`
`United States Patent
`LaPier et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,333,931 B1
`*Dec. 25, 2001
`
`US006333931B1
`
`(54) METHOD AND APPARATUS FOR
`INTERCONNECTING A
`CIRCUIT'SWITCHED TELEPHONY
`
`_
`
`ggyggggggggggggggggggg
`
`THEREOF
`
`(75) Inventorsl gjlsgidlf- 1:111:98 NewAProvidi/rllf, NJ
`;
`avl
`.
`ran, cton,
`(US); Clay H. Neighbors, Reston;
`Khaled A. Gouda, Herndon, both of
`VA (Us); Andrew Adamiall, Potomac,
`MD (Us)
`
`( * ) Notice:
`
`(73) Assignee: Cisco Technology, Inc., San Jose, CA
`(Us)
`This patent issued on a Continued pros_
`ecution application ?led under 37 CFR
`153w), and is Subject to the twenty year
`atent term revisions of 35 U S C
`I1)54(a)(2)
`p
`'
`'
`'
`'
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U_S_C_ 154(k)) byo days
`
`_
`(21) Appl No 09/221,912
`"
`'
`(22) Filed;
`Dec. 28, 1998
`
`7
`(51) Int. Cl. ................................................... .. H04Q 11/00
`(52) US. Cl. ......................... .. 370/385; 370/410; 370/352
`(58) Field Of Search ................................... .. 370/352—563,
`370/385, 466, 467, 410, 522; 379/8817,
`202, 201-01—201-05, 201-11, 20112, 202-01,
`203-01; 348/ 14
`
`(56)
`
`_
`References Clted
`Us‘ PATENT DOCUMENTS
`
`5,828,666 * 10/1998 Focsaneau et al. ................ .. 370/389
`5,862,339 * 1/1999 Bonnaure et al. ..
`709/227
`2’333’333 i 31333 ‘755mm """"" " 5331551
`
`,
`
`,
`
`1 e e a .
`
`. . . . . . . .
`
`. . . ..
`
`
`
`* 6,021,126 * 10/1998 White et al. . . . . . . . . e.
`
`
`
`. . . .. 370/352
`
`6,084,892 * 7/2000 Benash et al.
`370/701
`6,111,893 * 8/2000 V If
`t
`l.
`..
`370/466
`6,112,305 * 8/2000 13211215111661? ....................... .. 713/156
`_
`_
`* cued by exammer
`Primary Examiner_chau Nguyen
`Assistant Examiner—Jasper Kwoh
`(74) Attorney, Agent, or Firm—Hickman Palermo Truong
`& Becker LLP; Christo her J. Palermo
`p
`(57)
`ABSTRACT
`
`methodand apparatus are disclosed for interconnecting a
`circuit-switched telephone‘ network and a packet-switched
`data network for communication among them, A signaling
`trunk of a circuit-switched telephone network is coupled to
`a signaling access server that is also coupled to a signaling,
`management and control network. One or more voice trunks
`of the telephone network are coupled to one or more network
`access servers, which are also coupled to the control network
`to thereby receive instructions from the signaling access
`server. The network access servers are also coupled to a
`packet-switched data network, such as the global packet
`. h d
`k k
`I
`Th .g
`1. g
`switc e networ nown as nternet.
`e 51 na in access
`server can receive call setup messages and other messages in
`standard telephone network protocol formats and convert
`them into can Setup messages in a Special protocol for
`Communications between the Signaling access Server and the
`network access servers, The signaling access server can
`instruct the network access servers to establish a call,
`containing voiceband information such as modem-based
`data calls, from the voice trunks to the data network.
`A d'glyhpbl' 'hdlph
`k dh
`ccor in
`,t e u 1c switc e te e one networ an t e
`Internet may be interconnected and may inter-communicate
`without modifying the protocols of either one.
