(12) United States Patent
`Bahut
`
`USOO677855OB1
`US 6,778,550 B1
`Aug. 17, 2004
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) METHOD AND APPARATUS FORTDM/
`TDMA COMMUNICATIONS
`
`(75) Inventor: Donald Edgar Blahut, Holmdel, NJ
`(US)
`(73) Assignee: Lucent Technologies Inc., Murray Hill,
`NJ (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(21) Appl. No.: 09/515,947
`(22) Filed:
`Feb. 29, 2000
`(51) Int. Cl.................................................... H04J 3700
`(52) U.S. Cl. ........................ 370/443; 370/449; 370/458
`(58) Field of Search ................................. 370/329, 337,
`370/341, 347, 442, 458, 468, 473, 477,
`478,485, 486, 487, 489, 496, 463, 461,
`443, 449; 359/123, 135
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`5,481,542 A * 1/1996 Logston et al. ............ 370/94.2
`5,570,355 A * 10/1996 Dail et al. ................. 370/60.1
`5,926,476 A * 7/1999 Ghaibeh ..................... 370/395
`5,926,478 A * 7/1999 Ghaibeh et al. ............ 370/395
`5,953,344 A * 9/1999 Dail et al. .................. 370/443
`5,956.338 A * 9/1999 Ghaibeh ..................... 370/395
`6,023,467 A * 2/2000 Abdelhamid et al. ....... 370/395
`6,055.242 A
`4/2000 Doshi et al. ................ 370/458
`6,370,153 B1 * 4/2002 Eng ........................... 370/438
`6,563.829 B1 * 5/2003 Lyles et al. ............ 370/395.21
`
`FOREIGN PATENT DOCUMENTS
`
`WO
`
`WO OO/O1127
`
`1/2000
`
`OTHER PUBLICATIONS
`
`J.E. Dail et al., IEEE Communications Magazine “Adaptive
`Digital Access Protocol: A MAC Protocol for Multiservice
`Broadband Access Networks”, vol. 34, No. 3, Mar. 1996, pp.
`104-112.
`
`* cited by examiner
`Primary Examiner-Chau Nguyen
`Assistant Examiner-Duc Duong
`(74) Attorney, Agent, or Firm-Stephen M. Gurey
`(57)
`ABSTRACT
`In a power splitting passive optical network, TDM/TDMA
`communication is employed for downstream/upstream
`transmission over a fiber. A framed Structure is used for the
`downstream transmission from an optical line card (OLC) at
`the network end to a plurality of optical network units
`(ONUs), which are each connected to end user equipment.
`A framed Structure is also employed for upstream burst
`transmission from the plural ONUs to the OLC. In the
`upstream direction each ONU transmits at most one burst
`per frame that includes a header and a payload containing a
`variable number of bytes that is adjustable on a continuum.
`The length of each payload in a burst transmitted upstream
`by an ONU is determined as a function of the bandwidth
`requirements of the end user equipment connected to that
`ONU as well as the bandwidth requirements of the end user
`equipment at the other ONUs.
`
`24 Claims, 4 Drawing Sheets
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`403
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`ATM
`FABRIC
`INTERFACE
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`404
`TRANSMIT DOWNSTREAM DATA
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`DOWNSTREAM
`FRAME GENERATOR/
`DLE CELL
`GENERATOR
`BANDWIDTH
`UPSTREAM
`MANAGEMENT
`MESSAGES
`405
`MICROPROCESSOR
`CONTROLLER
`ESSAGES
`INTRAPON M
`406
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`407
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`(1550nm)
`TRANSMITTED
`BW MGMT
`MESSAGES
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`BURST MODE
`RECEIVER
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`DATA
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`UPSTREAM
`CIRCUITRY
`415
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`DS1 MULTIPLEXOR
`ATM PACKETIZER
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`RECOWERED
`UPSTREAM
`DATA
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`Comcast, Ex. 1209
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`1
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`

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`US. Patent
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`US 6,778,550 B1
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`U.S. Patent
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`Aug. 17, 2004
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`Sheet 2 of 4
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`US 6,778,550 B1
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`FIC. 2
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`203-1
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`202-1
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`SONET
`NTERFACE
`202-M
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`SONET
`INTERFACE
`DS1
`INTERFACE
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`ATM
`SWITCHING
`FABRIC
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`204-1
`DS1
`INTERFACE
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`204-0
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`201
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`205-N
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`115
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`FIG. 3
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`302
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`DS2
`BYTES
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`303
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`304
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`305
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`PAYLOAD
`SIZE
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`CELL
`POINTER
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`BYTE
`OFFSET
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`/
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`501
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`3
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`

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`U.S. Patent
