United States Patent (19)
`Wasilewski et al.
`
`54 METHOD AND APPARATUS FOR
`PROVIDING CONDITIONAL ACCESS IN
`CONNECTION-ORIENTED, INTERACTIVE
`NETWORKS WITH A MULTIPLICITY OF
`SERVICE PROVIDERS
`
`Inventors: Anthony John Wasilewski, Alpharetta;
`Douglas F. Woodhead, Lawrenceville;
`Gary Lee Logston, Tucker, all of
`Ga.
`Assignee: Scientific-Atlanta, Inc., Norcross,
`Ga.
`
`Appl. No. 580,759
`Filed:
`Dec. 29, 1995
`
`Related U.S. Application Data
`
`Provisional application No. 60/007,962 Dec. 4, 1995.
`Int. Cl." ............................. H04L 9/00; H04K 1/00
`U.S. Cl. ................................. 380/21: 380/23: 380/25;
`380/49
`Field of Search .................................. 380/21, 23, 25,
`380/49
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`33,189 3/1861. Lee et al. .................................. 380/20
`4,405,829 9/1983 Rivest et al. ............................., 380/30
`4,531,020 7/1985 Wechselberger et al.
`178/2.2.08
`4,613,901
`9/1986 Gilhousen et al. ......
`... 358/122
`4,634,807
`1/1987 Chorley et al. ............................. 380/4
`4,912,762 3/1990 Lee et al. ......
`... 380/24
`5,029,207 7/1991 Gammie .................................... 380/10
`5,124,117 6/1992 Tatebayashi et al. ..................... 380/21
`5,237,610 8/1993 Gammie .................................... 380/10
`5,341,525 8/1994 Wasilewski et al. ..................... 380/20
`5,381,481
`1/1995 Gammie et al. .......................... 380/49
`5,400,401 3/1995 Wasilewski et al. ....................... 380/9
`5,420,866 5/1995 Wasilewski .......
`370/110.1
`5,473,692 12/1995 Davis ........................................ 380/25
`5,481,542
`1/1996 Logston et al.
`... 370/94.2
`5,481,613
`1/1996 Ford et al. ................................ 380/30
`5,557,678 9/1996 Ganesan .................................... 380/21
`(List continued on next page.)
`
`USOO5870474A
`Patent Number:
`11
`(45) Date of Patent:
`
`5,870,474
`Feb. 9, 1999
`
`OTHER PUBLICATIONS
`Diffle, et al., “Authentication and Authenticated Key
`Exchanges,” Designs, Codes and Cryptography 2,107-125
`(1992).
`Bruce Schneier, “Applied Cryptography,” 357-363 (1995).
`Menezes et al., “Handbook of Applied Cryptography,”
`506–525 (1996).
`MPEG Systems International Standards Reference (ISO/
`IEC JTC1/SC29/WG 11 NO801, Nov., 1994, ISO Reference
`No. 13818-1
`Wasilewski, A., “Universal Multi-Program Multiplex and
`Transport for MPEG-2 Systems”, International Organiza
`tion for Standardization, ISO/IEC JTC1/SC29/WG 11
`MPEG 93/170, (Jan. 1993).
`Wasilewski, A., An MPEG-2 Multi-Program Multiplex
`Syntax International Organization for Standardization, ISO/
`IEC JTC1/SC29/WG 11 MPEG 93/170, (Jan. 1994).
`(List continued on next page.)
`Primary Examiner Thomas H. Tarcza
`Assistant Examiner Hrayr A. Sayadian
`Attorney, Agent, or Firm-Kenneth M. Massaroni; Kelly A.
`Gardner, Hubert J. Barnhardt, III
`57
`ABSTRACT
`A control System provides Secure transmission of programs,
`including at least one of Video, audio, and data, between a
`Service provider and a customer's Set top unit over a digital
`network. Program bearing data packets are recieved in a first
`network protocol over a first data link and removed from the
`first network protocol. Packets representing a particular
`program requested by a customer having a Set top unit are
`Selected. Conditional access is provided to the Selected
`program. In particular, program bearing packets are
`encrypted according to a first encryption algorithm using a
`first key, which is then encrypted according to a Second
`encryption algorithm using a Second key. The first keys are
`transported in packets to the customer's Set top units along
`with the program packets. A public key cryptographic tech
`nique encrypts the Second key Such that the public key used
`in the encryption corresponds to the private key of the
`customer's Set top unit. After the conditional access layers
`have been added, the packets are encapsulated and output in
`a Second network protocol destined for the Set top unit.