`
`5,793,771 * 8/2000 Darland et a1. .................... .. 370/467
`
`19 Claims, 23 Drawing Sheets
`
`20
`_
`
`Signaling Access
`Server
`
`16
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`i
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`
`a
`a
`
`122
`
`4
`
`Servers
`
`14
`
`Cisco Systems, Inc., EX 1147, Page 1
`
`
`
`US. Patent
`
`Dec. 25, 2001
`
`Sheet 1 0f 23
`
`US 6,333,931 B1
`
`
`
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`ManagementNetwork
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`Cisco Systems, Inc., EX 1147, Page 2
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`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 2 0f 23
`
`US 6,333,931 B1
`
`FIG. 1B
`
`112
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`Cisco Systems, Inc., EX 1147, Page 3
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`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 3 0f 23
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`US 6,333,931 B1
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`Cisco Systems, Inc., EX 1147, Page 4
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`Dec. 25,2001
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`US 6,333,931 B1
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`US 6,333,931 B1
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`Cisco Systems, Inc., EX 1147, Page 7
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`Cisco Systems, Inc., EX 1147, Page 7
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`U.S. Patent
`
`Dec. 25,2001
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`Sheet 7 0f 23
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`US 6,333,931 B1
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`
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`US 6,333,931 B1
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`Cisco Systems, Inc., EX 1147, Page 9
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`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 9 0f 23
`
`US 6,333,931 B1
`
`FIG. 5
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`Cisco Systems, Inc., EX 1147, Page 10
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 10 0f 23
`
`US 6,333,931 B1
`
`FIG. 6
`
`602
`
`SERV|CE(COT) from SAS
`
`T _ _ _ _ J _ _ _ _ _ L
`
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`COT function?
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`Cisco Systems, Inc., EX 1147, Page 11
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 11 0f 23
`
`US 6,333,931 B1
`
`FIG . 7A
`
`704
`
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`
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`Cisco Systems, Inc., EX 1147, Page 12
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 12 0f 23
`
`US 6,333,931 B1
`
`FIG. 7B
`
`704
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`1 18
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`Cisco Systems, Inc., EX 1147, Page 13
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 13 0f 23
`
`US 6,333,931 B1
`
`FIG. 7C
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`1 18
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`
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`
`Cisco Systems, Inc., EX 1147, Page 14
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 14 0f 23
`
`US 6,333,931 B1
`
`FIG. 8A
`
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`Cisco Systems, Inc., EX 1147, Page 15
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 15 0f 23
`
`US 6,333,931 B1
`
`FIG. 8B
`
`Conversation
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`Cisco Systems, Inc., EX 1147, Page 16
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 16 0f 23
`
`US 6,333,931 B1
`
`FIG . 8C
`
`hr 20
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`
`Cisco Systems, Inc., EX 1147, Page 17
`
`
`
`U.S. Patent
`
`Dec. 25,2001
`
`Sheet 17 0f 23
`
`US 6,333,931 B1
`
`FIG. 8D
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`
`Cisco Systems, Inc., EX 1147, Page 18
`
`
`
`U.S. Patent
`
`2
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`US 6,333,931 B1
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`Cisco Systems, Inc., EX 1147, Page 19
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`Cisco Systems, Inc., EX 1147, Page 19
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`Cisco Systems, Inc., EX 1147, Page 20
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`Cisco Systems, Inc., EX 1147, Page 20
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`
`
`
`
`
`US. Patent
`
`Dec. 25, 2001
`
`Sheet 20 of 23
`
`US 6,333,931 B1
`
`Global
`
`NAS
`
`Configuration
`
`FIG. 11
`
`1106
`
`1104
`
`1108
`
`118
`
`1112
`
`-
`
`1 Faults + Measurements
`
`1100
`
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`
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`
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`
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`
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`
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`EMS
`
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`
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`
`1110
`
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`
`912
`
`112
`
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`
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`
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`
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`
`Configuration
`
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`\
`1118
`
`1116
`
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`
`SAS
`
`Configuration
`
`Cisco Systems, Inc., EX 1147, Page 21
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`Cisco Systems, Inc., EX 1147, Page 21
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`
`
`US. Patent
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`Dec. 25, 2001
`
`Sheet 21 0f 23
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`US 6,333,931 B1
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`FIG. 12
`
`‘
`
`Dial Application
`
`Call Control APl/SPI
`
`
`
`NAS/SAS
`
`Service
`
`Modem/ PPP
`
`Service
`
`
`
`
`
`
`
`Voice/FAX
`
`
`
`Signaling
`Protocol
`Provider
`Provider
`
`
`
`
` L2TP/L2F
`DSOflrunk
`Service
`
`
`
`Control
`
`
` IOS Kernel
`
`
`
`
`lP Routing
