Method and apparatus for determining propagation delay in a network

The International Telecommunications Union (ITU) has introduced the G.709 standard for “Interface for the Optical Transport Network (OTN)”. This standard includes a set of recommendations by the ITU and provides for data to be transmitted in specific frames, each of which including a header.

G.709 includes recommendations directed toward networks in which multiple users transmit traffic over one or more portions of the same network. For example, a link including four nodes connected in series may carry traffic for a first user between the first and second nodes only, while a second user may transmit traffic from the first to the fourth nodes. In order to permit monitoring of each user's traffic, G.709 specifies that up to six Tandem Connection Monitoring (TCM) fields be provided in each header, such that each of up to six users is assigned a corresponding TCM field. The TCM fields can be processed at each node along a link carrying the traffic of a particular customer and terminated at end point nodes of the link. In addition, TCM fields may be used to designate portions of a network that may be monitored or controlled independently of each other.

Data carried in each TCM field may be calculated based on one or more parameters. One of these parameters is the propagation delay between nodes along the link. Conventionally, in order to determine the propagation delay, an initiating or initiator node sends a predetermined series of bits for each TCM to a peer node. The peer node, in turn, stores information indicative of the timing of the received bits for subsequent transmission back to the initiator node. As noted above, there may be up to six TCM fields in a given header, and, therefore, additional memory may be required in the peer node in order to store the timing-related information for each TCM field. If more TCMs are provided, the amount of storage or memory for storing such data would necessarily increase accordingly. Alternatively, the propagation delay may only be determined in connection with a single TCM field.

Moreover, propagation delay calculations are typically carried out by software, which may require a relatively long time to perform such calculations.

Accordingly, a simpler approach to determining propagation delay that is faster and requires less peer node memory is desired, for example, in a G.709 compliant network.

Consistent with an aspect of the present disclosure, a method is provided that comprises transmitting a first plurality of frames from a first node provided at a first location, and receiving a second plurality of frames from a second node provided at a second location spaced from the first location. The method further includes storing a value of a first temporal location in a first one of the second plurality of frames. The first temporal location corresponds to a first time when a portion of said one of the first plurality of frames is transmitted from the first node to the second node. In addition, the method includes storing a first frame number identifying the first one of the second plurality of frames and receiving a second value of a temporal location in a second one of the second plurality of frames. The second temporal location corresponds to a second time when the portion of said one of the first plurality of frames is received at the second node. Also, the method includes receiving a second frame number identifying the second one of the second plurality of frames, determining a length of time associated with a transmission of the portion of the said one of the first plurality of frames based on the first value, the first frame number, the second value, and the second frame number.

Consistent with an additional aspect of the present disclosure, a communication system is provided that comprises a first node provided at a first location and configured to transmit a first plurality of frames. The system also includes a second node provided at a second location and configured to receive the first plurality of frames and transmit a second plurality of frames to the first node. The first node is configured to receive the second plurality of frames. A memory is also provided that is configured to store a first value of a first temporal location in a first one of the second plurality of frames. The first temporal location corresponds to a first time when a portion of said one of the first plurality of frames is transmitted from the first node to the second node. The memory also stores a first frame number identifying the first one of the second plurality of frames. In addition, the first node receives a second value of a second temporal location in a second one of the second plurality of frames. The second temporal location corresponds to a second time when the portion of said one of the first plurality of frames is received at the second node. The first node also receives a second frame number identifying the second one of the second plurality of frames. Also, circuitry is provided that is configured to determine a length of time associated with a transmission of the portion of said one of the first plurality of frames based on the first value, the first frame number, the second value, and the second frame number.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, and together with the description, serve to explain the principles of the present disclosure.

FIG. 1 illustrates a system consistent with an aspect of the present disclosure;

FIG. 2 illustrates a detailed block diagram of initiator and peer nodes consistent with an additional aspect of the present disclosure;

FIG. 3 illustrates a frame consistent with the present disclosure;

FIG. 4 illustrates a timing of frames supplied to and output from an initiator node consistent with the present disclosure;

FIG. 5 illustrates a timing of frames supplied to and output from a peer node consistent with the present disclosure;

FIG. 6 illustrates a flow chart of a method consistent with an aspect of the present disclosure;

FIG. 7 a illustrates a link consistent with an additional aspect of the present disclosure; and

FIG. 7 b illustrates an example of a frame compliant with the G.709 standard.

