Patent Publication Number: US-6338046-B1

Title: System and method for determining charges for usage of a network connection

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to communication systems, and more particularly, to a system and method for determining charges for usage of network resources. 
     BACKGROUND OF THE INVENTION 
     The state of communications technology, particularly that which affects the Internet, is currently in flux and subject to rapid and often uncoordinated growth. The ubiquity and diversity of personal computers and set-top boxes has placed significant pressure on the providers of communications system infrastructure to accommodate the alarming increase in the number of new users that demand immediate access to Internet and other network resources. The rapid development of new and sophisticated software made available to users of such services places additional demands on system infrastructure. 
     Charging for Internet and other network services is a practice that is gaining acceptance and popularity. By way of example, traditional on-line service., such as those offered by Internet providers, typical charge customers a monthly fee for access to basic services and resources, such as proprietary and public databases of information. Such traditional service providers may also advertise any number of products or services which are purchasable on-line by the user. 
     Other forms of Internet commercialization currently being considered or implemented include offering of video and audio conferencing services, and a variety of other real-time and non-real-time services. The providers of these services, as well as the providers of communications system infrastructure, are currently facing a number of complex issues, including management of network capacity, load, and traffic to support real-time, non-real-time, and high-bandwidth services, and implementing a viable billing scheme that accounts for the use of such services. 
     Conventional billing schemes which have been used by the operators of broadband networks typically involve computing connection usage charges based on the bandwidth provided to a customer. Such bulk charging methods fail to accurately distinguish between, and account for, the actual levels of service provided to the customer (e.g., maximum peak rates, acceptable cell loss ratios, real-time versus non-real-time services), and provide only a crude estimate of the actual resources used by the customer. Network operators are currently limited in their ability to asses the real cost of providing various types of network resources and services, which typically results in the overcharging or undercharging of customers Notwithstanding the complexity of conventional network traffic management schemes, such as those applicable to asynchronous transfer mode (ATM) networks, current ATM specifications and recommendations fail to adequately address the need of service providers for a billing methodology that provides for accurate and reliable charging of services utilized by user&#39;s of the network. 
     Accordingly, there is a need in the communications industry for a system and method for implementing a flexible and effective charging capability that accounts for the particular use of a network service connections and other resources by users of a network. The present invention fulfills this and other needs. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method for charging for usage of network service connections. The network includes a number of nodes which facilitate the transfer of information from a source location to a destination location. At the time a connection is established with a node, a billing message or cell is transmitted to the node over the connection. The billing cell contains billing and connection information which is copied by the node and used to produce billing information. Information cells are transmitted over the same connection subsequent to the transmission of the billing cell. A terminate billing cell or conventional means may be used to drop the connection when the connection is no longer needed. The node computes the cost of using the connection based on the billing and connection information copied from the billing cell The connection usage computation may also take into account other factors, such as the connection time and the amount of data transferred over the connection. 
     Upon establishing a subsequent connection with another node, the previous node transmits the billing cell to the new node which copies the billing cell content and, after receiving the information cells over the new connection, perform; similar connection usage computations for use of the new connection. This process is repeated by each of the nodes involved in the transfer of information between the source and destination. The nodes also transmit their respective charging information to a common network billing system which generates-the total charge in a final bill that is forwarded to the user. The billing cell may be transmitted from the destination back to the source to verify proper operation of the billing procedure. Fixed rate and variable rate billing schemes may be implemented and augmented as needed by each network operator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a diagrammatic representation of a network including a number of network nodes or switches, each of which is maintained by a different operator; 
     FIG. 2 is an illustration of two independent nodes of a network which operate in accordance with one embodiment of the present invention to determine the cost of communicating information over a network connection implicating the network nodes; 
     FIG. 3 illustrates an embodiment of a billing system for a network constituted by a number of independent network nodes maintained by autonomous operators; 
     FIG. 4 illustrates an embodiment of a charging scheme in which the cost of communicating over each of a number of virtual connections through a network node may be independently accounted for; 
     FIG. 5 illustrates another embodiment of a charging scheme which is based on a fixed cost approach; 
     FIG. 6 is a flow diagram illustrating a general procedure for communicating cells of information between a user/network interface and a network using a nominal bit rate service; 
     FIG. 7 illustrates in greater detail a procedure for transmitting cells of information between a user/network interface and a network using a nominal bit rate service; 
     FIG. 8 illustrates in flow diagram form a general procedure for filtering cells at a network node in accordance with an embodiment of a nominal bit rate service; 
     FIG. 9 is a block diagram of an embodiment of a system for filtering cells at a network node in accordance with a nominal bit rate service; 
     FIG. 10 is a block diagram of an alternative embodiment of a system for filtering cells at a network node in accordance with a nominal bit rate service; 
     FIG. 11 is a block diagram illustrating one embodiment of an ATM network implementing the NBR methodology; 
     FIG. 12 illustrates one embodiment of a network load status cell in accordance with the NBR service; 
     FIG. 13 is a block diagram of a representative ATM node within an ATM network configured to implement the NBR methodology; 
     FIG. 14 illustrates in flow diagram form a general methodology by which NBR status cells can be used to provide feedback to a source end-station to optimize cell transfer rates; 
     FIG. 15 illustrates one embodiment for determining the priority level feedback PL fb  wherein the minimum PL a  value is sought over a predetermined observation time period t 0 ; 
     FIG. 16 illustrates another embodiment for determining the priority level feedback PL fb  wherein a typical minimum PL a  value is determined by determining a minimum PL a  value occurring a predetermined number of times during a predetermined time period to; 
     FIG. 17 illustrates an embodiment for determining the priority level feedback PL fb  wherein the minimum PL a  value is determined by averaging the PL a  values over a time period t 0 ; 
     FIG. 18 illustrates yet another embodiment for determining the priority level feedback PL fb  wherein the minimum PL a  value is determined by calculating a weighted average of PL a  over a time period to; 
     FIGS. 19-20 are graphical depictions of the relationship of average cell loss ratio, P loss , as a function of priority level for four specific load levels; 
     FIG. 21 is a graphical illustration of the results of a simulation which compares the constant bit rate connections to those using feedback sources; and 
     FIG. 22 is a graphical illustration of the throughput/capacity from each of the feedback sources as a function of time. 
    
