Abstract:
The present invention provides a new type of router, called a PACS router, to build an improved IP network in which high-quality services such as voice and best-effort data services can be provided with higher reliability and lower cost. A PACS router includes a packet-switching fabric and a circuit-switching fabric interconnected by channelized high-speed links. A network built with PACS routers uses signaling and routing protocols to segregate traffic into different classes and route them using circuit-channels and packet-channels depending on quality of service requirements. Routing high-quality service over circuit-channels eliminates transit delay typically incurred in packet-switching fabrics, enhances reliability from software malfunction in the router and enhances network scalability by not having to terminate all traffic into packet-switching fabrics in every router. The circuit-switching fabrics in the PACS routers provide further immunity by fast rerouting of failed circuits via alternate routes when a network fault is detected.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. provisional application Ser. No. 60/550,203, filed 5 Mar. 2004, entitled “Method and Apparatus for Improved IP Networks and High-Quality Services,” which is incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to communications networks in general, and, more particularly, to IP routers and circuit switches. 
   BACKGROUND OF THE INVENTION 
   The telephony network has evolved over many decades; particularly since the 1960s, when stored-program-controlled switches with specific engineering requirements for voice telephony were deployed. Engineering requirements that are critical to achieve high-quality voice telephony include: (1) guaranteed bandwidth, (2) minimum network delay, and (3) a very low probability (≦0.1%) that a network would block an attempted call or drop an ongoing call. Transmissions that satisfy these engineering requirements are defined for the purposes of this specification as “high-quality traffic”. 
   The Internet, on the other hand, has evolved over the last decade to transport mostly computer data originating from web browsing, document and music downloading, and e-commerce. Yet traffic on the Internet surpassed the total traffic on the voice telephony network several years ago. While the volume of traffic on the Internet is increasing, network requirements for computer data traffic need not be as stringent as for voice telephony, however. 
   The operations and maintenance of the separate voice telephony network is expensive given that the ratio of telephony traffic to Internet data traffic will continue to decrease. 
   While Internet Protocol (IP) telephony, also known as voice over IP (VoIP) service, has been introduced several years ago, the quality of VoIP service has not yet been at par with the quality of service on the pre-existing voice telephony network. The Internet substantially operates in a “best-effort” delivery mode for packets that are traversing its network. Packets are forwarded from one node (i.e., a signal termination or transit point in a network, such as an IP router in an IP network or a cross-connect in a voice network) in the network to the next node with no regard for the capacity available in the link between the nodes or delay that the packets might encounter, and without a guarantee that the packets will be delivered to their final destination. Transmissions made in a best-effort manner, as described above, are defined for the purposes of this specification as “best-effort traffic”. 
   The quality of VoIP services is degraded by multiple factors that include: (a) latency for a packet to travel from source to destination (i.e. transit delay), (b) variation in latency from one packet to another commonly known as jitter, and (c) loss of packets. In an effort to mitigate the degradation in service quality due to these factors, some characteristics of the circuit-switched telephony network have been incorporated into the VoIP service network. For example, Multi-Protocol Label Switching (MPLS) has been developed and deployed in some best-effort packet switching networks in order to mimic the circuit-switching features of high-quality service. In spite of these improvements for VoIP implementation in IP networks, the basic nature of the packet switching devices, wherein the header of each packet is examined and then the packet is forwarded to a next destination (i.e., the next “hop”), remains a fundamental challenge to providing ubiquitous high-quality VoIP service over an IP network. On the other hand, costly maintenance and operation of the old circuit-switched voice telephony network for a diminishing fraction of the total information traffic offers a challenge as well as an opportunity to offer both best-effort traffic and high-quality traffic in an integrated network. 
   A typical telephony network comprises a set of stored program controlled (SPC) switching systems, known as Class V switches, which are interconnected by digital links, referred to as “trunk” lines. The Class V switches provide the interconnection between customer voice access lines and the trunk-lines. Each Class V switch can terminate up to tens of thousands of customer access voice lines. A circuit (i.e., the connection from a first customer access line to a second customer access line through the network) is established through one or more Class V switches, and the connectivity is based on a signaling scheme, such as Signaling System 7 (SS7). 
   Each voice channel is carried on a single 64 Kb/s digital stream known as a DS0 signal. Each trunk-line carries multiple DS0 signals in a higher level of digital stream; 24 DS0 signals compose a DS1 signal in North America, while 32 DS0 signals compose an E1 signal in Europe and elsewhere. The plurality of DS0 signals are multiplexed into the higher level signals using Time Division Multiplexing (TDM). Hence the digital voice telephony network is also called TDM network. The DS1 signals are further multiplexed to higher bit streams using an older asynchronous TDM scheme to DS2 (6.312 Mb/s) and DS3 (44.712 Mb/s) digital signals. 
   A newer TDM technique known as Synchronous Optical Network (SONET) mostly used in North America and Synchronous Digital Hierarchy (SDH) in other parts of the world are used to multiplex the lower level digital signals, such as DS1 and DS3, to a much higher-speed signal, such as OC-48 (2.5 Gb/s) or OC-192 (10 Gb/s), a number of which in turn are wave division multiplexed by a Wavelength Division Multiplexing (WDM) system for long distance transport over optical fiber. Even though SONET and SDH technologies are very similar now, historically they were developed for multiplexing 24-cahnnel and 32-channel digital streams to synchronous optical signals. However, the two hierarchies and standards evolved to converge so that there is practically no difference between the two. Thus henceforth only 24-channel terminology will be used even though all aspects of this application shall apply equally to the 32-channel system. 
   A transport network multiplexes low-level signals, transports the multiplexed high-speed signal over a long distance, and then demultiplexes it to extract the lower level signal for connection to Class V switches and other service access systems at the distant location. In addition to the management of the interconnectivity of the service access systems, a transport network provides another key network function, namely, network protection and restoration. In the case of a network link or equipment failure, the transport network finds alternate route to restore affected connections via alternate links or routes among the service access systems within a fraction of a second, thereby making the service systems immune to network failures. 