`
`
`
`
`
`
`
`
`DSP Complex (Modems, Voice/Fax)
`
`
`
`
`
`Wle
`
`Voice Network Trunks
`
`Cisco Systems, Inc., EX 1147, Page 22
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`Cisco Systems, Inc., EX 1147, Page 22
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`
`
`US. Patent
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`Dec. 25, 2001
`
`Sheet 22 0f 23
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`US 6,333,931 B1
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`FIG. 13
`
`1322
`
`1324
`
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`
`1302
`
`1310
`
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`
`Cisco Systems, Inc., EX 1147, Page 23
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`
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`
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`
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`
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`
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`
`Cisco Systems, Inc., EX 1147, Page 23
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`
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`US. Patent
`
`Dec. 25, 2001
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`Sheet 23 0f 23
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`US 6,333,931 B1
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`FIG. 14
`
`VD SUBSYSTEM
`
`905
`
`|/O Channel
`
`l/O Channel
`
`l/O Channel
`
`I ControllerROAA-i
`
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`
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`
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`
`External Protocol
`
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`
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`
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`
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`
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`
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`
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`M22
`
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`
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`
`Call Control
`M25
`
`Cisco Systems, Inc., EX 1147, Page 24
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`Cisco Systems, Inc., EX 1147, Page 24
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`
`
`US 6,333,931 B1
`
`1
`METHOD AND APPARATUS FOR
`INTERCONNECTING A CIRCUIT-
`SWITCHED TELEPHONY NETWORK AND A
`PACKET-SWITCHED DATA NETWORK, AND
`APPLICATIONS THEREOF
`
`FIELD OF THE INVENTION
`
`The present invention generally relates to telecommuni-
`cations. The invention relates more specifically to intercon-
`necting a circuit-switched telephone network to a packet-
`switched data network, and methods of using the
`interconnected networks.
`
`BACKGROUND OF THE INVENTION
`
`The public switched telephony network (PSTN) is made
`up of carriers who have agreements to interconnect with one
`another and switch calls to among one another. The PSTN
`comprises numerous switching systems, which are stored
`program controlled computers comprising line terminations.
`The switching systems connect to other switching systems,
`forming indirect links from customer premises to customer
`premises around the world.
`In a common channel signaling (CCS) network, switching
`systems interconnect using bearer channels and signaling
`channels. Bearer channels generally are 64 kbit/second
`channels that carry voice traffic such as modem calls.
`Signaling channels carry call
`information in a separate
`network; this is known as out-of-band signaling because call
`signaling information is communicated outside the audible
`voice band of calls. ITU Recommendation Q.700 describes
`this type of PSTN in detail.
`to the signaling network.
`Only carriers may connect
`However, as a result of national and international deregu-
`lation of telephone systems, many new carriers are entering
`or are about to enter the field. For example, in the United
`States, in the past there have been approximately seven (7)
`major signaling carriers and numerous independent carriers,
`but in the future, there will be many new local exchange
`carriers. Internet Service Providers (ISPs), for example, are
`expected to rush into the local exchange switching market.
`In a regulated environment, the major signaling carriers
`could establish common standards and rely on trust of one
`another to ensure that proper signals were carried in the
`signaling network. In a deregulated environment, the sig-
`naling network is perceived as a less trustworthy environ-
`ment. In particular, in the past, the physical security of SS7
`switching facilities provided the primary overall security of
`an SS7 switching network. Now, there is less assurance that
`new carriers will maintain the same level of physical facility
`security that the major carriers have enforced in the past.
`Further, in the deregulated environment, a carrier cannot
`be trusted to connect to the network and use it in a proper
`manner. For example, a carrier could generate calls for no
`reason. Acarrier could claim to have reachability to another
`location when in fact it doesn’t have that capability. Acarrier
`could express state information about its circuits incorrectly.
`A carrier could generate Initial Address Messages to distant
`switches and then wait for them to time out. All these
`
`operations are improper and potentially harmful to other
`network elements and to the private data networks that are
`connected to such network elements.
`
`In data networks, firewalls are available that are based on
`UNIX computer systems, and may operate in standalone
`fashion or are integrated into routers. Firewalls can be used
`to selectively admit or deny data packets based on rules or
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`policies. Such firewalls, however, cannot be used in SS7
`telephone networks or their component systems because
`their hardware and software is incompatible. For example,
`lower SS7 messaging layers require unique firmware and
`hardware for terminating trunks. Further, SS7 is a message-
`oriented protocol that requires the use of special messages
`for graceful startup and shutdown of links. These messages
`are not supported in available firewalls.
