Patent Publication Number: US-6671271-B1

Title: Sonet synchronous payload envelope pointer control system

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to communication systems, and more specifically to a multiported integrated circuit for interfacing a local STM switch fabric and an ATM switch fabric unit to a SONET ring. 
     As it is generally known, SONET (Synchronous Optical Network) defines a set of standards for a synchronous optical hierarchy that has the flexibility to transport many digital signals having different capacities. A corresponding international synchronous digital hierarchy (SDH) standard provides a set of definitions analogous to those of SONET. The synchronous nature of SONET is provided by a receive side and a transmit side clock in each network element (NE). In order to synchronize the receive and transmit clocks, a SONET network element, such as an add-drop multiplexer, includes circuitry to recover clock signals from various sources that may be available, and to distribute highly accurate clocks internally based on such recovery. 
     A central timing source provides a Building Integrated Time Source, also referred to as a “BITS” clock, that may be provided out-of-band to each network element in a SONET ring. If a network element is for some reason not able to receive the BITS clock directly, an embedded clock may be recovered by that device from an incoming line that should reflect the centrally provided BITS clock. 
     The basic building block in SONET is a synchronous transport signal level-1 (STS-1), which is transported as a 51.840-Mb/s serial transmission using an optical carrier level-1 (OC-1) optical signal. Higher data rates are transported using SONET by multiplexing N lower level signals together. To this end, SONET defines optical and electrical signals designated as OC-N (Optical Carrier level-N) and STS-N (Synchronous transport signal level-N), where OC-N and STS-N have the same data rate for a given value of N. Accordingly, just as STS-1 and OC-1 share a common data rate of 51.84 Mb/s, OC-3/STS-3 both have a data rate of 155.52 Mb/s. 
     Information transported via an STS-1 signal is organized as frames, each having 6480 bits (810 bytes). An STS-1 frame includes transport overhead and a Synchronous Payload Envelope (SPE). The SPE includes a payload, which is typically mapped into the SPE by what is referred to as path terminating equipment at what is known as the path layer of the SONET architecture. Line terminating equipment, such as an OC-N to OC-M multiplexer, is used to place an SPE into a frame, along with certain line overhead (LOH) bytes. The LOH bytes provide information for line protection and maintenance purposes. The section layer in SONET transports the STS-N frame over a physical medium, such as optical fiber, and is associated with a number of section overhead (SOH) bytes. The SOH bytes are used for framing, section monitoring, and section level equipment communication. Finally, a physical layer in SONET transports the bits serially as either electrical or optical entities. 
     The SPE portion of an STS-1 frame is contained within an area of an STS-1 frame that is typically viewed as a matrix of bytes having 87 columns and 9 rows. Two columns of the matrix (30 and 59) contain fixed stuff bytes. Another column contains STS-1 POH. The payload of an SPE may have its first byte anywhere inside the SPE matrix, and, in fact may move around in this area between frames. The method by which the starting payload location is determined is responsive to the contents of transport overhead bytes in the frame referred to as H 1  and H 2 . H 1  and H 2  store an offset value referred to as a “pointer”, indicating a location in the STS-1 frame in which the first payload byte is located. 
     The pointer value enables a SONET network element to operate in the face of certain conditions which may, for example, cause the STS-1 frame rate to become faster or slower than the SPE insertion rate. This situation may arise when the clock of the NE must be derived from a relatively less accurate clock source, in order to continue operation, when a more accurate source, such as the BITS clock itself, has been lost. In such a case, an extra byte may need to be transmitted in what is known as a negative justification opportunity byte, or, one less byte may be transmitted in a given STS-1 frame so as to accommodate the SPE, thus causing the location of the beginning of the payload to vary. 
     Various digital signals, such as those defined in the well-known Digital Multiplex Hierarchy (DMH), may be included in the SPE payload. The DMH defines signals including DS-0 (referred to as a 64-kb/s time slot), DS-1 (1.544 Mb/s), and DS-3 (44.736 Mb/s). The SONET standard is sufficiently flexible to allow new data rates to be supported, as services require them. In a common implementation, DS-1s are mapped into virtual tributaries (VTs), which are in turn multiplexed into an STS-1 SPE, and are then multiplexed into an optical carrier-N (OC-N) optical line rate. 
     The payload of a particular SPE may be associated with one of four different sizes of virtual tributaries (VTs). The VTs are VT1.5 having a data rate of 1.728 Mb/s, VT2 at 2.304 Mb/s, VT3 at 3.456 Mb/s, and VT6 at 6.912 Mb/s. A superframe consists of four STS-1 frames, and is used to transmit a VT. The alignment of a VT within the bytes of the payload allocated for that VT is indicated by a pointer contained within two VT pointer bytes, which contain a pointer offset similar to the STS-1 pointer described above. 
     Existing add-drop multiplexers (ADMs) are SONET multiplexers that allow DS-1 and other DMH signals to be added into or dropped from an STS-1 signal. Traditional ADMs have two bi-directional ports, and may be used in self-healing ring (SHR) network architectures. An SHR uses a collection of network elements including ADMs in a physical closed loop so that each network element is connected with a duplex connection through its ports to two adjacent nodes. Any loss of connection due to a single failure of a network element or a connection between network elements may be automatically restored in this topology. Existing ADMs have additionally included a cross-connect matrix for directing STM signals from one interface to another. Such a cross-connect matrix is referred to as an STM switch fabric. The manner in which specific STM signals are directed between interfaces of the STM switch fabric depends on how the network bandwidth has been “provisioned” to the various customers using the network. The path of a signal through a given cross-connect matrix is statically defined based on provisioning information provided from a central office or “craft” technician. 
     As mentioned above, SONET provides substantial overhead information. SONET overhead information is accessed, generated, and processed by the equipment which terminates the particular overhead layer. More specifically, section terminating equipment operates on nine bytes of section overhead, which are used for communications between adjacent network elements. Section overhead supports functions such as: performance monitoring (STS-N signal), local orderwire, data communication channels (DCC) to carry information for OAM&amp;P, and framing. The section overhead is found in the first three rows of columns 1 through 9 of the SPE. 
     Line terminating equipment operates on line overhead, which is used for the STS-N signal between STS-N multiplexers. Line overhead consists of 18 overhead bytes, and supports functions such as: locating the SPE in the frame, multiplexing or concatenating signals, performance monitoring, automatic protection switching, and line maintenance. The line overhead is found in rows 4 to 9 of columns 1 through 9 of the SPE. 
     Path overhead bytes (POH) are associated with the path layer, and are included in the SPE. Path-level overhead, in the form of either VT path overhead or STS path overhead, is carried from end-to-end; it is added to DS-1 signals when they are mapped into virtual tributaries and for STS-1 payloads that travel end-to-end. VT path overhead (VT POH) terminating equipment operates on four evenly distributed VT path overhead bytes starting at the first byte of the VT payload, as indicated by the VT payload pointer. VT POH provides communication between the point of creation of an VT SPE and its point of disassembly. 
     STS path terminating equipment terminates STS path overhead (STS POH) consisting of nine evenly distributed bytes starting at the first byte of the STS SPE. STS POH provides for communication between the point of creation of an STS SPE and its point of disassembly. STS path overhead supports functions such as: performance monitoring of the STS SPE, signal labels (the content of the STS SPE, including status of mapped payloads), path status, and path trace. The path overhead is found in rows 1 to 9 of the first column of the SPE. 
     Asynchronous Transfer Mode (ATM) is a cell-based transport and switching technology. ATM provides high-capacity transmission of voice, data, and video within telecommunications and computing environments. ATM supports a variety of traffic types, including constant bit-rate (CBR) traffic—like full-motion video and voice—where delays and cell loss cannot be tolerated. ATM also supports variable bit-rate (VBR) applications—like LAN traffic and large file transfers—where delay can be tolerated. 
