets utilizing time division multiplexing with variable-length time slots to avoid transmission collisions. In an embodiment, the system further includes a time slot controller in communication with the OLT and the ONUs for changing the length of the ONU-specific variable-length time slots in response to upstream traffic demand from the ONUs. In a further embodiment, the time slot controller includes logic for increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU. In a further embodiment, the time slot controller includes logic for decreasing the length of a second ONU-specific time slot to accommodate for the increase in the length of the first ONU-specific time slot. Fig. 2 depicts an example point-to-multipoint PON 200. The point-to- multipoint PON includes an OLT 202 and multiple ONUs 204 that are connected by a passive optical distribution network. In an embodiment, the OLT is connected to a service station 210 such as a central office and/or a head-end station. Services provided at the service station may include data network access, voice network access, and/or video network access. Example connection protocols utilized between the service station and the OLT may include OC-x, ethernet, E1 T1 , DS3, and broadband video. In an embodiment, the ONUs are connected to an end user system or systems 214, which may include a local area network, personal computers, a PBX, telephones, set-top boxes, and/or televisions. Example connection protocols utilized between the end user systems and the ONUs may include 10/100 Mb/s ethernet, T1 , and plain old telephone service (POTS). The passive optical distribution network shown in Fig. 2 has a tree topology that includes a common optical fiber 210 (trunk fiber) and multiple ONU-specific fibers 216 that are connected by a passive optical splitter/coupler 212. An optical signal transmitted in the downstream direction (from the OLT 202 to the ONUs 204) is optically split into multiple ONU- specific optical signals that all carry the same information. Optical signals transmitted in the upstream direction (from the ONUs to the OLT) are optically coupled into the trunk fiber that is connected between the coupler and the OLT. As explained in more detail below, time division multiplexing with variable-length time slots is utilized in the upstream direction to prevent collisions of upstream transmissions from two or more ONUs. In the embodiment of Fig. 2, an optical signal in the downstream direction is transmitted at a different wavelength (or frequency) than an optical signal in the upstream direction. In an embodiment, downstream traffic is transmitted in the 1550 nm wavelength band and upstream traffic is transmitted in the 1310 nm wavelength band. Utilizing different wavelengths in the upstream and downstream directions allows a single optical fiber to simultaneously carry downstream and upstream traffic without interfering collisions. In an alternative embodiment, separate downstream and upstream fibers may be utilized for the passive optical distribution network. In addition, wavelength division multiplexing (WDM) may be used in the downstream and/or upstream directions to increase transmission bandwidth. Although the passive optical distribution network of Fig. 2 has a tree topology, alternative network topologies are possible. Alternative network topologies include a bus topology and a ring topology. In addition, although the distribution network of Fig. 2 depicts only single fiber connections between network components, redundant fibers may be added between network components to provide fault protection. Fig. 3 is an expanded view of an example OLT 302 in the point-to- multipoint PON of Fig. 2. Functional units included within the OLT are a packet controller 320, a time division multiplexing (TDM) controller 322, a time slot controller 323, an optical transmitter 324, and an optical receiver 326. The OLT may also include other well known functional units that are not depicted. The packet controller receives downstream digital data from a service station and formats the downstream digital data into variable-length packets. The packet controller may be embodied in hardware and/or software and is sometimes referred to as the media access control (MAC) unit. In an embodiment, each variable-length packet includes a fixed-length header at the front of the packet, a variable-length payload after the header, and a fixed- length error detection field (such as a frame check sequence (FCS) field) at the end of the packet. In an embodiment, the downstream variable-length packets are formatted according to the IEEE 802.3 standard (commonly referred to as ethernet) or any of the related IEEE 802.3x sub-standards. In an embodiment, the downstream variable-length packets are transmitted at a rate of 1 gigabit per second (Gb/s) as defined by IEEE 802.3z (commonly referred to as gigabit ethernet), although lower or higher transmission rates may be utilized. In an embodiment, much of the downstream digital data arrives at the packet controller 320 in IP datagrams that range in