`Accordingly, there is a need to improve signaling security
`and data channel security between signaling network enti-
`ties.
`
`there is a need to protect internetworks,
`In particular,
`LANs and WANs from undesirable, undefined, or malicious
`signals and messages originating in local exchange carriers
`or other carriers outside such data networks.
`
`There is also a need for a system and method that can
`interconnect a data network, and servers thereof, to a circuit-
`switched telephony network that uses time-division multi-
`plexed signaling, for a variety of purposes. In particular,
`there is a need for a system and method that can interconnect
`a data network to a circuit-switched telephony network that
`uses Signaling System 7 protocols for signaling.
`SUMMARY OF THE INVENTION
`
`The foregoing needs and objects, and other needs and
`objects that will become apparent from the following
`description, are fulfilled by the present invention, which
`comprises, in one aspect a telecommunications apparatus
`that can interconnect a circuit-switched telephone network
`and a packet-switched data network to enable communica-
`tion among the networks. The apparatus has a first server
`coupled to a signaling trunk of the telephone network to
`interchange call signaling messages therewith, and coupled
`to a control network. One or more second servers each are
`
`coupled to a voice trunk of the telephone network to
`communicate voice-band information thereon, and coupled
`to receive instructions from the first server through the
`control network, and coupled to the data network to com-
`municate data thereon. First and second software compo-
`nents are executed by and controlling the first server and the
`second server, respectively, and interact cooperatively to
`establish a call originating in the telephone network and
`containing voiceband information and terminating in the
`data network, based on call signaling information received
`from the signaling trunk by the first server.
`One feature of this aspect is that the first and second
`software components interact cooperatively to establish a
`call originating at one of the second servers and containing
`voiceband information and terminating in the telephone
`network, based on call signaling information generated by
`the first server and provided on the signaling trunk.
`Another feature is that the second servers are coupled to
`the data network by an Internet Protocol (IP) backbone
`apparatus, and the second servers are coupled to the first
`server and to a management network by the control network,
`and the control network, the management network, the first
`server, and the second server communicate using IP.
`In another feature, the signaling trunk and the voice trunk
`originate in a Common Channel Signaling telephone net-
`work. Another feature is that the signaling trunk is a Sig-
`naling System 7 (SS7) trunk, and the data network is a
`global, packet-switched network that operates using Internet
`Protocol (IP) and Transmission Control Protocol (TCP).
`According to another feature, the signaling trunk is coupled
`to a Signal Transfer Point of the telephone network and each
`of the second servers is coupled to receive the voiceband
`information from a switch of the telephone network.
`
`Cisco Systems, Inc., EX 1147, Page 25
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`Cisco Systems, Inc., EX 1147, Page 25
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`US 6,333,931 B1
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`3
`In yet another feature, the signaling trunk is coupled from
`the first server to a digital cross connect apparatus that is
`coupled to the telephone network, and each of the second
`servers is coupled by an associated voice trunk to the digital
`cross connect apparatus to receive the voiceband informa-
`tion therefrom. Still another feature is that the apparatus
`includes a cross-connect apparatus logically coupled
`between the telephone network and both the signaling trunk
`and the voice trunk, and means, coupled between the sig-
`naling trunk and the first server, for communicating call
`routing signals from the digital cross-connect apparatus
`through the signaling trunk and to the first server. The
`software components are configured to receive call signaling
`messages from the means for communicating and over the
`control network, and to communicate the call signaling
`messages back over the control network to the second
`servers.
`
`the software components
`Still another feature is that
`comprise instructions which, when executed by the first
`server, cause the first server to carry out
`the steps of
`receiving a call setup message from the telephone network
`that contains a master telephone number; sending a respon-
`sive message to the telephone network that contains a
`specific telephone number associated with one of the second
`servers; and setting up a call between the telephone network
`and one of the second servers that is associated with the
`
`the
`specific telephone number. A related feature is that
`software components further comprise instructions which,
`when executed by the first server, cause the first server to
`carry out the steps of receiving a call offer message from the
`telephone network that contains the specific telephone num-
`ber.