`
`50 Claims, 17 Drawing Sheets
`
`59
`
`ETHERNE
`ENERFACE
`
`5
`
`CONROL
`PROCESSOR
`
`DU, POR
`A.
`
`
`
`155
`CONTROL
`WOR)
`STORACE
`
`RANDOM
`NUMBER
`GENERATOR
`
`CHANNE
`BANK
`INTERFACE
`
`164
`
`CONTROL
`CARD
`
`22
`
`
`
`153
`
`TRIPE
`DES
`NCRPEER
`
`162
`
`PACKET
`MUX
`
`PACKET
`ENCRYPTION
`PROCESSOR
`
`58
`
`f
`
`PACK
`DISTR8UTION
`154
`166.
`BOCK
`
`DES
`Block
`DES
`
`66b
`
`|-
`|
`
`l
`
`Comcast, Ex. 1019
`
`1
`
`

`

`5,870.474
`Page 2
`
`U.S. PATENT DOCUMENTS
`5,557,765 9/1996 Lipner et al. ............................. 380/21
`5,559,889 9/1996 Easter et al. .............................. 380/30
`5,563,950 10/1996 Easter et al. ..
`... 380/30
`5,568,552 10/1996 Davis ............
`... 380/4
`5,583,939 12/1996 Chang et al. ............................. 380/21
`
`
`
`OTHER PUBLICATIONS
`Wasilewski, A., “Requirements and Method for High-Level
`Multiplexing of MPEG and Other Digital Service Citstreams
`with Universal Transport Layer”, International Organization
`for Standardization, ISO/IEC JTC1/SC2/WG 11 MPEG
`92/754, (Nov., 1992).
`
`2
`
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`U.S. Patent
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`U.S. Patent
`
`Feb. 9, 1999
`
`Sheet 8 of 17
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`5,870,474
`
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`

`

`U.S. Patent
`
`Feb. 9, 1999
`
`Sheet 9 of 17
`
`5,870,474
`
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`
`

`

`U.S. Patent
`
`Feb. 9, 1999
`
`Sheet 10 of 17
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`5,870,474
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`PACKET
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`Feb. 9, 1999
`Feb. 9, 1999
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`Sheet 12 of 17
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`5,870,474
`
`1
`METHOD AND APPARATUS FOR
`PROVIDING CONDITIONAL ACCESS IN
`CONNECTION-ORIENTED, INTERACTIVE
`NETWORKS WITH A MULTIPLCITY OF
`SERVICE PROVIDERS
`
`15
`
`2
`network. A provider of Level 2 services is defined by the
`FCC as an enhanced Services provider and is not regulated
`by the FCC. Significantly, these FCC regulations limit the
`control a Level 1 services provider may have over Level 2
`Services.
`In a Level 1/Level 2 system, which is under the jurisdic
`tion of the FCC, the SP resides in Level 2 and the control that
`the SP can exercise over Level 1 Services is restricted.
`However, in any System where a SP is delivering programs
`to a customer Over a network, the SP has a need to prevent
`the unauthorized access to the programs provided to the
`customer. For example, a nonSubscriber may attempt to
`illegitimately receive the programs intended for the use of
`paying Subscribers. This protection of programs through the
`prevention of unauthorized acceSS is referred to as “condi
`tional access.” AS used herein the terms “conditional access”
`and “conditional access layer” broadly refer to the control
`mechanisms, data Structures and commands that provide for
`Selective acceSS or denial of Specific Services. Prior art
`Systems have provided conditional access by encrypting the
`programs at the SPSite and decrypting the programs at the
`customer Site.
