Patent Publication Number: US-11659072-B2

Title: Apparatus for adapting a constant bit rate client signal into the path layer of a telecom signal

Description:
The present invention relates to data transmission. More particularly, the present invention relates to support for constant bit rate (CBR) clients by mapping a CBR digital client signal of an arbitrary rate into the payload of a telecom network server layer channel. 
     BACKGROUND 
     A constant bit rate (CBR) signal (e.g., a digital video signal) delivers bits at known fixed rate. It has become common to group consecutive data bits into 64-bit blocks that are then encapsulated into a 66-bit line code block (64B/66B encoding). The resulting block-coded stream then has a fixed rate of “W” bit/s (with some variance based on the CBR signal clock source accuracy). 
     The newly-launched MTN project in ITU-T SG15 initially assumes all client signals are Ethernet and lacks a direct method to support constant bit rate (CBR) clients. There are two categories of previous solutions for transporting CBR clients along a path from a source node to a sink node. One category creates a CBR path signal containing the client and some additional path overhead. It then uses overhead in the server signal to accommodate the difference between the path signal rate and the server payload channel rate. While there are various approaches within this category, the ITU-T Generic Mapping Procedure (GMP) is a commonly used solution for mapping a CBR digital client signal of an arbitrary rate into the payload of a telecom network server layer channel. The source uses the GMP overhead in each GMP window to send a count value (Cm) that tells the sink node how many payload data blocks it will send in the next window. The source node uses a modulo arithmetic algorithm based on Cm for inserting pad blocks to fill any channel bandwidth not required by the client signal. The sink uses the same algorithm to recover the data. However, the server channel for the MTN project in ITU-T SG15 does not provide for GMP overhead. Since the GMP overhead is relatively small and regularly spaced, this approach typically greatly simplifies the sink receiver process for deriving the client signal rate when it extracts the signal. A disadvantage of this approach is that it requires server section overhead, which must be processed at each node along the path. 
     The other category of solution operates in the packet domain. Fixed-sized portions of the CBR client signal stream are periodically encapsulated into Layer 2 or Layer 3 packets (e.g., Ethernet frames) sent from source to sink as the path signal. The sink then extracts the client data from the packets to reconstruct the client signal. Differences in clock domains along the path are accommodated by inserting or removing inter-packet idle blocks. This approach is popular in networks that primarily carry packet information with relatively little CBR traffic. 
     One drawback of this solution is the large amount of overhead bandwidth required for the packet encapsulation. Another drawback is that packet processing along the path, including the standard Ethernet Idle insertion/removal process (IMP), creates jitter due to irregular inter-packet arrival times at the sink. This adds significant complexity to the process of deriving the client signal rate at the sink, since average packet arrival time can be modified by intervening equipment. Also, using packets adds latency at the source and sink nodes and requires much larger buffers at the sink node. 
     As defined by the ITU-T (G.709 Optical Transport Networks), GMP requires a consistent fixed number of bits per GMP window. The server channel is point-to-point between nodes such that the GMP is terminated at the ingress to an intermediate node and generated anew at the node&#39;s egress port. Since the server channel for the MTN lacks GMP overhead, it would be desirable to move the GMP function into “path” overhead (POH) that is added to the client signal stream. POH passes through intermediate nodes without modification. Hence, placing GMP in the POH allows using legacy intermediate nodes without upgrade, since it avoids the need to add GMP processing to them. The problem with using GMP in the POH is that an intermediate node has a different clock domain than the source node, which makes it impossible to maintain a constant fixed number of bits for each GMP window. GMP only adjusts the amount of payload information sent per window, but the time period for the window is set by the source node based on its reference clock (REFCLK). 
     BRIEF DESCRIPTION 
     The present invention overcomes the intermediate clock domain problem by adding a mechanism that allows a small variable spacing between GMP windows. 
     The present invention allows using GMP in the path overhead (POH) for adapting the path stream to the source&#39;s server channel such that it can pass through intermediate nodes and provide the sink node with the frequency (rate) information that it can use for recovering the client signal. 
     The client stream consists of Ethernet-compliant 64B/66B blocks. POH is inserted into that stream as a special ordered set (OS) and identifiable 64B/66B data blocks to create the path signal stream. Unlike G.709, which relies on a framing pattern and fixed spacing for finding the GMP OH, the invention uses a 64B/66B OS block to identify the boundary of the GMP window, with the other POH/GMP blocks located in fixed locations within the GMP window. 