   While the transport network provides robust connectivity and management of the connectivity of voice-switches, such as Class V switches, it does not participate in making decisions on how a DS0 signal within a trunk-line is used to route a call. The voice-switches provide the routing function using a signaling scheme, such as SS7, and a different signaling scheme, such as tip-and-ring, between a voice-switch and a user terminal, such as a telephone. 
   The telephony network meets the key voice service requirements by its nature of being a circuit-switching network and by proper engineering of the network. A fixed bandwidth requirement to transmit a voice signal between two parties is automatically met by the nature of the circuit-switched network. When a connection is made it is of a fixed bandwidth typically 64 Kb/s between the two devices and the circuit is used exclusively for one call during the call session. Telephony network meets the minimum delay requirement also because of the nature of circuit-switching technology. A circuit switch device does not need to continuously process the circuit in the transit nodes once the connection is established. Thus, the delay for a voice signal from one end to the other is kept at a minimum, which is essentially the transit delay, required for the electrical or optical signal to travel over the transmission medium. The requirement on minimum call blocking probability is met by proper engineering. Using traffic characteristics such as incoming call volume, call duration etc. and traffic engineering techniques, an appropriate number of trunk-lines are configured in a switch to keep the call blocking probability to a required minimum level. In addition, call-dropping probability is kept at a minimum by using the network protection and restoration techniques in the transport network to further enhance the reliability of the trunk-lines. 
   An IP network is essentially a network of computers to exchange data usually between server and client computers. An IP network comprises a set of IP routers that are connected by high-speed SONET/SDH links on one side and the server and client computers on the other. The application programs running in the client/server computers generate information packets each with a destination address added at the header of the packet. The routers then forward each packet independently, using a set of routing protocols. 
   Unlike in a telephony network, there is no signaling to first establish an end-to-end fixed bandwidth connection for a packet stream in an IP network. Instead, each router has the means to generate and update a routing table, which includes all reachable IP addresses in the network. A router upon receiving a packet looks at the destination address in the packet; checks the routing table for best match with the destination address and decides which interface it will forward the packet to its next hop router. This process is repeated in each router on the route of a packet until it reaches the final destination. 
   The computer application program at the other end removes the added information bytes from the packets and then assembles the packets to generate the information for presentation to the user, whether it is a document, a picture or music. The packets from a single application may even arrive via different routes and in a different sequence than the one generated at the source. The application program stores these packets and rearranges the sequence for the information to be presented to the user. The process of storing the packets for rearrangement adds further delay to the information transport. In addition, the transmission links between routers are shared among different applications passing through a router. Thus, there is no guarantee that a specific application session will have a minimum bit rate capacity or bandwidth along its routes. 
   In an IP network, packets are discarded when there is a shortage in link capacity. When packets are discarded, the application layer program detects that packets are missing, and requests the sender of the missing packets to resend them. The process of resending also adds to the overall delay. Delay, however, is not detrimental to best-effort traffic. The IP network is not well suited, however, for applications wherein high-quality traffic, such as VoIP, video telephony and videoconferencing, is transmitted. 
   IP routers are being updated with some capabilities to deal with these impairments. For example, some applications may extend the headers of high-quality traffic packets with one or more markers that designate these packets for special handling. These markers may include: source-routing, in which the source specifies the entire route to be taken by the packets; a quality of service (QoS) indicator, which specifies routing priority for high-quality traffic packets over best-effort traffic packets; or routing protocols, such as MPLS or VPN, which emulate circuit behavior within routers. These features mitigate, to some extent, the IP network deficiency for high-quality traffic. It is well known, however, that they tend to degrade the performance of routers, which are most efficient for routing packets that do not contain markers. 
   A media gateway function is required between an IP network and a TDM network in order to provide voice telephony service between a user connected to the IP network and another user connected to the TDM network. The media gateway transcodes between packet based voice, VoIP, onto a TDM network. 
   In addition to the media gateway function, it is also possible to incorporate packet switching fabric for VoIP services in the same system. The two switching fabrics may be implemented in a single hardware or in separate hardware but under a common control and signaling infrastructure. A set of signaling and routing protocols for call control such as SS7, SIP, MGCP and others can unify services in an integrated network for both traditional and IP based access means such as voice access over wire pairs and VoIP access such as cable modem and DSL. While the integrated media gateway and circuit-switching system technology offers a means for interworking between the TDM voice network and IP network, voice calls in the IP network are still routed hop-by-hop and packet-by-packet in the IP network. 
   The telephony network meets the stringent reliability and latency requirements using circuit-switching technology (also known as TDM technology), while an IP network provides flexible and low-cost data communications that do not impose stringent requirements for voice telephony. However, operations and maintenance of a separate voice network for a small fraction of the total traffic is expensive. While an integrated media gateway and voice-switch allows voice calls to flow seamlessly from IP network to TDM network, the cost and complexity to route both high-quality and best-effort traffic in the IP network still remain significant issues. Furthermore, efficient interconnection of voice-switches continues to be problematic. In a network, each switch needs to communicate with every other switch for call set up as well as call routing. However, it is impractical to interconnect every switch with every other switch since such interconnection would require that the number of links grow as the square of the number of switches in a network. 
   A traditional voice network typically deploys tandem switches, which are intermediary switches that facilitate the interconnection of voice switches. For example, a tandem switch is used to route a call between two voice-switches to which it is connected when the two voice-switches do not have a direct link between them. The tandem switch routs the call in response to a request from one of the two voice-switches. Several layers of tandem switches typically exist in a traditional voice network for providing such connectivity. 
   An IP network on the other hand is a flat network that routes traffic hop-by-hop. For example, traffic between two routers A and D that are not directly connected but connected through a chain of routers B and C, with links AB, BC and CD, would flow from A to B over link AB, B to C over link BC and finally from C to D over link CD. At each intermediate node B and C the transport links are terminated and all packets are extracted at the intermediate nodes and reassembled in the next link with other traffic. The termination of the transport links and routing via the packet-switching fabrics in routers at each intermediate node is expensive and it is difficult to build larger packet fabric to route transit traffic. In addition, such hop-by-hop transfer adds unnecessary delay in each intermediate node. Thus there is a need for a device to build an integrated network that would meet the same robustness and delay requirements for high-quality traffic while providing flexible, low-cost and scalable transport of best-effort traffic. 