     ATM establishes virtual connections which may be shared by multiple users. Each ATM virtual connection is identified by a combination of a Virtual Channel Identifier and a Virtual Path Identifier, referred to as a VCI/VPI value. ATM is a transport technology that formats all information content carried by the network into 53-byte cells. Since these cells are short in length and standard in size, they can be switched through network elements known as ATM switches with little delay, using what is referred to as an ATM switch fabric. Since various types of traffic can be carried on the same network, bandwidth utilization can be very high. These characteristics make the network very flexible and cost effective. 
     An ATM switch fabric operates to direct ATM cells from one interface to another. For a given received cell, the specific output interface of the ATM switch fabric is determined in response to a VCI/VPI value contained within the cell. Virtual channel and virtual path routing information is dynamically modified in the switch fabric as connections are established and torn down in the network. In this way the ATM switch fabric operates in response to dynamically changeable virtual connection information. 
     ATM cells may be encapsulated and transmitted over SONET for example using STS-1 or STS-3c, which is a concatenation of three STS-1 signals. STS-1 transports may generally be concatenated, and the combination then referred to as STS-Nc, where N is the number of STS-1 signals that are combined. In the case of STS-3c, the SPE of the resultant STS-3c frame consists of 3×783 bytes, together with POH. The concatenated STS-1s are multiplexed, switched, and transported as a single unit. An overhead byte of the STS-3c frame transport overhead, referred to as the H 4  byte, contains an offset indicating the number of bytes between the H 4  byte and the first ATM cell that is contained in the SPE. 
     Existing SONET ADMs include a line unit or line units, which provide connectivity to a SONET ring, together with service units which provide connections to service interfaces. During operation of existing SONET ADMs, each line unit or service unit is capable of directing traffic in one of two directions: either the SONET side of device or the service side of the device. This limited functionality is adequate where native STM is the predominant communication signaling technique, but does not include any functionality which would facilitate ATM, such as ATM cell relay transport and/or ATM Internetworking Services (IWS). As ATM networks become more commonplace, such limitations will make SONET-only ADM type devices less and less desirable, since ATM functionality will be in greater demand. 
     Accordingly, it would be desirable to have a line unit and/or service unit for a SONET ADM which provides interfaces to support ATM functionality such as ATM cell relay transport and/or ATM Internetworking Services. The line unit or service unit should facilitate the distribution and processing of ATM cells within the ADM, such that ATM cells may be transmitted to and received from the line unit and/or service units within the device. Such a system should further be capable of extracting ATM cells from a SONET interface. In addition, logic designs facilitating both ATM and STM support within the device should be applicable to both line units and service units. 
     BRIEF SUMMARY OF THE INVENTION 
     A multiported integrated circuit is disclosed, including an interface to an electrical signal derived from an optical SONET ring. The integrated circuit removes overhead information from the received electrical signal, and performs pointer processing at both the STS and VT level. The integrated circuit operates to extract STM traffic and ATM traffic from the received electrical signal, in response to provisioning information received from a management and control unit. The extracted STM traffic is then combined with path switching information, and forwarded out a pair of interfaces, which enables use of a local STM switch fabric and a partner switch fabric within a partner line unit module. In a preferred embodiment, the path switching information is written into line overhead bytes for the STS signal for which it applies, or into VT pointer bytes of the VT signal for which it applies. The receiving switch fabrics may then perform path selection in response to the path switching information within the overhead bytes of the received STM signals. 
     The path switching information may be combined with the extracted ATM traffic within the integrated circuit, or external to the integrated circuit. In a preferred embodiment the path switching information is forwarded from the integrated circuit independently of the extracted ATM traffic. The path switching information is then combined into a serial signal for transmission to an ATM switch fabric unit. The path switching information reflects performance monitoring and error detection with regard to the STS, VT or ATM traffic provided by the integrated circuit to its interfaces. The integrated circuit includes two output interfaces for each of the extracted STM and ATM traffic streams. The two outputs for the extracted STM traffic are operable to couple with a local STM switch fabric and an STM switch fabric on a partner line unit. The two outputs for the extracted ATM traffic are operable to couple with partner ATM switch fabric units within an active/standby pair. The path switching information permits selection of input paths on either an STS or VT level within each STM switch fabric, and on a virtual path level within the ATM switch fabrics. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood by reference to the following detailed description of the invention in conjunction with the drawings, of which: 
     FIG. 1 shows module partitioning in an illustrative embodiment of the disclosed network element; 
     FIG. 2 shows the layout of slots in an illustrative embodiment of the disclosed network element interconnection system; 
     FIG. 3 illustrates multiple star interconnection configurations employed in the disclosed network element; 
     FIG. 4 shows an illustrative configuration of the disclosed network element providing synchronous transfer mode (STM) support; 
     FIG. 5 shows an illustrative configuration of the disclosed network element providing asynchronous transfer mode (ATM) support; 
     FIG. 6 shows an illustrative configuration of the disclosed network element providing asynchronous transfer mode (ATM) support and synchronous transfer mode (STM) support; 
     FIG. 7 is a functional block diagram of an illustrative line unit employed in the presently disclosed network element; 
     FIG. 8 is a block diagram illustrating the STM and ATM inbound datapath within the line unit; 
     FIG. 9 is a block diagram illustrating the STM and ATM outbound datapath within the line unit; 
     FIG. 10 is a block diagram of a control architecture for the line unit; 
     FIG. 11 is a block diagram of a signal routing ASIC; 
     FIG. 12 is a block diagram of an extended synchronization module; 
     FIG. 13 is a block diagram of software and hardware used to download a software image into a management and control unit; 
     FIG. 14 is a block diagram showing hardware components within a management and control unit and an intelligent service unit; 
     FIG. 15 is a block diagram showing interfaces of a management and control unit; 
     FIG. 16 is a block diagram showing a gateway network element and a remote network element; and 
     FIG. 17 is a flow chart showing steps performed to download a software image to a network element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A network element (NE) architecture is disclosed which conveniently and efficiently combines the functions of a SONET add-drop multiplexer (ADM) with the functions of an ATM switch. As shown in FIG. 1, the disclosed network element  10  may be physically embodied as a set of hardware “units” interconnected across an interconnection system, also referred to as a backplane. The units, for example, include line units (LU)  30 , a management and control unit (MCU)  32 , ATM switch fabric units (ATMU)  34 , and service units (SU)  36 . The ATMU  34  provides ATM cell-relay related functionality, such as: VP and VC switching, segmentation and reassembly, signaling, routing, call control, traffic management, and Operations Administration, Management, and Provisioning (OAMP). In this regard, the ATMU may specifically provide address translation, an application programming interface (API) for user to network interface (UNI) signaling, an interim local management interface (ILMI) server, full ILMI and user network interface/network to network interface (UNI/NNI) signaling stacks, a private node-to-node interface (PNNI) server for routing, and connection admission. In an alternative embodiment, such ATM functionality may be split across two separate unit types: ATM Switch units (ATMSU) containing primarily the ATM switch fabric, and ATM Processing units (ATMPRU) providing other ATM functions. 
     In another alternative embodiment, an internet protocol (IP) switch fabric unit may be substituted for one of the ATM switch fabric units. The IP switch fabric unit performs switching (also known as “routing”) at the IP layer of the TCP/IP protocol stack, using what is referred to herein as an IP switch fabric. The IP switch fabric unit may also provide an ATM switch fabric, together with such ATM functionality as described above. 
     The network element  10  provides (1) a SONET line interface on a line side  15 , (2) connection of ATM traffic from either the line side  15  or a service side  20  to an ATM switch fabric unit  34 , (3) connection of STM traffic from either the line side  15  or the service side  20  to STS/VT switch fabrics within the line units  30 , (4) various service interfaces on the service side  30  through the specific service units  36 . The ATM switch fabric unit  34  performs activities associated with ATM cell-relay. These for example include VP and VC switching, signaling, routing, call control, and traffic management. 