    
     DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS 
     In the following description of the various embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration various embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present invention. 
     Various embodiments of the present invention will be described hereinbelow within the context of a broadband network environment which employs a packet switching mode of information transfer. In particular, several embodiments and aspects of the invention will be described within the context of an asynchronous transfer mode (ATM) network environment, and within the context of a novel service model which may be implementable within existing ATM networks. It is to be understood that the various embodiments of the invention described with regard to specific types of network environments are provided for illustrative purposes only, and are not to be construed as limiting the scope of the invention. 
     In FIG. 1, there is illustrated a depiction of a network  20  over which information is transported by use of a number of independent network nodes or switches  24 . A user  26  of the network  20  typically utilizes a user/network interface (UNI) or other network interfacing device to access the network  20 . It is noted that a user  26  may access the network  20  through a communication system  28  separate from the network  20 , such as a local or wide area network for example. After establishing connectivity with the network  20 , a user  26  may interact with the network  20  to perform a number of operations, such as transporting data to another user  26  of the network  20 . The manner in which such data is transported through the network  20  is typically transparent to the user  26  and dependent on the particular architecture and protocol of the network  20 . 
     A system and method directed to charging for network connection usage implemented in accordance with the principles of the present invention permits different network operators  22  to implement charging strategies for determining the cost of using network connections  30  and other resources within their jurisdiction independent from the charging strategies employed by other network operators  22 . Such a charging methodology provides network operators  22  the opportunity to develop and implement a charging strategy that takes into consideration a number of service factors which impact the performance of network resources as perceived by the paying user of the network. 
     A non-exhaustive list of possible factors that may be used as variables within usage charging formulae include service class type, quality of service, any individual or combination of ATM traffic parameters, connection time, and other traffic flow parameters, for example. Those skilled in the art will immediately appreciate the substantial increase in flexibility and credibility provided by a billing scheme which considers such factors when computing the cost of network resource usage, especially when contrasted to the relatively primitive approach of merely accounting for the number of bytes transferred between any two network operators or the amount of bandwidth provided to a customer. 
     In FIG. 2, there is illustrated one embodiment of the present invention which employs a billing message that incorporates connection/level of service information and other billing information. The billing message is utilized by the nodes  24  of different network operators  22  when determining the cost of using a connection or other resource established through a particular node  24 . In this embodiment, the billing message or billing cell  31  is transmitted over the same connection used to transfer the actual data. The billing cell  31  may be transmitted at the time the connection is established and prior to the transfer of data over the connection. 
     By way of example, and with continued reference to FIG. 2, it is assumed that a user  26  wishes to transfer information between an information source  27  and a destination  29 . The user  26  accesses the network  20  and establishes a connection  33 , such as a virtual connection within the context of an ATM network, with a node  24   a.  It is noted that the node  24   a  is under the control or otherwise maintained by an operator OP-A. At the time of, or following, establishing the connection  33  between the source  27  and the node  24   a,  the source  27  transmits a billing cell  31  prior to transmitting cells of information over the connection  33 . 
     The billing cell  31  includes a header  36  which uniquely identifies this cell as providing billing information, as distinguished from cells of a conventional type. It is noted that the node  24   a,  which may constitute a standard ATM switch, does not inspect the actual data field of the billing cell  31 , and therefore reads the header information as a means of distinguishing between billing cells and non-billing cells. It is further noted that the payload type (PT) field of a standard ATM cell comprises two bits which may be set by the source unit  27  to identify a particular cell as a billing cell  31 . Alternatively, Operation and Maintenance (OAM) cells may be used to transfer billing data. 
     In addition to a header  36 , a billing cell  31  carries with it two other types of information, namely, billing address information  25  and connection information  38 . The billing address information  25  is used to determine the recipient of the charging information associated with use of a particular connection  33 . The node  24   a  cooperates with a billing unit  34  which processes the billing information transported by the billing cell  31 . It is understood that the billing unit  34  may reside within the node  24   a  or may operate peripherally to the node  24   a  Further, the functions performed by the billing unit  34  may instead be performed by a processor of the node  24   a.    
     When the billing cell  31  is received by the node  24   a , the connection information  38  is copied from the billing cell  31  to the billing unit  34 . Cells having conventional payloads are typically received by the node  24   a  over the connection  33  subsequent to the receipt of the billing cell  31 . When the connection  33  is no longer required, a terminate billing message or cell (not shown) may be transmitted by the source  27  to the node  24   a  Alternatively, the connection  33  may be dropped in accordance with the network&#39;s standard procedure for terminating a connection 
     After receiving the terminate billing cell or dropping the connection  33  in a conventional manner, the billing unit  34  processes the connection information  38  and billing address information  25  provided by the billing cell  31  to compute the cost of using the connection  33 . In one embodiment, a timer  39  of the node  24   a  may be employed to account for the duration of time in which the connection  33  was used. The connection time information may be used by the billing unit  34  in combination with the connection information  38  when computing connection usage charges. In another embodiment, the number of bytes of data transferred during the existence of the connection  33  may also be considered when computing the cost of using the connection  33 . 
     The relevant charging information acquired by the billing unit  34  may then be used to locally compute the cost of connection usage. Alternatively, unprocessed charging information may be transported to an external billing system which may perform the computations necessary to determine the cost of using the connection  33 . The external billing system may be used as a common site for acquiring and/or processing charging information received from other nodes of the network  20 . 
     The connection information carried by the billing cell  31  and copied by the node  24   a  is intended to describe the general level of service expected or required by a network user when using a particular connection, such as by communicating data between the source  27  and node  24   a  In this regard, various parameters that effect the service that a customer directly perceives, such as maximum peak rate and acceptable cell loss ratio for example, may be included in the connection information  38  of the billing cell  31 . In a traditional ATM network environment, for example, the connection information  38  may include the specific class of service implicated (e.g., CBR, VBR, UBR, and ABR, which will be described in detail hereinbelow), or other specific ATM traffic parameters. It is to be understood that other types of information which can be used to characterize the performance of the connection  33  as perceived by a user may constitute or be included with the connection information  38  carried by the billing cell  31 . 
     A significant advantage realized when using a billing cell  31  in a manner described herein concerns the increase in flexibility provided to each independent network operator, such as operators OP-A and OP-B, when implementing a charging scheme that accounts for connection usage associated wills various switches or nodes maintained by such operators. For example, and as previously described, the operator OP-A may compute the cost of using the connection  33  in a desired manner for such usage independent of the charging methodologies employed by other network operators. 
     By way of example, a connection  35  may be established between the node  24   a  and a node  24   b  in an effort to transport data between the source  27  and the destination  29 . When establishing the connection  35  with node  24   b,  node  24   a  transmits the billing cell  31  to the node  24   b  followed by the actual data using the connection  35 . The node  24   b  copies the contents of the billing cell  31  and computes charging information for usage of the connection  35  in a manner similarly described hereinabove. The charging information acquired or produced by the billing unit  34   b  of the node  24   b  may then be transmitted to a common billing system. It is noted that both operators OP-A and OP-B may compute charging information for a customer&#39;s use of the connection  35  established between nodes  24   a  and  24   b.  Importantly, the manner by which each network operator computes the relevant connection charge may be determined independent of the charging methodologies adopted by other operators. 
     Finally, the node  24   b  establishes a connection  37  with a destination  29 , and then transmits the billing cell  31  and data cells over the connection  37 . In one embodiment, the destination unit  29  transmits the billing cell  31  back to the source unit  27  as a means of confirming the operability of the billing system. It is noted that information identifying each node that processes the information carried by the billing cell  31  may be encoded in the billing cell  31  as the billing cell  31  travels through the various connections between the source  27  and destination  29 . As such, the billing cell  31  may be used as a means of tracking the actual route of the data as it passes through the network  20  from one location to another. 
     Turning now to FIG. 3, there is illustrated a depiction of a network  20  which utilizes a common network billing system  40  and a number of independent billing systems  34  which acquire connection cost information for corresponding independent network nodes  24 . In this illustrative example, a user  26   a  accesses the network  20  and wishes to transmit data to another user  26   b  also having access to the network  20 . The user  26   a  at the source location establishes a connection  32   a  with node  24   a  and transmits a billing cell to the node  24   a.    
     The billing unit  34   a  generates charging information using the billing cell information received at the node  24   a  in accordance with a charging strategy determined by the operator  22   a . Data cells are then transferred over the connection  32   a  between the user source  26   a  and node  24   a  After termination of the connection  32   a , such as by dropping the connection in accordance with a traditional procedure or by transmission of a terminate billing message, the billing unit  34   a  associated with the node  24   a  computes the connection charge for use of the connection  32   a  and transmits the connection cost information to the network billing system  40 . As was mentioned previously, the billing unit  34   a  may alternatively acquire billing information from the node  24   a  and transmit the unprocessed billing information to the network billing system  40  which, in turn, computes the usage charges associated with the connection  32   a.    
     The billing cell  31  processed by the node  24   a  is transmitted to the node  24   b  upon establishing the connection  32   b . The billing unit  34   b  generates billing information based on the billing cell contents and the charging strategy imposed by the operator  22   b . Data is transferred between node  24   a  and  24   b  and, upon termination of connection  32   b , the billing information acquired and/or computed by the billing unit  34   b  is transmitted to the network billing system  40 . Similar operations occur at nodes  24   c  and  24   d,  with the billing cell  31  and user  26   a  data ultimately being received at the destination by the user  26   b.  The billing cell  31  may then be transmitted back to the source user  26   a  to verify proper operation of the billing procedure. After acquiring the relevant billing information from each of the billing units  34   a - 34   d  associated with each of the nodes  24   a - 24   d  defining the connections between the users  26   a  and  26   b,  the network billing system  40  computes the total cost for transporting the data between source and destination locations  26   a ,  26   b,  and transmits the total computed connection cost as a bill to the user  26   a.    
