Abstract:
A network device transfers variable-length data frames across a synchronous network employing a multiple-channel synchronous transport signal. Encapsulation logic encapsulates each data frame in a point-to-point frame including a body portion of the data frame and a length value located in a beginning portion of the point-to-point frame. Segmentation logic divides each point-to-point frame into fixed-sized segments, a first segment carrying the beginning portion of the point-to-point frame. Transmitting circuitry transmits the frame segments as payloads of at least one channel of the synchronous transport signal, the payloads being marked so as to be identifiable. Receiving circuitry receives payloads of at least one channel of the synchronous transport signal and from each received set of payloads regenerates a corresponding point-to-point frame using the length value from the first segment thereof. De-capsulation logic de-capsulates each regenerated point-to-point frame to recover the corresponding data frame.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   None 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   The present invention is related to the field of data communications, and more particularly to the transmission of local area network data traffic through a synchronous communications network. 
   Traditional data communications among computers has been carried out using local-area networks (LANs). Ethernet LANs, for example, have been very widely used in the data communications field. Generally, data communication over LANs has employed data units or “packets” having a variable size. Explicit address information is included in a header portion of each packet to identify the recipient. Specialized control logic at the various nodes of a network are responsible for detecting incoming packets, temporarily storing them in variable-size buffers, and determining from the address information which node or nodes the packet is destined for. 
   As LAN technology has developed, there has also been development in the technologies used in traditional telephony communications. Pulse code modulation (PCM) of voice signals and synchronous time-division multiplexed (TDM) transmission channels have been in use for many years. With advances in optical fiber technology, communications carriers have devised very high data rate signals for fiber optic transmission that carry thousands of individual TDM channels using hierarchical TDM techniques. In particular, a set of standardized synchronous transport signals (STSs) are utilized in networks that adhere to Synchronous Optical Network (SONET) standards. Synchronous digital signals having data rates ranging from about 51 Mb/s to over 100 Gb/s are defined in SONET, each signal generally incorporating an integer number of basic “STS- 1 ” signals. 
   There has been increasing interest in and need for communications equipment that can interface with traditional and emerging LANs, on the one hand, and the high-speed synchronous communications networks of the type traditionally deployed in telephony communications. The telephony networks, for example, are used for inter-LAN communications in wide-area networks (WANs), and therefore special interfaces are required to translate between LAN hardware and protocols and the hardware and protocols of the synchronous networks. Additionally, SONET-compliant equipment has been incorporated into portions of private and semi-private networks where the expense of such equipment is justified by the performance it provides, for example in backbone segments that are required to carry very high volumes of data traffic. 
   In these hybrid networks, there has been increased use of techniques that can be classified as “LAN emulation”. In a typical application, two or more disjoint LAN segments, for example segments residing in different buildings, are connected by one or more high-speed network segments of the type traditionally used in longer-haul networks such as the telephony networks. For example, one or more fiber optic links carrying SONET traffic may be used for such a high-speed link. The equipment that interfaces the separate LAN segments to the high-speed segments operates such that the collection of separate LAN segments appear to the connected host computers as a single LAN. This type of operation has numerous benefits, including the protection of investments in LAN hardware and software while providing greater connectivity and network capacity than would be possible using LAN technology alone. 
   In some networks of the type described above, the high-speed segments may provide data transport services to a number of different sets of users. For example, different businesses within a building or complex may utilize the high-speed network. There may be several independent emulated LANs, for example, that share the use of the high-speed network. In such cases, it may be necessary to allocate the usable capacity or bandwidth of the high-speed network among these multiple entities, to ensure that each enjoys a specified capacity without regard to the use of the network by the other entities. 
   Another issue to be addressed in networks of the foregoing type is the manner in which data traffic should be carried. The synchronous signals in the high-speed network are typically organized into fixed-size frames. The signal may retain a TDM aspect, in which case there must be a conversion between the connected LANs and discrete channels of the signal. In some cases, some part or all of the synchronous signal may have a “concatenated” format, meaning that the boundaries that typically exist among discrete TDM channels are not present, and the signal can be treated as a single fixed-rate stream available to carry data traffic. An example of such a signal is an Optical Carrier 3c, or OC-3c, which is a 155 Mb/s signal having no TDM sub-structure. 