`
`Another related feature is that the software components
`further comprise instructions which, when executed by the
`first server, cause the first server to carry out the steps of
`determining that the one of the second servers has insuffi-
`cient resources to set up the call; communicating a call
`release message from the first server to the telephone
`network; communicating a second responsive message from
`the first server to the telephone network, wherein the second
`responsive message contains a second specific telephone
`number that is associated with a second one of the second
`servers.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention is illustrated by way of example,
`and not by way of limitation, in the figures of the accom-
`panying drawings and in which like reference numerals refer
`to similar elements and in which:
`
`FIG. 1A is a block diagram of a preferred embodiment of
`a call monitoring system coupled to a telephony network and
`a data network;
`FIG. 1B is a network diagram of further details of the
`system of FIG. 1A;
`FIG. 1C is a block diagram of the logical placement of a
`preferred embodiment of the system of FIG. 1A;
`FIG. 2A discloses an alternate configuration of the system
`of FIG. 1C;
`FIG. 2B discloses another alternate configuration of the
`system of FIG. 1C;
`FIG. 2C is a block diagram of a process of routing a call
`using the system of FIG. 1A;
`FIG. 3 is a block diagram of a configuration of multiple
`Network Access Servers and a Signaling Access Server;
`FIG. 4 is a state diagram of processes executed by a
`Network Access Server in processing calls;
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`4
`FIG. 5 is a state diagram of processes carried out by a
`Signaling Access Server;
`FIG. 6 is a state diagram of processes carried out by the
`Network Access Server to perform continuity testing;
`FIG. 7A is a call flow diagram showing messages com-
`municated in carrying out certain call processing functions;
`FIG. 7B illustrates messages exchanged as in FIG. 7A,
`with an alerting function applied to the call;
`FIG. 7C illustrates messages exchanged as in FIG. 7A,
`when establishing a call that is originated by a Network
`Access Server;
`FIG. 8A illustrates messages exchanged in clearing or
`completing a call;
`FIG. 8B illustrates messages exchanged when a call is
`cleared from the Network Access Server side of the con-
`nection;
`FIG. 8C illustrates messages exchanged in establishing a
`call originated by the telephone network and with successful
`continuity testing;
`FIG. 8D is a diagram of messages communicated among
`the network, Signaling Access Server, and Network Access
`Server in setting up a call, originated at the network, with
`continuity testing enabled but when continuity fails;
`FIG. 9 is a block diagram of internal components of
`Signaling Access Server including the means of intercom-
`munication among the components and the output produced
`by the components;
`FIG. 10 is a block diagram of a process of converting call
`detail records;
`FIG. 11 is a block diagram of an integrated Element
`Management System;
`FIG. 12 is a block diagram of an embodiment of software
`elements that may be executed by Network Access Server;
`FIG. 13 is a block diagram of hardware components of a
`Signaling Access Server;
`FIG. 14 is a block diagram of software elements relating
`to protocol conversion that may be executed by a Signaling
`Access Server.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`A method and apparatus for interconnecting a circuit-
`switched telephony network and a packet-switched data
`network are described. In the following description, for the
`purposes of explanation, numerous specific details are set
`forth in order to provide a thorough understanding of the
`present
`invention.
`It will be apparent, however,
`to one
`skilled in the art that the present invention may be practiced
`without these specific details. In other instances, well-known
`structures and devices are shown in block diagram form in
`order to avoid unnecessarily obscuring the present inven-
`tion.
`
`System Overview
`
`The present disclosure will describe an invention which,
`in one embodiment, provides a distributed system for inter-
`connecting one or more Network Access Servers, which are
`coupled to a data network such as the Internet, to a circuit-
`switched Time Division Multiplexing telephone network
`that uses Signaling System 7 protocols for signaling. The
`interconnection is achieved, in part, using a protocol con-
`verter that
`is configured as a Signaling Access Server
`(“SA ”) and that interworks with a Network Access Server
`(“NAS”). “Interworks” means, for example, that the Signal-
`
`Cisco Systems, Inc., EX 1147, Page 26
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`Cisco Systems, Inc., EX 1147, Page 26
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`
`
`US 6,333,931 B1
`
`5
`ing Access Server commands and manages the Network
`Access Servers using a signaling and control network; it is
`also possible for the NAS to place outgoing calls through the
`SAS to an SS7 network.