     FIGS. 4 and 5 illustrate two embodiments of a billing procedure in accordance with the principles of the present invention. The procedure illustrated in FIG. 4 represents a billing scheme in which two types of billing cells are employed for purposes of initiating and terminating a connection charging operation. The procedure illustrated in FIG. 4 may be employed when it is desired to increase the flexibility of a network billing scheme. The procedure illustrated in FIG. 5, in contrast to that depicted in FIG. 4, may be employed when connection usage charges are computed on a fixed rate basis. If the amount of data to be sent over the network can be categorized as belonging to a pre-defined class which results in the transfer of small quantities of data, such as an amount of data falling within a certain threshold range, a fixed tariff or cost approach may be appropriate. The magnitude of the fixed cost is typically dependent on a number of factors, including the amount of data to be transferred and the level of service expected by the user. 
     Referring to FIG. 4 in greater detail, it is assumed that the switch  24  receives information over two virtual connections, VC 1  and VC 2 . The first type of information received by the switch  24  over the virtual connection VC 1  constitutes a billing cell  50  which contains information indicative of the level of service pertaining to the connection VC 1 . Upon receiving billing cell  50 , the node  24  copies the contents of billing cell  50  which is processed by the billing unit  34  of the node  24  in a manner discussed previously. A number of information cells  52  transmitted over the virtual connection VC 1  are subsequently received by the node  24 . 
     At some later time, a billing cell  54  associated with a second virtual connection, VC 2 , is received by the node  24 , the contents of which is copied and processed by the billing unit  34 . Subsequent data cells  56  transmitted over the virtual connection VC 2  are subsequently received by the node  24 . A terminate billing cell  58  is eventually received by the node  24  over the connection VC 2 . The node  24 , in response to the terminate billing cell  58 , drops the connection VC 2 . A terminate billing cell  60  is subsequently received by the node  24  after all information cell  52  are transferred through the virtual connection VC 1 . The node  24 , in response to the terminate billing cell  60 , drops the connection VC 1 . 
     Billing cells  50 ,  58  and terminate billing cells  54 ,  60  associated with virtual connections VC 1  and VC 2 , respectively, are transmitted to the network billing system  40  by the node  24 . It is noted that the connection cost information computed by the node  24  is included in the billing cells  50 ,  54  transmitted to the network billing system  40 . When the node  24  establishes another set of virtual connections with another node or nodes, billing cells  50 ,  54  are then transmitted over these connections. Information cells  52 ,  56  are then transmitted subsequent to the billing cells  50 ,  54 , followed by terminate billing cells  60 ,  58 . Subsequent nodes of the network process the cells  50 ,  52 ,  54 ,  56 ,  58 ,  60  in a similar manner. 
     The billing procedure illustrated in FIG. 5 differs from that described previously with respect to FIG. 4 in that a terminate billing cell is not required to signal completion of a particular data transmission over a particular connection. The billing scheme depicted in FIG. 5 may be employed when a fixed tariff or cost billing approach is employed for computing connection usage charges. In this example, a first virtual connection, VC 1 , is established with a node  24 . A billing cell  70  is initially received by the node  24  over virtual connection VC 1  followed by information cells  72 . After receiving the last information cell  72  over the connection VC 1 , the virtual connection VC 1  is dropped using a known termination procedure. 
     The node  24  also receives a billing cell  74  and subsequent information cells  76  over a second virtual connection VC 2 . Upon receiving the last information cell  76 , the virtual connection VC 2  is dropped using a standard connection termination procedure. The billing unit  34  of the node  24  computes the charges for use of virtual connections VC 1  and VC 2 , this information being communicated to a network billing system  40  using billing cells  70  and  74 . Subsequent virtual connections are established with another node or nodes of the network  20 , with data flowing over the virtual connections from node  24  in a manner depicted generally in FIG.  5 . 
     Those skilled in the art will appreciate that the above-described charging methodology may be employed in networks having differing architectures, protocols, and transfer modes. The above-described methodology may be particularly advantageous when used to compute connection usage information within an ATM network environment. It is to be understood that the present invention is not limited in application to any particular type of network By way of example, and not of limitation, the charging methodology of the present invention may be implemented within a new network service environment which is described in detail hereinbelow. 
     The service concept described hereinbelow for illustrative purposes may be viewed as a Simple Integrated Media Access (SIMA) service model. The SIMA service model incorporates the basic properties of ATM with the addition of eight priority levels as defined within the context of a new service concept termed ;a nominal bit rate (NBR) service. In general, the NBR service provides for the simple and efficient division of network capacity amongst different connections and the charging of user fees for the use of such connections. 
     For purposes of illustrating various advantages associated with a network implemented using the SIMA service model, a brief description of various conventional ATM service models is provided. A conventional ATM service architecture typically provides a number of predefined quality of service classes, often referred to as service categories. Each of the service categories includes a number of quality of service (QoS) parameters which define the nature of the respective service category. In other words, a specified service category provides performance to an ATM virtual connection (VCC or VPC) in a manner specified by a subset of the ATM performance parameters. The service categories defined in the ATM Forum specification referenced hereinbelow include, for example, a constant bit rate (CBR) category, a real-time variable bit rate (rt-VBR) category, a non-real-time variable bit rate (nrt-VBR) category, an unspecified bit rate (UBR) category, and an available bit rate (ABR) category. 
     The constant bit rate service class is intended to support real-time applications that require a fixed quantity of bandwidth during the existence of the connection. A particular quality of service is negotiated to provide the CBR service, where the QoS parameters include characterization of the peak cell rate (PCR), the cell loss rate (CLR), the cell transfer delay (CTD), and the cell delay variation (CDV). Conventional ATM traffic management schemes guarantee that the user-contracted QoS is maintained in order to support, for example, real-time applications, such as circuit emulation and voice/video applications, which require tightly constrained delay variations. 
     The non-real-time VBR service class is intended to support non-real-time applications, where the resulting network traffic can be characterized as having, frequent data bursts. Similarly, the real-time variable bit rate service category may be used to support “bursty” network traffic conditions. The rt-VBR service category differs from the nrt-VBR service category in that the former is intended to support real-time applications, such as voice and video applications. Both the real-time and non-real-time VBR service categories are characterized in terms of a peak cell rate (PCR), a sustainable cell rate (SCR), and a maximum burst size (MBS). 
     The unspecified bit rate (UBR) service category is often regarded as a “best effort service,” in that it does not specify traffic-related service guarantees. As such, the UBR service category is intended to support non-real-time applications, including traditional computer communications applications such as file transfers and e-mail. 
     The available bit rate (ABR) service category provides for the allocation of available bandwidth to users by controlling the rate of traffic through use of a feedback mechanism. The feedback mechanism permits cell transmission rates to be varied in an effort to control or avoid traffic congestion, and to more effectively utilize available bandwidth. A resource management (RM) cell precedes, the transmission of data cells, which is transmitted from source to destination and back to the source, in order to provide traffic information to the source. 
     Although the current ATM service architecture described above would appear to provide, at least at a conceptual level, viable solutions to the many problems facing the communications industry, ATM, as currently defined, requires implementation of a complex traffic management scheme in order meet the objectives articulated in the various ATM specifications and recommendations currently being considered. In order to effectively manage traffic flow in a network, conventional ATM traffic management schemes must assess a prodigious number of traffic condition indicators, including service class parameters, traffic parameters, quality of service parameters and the like. A non-exhaustive listing of such parameters and other ATM traffic management considerations is provided in ITU-T Recommendation 1.371, entitled Traffic Control and Congestion Control in B-ISDN, and in Traffic Management Specification, version 4.0 (af-tm-0056.000, April 1996), published by the Technical Committee of the ATM Forum. 
     One significant network traffic consideration is the amount of currently available bandwidth on a particular virtual connection Except for the ABR service category, the existing ATM service classes do not suggest utilizing this network load information. The ABR service category provides for dynamic allocation of available bandwidth in response to network load information returned to the user. 
     However, the ABR service class provides a complex arrangement of feedback information, including a current cell rate, explicit rate, minimum cell rate, direction indication, congestion indication, as well as others. This complex arrangement increases the complexity of the service class architecture. Also, the feedback mechanism defined for the ABR service class provides for allocation of bandwidth bounded between a defined minimum cell rate and a peak cell rate. Therefore, cell rate guarantees continue to exist which increases the complexity of the traffic management scheme. Furthermore, the conventional ATM service classes, including the ABR service class, do not provide solutions for determining network load conditions, and managing cell transfer rates in accordance with those network load conditions. 
     In contrast to these conventional ATM service models, the SIMA service model provides for a network arrangement and method that is not conceptually complicated to implement A network implementing a SIMA service model further provides for the effective division of network capacity during bandwidth overload situations, while providing network load information to the user with minimal complexity and negligible delays. A network that embraces a basic version of the SIMA service model does not need to perform many of the traditional and burdensome traffic management functions involving traffic descriptors, quality of service parameters, service classes, connection admission control (CAC), or usage parameter control (UPC). 
     All of these functions are effectively replaced by functions performed by two autonomous units: a measuring unit, provided at a user/network interface, and a cell scheduling and buffering unit, provided at a network node. The SIMA service concept, from a user&#39;s perspective, is simple and understandable, because there are no pre-defined traffic or quality parameters associated with each connection, and charging for connection usage is based solely on the value of NBR and the duration of the connection. 
     A typical implementation of a SIMA service utilizes two primary components: access nodes and core network nodes, which have fundamentally different functional responsibilities. For example, access nodes, which may be a user/network interface, perform the task of measuring traffic for every connection, whereas at the core network nodes, the traffic control functions do not need to know anything about the properties of individual connections. 
     The elegant simplicity of the SIMA service model offers obvious advantages to the manufacturers of infrastructure hardware and software. For example, ATM switches or crossconnects can be built using individual cell scheduling and buffering units, switching fabrics, and routing functions. By using ATM virtual paths or IP switching technology, the routing tasks may be reduced in complexity. In addition, packet discarding and priority feedback features may be included in the cell scheduling and buffering units without negatively impacting their automaticity. Also, simple implementation of network nodes may result in the availability of a relatively inexpensive, high capacity network infrastructure. 
     The more complex unit of the SIMA service infrastructure concerns the access nodes. Such access nodes will typically include a measuring unit to measure the traffic stream of every connection in real-time, and a computation unit for determining a priority to be assigned to every cell. These additional features should be implementable at a level of difficulty no greater than that for effecting UPC in conventional ATM networks. 
     An embodiment of an NBR system may be advantageously employed to provide priority feedback information, so that cell-sourcing units are able to regulate their cell transfer rates (CTR) and realize acceptable cell loss probabilities. Some information regarding the connection threshold level is periodically updated and fed back to the cell-sending unit. Each ATM node calculates a typical allowable priority level that can be inserted into special status cells used by the source to probe the connection. 
     In one embodiment, this feedback feature is used to optimize the CTR by maintaining the highest priority possible without exceeding the worst-case allowable priority accepted at the nodes of the virtual connection. This embodiment, therefore, provides priority level feedback (PL fb ) which informs the source end-system of a typical priority level that is still accepted by the nodes of a connection. 