   Protocols, such as High-Level Data Link Control (HDLC), have been used to transmit variable-length frames in an otherwise undifferentiated signal such as an OC-3c signal. Like other framing protocols, HDLC employs special “escape” characters and “escape sequences” that convey signaling information such as frame boundaries. One problem with such techniques is that the pattern of the escape character generally occurs in the data stream being framed, and if transmitted without modification would be erroneously interpreted as an escape character. To deal with this situation, a technique called “character stuffing” is used. Basically, every occurrence of the escape character in the data stream is replaced with a multi-character escape sequence that signifies that the receiver should insert the pattern of the escape character in the received data stream. 
   One significant drawback of employing HDLC or similar framing is the unpredictable expansion of the data rate that results from character stuffing. If a user data signal is specified to have a given fixed rate, for example, then the rate in the network is increased in proportion to the rate at which the escape character pattern appears in the data. Although on average such expansion may be very small, there may be realistic worst-case patterns that can result in expansion of 10% or more for non-negligible periods of time. If bandwidth is allocated based on such worst-case traffic, the link is generally under-utilized, which is inefficient. If bandwidth allocation is based on average traffic, then there may be an unacceptable rate of traffic loss when worst-case data patterns occur. 
   A number of links have been used to carry a single stream of packets or LAN frames. The stream is divided into separate logical channels, each of which is carried over a corresponding link. This technique has the effect of providing a desired overall transmission capacity by employing a number of lower-capacity links. Systems employing such techniques are generally referred to as “multilink” systems. It is generally necessary to append control information to the data transmitted over each link to enable a receiver to reconstruct the original data stream. On each link, it is necessary to frame or otherwise delineate the data and control information just as in the case of non-multilink transmission. When HDLC or similar framing is employed, the above-described problem of excessive rate expansion may exist. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with the present invention, methods and apparatus for transmitting data frames in a synchronous network are disclosed. Low-overhead encapsulation of the data frames is used along with multi-link transmission to obtain flexible allocation and efficient use of transmission bandwidth without incurring the unpredictable rate expansion of HDLC or similar framing protocols. 
   Variable-length data frames, such as from a local-area network, are transferred across a synchronous network employing a multiple-channel synchronous transport signal, wherein each channel of the synchronous transport signal carries data in fixed-size payloads. The channels may be virtual tributaries (VTs) of a SONET STS signal, for example, which carry 24-byte payloads. The data frames are first encapsulated in corresponding point-to-point frames, each including a body portion of the corresponding data frame and a length value. Preferably, the length value is located at a predetermined position in a beginning portion of the point-to-point frame. Each point-to-point frame is divided into a set of fixed-sized segments. The segments of each point-to-point frame are transmitted as payloads of at least one channel of the synchronous transport signal. When SONET VTs are utilized, the payloads are carried as VT synchronous payload envelopes (SPEs). The payloads are marked so as to be identifiable in a stream of payloads in the channel or channels of the synchronous transport signal. 
   Sets of payloads of the channel(s) are received by a receiver, and from each received set of payloads the corresponding point-to-point frame is regenerated based at least in part on the length value included in the first segment. That is, the length value helps the receiver identify the last data unit of the point-to-point frame within the segment carrying the last part of the point-to-point frame. The regenerated point-to-point frames are then de-capsulated to recover the corresponding data frames. 
   By using encapsulation and an explicit length value, the rate variability of HDLC and similar character-stuffing protocols is avoided, while relatively low overhead is retained for efficient use of transmission bandwidth. 
   In one transmission scheme, the synchronous network is used by multiple users, such as multiple LAN bridges or bridge groups. The channels of the synchronous transport signal are grouped or “bundled”, and each bundle is used to carry the data frames of a different set of network users. Each distinct point-to-point frame is carried on only a single channel of a bundle, and a load balancing process is employed to distribute the frames of the users across all the channels of each bundle so as to minimize delay differences, or dispersion, through the synchronous network. Resequencing logic is used within the receivers to re-order received frames as necessary to recover the original frame ordering. 
   Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood by reference to the following Detailed Description in conjunction with the Drawing, of which: 
       FIG. 1  is a generalized block diagram of a network having a Synchronous Optical Network (SONET) ring providing transport of data frames among multiple local area network (LAN) bridges in accordance with the present invention; 
       FIG. 2  is a diagram depicting a layered partitioning of processing for transporting frames in the network of  FIG. 1 ; 
       FIG. 3  illustrates the use of multiple bundles of SONET virtual tributaries (VTs) for LAN traffic transport in the SONET ring of  FIG. 1 ; 
       FIG. 4  is a block diagram of a version of the network of  FIG. 1  employing full multicast connectivity to transport data frames among the LAN bridges; 
       FIG. 5  is a block diagram of a version of the network of  FIG. 1  employing shared VT connectivity to transport data frames among the bridges; 