     The present invention uses the fact that the GMP Cm is, by definition, the count of the number of 64B/66B data (i.e., non-stuff) blocks that the source node will transmit in the next GMP window. Consequently, the sink node can accommodate having the GMP window extended by an arbitrary number of blocks since the Cm allows the Sink to determine when the Source has finished sending all the data (i.e., non-pad data) for that window. This insight allows using the approach of adding a small block of 64B/66B Idle blocks to each GMP window such that an intermediate node can increase or decrease the number of Idle blocks in order to do rate adaptation according to the standard Ethernet Idle insertion/removal process (IMP). The required Idle block length inserted by the Source will be a function of the chosen frame length such that the maximum 200 ppm clock difference between the nodes can be accommodated. 
     The sink node recovers the client signal rate through the combination of the received GMP overhead and average number of idles it receives. GMP further helps the receiver PLL by its smoother distribution of stuff/pad blocks. 
     The present invention uses existing nodes, which perform this padding adjustment by adding or removing Ethernet Idle blocks (i.e., the standard IMP process). 
     The sink node that extracts the CBR client signal must determine the CBR signal rate in order to re-create the output signal at exactly the correct rate. In accordance with the present invention, the sink node uses a combination of the rate it recovers/observes from the section signal coming into that node, the amount of IMP padding between the section and path signal, and the amount of GMP padding between the client and path signal to re-create the output signal at exactly the correct rate. 
     According to an aspect of the invention, a method for rate adapting a constant bit rate client signal into a signal stream in a 64B/66B-block telecom signal communication link including at a source node encoding into a control 64B/66B block an ordered set block-designator, at the source node encoding into a plurality of path overhead 64B/66B data blocks a count of data blocks to be encoded in a signal 64B/66B block, at the source node encoding into each of a plurality of signal 64B/66B blocks a total number of data blocks from the constant bit rate client signal equal to the count sent in the path overhead 64B/66B data blocks and a number of 64B/66B pad blocks, assembling the plurality of path overhead 64B/66B data blocks, the plurality of signal 64B/66B blocks and the control 64B/66B block into a path signal frame, the control 64B/66B block occupying a last position in the path signal frame, appending a set of number of 64B/66B idle blocks including a number of 64B/66B idle blocks at a position past the control 64B/66B block following the end of the path signal frame to match a link bit rate of a first link segment, and transmitting the path signal frame and the appended number of 64B/66B idle blocks from the source node into the signal stream at the first link segment at the link bit rate. 
     According to an aspect of the invention, the count of data blocks to be encoded in a path overhead 64B/66B block is variable, and the appended number of idle blocks is fixed. 
     According to an aspect of the invention, the count of data blocks to be encoded in a path overhead 64B/66B block is fixed, and the appended number of idle blocks is variable. 
     According to an aspect of the invention, the method includes receiving the path signal frame with the appended number of idle character 64B/66B blocks from the first link segment at an intermediate node, adapting the link bit rate to a bit rate internal to the intermediate node by conditionally appending additional idle character 64B/66B blocks to the set of idle character 64B/66B blocks when the link bit rate is slower than a bit rate in the intermediate node and by conditionally deleting idle character 64B/66B blocks from the set of idle character 64B/66B blocks when the link rate is faster than the bit rate in the intermediate node to form a modified set of idle character 64B/66B blocks, and transmitting the path signal frame and the modified set of idle character 64B/66B blocks into the signal stream at a second link segment from the intermediate node at the link bit rate. 
     According to an aspect of the invention, the method includes receiving the path signal frame and the modified set of idle character 64B/66B blocks at the link bit rate from the second link segment at a sink node, extracting in the sink node the count of encoded data blocks from the plurality of path overhead 64B/66B data blocks, extracting in the sink node the encoded data blocks from the modified signal 64B/66B block, regenerating from the extracted encoded data blocks the constant bit rate client signal, and determining in the sink node a bit rate of the constant bit rate client signal from a sink node reference clock and both the extracted count of encoded data blocks, and the number of idle character 64B/66B blocks in the modified set of idle character 64B/66B blocks and adjusting the rate of a constant bit rate client signal clock for transmitting the constant bit rate client signal at the bit rate of the constant bit rate client signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention will be explained in more detail in the following with reference to embodiments and to the drawing in which are shown: 
         FIG.  1    is a diagram showing a basic network illustration using GMP in the POH for adapting a Path stream to a source server channel. 