     In order to provide the priority level feedback, the allowable priority level PL a  is determined at each node in the connection. A priority level feedback level PL fb  records the worst-case PL a  from the source to the destination, which is stored as a field of the status cell. An embodiment may be employed to effectively determine the value of the lowest allowable priority (i.e., the highest allowable priority “level”, PL fb ) currently being accepted on the connection. 
     Subsequent ATM nodes compare the PL fb  in the special status cell to the current priority level PL a  that is being accepted at the node (i.e., not discarded at the node). If the status cell includes a PL fb  having a larger value than the current allowable priority level at the node, then PL fb  in the status cell is replaced with a new, smaller value corresponding to the PL a  value at the node. The connection destination will thus receive the smallest PL fb  of the connection, indicating what is typically the highest priority level PL (corresponding to the worst priority) that will successfully traverse the connection without cell discarding. The destination unit then sends this network load information back to the cell source, which allows the user to adjust the CTR in order to reduce the likelihood that subsequently issued data cells will be lost through cell discarding at the nodes of the connection. 
     Referring now to FIG. 6, there is shown a general methodology for transmitting information between a user/network interface and a network over an NBR service connection. Initially, a user negotiates or selects  140  a nominal bit rate with the network operator, which may be performed prior to, or at the time of; establishing the connection. In one embodiment, the user informs the network operator that a desired NBR is required, and the requested connection bandwidth is allocated to the user. The network operator, in accordance with this embodiment, need not perform the task of analyzing current network load conditions existing at the core network nodes prior to establishing or releasing an NBR connection. In an alternative embodiment, the network operator performs the task of determining network load status prior to establishing or releasing an NBR connection, although this task may not be necessary in a properly dimensioned network supporting an NBR service. 
     Depending on a particular application, the user selects  142  a real-time-or a non-real-time network connection. The process of determining the priority level (PL) of each cell, which indicates the importance or criticality of the cell relative to other cells, involves measuring  144  the actual or measured bit rate (R) of the selected real-time or non-real-time connection at the UNI. The priority level of each cell is determined  146  at the UNI using a ratio of MBR and NBR. 
     After computing the priority level of each cell at the UNI, the cells are transmitted  148  to the network, such as to a node of the network. A network node, upon arrival of a cell transmitted from the UNI, performs a cell filtering process by which the node determines whether to accept or discard a particular cell. The cell filtering process involves determining  150  the state of one or more buffers or memories of the network node to determine a buffer or memory occupancy level. The node accepts or discards  152  a cell based on the priority level of the cell and the state of the node buffer. Cells that meet the filtering criteria determined at the node are accepted, buffered, and eventually transmitted  154  to another node in the network or another network in a manner consistent with the expected quality of service for the connection. 
     FIGS. 7-8 illustrate a procedure for scheduling and buffering cells in accordance with one embodiment of an NBR service methodology. Referring now to FIG. 7, a user establishes  160  an NBR with a network operator. It may be desirable, although not required, to initially set the service class  162  to a non-real-time (nrt) service class as a default setting. Depending on a particular application, the user may require a real-time (rt) service class  164 , which may be set by the user directly or, typically, by the user&#39;s application or communications software. If the user requires a real-time connection, each cell transmitted from the user&#39;s UNI will have the service class bit in the cell header set to indicate that the payload of the cell contains real-time information  170 . It is noted that within the context of a network implemented in accordance with the NBR concept, real-time service class connections are expected to support virtually any real-time application without the need to specify particular cell transfer delay (CTD) and cell delay variation (CDV) parameters. As such, the conventional procedure of setting CTD and CDV bits of the cell header to appropriate values to accommodate the real-time service requirements of the connection is altogether obviated. 
     If the user does not require a real-time service connection, the default non-real-time service class condition remains operative. As such, the rt/nrt service class bit of each cell header is set to indicate that the payload of the cell includes non-real-time information  166 . It is noted that the NBR service disclosed herein does not utilize the cell loss priority (CLP) scheme used by conventional ATM traffic management approaches. As such, the CLP bit in the cell header may instead be used to discern between real-time and non-real-time payloads. 
     In the above described embodiment, each cell transmitted over a connection is designated as either a real-time cell or a non-real-time cell, such as by appropriately setting the rt/nrt service class bit of the cell header. In an alternative embodiment, depending on a user&#39;s requirements, a connection may be designated as being either a real-time or non-real-time connection, and the cells communicated over such a connection need not be individually assigned a real-time or non-real-time status. Each node for a given connection, for example, may perform a table look up procedure upon arrival of a cell at the node to determine whether the cell is associated with a real-time or a non-real-time connection. Thus, in accordance with this embodiment, a cell header bit need not be reserved for distinguishing between real-time and non-real-time cells. 
     After the rt/nrt service class header bit has been set in the above-described manner, the actual bit rate of a particular cell to be transmitted between the UNI and the network is measured 174. Since, in practice, the actual bit rate may be subject to significant variability over time, a measuring unit of the UNI employs an averaging measuring principle to determine the actual or instantaneous bit rate, 
     In general, the UNI measures 174 the actual bit rate of a cell, such as cell i , by approximating the actual or instantaneous bit rate of the connection within a measuring period having a duration that is appropriate for the particular connection (e.g., a real-time or non-real-time connection). The instantaneous bit rate, MBR i , may be determined using a known technique. 
     Having determined 174 the measured bit rate, MBR i , of the i:th cell, the priority level of the i:th cell is computed using the measured bit rate, MBR i , and the nominal bit rate, NBR. In accordance with one embodiment, it is assumed that a cell may be distinguished from other cells using a cell prioritization scheme that employs eight priority levels. In order to indicate which of the eight priority levels is attributed to a particular cell, each cell allocates three bits for this purpose. 
     In accordance with current ATM specifications, an ATM cell is specified as a unit of transmission having a fixed-size frame consisting of a 5-octet header and a 48-octet payload. It is appreciated that the necessity to allocate three bits in the cell header for the purpose of designating cell priority level may require utilization of currently defined ATM header bits. By way of example, it may be possible to utilize the current Generic Flow Control (GFC) field which constitutes a total of four bits. In this case, three bits may be allocated for designating cell priority level and one bit may be designated as the rt/nrt service class bit. It may be possible, in accordance with another embodiment, to allocate other header bits for the purpose of indicating one of eight priority levels and rt/nrt service class by deviating from the five-octet header ATM specification. 
     As such, other header bits may be redefined to represent cell priority level and service class designations. Alternatively, one or more bits required to specify cell priority level and/or service class may be situated outside of the currently defined ATM cell header. The need to make a minor modification to the existing ATM cell header definition is significantly offset by the substantial advantages offered by employing the NBR service scheme, such as a significant reduction in network and traffic management overhead and complexity. 
     It is understood that the number of priority levels may be less than eight or greater than eight. By way of example, if it is assumed that four cell header bits are allocated for purposes of indicating a cell&#39;s priority level, as many as 2 4  (i.e. 2 n-bits ) or 16 priority levels may be defined. Increasing the number of priority levels within the context of an NBR service permits the network operator to make finer adjustments to the bandwidth of a particular connection when managing network traffic. The price for this finer level of traffic control is the additional cell header bit or bits needed to resolve a greater number of priority levels. 
     A priority level computing unit determines 176 the priority level of each cell, such as cell i , using the computed value of MBR i  and the value of NBR. In accordance with one embodiment, and assuming that the measured bit rate is MBR i  when the i:th cell is transmitted to the network, the priority level (PL i ) of cell; may be calculated using the following equation:                x   =     4.5   +       ln        (       MBR   i     /   NBR     )         ln        (   2   )                  
            PL   i     =     {           7           if                 x     ≥   7               ⌊   x   ⌋             if                 0     &lt;   x   &lt;   7             0           if                 x     ≤   0           ,                 [   1   ]                         
     where, └x┘ represents the integer part of x. As will be discussed hereinbelow in accordance with an embodiment in which both NBR and traditional ATM service connections are accommodated, the zero priority level, PL=0, is reserved for those connections that use an ordinary ATM service with guaranteed bandwidth and quality of service. Accordingly, Equation [1] above may be modified so as to produce cell priority levels ranging between PL=1 and PL=7, such that └x┘ represents the integer part of x, if 1&lt;x&lt;7. It is to be understood that the order of priority values can depart from the scheme described herein. Thus, it is possible to define, for example, that the priority value “7” corresponds to the highest priority and that the priority value “0” corresponds to the lowest priority. 
     It can be seen by application of Equation [1] above that if a connection is exploiting network capacity in excess to the connection&#39;s negotiated NBR value, the priority level of cell; is at least 4. It can further be seen that if the momentary bit rate at the UNI is less than the negotiated value of NBR, PL is at most 4. The priority level scheme in accordance with this embodiment thus permits adjustment of the relative capacity used by a connection in steps of 2. From Equation [1] above, it can be seen that for an NBR of 100 kbit/s, an MBR higher than 566 kbit/sec results in a PL of 7, and an MBR lower than 8.8 kbit/s results in a PL of 0. 
     The three priority level bits allocated in the cell header are set 178 for each ATM cell transferred from the UNI. The ATM cells are then transmitted  180  to targeted network nodes j  identified by node addressing information provided in the cell header. 
     It is noted that if a user is not satisfied with the quality of service of the connection, the user may elect between at least three alternatives. First, the user may elect to keep the average bit rate unchanging, but reduce the variation of traffic process. Second, the user may elect to decrease the average bit rate, or to increase the nominal bit rate. Increasing the NBR will, however, generally result in a concomitant increase in cost for a higher speed connection. Finally, the user may change the network operator. 
     In FIG. 8, there is illustrated in flow diagram form a general methodology by which a network node processes cells containing priority level information received from a UNI in accordance with one embodiment of an NBR system. FIG. 9 illustrates an embodiment of various components of a network node employed to effectuate the methodology illustrated in FIG.  8 . It is assumed that a cell, such as cell i , has been processed at a UNI and includes priority level information derived in a manner described hereinabove. 
     Cell i  is transmitted from the UNI to a network node and is received at a filter  188  of the node. A memory manager  189  checks the status  181  of the memory  190  in order to determine the occupancy in the memory  190 . The memory manager  189  determines  182  the allowable priority level (PL a ) based on the occupancy state of the memory  190 . In general, the memory manager  189  establishes a high allowable priority which translates to a low allowable priority “level,” for example PL a =0 or 2, when the occupancy level of the memory  190  is high (i.e., few available memory locations). When the memory manager  189  determines that the memory  190  has ample capacity for receiving new cells, the memory manager  189  establishes a low allowable priority which translates to a high allowable priority “level,” for example PL a =6 or 7. As will be appreciated by those skilled in the art, the calculation of PL a  could alternatively be based on unoccupied buffer capacity rather than on buffer occupancy. 