size up to a maximum of 65,535 bytes. The packet controller reads header information from the IP datagrams and generates variable-length packets that include the IP datagrams as the payload. In an embodiment, the length of each variable- length packet is related to the length of the IP datagram that is placed into the payload. That is, if a downstream IP datagram is 100 bytes, then the variable- length packet will include 100 bytes of payload plus the packet overhead (the header and the error detection field) and if the IP datagram is 1 ,000 bytes then the variable-length packet will include 1 ,000 bytes of payload plus the packet overhead. In an embodiment in which the packets are formatted according to IEEE 802.3, the maximum length of a packet is 1 ,518 bytes (1 ,500 bytes of payload and 18 bytes of packet overhead). If an IP datagram exceeds 1 ,500 bytes, then the IP datagram is broken down into multiple IP datagrams that are carried in multiple variable-length packets. In contrast, an ATM-based point-to-multipoint PON breaks IP datagrams down into 48 byte segments regardless of the size of the original IP datagram and then adds the 5 byte header to create each ATM cell. When network traffic predominantly consists of IP traffic, the use of ATM as the data link protocol in a point-to- multipoint PON significantly increases the amount of bandwidth that is consumed by overhead. Although IP is described as a common higher layer protocol, other protocols, such as IPX and Appletalk, may be carried over the PON. The TDM controller 322 of the OLT 302 depicted in Fig. 3 controls the downstream flow of traffic from the OLT to the ONUs. Specifically, the TDM controller controls downstream framing and allocates bandwidth to variable- length packets that need to be transmitted downstream. The TDM controller may be embodied in hardware and/or software. The time slot controller 323 includes logic for controlling the length of the variable-length time slots that are utilized for time division multiplexing the upstream traffic from the ONUs. Specifically, the time slot controller provisions transmission capacity to each ONU by dictating the length (defined as a transmission interval) of each ONU-specific variable-length time slot. As the upstream traffic demand from the ONUs changes, the time slot controller changes the length of the ONU-specific variable-length time slots to best accommodate the changing upstream traffic demand. A more detailed description of the function of the time slot controller is provided below with reference to Figs. 7A - 7C. The optical transmitter 324 and the optical receiver 326 provide the interface between optical and electrical signals. Optical transmitters and receivers are well known in the field of point-to-multipoint PONs and are not described in further detail. Fig. 4 depicts an example of downstream traffic that is transmitted from the OLT to the ONUs in variable-length packets. In an embodiment, the downstream traffic is segmented into downstream frames that are fixed transmission intervals. Each of the downstream frames carries multiple variable-length packets. In an embodiment, clocking information, in the form of a synchronization marker 438, represents the beginning of each downstream frame. In an embodiment, the synchronization marker is a 1 byte code that is transmitted every 2 ms in order to synchronize the ONUs with the OLT. In an embodiment, a synchronization marker is transmitted every 2ms. In the embodiment of Fig. 4, each variable-length packet is intended to be read by a particular ONU, as indicated by the numbers, 1 through N, above each packet. In an embodiment, the variable-length packets are formatted according to the IEEE 802.3 standard and are transmitted downstream at 1 Gb/s. The expanded view of one variable-length packet 430 shows the header 432, the variable-length payload 434, and the error detection field 436 of the packet. Because the packets have variable-length payloads, the size of each packet is related to the size of the payload, for example the IP datagram, carried within the payload. Although each variable-length packet in Fig. 4 is intended to be read by a particular ONU (unicast packet), some packets may be intended to be read by all of the ONUs (broadcast packets), or a particular group of ONUs (multicast packets). Fig. 5 is an expanded view of an example ONU 504 in the point-to- multipoint PON 200 of Fig. 2. Functional units included within the ONU are a packet controller 520, a TDM controller 522, an optical transmitter 524, and an optical receiver 526. The ONUs may also include other well known functional units that are not depicted. The packet controller receives upstream digital data from end user systems and formats the upstream digital data into variable-length packets, with each variable-length packet including a header, a payload, and an error detection field as described above with reference to the downstream traffic. The packet controller is embodied in hardware and/or software and is sometimes referred to as the MAC unit. As with the