United States Patent
`
`[19]
`
`4,533,948
`[11] Patent Number:
`McNamara et al.
`[45] Date of Patent:
`Aug. 6, 1985
`
`
`[54] CATV COMMUNICATION SYSTEM
`
`[75]
`
`Inventors: Robert P- McNamara; Gregory 3-
`Ennis, both of San Jose; Richard J.
`Feiertag, Sunnyvale; Robert K.
`Bauer, Fremont, all of Calif.
`
`.
`[73] ASSIgnee=
`
`_
`Seller? 1:53??th Corporation,
`ew or ,
`.
`.
`
`1le APPI- N04 373,755
`[22] Filed:
`Apr. 30, 1982
`
`Int. 01.3 ............................................... H04N 7/16
`[51]
`[52] U.S. C1. ...................................... 358/122; 358/86;
`.
`353/114; 455/5; 340/8255; 375/2-1
`[58] Fleld of Search ......................... 358/114, 122, 86;
`375/2-2’ 2-1; 340/8255; 364/200! 900;
`178/2201, 22-02, 22'03’ 220:” 22‘05’ 2211)?
`22221212231058,2222969522211170’2222118’2222-1192-{525/5’
`‘
`’
`'
`’
`'
`’
`'
`’
`‘
`’
`‘
`’
`6 3:
`’
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`‘
`:nesen ................................. 137588/58g
`é/i9'619
`ace """""" /
`/ 9 2
`7/1973 Currey
`325/53
`1/1974 McVoy ..
`340/151
`3/1974 Feistel
`178/2208
`4/1974 Osborne ................................ 325/53
`
`
`
`3,4:3,§21
`3’6 8’ 07
`3 750 022
`3:786:424
`3,798,605
`3,803,491
`
`3,859,596
`1/1975 Jannery ................................. 325/31
`3,934,079
`
`1/1976 Barnhart .....
`178/51
`
`3,943,447
`.. 325/308
`3/1976 Shomo, 111 .
`3,997,718 12/1976 Ricketts ......
`178/6.8
`
`
`4,031,543
`6/1977 Holz
`358/86
`8/1977 Ellis ........................
`.. 325/308
`4,041,398
`
`1/1981 Matsumoto .................. 358/122
`4,245,245
`........................ 178/2208
`4,310,720
`1/1982 Check, Jr.
`Primary Examiner—Robert L. Griffin
`Assistant Examiner—Timothy K. Greer
`Attorney, Agent, or Firm—Allan J. Jacobson
`
`ABSTRACT
`1571
`A two way digital communication arrangement utilizes
`a CATV system to provide bidirectional data transport
`service between any two points Within the CATV sys-
`term. The headend receives an upstream message and
`selectively rebroadcasts such message on the down-
`stream portion of the spectrum. System intelligence is
`thus distributed throughout the system as server and
`subscriber nodes can be located anywhere in the CATV
`network. In order to obtain access to the CATV com-
`munication resources, user equipment at each node
`must attach a frame verifier (FV) code to each respec-
`tive message. The headend examines the FV and per_
`mits rebroadcast of messages only if the FV code indi-
`.
`,
`cates that the user ‘5 “thawed-
`
`11 Claims, 17 Drawing Figures
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`Petitioner Cisco Systems - Exhibit 1012 - Page 1
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 1
`
`

`

`US. Patent Aug. 6, 1985
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`Petitioner Cisco Systems - Exhibit 1012 - Page 2
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 2
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 3
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 4
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 4
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 5
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 5
`
`
`

`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 6
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 6
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 7
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 7
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 8
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 8
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 9
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 9
`
`

`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 10
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 10
`
`

`

`US. Patent Aug. 6, 1985
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`Petitioner Cisco Systems - Exhibit 1012 - Page 11
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 11
`
`

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`Petitioner Cisco Systems - Exhibit 1012 - Page 12
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 12
`
`

`

`US. Patent Aug. 6, 1985
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`Petitioner Cisco Systems - Exhibit 1012 - Page 13
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 13
`
`