`For example, Lee et al., U.S. Pat. No. Re. 33,189, dis
`closes a System using an encryption mechanism for provid
`ing conditional acceSS in a Satellite television System and is
`hereby incorporated by reference. In Lee, a program is
`Scrambled at a SP Site using a frequently changing random
`number. The random numbers are encrypted with a key and
`broadcast along with the program to customer Sites. Cus
`tomers who have paid receive the key, encrypted with the
`unique ID that is embedded in their set top unit (STU). These
`customers STUS can decrypt the key using the unique ID
`embedded therein. The customers’ STU can then decrypt the
`encrypted random numbers, as they are broadcast, and use
`the random numbers, along with the key, to decrypt the
`program. AS noted above, the key in the Lee invention must
`be securely transmitted; otherwise, an unauthorized user
`could get access to the key and gain access to the broadcast
`programs. Lee protects the key by using the unique ID of the
`STU to encrypt it. Such a technique works fine in a broadcast
`environment where there is a single broadcaster to multiple
`users. In that environment, the broadcaster can take adequate
`measures to protect the list of valid customer STU IDs.
`However, in a telephone architecture regulated by the FCC,
`as described above, multiple service providers (i.e.,
`broadcasters) must have access to the multiple users. In Such
`an environment, the list of unique STU IDs is vulnerable to
`discovery by unauthorized parties, and the Security of the
`System may be breached. Additionally the Lee System is
`appropriate for a broadcast environment in which the SPS
`have the only reasonable means to address the STUs.
`Therefore, the System is not Susceptible to compromise by
`unauthorized users addressing the STUs. However, in a
`digital network environment where STUs are uniquely
`addressable, and multiple SPs have access to multiple STUs,
`an unauthorized user could put information on the network
`addressed to individual STUs and thereby compromise the
`System. Applicants have recognized that a conditional access
`System in a digital network environment must have a mecha
`nism that allows the STU to authenticate the identity of the
`SP. Thus, applicants have recognized that an improved
`encryption technique is needed.
`Moreover, while encryption has provided conditional
`access, the problem of where to perform the conditional
`acceSS in an FCC regulated System remains unresolved.
`Applicants have recognized that a Solution that performs the
`conditional access within the L1 portion of the System is
`unnecessarily complicated by FCC regulation.
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`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`This application claims priority of earlier filed U.S. pro
`visional application Ser. No. 60/007,962, filed Dec. 4, 1995,
`entitled “An Apparatus for Providing Conditional Access in
`Connection-Oriented, Interactive Networks With a Multi
`plicity of Service Providers.”
`FIELD OF THE INVENTION
`The present invention relates to a control System for
`providing interactive information Services, Such as Video,
`audio, library, interactive games, and the like over a digital
`network. Particular video applications include “movie on
`demand,” on-line data retrieval, and home Shopping. More
`particularly, the invention relates to a control System for
`providing Secure transmission of these information Services
`between a Service provider and a customer's Set top unit over
`a digital network.
`BACKGROUND OF THE INVENTION
`Recent advances in digital Signal processing techniques
`and, in particular, advancements in digital compression
`techniques, have led to an abundance of proposals for
`providing new digital Services to the customer's home via
`existing telephone and coaxial cable lines. For example,
`proposals have been made to provide hundreds of CATV
`channels to customers by compressing digital Video, trans
`mitting the compressed digital Video over conventional
`coaxial CATV cables, and then decompressing the video at
`the customer's Set top unit. Another proposed application of
`this technology is a “movie on demand Video System in
`which a customer communicates directly with a Video
`service provider via the telephone lines or coaxial CATV
`40
`cables to request a particular video program from a video
`library, and the requested Video program is routed to the
`caller's home via the telephone lines or via the coaxial
`CATV cables for immediate viewing.
`Such an exemplary System typically has three distinct
`segments: (1) a service provider (SP), which provides the
`Video, audio, interactive games and the like (collectively
`referred to hereinafter as “programs”) to the System; (2) a
`customer, who purchases the programs from the Service
`provider; and, (3) a network operator, which provides a
`transmission path or connection between the SP and the
`customer for delivery of the programs. Alayer of complexity
`is added to the operation and design of the System if the
`network operator is defined as a telephone company by the
`Federal Communications Commission (FCC). In such a
`case, the network operator is Subject to regulation under the
`jurisdiction of the FCC. The system will then be further
`categorized into Level 1 Services (L1) and Level 2 Services
`(L2). Level 1 Services provide the information Session
`connection and define the portion of the System responsible
`for Setting up and maintaining interactive communication
`Sessions between customers and SPS. Level 1 Services are
`provided by the network operator and are regulated by the
`FCC. Level 2 services, on the other hand, define the portion
`of the System responsible for providing the programs
`requested to the L1 portion of the system from the SP and
`for terminating the Service at the customer end of the
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`Applicants have recognized that conditional access
`should be performed while a program is still in control of the
`Level 2 service provider, i.e., before it is delivered to the L1
`portion of the System. Access to the program and Vital
`conditional access information can be closely controlled by
`a Service provider. Unfortunately, the file Server equipment
`currently available to Service providers does not provide the
`necessary functionality to perform conditional access before
`a program is output from the file Server. As a result, there is
`a need for method and apparatus to provide conditional
`access to a program after it exits a file Server, but before it
`enters the L1 portion of the System.