downstream traffic, in an embodiment, the upstream variable-length packets are formatted according to the IEEE 802.3 standard and transmitted at a rate of 1 Gb/s. In an embodiment, much of the upstream digital data arrives at the packet controller in IP datagrams. In an embodiment, the packet controller reads header information from the upstream IP datagrams and generates variable-length packets that include the IP datagrams as the payload. In an embodiment, the length of each upstream variable-length packet is related to the length of the respective IP datagram. In many implementations, the upstream traffic arrives at the ONUs via an ethernet connection and therefore the traffic does not need to be reformatted into ethernet packets. The TDM controller 522 of each ONU 504, as depicted in Fig. 5, controls the upstream flow of traffic from each ONU to the OLT. Specifically, the TDM controller for each ONU, in conjunction with the OLT, ensures that the respective ONU transmits upstream variable-length packets in the designated variable-length time slot of a time division multiple access (TDMA) protocol. In order to synchronize transmissions between multiple ONUs, the ONUs utilize timing information from the OLT to maintain synchronized clocks. During operation, each ONU is allocated ONU-specific variable-length time slots by the OLT that are established so that upstream transmissions from the multiple ONUs do not collide with each other after they are combined into the trunk fiber. That is, the ONU-specific variable-length time slots do not overlap in time on the trunk fiber. It should be noted that prior art ethernet networks utilize CSMA/CD as the media access control protocol to ensure that all transmissions over a shared media reach their ultimate destination without collisions. CSMA/CD limits the maximum separation distance between the ONUs and therefore limits the viability of an ethernet and CSMA/CD based point-to-multipoint PON as a local access network architecture. Utilizing time division multiplexing as a media access control protocol, the separation distance between the ONUs is not limited by the CSMA/CD collision domain. Fig. 6 depicts an example of upstream traffic that is time division multiplexed into the common optical fiber 210 shown in Fig. 2 in order to avoid collisions between upstream traffic from the multiple ONUs 204. In the embodiment of Fig. 6, the upstream traffic is segmented into upstream frames and each upstream frame is further segmented into ONU-specific variable- length time slots. Although the variable-length time slots in Fig. 6 are depicted as having equal lengths, the length of the time slots can be changed as described below with reference to Figs. 7A - 7C. In an embodiment, the upstream frames are formed by a continuous transmission interval of, for example, 2ms. In an embodiment, the start of each system frame is identified by a frame header (not shown). The upstream frames can be fixed length or can be varied length to accommodate for different traffic demands or to create different traffic patterns. The ONU-specific variable-length time slots are transmission intervals within each upstream frame that are dedicated to the transmission of packets from specific ONUs. In an embodiment, each ONU has a dedicated ONU- specific variable-length time slot within each upstream frame. For example, referring to Fig. 6, each upstream frame is divided into N time slots, with each variable-length time slot being related to the respective 1 through N ONUs. In an embodiment that includes 2ms upstream frames and 32 ONUs that have equal bandwidth allocations, each time slot represents less than approximately 62.5μs of transmission time. At an upstream transmission rate of 1 Gb/s each time slot can carry approximately 7,800 bytes. The TDM controller for each ONU, in conjunction with timing information from the OLT, controls the transmission timing of the variable- length packets within the dedicated variable-length time slots and the time slot controller within the OLT dictates the length of each ONU-specific time slot within the system frames. Fig. 6 depicts an expanded view of an ONU- specific variable-length time slot (e.g. the time slot dedicated to ONU-4) that includes two variable-length packets 640 and 642 and some time slot overhead 644. In an embodiment, the time slot overhead includes a guard band, timing indicators, and signal power indicators. Although Fig. 6 only depicts two variable-length packets within the ONU-specific time slot, more variable-length packets may be transmitted within each time slot. Likewise, a time slot may be filled with an idle signal if there is no traffic to transmit from the ONU. Fig. 6 also depicts an expanded view of variable-length packet 642 within the ONU-specific variable-length time slot. The expanded view of the variable-length packet 642 shows the header 632, the variable-length payload 634, and the error detection field 636. In the embodiment of Fig. 6, the payload of the variable-length packet is an IP datagram or a portion of an IP datagram