         FIG.  2    is a diagram illustrating a first way for the source node to derive the path signal rate in accordance with an aspect of the present invention; 
         FIG.  3    is a diagram illustrating a second way for the source node to derive the path signal rate in accordance with an aspect of the present invention; 
         FIGS.  4 A,  4 B and  4 C  are diagrams that shows the structure of a 64B/66B block; 
         FIG.  5 A  is a diagram showing a representative path signal frame having client data disposed within the frame in a first manner; 
         FIG.  5 B  is a diagram showing a representative path signal frame having client data disposed within the frame in a second manner; 
         FIG.  5 C  is a diagram showing a representative path signal frame having client data disposed within the frame in a third manner; 
         FIG.  6 A  is a block diagram of an example of a source node configured in accordance with an aspect of the present invention; 
         FIG.  6 B  is a block diagram of another example of a source node configured in accordance with an aspect of the present invention; and 
         FIG.  7    is a block diagram of an example of a sink node configured in accordance with an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Persons of ordinary skill in the art will realize that the following description is illustrative only and not in any way limiting. Other embodiments will readily suggest themselves to such skilled persons. 
     Referring first to  FIG.  1    a diagram illustrates a typical data flow in a network  10  in accordance with the present invention from a source node  12 , through an intermediate node  14 , and ultimately to a sink or destination node  16 . 
     There are two nested channels used to carry a CBR signal through the network. The first channel extends end-to-end, i.e., from where the CBR signal enters the network in the source node  12  and through the one or more intermediate nodes  14  to where it exits the network in the sink or destination node  16 . This channel is referred to as the “path” layer channel herein and is indicated in brackets at reference numeral  18  in  FIG.  1   . 
     The CBR signal plus overhead information inserted by the present invention is carried hop-by-hop over a network composed of multiple pieces of switching equipment (nodes), with nodes connected to each other by some physical media channel. This physical media channel (the second of the two cascaded channels) is referred to as the “section” layer channel herein. A first section layer channel connects the source node  12  to the intermediate node  14  and is indicated in brackets at reference numeral  20  in  FIG.  1   . A second section layer channel connects the intermediate node  14  to the sink or destination node  16  and is indicated in brackets at reference numeral  22  in  FIG.  1   . 
     A set of 64B/66B-encoded CBR client signals  24  are delivered to the source node  12 , which after adding the appropriate performance monitoring overheads, inserts the set of 64B/66B-encoded CBR client signals  24  into a link  26  towards the intermediate node  14 . The section layer  20  encompasses all of the information carried over the link  26 . For purposes of this disclosure, it is assumed that the incoming client signals  24  have been adapted into 64B/66B format in such a way that all the 64B/66B blocks are data blocks. 
     The intermediate node  14  is typically connected to multiple source nodes  12  and multiple sink nodes  14 . Client signals are switched by the intermediate node  14  onto a set of egress links (one of which is identified by reference numeral  30 ), connected to multiple sink nodes. The particular sink node  16  shown in  FIG.  1    is designated as the destination sink node for the 64B/66B-encoded CBR client signals  24  and extracts the performance monitoring overheads and recovers the 64B/66B-encoded CBR client signals  24 . 
     Managing this traffic from multiple source nodes to multiple sink nodes is typically handled using the FlexE calendar slot technique known in the art. This layer of the data transport is not shown in  FIG.  1    in order to avoid overcomplicating the disclosure and obscuring the invention. Persons of ordinary skill in the art will appreciate that the clock timing concepts for data transmission disclosed herein reflect the calendar slot timing employed in the FlexE technique, which will be referred to as is necessary for an understanding of the invention. 
     The section layer is based on FlexE, in which the time-division multiplexed channels are referred to as calendar slots. The term comes from the fact that each calendar slot appears multiple times per FlexE frame on a fixed interval between appearances of the same calendar slot. 