     FIG. 1  depicts an IP network according to the prior art. IP network  100  is a packet-switched network, i.e. information is carried from one network element to another by means of breaking messages up into discrete, variable length packets. Each packet contains a header section, which includes information about the destination address, source address, packet&#39;s priority, etc., and a payload section, which contains the data that makes up a portion of the message. IP network  100  comprises IP routers,  102   1  through  102   5 , which are interconnected in a network configuration via links  104   1  through  104   7 . For example, IP router  102   1  is connected to IP routers  102   2 ,  102   3 , and  102   5 , via links  104   1 ,  104   4 , and  104   6 , respectively. Each of links  104   1  through  104   7  may be an aggregate link of multiple OC-48 or OC-192 links. 
   IP router  102   i , where i=1 through 5, provides connectivity between user equipment  106   1  through  106   3  using packetized data transmissions over network links  104   1  through  104   7  . IP router  102   i  comprises logic circuitry, memory, and routing information and protocols that enable IP router  102   i  to receive an information packet, examine the packet, determine the destination for the packet, and decide on an immediate, potentially intermediate, destination for the packet. IP router  102   i  then transmits the packet to its immediate destination over either a network link (if the final destination is not connected to IP router  102   i ) or an access link (if the destination is directly connected to IP router  102   i ). IP router  102   i  is described below and with respect to  FIG. 2 . 
   Network link  104   i  comprises a plurality of OC-48 and OC-192 signal lines, which carry high-bandwidth transmissions between IP routers (e.g., network link  104   1  provides transmissions between IP router  102   1  and  102   2 ). In many instances, IP router  104   i  will aggregate packets that originated as part of different signals into a single transmission at OC-48 or OC-192 rates. 
   IP network  100  operates in a best-effort, hop-by-hop manner. For example, a transmission by user equipment  106   1 , which has an intended destination  106   3 , is spread over multiple packets of data. Each packet includes header information, which contains the intended address for that packet. IP router  102   1  receives each packet from user equipment  106   1 , and examines the destination header of each packet and decides which router to forward the packet to based on an internal connectivity table (i.e., a routing table). Each packet of the transmission may be sent to a different IP router, which is connected to  102   1 , and may be joined with other packets to compose an OC-48 or OC-192 signal. Each IP router that receives a packet then goes through the same process, wherein it examines the destination header and decides which IP router to which it is connected should receive the packet (i.e., decides on the next hop). Hop-by-hop routing continues until each packet is received by IP router  102   4 , which is connected to user equipment  106   3 . The packets that compose the transmission may arrive at user equipment  106   3  in any order and each packet can incur various time delays that affect the total delay in user equipment  106   3  receiving the transmission in its entirety. 
   The simplicity of best-effort, hop-by-hop routing and uniformity of signaling and routing protocols make IP network  100  easy to operate. However, IP network  100  is deficient in delivering high-quality services efficiently, in providing scalability and robustness because each intermediate router must route transit traffic packet by packet. 
     FIG. 2  depicts a schematic diagram of the salient components of an IP router in accordance with the prior art. Router  102   4 , which is representative of IP routers  102   1  through  102   5 , comprises packet-switching fabric  210   4 , processor  212   4 , and link interfaces  222 . Processor  212   4  comprises logic circuitry and memory and includes routing protocol  214   4 , link state database  216   4 , routing table  218   4 , label switching path database  220   4 , and signaling protocol  218   4 . 
   Packet switching fabric  210   4  is a matrix of electronic switches and logic circuitry that receives a packet, reads the destination header of the packet, compares it to the closest match in routing table  218   4  to determine the next hop destination, and sends the packet out on the appropriate network link where a link interface assembles the packet into a signal such as a SONET signal ready for transport to the next IP router. 
   Signaling protocol unit  218   4  sends and receives protocol messages through the packet fabric using the links  104   3 ,  104   5 , and  104   7 . Based on these messages each router learns the existence of other routers connected in the network and how they are connected i.e., the network topology. As a result, each router creates and maintains the topology information in link state database  216   4 . Link state database  216   4  is used to generate routing table  218   4  using a variety of algorithms such as shortest path routing. In addition, if the router has the capability to route a set of packets grouped in a data stream called a Forwarding Equivalency Class (FEC) based on certain criteria such as a higher QoS requirement using labels instead of destination headers. Then IP router  102   4  uses signaling protocol such as RSVP and CR-LDP to create label switching paths (LSP) database  220   4 . LSP database  220   4  is then used to route packets based on the label header instead of the destination header of the packet. 
     FIG. 3  depicts a schematic diagram of the salient components of a voice network in accordance with the prior art. Voice network  300  comprises voice-switches  312   1  through  312   5  which are connected via cross-connects  310   1  through  310   5 . Cross-connects  310   1  through  310   5  are interconnected by network links  304   1  through  304   7 . Voice-switches  312   1  through  312   5  are connected to cross-connects  310   1  through  310   5  by trunk links  318 . 
   Cross-connect  310   i , where i=1 through 5, is a circuit-switching fabric and an associated fabric controller that interconnects any one of N inputs to any one of M outputs. Cross-connect  310   i  will be described below and with respect to  FIG. 4 . 
   Voice-switch  312   i , where i=1 through 5, is a DSO-signal-level voice-switching fabric which provides connections between voice terminals  306   1  and  306   2 . The connections between user terminals  306   1  and  306   2  are made via voice-switches using signaling such as SS7 signaling. Cross-connect  310   i  provides multiplexing of low-speed signals from the voice-switches to high-speed signals such as OC-48 and OC-192 for transmission to other nodes; provides demultiplexing of a high-speed signal such as OC-48 and OC-192 arriving from a first node; connects one or more low-speed signals from the arriving high-speed signal to the voice-switch connected to cross-connect  310   i ; and provides multiplexing the remaining low-speed signals within the arriving high-speed signal and one or more low-speed signals arriving from the voice-switch  312   i  into a high-speed signal such as OC-48 and OC-192 for transmission to a second node in the network. 