       FIG. 6  illustrates the use of a single VT bundle and a scheduler to carry data frames of multiple bridges of a bridge group in the shared VT connectivity network of  FIG. 5 ; 
       FIG. 7  is a diagram illustrating a technique by which data frames are encapsulated and segmented into multiple VT payloads for transmission in the networks of  FIGS. 1 ,  4  or  5 ; 
       FIG. 8  is a block diagram of logic for encapsulation and segmentation of data frames and distribution of data frame segments among available VTs in the networks of  FIGS. 1 ,  4  or  5 ; 
       FIG. 9  is a flow diagram of a process for selecting from among the VTs of a VT bundle to carry data frames in the logic of  FIG. 8 ; and 
       FIG. 10  is a block diagram of logic for receiving, re-sequencing, and de-capsulating encapsulated frames in the network of  FIGS. 1 ,  4  or  5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , bridges  10 - 1  through  10 - 4  are connected to corresponding local area network (LAN) segments  12 - 1  through  12 - 4 . Each LAN segment  12  has one or more host computers  14  or similar nodes connected to it. The bridges  10  are coupled to a Synchronous Optical Network (SONET) ring  16  via respective SONET termination and add-drop circuitry  18 . The SONET ring  16  and add/drop circuitry  18  provide data frame transport services to the bridges  10  such that the set of LAN segments  12  collectively appear as a single LAN from the perspective of any of the hosts  14 . That is, a host  14  communicates with another host  14  residing on a different LAN segment  12  in the same manner as if the other host  14  resided on the same LAN segment  12 . The bridges  10  and add/drop circuitry  18  perform frame forwarding and filtering on behalf of the LAN segments  12  to achieve this logical joining of the LAN segments  12  into a single LAN. 
   The hosts  14  on each LAN segment  12  communicate using any of a variety of LAN protocols. Such protocols typically employ data “packets” or “frames” that are transmitted non-synchronously. Although all the hosts  14  on a segment  12  use the same raw signaling rate, such as 10 Mb/s for example, their respective clocks are generally not synchronized. Additionally, the time periods in which each host  14  transmits are not fixed, but rather are decided dynamically according to some type of selection process. The frame sizes may be fixed or variable, and each frame typically includes one or more addresses indicating the destination(s) of the frame. For the sake of specificity in this description, it is assumed in the following description that the LAN segments  12  employ the Ethernet communications protocol. 
   When a bridge  10  receives a frame from a LAN segment  12 , it is responsible for determining whether the destination host  14  identified in the frame resides on another LAN segment  12 , and if so then forwarding the frame to the corresponding bridge  10  via the SONET ring  16 . Thus, if bridge  10 - 1  receives a frame from LAN segment  12 - 1  that is destined for a host  14  on LAN segment  12 - 4 , bridge  10 - 1  forwards this frame to bridge  10 - 4 . A bridge  10  receiving a frame from the SONET ring  16  is responsible for determining whether the frame is to be transmitted on the LAN segment  12  to which the bridge  10  is connected, and if so then taking such action. Continuing with the above example, bridge  10 - 4  transmits the frame received from bridge  10 - 1  on the LAN segment  12 - 4  for delivery to the destination host  14 . As described below, there are a variety of ways in which this general operational scheme can be carried out. 
   In  FIG. 1 , each bridge  10 - 1  through  10 - 4  is a member of a single “bridge group” defined in the system. Although not shown in  FIG. 1 , there may be additional, independent bridge groups that rely on the transport services of the SONET ring  16  for exchanging their traffic. Such additional bridge groups may have separate connections to the ring  16  or may share the connections used by the bridges  10 - 1  through  10 - 4 . The bridges of different groups do not exchange LAN frames. Different bridge groups may be associated with different customers of a SONET transport provider, for example, or may form different sub-networks in a larger network of a single organization. 
   Data traffic on the SONET ring  16  is carried in time-division multiplexed (TDM) fashion in units referred to as Synchronous Transport Signals (STSs). There are several standard STSs that have a hierarchical relationship with respect to each other. The basic or lowest-rate STS is known as STS- 1  and has a specified bit rate of approximately 51 Mb/s. An STS- 1  frame can be described as a 9-row by 90-column array of 810 bytes or “octets”. Different types of overhead information occupy several predetermined columns, and the remaining columns contain a data payload. There are also several higher-level STSs, such as STS- 3 , STS- 12 , and STS- 48 , for example. Each of these higher-level STSs is formed by multiplexing a number of STS- 1  signals together, and has a bit rate substantially equal to the corresponding multiple of the STS- 1  bit rate. The following description focuses on the use of a single STS- 1  in the SONET ring  16  to carry LAN traffic among the bridges  10 . It will be appreciated that the techniques described herein can be extended to systems employing higher-level STS signals. 
   In addition to the aggregation of STS- 1  signals into higher-level STS signals, there are standard ways in which an STS- 1  can be subdivided into multiple lower-capacity channels. One of these is the use of virtual tributaries (VTs). When carrying a VT-structured payload, an STS- 1  carries 7 VT groups, with each group using a corresponding set of 12 columns of the STS- 1  payload. Each group can be structured in one of four ways as follows:
         4 “VT 1.5” signals, each 1.7 Mb/s and using 3 columns   3 “VT 2” signals, each 2.3 Mb/s and using 4 columns   2 “VT 3” signals, each 3.5 Mb/s and using 6 columns   1 “VT 6” signal, 6.9 Mb/s and using all 12 columns       