     The line units  30  may support various SONET optical media line interfaces, such as OC-3, OC-12, or any other suitable optical interface. The line unit provides interfaces to the ATM switch fabric unit  34  from a SONET ring on the line side  15 , as well as to the various service units  36 . The line unit also includes an STM switch fabric  39  capable of performing STM switching at both the STS and VT levels. Examples of ATM switch fabric unit  34  include modules providing connectivity to the line units  30 , as well as to the service units  36 , and which also include an ATM switch fabric  41 . 
     Service units  36  include modules supporting various telecommunication or data communication interfaces to the service side  20 , including for example DS-1, DS-3, Token Ring, FDDI (fiber distributed data interface), 10BaseT, 100BaseT, 10BaseF, or 100BaseF Ethernet, EC-1 (electrical carrier  1 , also referred to as STS-1 electrical, or STS-1/EC-1), OC-3, OC-12 or any other suitable service. The service units  36  may communicate with the ATM switch fabric  41  installed in the ATM switch fabric unit  34  of the network element, as well as with the STM switch fabrics  39  contained within the line units  30 . Accordingly, service units  36  for the network element  10  contain interfaces within the network element which may be considered to fall within three categories: STM, ATM, and STM/ATM. For example, STM service unit  36   a  formats data received from the service side  20  into STS frames to be forwarded to the line units  30 , while ATM service unit  36   c  formats data received from the service side  20  into ATM cell streams to be forwarded to the ATM switch fabric unit  34 . Since each service unit  36  is connected to both ATM and STM services, a single STM/ATM service unit  36   b  may simultaneously use ATM and STM switching services by selectively communicating the data it receives to the line units  30  and/or the ATM switch fabric unit  34 . 
     Management and control unit  32  includes a subsystem employing a microprocessor  33  coupled to microprocessor bus  37 . The illustrative MCU  32  provides a basic control infrastructure for the network element  10 , using what is referred to herein as a “serialized hardbus”. During operation of the network element  10 , the components of the serialized hardbus convert a parallel bus communication protocol into a master/slave, serialized communication between the MCU  32  and the other units in the network element  10 . A serialized hardbus master logic unit  35  is coupled to microprocessor bus  37  of the MCU and maps control and status registers and/or memory locations of other units in the network element onto the microprocessor memory map. Units within the device  10  which are managed by the MCU  32  include serialized hardbus termination logic. The serialized hardbus also provides notification of certain autonomous events and/or alarms, occurring on or detected by other units, by interrupting the microprocessor  33 . 
     In addition to the serialized hardbus, the MCU  32  uses a SONET overhead link from each line unit  30  and service unit  36 , to support maintenance communications such as Data Communications Channel (DCC) and OrderWire information. The MCU  32  further includes various management and control capabilities, such as DCC and Orderwire processing functionality, which may include a combination of hardware and software, and which are used to process the received maintenance information. 
     FIG. 2 shows an illustrative slot layout  40  for the disclosed interconnection system. The slot layout  40  of FIG. 2 includes several different types of slots, specifically line unit slots  42 , management and control unit slot  44 , and service unit slots  46 . The service unit slots  46  include two slots  48  which may also be used as ATM switch fabric slots. The slots  48  are accordingly also referred to as switch fabric slots, or ATM switch fabric slots. The slots shown in FIG. 2 are each operable to receive hardware units as described in FIG. 1 of the corresponding type. For example, line unit slots  42  are operable to receive line units  30 , service unit slots  46  are operable to receive service units  36 , ATM switch fabric unit slots  48  are operable to receive ATM switch fabric units  34  or service units  36 , and MCU slot  44  is operable to receive the MCU  32 . 
     FIG. 3 illustrates the multi-star architecture of the disclosed interconnection system. As shown in FIG. 3, a number of point-to-point connections  50  form star configurations having the management &amp; control unit slot  44 , line unit slots  42 , and ATM switch unit slots  48  as hubs. In a preferred embodiment, the point-to-point connections  50  are implemented using a low voltage, complementary signaling technique to achieve high speeds, such as Low Voltage Differential Signaling (LVDS). In addition, the point-to-point connections  50  are terminated in the backplane, without the presence of a service unit within any of the respective service unit slots  46 . Accordingly, no additional physical termination is required for empty one of the service unit slots  46 . 
     In the depiction of FIG. 3, and as shown in FIG. 2, two of the service unit slots  46  are the same as ATM switch fabric slots  48 . Accordingly, when ATM switch fabric slots  48  are used to connect ATM switch fabric units, these same slots may not simultaneously be used to install service units. However, as shown in FIGS. 2 and 3, those two “multifunction” slots may alternatively be used to connect service units to the device in configurations where the ATM switch fabric units are not needed. 
     More specifically, in a preferred embodiment, each line unit slot  42  has two star configurations emanating from it. An STM datapath star communicates STM type data with each of the service unit slots  46 . The second star emanating from each line unit slot  42  is a synchronization star for conveying a frequency reference and a frame alignment pulse (FP) to the service unit slots  46 , in order to synchronize STM communication between each service unit and the STM switch fabrics within the line units. An extended synchronization module (ESM) may be provided to accept and distribute an extracted clock from the service unit slots  46  or, alternatively, a BITS clock from further interconnections within the device. The ESM is for example implemented on a sub-board module which is electrically and mechanically coupled to an alternative, enhanced version of the illustrative line unit. 
     In addition to the STM datapath and synchronization interconnection stars, a private datapath  52  is provided for communicating STM data between active and standby line units installed in respective ones of the line unit slots  42 . The private datapath  52  facilitates a pass through path between the line units which may be required under certain conditions. 
     Further, in the illustrative embodiment of FIG. 3, the line unit slots  42  each have a datapath connection  54  to each of the ATM switch fabric slots  48 . This interface supports ATM cell stream traffic between the ATM switch fabric slots  48  and the line unit slots  42 . During operation of the device, ATM traffic from both line unit slots  42  (active and standby) is forwarded to each of the ATM switch fabric slots  48 . Accordingly, per unit fault protection of the units in the line unit slots  42  and ATM switch fabric slots  48  may be provided independently, on a unit by unit basis. 
     Further, in the illustrative embodiment of FIG. 3, the MCU slot  44  has two star interconnect configurations emanating from it. First, an MCU control star is provided to support the serialized hardbus between the MCU and each other unit in the device. Second, a SONET overhead star supports communication of SONET overhead information, such as DCC and Orderwire data, as well as other information in the section, line, or path overhead portions of a SONET signal, between the MCU and other units in the device. The SONET overhead star connects the MCU slot  44  to each other slot in the device, via respective ones of the point to point connections  50 , thus enabling communication of SONET overhead between the MCU and any other unit in the device, including service units installed in the service unit slots  46 . The MCU slot  44  is further provided with a software image download interconnection with the ATM switch fabric slots  48  to facilitate communication of software executable image data between an ATM switch fabric unit installed in the ATM switch fabric slots  48  and the MCU, for example using the High-Level Data Link Control (HDLC) protocol. Such software executable image data may further be communicated from the MCU to individual ones of the services units installed in the service unit slots, over the serialized hardbus. 
     The ATM switch fabric slots  48  each have a datapath star emanating from them to all other service unit slots  46 . Because these ATM datapath stars are routed to the other service unit slots  46  independently from any other star configuration, the cost of adding ATM features is independent of the costs associated with other functionality provided by the device. 
     The disclosed interconnection system supports ATM VP Path Switching within an active ATM switch fabric unit coupled to one of the ATM switch fabric slots  48 , through the datapath connection  54 . The datapath connection  54  between each of the ATM switch fabric slots  48  and both line unit slots  42  enables maintenance of a current switch state in both of the ATM switch fabrics within ATM switch fabric units in the switch fabric slots  48 . Unlike support provided in the device for the SONET Unidirectional Path Switched Ring (UPSR), in which the pass through path is independently carried between the line units over the private datapath  52 , the VP pass through path is internal to an active ATM switch fabric unit. Accordingly, each ATM switch fabric slot  48  supports full ATM bandwidth to and from each line unit slot  42  simultaneously. 