   Cross-connect  310   i  is typically operated via external commands from a Network Management System (not shown). Based on forecasted demands and traffic patterns a network design is developed which provides the number of low-speed links  318  connecting voice-switches  312   i  to cross-connect  310   i . Based on this connectivity design, the cross-connect management systems configure cross-connects  310   1  through  310   5  to implement the network design (i.e., the switch connectivity). The connectivity typically remains static for several months until a new network design based on new demand forecast is implemented. 
   Cross-connect  310   i  also provides another key transport function, namely, restoration from catastrophic network failures such as a fiber cut. When a failure is detected, cross-connects  310   1  through  310   5  autonomously detect the failure and reconnect the failed links via an alternate route using network links reserved for restoration. 
     FIG. 4  depicts a schematic diagram of the salient components of a cross-connect in accordance with the prior art. Cross-connect  310   2 , which is representative of each of cross-connects  310   1  through  310   5 , comprises circuit-switching fabric  414   2 , fabric controller  416   2 , and TDM interfaces  418   1  through  418   4 . 
   Circuit-switching fabric  414   2  is a matrix of electronic logic that interconnects any one of N inputs to any one of M outputs. 
   Fabric controller  416   2  is a processor, which provides control over the connectivity between each of the N inputs and each of the M outputs of circuit-switching fabric  414   2 . Fabric controller  416   2  interprets instructions provided by the Network Management System and translates these instructions into the specific switch configuration required to establish appropriate connectivity within circuit-switching fabric  414   2 . 
   TDM interfaces  418   1  through  418   3  terminate overheads of incoming signals on network links  304   1 ,  304   2 , and  304   3 . Overheads include section overheads and line overheads of OC-48 and OC-192 signals arriving at TDM interfaces  418   1  through  418   3 , which are first terminated at the interfaces for network performance monitoring, multiplexing lower level digital bit streams into higher level signals such as OC-48, demultiplexing of higher level signals coming from the network links into lower level digital signals or tributaries. Lower level digital signals in appropriate formats from the TDM interfaces  418   1  through  418   3  are then sent to circuit-switching fabric  414   2 . Cross-connect  310   2  does not terminate SONET Path overhead since packets inside the SONET payloads are not extracted in the cross-connect  310   2 . 
   Circuit-switching fabric  414   2  then connects the lower level digital signals (i.e., tributaries) to appropriate interfaces for multiplexing and transporting to the next node. Upon receiving a command from a network management system or detecting a network failure, affected circuit-switching fabrics of cross-connects  310   1  through  310   5  change their corresponding circuit-switching fabric  414   1  through  414   5  configurations to, in the case of a received command, provide the connectivity specified by the network management system, or, in the case of a detected network failure, a pre-configured protection circuit. Cross-connects  310   1  through  310   5  do not participate in signaling that voice-switch  312   i  uses to set up and control voice calls. 
   SUMMARY OF THE INVENTION 
   The present invention provides methods and systems for providing communications services in a communications network without some of the costs and disadvantages for doing so in the prior art. In the present invention, methods and systems for providing both best-effort communications data and delay-sensitive, high-quality communications data in a single integrated communications network, which comprises a new type of router, hereinafter referred to as a “Packet-switching and Circuit-switching System (PACS) router”. A communications network comprising PACS routers provides improved performance in some metrics, such as performance, cost efficiency, reliability and scalability, over communications systems in the prior art. 
   A PACS router comprises a plurality of switching devices, at least one circuit-switching fabric and at least one packet-switching fabric, and channelized high-speed links between the packet-switching fabrics and the circuit-switching fabrics. There are three key elements of the present invention. First, a packet-switching fabric in a first PACS router is connected to a packet-switching fabric in a second PACS router via circuit-switching fabrics in each of the PACS routers. Second, a link between a packet-switching fabric and a circuit-switching fabric in a PACS router is channelized, wherein multiple low-speed signals are multiplexed, using TDM, into a high-speed signal. Channelized links enable the circuit-switching fabric to route low-speed signals without terminating their payloads to extract packets from the signals. Therefore, the circuit-switching fabric can route transit traffic (i.e., traffic which originates and terminates in other PACS routers) without being required to extract packets from the signals&#39; payloads. Third, a PACS router utilizes a set of signaling and routing protocols that enable its packet-switching fabric and circuit-switching fabric to route both best-effort traffic and high-quality traffic with higher efficiency, better reliability, and lower cost. 
   According to one aspect of the present invention, a PACS router characterizes a low-speed channel within a channelized high-speed link as: (1) a packet-channel when best-effort traffic is being routed by the low-speed channel; or (2) a circuit-channel when high-quality traffic is being routed by the low-speed channel. In a PACS router, only the packet-channels are terminated; packets are extracted; and then each packet only within the packet-channel is routed by the packet fabric at each intermediate PACS router on the route of the packets. Circuit-channels, on the other hand, are not terminated at intermediate PACS routers. The circuit-channels are routed via the circuit-switching fabrics at intermediate PACS routers so that they remain as circuits between the originating PACS router and the final destination PACS router. 
   In another aspect of the present invention, an end PACS router, (i.e., a PACS router which receives both best effort traffic and high-quality traffic from networking devices and user terminals to which it is directly connected via acces links), segregates best-effort traffic into at least one packet-channel and high-quality traffic into at least one circuit-channel. It should be noted that an end PACS router is simultaneously an intermediate PACS router for traffic that originates and terminates in other PACS routers to which it is connected via network links. Packet-channels are terminated into the packet-switching fabrics at intermediate PACS routers to extract the packets in these channels, as is known in the prior art. The extracted packets are then forwarded in hop-by-hop fashion to other PACS routers. A PACS router routes high-quality traffic to another PACS router via circuit-switching fabrics without terminating the channels into packet fabrics at intermediate PACS routers. 
