(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0110245 A1
`(43) Pub. Date:
`Aug. 15, 2002
`Gruia
`
`US 2002O110245A1
`
`(54)
`
`(76)
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`(21)
`(22)
`
`METHOD AND SYSTEM FOR
`SYNCHRONIZING SECURITY KEYS INA
`POINT-TO-MULTIPOINT PASSIVE OPTICAL
`NETWORK
`
`Inventor: Dumitru Gruia, San Ramon, CA (US)
`Correspondence Address:
`Mark A. Wilson
`Wilson & Ham
`PMB: 348
`2530 Berryessa Road
`San Jose, CA 95132 (US)
`Appl. No.:
`09/783,239
`
`Filed:
`
`Feb. 13, 2001
`
`Publication Classification
`
`(51)
`
`Int. Cl. ............................ H04L 9/00; H04K 1/00
`
`(52) U.S. Cl. ............................................ 380/278; 380/274
`
`(57)
`
`ABSTRACT
`
`Security key Synchronization is maintained between nodes
`in an optical communications System utilizing out-of-band
`Signaling to indicate that a new key is being used to encrypt
`Subsequent information blocks at the transmitting point and
`that the new key should be used to decrypt Subsequent
`information blocks at the receiving point. A Switch-to-new
`key code can be Selected from a group of unused codes in an
`eight bit to ten bit encoding Scheme. The Switch-to-new-key
`code can replace an idle code that is used to create Sufficient
`spacing between information blockS. Receipt of the Switch
`to-new-key code indicates that the new key is being used to
`encrypt Subsequent information blocks at the transmitting
`point and triggers a Switch to the new key for decrypting
`Subsequent information blocks at the receiving point.
`
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`1
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`Patent Application Publication Aug. 15, 2002 Sheet 1 of 10
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`US 2002/0110245 A1
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`Patent Application Publication Aug. 15, 2002 Sheet 2 of 10
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`US 2002/0110245 A1
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`Patent Application Publication Aug. 15, 2002 Sheet 3 of 10
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`Patent Application Publication Aug. 15, 2002 Sheet 4 of 10
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`US 2002/0110245 A1
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`Patent Application Publication Aug. 15, 2002 Sheet 5 of 10
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`US 2002/0110245 A1
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`Patent Application Publication Aug. 15, 2002. Sheet 6 of 10
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`US 2002/0110245 A1
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`Patent Application Publication Aug. 15, 2002. Sheet 7 of 10
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`Patent Application Publication Aug. 15, 2002. Sheet 8 of 10
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`Patent Application Publication Aug. 15, 2002. Sheet 9 of 10
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`Patent Application Publication Aug. 15, 2002. Sheet 10 of 10
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`US 2002/0110245 A1
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`Distribute a new key between a first node and a second node
`
`Signal, to one of the first and Second nodes, a Switch to a new key with a
`Switch-to-new-key code that is not part of a header or a payload of any
`information blocks that are transmitted between the first and second nodes
`FIG. 13
`1304
`
`Generate a new key at either a Source node or a destination node
`1402
`
`Transmit the new key from the node where the new key was
`generated to the other of the Source and destination nodes
`
`
`
`Generate a Switch-to-new-key code that is not part of the
`header or the payload of any information blocks that are
`transmitted from the Source node to the destination node
`
`1404
`
`1406
`
`Transmit the Switch-to-new-key code from the SOurce node
`to the destination node
`
`1408
`Encrypt, with the new key, the payload of the information blocks that are
`transmitted from the Source node after the Switch-to-new-key code is transmitted
`1410
`Decrypt, with the new key, the payload of the information blocks that are
`received at the destination node after the Switch-to-new-key code is received
`FIG. 14
`1412
`
`11
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`

`

`US 2002/0110245 A1
`
`Aug. 15, 2002
`
`METHOD AND SYSTEM FOR SYNCHRONIZING
`SECURITY KEYS IN A POINT-TO-MULTIPOINT
`PASSIVE OPTICAL NETWORK
`
`FIELD OF THE INVENTION
`0001. The invention relates generally to broadband opti
`cal communications networks, and more particularly to
`encryption messaging in point-to-multipoint passive optical
`networks.