`The problem is complicated further when considered in
`the context of a typical digital network environment. In Such
`an environment it is expected that the SPS will store pro
`grams on file servers in the form of Moving Picture Expert
`Group (MPEG-2) Systems transport packets, as defined in
`MPEG-2 Systems International Standards Reference (ISO/
`IEC JTC1/SC29/WG 11 NO801, November 1994, ISO Ref
`erence No. 13818-1), which is hereby incorporated by
`reference. Importantly, although the MPEG-2 Systems Inter
`national Standards Reference does not standardize on a
`particular method of conditional acceSS, it does contemplate
`the addition of conditional access to the MPEG-2 transport
`packets. Thus, to conform to the MPEG-2 standard, it is
`necessary that conditional acceSS be added to programs at
`the MPEG-2 transport packet layer rather than at a higher
`network protocol layer. However, when a program leaves a
`service provider's file server, it will not be in a convenient
`format for applying conditional access. Rather, the program,
`in the form of MPEG-2 transport packets, will leave the file
`Server enveloped in a first network protocol. Additionally, in
`Some applications, the packets may then need to be
`re-mapped into a second network protocol to conform to the
`network protocol provided by the network operator. Thus, in
`this context there is a need for method and apparatus for
`removing the MPEG-2 transport packets of a particular
`program from a first network protocol, providing conditional
`access to the MPEG-2 transport packets, and then mapping
`the MPEG-2 transport packets back into the first network
`protocol or into a Second network protocol.
`SUMMARY OF THE INVENTION
`The present invention meets the needs discussed above by
`providing method and apparatus between the SPS and the
`Level 1 Services provider that accepts programs destined for
`an STU in the form of MPEG-2 transport packets enveloped
`in one of a plurality of network protocols. According to the
`present invention, the packets are removed from a first
`network protocol. Conditional access layers are applied to
`the packets. After applying the conditional acceSS layers, the
`packets are encapsulated and output in a Second network
`protocol destined for the STU.
`According to an aspect of the present invention a method
`of providing conditional access to a Selected program is
`provided. Packets representing a program requested by a
`customer having an STU are Selected. Those program bear
`ing packets are encrypted according to a first encryption
`algorithm using a first key. The first key used to encrypt the
`program is, in turn, encrypted according to a Second encryp
`tion algorithm using a Second key. The first keys are trans
`ported in packets to the customer's STU along with the
`program packets. The Second key is, in turn, encrypted using
`a public-key cryptographic technique Such that the public
`key used in the encryption corresponds to the private key of
`the customer's STU. The encrypted second key is then
`transported via packets to the STU along with the program
`and first key packets.
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`According to another aspect of the present invention, the
`apparatus provides means for receiving program bearing
`packets in a first network protocol from a first data link and
`removing the packets from the first network protocol. The
`apparatus Selects all packets comprising a particular pro
`gram requested by a customer. Conditional acceSS is then
`applied to the requested program at the packet layer in
`accordance with the method described above. The apparatus
`then encapsulates all packets in a Second network protocol
`and outputs them over a Second data link for delivery to the
`customer’s STU.
`According to a further aspect of the present invention, a
`method and apparatus are provided for generating a message
`authentication code comprised of a hash of the first key and
`the second key, such that the STU can determine if the
`packets bearing the first key has been tampered with during
`transmission. An Additional method and apparatus are pro
`Vided for applying a digital Signature to the encrypted
`Second key, Such that the authorized customer can determine
`the identity of the provider of the encrypted Second key,
`thereby preventing unauthorized users from addressing
`STUS.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The foregoing Summary, as well as the following detailed
`description of the preferred embodiment, is better under
`stood when read in conjunction with the appended drawings.