`

`1
`
`CATV COMMUNICATION SYSTEM
`
`FIELD OF THE INVENTION
`
`4,533,948
`
`This invention relates to digital communication utiliz-
`ing a two way cable television (CATV) network.
`BACKGROUND OF THE INVENTION
`
`Two way CATV systems are well known. Tech-
`niques for utilizing the bidirectional nature of such net-
`works for digital data transmission have been devel-
`oped. For example see US. Pat. No. 3,803,491 to Os-
`born and US. Pat. No. 4,245,245 to Matsumato et al. A
`wide variety of consumer services such as home bank-
`ing, electronic mail and newspapers, shop at home, and
`the like, are envisioned to become commonplace.
`However, the systems developed to date have failed
`to achieve widespread use. One of the reasons for the
`lack of general acceptance is that prior art systems cen-
`tralize digital communication at
`the headend of the
`CATV system. That is digital messages are exchanged
`between the headend and the user nodes. Such concen-
`tration of network intelligence at the headend node has
`several disadvantages.
`Firstly, a centralized network design requires that
`many participants, particularly the cable operator, the
`service provider, and the equipment manufacturer, un-
`dertake coordinated activities simultaneously to assure
`that equipment and data formats are compatible. The
`reluctance of each individual party to act until a settled
`system architecture emerges has been an important
`factor in the delayed development of two way CATV
`data services. Also, a centralized network architecture
`results in complex and cumbersome headend equip-
`ment. The headend software in such prior art systems is
`typically multi-tasking in order to process different data
`services simultaneously. Therefore, adding new ser~
`vices to existing services can be difficult. Furthermore,
`as entirely new services are added to the system, the
`capability of a centralized system may be exceeded,
`requiring that the entire headend architecture be rede-
`signed to accommodate all of the desired services.
`Furthermore, system reliability is compromised when
`system intelligence is centralized: A single failure at the
`headend can disable all of the two way CATV services.
`Finally,
`in a centralized system,
`the cable system
`operator is closely involved with the service providers
`and is burdened with such problems as information
`privacy, data integrity and disputes over rights of access
`to consumers by competing service providers.
`SUMMARY OF THE INVENTION
`
`The present invention is embodied in a decentralized
`communication arrangement wherein a node originat-
`ing a message (a source node) and a node receiving a
`message (a destination node) can be located at any re-
`spective points in the CATV system.
`In accordance with one aspect of the present inven-
`tion a source node transmits a message towards the
`headend in the upstream portion of the cable spectrum.
`The headend selectively rebroadcasts the upstream
`message in the downstream portion of the cable spec-
`trum, thus providing an arrangment whereby a source
`node is able to transmit a message to a destination node,
`wherever located.
`The CATV communication network is deployed to
`the general public. Therefore, another aspect of the
`present invention includes a mechanism by which the
`
`2
`access to CATV communication resources is controlled
`so that'unauthorized users are denied access and autho-
`rized users are granted access. In accordance with the
`latter aspect of the present invention, a source node
`further transmits a verification message, referred to
`herein as a frame verifier (FV) code, as part of the
`upstream message.
`In accordance with yet another aspect of the present
`invention, the headend apparatus examines the frame
`verifier code and rebroadcasts the received upstream
`message in the downstream portion of the cable spec-
`trum only if the frame verifier code indicates that the
`source node is an authorized user thereby granting the
`user access to the CATV resources. Conversely, the
`headend apparatus does not rebroadcast the upstream
`message if the frame verifier message indicates that the
`source node is not an authorized user thereby denying
`the user access to the CATV resources.
`
`10
`
`15
`
`20
`
`ADVANTAGES OF THE PRESENT INVENTION
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`As previously stated, centralized network designs
`have not been widely established, in part because such
`designs require many participants to undertake coordi-
`nated activities simultaneously. Using the present inven-
`tion, the CATV system operator can provide transpar-
`ent data transport service, which data transport service
`can in turn be utilized by individual entreprenturs, or
`the CATV operator, to provide specific value added
`services.
`Therefore, a communication arrangement embodying
`the present invention provides for decentralized system
`intelligence. System growth at the headend or at the
`nodes is modular, following either rapid or slow devel-
`opment of the overall system.
`In other words,
`the
`CATV system operator can establish a communication
`system offering defined interfaces for transparent data
`transport service at the user nodes. The channel capac-
`ity of the headend may be expanded, but its architecture
`(both hardware and software) remains the same as the
`overall system develops. The further development of
`various consumer services and information appliances,
`both of known types, and of those yet to be invented,
`can continue at the node interfaces, and without further
`architectural changes at the headend.
`Decentralized network intelligence in accordance
`with the present invention results in less complex hea-
`dend equipment. The headend can be initially equipped
`with a few data channels. Additional data channel ca-
`pacity can be easily added as the communication data
`traffic load increases.
`New services are readily accommodated in the pres-
`ent system by adding equipment at the server nodes
`which may be located anywhere in the network. For
`example, a new server node for electronic funds transfer
`can be located at the bank providing such service.
`Although the complexity of each server node de-
`pends on the complexity of the specific service, server
`node software will generally be simplified (compared to
`a centralized system) due to the single task nature of a
`single service.
`System reliability is enhanced by use of the present
`invention because equipment failure at one server node
`affects only that service and does not interrupt the ser-
`vices provided by the remaining server nodes. Simi-
`larly, a heavy data traffic load for one service does not
`substantially effect the service response time of the
`other server nodes.
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 14
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 14
`
`

