United States Patent [191
`McNamara et a1.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,533,948
`Aug. 6, 1985
`
`[54] CATV COMMUNICATION SYSTEM
`
`[75] Invenmrs Robert P- McNamara; Gregory B-
`Ennis, both of San Jose; Richard J.
`Feiertag, Sunnyvale; Robert K.
`Bauery Fremont, all of Calif,
`
`[73] Assigne? Seller? 128??“ Corporation,
`CW or ’
`i
`'
`[21] APPL NW 373,765
`[22] Filed:
`APR 30, 1982
`
`[51] Int. 01.1 ............................................. .. H04N 7/16
`[52] US. Cl. .................................... .. 358/122; 358/86;
`_
`358/114; 455/5; 340/8255; 375/2-1
`[58] Field of Search ....................... .. 358/114, 122, 86;
`375/22’ 2-1; 340/8255; 364/2001 90o;
`178/201’ 22-02’ 2203’ 2204’ 22-05’ 2206’
`22‘O7’ 2208’ 22069’ 22'10’ 22'1181’ 2222'1192”42525‘12’
`22'14’ 22'15’ 22'1 ’ 22'17’ 22‘ ’
`‘
`’
`6/ 3’
`'
`’
`References Cited
`Us. PATENT DOCUMENTS
`‘
`3,423,52l l/l969 Fnesen ............................... .. 137588/58.g
`gig; ' ' ' ‘ ' '
`' ' ' " 32523
`3:786:424 1/1974 McVoyy '1'
`....::"340/151
`3,798,605 3/1974 Feistel
`178/2208
`3,803,491 4/1974 Osborne .............................. .. 325/53
`
`[56]
`
`3,859,596 1/1975 Jannery ............................... .. 325/31
`3,934,079 1/1976 Barnhart
`178/511
`3,943,447 3/1976 Shomo, III
`325/308
`3,997,718 12/1976 Ricketts
`178/6.8
`4,031,543 6/1977 Holz ..... ..
`358/86
`4,041,398 8/1977 Ellis .......... ..
`325/308
`4,245,245 l/1981 Matsumoto
`.... .. 358/122
`4,310,720 1/1932 Check, Jr. ...................... .. 178/2202
`Primary Examiner-Robert L. Grif?n
`Assistant Examiner-Timothy K. Greer
`Attorney, Agent, or Firm—-A11an J. Jacobson
`
`ABSTRACT
`[57]
`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 veri?er (FV) code to each respec
`?ve message. The headend examines the FV and Pet;
`mits rebroadcast of messages only if the FV code indi
`Cates that th? use‘ is authorized
`
`11 Claims, 17 Drawing Figures
`
`ARRIS883IPRI0000807
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`

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`U.S. Patent Aug. 6, 1985
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`Sheet 1 of 12
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`US. Patent Aug.6, 1985
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`U.S. Patent Aug. 6, 1985
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`U.S. Patent Aug. 6, 1985
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`U.S. Patent Aug. 6, 1985
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`US. Patent Aug. 6, 1985
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`U.S. Patent Aug. 6, 1985
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`U.S. Patent Aug. 6, 1985
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`ARRIS883IPRI0000819
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`

`
`CATV COMMUNICATION SYSTEM
`
`FIELD OF THE INVENTION
`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 dif?cult. 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.
`
`25
`
`30
`
`45
`
`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
`
`55
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`65
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`1
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`4,533,948
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`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 veri?cation message, referred to
`herein as a frame veri?er (FV) code, as part of the
`upstream message.
`In accordance with yet another aspect of the present
`invention, the headend apparatus examines the frame
`veri?er code and rebroadcasts the received upstream
`message in the downstream portion of the cable spec
`trum only if the frame veri?er 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 veri?er message indicates that the
`source node is not an authorized user thereby denying
`the user access to the CATV resources.
`
`ADVANTAGES OF THE PRESENT INVENTION
`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 speci?c 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 de?ned 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
`traf?c 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 speci?c service, server
`node software will generally be simpli?ed (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 traf?c load for one service does not
`substantially effect the service response time of the
`other server nodes.
`
`ARRIS883IPRI0000820
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`