Consistent with the present disclosure, a first or initiator node supplies frames to a second or peer node. The peer node also supplies frames to the initiator node. A propagation delay in the transmission of a frame from the initiator node to the peer node is determined by initially identifying a frame number and byte offset of a first incoming frame from the peer node at a time when the initiator node outputs a portion of a transmitted frame. The portion of the transmitted frame may be the first byte of a sub-frame within the transmitted frame. At the peer node, the frame number and byte offset of a second frame to be supplied to the initiator node is identified at a later time when the frame portion transmitted by the initiator node is received by the peer node, and such information is transmitted to the initiator node. Thus, since the frames output and received by the initiator node are typically of fixed duration, the frame number and byte offset of the incoming frame represents the time when the initiator node outputs the frame portion (a transmit time). In addition, the frame number and byte offset of the second frame represents the time at which the frame portion is received by the peer node (a receive time). Accordingly, by comparing the frame numbers and byte offsets of the first and second frames received from the peer node, a difference between transmit and receive times can be obtained, and such time difference corresponds to the propagation delay in transmission between the peer and initiator nodes.

Moreover, the propagation delay, thus determined, may serve as a common propagation delay or reference for each TCM, for example. As a result, the peer node need only store the timing information associated with only propagation delay measurement, instead storing such information for each TCM. Thus, additional peer node memory is unnecessary, such that system design is simplified and costs are reduced.

Reference will now be made in detail to the present embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 1 illustrates a communication system 100 consistent with an aspect of the present disclosure. System 100 may include a plurality of nodes, such as nodes 102 , 104 , and 106 . Nodes 102 and 104 communicate with one another over link 108 , while nodes 104 and 106 communicate with one another over link 110 . As discussed in greater detail below, data, in the form of frames may be output from node 102 , for example, and supplied to node 104 . The data may optionally be forwarded from node 104 to node 106 .

FIG. 2 illustrates nodes 102 and 104 in greater detail. For ease of explanation, nodes 102 and 104 may be referred to herein as initiator and peer nodes, respectively. As noted above, nodes 102 and 104 communicate with one another over link 108 . Link 108 includes a downstream path 235 and an upstream path 237 . Initiator node 102 may include a transmitter, which includes circuitry that encodes or otherwise process data and outputs and supplies an optical signal that carries the data as a first plurality of successive frames. The optical signal travels along downstream path 235 , which may include an optical fiber, for example, to a receiver 210 in peer node 104 . Receiver 210 may include circuitry for converting the optical signal to a corresponding electrical signal, as well as circuitry for processing that electrical signal and/or the data carried by it. For example, data carried by the electrical signal may be switched by switch 218 , which is also provided node 104 , and output from one of a plurality of output ports 222 - 1 to 222 - n.

Peer node 104 include transmitter 214 , which may be similar in construction and operation as transmitter 202 . Namely, transmitter 214 may also include circuitry that encodes or otherwise processes data and outputs and supplies an optical signal that carries the data as a second plurality of successive frames. The optical signal travels along upstream path 237 , which may also include an optical fiber, for example, to a receiver 208 in initiator node 102 . Receiver 208 may include circuitry for converting the optical signal to a corresponding electrical signal, as well as circuitry for processing that electrical signal and/or the data carried by it.

As further shown in FIG. 2 , processor circuitry 204 and 212 may be provided in the initiator ( 102 ) and peer ( 104 ) nodes, respectively. In addition, memory 206 is shown be coupled to processor circuitry 204 . Processor circuitry 204 and 212 may control the operation of transmitter 202 and receiver 208 and may also collect information received from upstream path 237 . The operation of processor circuitry 204 and 212 is described in greater detail below.

FIG. 3 illustrates an example of frame 300 that may be transmitted from either initiator node 102 or peer node 104 . Frame 300 includes a payload portion 302 and an overhead portion 304 . Frame 300 may be one of a series of frames, and overhead portion 304 may include a frame number 306 that identifies that frame within the series. For example, the first frame in the series may have a frame number of “1” the second “2” and so on. The series may be several thousand frames in length, and the frame numbers, accordingly, may number the frames up to the length of the series. A new sequence of frame numbers may be inserted into each of frames 300 after a particular series of frames has been transmitted. The frames output from node 102 , as well as those output from 104 , may be transmitted at a rate substantially equal to 40 Gbits/second.

The operation of nodes 102 and 104 will next be described with reference to FIGS. 4-6 . FIGS. 4 and 5 illustrate timing diagrams consistent with the present disclosure and FIG. 6 illustrates a flow chart 600 consistent with a further aspect of the present disclosure.