     If the priority level of cell; is greater than the allowable priority level, PL a , as determined  183  by the memory manager  189 , the filter  188  discards  184  cell i . If, on the other hand, the priority level of cell i  is equal to or less than the allowable priority level PL a  the filter  188  accepts  185  cell i . The memory manager  189  coordinates the transfer  186  of cell i  to the memory  190  and updates an index table  191  coupled to the memory manager  189  to include a new index table entry for newly accepted cell i . In one embodiment, the index table  191  stores the location of the accepted cell i  in the memory  190 , and also stores a cell-type indicator which specifies whether cell i  is a real-time cell or a non-real-time cell. As such, the memory  190  may store both real-time and non-real-time cells. 
     The memory manager  189 , in cooperation with the index table  191 , manages cell transfer operations from the memory  190  to the output of the memory  190  by giving preference to the real-time cells over the non-real-time cells. By way of example, the memory manager  189 , upon determining the presence of both rt-cells and nrt-cells stored in the memory  190 , transfers all of the rt-cells to the output of the memory  190  prior to transferring out any of the nrt-cells. 
     In accordance with another embodiment, as is illustrated in FIG. 10, a memory manager  189  determines the status of a real-time buffer (rt-buffer)  193  and a non-real-time buffer (nrt-buffer)  194 . The memory manager  189  determines, in a manner similar to that previously described, the allowable priority level PL a , for the filter  188  based on the status of the rt-buffer  193  and the nrt-buffer  194 . If the priority level of cell; is greater than the allowable priority level, PL a , the filter  188  discards cell i . If, on the other hand, the priority level of cell i  is equal to or less than the allowable priority level, PL a , cell i  is accepted. 
     In accordance with another embodiment, the network node may apply a buffer filtering scheme which performs the filtering function based on packets of cells, rather than on individual cells. By way of example, the filtering procedure described hereinabove may be applied to the first cell of each packet. If the first cell is discarded by the node, then all of the cells of the packet following the first cell are discarded as well. If, however, the first cell of a packet is accepted, then the priority of all other cells belonging to that packet may be increased, for example by changing the priority level from PL=5 to PL=3. A gain of even one priority level, such as from PL=4 to PL=3, is believed to be sufficient to ensure that there will only be very few partially transmitted packets. 
     A cell-type detector  192  receives the accepted cell, cell i , from the filter  188  and determines whether cell; is an rt-cell or an nrt-cell. As discussed previously, the header of cell; includes a header bit, such as the CLP bit, which indicates whether or not cell; is an rt-cell or an nrt-cell. The cell-type detector  192 , upon determining the service class type of the cell;, transfers the cell i  to either the rt-buffer  193  or the nrt-buffer  194 . In a manner similar to that described previously with respect to FIGS. 8 and 9, the memory manager  189  coordinates the output of rt-cells and nrt-cells respectively from the rt-buffer  193  and the nrt-buffer  194 , giving preference to the rt-cells. 
     In accordance with another embodiment, it may be desirable, for purposes of enhancing network expansion and traffic control, to request that each user of the network purchase a maximum NBR The maximum NBR value is intended to remain substantially constant In addition, it may be desirable to request that each user select an appropriate instantaneous NBR, which should be no greater that the selected maximum NBR. The selection of an appropriate instantaneous NBR generally involves a compromise between price and quality of service. The service quality detected by a user depends largely on three parameters, namely the NBR, the average bit rate, and the amount of traffic variations. Although a user may change any of these parameters, the only information that the network needs to know at the initiation of cell transmission is the NBR and the service class (real-time or non-real-time) of the connection. 
     In accordance with another embodiment, a SIMA service model provides for the accommodation of both NBR and traditional ATM service connections. It is appreciated that traditional ATM services which offer guaranteed connections may be desirable for certain applications. It is anticipated, however, that the quality of service offered by the NBR service will meet or exceed a user&#39;s expectations for virtually all real-time and non-real-time applications. 
     A SIMA service which provides for both NBR and traditional ATM services requires that the network operator dedicate a UPC device for each conventional ATM connection, or possibly for each virtual path. All of the cells transmitted using traditional ATM service connections are designated with the highest priority of PL=0 and with a real-time (rt) service class designation. In accordance with this approach, the zero priority level is reserved for those connections that use an ordinary ATM service with guaranteed bandwidth and quality of service. Accordingly, the priority determination Equation [1] above is modified so as to produce cell priority levels ranging between PL=1 and PL=7, such that └x┘ represents the integer part of x, if 1&lt;x&lt;7. It is noted that if the network operator wants to mark excessive cells as CLP=1 cells, those cells may be marked with a lower priority, such as PL=6, for example. 
     A point of possible incompatibility with traditional ATM technology involves the necessity of three bits for each ATM cell for the determination of cell priority, or two bits if the current cell loss priority, CLP, bit in the cell header is used. In addition, one bit is needed to distinguish between real-time and non-real-time connections. The rt/nrt service bit may be, but is not required to be, included in every cell. It may be possible to utilize the current Generic Flow Control (GFC) field which constitutes a total of four bits. In this case, three bits may be allocated for designating cell priority level and one bit may be designated as the rt/nrt service class bit. 
     FIG. 11 is a block diagram illustrating one embodiment of an ATM network  200  implementing the NBR methodology. The exemplary ATM network  200  used for purposes of this description is depicted as a network having two intermediary ATM nodes  202  and  204 . However, as will be appreciated by those skilled in the art, other network structures may be exploited, such as multipoint, star, ring, loop and mesh network topologies used in networks ranging from local area networks (LAN) to proliferative global area networks (GAN) such as the Internet. 
     The network  200  includes a source end-system  206  for transmitting digital information to the destination end-system  208 . The information transmitted in such a network typically passes through various network nodes, such as nodes  202  and  204 , as it advances towards its destination. These nodes represent network data communications elements such as routers, switches or multiplexers. Connecting the end-systems and nodes are circuit connections that provide the means for which the digital information may be passed. Connection links  210 ,  212  and  214  represent the connections for data sent from the source end-system  206  to the destination  208 , while connection links  216 ,  218  and  220  represent those connections which provide information in return. 
     FIG. 11 further illustrates an ATM cell flow in the ATM network  200  implementing the NBR methodology. As data is sent to the destination end-system  208  by way of a flow of ATM cells along connections  210 ,  212  and  214 , network load information may be returned to the source end-system  206  via connections  216 ,  218  and  220 . The NBR system operates on a priority basis, thereby resulting in the presentation of NBR network load information as priority level information. Information regarding current allowable priority levels at the nodes is provided to the source end-system  206  in order to provide status and allow optimization of the cell transfer rate (CTR). 
     In one embodiment, the network load information is provided to the source end-system  206  in the form of special ATM status cells periodically issued from the source end-system  206 . The status cells are part of the normal connection cell flow in the sense that they are included in the normal MBR calculation of the connection, and the status cell priority level is calculated in the manner previously described in connection with FIG.  7 . FIG. 11 illustrates the progress of a typical status cell in accordance with the present invention, seen at six different intervals as it advances from source end-system  206  to the destination end-system  208 . The status cell  222   a-f  is depicted at times t=1 through t=6 respectively, which corresponds to the location/time relationship as the status cell traverses a connection. 
     Referring now to FIG. 12, one embodiment of a network load status cell  250  is shown. ATM standards define an ATM cell as a fixed-size cell with a length of 53 octets comprised of a 5-octet header and a 48-octet payload. The status cell  250  is modeled after the ATM standard cell, and includes a 5-octet header  252  and a 48-octet payload  254 . Within the payload section  254  of the status cell  250  are a pair of priority level feedback (PL fb ) cells, labeled PL fb,f    256  (forward priority, level feedback) and PL fb,b    258  (backwards priority level feedback), that are used to store priority level information as the status cell  250  travels from source-to-destination and destination-to-source respectively. The PL fb,f  field  256  gathers the worst-case PL a  on the connection identified by the value of the lowest allowable priority (i.e., the highest allowable priority “level”, PL fb ) currently being accepted from the source to the destination. In one embodiment, the source end-system  206  initially sets the PL fb,f  field  256  to the lowest priority, corresponding to a priority level value of “7”. 
     As each node receives the status cell  250 , it examines its current load level at the outgoing link of the connection. The load condition at a particular node is denoted PL fb,n , which identifies the highest priority, and therefore the lowest allowable priority level PL a , for the ATM node. The current load condition PL fb,n  at the node is compared to the value available in the PL fb,f  field  256 , where the PL fb,f  field  256  reflects the lowest allowable priority level PL a , identified in a node on the connection. Where PL fb,n &lt;PL fb,f , the value in the PL fb,f  field  256  is decreased to reflect the lowest allowable priority level identified up to this point in the connection, and is therefore decremented to equal the PL a  value of the node. Where PL fb,n &gt;PL fb,f , the node does not change the value in the PL fb,f  field  256 . 
     Each network node detects the status cell  250  based on identifying information in the cell header  252 . Among the ATM header fields is a 3-bit payload type (PT) field  260  used to discriminate between a cell payload carrying user information from a cell payload carrying management information. The PT field  260  of the header  252  in FIG. 12 is used to distinguish a status cell  250  from a standard data cell. Any desired combination of bits in the PT field  260  could be used to identify a status cell  250 . Alternatively, a separate bit in another location of the header  252  can be used to distinguish status cells  250  and standard data cells. 
     After the destination end-system  208  has received the status cell  250 , it returns the status cell  250  to the source end-system to allow it to examine the value collected in the PL fb,f  field  256 . In one embodiment, the value in the PL fb,f  field  256  is placed into the backward priority level feedback field shown as PL fb,b    258 . This allows the PL fb,f  field  256  to gather priority level status information as the status cell  250  travels from the destination end-system  208  to the source end-system  206  in a manner similar to the source-to-destination status collection. Consequently, the destination end-system sets the PL fb,f  field  256  to the lowest priority, corresponding to a priority level value of “7”, and the status cell  250  is transmitted back into the network to return to the source end-system  206 . During the return journey, the PL fb,f  field  256  will again gather network load status information, this time for the connection from the destination end-system  208  to the source end-system  206 . The previously collected load information stored in the PL fb,b    258  field will remain static for analysis at the source end-system  206 . 
     The source end-system  206  is configured such that the failure to receive the status cell within a predetermined time period will result in an automatic decrease of the cell transfer rate. This is based on the assumption that loss of a status cell indicates that it was discarded in view of the cell transfer rate being too high, and therefore should be lowered. 
     In another embodiment, several pairs of PL fb,f /PL fb,b  fields can be provided in the payload  254 . This can provide the user with information regarding the network load conditions with respect to various parameters, such as various time periods. For example, first, second and third pairs of PL fb,f /PL fb,b  fields can provide the network load conditions during the last 100 milliseconds, 10 seconds, and 10 minutes respectively. 
     Referring now to FIG.  11  and Table 1 below, an example of the calculation of the forward priority level feedback PL fb,f    256  and the backwards priority level feedback PL fb,b    258  is described. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 t &lt; 1 
                 t = 1 
                 t = 2 
                 t = 3 
                 t = 4 
                 t = 5 
                 t = 6 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 PL a(NODE 202)   
                 5 
                 5 
                 5 
                 4 
                 4 
                 4 
                 4 
               