   In yet another aspect of the present invention, a PACS router comprises a processor, which implements a control system to manage channel inventory, network connectivity, and traffic routing. The processor uses signaling and routing protocols to route both packet-channels and circuit-channels in the PACS network. A PACS router uses packet-channels to transmit control, protocol, and management packets. 
   In another aspect of the present invention, a PACS router utilizes its circuit-switching fabric to provide fast restoration of traffic affected by a link failure, such as a fiber break. A PACS router provides faster failure detection than routers known in the prior art. Once a PACS router detects a failure, it reroutes affected traffic via its circuit-switching fabric and circuit-switching fabrics of other PACS routers in the network to restore connectivity. Therefore, a network comprising PACS routers is more stable and robust a network comprising routers known in the prior art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts an IP network according to the prior art. 
       FIG. 2  depicts a schematic diagram of the salient components of an IP router in accordance with the prior art. 
       FIG. 3  depicts a schematic diagram of the salient components of a voice network in accordance with the prior art. 
       FIG. 4  depicts a schematic diagram of the salient components of a cross-connect in accordance with the prior art. 
       FIG. 5  depicts a block diagram of the salient aspects of a communications network in accordance with the illustrative embodiment of the present invention. 
       FIG. 6A  depicts a block diagram of the salient aspects of a PACS router in accordance with the illustrative embodiment of the present invention. 
       FIG. 6B  depicts a block diagram of the salient aspects of a PACS router in accordance with an alternative embodiment of the present invention. 
       FIG. 7  depicts a block diagram of the salient aspects of processor  620   i  in accordance with the illustrative embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 5  depicts a block diagram of the salient aspects of a communications network in accordance with the illustrative embodiment of the present invention. PACS network  500  comprises PACS routers  502   1  through  502   5  and network links  504   1  through  504   7 . PACS network  500  is interconnected to user equipment  506  via access links  508 . 
   PACS router  502   i , where i=1 through 5, is a network router capable of switching signals that comprise either information circuits or information packets to any PACS router or user equipment to which it is connected based on a quality-demand associated with each signal. PACS router  502   i  is described below and with respect to  FIG. 6 . 
   Network link  504   i , where i=1 through 7, is a multi-channel, bi-directional communications link which comprises a plurality of circuit-channels and a plurality of packet-channels. Network link  504   i  interconnects transmitters of a first PACS router with receivers of a second PACS router to provide communication in one direction, and interconnects transmitters of the second PACS router with receivers of the first PACS router to complete the bi-directional communications link. For example, network link  504   4  interconnects transmitters of PACS router  502   1  with receivers of PACS router  502   3 , and interconnects transmitters of PACS router  502   3  with receivers of PACS router  502   1 . 
   User equipment  506  is a superset of network devices  106  and user terminals  306 , as described above and with reference to  FIG. 1  and  FIG. 3 . It will be clear to those skilled in the art how to make and use user equipment  506 . 
   PACS access link  508  combines the functionality of access link  108  and access link  308 . Access link  108  and access link  308  are described above and with reference to  FIG. 1  and  FIG. 3 . PACS access link  508  provides interconnection between a PACS router, for example  502   1 , and user equipment  506 . In some alternative embodiments, PACS access link  508  includes intermediate equipment such as a gateway switch, voice-over-IP access router, or access router between PACS network  500  and user equipment  506 . It will be clear to those skilled in the art, after reading this specification, how to make and use access links  508 . 
   Although PACS network  500  comprises four PACS routers, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which PACS network  500  comprises any number of PACS routers. Furthermore, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which PACS network  500  is interconnected to any number of user equipments  506 . 
     FIG. 6A  depicts a block diagram of the salient aspects of a PACS router in accordance with the illustrative embodiment of the present invention. PACS router  502   i  comprises circuit-switching fabric  614   i , packet switching fabric  610   i , processor  620   i , channelized high-speed link  608 , and TDM interfaces  418 . Processor  620   i  is connected with the packet-switching fabric  610   i  and the circuit-switching fabric  614   i  via control links  618 . 
   Channelized high-speed link  608  provides connectivity between packet-switching fabric  610   i  and circuit-switching fabric  614   i . Channelized high-speed link  608  is a chip-to-chip electrical interface, which carries data in parallel format instead of in high-speed serial format. Channelized high-speed link  608  logically comprises low-speed circuit-channels and packet-channels between packet-switching fabric  610   i  and circuit-switching fabric  614   i . It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein channelized high-speed link  608  is a high-speed serial link. 
   Circuit-switching fabric  614   i  is a matrix of electronic logic and switches that interconnect any one of N inputs to any one of M outputs where the inputs and outputs are DS1 signals. In the illustrative embodiment, N is equal to 256 and M is also equal to 256. It will be clear to one skilled in the art, after reading this specification, how to make and use alternative embodiments that comprise a circuit-switching fabric wherein N and M are any positive integer. Further, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein the N inputs and M outputs of circuit-switching fabric  614   i  comprise:
         (i) any asynchronous signal (e.g., DS0, DS1, DS3, etc.); or   (ii) any SONET signal (e.g., STS-N, STS-Nc, etc.); or   (iii) any combination of (i) or (ii).       

   The connectivity of the N inputs to the M outputs is controlled by processor  620   i , which is described below and with respect to  FIG. 7 . 
   Circuit-switching fabric  614   i  provides several functions: establishing trunk-line connections between packet-switching fabric  610   i  and packet-switching fabrics located in other PACS routers; switching low-speed circuit-channels arriving from one TDM interface  418  to another TDM interface  418 ; switching packet-channels that arrive from each of TDM interfaces  418  to channelized high-speed link  608 . In conjunction with cross-connect fabrics of other PACS routers, circuit-switching fabric  614   i  can form semi-static circuits, which maintain their connectivity for periods that can exceed several months. 