`
`BACKGROUND OF THE INVENTION
`0002 The explosion of the Internet and the desire to
`provide multiple communications and entertainment Ser
`vices 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 communica
`tions 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.
`0003 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 informa
`tion 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. Because all of the ONU-specific
`information blocks are transmitted to each ONU, it is
`possible for each ONU to read information blocks that are
`intended for the other ONUs. In order to prevent ONU
`Specific information blocks from being read by the wrong
`ONUs, the information blocks intended for each ONU are
`encrypted and decrypted with encryption/decryption keys
`that are specific to each ONU. For example, information
`blocks intended for ONU-1 are encrypted and decrypted
`with a key that is specific to ONU-1, information blocks
`intended for ONU-2 are encrypted and decrypted with a key
`that is specific to ONU-2, and information blocks intended
`for ONU-3 are encrypted and decrypted with a key that is
`specific to ONU-3. Although ONU-1 receives encrypted
`information blockS 1, 2, and 3, it can only decrypt informa
`tion block 1 with its ONU-specific key. Likewise, ONU-2
`can only decrypt information block 2 and ONU-3 can only
`decrypt information block 3.
`0004 Although encrypting and decrypting downstream
`information blocks with ONU-specific keys works well to
`create Secure downstream connections between the OLT and
`each ONU, the longer the same key is used to encrypt and
`decrypt a stream of information blocks, the easier it is for an
`intruder to figure out the key and decrypt the encrypted
`information blocks. One technique for improving a Secure
`downstream connection between an OLT and an ONU
`involves continuously changing the key used between the
`OLT and the ONU for encryption and decryption. While
`
`continuously changing the key used between an OLT and an
`ONU improves security, the OLT and the ONU must be
`continuously Synchronized So that they are always using the
`Same key to encrypt and decrypt the same information
`blocks. If the OLT and the ONUs are not using the same keys
`to encrypt and decrypt the same information blocks, then the
`ONU will not be able to decrypt the encrypted downstream
`information blocks.
`0005. In an ATM based point-to-multipoint PON as
`described in the Full Service Access Network (FSAN)
`specification 983.1 developed through the International
`Telecommunications Union (ITU), Security messages are
`exchanged between the OLT and the ONUs in 53 byte ATM
`cells that are dedicated to carrying operations and mainte
`nance (OAM) information (OAM cells). According to the
`FSAN specifications and as depicted in FIG. 2, a key
`request 208 is sent in an OAM cell from the OLT to an ONU.
`In response to the key request, the ONU sends a new key 210
`to the OLT in another OAM cell. Once the key has been sent
`to the OLT, the OLT sends a key synchronization signal 212
`(in an OAM cell), which causes the ONU to switch to the
`new key for decrypting Subsequent downstream cells. The
`ONU sends an acknowledge signal 214 to the OLT in an
`OAM cell to acknowledge that the key switch has been
`made. The process of passing a key and Synchronizing the
`key Switch is repeated for each ONU that is connected to the
`OLT.
`0006 Although the Security messaging technique speci
`fied in the FSAN specification works well, the security
`messaging transmissions consume bandwidth that could be
`used for other data transmissions. While the amount of
`bandwidth consumed by Security messaging may be Small
`for a single exchange between an OLT and an ONU, the
`amount of bandwidth consumed by Security messaging
`increases directly with the number of ONUs in the point
`to-multipoint PON and with the rate of key changing.
`0007. In view of the bandwidth consumed by security
`messaging, what is needed is a Security messaging System
`that consumes less bandwidth.
`
`SUMMARY OF THE INVENTION
`0008. A method and system for maintaining security key
`Synchronization between nodes in a communications System
`involves utilizing out-of-band Signaling to indicate that a
`new key is being used to encrypt Subsequent information
`blocks at the transmitting point and that the new key should
`be used to decrypt Subsequent information blocks at the
`receiving point. In an embodiment, a Switch-to-new-key
`code is Selected from a group of unused codes in an eight bit
`to ten bit encoding Scheme. The Switch-to-new-key code
`replaces an idle code that is used to create Sufficient spacing
`between information blocks. Receipt of the Switch-to-new
`key code indicates that the new key is being used to encrypt
`Subsequent information blocks at the transmitting point and
`triggers a Switch to the new key for decrypting Subsequent
`information blocks at the receiving point.