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`Aug. 17, 2004
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`Sheet 3 of 4
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`US 6,778,550 B1
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`FIC. 4
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`403
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`404
`TRANSMIT DOWNSTREAM DATA
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`ATM
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`t
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`DOWNSTREAM
`FRAME GENERATOR/
`DLE CEL
`GENERATOR
`UPSTREAM BANDWDTH
`MANAGEMENT
`MESSAGES
`405
`MICROPROCESSOR
`CONTROLLER
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`(1550nm)
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`TRANSMITTED
`BW MGMT
`MESSAGES
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`407
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`410
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`A
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`UPSTREAM
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`CIRCUITRY MEMORY
`415
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`- - - - - - - - - - - - - - - - -
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`BURST MODE
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`RECEIVER
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`RECOVERED
`UPSTREAM
`DATA
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`4
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`U.S. Patent
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`Aug. 17, 2004
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`Sheet 4 of 4
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`US 6,778,550 B1
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`1
`METHOD AND APPARATUS FORTDM/
`TDMA COMMUNICATIONS
`
`US 6,778,550 B1
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`TECHNICAL FIELD
`This invention relates to Time Division Multiplexing/
`Time Division Multiple Access (TDM/TDMA) communi
`cations.
`
`2
`used for upstream transmission. Prior art PSPON based
`systems use a framed structure, in the downstream direction,
`that consists of 2968 bytes at a bit rate of 155.52 mbits/sec.
`These 2968 bytes in each downstream frame, represent 56
`ATM cells/frame, each cell consisting of a 5-byte header and
`a payload of 48 bytes. Addressing information is included
`within that 5-byte header, which enables each of the up to 32
`end user ONUs to select for reception by its connected
`terminals only those ATM cells that are broadcast or spe
`cifically addressed to it. In the upstream direction, the ONUs
`transmitting to the OLC consecutively transmit bursts, each
`burst containing a single ATM cell. ASSuming an additional
`3-byte burst header, each burst is thus 56 bytes long. Each
`2968 byte frame, therefore, contains 53 bursts, each being 56
`bytes. If each ONU transmits one burst per frame, approxi
`mately 2.777 mbits/sec of user ATM upstream bandwidth is
`available to each end user terminal. Disadvantageously, a
`finer bandwidth granularity (e.g., less than 2.777 mbits/sec)
`per end user terminal requires assigning p bursts every m
`frames, requiring a complicated upstream bandwidth man
`agement procedure. Further, if an end user terminal requires
`a higher upstream bandwidth (e.g., higher than 2.777 mbits/
`sec), the ONU must manage multiple bursts per frame from
`such a terminal. Digital voice communications in prior art
`system has further inefficiencies. Specifically, Since the
`bandwidth requirement for a digital voice channel is only 64
`kbits/sec (equivalent to one byte per frame), 47 of the 48
`payload bytes in each ATM cell containing digitized Voice in
`each upstream burst remain unused (assuming one burst
`every 125 usec). If, alternatively, 48 voice samples are
`accumulated over a 6 msec period before being transmitted,
`echo cancellation will likely need to be implemented due to
`the delay imposed on the transmitted Voice samples.
`Furthermore, the 8000 samples/sec associated with digital
`voice circuits cannot be simply generated from the non-8000
`frames/sec frame rate.
`A need therefore exists to better control the bandwidth
`allocation to end user terminals of all types, and especially
`as applied to digital transmission of voice signals.
`SUMMARY OF THE INVENTION
`In accordance with the present invention, efficient band
`width allocation is achieved by using variable length bursts
`for upstream transmission. Rather than setting the length of
`each upstream burst at a fixed length, the length of each burst
`is determined in accordance with the actual bandwidth
`requirements of the transmitting end user terminal. In
`particular, and depending upon the overall bandwidth
`requirements of all the end user terminals transmitting over
`the upstream channel, the number of payload bytes per burst
`can vary between Zero and the total number of payload bytes
`allocated per frame. The latter would occur if only one end
`user terminal is connected to the channel for upstream
`communication. In the more likely Scenario of multiple end
`user terminals communicating over the channel, the total
`number of bytes per upstream frame are divided among all
`the end user transmitting terminals in accordance with their
`current bandwidth requirements and the overall bandwidth
`capacity of the channel. Each end user transmitting terminal
`then transmits one and only one burst each frame. That burst
`contains all the digital information that the end user is
`transmitting upstream to the network end including, for
`example, video, data, and digital voice. Advantageously, a
`high degree of granularity in allocating bandwidth can be
`achieved since the burst length can be adjusted in one byte
`increments.