`For the purpose of illustrating the invention, there is shown
`in the drawings an embodiment that is presently preferred,
`it being understood, however, that the invention is not
`limited to the Specific methods and instrumentalities dis
`closed. In the drawings:
`FIG. 1 illustrates an exemplary digital video distribution
`System in which the present invention may be employed.
`FIG. 2 is a block diagram providing further details of a
`Server access and broadband encrypter re-mapper in accor
`dance with a presently preferred embodiment of the inven
`tion.
`FIG. 2A is a block diagram illustrating further details of
`a presently preferred embodiment of an FDDI input card.
`FIG. 2B is a block diagram illustrating further details of
`a presently preferred embodiment of a SONET-ATM output
`card.
`FIG. 2C is a block diagram illustrating further details of
`a presently preferred embodiment of a conditional access
`card.
`FIG. 2D is a block diagram illustrating the operation of
`the control card.
`FIG. 3 is a functional block diagram illustrating the
`conditional access Scheme provided in accordance with the
`present invention.
`FIG. 3A is a functional block diagram illustrating the
`process of message authentication of control words in accor
`dance with the present invention.
`FIG. 3B is a functional block diagram illustrating the
`process of adding a digital Signature to an MSK in accor
`dance with the present invention.
`FIG. 4 graphically illustrates the structure and content of
`an exemplary MPEG-2 transport packet.
`FIG. 5 graphically illustrates the mapping of MPEG-2
`transport packets into ATM cells in accordance with the
`present invention.
`FIG. 6 graphically illustrates the mapping of MPEG-2
`transport packets into an FDDI frame in accordance with the
`present invention.
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`FIG. 7 graphically illustrates the mapping of MPEG-2
`transport packets into a DS-3 frame in accordance with the
`present invention.
`FIG. 8 graphically illustrated the mapping of MPEG-2
`transport packets into a UNISON frame in accordance with
`the present invention.
`FIG.9 graphically illustrates the transport overhead struc
`ture utilized in the UNISON-1 STS-3c frame structure.
`FIG. 10 graphically illustrates the Synchronous Payload
`Envelope (SPE) structure used for transmitting MPEG-2
`transport packets in accordance with the UNISON-1 STS-3c
`frame Structure.
`FIG. 11 illustrates a functional block diagram of an
`exemplary Set top unit implementing the conditional acceSS
`method of the present invention.
`FIG. 12 is a functional block diagram illustrating the
`context and operation of the Conditional AcceSS Manager.
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`Referring to the drawings wherein like numerals indicate
`like elements throughout, there is shown in FIG. 1 a block
`diagram of the components of an exemplary digital infor
`mation distribution system 10 (“distribution system') in
`which the present invention may be incorporated. A similar
`system is described in U.S. Pat. No. 5,481,542, which is
`assigned to the same assignee as the present invention and
`is hereby incorporated by reference in its entirety. The
`distribution system 10 provides a mechanism whereby data,
`Such as compressed digital Video data from a Service pro
`vider (SP) 110, is transmitted over a broadband transmission
`network under the control of a network operator 120 to a
`customer 130 for presentation to the customer's STU 90. As
`used herein, the term Set top unit refers to any customer
`device capable of receiving and decoding digital Services,
`Such as personal computers, home control terminals, decod
`erS and the like. In the case of a Video Service, for example,
`the received information could be displayed on the custom
`er's television or computer Screen. A bi-directional commu
`40
`nication path is also established and maintained between the
`SP 110 and the customer 130 by the network operator 120,
`which allows the customer 130 to interact with the service
`provider. For example, the customer 130 may wish to select
`programs from a menu, control the playback of a program,
`or interact with a Video game.
`Various aspects of the distribution system 10 incorporat
`ing the present invention are described below. First, an
`overview of the components of the distribution system 10
`are described. Following the overview, detailed information
`concerning the various components of the distribution SyS
`tem 10 that incorporate the present invention is provided.