   Circuit-switching fabric  614   i  interfaces with packet-switching fabric  610   i  to provide efficient transport of services over PACS network  500 , and to provide multiplexing and restoration functions. PACS router  502   i  controls and manages the connectivity of circuit-switching fabric  614   i  using signaling and routing protocols described below and with respect to  FIG. 7 . 
   Although the illustrative embodiment comprises a circuit-switching fabric, which comprises a matrix of electronic logic, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which an all-optical switching matrix is used instead of a matrix of electronic logic. 
   Packet-switching fabric  610   i  is a matrix of electronic switches and logic circuitry. Packet-switching fabric  610   i  receives a packet, reads the destination header of the packet, determines the next hop destination, and sends the packet out on the appropriate packet-channel or circuit-channel in channelized high-speed link  608 . 
   Packet-switching fabric  610   i  is connected with user equipment  506  via PACS access link  508 . Packet switching fabric  610   i  receives data for various services such as voice, internet, and video from the user equipment  506 . Controlled by processor  620   i , the packet-switching fabric segregates traffic and sends high-quality service packets such as voice to a circuit-channel and best-effort data to a packet-channel. The Packet-switching fabric  610   i  receives data from both packet-channels and circuit-channels arriving from channelized high-speed link  608  and routes packets to appropriate user equipment  506  via PACS access link  508 . 
   Packet-switching fabric  610   i  receives best-effort traffic and high-quality traffic in the form of packets. Packet-switching fabric  610   i  segregates these packets according to a quality of service requirement and/or destination. For example, and referring to  FIGS. 5 and 6A , all packetized high-quality traffic transmitted by PACS router  502   1  which has the final destination of  502   4  is grouped into a single packet flow, also known as Forwarding Equivalency Class (FEC). This packet flow is then fed into a circuit-channel inside network link  504   1  that connects to PACS router  504   2 . Other packets that are to be routed through packet-switching fabric  610   2  of PACS router  502   2  are grouped into a different FEC, which is fed into a packet-channel inside network link  504   1 . PACS router  502   1  determines the bandwidth necessary for each packet-channel and each circuit-channel depending on the traffic volume to be carried on these channels. 
   When a new high-quality traffic request is received by PACS router  502   1 , PACS router  502   1  first determines whether there is an existing circuit-channel between PACS router  502   1  and PACS router  502   4  that has enough spare capacity for a new high-quality traffic request that arrives at PACS router  502   1  with the destination of PACS router  502   4 . If the existing circuit-channel has enough spare capacity, it is utilized to set up the call between PACS router  502   1  and PACS router  502   4 . If, on the other hand, there is insufficient capacity on an existing circuit-channel to carry the requested high-quality traffic, PACS router  502   1  establishes a new circuit-channel between PACS router  502   1  and PACS router  502   4  via, for example, PACS router  502   2 . 
   As described above, and with respect to  FIG. 5 , each circuit-channel bypasses the packet-switching fabrics of all intermediate PACS routers in a high-quality traffic circuit, thereby avoiding packet-forwarding delays that would be caused by the intermediate PACS routers. For example, a high-quality traffic circuit established between PACS router  502   1  and PACS router  502   4 , bypasses packet-switching fabric  610   2  of PACS router  502   2 , and thus avoids packet-forwarding delay that would be caused by intermediate PACS router  502   2 . In addition to the elimination of packet-forwarding delay at intermediate PACS router  502   2 , the packet-switching fabrics in each of PACS routers  502   1 ,  502   2 , and  502   4  do not need to handle packets for high-quality traffic in a different manner than packets for best-effort traffic. This eliminates the complexity of providing high-quality traffic transmission in PACS network  500 . 
   Normally, best-effort traffic is routed in hop-by-hop fashion in PACS network  500 . For example, best-effort traffic transmitted from PACS router  502   1  to PACS router  502   4  is first routed from packet-switching fabric  610   1  to packet-switching fabric  610   2  via a first packet-channel that interconnects packet-switching fabric  610   1  to packet-switching fabric  610   2 . The best-effort traffic is then routed again from packet-switching fabric  610   2  to packet-switching fabric  610   4  via a second packet-channel that interconnects packet-switching fabric  610   2  to packet-switching fabric  610   4 . Network scalability suffers due to the two-stage nature of this type of traffic routing. 
   PACS network  500  enables an alternative type of traffic routing, wherein a packet channel bypasses packet-switching fabrics in intermediary PACS routers. For example, a packet channel comprising best-effort traffic can be routed from PACS router  502   1  to PACS router  502   4  in a manner that bypasses packet-switching fabric  610   2  of PACS router  502   2 . In this routing scheme, the packet-channel, which originates in packet-switching fabric  610   1 , passes through circuit-switching fabric  614   1  of PACS router  502   1  and terminates at packet-switching fabric  610   4  of the PACS router  502   4 . This bypass routing scheme is particularly advantageous when there is sufficient best-effort traffic to justify the dedication of a packet-channel to this circuit (i.e., the packet-channel circuit between PACS router  502   1  and PACS router  502   4  as described in the example above). The bypass routing scheme enables better scalability, lower cost and higher reliability. 
   PACS network  500  makes use of IP-based signaling such as MPLS (Generalized MPLS designed for application in layers other than IP such as circuit) to set up the circuit-channel through PACS routers  502   1 ,  502   2 , and  502   4 . PACS router  502   1  also uses IP based routing such as OSPF to decide the most appropriate route for the circuit-channel. For example, the route through PACS routers  502   1 ,  502   2 , and  502   4  may not have sufficient link capacity to set up a new circuit. Using protocols such as OSPF, PACS routers  502   1  through  502   5  always maintain up-to-date databases for the availability and the status of network resources. Based on the network resources and cost associated with the links, PACS routers compute efficient routes for the channels. Then, using signaling schemes such as GMPLS, circuit-channels are set up through the circuit-switching fabrics  614   1  through  614   5  of PACS routers  502   1  through  502   5 . 