`For the specific embodiment of the PSPON topology in
`which frames are conveniently transmitted at 8000 frames
`
`15
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`BACKGROUND OF THE INVENTION
`In systems employing TDM/TDMA for the transmission
`of signals, TDM is typically used to transmit signals down
`stream from a network end to a plurality of end-user
`terminals at a home or business over a single channel. At the
`home or business end, a receiving terminal receives all
`downstream transmissions that are directed both to it and the
`other end-user terminals. However, in each received frame
`of data bytes, only those bytes that are properly intended for
`a particular receiving terminal are delivered to that terminal
`for processing. Typically, this can be done by assigning
`different time-slots in each frame to specific receiving
`terminals. Each receiving terminal thus only "looks' in its
`assigned time-slot for the bytes directed to it. Alternatively,
`the downstream signal, if originating, for example, from a
`broadband asynchronous transfer mode (ATM) network,
`may consist of a sequence of ATM cells which each include
`header information indicating an address of the destination
`(s) to which the cell is directed. A receiver terminal then only
`“picks out” the ATM cells that are addressed to it or are
`broadcast to many, and discards the other ATM cells
`addressed elsewhere.
`In the upstream direction, TDMA transmission is used for
`transmitting the outputs of multiple end-user terminals back
`to the network end. One way this is implemented is to allow
`an end-user terminal to transmit back to the network end
`only during a specific time-slot each frame. At the network
`end, therefore, the bytes received from the multiple end-user
`transmitting terminals are demultiplexed into separate plural
`data streams in accordance with the time-slots each frame
`during which they are received.
`Whereas the one-to-many aspect of TDM downstream
`transmission on a single channel is implemented in a rela
`tively straight-forward manner at both the network end and
`the end-user terminal end, upstream transmission from a
`plurality of end user terminals to a single network end
`presents several technical difficulties with respect to the
`management of the available upstream bandwidth. This is
`particularly true in digital broadband access networks that
`employ optical fiber-to-the-home (FTTH), utilizing a power
`50
`splitting passive optical network (PSPON) topology. As
`presently configured, each PSPON fiber can support up to 32
`homes or businesses. In Such a system, bidirectional com
`munications over a single fiber is achieved using coarse
`wavelength division multiplexing (CWDM), in which one
`wavelength, 1550 nm, is used for downstream communica
`tions to all home/business end user terminals that are con
`nected to that fiber. Another wavelength, 1310 nm, is then
`used for transmission to the network end of upstream data
`from all those connected homes/business terminals. That
`data, in both directions, can include Video, data (e.g.,
`Internet-type data), and digitized voice. In Such systems, the
`fiber is terminated at the home/business by an Optical
`Network Unit (ONU), and at the network end by an Optical
`Line Card (OLC).
`In such systems, ATM in a frame structure is employed for
`downstream transmission while ATM transmitted in bursts is
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`per Second and in which the upstream frame comprises 2430
`bytes, bandwidth can be assigned in one bytex8000/sec
`increments or equivalently, 64 kbit/sec increments.
`Advantageously, included within the payload of each
`upstream burst transmitted by the ONU that is connected to
`one or more end user terminals then being used, is one byte
`per each active digital voice channel required by that end
`user. Thus, if there is no current active telephonic
`conversation, no bytes are used, whereas if one voice
`channel is active, a single byte in the upstream burst is
`allocated to and used for digital voice transmission. Addi
`tional voice circuits associated with that same ONU are
`transmitted in additional bytes in the upstream burst. The
`bandwidth allocated for each Such digital voice channel is
`thus an efficient 64 kbit/sec for the specific example of a
`frame arrangement noted above.