     The section layers  20  and  22  are carried by the link  26  between the source node  12  and the intermediate node  14  and the link  30  between the intermediate node  14  and the sink node  16 , respectively. Persons of ordinary skill in the art will appreciate that performance monitor overhead unrelated to the present invention (not shown) for link  26  is inserted before the data leaves the source node  12  and is monitored in the intermediate node  14 . Similarly, performance monitor overhead unrelated to the present invention (not shown) for link  30  is inserted before the data leaves the intermediate node  14  and is monitored in the sink node  16 . The section layer  20 , or  22 , originates at the transmit end of a link  26 , or  30 , and is terminated at the receive end either in the intermediate node (link  26 ) or the sink node (link  30 ), respectively. 
     Each 64B/66B-encoded CBR client signal  24  is also associated with a path layer  18  running from the source node  12  to the sink node  16 . The path layer  18  spans from the source node  12  the sink node  16 . The intermediate node  14  treats the CBR client signal  24  and the associated path layer  18  overhead information as a single unit. They are switched from a link  26 , also known as ingress link  26  to link  30 , also known as egress link  30 , together, indivisibly. 
     At the ingress of the source node  12 , the client data received in the 64B/66B-encoded CBR client signal  24  is prepared for forwarding to the intermediate node  14  within the dashed line designated by reference numeral  32 . The insert POH at reference numeral  34  inserts path level performance monitor overhead information for the client signal  24 . The path level performance monitor overhead includes several components, including for the purposes of the present invention the number Cm which is an identification of the number of blocks of client data that will be transmitted in the next frame. In reference numeral  36 , rate adaptation is inserted via GMP and the number of idle blocks inserted is identified. Idle blocks are inserted to adapt the client signal  24  to the payload capacity of the calendar slots of the FlexE multiplexing protocol and to the clock rate of the link  26  connecting the source node  12  to the intermediate node  14 . The clock rate of the link  26  is known by the source node  12  which transmits into the link  26 . The client signal  24  as augmented by the POH and inserted idle blocks is transmitted by the source node  12  to the intermediate node  14  through the link  26 . 
     As will be shown with reference to  FIGS.  4 A,  4 B, and  4 C , a control block header, and an ordered set block-designator are encoded into a control 64B/66B block at the source node  12  as part of insert POH block  34 . A data block header and a count of data blocks to be encoded in a signal 64B/66B block are encoded into a plurality of path overhead 64B/66B data blocks. A data block header, a total number of data blocks from the CBR client signal  24  equal to the count sent in the path overhead 64B/66B data blocks and a number of 64b/66B pad blocks are encoded into each of a plurality of signal 64B/66B blocks. The data blocks and path overhead blocks are preferably distributed rather than lumped together for greater immunity to error bursts because they are used later to help reconstruct the client signal clock for the client CBR signal at the sink node. 
     The plurality of path overhead data 64B/66B blocks, the plurality of signal 64B/66B blocks and the control 64B/66B block are assembled into a path signal frame. The control 64B/66B block occupies a last position in the path signal frame. A set of idle character 64B/66B blocks having a number of idle character 64B/66B blocks selected to produce an assembly that matches a link bit rate of a first link segment are appended at a position past the control 64B/66B block following the end of the path signal frame. The path signal frame and the appended number of idle character 64B/66B blocks are transmitted from the source node into the signal stream at the first link segment  26  at the respective link bit rate. 
     In the intermediate node  14  the encoded client signals sent by the source node  12  are adapted to the clock rate of the intermediate node  14  in reference numeral  38 , which inserts or deletes idle character 64B/66B blocks from the data stream as necessary to match the data stream rate to the clock rate of the intermediate node  14 . The path signal frame with the appended number of idle character 64B/66B blocks is received from the first link segment  26  at the intermediate node  14 , the link bit rate is adapted to a bit rate internal to the intermediate node  14  by appending additional idle character 64B/66B blocks to the set of idle character 64B/66B blocks when the link bit rate is slower than a bit rate in the intermediate node and by deleting idle character 64B/66B blocks from the set of idle character 64B/66B blocks when the link rate is faster than the bit rate in the intermediate node to form a modified set of idle character 64B/66B blocks. After distribution by calendar slot switch  40 , to be discussed further below, the modified set of idle character 64B/66B blocks is further modified in reference numeral  44  to adapt the clock rate of the intermediate node  14  to the rate of link  30 , and the path signal frame and the further modified set of idle character 64B/66B blocks is transmitted into the signal stream at a second link segment  30  from the intermediate node  14  at the respective link bit rate. In particular the link bit rate is adapted from the bit rate internal of intermediate node  14  to the link bit rate of link  30  by appending additional idle character 64B/66B blocks to the set of idle character 64B/66B blocks when the bit rate in the intermediate node  14  is slower than the bit rate of link  30  and by deleting idle character 64B/66B blocks from the set of idle character 64B/66B blocks when the bit rate in the intermediate node  14  is faster than the bit rate of link  30  to form the further modified set of idle character 64B/66B blocks 
     The intermediate node  14  includes a calendar slot switch  40  for distributing the client data in calendar time slots according to the intended sink node in accordance with the FlexE scheme as known in the art. Link  42  is shown sending data to another sink node (not shown). 