   In general, different groups in the same STS- 1  signal can be structured differently. For example, there may be 3 groups using VT 1.5 structuring and 3 groups using VT 6 structuring. As described below, the presently disclosed system employs 7 groups of VT 1.5 signals, for a total of 28 VT 1.5s in the STS- 1  payload. These VTs are selectively allocated to carry the traffic of different bridges and/or bridge groups. The use of VT 1.5 signals provides finer control over the allocation of transport bandwidth than if larger-capacity VTs were used. It will be appreciated that other structures for an STS- 1  signal carrying LAN traffic may be advantageously employed in alternative embodiments. 
   The VTs can be allocated to carry the inter-bridge LAN traffic in any of a variety of ways. One useful abstraction in an allocation scheme is that of a VT “bundle”, or a set of VTs allocated to the same logical connection in the ring  16 . If traffic between two bridges  10 , for example, is expected to require 5 Mb/s of transport bandwidth, a bundle of 3 VT 1.5s can be defined to carry this traffic. This bundle is separate from other bundles that may carry traffic for other bridges in the same or different bridge groups. The number of bundles used in any particular system depends on a variety of factors, including the number of bridge groups, the number of bridges sharing each bundle, etc. Preferably there is software control over the creation and definition of the VT bundles as well as the assignment of individual VTs to the VT bundles, to provide desirable system flexibility. 
     FIG. 2  generally illustrates the multi-layer processing that occurs at each node of the ring  16 . In the illustrated embodiment, each node includes separate bridges  10 A– 10 D to support four independent local LAN segments  12 A– 12 D. The four bridges  10 A– 10 D share the use of the SONET termination and add/drop circuitry  18 , which is shown to include ENCAP/SAR logic  20 , VT/STS logic  22  and SONET physical layer (PHY) circuitry  24 . The ENCAP/SAR logic  20  performs frame encapsulation, decapsulation, segmentation and reassembly, which are described below. Each bridge  10 A– 10 D includes Ethernet media access control/physical layer (MAC/PHY) circuitry  26  and forwarding/filtering logic  28 . 
   When an Ethernet frame is received from a LAN segment  12  by the corresponding MAC/PHY circuitry  26 , it is first processed by forwarding/filter logic  28  to determine whether the destination address indicates that the frame is to be forwarded on the ring  16 . This is the case, for example, when the frame is a unicast frame and the destination is known to reside on a remote LAN segment  12  (i.e., a LAN segment  12  attached to another node of the ring  16 ), or when the frame is a multicast frame or has an unknown destination and therefore must be multicast to one or more remote LAN segments  12 . If the frame is to be forwarded on the ring  16 , it is provided to the ENCAP/SAR logic  20 , which encapsulates the Ethernet frame in a format described below. The encapsulated frame is then divided into segments (or “segmented”), and the segments are provided to the VT/STS logic  22  to be carried over the ring  16  in VT payloads, also described further below. The VT/STS logic  22  incorporates the segments as VT payloads of one or more VTs and STS- 1  frames, and the VTs and STS- 1  frames are provided to the SONET PHY circuitry  24  for transmission on an outgoing segment of the ring  16 . 
   On the receive side, the optical signal from the ring  16  is received by the SONET PHY circuitry  24  and converted into a corresponding serial electrical data signal. The VT/STS logic  22  identifies the STS- 1  frames in this signal and performs various STS overhead processing tasks. Additionally, the VT/STS logic  22  may utilize SONET functionality referred to as “drop and continue” to route received frames to the ENCAP/SAR logic  20  (a local “drop”) and to the next node in the ring  16  (a “continue”). The drop and continue functionality is used with a “full multicast” connectivity scheme employed in the ring to create connections among the bridges  10 , as described below. 
   For dropped SONET traffic, the VT/STS logic  22  recovers the individual VT payloads and provides them to the ENCAP/SAR logic  20 , where they are reassembled into encapsulated Ethernet frames. The ENCAP/SAR logic  20  may implement functions in support of a “shared VT” connectivity scheme that is an alternative way of creating connections among the bridges  10 , as described further below. Reassembled frames are de-capsulated and provided to the forwarding/filtering logic  28 . If the frame is to be forwarded to one of the segments  12 A– 12 D attached to the receiving node, the frame is provided to the MAC/PHY circuitry  26  for such purpose. 
   The VT/STS logic  22  carries out SONET line-layer and section-layer functions, including framing, performance monitoring, protection switching, and inserting/extracting the SONET synchronous payload envelope (SPE). The VT/STS logic  22  also processes VT overhead and includes 28 circuits (not shown) that operate in parallel on the 28 separate VT 1.5 signals in the SONET SPE. Each of these circuits performs VT path and transport functions, including VT framing, VT performance monitoring, VT signal label checking, and inserting/extracting the VT SPE. Data from received VT SPEs is passed to the ENCAP/SAR logic  20  via local receive buffers (described below; not shown in  FIG. 2 ), and outgoing data from local transmit buffers (not shown) is used to generate outgoing VT SPEs. 
     FIG. 3  shows one example of the use of VT bundles. Three bundles  30  are defined. VT bundle  30 - 1  includes the fourteen odd-numbered VTs in the set of twenty-eight total. VT bundle  30 - 2  includes the even VTs in the range from #2 through #14, and VT bundle  30 - 3  includes VTs #16, #18 and #28. VTs #20, #22, #24, #26 are not used. By virtue of the illustrated bundling scheme, the bridge(s) to which VT bundle  30 - 1  is assigned is/are allocated one half the total STS- 1  payload bandwidth. The bridges to which VT bundles  30 - 2  and  30 - 3  are assigned are allocated one quarter and three twenty-eighths, respectively, of the STS- 1  payload bandwidth. 
     FIG. 4  shows one arrangement for using VTs and VT bundles to carry the inter-bridge LAN traffic of a bridge group. In this arrangement, referred to as “full multicast connectivity”, different VT bundles are used to carry LAN traffic sourced by different bridges in a bridge group, and every VT bundle is received by all other bridges in the bridge group. Thus, in  FIG. 4 , VT bundle “x” carries the traffic sourced by bridge “x” (for “x” from 1–4). Each bundle is always available to the corresponding source bridge  10  for carrying data. Any number of the bridges  10  in a group can therefore transmit simultaneously without mutual interference. As in the example of  FIG. 3 , different bridge groups may have different-sized bundles, and therefore have different allocations of bandwidth in the ring  16 . 
   In the full multicast scheme, the SONET termination and add-drop circuitry  18  at each node provides certain VT routing functionality as follows:
         1. Receives VT bundles from the local bridge(s) and inserts the VTs of this bundle into an “outgoing” STS- 1  being transmitted to the next node in the ring  16 . This operation is referred to as an “add” operation.   2. Receives VT bundles from the other bridges of the bridge group via an “incoming” STS- 1  from the previous node in the ring  16 , and (i) provides the received VT bundles as input to the local bridge  10 , and (ii) sends the received VT bundles to the next node via the outgoing STS- 1  . This operation is referred to as “drop and continue”. Different nodes may perform different combinations of drop and continue functions. For example, if there is no local bridge that is a member of a bridge group for a VT bundle, then the traffic is only “continued” by the add/drop circuitry  18 . If the local bridge is the last bridge in the ring that is a member of the bridge group, then the traffic is only “dropped” and not continued. The functions to be performed at each node are established by provisioning.       