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`4,533,948
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`3
`nals. However, a signal transmitted from an individual
`Finally, a decentralized intelligence communication
`subscriber terminal in the return direction is heard only
`system tends to disassociate the CATV operator from
`by that portion of the other subscriber terminals that are
`the service provider. Issues involving information pri
`in the signal propagation path from the transmitting
`vacy and data integrity become the responsibility of the
`subscriber terminal to the headend. Therefore, the sig
`service vendor. The CATV operator simply offers
`nal transmitted from a particular subscriber terminal is
`transparent data transport service to be used as desired
`heard by only a portion of the other subscriber termi
`by the service provider.
`nals.
`BRIEF DESCRIPTION OF THE DRAWINGS
`In accordance with the present invention, each signal
`FIG. 1 is a graphical representation of bandwidth
`frequency in the return band 2 is paired with a corre
`sponding signal frequency in the forward band 4. The
`allocation in a two way CATV system;
`headend includes apparatus for receiving the return
`FIG. 2 is a block diagram of headend apparatus em
`bodying the present invention;
`signal in the upstream band, and selectively rebroad
`casting the signal at a higher corresponding frequency
`FIG. 2 is a is a block diagram of the modern portion
`of a subscriber terminal unit including a network access
`in the forward band. In such manner, a signal from an
`unit;
`individual subscriber terminal (in the return band) is
`FIG. 3 is a block diagram illustrating a CATV com
`heard by all the other subscriber terminals (in the for
`munication system embodying the present invention;
`ward band) thereby permitting any individual sub
`FIG. 4 is a block diagram illustrating two CATV
`scriber terminal to transmit a message to any other
`systems linked together in a CATV communication
`subscriber terminal within the CATV system.
`system embodying the present invention;
`Digital signals are transmitted in the present system
`FIG. 5 is a representation of a data encryption and
`by the use of frequency shift keyed (FSK) modulation.
`decryption process used in conjunction with the present
`A digital signal has one of two binary logic states, i.e. l
`invention;
`or 0. When the digital signal is at a logical 1, an FSK
`FIG. 6 is a ?ow chart representing a program for
`modulator transmits a signal of ?rst frequency and
`generating channel access codes in the network access
`when the binary signal is at a logical 0, an F SK modula
`controller in accordance with the present invention;
`tor transmits a signal at a second frequency. Similarly an
`FIG. 7 is a ?ow chart representing a program for
`FSK demodulator is responsive to an FSK signal to
`generating frame veri?er codes in a network user node
`reproduce the original digital signal.
`embodying the present invention;
`A headend apparatus in accordance with the present
`FIG. 8 is a flow chart representing a program for
`invention is shown in FIG. 2. A data channel access
`checking a frame veri?er code in the data channel ac
`monitor (DCAM) 10 comprises individual data channel
`cess monitor at the headend of a CATV communica
`access monitor modules 11a, 11b, etc., a network access
`tions system embodying the present invention;
`controller interface processor 18 and a modem 20. An
`FIG. 9 is a representation of a generalized protocol
`individual data channel access monitor module 11a
`architecture for use in conjunction with the present
`comprises FSK demodulator 12, frame veri?er logic 14,
`invention;
`and FSK modulator 16. The FSK demodulator 12 is
`FIG. 10 llustrates the message format used in con
`tuned to a particular frequency in the return band. The
`junction with the present invention; and
`FSK modulator 16 is tuned to a corresponding paired
`FIGS. 11a thru 11f illustrate the sequence of mes
`frequency in the forward band. The frame veri?er logic
`sages exchanged in order to initiate and terminate a
`14 examines the received data and selectively connects
`communication session between a source node and a
`the output of the FSK demodulator 12 to the input of
`destination node in a CATV system embodying the
`the FSK modulator 16. The FSK modulator 16 re
`present invention.
`broadcasts the received signal in real time at a corre
`45
`sponding higher frequency in the forward portion of the
`DESCRIPTION OF AN EMBODIMENT OF THE
`cable spectrum. The frame veri?er logic 14 also inter
`INVENTION
`faces with the network access controller interface pro
`A typical CATV system is capable of propagating a
`cessor 18 which provides two way communication to
`range of signal frequencies, for example, from 5 MHZ
`the network access controller via modem 20. The oper
`to 400 MHZ. Signal frequencies above 50 MHZ are
`ation of the frame veri?er logic 14 will be described in
`reserved for distributing signals from the headend to the
`more detail in conjunction with the description of FIG.
`subscriber terminals (i.e. in the downstream or forward
`8.
`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
`
`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 veri?er (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.
`
`25
`
`35
`
`55
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`60
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`65
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`ARRIS883IPRI0000821
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`

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`5
`
`25
`
`45
`
`4,533,948
`t
`5
`At the headend 10 (FIG. 2), FSK demodulator 12
`receives the encoded frame message. The frame mes
`sage is examined in frame veri?er logic 14. If the re
`ceived frame veri?er (FV) code indicates that the user
`is unauthorized, the frame veri?er logic 14 blocks the
`further transmission of the frame message. However, if
`the frame veri?er code indicates that the user is autho
`rized, then the frame veri?er 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 11a (FIG. 2).
`As the data traf?c load increases, the CATV system
`operator may increase capacity by adding additional
`modules 11b, 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).
`Brie?y, 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 ampli?ers 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 traf?c 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
`40
`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 retuning the individ
`ual user modems (FSK modulator and FSK demodula
`tor) so that the data traf?c 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, a 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
`
`55
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`ARRIS883IPRI0000822
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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 traf?c
`hours, and (2) channel traf?c statistics are provided to
`the NRM 36 for purposes of traf?c 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 10a NRM 36, NTM 32 and
`NAC 34. The other CATV system includes DCAM
`15
`10b.
`In its simplest form, links 50a. and 50b perform one to
`one mapping of speci?c 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 500 and 5017
`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 ?rst 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
`65
`cess:
`l. 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.
`
`4,533,948
`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 a signal path connection between
`them. The two nodes 40 and 44 can continue a commu~
`nication exchange until the session is terminated by
`appropriate messages.
`In order to protect the security of channel access
`codes, and prevent eavesdroppers from discovering
`valid FV codes, a double data encryption/decryption
`scheme is provided. The basic data encryption used is
`the Data Encryption Standard (DES) adopted by the
`National Bureau of Standards, Washington, DC.
`Brie?y, as illustrated in FIG. 5, the DES scheme per
`mits a 64 bit input 104 to be either encrypted or de
`crypted into a 64 bit output 106 by use of a 56 bit key
`102. Control line 108 determines whether an encryption
`or a decryption process is taking place.
`To encrypt a 64 bit input 105, the control line 108 is
`set for an encryption. A 56 bit key 102 is loaded into the
`logic 100. Then, a 64 bit input 104 is loaded into the
`logic, and a 64 bit output 106 of encrypted data thereaf
`ter becomes available. For decryption, the control line
`108 is set for a decryption, a 56 bit key 102 is loaded into
`the logic 100, a 64 bit encrypted input is loaded into the
`logic 100, and the 64 bit output 106 is thereafter avail
`able. Standard integrated circuits are commercially
`available for implementing DES in hardware. How
`ever, it is anticipated that the DES encryption and de
`cryption will be carried out in software.
`A flow chart representin