     The calendar slot switch  40  is a switch fabric that connects a path layer signal being carried over a set of calendar slots on an input port to a set of calendar slots on an output port. It is conceptually similar to any fabric for switching/cross-connecting constant rate signals. The main difference from other fabrics is that calendar slot switch  40  must use the I/D rate adapters  38  and  44  for rate adaptation. The I/D rate adapters  38  and  44  insert or remove idle blocks from between path signal frames as shown in  FIGS.  5 A,  5 B, and  5 C  so that its resulting data rate matches the actual calendar slot rate of the switch fabric in the calendar slot switch  40  and then of the link  30  at the output port of intermediate node  14 . 
     The path signal frame and the further modified set of idle character 64B/66B blocks is received at the respective link bit rate from the second link segment  30  at the sink node  16 . In the sink node  16  the count of encoded data blocks is extracted from the plurality of path overhead 64B/66B data blocks. The encoded data blocks are extracted from the further modified signal 64B/66B block. The constant bit rate client signal is regenerated from the extracted encoded data blocks. A bit rate of the constant bit rate client signal is determined from the recovered bit rate of the incoming link  30 , the extracted count (Cm) of encoded data blocks, and the number of idle character 64B/66B blocks in the further modified set of idle character 64B/66B blocks and adjusting the rate of a constant bit rate client signal clock for transmitting the constant bit rate client signal at the bit rate of the constant bit rate client signal  24  that was provided to source node  12 . 
     In reference numeral  46  in the sink node  16  the path level performance monitor overhead information for the CBR client signal  24  is extracted from the client data signal. Included in that information is the number Cm that identifies how many data blocks are to be recovered from the next frame. The number Cm of data blocks to be recovered from the current frame has already been extracted from the previous frame by reference numeral  46 . 
     At reference numeral  48 , the GMP overhead (Cm) is recovered, the number of received idle blocks is counted, and the GMP pad blocks and all idle blocks are discarded. The output of block  48  is the resulting client 64B/66B CBR encoded signal as shown at reference numeral  50 . 
     As can be appreciated by persons of ordinary skill in the art, the intermediate node  14  passes the client 64B/66B encoded signal through and only removes or adds idle blocks as necessary to adapt the rate of the incoming signal to its own clock rate and the clock rate of the link  30  between it and the sink node  16 . The intermediate node  14  does not need to consume processing power to unpack and repackage the client 64B/66B encoded signal. 
     The POH insertion and rate adaptation performed at reference numerals  34  and  36  adapt the rate of the path overhead augmented 64B/66B-encoded client signal to the payload capacity of the FlexE calendar slots associated with the selected path layer (not shown) and to the clock rate of the link  26 . In accordance with a first aspect of the invention as illustrated in  FIG.  2   , which shows a first embodiment of a more detailed reference number  32 , the number Cm of data blocks is variable and a variable number of 64B/66B pad blocks are added to the frame at reference numeral  52  to achieve a nominal rate stream that has a bit rate that is a fixed amount lower than the payload capacity of the FlexE calendar slots for the selected path. The rest of the payload capacity is filled at reference numeral  54  by inserting a fixed number of 64B/66B idle blocks following the frame. In other words, the source node  12  inserts a variable number of 64B/66B pad blocks into the client data within the frame such that when the fixed/constant number of idle blocks is added at the end of the frame, the length in time of the resulting signal exactly matches the rate of the FlexE calendar slots that will carry it. In accordance with this aspect of the invention, the source node  12  derives the clock rate of the path layer signal  18  from the FlexE channel (“CalSlot”) rate, and uses dynamic GMP for client mapping and source rate adaptation. The source node  12  transmits a constant minimum number of idle blocks per frame. No IMP is performed at the source node  12 . The source node  12  GMP includes word fraction information to help the receiver phase locked loop (PLL). The sink node  16  determines the original client signal rate by examining the combination of the dynamic GMP information and the average number of idles it receives relative to the known number of idles inserted by the source node. The difference between the known number of idles inserted by the source node and the average number of received idles is that the one or more intermediate nodes has modified the number of idles. 