`0009 Amethod for maintaining synchronization between
`a key used by a first node to encrypt information and a key
`used by a Second node to decrypt information includes
`distributing a new key between a first node and a Second
`node, Signaling, to one of the first and Second nodes, a Switch
`to the new key with a Switch-to-new-key code that is not part
`
`12
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`US 2002/0110245 A1
`
`Aug. 15, 2002
`
`of the header or the payload of any of the information blocks
`that are being transmitted between the first and Second
`nodes.
`0010. In an embodiment of the method, the first node is
`an optical line terminal (OLT) of a point-to-multipoint
`optical communications network and the Second node is one
`of multiple optical network units (ONUs) in the point-to
`multipoint optical communications network. A further
`embodiment of the method includes a step of broadcasting
`the Switch-to-new-key code to all of the multiple ONUs. A
`further embodiment of the method includes a step of Switch
`ing to new keys at the ONUs in response to the broadcast of
`the Switch-to-new-key code. In an embodiment, information
`is formatted according to the IEEE 802.3 protocol. In an
`embodiment, an unused ten bit code in an eight bit to ten bit
`encoding Scheme is used to generate the Switch-to-new-key
`code. In an embodiment, an idle code between two packets
`is replaced with the Switch-to-new-key code.
`0.011) A system for maintaining synchronization between
`a key used by a first node to encrypt information and a key
`used by a Second node to decrypt information includes an
`OLT and a group of ONUs. The OLT includes an encryption
`controller and a key Synchronization unit. The encryption
`controller encrypts information within information blockS
`using ONU-Specific keys. The key Synchronization unit
`generates a Switch-to-new-key code that is not part of a
`header or a payload of any information blocks that are
`transmitted from the OLT to the group of ONUs and causes
`the OLT encryption controller to use new ONU-specific keys
`to encrypt information within information blocks that are
`transmitted after the Switch-to-new-key code is transmitted
`to the group of ONUs. Each of the ONUs includes a key
`generator, an ONU encryption controller, and a key Syn
`chronization unit. The key generator generates a new ONU
`specific key that is transmitted to the OLT. The ONU
`encryption controller decrypts information within informa
`tion blocks using an ONU-Specific key and the key Synchro
`nization unit identifies the Switch-to-new-code that is trans
`mitted from the OLT and causes the ONU encryption
`controller to use the new ONU-specific key to decrypt
`information within the information blocks after the Switch
`to-new-key code is received from the OLT.
`0012. Other aspects and advantages of the present inven
`tion 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
`0013 FIG. 1A depicts the downstream flow of traffic
`from an OLT to multiple ONUs in a point-to-multipoint
`PON.
`0014 FIG. 1B depicts the upstream flow of traffic from
`multiple ONUs to an OLT in a point-to-multipoint PON.
`0.015 FIG.2 depicts the security messaging protocol that
`is defined by the FSAN specification in accordance with the
`prior art.
`0016 FIG. 3 depicts a point-to-multipoint PON with a
`tree topology.
`0017 FIG. 4 depicts functional blocks of an OLT that is
`used to carry out Security messaging, in accordance with an
`embodiment of the invention.
`
`0018 FIG.5 depicts functional blocks of an ONU that is
`used to carry out Security messaging, in accordance with an
`embodiment of the invention.
`0019 FIG. 6 depicts a security messaging technique that
`utilizes out-of-band Signaling to maintain Synchronization
`between keys used to encrypt and decrypt information in
`accordance with an embodiment of the invention.
`0020 FIG. 7 depicts six consecutive idle codes that
`separate packets as required by the 1000BASE-X specifi
`cation of the IEEE 802.3 protocol.
`0021
`FIG. 8 depicts a Switch-to-new-key code that has
`been inserted between two packets in the place of an idle
`code in accordance with an embodiment of the invention.
`0022 FIG. 9 depicts multiple switch-to-new-key codes
`that have been inserted between two packets in the place of
`idle codes in accordance with an embodiment of the inven
`tion.
`0023 FIG. 10 depicts Switch-to-new-key codes that have
`been inserted in the place of idle codes in at least two
`different idle Spaces between packets in accordance with an
`embodiment of the invention.
`0024 FIG. 11 depicts a Switch-to-new-key code that is
`inserted at the beginning of an upstream time slot in accor
`dance with an embodiment of the invention.