and the length of the variable-length packet is related to the length of the IP datagram. Figs. 7A - 7C are examples of how the lengths of the ONU-specific variable-length time slots are changed by the time slot controller in response to upstream traffic demand from the ONUs. In Fig. 6, the ONU-specific variable-length time slots all have the same length and in Figs. 7A - 7C, the ONU-specific variable-length time slots have different lengths that change to accommodate for changes in upstream traffic demand from the ONUs. Referring to the example of Fig. 7A, the length of ONU-specific time slot 4 has been increased from the time slot distribution of Fig. 6 and the lengths of ONU-specific time slots 2 and 3 have been reduced from the time slot distribution of Fig. 6. In an embodiment where the length of the upstream frames are fixed, an increase in the length of one ONU-specific variable- length time slot requires an equal decrease in the overall length of the other ONU-specific variable-length time slots in order to keep the total length of the time slots within the upstream frame. Because the length of ONU-specific time slot 4 has been increased, ONU-4 can transmit more data over a multiple frame interval than it could with the time slot distribution of Fig. 6. Likewise, because the lengths of ONU-specific time slots 2 and 3 have been decreased, ONU-2 and ONU-3 can transmit less data over a multiple frame interval than they could with the time slot distribution of Fig. 6. Fig. 7B represents another change in the distribution of the ONU- specific variable-length time slots within an interval of upstream frames. As shown in Fig. 7B, ONU-specific variable-length time slots 1 and 2 have increased in size from the distribution of Fig. 7A, ONU-specific variable-length time slot 4 has decreased in size from the distribution of Fig. 7A, and ONU- specific variable-length time slots 3 and N are unchanged in size from the distribution of Fig. 7C. Fig. 7C represents another changed distribution of the ONU-specific variable-length time slots within an interval of upstream frames. As shown in Fig. 7C, ONU-specific variable-length time slot 4 has increased in size from the distribution of Fig. 7B, ONU-specific variable-length time slots 1 , 2, and N have stayed the same as the distribution of Fig. 7B, and ONU-specific variable-length time slot 3 is eliminated. In an embodiment, an entire ONU- specific variable-length time slot can be eliminated if the ONU has no upstream traffic to transmit. In another embodiment, an ONU-specific variable-length time slot may carry only a small amount of traffic that includes, for example, an idle signal or upstream operations and maintenance information. A technique for changing the length of the variable-length time slots in a synchronized manner is described with reference to Figs. 8 and 9. The technique utilizes time slot tables and synchronizing frames to change the length of the variable-length time slots in a synchronized manner. A time slot table is a set of information sent to the ONUs from the time slot controller of the OLT that contains time slot assignment information for each ONU. In an embodiment, the time slot table includes the time slot number, the start position, and the length of each variable-length time slot for each ONU. In an embodiment, the timing information in the time slot table is identified as a number of counts from the beginning of each upstream frame indicator. For example, a 2 ms upstream frame timed with a 25 MHz clock consists of 50,000 counts. In order to successfully change the time slot allocations for the ONUs, the time slot allocations for all of the ONUs must be changed simultaneously. Therefore, all of the ONUs must have received the same time slot table information before a change can take place and the changeover to the new time slot table must take place simultaneously across all ONUs. According to the technique for changing the length of the variable- length time slots, synchronizing frames, referred to as "super frames" are sent downstream by the OLT. In an embodiment, the super frames are generated by the TDM controller 322 and sent downstream at fixed time intervals. In an embodiment, the super frames are identified by a unique 10 bit super frame indicator. The super frames identify that the latest time slot table should be used by the ONUs for upstream transmissions. If the time slot controller of the OLT has generated a new time slot table and if the ONUs have received the new time slot table, then the ONUs begin using the new time slot table for upstream transmissions immediately after receiving a super frame indicator. The new time slot table is utilized to allocate time slots for all subsequent upstream frames until the next super frame is received by the ONUs. In an embodiment, a new time slot table is generated for every super frame and in an alternative embodiment, new time slot tables are generated as needed to compensate for changes in the upstream traffic load. In an embodiment, the super frames indicators are sent every 60 ms