     Additionally, the ATM switch fabric slots  48  connect with an ATM Control Bus which extends to two of the service unit slots  46  which are adjacent to the ATM switch fabric slots  48 , such as the service unit slots labeled SU 09  and SU 010  in FIG.  2 . This bus enables division of the ATM switch fabric unit functionality into separate ATM Switching Unit and ATM Processing Unit modules. Such a division permits the number of modules and power allocation for the ATM function to be doubled by occupying 4 slots with ATM switch fabric units instead of 2. 
     For protected services, service unit slots  46  are allocated in pairs, to support operation of redundant service units. However, each one of the service unit slots  46  has independent connections to both the ATM switch fabric units (in the ATM switch fabric slots  48 ), and to both the STM switch fabrics in the line units (in the line unit slots  42 ). This allows for a configuration of up to 12 different unprotected service units to be supported simultaneously in an STM configuration, and 10 different unprotected service units in an STM/STM configuration. Configurable backplane private connections  51  between selected pairs of service unit slots  46  are also provided, to support active/standby service unit pairs, as well as active standby pairs of ATM switch fabric units in ATM switch fabric slots  48 . 
     FIGS. 4 through 6 illustrate various system configurations that may be obtained using the presently disclosed architecture. The disclosed network element can be configured as an STM system as shown in FIG. 4, an ATM system as in FIG. 5, or an STM/ATM hybrid system as shown in FIG. 6, depending on the complement of units which are employed. 
     The exemplary configurations of FIGS. 4-6 include the following hardware components: 
     1) line units (LUs)  60   a  and  60   b , which operate as an active/standby pair, each of which terminates a SONET ring, for example through an OC-12 connection. 
     2) a number of service units (SUs), illustrated by the following specific examples: 
     STM Service Units  64 , which map STM services provided by the device to their respective STM service interfaces. Similarly, ATM subtending ring service units  80   e  and  80   f  provide a service side interface to a SONET ring carrying ATM cells, and STM/ATM subtending ring service units  90   a  and  90   b  provide a service side interface to a SONET ring carrying both STM and ATM traffic. 
     Service units  90   a  and  90   b  may also be referred to as “Hybrid Service Units”, because they provide both STM and ATM services to their respective service interfaces. A hybrid service unit includes three interfaces: an STM internal interface (to the STM switch fabric), an ATM internal interface (to the ATM switch fabric), and a service interface. During operation of a hybrid service unit, data units such as packets or cells are forwarded from the interface at which they are received to either of the other interfaces. Such forwarding is performed by one or more application specific integrated circuits (ASICs) and/or a microprocessor based subsystem. For example, in another illustrative embodiment, an Ethernet hybrid service unit is provided. In the Ethernet hybrid service unit, data units carried in Ethernet frames are received at an Ethernet service interface, and are selectively forwarded to either the STM or the ATM interfaces, based on information such as addressing included within the header of each received Ethernet frame. Similarly, each data unit received at the ATM or STM internal interfaces may be forwarded to either the other internal interface, or to the Ethernet service interface, responsive to information contained within each data unit, or to provisioning of individual signals. Hybrid service units may include any suitable external service interfaces to the device, including but not limited to data communications network, such as Ethernet or ATM, or any traditional telecommunications system, such as the digital multiplex hierarchy (DMH) or SONET. 
     ATM Interworking Service Units  80   a ,  80   c  and  80   d , which adapt traditional datacom (10/100BaseT for example) or telecom (OC-3 or DS-1 for example) service interfaces to the ATM protocol. 
     Native ATM Service Unit  80   b , which provides an interface to the ATM services provided by the device to an ATM service interface, for example an OC-3 based cell relay connection. 
     3) ATM switch fabric units (ATMUs)  70   a  and  70   b , which form an active/standby pair, and which handle ATM VP/VC switching and other activities associated with ATM cell relay, such as signaling, routing, call control, and traffic management. 
     4) A Management and Control Unit (MCU)  62 , which manages and controls the units within the network element  10 . This unit provides all the administrative interfaces to the device and processes all the SONET overhead bytes. 
     The STM system of FIG. 4 includes the two LUs  60   a  and  60   b , the MCU  62 , and some number of STM service units  64   a - 64   e . Service units  64   a  and  64   b  are an active/standby pair supporting a SONET DS-1 service interface, service units  64   c  and  64   d  are an active/standby pair supporting a SONET DS-3 service interface, and service unit  64   e  supports a SONET OC-3 service interface. The line units  60   a  and  60   b  attach via OC-12 to a SONET ring  61 . During operation of the embodiment shown in FIG. 4, STM signals are routed by the STM switch fabric in the active one of the line units  60 , between the ring  61  and active ones of the service units  64  over individual ones of point to point STS-3 serial links  66 . 
     FIG. 5 shows an ATM configuration of the disclosed network element, including the two LUs  60   a  and  60   b , the ATMUs  70   a  and  70   b , the MCU  62  and ATM service units  80   a  through  80   f . The ATM Interworking service unit  80   a  includes a service interface to a 10/100BaseT LAN, the native ATM service unit  80   b  includes a service interface to an OC-3 based cell relay connection, and a pair of ATM Interworking Service Units  80   c  and  80   d  include service interfaces to a DS-1 frame relay connection. The service units in FIG. 5 are connected via an active/standby pair of ATMUs  70   a  and  70   b  to a pair of OC-12 LUs  60   a  and  60   b . The service units  80   c  and  80   d  are configured as an active/standby pair. The ATM subtending ring service units  80   e  and  80   f  provide a service side interface to a SONET ring carrying ATM cells. 
     During operation of the embodiment shown in FIG. 5, ATM cells carried over STS signals within the SONET ring  61  are routed by the line units  60   a - 60   b  over STS-12 datapath connections  81  to each of the ATM switch fabrics within the ATM switch fabric units  70   a  and  70   b . The ATM switch fabric units  70   a  and  70   b  in turn direct the ATM cells, based on VPI/VCI values within the cell headers, to the appropriate destination service units as indicated by ATM virtual connections established through the ATM switch fabric. 
     A hybrid STM/ATM configuration of the disclosed network element is shown in FIG.  6 . In FIG. 6, the STS signals from the ring  61  which contain encapsulated ATM cells are routed by the line units  60   a - 60   b  to the ATM switch fabric units  70   a - 70   b . STS signals from the ring  61  that are provisioned to pass through service interfaces of the device are routed by the line units  60   a - 60   b  to the appropriate service units using the STM switch fabrics contained within the line units  60   a - 60   b . The service units  64   a - 64   b  communicate STM frames with the line units  60   a - 60   b , while the service units  80   a - 80   b  communicate ATM cells with the ATM switch fabric units  70   a - 70   b . The service units  90   a - 90   b  communicate ATM cells with the ATM switch fabric units  80   a - 80   b , and also communicate STM frames with the line units  60   a - 60   b.    
     FIG. 7 is a functional block diagram of an illustrative line unit  100 . The line unit  100  provides an optical interface to a SONET ring at the line side of the disclosed network element. The line unit  100  is shown including a signal routing ASIC  102 , which is coupled to an optical receiver  104 , an optical transmitter  106 , ATMU Low Voltage Differential Signaling (LVDS) receivers  108 , ATMU transmitters  110 , and an STM switch fabric ASIC  114 . ATMU transmitters  110  and ATMU receivers  108  are, for example, shown using LVDS devices. The STM switch fabric ASIC  114  is shown including service unit transmit interfaces  116  and service unit receive interfaces  118 . 
     During operation of the line unit  100 , the optical receiver  104  receives for example a SONET formatted OC-12 or OC-3 optical signal, carrying an STS-12 or STS-3 signal respectively, or any suitably formatted signal. The optical receiver  104  passes electrical clock signals  124  and data signals  126  that reflect the received SONET signal to the signal routing ASIC  102 . The signal routing ASIC  102  extracts STS frames from the STS signal, performs pointer interpretation to locate the beginnings of payloads and virtual tributaries within the received frames, and also extracts line, section and path overhead data. The extracted overhead data  128  is sent by the signal routing ASIC  102  to a management and control unit (MCU) for further processing. 