               
                 PL a(NODE 204)   
                 — 
                 — 
                 — 
                 6 
                 6 
                 6 
                 6 
               
               
                 PL fb,f   
                 7 
                 7 
                 5 
                 5 
                 5 
                 5 
                 4 
               
               
                 PL fb,b   
                 — 
                 — 
                 — 
                 — 
                 5 
                 5 
                 5 
               
               
                   
               
            
           
         
       
     
     As illustrated in FIG. 11, the status cell  222   a-f  is shown at times t=1 through t=6 respectively. Table 1 above illustrates the PL fb,f    256  and PL fb,b    258  as compared to the allowable priority level PL a  at nodes  202  and  204  at times t&lt;1 through t=6. At time t&lt;1, the PL fb,f  is initialized to the lowest priority, thereby having a preset priority level value of “7”. At time t−1, the status cell  222   a  is transmitted from the source end-system  206  to the ATM node  202 , at which time the PL fb,f  still has the value of “7”. Because the node  202  has a PL a  value of “5”, PL fb,f    256  within status cell  222   b  is decremented to a value of “5” at time t=2 to reflect the current worst-case PL a  value of the connection Node  204  has a PL a  value of “6” at time t=3, which is greater than the current state of PL fb,f    256  which is equal to “5”. Therefore, PL fb,f    256  remains unchanged at time t=3, when status cell  222   c  exits ATM node  204 . 
     Between times t=3 and t=4, PL fb,f    256  is placed into the backward priority level feedback field PL fb,b    258 . At time t=4, status cell  222   d  therefore includes a PL fb,b  field  258  that stores the value “5” which corresponds to the worst-case allowable priority level of the source-to-destination connection. Because the node  204  still has a PL a  value of “6”, PL fb,f    256  of status cell  222   e  remains unchanged at a value of “5” at time t=5. However, at some time between t=2 and t=3, the PL a  value at node  202  changed from “5” to “4”, causing PL fb,f    256  in cell  222   f  to also be decreased to a value of “4”. As can be seen from Table 1, PL fb,b    258  remains static during the return journey (i.e., t=4 through t=6), so that the source-to-destination PL fb,f  can be reported to the source end-system  206 . 
     FIG. 13 is a block diagram of an ATM node  300 , representative of any of the nodes  202 ,  204  or additional nodes in the ATM network  200 , configured for the NBR methodology. Each node may have multiple inputs from other nodes or end-stations, seen generally as links  302 . The switch  304  receives each of the links  302  containing multiplexed information streams, and rearranges the information streams between input and output, as is generally known in the art In the example of FIG. 13, switch  304  receives cell  306   a  at link  308 , and provides the cell  306   b  at its output. 
     The ATM node  300  determines  310  whether the cell  306   b  is a standard data cell or a status cell configured for the NBR methodology. In one embodiment, this is accomplished by comparing a known value to a payload type value in the payload type (PT) field  260  in the header  252 . Where the cell  306   b  is not an NBR status cell, it is a standard ATM information cell which is transferred It the cell scheduling and buffering circuitry  312 , generally described in connection with FIGS. 9 and 10, which accepts and discards the cells according to the priority level of the cell and the current buffer occupancy level. Where the cell  306   b  is an NBR status cell, the PL fb,f  field  256  is appropriately set  314  in the cell  306   c  in accordance with the current allowed priority level PL 3 . 
     The various PL a  values for each of the links coupled to the ATM node  300  are stored in a storage table  316 . The PL fb,f  field  256  is set  314  to a value equivalent to the PL a  value in table  316  when PL a  is less than the value currently residing in the PL fb,f  field  256 . Otherwise, the PL fb,f  field  256  remains unchanged Whether the PL fb,f  field  256  is modified or not, the status cell  306   d  is provided to the cell scheduling and buffering circuitry  312  to be filtered and buffered like any standard ATM cell. Cells output the node  300  at output links  318 , where the status cell  306   e  of this example is shown exiting node  300  via link  320 . In one embodiment, a cell scheduling and buffering block  312  is provided for each output of the node, such that each cell scheduling and buffering block  312  operates independently of other cell scheduling and buffering blocks (not shown) at the node. 
     In FIG. 14, there is illustrated in flow diagram form, in accordance with one embodiment, a general methodology by which NBR status cells can be used to provide feedback to a source end-station so that cell transfer rates may be optimized. A status cell is transmitted  400  from the source end-system  206  to the destination end-system  208 . Intermediary nodes, such as nodes  202  and  204  of FIG. 11, detect  402  status cells sent by users at the source. The allowable priority level PL a  is stored  404  as PL fb,n  at each of the intermediary nodes. The highest allowable priority level identified at all of the nodes traversed prior to the current node is available in the PL fb,f  field  256  of the status cell, which can then be compared  406  to PL fb,n  at each node. Where PL fb,n &gt;PL fb,f , the node does not change  408  the value in the PL fb,f  field  256 . Where PL fb,n &lt;PL fb,f , the value in the PL fb,f  field  256  is set  410  to reflect the lowest allowable priority level identified up to this point in the connection, and is therefore decremented to equal PL fb,n  at the node. 
     The status cell may encounter more nodes  412  as it progresses towards its destination. Where more intermediary nodes are in the path of the status cell, each of the intermediary nodes must detect  402  the status cell, and the PL fb  f field  256  is set accordingly  404 ,  406 ,  408 ,  410 . Where no more intermediary nodes are encountered from source to destination, the status cell, and the current load indication PL fb  is returned  414  to the source end system. In one embodiment, the value in the PL fb,f  field  256  is placed into the backward priority level feedback field PL fb,b    258  prior to the cell&#39;s departure from the destination end-system  208 . This allows new network load information to be gathered in the PL fb,f  field  256  on the return connection without corrupting the network load information gathered on the source-to-destination connection. 
     When the source end-system  206  receives the returned status information, it can modify its current traffic parameters to optimize cell transmission. Where the returned status indicates that the cell traffic is relatively low  416 , the user may decrease  418  the priority of newly issued ATM data cells to reflect a priority level that will likely be accepted at each of the nodes of the connection. Similarly, where the cell traffic appears to be relatively high  420 , the user may increase  422  the priority of newly issued data cells. This allows adjustment of the confidence level that data cells will not be discarded at any node along the connection. The returned status is therefore used to allow the user to adjust the cell transfer rate (CTR) of cells exiting the source end-system  206  to the network. 
     FIGS. 15-18 illustrate various embodiments for determining the priority level feedback (PL fb ) in a switch. Referring first to FIG. 15, a first embodiment for determining PL fb  is illustrated. In this embodiment, the PL a  values calculated at the ATM node as cells pass through the switch are monitored over a predetermined observation time period to. The minimum PL a  value occurring during this predetermined time period to is recognized as the lowest accepted priority level PL a  (i.e., the highest priority) for this period, thereby indicating the threshold priority level required to ensure acceptance of cells for that period. 
     In one embodiment, the period over which the minimum PL a  value is monitored is calculated by monitoring the number of cells entering the ATM node, and selecting the minimum PL a  from a “window” of ATM cells corresponding to a predetermined number of cells entering the ATM node. The window of cells has a size 10 determined by: 
     
       
           l   0 =( c )( t   0 )  [2] 
       
     
     where, c is the rate of cells arriving at the ATM node, and t 0  is the observation time period. The smallest accepted priority level PL a  value inside this window of size 1 represented by dimension line  500 , is compared with the PL fb,f  value carried in each cell. In the example of FIG. 15, a PL a  of “5” at time slot  502  is the minimum PL a  during the window  10 . The PL fb  value is therefore determined by the following mathematical equation: 
     