     According to another aspect of the invention as shown with reference to  FIG.  3   , which shows a second embodiment of a more detailed reference number  32 , a variable number of 64B/66B pad blocks are inserted to construct a stuff augmented stream with a bit rate that varies with that of the 64B/66B encoded client signal  24 . As in the embodiment of  FIG.  2   , the POH insertion is performed at reference numeral  34 . A fixed number of 64B/66B Data blocks (Cm) and 64B/66B pad blocks are added to the frame at reference numeral  56  to achieve a stream that has a bit rate that is a variable amount lower than the payload capacity of the FlexE calendar slots for the selected path  18 . The rest of the payload capacity is filled by inserting a variable number of 64B/66B idle blocks in each frame as shown at reference numeral  58  to fill the assigned Calendar Slots into FlexE type Cal Slots. In accordance with this aspect of the invention, the source node  12  derives the path signal rate from the client rate, uses static GMP for mapping, and uses IMP for source rate adaptation. The predetermined constant GMP Cm is used in order to create a path signal that is slightly slower than the nominal server channel rate. The standard FlexE “Shim” layer then uses IMP to add idle blocks between frames in order to fill any remaining bandwidth in the link  26 . The sink node  16  will determine the original client rate based only on the average number of received 64B/66B idle blocks. In this embodiment GMP is primarily used to provide a smooth delivery of payload blocks within the path frame payload at a fixed rate per path frame. 
     While the standard link protocol provides that the bit rate of the section links  26  or  30  is nominally the same between each pair of connected nodes, differences in clock source accuracy at each node cause small frequency variations in rate for each node-to-node link. Consequently, each node needs to make some adjustment to the number of 64B/66B idle blocks so that it adds the appropriate number of 64B/66B idle blocks between the path signal frames to match the section layer channel rate of the next hop. 
     The per client idle I/D rate adapt block  38  of intermediate node  14  inserts or deletes idle blocks on a per-client basis. The bitrate of the ingress stream over link  26  is adjusted to match the clock in the intermediate node  14  controlling the payload capacity of the calendar slots in the FlexE scheme set by the calendar slot switch block  40 . The calendar slot switch block  40  switches client signals delivered by one set of calendar slots through ingress link  26  to the corresponding FlexE calendar slots of a destination set of egress links  30 . Typically, the capacity of calendar slots in the switch  40  matches that of the egress link  30 . In that case, the rate adaptation block  44  may be omitted. In the case where the calendar slot rates of the calendar slot switch  40  and the egress link  30  are not the same, the rate adaptation block  44  inserts or deletes idle blocks in the client stream to match the rate of the resulting stream to that of the payload capacity of the calendar slots at the egress link  30 . 
     The end-to-end path layer  18  carrying the CBR client is sent by the source node  12  with a bit rate of “X” bit/sec. The bit rate of a section layer channel  20  or  22  that carries the above path layer channel between nodes is “Z” bit/sec, where the rate of Z is somewhat higher than that of X. The present invention adds identifiable padding blocks to the path stream to accommodate the difference between the X and Z rates. According to one aspect of the invention, special Ethernet control blocks (Ethernet Idle or Ordered set blocks) are used for the padding blocks. According to a second aspect of the invention, the identifiable padding blocks are GMP pad blocks. 
       FIGS.  4 A,  4 B and  4 C  are diagrams that shows the structure of a 64B/66B block.  FIG.  4 A  shows a control block  60 . The block contains 64 information bits that are preceded by a 2-bit header identified at reference numeral  62 , sometimes called the “sync header.” If the 64B/66B block includes control information, the header is a control block header and is set to 10 as shown in  FIG.  4 A , and the 8-bit field at byte 1 following the header identifies the control block type. The block identified by reference numeral  60  is a control block. For the purpose of the present invention, the only control block type of interest is the ordered set block, designated by block type 0x4B. An ordered set (OS) 64B/66B block is shown in  FIG.  4 A . 