   By the above mechanism, every bridge  10  receives the LAN traffic generated by all other bridges  10  in the same bridge group. In particular, the traffic from any given source bridge  10  is transmitted on a VT bundle uniquely associated with that source bridge  10 . Each set of add/drop circuitry  18  identifies the source of incoming traffic based on the identities of the VTs carrying the traffic. Locally generated traffic is simply placed on the VT bundle associated with the local bridge  10 . Traffic on VT bundles associated with other bridges  10  is dropped locally and also continued via the outgoing STS- 1 . 
   The number of VTs available in the SONET ring  16  establishes certain configuration constraints when the full multicast technique is employed. In particular, there are bounds on the number of bridges allowed in a bridge group, the number of bridge groups, and the amount of bandwidth (in increments of VT 1.5) allocated to each full multicast channel. For example, if it is assumed for simplicity that all bridge groups are similar to each other in terms of the maximum number of bridges, the number of VTs allocated per bridge, and the type of VTs used, then the following relationship must hold:
 
[(# groups)×(# bridges per group)×(# VTs per bridge)]≦total number of VTs available to carry bridge traffic
 
   Thus, when there are 28 VT 1.5s available, for example, a configuration such as (7 groups, 2 bridges/group, 2 VT 1.5s per bridge) is legal, whereas (5 groups, 3 bridges/group, 2 VT 1.5s per bridge) is illegal. In general, there is no requirement that all bridge groups be so similar, and therefore the general constraint is simply that the sum of all VTs, regardless of how they are allocated to bridges or groups, must be no greater than the total number of available VTs. 
   The full multicast connectivity scheme is well suited to carry multicast or broadcast messages, which are used in a variety of contexts to distribute messages to a number of different recipients in a LAN. The full multicast scheme can also be used to carry unicast traffic. Preferably, each bridge  10  employs a filter between the SONET ring  16  and the local LAN segment  12  to avoid unnecessarily transmitting frames on the local segment  12  that are destined for a host  14  residing on a different LAN segment  12 . Each bridge  10  maintains a list of network addresses of hosts  14  that are known to the bridge  10 , and each address is associated with the LAN segment  12  to which the corresponding host  14  is attached. Each bridge  10  compares the destination addresses of received frames to the addresses in the list. If the address is known and the destination host resides on the local LAN segment  12 , the frame is transmitted on the local LAN segment  12  to be received by the destination host  14 . If the address is unknown, then the frame is forwarded to the local LAN segment  12 . If the address is known but the destination host resides on another LAN segment  12 , the bridge  10  simply drops the frame. 
     FIG. 5  shows an alternative method of allocating the VTs in the ring  16  to carry the LAN traffic of the different bridges  10 . In this scheme, which is referred to as “shared VT connectivity”, VT bundles are associated with respective bridge groups, rather than with particular source bridges, and the bridges of each bridge group share the use of the respective VT bundle. 
     FIG. 5  shows an example of the use of a single VT bundle by multiple bridges  10  of the same bridge group. A unicast frame is sent from bridge  10 - 1  to bridge  10 - 3  along a two-hop path  32 , and a separate unicast frame is sent from bridge  10 - 3  to bridge  10 - 4  along a one-hop path  34 . Each node transmits on one or more VTs of the same VT bundle. At each node, scheduling logic (discussed below; not shown in  FIG. 5 ) for the bridge group is responsible for allocating the use of the VT bundle. One major function of the scheduling logic at each node is to determine at periodic scheduling intervals whether the outgoing VTs are to carry frames originated by the local bridge  10  or frames originated by bridges  10  residing at other nodes of the ring  16 . Examples of the latter include the frames received on path  32  at node #2 (i.e., the node including bridge  10 - 2 ). At node #3, the scheduling logic is free to use the same VTs for the frames on path  34  as are being used for the frames on path  32 , because the frames on path  32  are not being sent further in the ring  16 . 
   The shared connectivity model of  FIG. 5  requires the use of a scheduler to allocate the use of the ring  16  among the bridges of a bridge group. A scheduler can be implemented using a table of “slots” and sequencing logic that uses information in the slots to make moment-by-moment scheduling decisions. For example, a scheduling table for a bridge group may be configured as follows: 
   
     
       
             
             
             
           
         
             
                 
             
             
               Bridge 
               Allocation 
               Internal/External 
             
             
                 
             
           
           
             
               1 
               2000 
               2000/9500 
             
             
               2 
               2500 
               2500/9000 
             
             
               3 
               3000 
               3000/8500 
             
             
               4 
               4000 
               4000/7500 
             
             
                 
             
           
        
       
     
   