`In order to manage the allocation of upstream bandwidth
`among the plural end user terminals transmitting upstream
`information, the terminating terminal associated with each
`end user, Such as the ONU, is assigned the timing and length
`of its upstream TDMA slot. This is effected through the
`broadcast of downstream cells containing Slot assignment or
`modification messages to the terminating terminal. The Slot
`assignment messages include information used for assigning
`a slot of a specified length to a particular end user termi
`nating terminal. Such information thus includes where (i.e.,
`from which byte position) within each upstream frame that
`the terminating terminal is to transmit, and how many bytes
`the payload of each burst from that terminal is to be. An
`assignment message is used to assign an upstream slot to a
`newly installed end user terminating terminal and to reassign
`(i.e., confirm) an existing assignment as a fault recovery
`mechanism. A modification message is used to change the
`length of an existing slot assignment and/or the number of
`digital voice channels associated with the terminating ter
`minal to which the message is directed. Such a message, by
`necessity, is also used to move the location of all assigned
`slots located after that modified slot in the frame.
`
`4
`32 optical fibers, 105-1-105-32. It should be obvious that as
`technology develops lower-loSS optical fibers, and/or if
`spans are limited to shorter lengths, the power transmitted in
`an optical fiber could be split into more than 32 fibers to
`Server a greater number of end users. Each Such optical fiber
`105-1, for example as shown, is connected to a terminal
`known as an Optical Network Unit (ONU) 106 at a business
`or residence location. At the illustrated residence endpoint,
`three examples of end user terminal equipment are shown
`connected to ONU 106. These include a telephone station set
`107 connected over a conventional unshielded twisted pair
`(UTP) of wires 108 to a telephone interface (not shown) in
`ONU 106; a personal computer (PC) 109 connected to ONU
`106 over a data link 110, Such as an Ethernet, to an Ethernet
`interface in ONU 106: and a standard television 111 con
`nected to ONU 106 via a standard coaxial cable 112. In order
`to drive a standard television, ONU 106 includes (not
`shown) an MPEG decoder and an NTSC encoder, for
`generating a TV compatible signal from a received digital
`MPEG signal being transmitted downstream from the net
`work end 103.
`At the network end 103, fiber 102 terminates in a Optical
`Line Terminal (OLT) 113 which also terminates a plurality
`of other PSPON fiberS. Each Such other PSPON fiber is
`connected through another power Splitter to what may be up
`to 32 other residences/businesses. Included within OLT 113
`are a plurality of Optical Line Cards (OLCs) (not shown in
`FIG. 1), which each individually terminate a single PSPON
`fiber. OLT 113 is connected to two networks: a broadband
`network 114, Such as an ATM network; and a Public
`Switched Telephone Network (PSTN) 115. The broadband
`network 114, which in this illustrative embodiment is an
`ATM network, is typically connected to OLT 113 via a
`plurality of SONET fibers, collectively designated as 120.
`Servers connected to the ATM network 114, Such as an
`Internet server 116 and a video server 117, deliver service,
`in ATM format, onto network 114. Both IP data and MPEG
`Video Services are currently Supported by existing ATM
`Standards. Accordingly, no further description of IP data or
`MPEG video over ATM is given herein. OLT 113 is con
`nected to PSTN network 115 via, for example, a plurality of
`DS1 circuits, collectively 121, that each deliver 24 DS0
`voice channels. Rather than a virtual connection, PSTN
`network 115 establishes a conventional circuit-switched
`telephone connection to end-users at telephone Station Sets,
`Such as 118, connected to the network.
`In the embodiment of the present invention, downstream
`transmission between the network end 103 over PSPON
`fiber 102 to the up to 32 fibers 105-1-105-32, which are each
`connected to an ONU, is in a TDM fixed frame format that
`consists of a total 2430 bytes transmitted at an 8000 frames/
`secrate. Downstream optical transmission over the fiber 102
`is at a 1550 nm wavelength. Each frame includes a 3 byte
`framing pattern, leaving a 2427 byte payload. For purposes
`of the present embodiment, all downstream payload data is
`formatted as ATM cells. Each cell includes 48 bytes of
`payload with a 5 byte ATM header. Since each ATM cell is
`53 bytes long and the frame payload is not integrally
`divisible by 53, each frame boundary is typically spanned by
`an ATM cell. Each downstream ATM cell contains what may
`be either video MPEG data, IP data, or other data, and a
`destination address (contained within the header) for that
`cell. That cell may be directed to more than one end user
`terminal. For example, a cell originating from a Video Server
`may be broadcast to all or plural Selected end users’ televi
`Sion sets that are connected to different ONUs via fibers
`105-1-105-32. That same cell may also be transmitted over
`
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`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1 is a block diagram of a PSPON fiber system
`incorporating the present invention;
`FIG. 2 is a block diagram of the Optical Line Terminal
`(OLT) at the network end in FIG. 1 that interconnects an
`ATM network and a Public Switched Telephone Network
`with the PSPON fibers;
`FIG. 3 shows the fields within a word in a slot assignment
`memory located at the network end, which Stores for each
`ONU the format of the transmitted upstream burst, including
`its payload size and the number of bytes associated with
`Voice circuits in that payload;
`FIG. 4 is a block diagram of Optical Line Card (OLC) at
`the network end of the system of FIG. 1; and
`FIG. 5 is a block diagram of the ONU terminal at the
`outside plant in the system of FIG. 1 that interconnects the
`PSPON fiber with each end user's terminal equipment.