and each super frame contains 30 upstream frames of 2 ms each frame. With 60 ms super frames, the length of the variable-length ONU-specific time slots can be changed every 60 ms. Fig. 8 represents a process flow diagram of the time slot control technique and Fig. 9 represents the timing of upstream traffic in terms of super frames, frames, and time slots relative to the steps, from the process flow diagram of Fig. 8. Referring to Fig. 8, at step 802, a super frame indicator arrives at the ONUs. At step AN, the ONUs start operating using time slot table N. Time slot table N is given as a starting time slot table for example purposes and it is assumed that the ONUs have previously received the time slot table from the time slot controller of the OLT. Referring to Fig. 9, step A N occurs at the start of the first super frame and the time slots are evenly distributed as shown in Fig. 6. At step BN, the OLT transmits time slot table N+1 to the ONUs and the ONUs receive the new time slot table N+1. Referring to Fig. 9, step BN occurs over the indicated time interval. At step CN, the ONUs confirm receipt of time slot table N+1 and transmit current ONU-specific traffic demand data to the OLT. Current traffic demand data may include queue size, delay information, and bandwidth reservation information. Referring to Fig. 9, step CN occurs over the indicated time interval. At step D N , the OLT receives the current traffic load data from the ONUs and the OLT generates a new time slot table, time slot table N+2, taking into consideration the current traffic load data from the ONUs. Referring to Fig. 9, step DN occurs over the indicated time interval. At decision point 804, it is determined whether or not a new super frame indicator has arrived at the ONUs. If a new super frame indicator has not arrived at the ONUs, then no change in the active time slot table occurs. However, if a new super frame indicator has arrived at the ONUs, then at step AN + I , the ONUs start operating using time slot table N+1. Referring to Fig. 9, step A + I occurs at the start of the second super frame and the time slots are distributed on a per frame basis as shown in Fig. 7A. At step B N+ ι, the OLT transmits time slot table N+2 to the ONUs and the ONUs receive the new time slot table N+2. Referring to Fig. 9, step B +I occurs over the indicated time interval. At step CN + I , the ONUs confirm receipt of time slot table N+2 and transmit current ONU-specific traffic load data to the OLT. Referring to Fig. 9, step CN + I occurs over the indicated time interval. At step DN+I, the OLT receives the current traffic load data from the ONUs and the OLT generates a new time slot table, time slot table N+3, taking into consideration the current traffic load data from the ONUs. Referring to Fig. 9, step DN+I occurs over the indicated time interval. At decision point 806, it is determined whether or not a new super frame indicator has arrived at the ONUs. If a new super frame indicator has not arrived at the ONUs, then no change in the active time slot table occurs. However, if a new super frame indicator has arrived at the ONUs, then at step AN +2 , the ONUs start operating using time slot table N+2. Referring to Fig. 9, step AN+2 occurs at the start of the third super frame and the time slots are distributed on a per frame basis as shown in Fig. 7B. The process continues as described, such that the length of the time slots is continuously adjusted in response to current traffic load data in a synchronized manner. In an alternative embodiment, the steps Bx, Cx, and D x are not entirely serial operations as shown in Fig. 9. That is, some of he operations may occur simultaneously. In another alternative embodiment, the next time slot table is generated by the OLT and transmitted to the ONUs in the same super frame instead of successive super frames. In an embodiment, the length of the upstream frames can be changed as needed. In an embodiment, the length of each upstream frame is a multiple of the super frame, for example, 1/10 th , 1/15 th , 1/20 th , or 1/25 th of a super frame. The multiple can be changed to respond to traffic demand and/or to create a particular traffic pattern. In another embodiment, the length of the upstream frames is a multiple of 125 μs. The length of the upstream frames are a multiple of 125 μs so that the PON is readily compatible with synchronized telecommunications networks, which use 125 μs frames. In an embodiment, both the upstream frames and the super frames are multiples of 125 μs. A method for exchanging information between an OLT and multiple ONUs in a point-to-multipoint PON is depicted in the process flow diagram of Fig. 10. In a step 1002, downstream data is transmitted from the OLT to the ONUs in variable-length downstream packets. In a step 1004, upstream data is transmitted from the ONUs to the OLT in ONU-specific variable-length time slots utilizing time division multiplexing to avoid transmission collisions, wherein the ONU-specific variable-length time slots are filled with variable- length upstream packets.