   The data in the “Allocation” column signifies that for every scheduling cycle, bridge  1  is allowed to transmit up to 2000 bytes; bridge  2  up to 2500, etc. Each bridge does not require all of the allocation information in the table. The scheduler at any given bridge needs only the allocation for that bridge and the total allocation to all other bridges. The information available at each bridge is shown in the “Internal/External” column of the above table. Bridge  1 , for example, is provided with an “Internal” number of 2000 (which is the allocation to bridge  1 ) and an “External” number of 9500 (which is the sum of the “Allocation” entries for bridges  2 ,  3  and  4 ). All other bridges in the bridge group are configured in a similar fashion. 
     FIG. 6  shows an example of the operation of a scheduler at bridge  1 . It is assumed that bridge  1  initially has four “Internal” frames ready for transmission, of sizes 512, 1518, 1131, and 512 bytes respectively. Also, one “External” frame of size 1317 bytes has been received and requires forwarding in the ring  16 . 
   The scheduler compares the size of “Internal” frame  1  to the value in slot  1 . In this case, the allocation exceeds the size of the frame, so the frame is transmitted. The count in slot  1  is then decremented by the number of bytes in the frame. In this case, the count is decremented by 512 to 1488. 
   The scheduler then compares the size of “Internal” frame  2  to the count in slot  1 . In this case, the allocation is insufficient, so frame  2  cannot be transmitted yet. The scheduler thus proceeds to the “External” frames and slot  2 . Because the allocation of 9500 bytes is greater than the size of “External” frame  1 , this frame is transmitted and the byte count is decremented by 1317 to 8183. There are no additional “External” frames to be sent. 
   At this point, the scheduler has made one complete pass through all the slots, so slot  1  is incremented by 2000 to 3488. Now, there is sufficient allocation for the 1518-byte “Internal” frame  2 , so this frame is transmitted and the count for slot  1  is decremented to 1970. Subsequently, it will be possible for the remaining “Internal” frames  3  and  4  to be sent before the scheduler moves to slot  2 . This process repeats indefinitely. If there are no frames to be sent during one complete pass of the scheduler, the slot byte counts are not incremented. 
     FIG. 7  illustrates a mapping between MAC frames and VT payloads that is used for transport in the ring  16 . 
   The MAC frame  36  is shown as having a body  38  and a MAC frame check sequence (FCS)  40 , where the body  38  includes several fields (not shown in  FIG. 7 ) such as source address, destination address, length, etc. as is known in the art. 
   The body  38  of the MAC frame  36  is encapsulated as a “MAC payload”  42  of a point-to-point (PT-PT) frame  44  used as an intermediate message unit in the system of  FIG. 1 . Due to the use of VT payload structuring to identify traffic, the PT-PT frame  44  requires no additional addressing. The PT-PT frame  44  includes a small amount of additional overhead in the form of a two-byte length field  46 , a two-byte sequence number field  48 , and a two-byte PT-PT FCS  50 . The length field  46  specifies the length of the MAC payload  42  only, because the sequence number field  48  and PT-PT FCS field  50  have fixed sizes. 
   The PT-PT frame  44  is segmented into 24-byte segments  52 - 1  through  52 -N. As indicated by the dotted line in segment  52 -N, the last segment may be only partially full, owing to the variable size of the MAC payload  42 ; such segments are padded out to 24 bytes. 
   Different schemes of distributing the segments  52  to particular VTs of a bundle and/or particular containers within a VT can be used. Different VTs of a bundle can be thought of as different channels within the SONET ring  16 . One general approach is to distribute fixed-size groups of segments  52  across all VTs of the bundle. Thus, if there are 3 VTs in a bundle, for example, each PT-PT frame  44  is transmitted in 3-segment pieces in successive time slots of all three VTs. For example, segments 1–3 are sent in time slot  1  of all three VTs, segments 4–6 are sent in time slot  2 , etc. Another general approach is to send each PT-PT frame  44  via a single VT of the bundle, using as many successive time slots as there are segments  52  of the frame, and to dynamically distribute different frames among the different VTs of the bundle. One arrangement that uses such an approach is described in detail below. 
   The segments  52  are incorporated as 24-byte payloads in respective VT SPEs for transmission in the ring  16 . In the illustrated embodiment, each segment  52  is incorporated as a 24-byte payload  54  within a four-payload superstructure referred to as a VT 1.5 “superframe”  56 . In a VT superframe, several VT path overhead (POH) bytes  58  are common to a group of four VT SPEs. That is, these bytes are sent only once for each group of four VT SPEs. Each VT superframe  56  carries a single set of VT POH  58 , four payloads  54  and four framing (F) bytes  60 . 
   The framing bytes  60  are used to demarcate each PT-PT frame  44  in the stream of VT payloads  54  carried by a stream of VT superframes  56 . In particular, each framing byte  60  can be one of three values as follows: (1) Start of Frame (SOF), appearing with the VT payload  54  carrying the first segment  52 - 1  of a PT-PT frame  44 ; (2) Continuation of Frame (COF), appearing with the VT payloads  54  carrying the second segment  52 - 2  through the next-to-last segment  52 -(N−1) of a PT-PT frame  44 ; and (3) End of Frame (EOF), appearing with the VT payload  54  carrying the last segment  52 -N of a PT-PT frame  44 . 
   The beginning of each PT-PT frame  44  is aligned with the beginning of an SOF VT payload  54 . The location of the last byte of the PT-PT frame  44  in the EOF VT payload  54  is determined by adding 6 (the total number of bytes in the length field  46 , the sequence number field  48 , and the PT-PT FCS field  50 ) to the value in the length field  46 , modulo  24 . As shown, a single VT superframe  56  may include the last segment  52 -N of one PT-PT frame  44  and the first segment of the next PT-PT frame  44 . The transition between separate PT-PT frames  44  within a VT superframe  56  is indicated by the pattern of (EOF) (SOF) in two successive F bytes  60 . 
     FIG. 8  shows the structure of the portion of the ENCAP/SAR logic  20  responsible for encapsulating MAC frames  36  in PT-PT frames  44 , segmenting the PT-PT frames  44  into segments  52 , and providing the segments  52  to the VT/STS logic  22  of  FIG. 5 . The portion of the ENCAP/SAR logic  20  that accepts received segments  52  and delivers de-capsulated MAC frames  36  to the bridges  10  is described below with reference to  FIG. 10 . 