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`DETAILED DESCRIPTION
`With respect to FIG. 1, a fiber-to-the-home communica
`tions system 101 is shown that incorporates TDM/TDMA
`transmission over a PSPON fiber 102, in accordance with
`the invention. That fiber 102 interconnects the network end
`103 to a passive optical splitter 104. Splitter 104 passively
`Splits the power in the downstream optical Signal transmitted
`from the network end 103 into up to, for this embodiment,
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`the other PSPON fibers to different splitters and ONUs
`connected thereto. On the other hand, a cell containing IP
`data may be addressed, for example, to only the Single end
`user's PC 109 connected to ONU 106. Digitized voice data
`from the station sets connected to PSTN 115 are broadcast
`on fiber 102 to each ONU connected to fibers 105-1-105-32.
`Specifically, two DS1 frames, each consisting of 24 digitized
`DSO voice channels are combined in a 48 byte channel
`payload for a dedicated ATM virtual circuit (VC). By
`transmitting downstream one Such ATM cell per frame,
`forty-eight 64 kbit/sec downstream channels are provided
`corresponding to voice channels. Each ONU then extracts
`from that ATM cell in each frame the byte that is assigned
`to its active voice channel. If more, N, active telephone
`station sets are connected to an ONU, then N such bytes are
`extracted from that cell each frame, each byte being asso
`ciated with one of the Voice channels.
`FIG. 2 illustrates a block diagram of OLT 113. It includes
`an ATM Switching fabric 201 to which is connected a
`plurality of SONET interfaces 202-1-202-M. SONET fibers
`120, connected to ATM network 114 in FIG. 1, are con
`nected to SONET interfaces 202. Each ATM cell received
`from ATM network 114 via one of the a SONET fibers 120
`is routed through the ATM Switching fabric 201 to one or
`more appropriate Optical Line Cards (OLCs) 203-1-203-N
`25
`in accordance with the cells ATM address. Each Such OLC
`interfaces with a PSPON fiber, Such as fiber 102 in FIG. 1.
`As previously noted, each PSPON fiber interfaces with a
`Splitter in the outside plant, which passively splits the
`downstream signal into up to 32 equal optical Signals. Thus,
`each OLC within OLT 113 is connected to a separate PSPON
`fiber 102, which in turn Supports Service provisioning to up
`to 32 different residences/businesses. Also connected to the
`input of Switching fabric 201 are a plurality of DS1 inter
`faces 204-1-204O, which each support 48 DS0 digital voice
`circuits from a pair of DS1 inputs.
`In the downstream direction, ATM Switching fabric 201
`receives an ATM cell from the ATM network (originating,
`for example, from a video server or Internet Server) and
`routes it, in accordance with its address, to the proper OLC
`40
`203-1-203-M associated with its intended destination. That
`cell is then transmitted in the payload of a frame (or in two
`frames if that cell spans two frames) to the corresponding
`Splitter at the outside plant location. That cell is then
`delivered to each connected ONU. Only if the cell is
`addressed to ONU 106, however, is it accepted by ONU 106
`and delivered to the appropriate connected end user
`terminal, such as the PC 109 or TV 111, as shown in FIG.
`1. With respect to the 48 DS0 voice circuits that are inputted
`to each DS1 interface 204, the 48 DSO voice circuits are
`50
`formatted as a single ATM cell and routed by the ATM
`Switching fabric 201 to the appropriate OLC for broadcast
`on that fiber. As noted, each ONU that is connected to that
`fiber extracts only the byte or bytes from that cell that is (are)
`associated with its voice channel(s).