     According to provisioning information provided to the line unit  100  by the MCU, the signals in the received STS signal are interpreted by the line unit  100  as carrying either ATM or STM traffic. The STS-1 signals provisioned as ATM traffic are processed by the line unit  100  to perform the ATM Transmission Convergence (TC) layer functions of cell delineation and STS channel identification. The resulting ATM cells are then formatted into a cell stream and sent to the active and standby ATM switch fabric units, via transmitters  110 . 
     The STS signals received by the line unit  100  that carry STM traffic are sent by the signal routing ASIC  102  via connections  130  to the STM switch fabric ASIC  114 , where STS/VT grooming and cross-connection takes place. The switch fabric ASIC  114  is further coupled via connection  132  to receive STM traffic from a signal routing ASIC of a “partner” line unit in an active/standby pair. Similarly, the output connection  134  of the signal routing ASIC  102  is used to pass STM traffic to the switch fabric of the partner line unit. 
     The switch fabric ASIC  114  receives STM traffic from the line side of the device through one of the two connections  130  and  132 , each of which for example provides full STS-12 bandwidth, and from the service side through service unit receive interfaces  118 , each of which for example provides STS-3 bandwidth. The switch fabric ASIC  114  outputs STM signals through the output  135  (STS-12) or any of the service unit transmit interfaces (STS-3)  116 . 
     Further during operation of the line unit  100  as shown in FIG. 7, the switch fabric ASIC  114  performs Unidirectional Path Switched Ring (UPSR) path selection, responsive to path performance information provided by the signal routing ASIC  102 , and passed to the switch fabric ASIC  114  within some number of over-written overhead bytes, in order to determine which of the connections  130  or  132  should be the path for individual STS or VT signals that are dropped at the service side of the device. The switch fabric ASIC  114  performs this selection in response to path performance criteria, in order to determine the path of highest quality from the two available paths. The switch fabric ASIC  114  performs this selection at either the STS-1 or VT 1.5 level for each path dropped to the service side of the device. 
     The line unit  100  also includes a back-up memory  141  which stores configuration information that may be used in the event of a failure of the MCU. The contents of the back-up memory  141  may also be used by a new MCU that is installed to replace the failed unit. The physical inventory EEPROM  142  is used to store information such as the serial number of the unit, a hardware revision number, and a software revision number. A serial bus terminator SBT  143  operates to connect the unit to the serial hardbus for communication with the MCU, and LEDS  144  provide a visual indication of the unit&#39;s status. 
     The line unit  100  further includes various clock related elements including an extended synchronization module (ESM)  120 , which, in combination, provide STM synchronization clocks to the line unit and other units within the device. Specifically, a timing reference switch  145  is controlled by the output of synchronization switch controller logic  146 . A number of inputs  147  to the synchronization switch controller logic  146  provide indication of whether the ESM  120  is present, whether the active/standby partner line unit is present, and whether the SONET signal on the line side is present on either the local or partner line unit. 
     The inputs  148  to the timing reference switch  145  include a SONET minimum clock (SMC)  149  generated by a clock source on the line unit, and a local line reference clock from the signal routing ASIC  102 , that is derived from the line side SONET ring. SMC and line reference signals are also provided to the timing reference switch  145  from the partner line unit as well. When the ESM daughter board is present, the reference signal from the ESM is always selected to pass through the timing reference switch  145 . 
     The output of the timing reference switch  145  is passed to distribution phase locked loop  151  which smoothes out any switching transients and converts the selected timing reference to a higher distribution frequency that is passed to the other units in the device, as well as to frame pulse generation logic  153 . The frame pulse generation logic  153  derives a frame pulse from the distribution frequency and passes that frame pulse to the other units in the device, as well as to the frame pulse generator of the partner line unit. The distribution frequency is also received by the board clock logic  155 , and passed to phase locked loops  157  and  159  for further frequency conversion to obtain frequencies needed to support the ATM and STM protocols within the logic of the signal routing ASIC  102  and switch fabric ASIC  114 . 
     FIG. 8 is a block diagram illustrating the operations performed in the STM and ATM inbound datapaths within the line unit  100  of FIG.  7 . An incoming SONET signal is first converted from optical to electrical signals (OC-12 to STS-12 for example) by optical to electrical conversion circuit  150 . The output of the circuit  150  is passed to the signal routing ASIC  102 , which performs section, line and path overhead extraction, as well as STM and VT pointer processing in a first functional block  152 . In response to provisioning information, the functional block  152  separates ATM and STM traffic contained within the received SONET signal. A first output of the functional block  152  is ATM traffic  154  extracted from the received SONET signal, which is passed to a second functional block  156  within the signal routing ASIC  102 . A second output of the functional block  152  is STM traffic  158  extracted from the received SONET signal, which is passed to both the STM switch fabric of the partner line unit, and also to the local switch fabric ASIC  114 . A third output of the functional block  152  is the extracted SONET overhead information  128 , which is passed to the MCU. 
     The signal routing ASIC  102  further provides path selection control information to the switch fabric ASIC  114 , as well as to the ATM switch fabric units. Such information reflects monitoring of incoming path performance by the signal routing ASIC  102 , for example through bit error monitoring, AIS (Alarm Indication Signal) monitoring, alarm detection, and or path label monitoring. The path selection information provided to the switch fabric ASIC  114  may be at the STS signal or VT level. Path selection information at the STS level is provided by writing a path status value over an STS line overhead byte within the STS signal. The status value reflects which of the input paths to the switch fabric ASIC  114  is to be used to receive the particular STS signal associated with the line overhead byte. The line overhead byte may be overwritten by the signal routing ASIC because it has previously extracted the line overhead and sent it to the MCU, prior to forwarding of the signal to the switch fabric ASIC  114 . For VT specific path selection information, the signal routing ASIC  102  overwrites one of the VT pointer bytes for the corresponding VT with path status information. The VT pointer byte may be overwritten by the signal routing ASIC  102  because it has previously performed VT (as well as STM frame) pointer processing. In this way STM traffic  158  carries path selection information to the STM switch fabrics of both the local and partner line units. The signal routing ASIC  102  must align the STM traffic  158  with the STM traffic  132  received from the signal routing ASIC of the partner line unit prior to reception by the switch fabric ASIC  114 . Such alignment must ensure that STS signals received by the switch fabric ASIC  114  are frame aligned for STS signals, and frame and SPE aligned for VTs. 
     The path selection control information provided by the signal routing ASIC  102  to the active and standby ATM switch fabric units is similarly reflective of incoming path performance and availability monitoring as described above. The path selection control information for the ATM switch fabric unit is output from the signal routing ASIC  102 , and transmitted as part of a serial data stream including an ATM cell stream to each of the ATM switch fabric units. The path selection information so provided is used by the ATM switch fabric units to perform ATM VP path selection. 
     The second functional block  156  in the signal routing ASIC  102  formats the ATM traffic  154  into an ATM cell stream. The resulting cell stream is stored in the output buffer  160  for transmission to the active and standby ATM switch fabric units. While the output buffer  160  is shown for example within the signal routing ASIC  102  in FIG. 8, they may also be implemented externally to the signal routing ASIC  102 . 
     The STM traffic  158  from the signal routing ASIC  102  is received by the switch fabric ASIC  114 , where the STS-1 signals it contains are groomed for the type of traffic they are carrying. For example, a VT mapped STS-1 signal in the STM traffic from the signal routing ASIC is broken up into 28 VTs  162 , which are organized into two sets of 14 VTs. Each set of 14 VTs is received from both the local STS switch fabric  163  and the partner STM switch fabric by the path selector logic  166  for subsequent broadcast to an active/standby service unit pair. In addition, an STS-1 signal  164  in the STM traffic from the local and partner signal routing ASICs is shown for purposes of example being passed to the path selector logic  166 , also for subsequent broadcast to an active/standby service unit pair. 