   A variety of known route optimization algorithms (e.g., Dijkstra, Bellman-Ford, Suurballe, K-shortest path) with up-to-date link state databases created and maintained by protocols such as OSPF are used to establish new packet and circuit-channels or use existing packet or circuit-channels to route traffic efficiently. Best-effort data routing are typically be hop-by-hop using packet-channels like the traditional router network. PACS router  502   1  aggregates best-effort data (particularly not sensitive to delay) into a packet-channel inside channelized high-speed link  608  between packet-switching fabric  610   1  and circuit-switching fabric  614   1 . The packet-channel is then routed via circuit-switching fabric  614   2  that is routed via packet-switching fabric  610   2 . Packet-switching fabric  610   2  then determines the next hop and maps the best-effort data packets with other best-effort data originating in or passing through PACS router  502   2  into another FEC that is fed into a second packet-channel originating from packet-switching fabric  610   2 . The second packet-channel is routed from PACS router  502   2  to PACS router  502   4  where the second packet-channel is terminated into packet-switching fabric  610   4 . 
   If there is enough best-effort service traffic that can be segregated between two distant PACS routers they can be mapped into a packet-channel routed through the circuit-switching fabrics in intermediate PACS routers to create bypass packet-channels that bypass packet-switching fabrics in one or more intermediate PACS routers. The flexible means of hop-by-hop or bypass routing of packet-channels provides a means of more efficient and better performing routing of best-effort services as well without adding any constraint in the network. There are several efficient known routing algorithms that can be used to make a decision on whether to utilize hop-by-hop packet-channels or bypass packet-channels for best-effort data. 
   Processor  620   i  is a general purpose processor and control system comprising digital logic, a fabric control system, memory, data bases, software, protocols, and algorithms which are required for communicating with other PACS routers and maintaining desired connectivity for both packet fabric  610   i  and circuit-switching fabric  614   i . Processor  620   i  is described below and with respect to  FIG. 7 . 
   TDM interfaces  418  provide connectivity between PACS router  502   i  and other PACS routers through network links  504   i , and connectivity between PACS router  502   i  and user equipment  506 . TDM interfaces also provide demultiplexing of high-speed signals such as OC-48 or OC-192 arriving from network interface  504   i  to feed to the circuit-switching fabric  614   i ; multiplexing of low-speed signals arriving from circuit-switching fabric  614   i  into high-speed signals such as OC-48 and OC-192 to be transmitted over a network link  504   i  in preparation for transport to cross-connect fabrics located in other PACS routers. 
     FIG. 6B  depicts a block diagram of the salient aspects of a PACS router in accordance with an alternative embodiment of the present invention. In the alternative embodiment, PACS router  502   i  further comprises voice-switch  312   i  and media gateway  622   i . Voice-switch  312   i  is connected to the circuit-switching fabric via trunk link  318 . Processor  620   i  is connected to circuit-switching fabric  614   i , packet-switching fabric  610   i , and voice-switch  312   i  via control links  618 . Media gateway  622   i  is connected with voice-switch  312   i  and packet-switching fabric  610   i  via trunk link  318  and PACS access link  508 , respectively. 
   In the alternative embodiment, voice-switch  312   i  is a Class V switch, which connects voice telephones and other voice circuit devices directly to PACS router  502   i . In some other alternative embodiments, voice-switch  312   i  is an alternative voice-switch such as a private automatic brunch exchange commonly known in the industry as PABX. 
   Media gateway  622   i  comprises electronic logic for converting voice circuits into packets, and transmitting the converted packets to packet-switching fabric  610   i . Media gateway  622   i  also converts packets carrying high-quality traffic into voice circuits and send to the voice-switch  312   i . Processor  620   i  determines if a call originating from user terminal  308  is to be connected at the other end to a user terminal, which is connected to another voice-switch  312   j  in PACS network  500 . The voice-switch then connects the voice circuit directly to the circuit-switching fabric  614   i  via trunk link  318 . If processor  620   i  determines that the call is to be connected to a networking device connected to a networking device  106 , which is connected to an IP router  102   j  in IP network  102  or to a PACS router  502   j  in PACS network  500 , then voice-switch  312   j  connects the voice circuit to media gateway  622   i  via trunk link  318 . 
   Circuit-switching fabric  614   i  routes the voice circuit received from voice-switch to another PACS router  502   j  directed by processor  620   i  via control link  318 . 
   Media gateway  622   i  converts voice signals from circuit format into packet format, and sends the packets (i.e., the converted voice signals) to packet-switching fabric  610   i . Packet-switching fabric  610   i  switches the packets into an appropriate circuit-channel within channelized high-speed link  608  to be routed to another PACS router via circuit-switching fabric  614   i . 
     FIG. 7  depicts a block diagram of the salient aspects of processor  620   i  in accordance with the illustrative embodiment of the present invention. Processor  620   i  comprises fabric controller  712   i , protocol processor  714   i , and memory  716   i . Memory  716   i  includes link-state database  720   i , routing table  732   I , and topology database  724   i . Protocol processor  714   i  and fabric controller  712   i  are connected to memory  716   i  via data links  718 . 
   Fabric controller  712   i  comprises digital logic and provides the control signals sent to configure packet fabric  610   I , circuit-switching fabric  614   I  and voice-switch  312   i . Fabric controller  712   i  provides the control signals in response to call control messages received from protocol processor  714   i . 
   Memory  716   i  is a general-purpose memory cell well-known to those skilled in the art. Memory  716   i  contains protocols and databases, which are necessary for the proper operation of PACS router  502   i  in PACS network  500 . These protocols and databases include link-state database  720   i , topology database  724   i , and routing table  722   i . 
   Protocol processor  714   i  comprises control logic and generates call control messages in response to connectivity requests generated by user equipment  506 . User equipment  506  makes use of a variety of signaling means such as Session Initiation Protocol (SIP) for call (also known as session) connection in PACS network  500  to generate call control messages. Packets marked as control packets are utilized to carry signaling and other control messages. Packet-switching fabric  610   i  sends signaling packets received from user equipment  506  to protocol processor  714   i . 