`In the upstream direction, in accordance with the
`invention, variable length bursts are transmitted by each
`ONU. Each ONU that is connected through the same splitter
`to a common PSPON fiber transmits one burst per 2430
`byte-length frame back to the network end at a wavelength
`of 1310 nm. Each upstream burst contains a 3 byte burst
`header and a payload consisting of between 0 and 2427
`bytes. As will be discussed, the 2430 bytes per upstream
`frame are divided among the actual number of plural ONUs
`that are connected and are operating. Each burst from each
`65
`connected ONU contains one byte per active voice channel
`that is concatenated with digital Video and IP data Signals,
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`each in ATM format, outputted by, for example, a Video
`Source Such as a camera, and a PC, respectively. Since the
`payload of each burst transmitted by each ONU may not
`have a length that incorporates both the number of bytes
`needed for its active Voice channels plus a full or an integral
`number of ATM cells, the bytes associated with such ATM
`cells are transmitted by the ONU each burst to the OLC at
`the network end, where they are accumulated to form ATM
`cells. At the OLC, each ATM cell is formed and routed over
`a SONET fiber 120 to the ATM network 114 for transmission
`to its intended destination indicated in its header address
`such as, for example, the video server 117 or Internet server
`116. Within the OLC, the digital voice channels from all the
`ONUs assigned the corresponding downstream VC are
`combined yielding two upstream DS1 frames which are
`outputted through an appropriate DS1 interface 204 to the
`circuit-switched PSTN network 115 for individual transmis
`Sion of each channel to a telephone Station Set.
`In the downstream direction, the OLC Sequentially trans
`mits 53-byte ATM cells within each frame, which are
`broadcast to each ONU. Each ONU determines whether a
`received cell is directed to it or not. Since each received
`ATM cell originates from the same OLC, each cell is timed
`to begin transmission as Soon as the previous cell has been
`transmitted. In the upstream direction, however, each burst
`originates from a different ONU. Transmission of each burst
`from each ONU is precisely timed so that it reaches the
`Splitter just as another ONU has finishing transmitting its
`burst. Since the loop transmission delays of the fibers
`105-1-105-32 connecting the ONU 106 and splitter 104
`differ in accordance with the length of each fiber loop, a
`ranging procedure is implemented as each ONU in the
`System is installed to compensate for loop transmission
`delay thereby eliminating the need for a time interval
`between bursts. Specifically, a ranging procedure is executed
`as each ONU is installed that results in a common loop
`delay, comprising the loop transmission delay plus a calcu
`lated and assigned “ranging delay.” The “ranging delays are
`calculated to also conveniently Synchronize the upstream
`frames with downstream frames. In the Specific embodiment
`described herein, that loop delay is set at two frames, or 250
`usec (based on 8000 frames/sec). Therefore, at the OLC, the
`received upstream communications consists of a continuous
`Sequence of upstream frames, each in response to down
`Stream frame transmitted two frame periods earlier, each of
`those upstream frames consisting of a sequence of concat
`enated bursts, one per active ONU. A ranging procedure that
`can be employed is described in co-pending patent
`application, Ser. No. 09/356,980 filed Jul. 19, 1999, entitled
`“Ranging Arrangement and Method for TDMA
`Communications,” which is incorporated herein by refer
`CCC.
`With variable length bursts being transmitted upstream,
`each ONU transmits only one burst per frame. Therefore, as
`noted, only one variable length upstream slot having a length
`between 3 and 2430 bytes is assigned per ONU. Timing for
`upstream frames is derived from the downstream Signal
`which includes, as noted, three framing bytes per every
`downstream 2430 byte frame. Once each ONU detects the
`framing byte pattern in the downstream Signal, upstream
`transmission is Synchronized to the downstream Signal. AS a
`system “grows”, through the addition of ONUs and associ
`ated end user terminals, bandwidth allocation is redistrib
`uted and Slot assignments are modified. Thus, when only a
`Single ONU is connected to a splitter, a Slot having a 3-byte
`header and a 2427-byte payload is assigned to that ONU. As
`more ONUS are activated, a Slot is assigned to each, thereby
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`US 6,778,550 B1
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`15
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`requiring both a re-allotment of bytes to the already active
`ONUs. Specifically, within the 2430 byte upstream frame, a
`Slot assignment for each ONU is made indicating at which
`byte within the 2430 byte frame that ONU's slot is to start
`and how many bytes that slot should be. As each ONU
`becomes active, it sends an out-of-band Signal, Such as a
`tone, back to the OLC, which in turn initiates a ranging
`procedure, previously described, to insure an equal loop
`delay per ONU. AS part of that ranging procedure, a ranging
`delay is determined for that ONU to ensure equal loop delay.