       FIG.  4 B  shows the organization of the 64B/66B control block  60  and three associated 64B/66B POH data blocks identified at reference numerals  64 . Byte positions 7 and 8 in the three POH data blocks  64  are used to transport the data count Cm along with error correction data for the number Cm. The Cm data and the error correction data are distributed across the three 64B/66B POH data blocks  64  so that in the event of a single disruptive event during data transmission the number Cm can still be recovered using the error correction data. 
     Referring now to  FIG.  4 C , if the 64B/66B block only contains data (i.e., it is a data character), then the header is a data block header and is set to the value 01 and the 64 bits following the header contain 8 bytes of data (e.g., bytes from an Ethernet packet). The upper 64B/66B data block shown in  FIG.  4 C  at reference numeral  66  contains only client data represented by Val 1 through Val 8 in byte positions 1 through 8. Data blocks can also contain additional POH fields including GMP overhead as shown in the lower 64B/66B POH block of  FIG.  4 C  as shown at reference numeral  62  (also represented as 64B/66B blocks  64  in  FIG.  4 B ). 
       FIGS.  5 A,  5 B and  5 C  show three different illustrative non-limiting arrangements for frames.  FIGS.  5 A and  5 B  are arrangements where N data blocks (identified as “Payload” at reference numerals  68 ) are distributed into segments.  FIG.  5 A  shows the N data blocks being divided into four segments each including N/4 data blocks. Each data block segment is followed by a 64B/66B POH block  70 . The 64B/66B POH block  72  at the end of the frame is the 64B/66B control block.  FIG.  5 B  shows the N data blocks being divided into three segments each including N/3 data blocks. Each data block segment is preceded by a 64B/66B POH block. The 64B/66B POH block at the end of the frame is the control block. 
       FIG.  5 C  shows an arrangement where N data blocks (identified as “Payload  68 ”) are grouped together, preceded by a group of three 64B/66B POH blocks. The 64B/66B POH block at the end of the frame is always the control block. 
     The frames in each of  FIGS.  5 A,  5 B, and  5 C  are followed by a number of 64B/66B idle blocks (identified at reference numerals  74 ) that, as previously explained are used to rate adapt the frames to slight variations in the bit rates of the source node  12 , the intermediate node  14 , the sink node  16  and the links  26 ,  30  that connect them. 
     The control POH block  72  is positioned at the end of each of the frames depicted in  FIGS.  5 A,  5 B, and  5 C . This is done because intermediate nodes in telcom systems are already configured to insert idle blocks in data streams. The intermediate nodes are configured to always insert any necessary idle blocks  74  immediately following a control block. If the control blocks  72  of the present invention  72  were located in any of the other POH block positions, there is a risk that an intermediate node could insert an idle block  74  at a position immediately following the control block. This would completely disrupt the ability of the sink node to correctly find the path signal frame. 
     Referring now to  FIG.  6 A , a block diagram shows an example of a source node  80  configured in accordance with an aspect of the present invention. The source node  80  implements the aspect of the invention shown in  FIG.  2   . In  FIG.  2   , the number of GMP pad blocks is varied to fill a GMP frame having a fixed period. If the 64B/66B client encoded signal is slow more pad blocks are added. If the client is fast, fewer pad blocks are added. An external frame pulse on line  86  generated by a reference clock inside the source node is applied to assure that the GMP frame has a fixed period. Since the GMP frame has a fixed period and a fixed number of blocks per frame, and the FlexE calendar slot has a fixed capacity per time unit, the difference between them can be filled with a fixed number of 64B/66B idle blocks. 
     The 64B/66B client data is received on line  82 . The path layer frame boundary is generated by GMP engine  84  that is time aligned by an external GMP window frame pulse on line  86 . The GMP window frame pulse is generated by a master timing clock (not shown) for the node  80 . 
     The GMP engine  84  determines the location of the POH blocks, and GMP pad blocks. The sum of payload data blocks and pad blocks per frame in a GMP frame is fixed. The number of payload data blocks and pad blocks per frame is fixed. The mix of payload data blocks and pad blocks per frame is variable, computed from the client data rate measured by the clock rate measuring circuit  88 . A fixed number of 64B/66B idle blocks are inserted from the idle insert block  90  per GMP frame period, irrespective of the fill level of 64B/66B encoded client data blocks in FIFO buffer  92 . The multiplexer controller  94  is controlled by the GMP engine  84  to direct the multiplexer  96  to select among payload data (64B/66B client data) from the FIFO buffer  92 , 64B/66B idle blocks from idle insert block  90 , 64B/66B pad blocks from pad insert block  98 , and 64B/66B POH blocks from POH insert block  100 . The output of the multiplexer  96  is presented to the FlexE calendar slots on line  98 . 