`
`FIG. 1A is a block diagram of the logical placement of a
`preferred embodiment of a call monitoring system 2 within
`a telephony network 4 and a data network. System 2
`comprises one or more Network Access Servers 118, located
`at one or more service provider points of presence. Each
`Network Access Server 118 is coupled to and provides
`termination for a pre-determined number of voice network
`trunks 10. Each Network Access Server 118 has a first
`
`Internet Protocol network interface coupled to a data net-
`work backbone 14 and a second Internet Protocol network
`
`interface coupled to the service provider’s management,
`signaling and control network 12. There may be more than
`two IP network interfaces so that
`the system supports
`redundant IP links for communication between the NAS and
`SAS.
`
`The data network backbone 14 is coupled to an external
`IP data network 122, such as the Internet. The data network
`backbone is owned and operated by the service provider,
`which may be an Internet Service Provider. The service
`provider’s management, signaling and control network 12 is
`generally maintained securely, according to telephone indus-
`try standards, and is generally more trusted than the IP data
`network 122.
`
`The voice network trunks 10 may originate from any type
`of Time Division Multiplexing network infrastructure, and
`may be ISDN Primary Rate Interface trunks, T1 trunks, E1,
`T3, E3, etc. The voice network trunks 10 are matched with
`compatible digital interfaces at each Network Access Server
`118. In the preferred embodiment, the voice network trunks
`10 comprise only bearer channels. Thus, the voice network
`trunks do not carry associated signaling channels. Instead,
`all signaling channels of telephone network 4 terminate at
`Signaling Access Server 112.
`One or more Signaling Access Servers 112 are coupled to
`one or more SS7 signaling trunks 20. Each Signaling Access
`Server 112 terminates the SS7 signaling links and handles
`voice network maintenance messages, as well as call control
`messages. Aparticular Signaling Access Server 112 may be
`co-located with the Network Access Servers 118, or located
`near a circuit switch or Service Control Point of a telephone
`service provider. One Signaling Access Server 112 may
`provide signaling and call processing services for a large
`number of Network Access Servers 118. Each Signaling
`Access Server 112 may be replicated for redundancy. From
`the point of view of the network 4, each Signaling Access
`Server 112 represents a terminating and originating Service
`Control Point.
`
`The SS7 signaling trunks 20 may be A-Links, or F-Links
`that have been separated from their associated voice trunks
`and delivered to the Signaling Access Server 112 using
`separate facilities. Alternatively, SS7 signaling may be back-
`hauled over voice network trunks 10.
`In still another
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`the SS7 signaling may be encapsulated by a
`alternative,
`separate apparatus and backhauled over the IP Control/
`Maintenance Network 16.
`
`60
`
`The Signaling Access Server 112 communicates with the
`Network Access Servers 118 over the management, signal-
`ing and control network 12. The Signaling Access Server
`112 is coupled to the management, signaling and control
`network 12 by connection 22. Preferably, Signaling Access
`Server 112 communicates over the network 12 with Network
`
`65
`
`Access Servers 118 using an agreed-upon protocol, which
`
`6
`may be similar to the UDP protocol. Advantageously, only
`layer 3 of the ISDN protocol is needed when the system
`operates using UDP.
`Functions of the Signaling Access Server 112 include SS7
`signaling; carrying out call control in communication with a
`Network Access Server; establishing and managing
`bi-directional calls;
`fault management; compiling and
`reporting on telephony statistics; implementing custom fea-
`tures; and providing call detail accounting.
`Each Network Access Server 118 provides bearer channel
`termination; implements modem pools; provides IP routing;
`implements authentication, authorization, and accounting
`(AAA) functions; generates telephony tones using digital
`signal processors; provides virtual private network manage-
`ment; manages faults that occur with bearer channels; and
`compiles and reports on statistical information relating to
`bearer channels. For example, a Network Access Server 118
`may provide routing functions that carry out authentication
`functions and call completion functions. Preferably, each
`Network Access Server 118 has a Q.931 protocol stack for
`use with ISDN Primary Rate Interface (PRI). Each NAS also
`contains a time division multiplexer (TDM) that chooses a
`channel and selects and assigns a free modem from among
`the modems in the NAS. Each NAS may accept a loopback
`circuit, which may be accomplished by installing a driver
`and modifying microcode that controls the TDM so as to
`permit circuits to go into a loopback.
`FIG. 1B is a network diagram of further details of the
`system of FIG. 1A. Signaling Access Server 112 is coupled
`to a local area network or LAN 108 that comprises numerous
`network devices, such as Network Access Servers
`118a—118c. An example of a Network Access Server is the
`Cisco Model ASS300, commercially available from Cisco
`Systems, Inc.