`

`4,533,948
`
`4
`nals. However, a signal transmitted from an individual
`subscriber terminal in the return direction is heard only
`by that portion of the other subscriber terminals that are
`in the signal propagation path from the transmitting
`subscriber terminal to the headend. Therefore, the sig-
`nal transmitted from a particular subscriber terminal is
`heard by only a portion of the other subscriber termi-
`nals.
`In accordance with the present invention, each signal
`frequency in the return band 2 is paired with a corre.
`sponding signal frequency in the forward band 4. The
`headend includes apparatus for receiving the return
`signal in the upstream band, and selectively rebroad-
`casting the signal at a higher corresponding frequency
`in the forward band. In such manner, a signal from an
`individual subscriber terminal (in the return band) is
`heard by all the other subscriber terminals (in the for-
`ward band)
`thereby permitting any individual sub-
`scriber terminal
`to transmit a message to any other
`subscriber terminal within the CATV system.
`Digital signals are transmitted in the present system
`by the use of frequency shift keyed (FSK) modulation.
`A digital signal has one of two binary logic states, i.e. 1
`or 0. When the digital signal is at a logical 1, an FSK
`modulator transmits a signal of first frequency and
`when the binary signal is at a logical 0, an FSK modula-
`tor transmits a signal at a second frequency. Similarly an
`FSK demodulator is responsive to an FSK signal to
`reproduce the original digital signal.
`A headend apparatus in accordance with the present
`invention is shown in FIG. 2. A data channel access
`monitor (DCAM) 10 comprises individual data channel
`access monitor modules 11a, 11b, etc., a network access
`controller interface processor 18 and a modem 20. An
`individual data channel access monitor module 11a
`comprises FSK demodulator 12, frame verifier logic 14,
`and FSK modulator 16. The FSK demodulator 12 is
`tuned to a particular frequency in the return band. The
`FSK modulator 16 is tuned to a corresponding paired
`frequency in the forward band. The frame verifier logic
`14 examines the received data and selectively connects
`the output of the FSK demodulator 12 to the input of
`the FSK modulator 16. The FSK modulator 16 re-
`broadcasts the received signal in real time at a corre-
`sponding higher frequency in the forward portion of the
`cable spectrum. The frame verifier logic 14 also inter-
`faces with the network access controller interface pro-
`cessor 18 which provides two way communication to
`the network access controller via modem 20. The oper-
`ation of the frame verifier logic 14 will be described in
`more detail in conjunction with the description of FIG.
`8.
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`Finally, a decentralized intelligence communication
`system tends to disassociate the CATV operator from
`the service provider. Issues involving information pri-
`vacy and data integrity become the responsibility of the
`service vendor. The CATV operator simply offers
`transparent data transport service to be used as desired
`by the service provider.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a graphical representation of bandwidth
`allocation in a two way CATV system;
`FIG. 2 is a block diagram of headend apparatus em-
`bodying the present invention;
`FIG. 2 is a is a block diagram of the modem portion
`of a subscriber terminal unit including a network access
`unit;
`FIG. 3 is a block diagram illustrating a CATV com-
`munication system embodying the present invention;
`FIG. 4 is a block diagram illustrating two CATV
`systems linked together in a CATV communication
`system embodying the present invention;
`FIG. 5 is a representation of a data encryption and
`decryption process used in conjunction with the present
`invention;
`FIG. 6 is a flow chart representing a program for
`generating channel access codes in the network access
`controller in accordance with the present invention;
`FIG. 7 is a flow chart representing a program for
`generating frame verifier codes in a network user node
`embodying the present invention;
`FIG, 8 is a flow chart representing a program for
`checking a frame verifier code in the data channel ac-
`cess monitor at the headend of a CATV communica-
`tions system embodying the present invention;
`FIG. 9 is a representation of a generalized protocol
`architecture for use in conjunction with the present
`invention;
`FIG. 10 llustrates the message format used in con-
`junction with the present invention; and
`FIGS. Ila thru 11f illustrate the sequence of mes-