     In both the embodiments shown in  FIG.  2    and  FIG.  3   , the pad blocks are distributed amongst the data blocks rather than being concentrated at one location. 
     Referring now to  FIG.  6 B , a block diagram shows another example of a source node configured in accordance with an aspect of the present invention. Referring now to  FIG.  6 B , a block diagram shows an example of a source node  110  configured in accordance with an aspect of the present invention. Certain elements of the source node  110  are common to source node  80  of  FIG.  6 A  and will be designated in  FIG.  6 B  using the same reference numerals used in  FIG.  6 A  for these elements. 
     The source node  110  implements the aspect of the invention shown in  FIG.  3   . The 64B/66B client data is received on line  82 . The path layer frame boundary is generated by a free-running GMP engine  84  with no external time alignment. The GMP engine  84  determines the location of the 64B/66B POH blocks, and GMP pad blocks. The number of payload data blocks and 64B/66B pad blocks per frame is fixed. The higher the client rate, the shorter time it will take to accumulate the payload 64B/66B client data blocks within a GMP frame. The lower the client rate, the longer it will take to accumulate the payload data 64B/66B client data blocks within a GMP frame. Thus, the period of the GMP frame is determined by the bit rate of the incoming 64B/66B client data blocks on line  82 . The multiplexer control  94  monitors the fill level of the FIFO buffer  92  that is accepting 64B/66B client data blocks over line  82 . When the level of the FIFO buffer  92  is low, extra 64B/66B idle blocks are inserted. When the level of 64B/66B client data blocks in the FIFO buffer  92  is high, a reduced number of 64B/66B idle blocks are inserted. 64B/66B idle blocks are inserted between path layer frames. The multiplexer  96  is controlled by the GMP engine  84  to select among payload Data (64B/66B client data blocks) from the FIFO buffer  92 , 64B/66B idle blocks from idle insert block  90 , 64B/66B pad blocks from pad insert block  98 , and 64B/66B POH blocks from POH insert block  96 . 
     Referring now to  FIG.  7   , a block diagram shows an example of a sink node  120  configured in accordance with an aspect of the present invention to receive streams generated by the source nodes shown in  FIGS.  1  through  3   . Incoming FlexE calendar slots carrying the client payload stream are received on line  122 . The clock rate measuring circuit  124  measures the bitrate of the incoming FlexE calendar slots carrying the client payload stream. This rate is scaled by the DSP engine  126  to recover the client payload rate, as a function of the number of idles and the value of Cm in the GMP overhead as detected in the recover GMP overhead and count idle circuit  128 . Using the Cm value and idle blocks identified by the recover GMP overhead and count idle circuit  128 , the extract client payload block  130  identifies the payload, idle, and pad blocks within the GMP frame. The 64B/66B pad blocks and idle blocks are discarded while client payload 64B/66B data blocks are written into the FIFO buffer  132 . The phase locked loop (PLL)  134  is controlled to read from FIFO buffer  132  on line  136  at the client payload rate. All other blocks in the FlexE calendar slots are discarded. 
     Persons of ordinary skill in the art will appreciate that the intermediate node  14  of  FIG.  2    is configured in a conventional manner. As is clear from the disclosure herein, the intermediate node only inserts or deletes 64B/66B idle blocks as is known in the art to synchronize the data flow timing between its input and output rates without regard to the contents of the 64B/66B data and 64B/66B POH blocks. 
     The invention provides several advantages over prior-art solutions. The rate adaptation of the CBR client signal into the section layer is located within the path layer signal rather than the section layer overhead. This results in no impact on section layer format. In addition, using IMP allows using GMP for improved performance while making the path signal transparent to intermediate nodes, and hence having no impact on them. Unlike previous solutions, the present invention allows using GMP wholly-contained within the path signal. This provides the advantages of GMP relative to IMP/packet solutions including minimizing the required sink FIFO buffer, and simplifying the sink recovery of the client clock. The present invention maximizes server channel bandwidth available for the client signal, especially relative to packet-based solutions. 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.