   Protocol processor  714   i  receives data from topology database  724   i , link state database  720   i  and routing table  722   i . With the received data and the requested call set up message received from packet-switching fabric  610   i , protocol processor  714   i  computes the call route and generates fabric control messages for PACS router  502   i  and PACS router control messages for PACS routers  502   j  to set up the call. Protocol processor  714   i  sends fabric control messages to fabric controller  712   i  and sends router control messages to packet-switching fabric  610   i . Router control messages are embedded in packets addressed to appropriate target PACS router  502   j . 
   Protocol processor  714   i  uses two types of router control messages. A first type of router control messages are used to route circuit-channels that start at the call originating PACS router  502   i , pass through the circuit-switching fabrics of a set of intermediate PACS routers  502   j  through  502   i  and terminate at a PACS router  502   n . Protocol processor  714   i  uses second types of router control messages to route individual packets over packet-channels that start at the call originating PACS router  502   i , pass through the all the circuit-switching fabrics over packet-channels and one or more packet-switching fabrics of a set of intermediate PACS routers  502   j  through  502   i  and terminate at a PACS router  502   n . 
   While a circuit-channel never passes through the packet-switching fabrics of the intermediate PACS routers, both circuit-channel and packet-channel are originated from the packet-switching fabric of the service originating PACS router  502   i  and are terminated at the packet-switching fabric of the service terminating PACS router  502   n . 
   It is to be noted that there is a scenario in which a circuit-channel may not pass through the packet-switching fabrics at the originating and terminating PACS routers. With respect to  FIG. 6B , if a call is started at the voice-switch  312   i  and ends at another voice-switch  312   n  then there is no need for the call to be converted into packet by the media gateway  622   i . In this type of router configuration, processor  620   i  sends the call directly via circuit-switching fabric  614   i . 
   A router control message consists of connection messages for circuit-switching fabric indicating which input should be connected to which output and packet routing messages for the packet-switching fabric. Packet routing messages include Open Shortest Path First (OSPF) protocol messages in which network connectivity information such as link state database information is transmitted from one PACS router to other. 
   Neighbor discovery is a mechanism by which a PACS router communicates with every PACS router that is directly connected via network link  504   i . TDM interface  418  on a network link  504   i  sends and receives neighbor discovery messages via SONET overhead bytes to generate neighbor connectivity information. The neighbor connectivity information is received by the protocol processor  714   i  via packet switching fabric  610   i . Protocol processor writes the entire neighbor connectivity information into the link-state database  720   i.    
   Protocol processor  714   i  distributes neighborhood connectivity information of PACS router  502   i  using OSPF messages. OSPF was developed for working with traditional routers. A modified OSPF known as OSPF-TE (Traffic Engineering) was developed to be used with other types of networks. OSPF-TE can be used for sending link-state information where links include both packet and circuit-channels. 
   Protocol processor  714   i  receives neighborhood connectivity information via packet routing messages from every other PACS router connected in PACS network  500  and writes the information into topology database  724   i . In addition, whenever there is a change in a link status such as connecting input to out in circuit-switching fabric in response to new call set up, existing call tear-down, and addition of best-effort service calls in an existing packet-channel in a packet-switching fabric, processor  714   i  sends update messages to every other PACS router. Processor  714   i  also writes the link status change information into topology database  724   i . Thus, topology database  724   i  always maintains up-to-date information on network connectivity, link status, link usage status, type of channels (circuit or packet) within a link, and usage status of circuit and packet-channels in all links in PACS network  500 . It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein many link status changes are collected and sent together when a pre-determined threshold is reached. In this way, a proper balance of the frequency of status change messages and optimality of network capacity usage can be maintained. 
   Protocol processor  714   i  uses routing algorithms to compute optimal routes for packets with different destination addresses and quality of service required that are contained in the packet overhead. Optimal route computation includes determination of next hop PACS router for best-effort packets; whether to use an existing packet-channel for best-effort services; whether to use an existing circuit-channel for high-quality services; whether to create a new packet-channel for best-effort services; and through which intermediate PACS routers and network links newly computed circuit and packet-channels should pass. Protocol processor  714   i  sends appropriate router control messages to other PACS routers to configure circuit-switching and packet-switching fabrics to establish new circuit and packet-channels. 
   Based on next hop router computation for best-effort services, processor  714   i  writes next hop information into routing table  722   i . When a best-effort data packet arrives at packet-switching fabric  610   i , fabric controller looks up routing table to determine next hop and the packet-channel to be used for sending the packet to the next hop PACS router. The mechanism of routing table lookup, instead of computation every time a packet needs to be forwarded, enables packet-switching fabric  610   i  to forward millions of packets every second. Fabric controller  712   i  uses data link  718  for routing table lookup. 
   When PACS router  502   i  detects a failure on one of its associated network links (e.g.,  504   i-1  or  504   i , as described above and with respect to  FIG. 6A ) or on one of its TDM interfaces  418 , PACS router  502   i  sends a failure detection message to protocol processor  714   i . Protocol processor  714   i  reads data from topology database  722   i  and determines if there is enough idle capacity or best-effort traffic that can be pre-empted in other network links in PACS network  500 . Protocol processor  714   i  then computes one or more alternate routes via circuit-switching fabrics in other PACS routers, sends routing messages to the PACS routers on the alternate routes. These routing messages instruct the PACS routers to configure their circuit-switching fabrics such that failed services originally being transported over the failed link are restored. This restoration mechanism, using only circuit-switching fabrics in PACS routers, results in fast and robust restoration of high-quality traffic. 
   In some alternative embodiments, packet-switching and circuit-switching fabrics are loosely coupled, wherein a stand-alone packet-switching router and a stand-alone circuit-switching cross-connect system are connected via high-speed electrical or optical links and control links. Control links provide the means of data transport and control mechanism required for the two packet-switching and circuit-switching fabrics to work in harmony as described in the illustrative embodiment. 
   It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
   Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.