`That ranging delay is determined by the OLC and transmit
`ted downstream from the OLC to the ONU, together with an
`assigned identity for that ONU. That ranging delay is then
`used by that ONU to artificially insert an electronic delay to
`its upstream burst transmissions so that all ONUs connected
`to a common Splitter have equal transmission delayS.
`Further, each ONU is assigned its upstream TDMA slot
`through a broadcast downstream of a Physical Layer Opera
`tion and Maintenance (PLOAM) cell. Such PLOAM cells
`can be formatted in many ways and messages to more than
`one ONU can be combined within a single PLOAM cell.
`Such PLOAM cells are also used to broadcast downstream
`to all ONUs whatever changes may need to be made to slot
`assignments in accordance with a received request from an
`ONU for more bandwidth, or a received indication that a
`particular ONU no longer needs all of the bandwidth
`assigned to it.
`In the Specific embodiment, each upstream slot manage
`ment message contains four fields: 1) a 1-byte message type
`field; 2) a 1-byte ONU identification field for indicating the
`particular ONU to which the message is associated; 3) a
`6-byte message contents field containing the message, and
`4) a cyclical redundant code (CRC) field for error correction.
`The specified sizes of each field are merely illustrative for
`the embodiment in which 32 ONUs are supported per PON
`35
`and the 2430 byte maximum size of a burst. Three message
`types are used for upstream slot management: 1) an assign
`ment message, which is used for the assignment of a slot to
`a particular ONU; 2) a modification message, which is used
`for the modification of an already assigned slot; and 3) an
`idle message, which is used for unused messages of a
`multi-message PLOAM cell.
`An assignment message contains three fields: 1) a 2-byte
`Start of Slot location, which is the byte offset into each
`upstream frame identifying the first byte of the assigned slot;
`2) a 2-byte burst payload size, which is the number of
`bytes per burst excluding the three overhead bytes; and 3)
`and a 2-byte ds0 channels, which is the number of leading
`bytes per upstream burst payload representing DS0 voice
`channels, one DS0 channel per payload byte. ASSignment
`messages are used to assign an upstream slot to newly
`installed ONUs. In addition, they are used to confirm an
`existing assignment as a fault recovery mechanism.
`A modification message is used to change the length of an
`existing upstream Slot assignment, and/or the number of
`DSO voice channels. These messages also move the location
`of all assigned slots located after that modified slot in the
`frame Since increasing or decreasing a slot Size necessitates
`changing the Starting byte position of each slot that follows
`as well a possibly the size of one or more slots. The
`modification message contains three fields: 1) a 2-byte
`Start of change location, which is the byte offset into each
`frame identifying the first byte position to be changed and is
`typically the Start-of-slot location of the slot being changed;
`2) a 2-byte change size, which is a signed quantity indi
`cating how many bytes the identified slot is being increased
`or decreased; and 3) a 2-byte ds0 channels, which as in the
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`8
`assignment message is the number of leading bytes per
`upstream burst payload representing DS0 channels, one DS0
`channel being allocated per payload byte. In response to a
`modification message, the targeted ONU (identified by an
`ONU identification parameter) changes its assigned slot size
`(i.e., its burst payload) by the change size parameter.
`Every other ONU, which also receives the broadcast
`message, compares its current Start of slot boundary to the
`Start of change parameter contained in every modification
`message (targeted to a different ONU). If it has a larger
`Start of slot boundary than the Start-of-change parameter,
`it modifies its start-of-slot location by the amount Specified
`by the change size parameter. Otherwise the modification
`message is ignored. Therefore, as noted, the modification
`message not only changes the length of the targeted slot, it
`also appropriately moves all Slots that are located after that
`targeted Slot in the frame. The modification messages are the
`primary mechanism for reallocating upstream bandwidth as
`part of the bandwidth management process.
`For an idle message, only the message type and the CRC
`are used, the remaining Seven bytes being unused.
`An ONU that becomes idle can either have its upstream
`slot removed, making that bandwidth available for active
`ONUs, or it can maintain its slot at a minimum length of
`three bytes (the burst header, with no payload). The former
`disadvantageously results in a continually changing list of
`upstream slot assignments and also requires a mechanism
`for reactivating idle ONUs. The latter minimizes these
`problems and imposes a maximum impact of only