   Encapsulation logic  62  receives MAC frames  36  ( FIG. 7 ) from the local bridge(s) over corresponding ports, such as the four ports labeled A, B, C and D in  FIG. 8 . For each MAC frame  36 , the encapsulation logic  62  calculates its length, a sequence number, and an FCS value, and generates the PT-PT frame  44  using these calculated values and the body  38  of the MAC frame  36 . For generating the sequence number, the encapsulation logic  62  maintains four separate counters, each being used to provide sequence numbers for a corresponding bridge port. Each counter is incremented for each PT-PT frame  44  generated for the corresponding bridge port. 
   Each PT-PT frame  44  generated by the encapsulation logic  62  is provided to demultiplexing logic  64  for distribution to one of twenty-eight VT FIFO buffers  66 , each associated with a single VT. The VT FIFO buffers  66  feed twenty-eight pairs of VT subframe buffers  68 , which provide VT subframes synchronously to the VT/STS logic  22  ( FIG. 2 ). For each VT, one buffer of the corresponding pair of subframe buffers  68  is being filled from the corresponding VT FIFO buffer  66  (assuming it contains a segment  52  to send) while the other buffer of the pair is being read and the data is being provided to the VT/STS logic  22  for inclusion in an outgoing STS- 1  frame. After each concurrent filling and reading operation is completed, the operation of the buffers switches—i.e., the newly emptied buffer is filled with the next segment  52  from the corresponding VT FIFO buffer  66  and the newly filled buffer is read and the data is provided to the VT STS logic  22  for inclusion in the next STS- 1  frame. Because of this alternating characteristic of their operation, the VT FIFO buffers  66  are also referred to as “ping-pong” buffers. 
   The demultiplexing logic  64  is controlled by VT selection logic  70 , which receives buffer status information from the VT FIFO buffers  66  and a 2-bit connection identifier (CID) from the encapsulation logic  62 . The buffer status information indicates the level of fullness of each VT FIFO buffer  66 . The CID identifies the bridge port (A, B, C or D) from which each MAC frame  36  is received, and therefore indirectly identifies the VT bundle that is to be used to carry each PT-PT frame  44 . The indirection arises from the preferred use of a configurable table (not shown) that associates each bridge port with a corresponding VT bundle. The use of such a table provides desirable flexibility in the allocation and use of VTs and VT bundles. Although generally less desirable, it is possible in alternative embodiments to establish a fixed association between the bridge ports and VT bundles. 
   In the illustrated embodiment, the FIFO buffers  66  co-reside in a single 2 MB memory. When all 28 VTs are used, this memory can simply be divided into 28 equal-sized pieces, each of which is used to implement a FIFO buffer  66  for a corresponding VT. If fewer VTs are used, it is possible to allocate more memory to each VT. In alternative embodiments, more or less total memory may be used for the VT FIFO buffers  66 . 
   The VT selection logic  70  distributes encapsulated frames  44  among the VT FIFO buffers  66  in a load-balancing manner in order to minimize the amount of buffering required to re-order the frames  44  at the receiver. Out-of-order delivery arises from the variable-sized nature of the PT-PT frames  44 . When a long PT-PT frame  44  is loaded into a VT FIFO buffer  66  and transmitted over several time slots of the corresponding VT, a later-created PT-PT frame  44  may be loaded into a separate VT FIFO buffer  66  and transmitted in a much shorter period of time. This later-created PT-PT frame  44  is completely received at the next network node before the earlier, longer PT-PT frame  44  is completely received. As described below, the sequence numbers  48  are used to correctly re-order the PT-PT frames  44  at a receiving node. However, it is desirable to minimize the average time interval between two successive packets, to thereby reduce the amount of buffering required to accomplish re-ordering at the receiver. This is accomplished by distributing the outgoing PT-PT-frames  44  among the VT buffers  66  in a balanced fashion. 
     FIG. 9  shows the selection process employed by the VT selection logic  70  of  FIG. 8  for PT-PT frames  44  destined for a given VT bundle. This process is generally replicated in some fashion for all VT bundles in use at a given time. For example, there may be four separate instances of logic implementing this process within the VT selection logic  70 . Alternatively, it may be desirable to use a single instance of such logic with variable input parameters so that it can be used for the frames of the different VT bundles. 
   It is assumed that the VTs have been ordered in some fashion so that a single VT is selected at any stage of the process even if multiple VTs satisfy a criterion of interest. This ordering can be referred to as a “priority” ordering. For example, it can be assumed without loss of generality that the VTs are priority-ordered by their respective VT numbers, i.e., VT #1 has highest priority, followed by VT #2, VT #3, etc. In this case, it happens that “higher priority” corresponds to “lower VT number”. In general, the set of VTs assigned to a given bundle may be non-contiguous. In the bundling scheme of  FIG. 3 , for example, every other VT is missing from VT bundle  30 - 1 . Thus, in the following description, when a “first” or “next” VT is referred to, this is to be understood as a search among only those VTs actually assigned to a bundle. If there is a search for the next VT after VT #5 in bundle  30 - 1 , for example, the first possible candidate is VT #7, which is the next-lower-priority VT assigned to bundle  30 - 1 . 
   In step  72  of  FIG. 9 , the VT selection logic  70  searches for the “first”, or highest-priority, VT in the bundle whose FIFO  66  is empty. If one or more FIFOs  66  are empty, then the first of these (i.e., the lowest-numbered) is selected. An identifier (ID) of the selected VT is used as the selection input to the demultiplexer  64 . For example, if VTs #3, #17, and #24 are all assigned to the bundle and empty, then the FIFO  66  for VT #3 is selected. 
   If none of the FIFOs  66  of the VTs assigned to the bundle is empty, then in step  74  the VT selection logic  70  searches for the first VT of the bundle whose FIFO  66  is not full. This assumes that at least one FIFO  66  is full. Assuming that this condition is met, then if there are also one or more non-full FIFOs  66 , the ID of the lowest-numbered VT having such a non-full FIFO  66  is used as the selection input to the demultiplexer  64 . For purposes of determining a buffer&#39;s fullness, a mark set at a predetermined point from the top of the FIFO can be used. For example, a buffer may be deemed full when it has less than 2 KB of free space. 