`sages exchanged in order to initiate and terminate a
`communication session between a source node and a
`destination node in a CATV system embodying the
`present invention.
`DESCRIPTION OF AN EMBODIMENT OF THE
`INVENTION
`
`A typical CATV system is capable of propagating a
`range of signal frequencies, for example, from 5 MHZ
`to 400 MHZ. Signal frequencies above 50 MHZ are
`reserved for distributing signals from the headend to the
`subscriber terminals (i.e. in the downstream or forward
`direction). Signal frequencies below 50 MHZ are re-
`served for propagating signals from individual sub-
`scriber terminals to the headhead (i.e. in the upstream or
`return direction).
`A bandwidth allocation for use in conjunction with
`the present invention is graphically illustrated in FIG. 1.
`The upstream band 2 is 25 MHZ wide and extends from
`5 MHZ to 30 MHZ. The downstream band 4 is also 25
`MHZ wide and may be selected from any convenient
`band of frequencies in the downstream portion of the
`frequency spectrum.
`The topology of a typical CATV system is that of an
`inverted tree. The headend is at the top of the inverted
`tree and the subscriber terminals are located throughout
`the trunk and branches. A signal from the headend in
`the forward direction is heard by all subscriber termi—
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`The modem portion of a subscriber terminal appara-
`tus in accordance with the present invention is shown in
`FIG. 2a. A network access unit modem 13 comprises an
`FSK modulator 19, an FSK demodulator 15, carrier
`sense and collision detection circuitry 3, frequency con-
`trol 17 and a microprocessor 21.
`In operation, a digital signal from a source 23 is re-
`ceived by microprocessor 21. Microprocessor 21 for-
`mats the digital data into a frame message and includes
`a frame verifier (FV) code (described in conjunction
`with the detailed description of FIG. 7) as part of the
`frame message format. The frame message is applied to
`FSK modulator 19 which transmits the frame message
`as an FSK signal on cable 25 in the upstream direction
`to the headend.
`
`Petitioner Cisco Systems - Exhibit 1012 - Page 15
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`Petitioner Cisco Systems - Exhibit 1012 - Page 15
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`5
`At the headend 10 (FIG. 2), FSK demodulator 12
`receives the encoded frame message. The frame mes-
`sage is examined in frame verifier logic 14. If the re-
`ceived frame verifier (FV) code indicates that the user
`is unauthorized, the frame verifier logic 14 blocks the
`further transmission of the frame message. However, if
`the frame verifier code indicates that the user is autho-
`rized, then the frame verifier logic continues to apply
`the frame message to FSK modulator 16 which trans—
`mits (rebroadcasts) the frame message in real time as an
`FSK signal in the downstream direction from the hea—
`dend 10 to all subscriber terminals.
`The frame message is received by a network access
`unit (NAU) modem similar to the NAU modem 13 in
`FIG. 2a. FSK demodulator 15 receives the rebroadcast
`frame message (FV) and forwards the received data to
`microprocessor 21. Note that the FSK demodulator 15
`permits the NAU modem to monitor its own transmis-
`sion as well as receive data from other network access
`units.
`In order to share CATV communication resources
`among many users, the allocated return spectrum space
`is divided in 80 FSK data channels, each capable of
`transmitting 128 Kb/s. The forward spectrum space is
`similarly divided into 80 FSK data channels, forming 80
`channel pairs in the system. However, a CATV system
`may have as little as one DCAM module 110 (FIG. 2).
`As the data traffic load increases, the CATV system
`operator may increase capacity by adding additional
`modules 111:, etc. Thus, communication capacity is
`increased without architectural changes at the headend.
`Each subscriber terminal NAU is assigned a home
`channel. Naturally, for two subscriber units to commu-
`nicate they must both be on the same data channel.
`Therefore, each NAU modem is frequency agile,
`i.e.
`able to change its upstream transmitting frequency (and
`its corresponding downstream receiving frequency)
`upon command from a system control computer called
`a network resource manager.
`Furthermore, the present system permits many users
`to share the same data channel. Channel sharing is
`achieved by a technique known to those skilled in the
`art as carrier sense multiple access with collision detec-
`tion (CSMA/CD).
`Briefly, CSMA/CD is a contention mechanism by
`which many users share a common data channel. All