     The path selector logic  166  selects the best receive path between the path from the local STM switch fabric, and the path  132  from the STM switch fabric in the partner line unit, in response to in-band path selection information provided by the signal routing ASICs  102  of both the active and standby line units. The path selection information is contained within overwritten overhead bytes in each particular STS or VT signal. The outputs of path selector logic  166  are then combined by multiplexers  167  for STS-3 transmission to the appropriate service units. 
     FIG. 9 is a functional block diagram illustrating operations performed in the STM and ATM outbound datapaths within the illustrative line unit  100  of FIG.  7 . FIG. 9 shows outbound datapath operation for a line unit provisioned to insert 1 VT-mapped STS-1 signal  190 , one DS-3 mapped STS-1 signal  192  and one ATM mapped STS-1 signal  194  onto an outgoing STS-12 signal  196 , which is converted to OC-12 by electrical to optical conversion circuit  175 . The 28 VTs of the VT-mapped STS-1 signal  190  are received from two separate active/standby pairs of DS-1 service units  184  and  186 . The DS-3 source of the DS-3 mapped STS-1 is an active/standby pair of DS-3 service units. The ATM mapped STS-1 signal  194  is received by the line unit  100  via cell streams  181  from the active/standby ATM switch fabric units within the device. 
     The 28 VTs from service units  184  and  186  pass through STS switch fabric  163  to path selector logic  166  where selection between active or standby service unit sources is performed in response to path selection information contained within the respective VT and STS signals received from the service units. In an illustrative embodiment, each one of the service units includes a signal routing ASIC such as signal routing ASIC  102 , which embeds path selection information within each VT or STS signal in response to path performance and availability monitoring. 
     The 28 VTs are then multiplexed onto a single STS-1 signal  190  by multiplexers  169 , and passed to the signal routing ASIC  102 . The DS-3 mapped STS-1 also goes through the STS switch fabric  163  to path selectors within path selector logic  166 , also for selection between active or standby service unit sources. The DS-3 mapped STS-1 is then passed as STS-1 signal  192  to the signal routing ASIC  102 . 
     Receive buffers  178  within the signal routing ASIC  102  receive the two ATM cell streams  181  from the active and standby ATM switch fabric units (ATMU-A and ATMU-S). While the receive buffers  178  are shown within the signal routing ASIC  102  in FIG. 9, they may also be provided externally to the signal routing ASIC  102 . A parity bit is included with each cell stream, and is used by the signal routing ASIC  102  to monitor the quality of the paths carrying the cell streams to the signal routing ASIC  102 . One of the cell streams  181  is selected, responsive to path quality information, to pass from receive buffering  178  to a first logic block  180 . Within the logic block  180 , ATM Transmission Convergence layer functions are performed, the cell stream is delineated, and the STS routing tags are stripped off. The resulting ATM-mapped STS-1 signal  194  is then processed by functional block  182 , and multiplexed onto the line side SONET ring with the other STS-1&#39;s  190  and  192 . Section, line, and path overhead information are inserted as necessary, and the combined traffic is sent to the electrical to optical conversion circuit  175  for optical transmission onto the SONET ring. For example, STM traffic  190  and  192  includes path overhead information carried through the STM switch fabric, and requires line and section overhead to be inserted by the signal routing ASIC  102 . ATM traffic  194  requires path, line and signal overhead bytes to be inserted by the signal routing ASIC  102 . 
     An illustrative control architecture for a line unit is shown in FIG.  10 . FIG. 10 shows a serial hardbus terminator  200  coupled to an address/data bus  202 . A number of general purpose I/O registers  208  are further coupled to the address/data bus  202 , and made accessible to the MCU over the serial hardbus via the serial hardbus terminator logic  200 . A memory map  206  consisting of accessible registers in the switch fabric ASIC  114 , and a memory map  204  consisting of accessible registers in the signal routing ASIC  102  are also made accessible to the MCU via the serial hardbus terminator logic  200 . During operation of the line unit, the MCU provides provisioning information through the serial hardbus to the line unit. The provisioning information determines the operation of the signal routing ASIC  102  and switch fabric ASIC  114 . For example, provisioning information from the MCU controls how the switch fabric ASIC  114  routes STS signals it receives, and determines which of the STS signals on the line side SONET ring are treated as containing ATM cells. In addition, the MCU detects failures and error conditions through interrupts received over the serial hardbus. In response to detection of such failures and error conditions, the MCU provides path selection information to the line unit over the serial hardbus, for example determining which one of an active/standby pair of service units is to be the source for a particular VT at any given time. 
     FIG. 11 shows an illustrative embodiment of the signal routing ASIC  102  provided within the line unit, and which may also be provided within a service unit. The signal routing ASIC  102  is capable of receiving data from and transmitting data to optical drivers that interface to a SONET ring on the line side or the service side of the device. The signal routing ASIC  102  further contains an ATM interface for processing ATM cells, such as are communicated between a line unit and an ATM switch fabric unit. Finally, the signal routing ASIC  102  includes an STM interface for transmitting data to and receiving data from an STM switch fabric ASIC. 
     The signal path for data received by the signal routing ASIC  102  from the SONET ring is first described. Data received from the SONET ring is converted from an optical signal to an electrical signal by an optical receiver outboard of the signal routing ASIC  102 , and is serially coupled to one input of a line selector  225  within the signal routing ASIC  102 . In a preferred embodiment, the serial input to the line selector  225  comprises a 155 megabit per second unidirectional path. The signal at the serial input to the line selector  225  may for example be 1 STS-3c signal, 3 STS-1 signals, 84 VTs, or any combination of STS-1 and VT signals within the specified signaling bandwidth. 
     The signals received from the SONET ring are passed from the output of the line selector  225  to both a synchronous payload envelope (SPE) splitter  227 , and an overhead bit drop (OHB) recovery circuit  229 . Overhead bits carried on the received STM signal are segregated by the OHB recovery circuit  229  and coupled to a RX OHB Serial Link interface  231 . In a preferred embodiment, the RX OHs Serial Link interface  231  transmits the overhead bits of the signals on the SONET ring over a 9.72 mbps serial link to the MCU for processing. 
     The SPE splitter  227 , in response to previously received provisioning information from the MCU, extracts STM, VT, and/or ATM traffic streams from the STS signals on the SONET ring, and passes each one of these traffic streams into a respective one of three elastic stores. In the case of ATM traffic, the ATM cells are unencapsulated prior to being stored in the ATM cell FIFO  233 . 
     More specifically, the output of the synchronized payload envelope splitter  227  is coupled to three elastic stores. The elastic stores are fabricated as first in first out storage (FIFOs) and serve a rate decoupling function. In the preferred embodiment, the first elastic store  235  receives STS-1 and STS-3c traffic and is used to perform rate decoupling for such traffic. The second elastic store  237  receives VT traffic. The third elastic store is an ATM cell FIFO  233 , and is employed to provide rate decoupling for ATM cell traffic. Outputs of the of the first and second elastic stores  235  and  237  are coupled to a synchronized payload envelope multiplexer (SPE MUX)  239 , which is employed to multiplex the various STS signals to the STM switch fabrics on each of the active/standby line unit pair. The output of the SPE MUX  239  is broadcast to the STM switch fabrics on both active and standby line units in the event two such cards are provided for purposes of redundancy. 
     The output of the ATM cell FIFO  233  is coupled to the ATM receive link interface (ATM Rx Link)  241 . The output of the ATM receive link interface  241  is coupled to the ATM switch fabric unit via a four bit wide parallel data bus which is clocked at 39 Mhz to a 156 Mb/s bandwidth between the signal routing ASIC and the ATM switch fabric. Since, in the illustrative embodiment of FIG. 11, the bandwidth between the signal routing ASIC  102  and the ATM switch fabric unit exceeds the ingress bandwidth of  155  Mb/s, the ATM cell FIFO  233  cannot overflow. In the foregoing manner, ingress SONET traffic carrying STS-1, STS-3c, VT or encapsulated ATM cells is forwarded to either an STM or ATM switch fabric as applicable. 