   If all of the VTs assigned to the bundle are neither empty nor full, then in step  76  the VT selection logic  70  looks for the next VT assigned to the bundle whose FIFO  66  satisfies a load-balancing criterion. The starting point for determining the next VT begins where a preceding selection left off. This operation is referred to as “semi-round-robin” selection. Although the basic selection mechanism is round-robin selection, the selection may be modified when the load-balancing criterion is applied. A mark register (not shown) is used to keep track of the VTs that have been previously selected in the semi-round-robin manner. At any given time, the first unmarked VT has the highest priority, and a search for the next VT may wrap around from VT #28 back to VT #1. For example, if assigned VTs in the range of #5 through #15 are marked, then the VTs are ordered (#16, #17, . . . , #28, #1, #2, #3, #4) for purposes of step  76 . The mark register is reset to all zeros upon all VTs becoming marked. 
   A load balancing criterion is used in step  76  so that frames are distributed evenly among the available VTs, as described above. The level of fullness of each FIFO buffer  66  is tracked, and new selections are made such that the difference in fullness between the fullest and emptiest FIFO buffers  66  is less than some predetermined amount. A useful value for this parameter is 2 kilobytes, for example. In such a case, the first VT whose selection does not result in a fullness difference of more than 2 Kb across all the FIFO buffers  66  is selected. 
     FIG. 10  shows the logic within the ENCAP/SAR logic  20  responsible for receiving VT segments  52  from the ring  16  and re-creating the originally transmitted MAC frames  36 . Twenty-eight sets of paired VT subframe buffers  78  work in a manner analogous to that of the subframe buffers  68  of  FIG. 8 . That is, one buffer in each pair is being filled by the VT/STS logic  22  while the other buffer is being read out for further processing, and when one filling/reading operation is complete, the operation of the buffers switches. 
   The segments  52  from the subframe buffers  78  are received by a set of VT bundle resequencers  80 . The resequencers  80  identify the PT-PT frames  44  within the stream of segments  52  from the VT subframe buffers  78  and perform error checking using the PT-PT FCS  50 . The main function of the resequencers  80 , however, is to re-order received PT-PT frames  44  as necessary to obtain the sequences as originally emitted by the encapsulation logic  62  of the transmitting node. One resequencer  80  is required for each VT bundle. There should be a sufficient number of resequencers  80  to handle the maximum number of VT bundles that may be employed. For example, if in a given embodiment up to ten VT bundles may be defined, then ten resequencers  80  should be included in the set of resequencers  80 . If fewer than ten VT bundles are actually configured in a given application, a corresponding number of resequencers  80  will be active and the remaining number will be idle. To the extent that the definitions of the VT bundles (i.e., the identities of the VTs included in each VT bundle) are configurable, then the resequencers  80  must be similarly configurable so as to operate on the correct sets of VTs. 
   Each resequencer  80  preferably operates using a “sliding window” algorithm. Each resequencer  80  maintains maximum and minimum values identifying a range, or “window”, of sequence numbers within which it is working at any given time, and these values are updated as resequencing operations progress. The size of this window reflects several considerations, notably the expected differences in delay (or dispersion) that PT-PT frames  44  may experience in the network and the amount of buffer storage space available for storing PT-PT frames  44  that are subject to resequencing. Dispersion can result when protection schemes such as unidirectional path-switched ring (UPSR) protection are used, because packets may traverse the ring in different directions and therefore experience different delays. An exemplary window size is 4 milliseconds. The window is generally advanced when a frame is received that either extends a sequence of earlier-received frames or that completes a sequence of earlier-received frames by filling a gap. As an example of the former, if the leading edge of the window has frame number  5  and no other frames have been received, then the leading edge is advanced to frame  6  if that is the next frame received. If the pattern of frames at the leading edge of the window is ( 10 ,  11 ,  13 ,  14 ) and frame  12  is received, then the leading edge of the window is advanced to frame  14 . 
   Monitoring circuitry is used to detect when gaps have not been filled in some predetermined time. When this situation occurs, the missing frame(s) are declared to be lost, and the window is advanced as though the gap(s) had been filled. The monitoring circuitry also detects the receipt of a PT-PT frame  44  having a sequence number that falls outside the window. Either of these situations is reported as an error condition to operating software. 
   The resequencers  80  provide the re-ordered streams of PT-PT frames  44  to decapsulation logic  82 , which extracts the MAC payloads  42  from the PT-PT frames  44 , re-generates the MAC FCS  40  for each MAC payload  42 , and re-generates the MAC frames  36  from the extracted MAC payloads  42  and the re-generated MAC FCS  40 . The re-generated MAC frames  36  are then provided to the respective bridge ports. 
   As previously mentioned, when the shared VT connectivity model is employed, the identity(s) of the destination bridge(s) for frames being carried in the ring  16  cannot be determined solely from the identity of the VT bundle carrying the frame. One method of performing the “drop and continue” function is to completely re-construct the original MAC frame  36  and employ MAC-layer logic. An example of this method is described above with reference to  FIG. 2  and the forwarding/filtering logic  28 . It may be desirable in alternative embodiments to perform this function at the level of the PT-PT frames  44 , so as to reduce delays by bypassing MAC-layer processing when possible. Additional information may be appended to each PT-PT frame  44  to enable such functionality. Specifically, a short field for identifying the destination bridge  10  may be included. By examining this field, a node receiving a PT-PT frame  44  can quickly determine whether the frame is to be sent further in the ring  16 . Information identifying whether a frame is unicast or multicast may also be used to enable a quick determination whether a frame is to be dropped locally in addition to being continued. Additionally, it may be useful to include a field identifying the source bridge  10  to enable “learning” of associations between MAC addresses and bridges  10  at each node. Additional useful information might include service type, such as “guaranteed” or “best effort”, which would be used in managing the flow of different classes of traffic. 
   It will be apparent to those skilled in the art that other modifications to and variations of the disclosed system are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.