Claims: WHAT IS CLAIMED IS: 1. A point-to-multipoint optical communications system comprising: an optical line terminal (OLT); and a plurality of optical network units (ONUs) connected to said OLT by a passive optical network in which downstream data is transmitted from said OLT to said ONUs over said passive optical network and upstream data is transmitted from said ONUs to said OLT over said passive optical network; said OLT transmitting downstream data over said passive optical network in variable-length downstream packets; said ONUs transmitting upstream data over said passive optical network within ONU-specific variable-length time slots utilizing time division multiplexing, wherein said ONU-specific variable-length time slots are filled with multiple variable-length upstream packets. 2. The system of claim 1 further including a time slot controller in communication with said OLT and said ONUs for changing the length of said ONU-specific variable-length time slots in response to upstream traffic demand from said ONUs. 3. The system of claim 2 wherein said time slot controller includes logic for increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU, said first ONU being one of said ONUs. 4. The system of claim 3 wherein said time slot controller includes logic for decreasing the length of a second ONU-specific time slot in response to said increase in the length of said first ONU-specific time slot. 5. The system of claim 2 wherein said time slot controller includes logic for decreasing the length of a first ONU-specific variable-length time slot in response to a decrease in upstream traffic demand from a first ONU, said first ONU being one of said multiple ONUs. 6. The system of claim 2 further including a time division multiplexing (TDM) controller within said OLT for generating super frames which are sent downstream to said ONUs to synchronize said upstream data transmissions. 7. The system of claim 6 wherein said time slot controller generates time slot tables in response to traffic demand data and wherein said ONUs begin using a new time slot table in response to receiving a super frame. 8. The system of claim 6 wherein upstream frames have lengths that are a multiple of said super frames. 9. The system of claim 1 wherein upstream frames are variable-length. 10. The system of claim 1 wherein upstream frames are a multiple of 125 μs. 11. The system of claim 1 wherein said variable-length downstream packets are formatted according to IEEE 802.3. 12. The system of claim 1 wherein said variable-length downstream packets include Internet protocol (IP) datagrams. 13. The system of claim 12 wherein the lengths of said variable-length downstream packets are related to the lengths of said IP datagrams. 14. The system of claim 1 wherein said variable-length upstream packets are formatted according to IEEE 802.3. 15. The system of claim 1 wherein said variable-length upstream packets include Internet protocol (IP) datagrams. 16. The system of claim 15 wherein the lengths of said variable-length upstream packets are related to the lengths of said IP datagrams. 17. A method for exchanging information between an optical line terminal (OLT) and multiple optical network units (ONUs) in a point-to-multipoint passive optical network comprising: transmitting downstream data from said OLT to said ONUs in variable- length downstream packets; transmitting upstream data from said ONUs to said OLT in ONU- specific variable-length time slots utilizing time division multiplexing to avoid transmission collisions, wherein said ONU-specific variable-length time slots are filled with variable-length upstream packets. 18. The method of claim 17 further including a step of changing the length of said ONU-specific variable-length time slots in response to upstream traffic demand from said ONUs. 19. The method of claim 18 further including a step of increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU, said first ONU being one of said ONUs. 20. The method of claim 19 further including a step of decreasing the length of a second ONU-specific time slot in response to said increase in the length of said first ONU-specific time slot. 21. The method of claim 17 further including a step of decreasing the length of a first ONU-specific variable-length time slot in response to a decrease in upstream traffic demand from a first ONU, said first ONU being one of said multiple ONUs. 22. The method of claim 17 further including a step of transmitting super frames downstream from said OLT to said ONUs to synchronize said upstream data transmissions. 