       
           PL   fb   =Min [( PL   a ) i ]  [3] 
       
     
     where i=1, 2, . . . , l 0 . 
     The size l 0  of the window may be set to any desired value to provide varying degrees of accuracy. Therefore, while 10 may be set as low as one, a particular embodiment sets the window size 10 to approximately correspond to the time delay of a round-trip cell path between the source and the destination. However, the observed time interval to may be set to a much larger time if providing long-term statistical priority feedback information to the source is desired Referring now to FIG. 16, another embodiment for determining PL fb  is illustrated. In this embodiment, the PL a  values calculated at the ATM node as cells pass through the switch are monitored over a predetermined observation time period t 0 , and the minimum PL a  value which occurs a predetermined number of times during this time period to is recognized as the lowest accepted priority level PL a . This method may be beneficial where single, unique events cause the minimum PL a  to inaccurately represent the typical minimum priority level at that ATM node during the period in question. For example, where the PL a  during a window l 0    500  includes values 5 and 6, but one of the PL a  values is 2, the connection may undeservingly be classified as a connection typically requiring a very high priority to avoid being discarded at a intermediary node. Therefore, FIG. 16 represents a manner of increasing the probability of providing a more typical minimum PL a . 
     As illustrated in FIG. 16, a predetermined number “M” of PL a  values must be recognized before modifying the PL fb,f  value. M equals three in FIG. 16, and PL a  is equal to 6 three times in the window l 0    500 , as represented by time slots  510 ,  512  and  514 . The PL a  value of 5 at time slot  516  was not selected because it only occurred once during the predetermined time period to. The PL a  value of 7 at time slots  518 ,  520 ,  522  and  524  were not selected because it was not the minimum accepted priority level value occurring at least M times. 
     FIG. 12 illustrates still another embodiment for determining PL fb . PL fb  may be calculated by averaging the PL a  values over a time period to. This method is most beneficial where there are relatively few large variations in PL a  over relatively long time periods. The PL fb  value can therefore be determined by the following mathematical equation:                PL   fb     =     Int        (       ∑     i   =   1       l   o              1     l   o            PL   a   i         )               [   4   ]                         
     where, l 0  is the number of cells inside the window, and Int represents the nearest integer value of the resulting average. 
     Referring again to FIG. 17, the average of the PL a  values is determined by applying Equation [4], where the PL a  values in the 7 (l 0 ) times slots  530 ,  532 ,  534 ,  536 ,  538 ,  540  and  542  of the window are averaged over the time to corresponding to the window of 10 cells. In the example of FIG. 17, the calculated average is approximately 6.3, resulting in a PL fb,f  of 6. 
     FIG. 13 illustrates yet another embodiment for determining PL fb . PL fb  may be determined by calculating a weighted average of PL a  over a time period t 0 . The PL a  values at the remote end of the window (corresponding to the least recently occurring ATM cells in the window) are afforded less weight in the calculation as the more recent PL a  values. More emphasis is placed on the more recent PL a  values, as it may provide a better indication of the trend under changing conditions. Mathematically, this is represented by:                PL   fb     =     Int        (       ∑     i   =   0       l   o              w   i          PL   a   i         )               [   5   ]                         
     where, the total sum of weights is            ∑   i          w   i       =   1.                   
     As seen in FIG. 18, the weighted average of the PL a  values is determined by applying Equation [5], where the PL a  values in the 7 times slots  550 ,  552 ,  554 ,  556 ,  558 ,  560  and  562  of the window are divided into four “weighted” segments. The first weighted segment  564  includes the PL a  values corresponding to the least recently occurring cells  550  and  552  which, in this example, are provided a weight value of 0.05. The second weighted segment  566  includes the PL a  values corresponding to cells  554  and  556  which, in this example, are provided a weight value of 0.1. The third weighted segment  568  includes the PL a  values corresponding to cells  558  and  560  which are provided a weight value of 0.2 in this example. Finally, a fourth segment  570  includes the PL a  values corresponding to cell  562  which has a weight value of 0.3. Applying Equation [5] to the example of FIG. 18, the calculated weighted average results in a PL fb,f  of 6. As will be appreciated by those skilled in the art from the above description, different quantities of weighted segments may be selected, as may different quantities of PL a  values per weighted segment. 
     For purposes of illustration, and not of limitation, examples are provided below to illustrate the relationship between the quality of service of an NBR or SIMA connection and throughput at different priority levels. The following examples demonstrate, in accordance with one embodiment, the relative differences,; in QoS with respect to adjacent priorities, such as the QoS associated with PL=4 in comparison to that of PL=3. It is noted that a higher cost or fee is typically assessed to users who request a higher priority for cells transmitted from the user&#39;s user/network interface. By way of example, the user fee may be doubled if the user wants to obtain one degree of higher priority for every cell without changing the actual bit rate. The resulting QoS of the connection, therefore, should be improved such that at least some users are willing to pay the additional charge. 
     EXAMPLE #1 
     In accordance with this illustrative example, the following assumptions and consideration are given. It is assumed that there are many identical traffic sources which generate traffic independent of the current or previous load conditions in the network. The following traffic parameters are assumed: the link capacity is C=1, which is useful in the examples as a means of normalization; the peak bit rate MBR=0. 1, which represents 10 percent of the link capacity, C; the ON probability at the burst (or packet) scale=0.2; and the average burst duration=1,000 time slots (i.e., the average packet size=100 cells). In addition, it is assumed that there is an upper ON/OFF layer, and that both the average ON-period and OFF-period of this layer are 100,000 time slots, The real time buffer  93  contains 200 cell locations and the non-real-time buffer  94  contains 5,000 cell locations. It is noted that the upper ON/OFF layer attempts to model the traffic process of connections, in which the determination of the number of connections is understood in the art to constitute a stochastic, random process. For example, if it is assumed that the total number of customers is represented by the variable x, then the average number of connections is x/2. More particularly, the number of connections is understood to be binomially distributed As such, 100,000 time slots represent the average holding time of a connection, and, also, the average idle period realizable by the user. As a consequence, a user is transmitting cells only if a connection is active both at the connection layer and the packet layer. A time scale parameter, a, can be obtained for the real-time and non-real-time connections: 
     
       
         α rt =0.025 
       
     
     
       
         α nrt =0.001 
       
     
     In this example, eight different connection types are assumed: four connections are real-time connections and four are non-real-time connections. Also, four different NBR values, which have been normalized with respect the link capacity of C=1, are assumed as: 0.2, 0.1, 0.05 and 0.025. The priorities corresponding to these NBR values, with MBR MAX =0.1, are 3, 4, 5 and 6, respectively. It should be noted, however, that not all cells are assigned these exact priorities, and that especially with non-real-time connections, many cells obtain better priority values because of the affects of the averaging measuring principle. The distribution of cells having different priority levels, represented as percentages, is presented below in Table 2: 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 REAL (SIMULATED) 
                   
               
               
                 PRIORITY 
                 PERCENTAGE OF 
                 PERCENTAGE BASED 
               
               
                 LEVEL 
                 OFFERED CELLS 
                 ON PEAK RATES 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 6.1 
                 0 
               
               
                 2 
                 7.9 
                 0 
               
               
                 3 
                 24.3 
                 25 
               
               
                 4 
                 23.5 
                 25 
               
               
                 5 
                 21.5 
                 25 
               
               
                 6 
                 16.8 
                 25 
               
               
                   
               
            
           
         
       
     