     Data destined for the line side SONET interface is received by the signal routing ASIC  102  from either the STM switch fabric or the ATM switch fabric. More specifically, an active/standby switch  243  receives STM data from both a first STM switch fabric, for example which is co-resident on the local line card, as well as from a second STM switch fabric, for example located on a separate line card, in the event the separate line card is present and configured with the first line card as a redundant pair. The active/standby switch  243  is controlled by provisioning information received from the MCU to select STM data from one of two STM switch fabrics to which is it coupled. The selected STM data is passed to an SPE multiplexer  245 . 
     Similarly, ATM cells destined for a SONET interface of the device are received at an ATM Transmit Link interface  247  either from a single ATM switch fabric or from two ATM switch fabrics (active and standby) in the event that two ATM switch fabrics are provided in a redundant configuration. In a preferred embodiment, ATM data is received from the respective active/standby ATM switch fabrics over a 4 bit wide parallel interface running at approximately 39 mbps. The ATM data received from one of the active/standby pair of ATM switch fabric units is selected, and passed to the ATM cell FIFO, which performs rate decoupling. The ATM data is then passed on to the SPE MUX  245 . The SPE MUX  245  multiplexes the ATM cell data from the ATM cell FIFO  249  together with the STM data from the active/standby switch  243  for subsequent STM transmission. 
     The output of the SPE MUX is then coupled to an overhead byte multiplexer (OHB MUX)  251 . The OHB MUX  251  also receives as an input management and control information, generated by the MCU, via a transmit OHB serial link interface  253  to the MCU. The OHB MUX  251  inserts the received overhead information in the appropriate overhead channels of the resulting SONET signal. 
     In a preferred embodiment, the output of the OHB MUX  251  is provided both in the form of an 8 bit parallel interface, as well as to a parallel to serial converter  257 , which provides a serial output interface  259 . The parallel interface to the signal routing ASIC  102  may be coupled to a parallel bus, for example in an alternative embodiment in which the signal routing ASIC is included within a service unit. The parallel data would then be processed as required by the particular service unit. The serial output  259  is illustratively coupled to an electrical to optical converter, as in the preferred embodiment in which the signal routing ASIC is employed within the line units of the device. The optical converter then passes the serial data to the line side SONET interface. In the foregoing manner, the signal routing ASIC  102  is capable of routing STM and ATM formatted traffic between a line side SONET interface, ATM switch unit interface, and STM switch fabric interface. 
     In an alternative embodiment of the signal routing ASIC  102  shown in FIG. 11, which is designed to support a line side interface to an OC-12 SONET signal, the second elastic store  237  is provided between the first elastic store  235  and the SPE multiplexer  239 . Such a configuration allows the second elastic store  237  to utilize the same clock domain as the output of the first elastic store  235  and the input of the SPE multiplexer  239 , in order to eliminate the clock jitter effect and reduce the number of gates needed to fabricate the signal routing ASIC  102 . 
     FIG. 12 shows an extended synchronization module (ESM), including a timing source selection ASIC  312 . The ASIC  312  is shown having inputs including a 12.96 Mhz clock source  300  from an ESM on a partner line unit, as well as 16 inputs  302  from each service unit in the device. A pair of 1.544 Mhz clocks, which are output from a T 1  receiver  308 , are also input to the ASIC  312 . The T 1  receiver  308  is operable to receive an externally generated 1.544 Mhz BITS clock  310 . 
     The ASIC  312  selects one of the input clocks it receives at each of multiplexers  311 , and detects which of the input clocks are not present in loss of signal (LOS) circuit  313 , and informs the digital phase locked loop  320  of any such loss. The selected clock is passed to frequency measuring logic  314 , fractional divider  316  and fractional divider  318  within the ASIC  312 . Frequency measuring logic  314  compares the selected clock with a target clock based on its own internal time base. The results of this comparison are passed to the digital phase locked loop  320 . The ASIC  312  may select a new clock source using multiplexers  311 , in response to feedback control signals from the digital phase locked loop  320 . 
     The fractional divider circuit  316  derives an 8 Khz reference clock from the selected clock source, which is passed to the digital phase locked loop  320 . The fractional divider  318  derives a 1.544 Mhz clock from the selected clock source, which is passed to the T 1  receiver  308 , in order to provide an alternative 1.544 Mhz source to the T 1  receiver  308 . 
     The digital phase locked loop  320  generates a 3.24 Mhz clock, that is passed to phase locked loop  322  which converts the signal to 12.96 Mhz. The converted signal is passed to protection switch logic  324  and to any ESM on the partner line unit. The protection switching logic  324  selects between either the 12.96 Mhz clock from phase locked loop  322  or the 12.96 Mhz clock from the ESM of the partner line unit. The selected 12.96 Mhz signal is passed through phase locked loop  326  for smoothing and into clock distribution buffer  328  for distribution to the other units in the device. 
     FIG. 13 shows components for performing a software image download to or from an MCU  400 . A craft personal computer (PC)  420 , operations support station (OSS)  422 , and remote network element (NE)  418  are shown communicably coupled with the MCU  400  via a first communications network  416 . A transmission control protocol/internet protocol (TCP/IP) functional unit  410  operates in conjunction with a file transfer protocol (FTP) functional unit  408  to support file transfers between the RAM disk  404 , and the OSS  422 , craft PC  420 , or network element  418 . 
     An Open Systems Interconnect (OSI) protocol stack  414  operates in conjunction with a file transfer access method (FTAM) functional unit  412  to support file transfer between the RAM disk  404  and the OSS  428  network element  426  on a second communications network  424 . A download agent  430  operates to maintain a software download management information base (MIB)  432 , as well as to process software download commands  427  received by the MCU. A software management agent  434  maintains version control over software images stored in either the RAM disk  404  or the FLASH disk  406 . Software image files received by the MCU  400  are copied from the RAM disk  404  to the FLASH disk  406 , and then loaded into appropriate ones of service units  402 , over serial hardbus connections  403 . The service units  402  each receive a respective software image initially into a “standby” RAM  440 , and subsequently copy it to an “active” FLASH RAM  442 . The image is then copied into storage referred to as the “executable” RAM  444 , from which the image may be executed on a microprocessor  446 . As the active FLASH RAM  442  and FLASH disk  406  are non-volatile stores, the software images stored within them persist following removal of power from the device. 
     FIG. 14 shows hardware components within a DCC processor  500  of an MCU card, and within an intelligent service unit  502 . The DCC processor  500  includes a FLASH disk  504 , CPU  506 , “boot” FLASH RAM  508 , RAM disk  510 , “executable” RAM  512 , “active” FLASH RAM  514 , “standby” RAM  516 , and serial hardbus master logic unit  518 . During operation, a software image received by the DCC processor  500  is initially stored in RAM disk  510 . The image is then copied to FLASH disk  504 . The FLASH disk  504  maintains a software image for the MCU, as well as for each intelligent service unit in the device, and for any ATM processor associated with an ATM switch fabric unit installed in the device. A received software image for the MCU itself is copied from the FLASH disk  504  to the standby RAM  516  and then to the active FLASH RAM  514 . The MCU software image is subsequently passed to the executable RAM  512 , from which it is executed on the CPU  506 . 
     A software image for the intelligent service unit  502  is copied from the FLASH disk  504  over the serial hardbus  519  to the standby RAM  524  of the intelligent service unit  502 , and then to the active FLASH RAM  528  and executable RAM  526 . During operation, the software image within the executable RAM  526  is executed on the CPU  520 . 
     FIG. 15 shows a number of interfaces to an MCU  550 , including an RS232 interface which may be connected to a craft terminal, a SONET overhead connection  564 , for receiving SDCC information from a SONET ring connected to the device, an X.25 connection  566 , a local communications network (LCN) interface  568 , an Ethernet connection  570  to which a craft PC may be connected, and an ATM virtual connection  572  which may be a permanent virtual circuit (PVC). The ATM virtual connection  572  is for example, one of a number of permanent virtual connections provisioned to carry software image data or other maintenance information between MCU  550  and MCUs of other network elements. 