23. The method of claim 22 further including steps of: generating a time slot table in response to traffic demand data; and triggering the use of said time slot table by said ONUs upon receiving a super frame from said OLT. 24. The method of claim 23 wherein upstream frames have lengths that are a multiple of said super frames. 25. The method of claim 17 wherein upstream frames are variable-length. 26. The method of claim 17 wherein upstream frames are a multiple of 125 μs. 27. The method of claim 17 wherein said variable-length downstream and upstream packets are formatted in accordance with the IEEE 802.3 protocol. 28. The method of claim 17 wherein said variable-length downstream and upstream packets include a header and a payload, and wherein the length of said variable-length packets is related to the length of an Internet protocol (IP) datagram that is included in the payload of said variable- length packets. 29. The method of claim 17 further including steps of: inserting downstream Internet protocol (IP) datagrams into said variable-length downstream packets; and inserting upstream IP datagrams into said variable-length upstream packets. 30. The method of claim 29 wherein said variable-length downstream and upstream packets are formatted in accordance with the IEEE 802.3 protocol. 31. The method of claim 17 wherein said step of transmitting downstream data includes transmitting downstream synchronization markers at constant time intervals. 32. The method of claim 17 wherein said ONU-specific variable-length time slots are filled with multiple variable-length packets. 33. A point-to-multipoint optical communications system comprising: an optical line terminal (OLT); and a plurality of optical network units (ONUs) connected to said OLT by a passive optical network in which downstream data is transmitted from said OLT to said ONUs and upstream data is transmitted from said ONUs to said OLT; said OLT including means for formatting downstream datagrams into variable-length downstream packets; each of said ONUs including: means for formatting upstream datagrams into variable-length upstream packets; and means for timing the transmission of said variable- length upstream packets to coincide with ONU-specific variable-length time slots in order to avoid collisions with upstream packets from other ONUs. 34. The system of claim 33 wherein said OLT includes a time slot controller in communication with said ONUs for changing the length of said ONU- specific variable-length time slots in response to upstream traffic demand from said ONUs. 35. The system of claim 34 wherein said time slot controller includes logic for increasing the length of a first ONU-specific time slot in response to an increase in upstream traffic demand from a first ONU, said first ONU being one of said ONUs. 36. The system of claim 35 wherein said time slot controller includes logic for decreasing the length of a second ONU-specific time slot in response to said increase in the length of said first ONU-specific time slot. 37. The system of claim 34 wherein said time slot controller includes logic for decreasing the length of a first ONU-specific variable-length time slot in response to a decrease in upstream traffic demand from a first ONU, said first ONU being one of said multiple ONUs. 38. The system of claim 33 further including a time division multiplexing (TDM) controller within said OLT for generating super frames which are sent downstream to said ONUs to synchronize said upstream data transmissions. 39. The system of claim 38 wherein said time slot controller generates time slot tables in response to traffic demand data and wherein said ONUs begin using a new time slot table in response to receiving a super frame. 40. The system of claim 33 wherein said variable-length downstream packets are formatted according to IEEE 802.3. 41.The system of claim 33 wherein said downstream datagrams are Internet protocol (IP) datagrams. 42. The system of claim 41 wherein the lengths of said variable-length downstream packets are related to the lengths of said IP datagrams. 43. The system of claim 33 wherein said variable-length upstream packets are formatted according to IEEE 802.3. 44. The system of claim 33 wherein said upstream datagrams are Internet protocol (IP) datagrams. 45. The system of claim 44 wherein the lengths of said variable-length upstream packets are related to the lengths of said IP datagrams.

Inventors: KRAMER GLEN, KUO JERCHEN, PESAVENTO GURRY, KRAMER, GLEN, KUO, JERCHEN, PESAVENTO, GURRY
Applicants: ALLOPTIC INC, ALLOPTIC, INC.
Application year: 2001
Main classification: G02B
IPC codes: H04B10, H04J3, H04J14, H04Q11