`users monitor the data channel to sense a carrier signal.
`A user node that desires to transmit a message waits
`until the channel is clear, and then transmits its message
`on the data channel. In the event that two users attempt
`to transmit at the same time, a collision occurs. The
`collision is detected by the users that have attempted to
`transmit. Each user then waits a random length of time
`before attempting to retransmit its respective message.
`A CATV system embodying the present invention is
`shown in FIG. 3. Such system comprises a headend
`including conventional one way CATV broadcasting
`equipment 22 which provides regular video program-
`ming material to all subscribers. The signal distribution
`path includes trunk cables 24, distribution amplifiers 26,
`feeder lines 28, and ultimately drop lines 29 to individ-
`ual system nodes 31.
`There are several types of individual system nodes.
`User nodes are nodes where access to CATV communi-
`cation resources is provided. Of the user nodes, there
`are two types: server nodes 46 (for service providers)
`and subscriber nodes 48 (for service consumers). An-
`other type of system node is a control node, where
`
`6
`control over the CATV communication system (e.g.
`network access control, billing for communication ser-
`vice, etc) is provided. Finally, there are network nodes
`including a link node 50 for communication between
`CATV networks, and a gateway node 52 for communi-
`cation between the CATV network and foreign net-
`works, such as the switched public telephone network.
`A server node 40 communicates with a subscriber
`node 44 through respective network access units
`(NAU) 38 and 42. In such cases, the CATV system
`provides basic data transport service so that the CATV
`system appears transparent to the server 40 and sub-
`scriber 44. For example, the service provider can pro—
`vide an asynchronous RS-232 server node apparatus 40
`and a compatible asynchronous RS—232 subscriber node
`apparatus 44.
`A server node 46 and a subscriber node 48 may incor-
`porate (in addition to a respective NAU) a higher level
`of communication service such as a full videotex imple-
`mentation including graphics capability. In such case,
`the server node 46 need only provide a videotex com-
`patible application service. The subscriber node 48
`hardware (and software) can thus be utilized by many
`different service providers.
`System control nodes comprise a data channel access
`monitor (DCAM) 10 at the headend, a network access
`controller (NAC) 34, a network resource manager
`(NRM) 36, and a network traffic monitor (NTM) 32.
`System control nodes communicate over the CATV
`system in the same manner as subscriber and server
`nodes. In addition there is a two way, out of band data
`channel 30 between the NAC 34 and the DCAM 10.
`Messages on the out of band channel 30 between the
`NAC 34 and the DCAM 10 are not generally broadcast
`on the CATV network. Also, system control nodes 34,
`34, and 36 may be located anywhere within the CATV
`system, except for the DCAM 10 which is located at the
`headend.
`The network access controller (NAC) 34 is a spe-
`cially programmed computer. The primary function of
`the NAC 34 is to grant or deny network access to user
`nodes. When network access is granted, a channel ac-
`cess code (CAC) is provided to the user node. When
`network access is denied, a reason is provided (e.g.,
`channel busy, etc.) to the user node. The generation and
`transmission of channel access codes is described in
`conjunction with the description of FIG. 6.
`The network resource manager (NRM) 36 is another
`specially programmed computer. An important func-
`tion of the NRM 36 is to allocate communication re-
`sources among the various users. One way this is
`achieved is by load leveling, ie by retum'ng the individ-
`ual user modems (FSK modulator and FSK demodula-
`tor) so that the data traffic load is more evenly distrib-
`uted among the available data channels.
`A second important function of the NRM 36 is to
`provide a directory look up service for user nodes. That
`is, the NRM 36 maintains a listing of currently assigned
`data channel frequencies (i.e. the original home channel
`frequency or a reassigned channel frequency) of each
`user node, as well as the address and symbolic name of
`that node. Thus, as will be further detailed in following
`descriptions, at user node can obtain the address and data
`channel frequency of a desired destination node by
`opening a communication session with the NRM 36.
`The network traffic monitor (NTM) 32 is a third
`specially programmed computer. The NTM 32 is a
`passive information collector that listens on all data