     During operation of the MCU  550 , software image files may be communicated to and from the MCU  550  using an FTP functional unit  556  in conjunction with a TCP/IP functional unit  558 , via the ATM VC  572 , Ethernet  570 , or LCN  568 . In addition, software image files may be communicated with the MCU  550  using the OSI stack functional unit  560  and FTAM functional unit  554  via SDCC over the SONET overhead connection  564 , or via the X.25  566 , LCN  568 , or Ethernet  570  interfaces. A command interpreter  552  operates to process Transaction Language  1  (TL 1 ) commands received from a craft terminal over the RS232 connection  562 . 
     FIG. 16 shows a “gateway” network element (GNE)  600 , operations support station (OSS)  604 , and a remote network element  602 . The network element  600  is referred to as a “gateway”, for example, because it provides a central management station such as the OSS  604 , a connection to a SONET ring to which the remote network element  602  is connected. To initiate a download of a software image from the gateway network element  600  to the remote network element  602 , a command is first issued by the OSS  604 . Processing of the command results in an FTP request, which causes the FTP functional unit  610  to operate with the TCP/IP functional unit  608  to form a number of IP packets containing the software image. A number of HDLC frames, which contain the IP packets storing the software image, are then passed by the MCU  606  to the ATM switch fabric unit  614 . The MCU  606  also passes a VCI/VPI value to the ATM switch fabric unit  614 . The VCI/VPI value indicates one of a number of provisioned PVCs that are used to pass software image files from the gateway network element to remote network elements. In a first embodiment, each of the provisioned PVCs connects the gateway network element with one remote network element. In this way a star configuration is formed with the gateway network element as a hub. In a second embodiment, a PVC is provisioned only to the next network element adjacent to the gateway network element. In this alternative embodiment, the software image file is passed from adjacent network element to adjacent network element, until it arrives at an MCU having an IP address matching the destination IP address of the IP packets carrying the software image file. In this way, the software image file is passed from network element to network element. An IP termination and forwarding function  612  in the ATM unit  614 , extracts the ATM VCI/VPI value contained in the HDLC frames, and performs segmentation of the file to form ATM cells, for example using a segmentation and reassembly (SAR) unit within the ATM switch fabric unit  614 . The IP termination and forwarding function  612  then passes the ATM cells to the switch fabric  616 , which determines either that the VCI/VFI value in the cells indicates a permanent virtual circuit (PVC)  620  between the GNE  600  and the remote NE  602  or, alternatively, between the GNE  600  and an adjacent network element, which may or may not be the remote NE  602 . The ATM switch fabric  616  forwards the cells over the PVC  620 , by way of the line unit  618 . 
     The cells are eventually received by the line unit  622  of the remote network element  602 . The line unit  622  forwards the received cells to the switch fabric  626  within the ATM switch fabric unit  624 . The switch fabric  626  recognizes that the PVC identified by the VCI/VPI value contained within the cells is terminated in the remote network element  602 , and accordingly forwards the received cells to an IP termination/forwarding function  628 . The IP termination/forwarding function then employs a segmentation and reassembly unit to reassemble the IP packets from the ATM cells, and forms a number of HDLC frames in which to forward the IP packets to the MCU  630 . The MCU  630  then examines the IP destination address in the IP packets, and determines that the IP destination address is an IP address of the MCU  630 . The FTP function then loads the software image file within the IP packets into a memory associated with the MCU  630 . 
     FIG. 17 shows steps performed during downloading of an executable software image from a gateway network element to a remote network element. As illustrated at step  650 , a software download command is received by the MCU of the gateway network element. The download command is for example, a TL 1  copy-file command which indicates a source of the software image file, as well as the destination network element to which the software image file is to be copied. The download command is for example, received over an interface to the MCU such as an RS232 connection to a craft terminal. Alternatively, the command may be received over an LCN, X.25, Ethernet connection to the MCU, or any other suitable connection. The software image file to be downloaded may, for example, be received within a number of IP packets, or, alternatively over an X.25, LCN, or Ethernet interface to the MCU of the gateway network element. 
     The download command is processed by software executing in the MCU of the gateway network element, which issues an FTP Request to an FTP functional unit also within the MCU. The FTP functional unit uses a TCP/IP functional unit to form a number of IP packets having IP destination addresses equal to an IP address of an MCU within the remote network element, and containing the software image file indicated in the download command received at step  650 . In an alternative embodiment, the IP destination address is a multicast address recognized by each device which uses a common executable software image, and which is being provided a new executable software image by the download being performed. 
     Software within the MCU of the gateway network element then determines a VCI/VPI value of a permanent virtual circuit (PVC). In a first embodiment, the PVC is between the gateway network element and the remote network element. In an alternative embodiment, the PVC is between the gateway network element and an adjacent network element (which may or may not be the remote network element in which the software image is to be loaded and executed). The MCU software then forms a number of HDLC frames containing the IP packets, as well as the VCI/VPI value of the PVC, and forwards these HDLC frames at step  654 , over a serial point to point connection to an ATM switch fabric unit within the local network element. 
     Upon receipt of the HDLC frames from the MCU, as illustrated at step  656 , software within the ATM switch fabric unit employs a SAR unit, also within the ATM switch fabric unit, to form a number of ATM cells having the VCI/VPI value provided from the MCU in their header. The cells are then passed to an ATM switch fabric within the ATM switch fabric unit, which forwards the cells to an output interface of the ATM switch fabric unit associated with the PVC. The selected output interface of the ATM switch fabric unit is, for example, coupled to a line unit within the local network element, which receives the cells, encapsulates them into an STS signal, and transmits them onto a SONET ring. 
     As depicted at step  658 , the remote network element, which is, for example, also coupled to the SONET ring, receives the STS signal containing the ATM cells storing the software image. The remote network element determines that the STS signal contains ATM cells. The remote network element further determines that the STS signal is provisioned such that the ATM cells it contains are extracted and forwarded to an ATM switch fabric unit within the remote network element. 
     The switch fabric within the ATM switch fabric unit, responsive to the VCI/VPI value in the received cells, forwards the received cells to an IP termination and forwarding functional unit within the ATM switch fabric unit. The IP termination/forwarding unit uses a SAR unit within the ATM switch fabric unit to reassemble the IP packets from the ATM cells. At step  660 , the IP packets are then encapsulated into a number of HDLC frames, which are forwarded to the MCU of the remote network element. 
     At step  662 , the MCU of the remote network element receives the HDLC frames containing the IP packets, and compares the destination IP address of those packets with an IP address of the MCU of the remote network element. In response to a match, the MCU software extracts the software image file within the IP packets and loads it into a memory within the MCU at step  664 . In the alternative embodiment in which the PVC from the gateway network unit is with an adjacent network element, and where that adjacent network element is not the remote network element for which the software image is destined, there would not be a match between the IP destination address of the packets and the IP address of the MCU of that adjacent network element. In that case, the MCU software of the adjacent network element would look up a VCI/VPI value associated with a PVC to an adjacent network element, and forward the packets back to the ATM switch fabric unit, along with the new VCI/VPI value. The ATM switch fabric unit would then form cells having the new VCI/VPI value in their headers, and forward the cells to an output interface associated with that PVC. The IP packets would then be received by the next adjacent network element, which would again determine if the destination IP address of the packets is the IP address of an MCU within that network element. In this way the IP packets continue to be forwarded from network element to network element until they reach the target network element, into which the software image is downloaded. 
     The functions herein described can be implemented in many forms, including one or more Application Specific Integrated Circuits or any other suitable hardware implementation, or some combination of hardware components and software. Where a portion of the functionality is provided using software, that software may be provided to the computer in many ways; including, but not limited to: (a) information permanently stored on non-writable storage media (e.g. read only memory devices within a computer such as ROM or CD-ROM disks readable by a computer I/O attachment); (b) information alterably stored on writable storage media (e.g. floppy disks and hard drives); or (c) information conveyed to a computer through communication media such as computer or telephone networks via a modem. 
     While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.