`0025 FIGS. 12A-12C depict an embodiment of an
`encryption messaging technique for two-way encryption
`that utilizes out-of-band signaling for key Synchronization.
`0026 FIG. 13 is a process flow diagram of a method for
`maintaining Security key Synchronization in accordance
`with an embodiment of the invention.
`0027 FIG. 14 is a process flow diagram of a method for
`maintaining Security key Synchronization in accordance
`with another embodiment of the invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`0028. A method and system for maintaining security key
`Synchronization between nodes in a communications System
`involves utilizing out-of-band Signaling to indicate that a
`new key is being used to encrypt Subsequent information
`blocks at the transmitting point and that the new key should
`be used to decrypt Subsequent information blocks at the
`receiving point. In an embodiment, a Switch-to-new-key
`code is Selected from a group of unused codes in an eight bit
`to ten bit encoding Scheme. The Switch-to-new-key code
`replaces an idle code that is used to create Sufficient spacing
`between information blocks.
`0029 Receipt of the Switch-to-new-key code indicates
`that the new key is being used to encrypt Subsequent
`information blocks at the transmitting point and triggerS a
`Switch to the new key for decrypting Subsequent information
`blocks at the receiving point.
`0030 FIG. 3 depicts an example point-to-multipoint
`PON 300. The point-to-multipoint PON includes an OLT
`302 and multiple ONUs 304 that are connected by a passive
`optical distribution network. In an embodiment, the OLT is
`connected to a service station 310 Such as a Central Office
`and/or a head-end Station. Services provided at the Service
`
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`US 2002/0110245 A1
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`Aug. 15, 2002
`
`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).
`0031. The passive optical distribution network shown in
`FIG. 3 has a tree topology that includes a common optical
`fiber 310 (trunk fiber) and multiple different fibers 316 that
`are connected by a passive optical Splitter/coupler 312 to the
`trunk fiber. An optical Signal transmitted in the downstream
`direction (from the OLT 302 to the ONUs 304) is optically
`Split into multiple ONU-specific optical Signals that all carry
`the Same information. Because of the broadcast nature of
`downstream transmissions in a point-to-multipoint PON, all
`of the ONUs always receive the same information from the
`OLT. Although all of the ONUs receive the same informa
`tion from the OLT, the actual receipt time of the Signals may
`vary slightly from ONU to ONU because of differences in
`travel distances.
`0.032
`Optical signals transmitted in the upstream direc
`tion (from the ONUs to the OLT) are optically coupled into
`the trunk fiber that is connected between the coupler and the
`OLT. The coupler is a directional coupler that passes
`upstream transmissions from the ONUs to the OLT and does
`not allow upstream transmissions to be received by any
`other ONUs. Time division multiplexing is utilized in the
`upstream direction to prevent collisions of upstream trans
`missions from two or more ONUs.
`0033. In the embodiment of FIG. 3, an optical signal in
`the downstream direction is transmitted at a different wave
`length (or frequency) than an optical signal in the upstream
`direction. In an embodiment, downstream traffic is transmit
`ted 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 down
`Stream and upstream fiberS may be utilized for the passive
`optical distribution network. In addition, wavelength divi
`sion multiplexing (WDM), multi-state modulation beyond
`the binary State, or other techniques may be used in the
`downstream and/or upstream directions to increase trans
`mission bandwidth.
`0034. Although the passive optical distribution network
`of FIG.3 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. 3 depicts only single fiber
`connections between network components, redundant fibers
`may be added between network components to provide fault
`protection.
`0035 FIG. 4 is an expanded view of an example OLT
`402 in the point-to-multipoint PON 300 of FIG. 3. Func
`tional units included within the OLT that are used to carry
`out Security messaging are a packet controller 420, a key
`
`generator 422, an encryption controller 424, a key Synchro
`nization unit 426, an optical transmitter 428, and an optical
`receiver 430. 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 information blockS
`referred to as packets. The packet controller may be embod
`ied in hardware and/or Software and is Sometimes referred to
`as the media access control (MAC) unit. In an embodiment,
`each 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 packets are formatted accord
`ing 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 packets are transmitted
`over optical fiber at a rate of 1 gigabit per Second (Gb/s) as
`defined by IEEE 802.3Z (commonly referred to as gigabit
`Ethernet) using the 1000BASE-X specification. Lower or
`higher transmission rates may be utilized in other embodi
`mentS.