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`Petitioner Cisco Systems - Exhibit 1012 - Page 16
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`Petitioner Cisco Systems - Exhibit 1012 - Page 16
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`4,533,948
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`7
`channels and collects information of usage of CATV
`communication resources. The information collected
`has two primary uses: (1) Billings for data communica-
`tion service are generated during non-peak traffic
`hours, and (2) channel traffic statistics are provided to
`the NRM 36 for purposes of traffic management in
`allocating CATV bandwidth, i.e. load leveling.
`A link node 50 provides intra-network communica-
`tion between two CATV networks to form a single
`address space for CATV digital communication. As
`shown in FIG. 4, two CATV networks 54 and 56 are
`interconnected by link nodes 500 and 50b. One CATV
`system 54 includes DCAM 1011 NRM 36, NTM 32 and
`NAC 34. The other CATV system includes DCAM
`10b.
`In its simplest form, links 50a. and 50b perform one to
`one mapping of specific messages between CATV sys-
`tem 54 and 56. Link 50a receives downstream data. The
`received data is applied to link 50b which in turn re~
`transmits the data in the upstream direction in CATV
`system 56. Thus, by the use of link nodes 50a and 501)
`the CATV system 54 and the CATV system 56 form a
`single data network having a common address space in
`which any node from either system may communicate
`with any other node.
`Within a single CATV data network, each node is
`assigned a unique 24 bit address. Messages that are in-
`tended for reception by a particular destination node,
`contain the address of the destination node. All nodes
`monitor at least one data channel. When the address of
`the destination node is recognized, the whole message is
`received for further processing.
`In addition to the address of a particular node, each
`node is assigned a secret 56 bit number called a node
`key. The node key is a security measure designed to
`prevent unauthorized users from obtaining access to
`CATV communication resources. Unlike the node ad-
`dress, the node key is never transmitted on the CATV
`system. Furthermore, the number of possible node keys,
`255,
`is very large and sparsely populated so that the
`probability of guessing a valid node key is very small.
`As a brief overview of system operation (FIG. 3),
`consider the typical situation, wherein a source node 44
`is to communicate with a destination node 40 (FIG. 3).
`The source node first obtains channel access by the
`following process:
`1. The source node signals the headend (DCAM) with
`a network access request (no FV code attached).
`2. The DCAM 10 forwards the network access request
`to the NAC 34, on the out of band channel 30 (no FV
`code required).
`3. The NAC 34 transmits an encrypted channel access
`code (CAC) to the source node. A valid FV code
`relative to the NAC 34 is transmitted with the NAC
`message so that it can pass through the DCAM at the
`headend.
`The source node decrypts the CAC, which is used by
`the source node to generate its own FV codes. The
`source node has thus obtained permission to utilize the
`requested data channel.
`After the source node 44 obtains channel access, it
`can then establish a signal path connection with a de-
`sired destination node 40 by the following routing pro-
`cess:
`
`1. The source node signals the NRM 36 (now with FV
`code attached). The message to the NRM 36 includes
`the symbolic name of the desired destination node.
`
`8
`2. The NRM 36 looks up the destination node name in
`its directory and responds with the channel
`fre-
`quency and address of the desired destination node. A
`valid FV, code, relative to the NRM 36 is transmitted
`with the NRM message so that it can pass through the
`DCAM at the headend.
`3. If the channel frequency of the destination node 40 is
`different than that of the source node 44, the source
`node 40 changes its frequency to that of the destina-
`tion node and repeats the above process for network
`access on the destination channel frequency.
`4. The source node 44 then signals the destination node
`40 with a session open request.
`5. The destination node 40 receives the session open
`request, and, if necessary, obtains a channel access
`code by the above stated process for obtaining chan-
`nel access. The destination node can then respond
`with a session open acknowledgement message.
`At the end of this process, the source node 44 and the
`destination node 40 are on the same data channel
`thereby establishing