     In FIG. 19, there is shown a graph illustrating the relationship of average cell loss ratio, P loss , as a function of priority level for four specific load levels,r. In particular, line- 800  represents an overall average load level of 0.72 for 9 connections of each connection type (i.e., real-time and non-real-time connection types). Line- 802  depicts an average load level of 0.80 for 10 connections of each connection type. Further, line- 804  represents an average load level of 0.88 for 11 connections of each connection type, and line- 806  represents an average load level of 0.96 for 12 connections of each connection type. It is noted that, in the case of line- 802  indicating a load level of 0.80, the cell loss ratios, P loss , for real-time and non-real-time cells are indicated by dotted and broken lines, respectively. 
     Given, for example, a traffic scenario where the operator wants to offer a cell loss ratio of 10 −6  to cells with priority 4, the total load can be approximately 0.75. It can be assumed that this average cell loss ratio is sufficient for most video applications. Give the same traffic load conditions, priority level 5 which corresponds to P loss ≈10 −4 , can meet the requirements of many voice applications, while priority 6, which corresponds to P loss ≈3.10 −3 , is suitable for a TCP/IP type of file transfer, provided that there is an adequate packet discarding scheme in place. 
     It should be emphasized, however, that the difference in cell loss ratio between adjacent priorities depends strongly on the offered traffic process and, in particular, the inherent control loops of the NBR or SIMA service. When the user perceives an unsatisfactory QoS, for example, the user can, and should, change either the actual bit rate or the nominal bit rate of the connection. In either case, tie priority distribution changes as well. Nevertheless, if this phenomenon is temporarily ignored, the basic behavior of priority distribution may be flier appreciated by making the following simplifying assumption: If it is assumed that all traffic variations are slow as compared to the measuring period and buffer size, then a well-known, conventional ATM approach to approximating cell loss ratio may be used, with the additional requirement that the eight NBR priority levels are taken into account. 
     If the loss ratio of cells with priority k is denoted by P loss,k , and the average loss ratio of cells with a priority of 0 to k is denoted by P* loss,k , then the following equation, which ignores buffering effect, provides that                  P     loss   ,   k     *     =         ∑     j   :       λ   j     &gt;   c              Pr        {       λ   k   *     =     λ   j       }          (       λ   j     -   c     )             ρ   k   *        c              
            P     loss   ,   0       =     P     loss   ,   0     *            
            P     loss   ,   0       =               ρ   k   *          P     loss   ,   k         -       ρ     k   -   1     *          P     loss   ,     k   -   1       *             ρ   k   *     -     ρ     k   -   1     *                       for                 k     =     1                 …                 7                 [   6   ]                         
     where, λ* k  represents the momentary bit rate level of all cells with a priority of 0 to k, ρ* k  represents the average offered load produced by these cells, and c represents the link capacity. The probability Pr{λ* k =λ j } can be calculated in a straightforward manner using known convolution techniques. 
     EXAMPLE #2 
     For purposes of further illustration, a second example is provided which assumes the same sources described in Example #1, except for the long ON and OFF periods. Because of the long periods reflected in Example #1, the peak rate always determines the cell priority. As the buffers are typically not capable of filtering any traffic variations, the allowed load in Example #2 is much lower than that in the original case of Example #1. 
     In FIG. 20, there is illustrated in graphical form a relationship between cell loss ratio as a function of priority level for different load levels, r. It is assumed in FIG. 20 that the peak cell rate of each connections depicted by solid lines  820 ,  822 ,  824  is 0.1, the peak cell rate of each connection depicted by the broken line- 826  is 0.2, and the peak cell rate of each connection depicted by the dotted line- 828  is 0.05. 
     FIG. 16 shows the cell loss probabilities obtained by application of Equation [6] for different priorities, depicted by the three solid lines, line- 820 , line- 822 , and line- 824 . In addition, two slightly different traffic cases are represented by the dotted line- 828  and the broken line- 826 . The effects of changing the traffic variations are reflected in the graph provided in FIG.  20 . The actual change in traffic variations is a direct consequence of doubling or halving bit rates and NBR values. 
     In a network that embraces the NBR/SIMA service concept, an increase of traffic variations has two main effects if the operator keeps the QoS of priority level 4 unchanged. First, the allowed load level is decreased in the same way as in conventional ATM, and second, the difference in cell loss ratio between adjacent priority level decreases. For purposes of providing a rough estimate of QoS based on FIGS. 19 and 20, it may be assumed that if priority level 4 offers a cell loss probability of 10 −6 , then the cell loss probability will be approximately 10 −4  to 10 −3  with priority level 5 depending on the overall traffic variations. The cell loss ratio with priority level 3 may be assumed to be less than 10 −9 , unless the traffic variations are very pronounced. 
     1 Although the above examples provide illustrations of relationships between QoS and priority levels, it may be unfruitful to attempt to exactly determine the allowed load or the cell loss difference between adjacent priority levels until user reactions to different QoS and usage charges are evaluated. In an NBR/SIMA service environment, a schedule of charges based on different QoS levels may be determined, in a certain sense, automatically. For example, if the difference in cell loss ratio between priority levels 4 and 5 is very small, it can be assumed that some of the connections will tend to move from priority level 4 to level 5 because of a lower assessed charge. This change indicates, apparently, that the cell loss ratio of priority level 4 decreases and the cell loss ratio of priority level 5 increases. It can be reasonably assumed that this type of movement continues until the QoS difference corresponds to the average user&#39;s expectation of a reasonable charging structure. 
     Similar concerns are raised with regard to the differences in charging which occur automatically during busy hours in contrast to idle hours. For example, it would appear reasonable to charge higher prices during periods of high load for a certain QoS, and charge lower prices during low load periods. However, it is believed desirable to avoid instigating a charging policy that assesses different fees for a certain NBR during busy and idle periods, which also avoids increasing the complexity of the charging scheme. The naturally occurring “supply and demand” effect may tend to automatically even out the load between busy and idle hours. It is anticipated that, if a user is displeased with the observable difference in QoS during busy and idle periods, the user may be motivated to pay a different rate during such periods. 
     EXAMPLE #3 
     Another important transmission control aspect is the modification and adjustment of the cell transfer rate, or similarly, the measured bit rate (MBR). It is; possible to provide those sources, which are able to change their bit rate, information about the current load condition on their respective connections. This example provides a performance outlook for these connections. 
     For the present example, the background traffic process generated lay the sources previously described in Examples #1 and #2 is used It is also assumed that there are 10 connections of each type, thereby providing an average background load of r=0.080. There are also three sources (hereinafter referred to as feedback sources FBS1, FBS2 and FBS3) that adjust their transmission rate according to the feedback information received via the network status cells. All of these feedback sources are assumed to have an NBR of 0.01. 
     The feedback sources are similar to each other, except for their time periods which are used in determining PL fb  information. More specifically, the feedback source time periods are 10,000 time slots for FBS1, 30,000 time slots for FBS2, and 100,000 time slots for FBS3. For comparison purposes to the feedback sources, three constant bit rate connections (source connections C4, C5 and C6) are also discussed, which have the following parameters (link capacity=1): 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Bit-rate/NBR 
                 NBR 
                 Priority Level (PL) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 C4 
                 1.4 
                 0.01 
                 4 
               
               
                   
                 C5 
                 2.8 
                 0.01 
                 5 
               
               
                   
                 C6 
                 5.5 
                 0.01 
                 6 
               
               
                   
                   
               
            
           
         
       
     
     The cell sources in Table 3 are set such that they transmit at a rate that is slightly less than the threshold that borders the next higher priority level. For example, using Equation [1] above, a bit rate/NBR ratio equal to 1.5 would result in a cell priority level of 5 (integer portion of 5.08), while a bit rate/NBR ratio of 1.4 yields a cell priority level of 4 (integer portion of 4.98). The feedback sources have been set to the same bit rate values in this example in order to optimally utilize the network capacity. 
     In FIG. 21, there is shown a graph illustrating the results of a simulation comparing the constant bit rate connections to those using feedback sources. The cell loss ratio is plotted on the vertical axis  850  as a function of accepted bit rate/NBR plotted on the horizontal axis  852 . The graph illustrates that constant bit rate sources, C4  854 , C5  856 , and C6  858 , obtain better loss vs. throughput characteristics when they are transmitting cells at lower priority level values (i.e., at higher priorities). However, the results indicate that feedback sources, which adjust their rate using feedback information from the status cells, are useful when the background traffic is changing slowly. The feedback sources FB1  870 , FB2  872  and FB3  874  can adapt to changes, while constant bit rate sources cannot take advantage of the changing load of the network. With fast changing background traffic, feedback sources are not able to adjust to changes fast enough, resulting in an increased cell loss ratio. 
     FIG. 17 also illustrates the use of one source that generally performs as a kind of TCP source  876  with NBR=0.01. This source halves its transmission rate when it receives information about a lost cell, and increases its transmission rate by 10% if it does not receive information regarding lost cells over a time period of 10,000 time slots. Comparison reveals that this kind of source loses more cells than corresponding feedback sources. This is expected, as this type of source has a slower reaction to changes. 
     Another aspect to be considered involves the ability of the connections to be adjusted in during periods of sudden capacity changes, as well as how the capacity division between different feedback connections will be managed. Referring now to FIG. 22, a graphical illustration of the throughput/capacity (vertical axis  900 ) from each of the sources is shown as a function of time (horizontal axis  902 ). FIG. 22 provides an example of four feedback sources (FB1  904 , FB2  906  FB3  908  and FB4  910 ) are transmitting cells to a network node, where FB1  904  and FB2  906  have an NBR=0.25, and FB3  908  and FB4  910  have an NBR=0.0625. At a time corresponding to 30,000 time slots, a uniform source begins to transmit, shown as step function  912 , with a cell rate of PCR=0.333 at an NBR=0.333. At a time corresponding to 60,000 time slots, the uniform source terminates its cell transmission. 
     As can be seen in FIG. 22, the feedback sources (FB) are able to adjust their transmission rates in view of the load step function  912  on the connection. All the feedback sources have roughly the same decrease of throughput when the step source  912  is switched on. After the step source is switched off, the feedback sources restore their original throughput 
     It will, of course, be understood that various modifications and additions can be made to the various embodiments discussed hereinabove without departing from the scope or spirit of the present invention. Accordingly, the scope of the present invention should not be limited by the particular embodiments discussed above, but should be defined only by the claims set forth below and equivalents thereof.