`0036) The key generator 422 is a functional unit that
`generates new keys for encryption and decryption. Typi
`cally, the key generator uses a random number generator to
`generate new keys. The encryption controller 424 is a
`functional unit that encrypts and decrypts the information
`within packets. In an embodiment, only the payload portions
`of packets are encrypted and decrypted although in other
`embodiments entire packets are encrypted and decrypted.
`When entire packets are encrypted, all of the received
`packets are decrypted and checked to see if they are valid
`packets that are intended for the respective ONU. In a
`System that implements only downstream encryption, the
`encryption controller of the OLT only performs encryption.
`In a System that implements downstream and upstream
`encryption, the encryption controller of the OLT performs
`both downstream encryption and upstream decryption. The
`key Synchronization 426 unit is a functional unit that main
`tains Synchronization between the keys that are used to
`encrypt information within packets and the keys that are
`used to decrypt information within packets. Example
`embodiments of the key Synchronization proceSS are
`described below with reference to FIGS. 6-13.
`0037. The optical transmitter 428 and the optical receiver
`430 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. 5 is an expanded view of an example
`ONU 504 in the point-to-multipoint PON 300 of FIG. 3.
`Functional units included within the ONUs that are used to
`carry out Security messaging are a packet controller 520, a
`key generator 522, an encryption controller 524, a key
`synchronization unit 526, an optical transmitter 528, and an
`optical receiver 530. 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 informa
`tion blocks referred to as packets, with each 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 down
`Stream traffic, in an embodiment, the upstream packets are
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`formatted according to the IEEE 802.3 standard and trans
`mitted at a rate of 1 Gb/s. Although ONU refers to optical
`network unit, ONU may also refer to a functionally equiva
`lent optical node unit.
`0.038. The key generator 522 is a functional unit that
`generates new ONU-specific keys for encryption and
`decryption. Typically, the key generator uses a random
`number generator to generate new ONU-specific keys. The
`encryption controller 524 is a functional unit that encrypts
`and decrypts the information within packets. In an embodi
`ment, only the payload portions of packets are encrypted and
`decrypted. In a System that implements only downstream
`encryption, the encryption controller of the ONU decrypts
`encrypted packets. In a System that implements downstream
`and upstream encryption, the encryption controller performs
`both downstream decryption and upstream encryption. The
`key Synchronization 526 unit is a functional unit that main
`tains Synchronization between the keys that are used to
`encrypt information within packets and the keys that are
`used to decrypt information within packets. Example
`embodiments of the key Synchronization proceSS are
`described below with reference to FIGS. 6-13.
`0039. The optical transmitter 528 and the optical receiver
`530 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.
`0040 FIG. 6 depicts an embodiment of a method for
`Security messaging in a point-to-multipoint PON that ulti
`lizes “out-of-band' Signaling to maintain Synchronization
`between keys used to encrypt and decrypt information. In
`the embodiment of FIG. 6, a new key request is generated
`by the encryption controller of the OLT for each ONU and
`the new key requests are transmitted from the OLT to the
`ONUs. In an embodiment, the new key requests are carried
`in packets that are addressed to specific ONUS. AS Shown in
`FIG. 6, a new key request 608 is transmitted from the OLT
`in an Ethernet packet having a header and a payload. In
`response to the ONU-Specific key requests, the key genera
`tor of each individual ONU generates a new ONU-specific
`key 610 and the new ONU-specific key is transmitted
`upstream to the OLT. In an embodiment, the new ONU
`Specific keys are transmitted upstream in the payload of
`packets. Referring to FIG. 3, a new ONU-specific key is
`transmitted from each of the ONUs in the point-to-multi
`point PON.
`0041. Once new ONU-specific keys have been passed
`from all of the ONUs to the OLT, the key synchronization
`unit of the OLT initiates a system-wide Switch to the new
`ONU-specific keys. The key synchronization unit of the
`OLT initiates the Switch to the new ONU-specific keys by
`generating and transmitting a Switch-to-new-key code that is
`not part of any of the packets that are being transmitted to
`the ONUs. That is, the Switch-to-new-key code is a special
`code that is transmitted between packets and that does not
`conform to a packet format. Referring to FIG. 6, an example
`Switch to-new-key code 616 is represented as a Signal that is
`transmitted between two packets. Embodiments of the
`Switch-to-new-key code are described below in more detail.