`I. System Overview
`When a customer 130 requests a program, the request is
`routed from the customer's STU 90 through a network
`access node 80 to the network control and management
`computer (NCMC) 100. The NCMC 100 then provides a
`communication connection between a particular Service
`provider 110a, 110b and the customer 130. To establish the
`connection, the NCMC 100 ensures that bandwidth is avail
`able on the digital network 70 and the network access node
`(NAN) 80. Thereafter, the NCMC 100 passes the customer
`request to the requested SP 110 via server gateway 61. The
`Server gateway 61 handles communications with various
`billing agencies to determine the customer's eligibility to
`receive the requested program and to determine the condi
`tional access requirements for the requested program. An SP
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`6
`stores programs on a file server 60 in the form of Moving
`Picture Experts Group (MPEG-2) Systems transport
`packets, containing compressed digital Video and audio data
`as well as other digital Service information. The requested
`MPEG-2 transport packets are then output over data link 40
`encapsulated in a network protocol. Ultimately, the packets
`are to be transmitted through the digital network 70 to a
`NAN 80 and then to the customer’s STU 90. The Service
`providers 110 want to ensure that programs entering the
`digital network 70 are viewed only by the customers who
`have been authorized by the server gateway 61. Thus there
`is a need to provide conditional access to programs before
`those programs enter the digital network 70. According to
`one aspect of the present invention, apparatus 20 and 30,
`referred to herein as Service Access and Broad Band
`Encrypter Re-mapper (SABER) 20 and Conditional Access
`Manager 30, are provided between the SP 110 and the digital
`network 70 to provide a means for adding conditional access
`to the program to be transmitted. In particular the SABER 20
`receives the MPEG-2 transport packets from the SP 110, via
`data link 140, encapsulated in the network protocol of that
`link. According to the present invention the SABER 20
`extracts the MPEG-2 transport packets, adds conditional
`access, and then re-encapsulates the packets in a Second
`protocol (which may be the same or different from the first
`protocol) for introduction into digital network 70. The CAM
`30 provides the SABER with information necessary to
`selectively apply the conditional access to the MPEG-2
`transport packets. The CAM 30 receives the conditional
`acceSS requirements and unique PID assignments for the
`requested program from the SP via the Server gateway 61.
`The second network protocol, that of data link 50, may be
`the same as or different from the first network protocol of
`data link 40. For example, data link 40 may conform to an
`FDDI network protocol, while data link 50 may conform to
`an ATM network protocol, or both data links 40, 50 may
`conform to an ATM network protocol. Among the protocols
`presently anticipated by the SABER 20 are SONET-ATM,
`FDDI, DS-3 and UNISON-1, all of which can be used to
`transfer Moving Picture Experts Group (MPEG-2) Systems
`transport packets through data link 40 or data link 50.
`However, the network protocols listed and described herein
`are merely illustrative, and should not be construed as
`limiting the invention to those protocols listed, other
`protocols, for example, a proprietary protocol, could func
`tion equally well.
`Significantly, if the digital network 70 were inherently
`Secure (for example, a completely fiber network) the
`SABER 20 could be located elsewhere in the system 10. In
`such a secure network, the SABER 20 might be located at
`the opposite end of the digital network 70, between the
`digital network 70 and the NANs 80.
`Network control and management computer (NCMC) 100
`manages sessions between the STUS 90 and the SPS 110.
`Among its duties, the NCMC 100 is responsible for provi
`Sioning the NAN 80, provisioning the STUS 90, providing
`routing information to the digital network 70 when
`appropriate, and for providing information Session manage
`ment between the STUs 90 and the SPs 110. In providing the
`session management, either the STUS 90 or the SPS 110 may
`Send requests for information Service connections to the
`NCMC 100. After receiving a request, the NCMC 100
`determines if there are resources available on the network 70
`for transporting the requested Services and, if So, establishes
`the requested service connection from the SP 110 to the STU
`90. The NCMC 100 then sends the Service information to
`both the STU 90 and the SP 110 to allow them to connect to
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`7
`the network and to begin the requested interactive informa
`tion service. The NCMC 100 may establish sessions in the
`manner described in U.S. Pat. No. 5,481,542, which is
`incorporated herein by reference in its entirety.
`II. Service Provider Complex
`The SPS 110 control the system that provides programs to
`the customer. To provide these programs, the SP employs
`one or more file Servers 60, a Server gateway 61, and in
`accordance with the present invention, a conditional acceSS
`manager (CAM)30 and a SABER 20. The file servers store
`programs in MPEG-2 transport packet format for delivery to
`customers. That is, when a customer requests a program
`from the file server 60, the