`0042. Once the Switch-to-new-key code is transmitted
`from the OLT, the encryption controller of the OLT encrypts
`Subsequently transmitted ONU-Specific packets using the
`
`new ONU-specific keys that were previously supplied to the
`OLT. Once the Switch-to-new-key code is received by the
`ONUs and identified by the respective key synchronization
`unit, the key Synchronization unit causes the encryption
`controller of the ONU to decrypt subsequent packets with
`the new ONU-specific key. The process of Switching keys is
`continuously repeated to prevent the same key from being
`used for an extended period of time.
`0043. As described above, an embodiment of the system
`and method utilizes gigabit Ethernet over optical fiber. The
`IEEE 802.3 specification for gigabit Ethernet over single
`mode and multimode mode optical fiber is defined in the
`1000BASE-X specification. The 1000BASE-X specification
`uses an eight bit-to-ten bit (8B/10B) encoding scheme in
`which eight bits of data (one byte) are encoded into ten bit
`codes. Among other reasons, the 8B/10B encoding is imple
`mented to ensure Sufficient Signal transitions for clock
`recovery at the receiver. Because eight bits can represent 256
`different data values while ten bits can represent 1,024
`different data values, there are more ten bit codes available
`than there are values to encode. According to the
`1000BASE-X specification, the available code space is
`divided into two groups of codes, the "D' group of codes
`and the “K” group of codes. The "D" group of codes are
`used to encode data bytes and the “K” group of codes (also
`referred to as the special codes) are used to encode special
`control characters. The Special codes are interpreted at the
`physical layer and provide for “out-of-band' Signaling, that
`is signaling that is not part of a packet. In order to ensure
`DC-balance in a bitstream, each byte value and each special
`code is represented by two different ten bit codes. Although
`there are two different ten bit codes designated for each byte
`value and for each special code, there are still many codes
`available that exhibit Sufficient Signal transitions and that
`have not been designated for use as a byte value or a special
`code by IEEE 802.3.
`0044) In addition to the 8B/10 encoding, the 1000BASE
`X Specification requires that each packet in a transmission be
`Separated by a minimum amount of time (96 us) in order to
`allow receivers enough time to recover between packets and
`to prepare to receive the next packet. Referring to FIG. 7,
`the minimum amount of Spacing between packets is created
`using a Series of Special codes referred to as idle codes 720.
`According to the 1000BASE-X specification, an idle code
`can be an idle 1 code (I1) or an idle 2 code (I2). The I1 and
`I2 codes each include two code words (/K28.5/D5.6/ and
`/K28.5/D16.2/, respectively) and the minimum spacing
`between packets of 96 uS is created by inserting at least Six
`consecutive idle codes between packets. In FIG. 7, each
`packet 722 is bordered by start-of packet (SOP) and end
`of-packet (EOP) control signals 724 and 726. The inner
`portion of the packet is defined as an “in-band' signal and
`the SOP, EOP, and idle codes are defined as “out-of-band'
`Signals. Both the in-band and out-of-band Signals are trans
`mitted using the same carrier wavelength.
`0045. In an embodiment of the method and system for
`maintaining key Synchronization, at least one of the unused
`ten bit code words is used to generate the Switch-to-new-key
`code. In an embodiment, the Switch-to-new-key code
`includes two ten bit code words so that the Switch-to-new
`key code has the same bit length as the idle codes. The
`Switch-to-new-key code is inserted in the place of one of the
`Six idle codes to initiate key Switching with an out-of-band
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`Signal. The Switch-to-new-key code indicates that Subse
`quent packets are encrypted using the new key and therefore
`should be decrypted using the new key. FIG. 8 depicts a
`Switch-to-new-key code 830 that has been inserted between
`two packets in the place of an idle code. AS described above,
`the purpose of the idle codes is to provide a minimum
`amount of Spacing between packets. By replacing an idle
`code with a Switch-to-new-key code of equal bit length, the
`minimum spacing between packets is main