As noted above, initiator node 102 may receive a plurality of frames from peer node 104 , as indicated by arrow 408 . FIG. 4 shows the timing of received ( 403 ) and output or transmitted frames 402 at time t=t 0 at initiator node 102 . Three of the received frames are labeled 403 - 1 , 403 - 2 , and 403 - 3 in FIG. 4 , while three of the transmitted frames are labeled 402 - 1 , 402 - 2 , and 402 - 3 . At time t=t 0 (note also line 404 indicating the portions of frames 402 - 2 and 403 - 2 occurring at time t=t 0 ), for example, a predetermined frame portion 414 in frame 402 - 2 may be output from initiator 102 onto upstream path 235 . Frame portion 414 may include a sub-frame in frame 402 - 2 .

At t=t 0 , processor circuit 204 stores in memory 206 , a first byte offset 405 , which may be a value of temporal location 412 in frame 403 - 2 received from peer node 102 (step 602 ). Byte offset 405 is typically the number bytes between the beginning 410 of frame 403 - 2 and temporal location 412 . Temporal location 412 may be identified by a byte number in frame 403 equal to the number of bytes of byte offset 405 . In addition, processor circuit 204 may store the frame number associated with frame 403 - 2 . Since each of the plurality of frames 403 has a fixed number of bytes and each byte has a given duration, the frame number and the byte offset 405 , i.e., the value of temporal location 412 , are indicative of or represent time to.

As noted above, frame 402 - 2 , along with other frames within the plurality of frames 402 is transmitted to peer node 104 , as indicated by arrow 406 in FIG. 4 . The plurality of frames 402 may be carried by an optical signal that is fed to receiver 210 of peer node 104 . At time t=t 1 (represented by line 504 in FIG. 5 ), frame portion 414 is received by peer node 104 , and processor circuit 212 in peer node 104 identifies a byte offset 505 (a value of a temporal location 512 in an outgoing frame 403 - 92 equal to a number of bytes between the beginning 510 of frame 403 - 92 and temporal location 512 ) at the time t 1 . Processor circuit 212 also identifies the frame number of frame 403 - 92 , which is one of the plurality of frames 403 output from peer node 104 . The frame number of frame 403 - 92 and byte offset 505 , i.e., the value of temporal location 512 , represent time t 1 for similar reasons as those noted above in connection with byte offset 405 and the frame number of frame 403 - 2 .

Processor circuit 212 controls transmitter 214 in node 104 to output byte offset 505 and the frame number of frame 403 - 92 on an optical signal that is fed to receiver 208 of node 102 via upstream path 237 . In one example, such frame number and byte offset information may be included in an overhead portion of one of the plurality of frames 403 . Thus, node 102 receives the frame number of frame 403 - 92 and byte offset 505 (step 604 ). Processor circuit 204 , next computes or determines a length of time or propagation delay (the difference between times t 0 and t 1 ) associated with the transmission of frame portion 414 based on byte offsets 405 and 505 , as well as frame numbers of frames 402 - 2 and 403 - 92 (step 606 ).

In particular, processor 204 may calculate the propagation delay PD in accordance with the following formula:

PD=[(FN — 1 *L +Byte_Offset — 1)−(FN — 2 *L +Byte_Offset — 2)]*Byte_Period,

where:

FN — 1=the frame number of frame 403 - 2 , FN — 2=the frame number of frame 403 - 92 , Byte_Offset — 1=byte offset 405 Byte_Offset — 2=byte offset 505 L=the length in bytes of each of frames 402 and 403 , and Byte_Period=the duration of each byte, i.e., the time period during which a byte is output from either transmitter 202 or transmitter 214 .

As noted above, the propagation delay determined in accordance with an aspect of the present disclosure may serve as a common propagation delay for each TCM terminated at a particular node. For example, as shown in FIG. 7 a , initiator node 710 may terminate fields TCM 1 to TCM 6 at first end 715 - 1 of link 715 , while peer node 720 terminates these TCMs at second end 715 - 2 of link 715 . Hardware and/or software in nodes 710 and 720 may use a propagation delay calculated in the manner discussed above to incorporate appropriate error or timing values in each of fields TCM 1 to TCM 6 . Different propagation delays need not be determined to calculate such error and timing values for each TCM field. Accordingly, peer node 720 need only store the timing information noted above for one propagation delay measurement, instead of storing such information for each TCM field. As a result, additional memory or storage is unnecessary, such that system design is simplified and costs are reduced. Nodes 710 and 720 may have a similar structure as nodes 102 and 104 discussed above.

If the TCM fields are terminated at different nodes, different propagation delay values may be calculated in a manner similar to that discussed above.

It is noted that the method for determining propagation delay discussed above may be carried out in hardware, such as hardwired logic, and need not be calculated with software. Hardware, such as hardwired logic, may operate faster than software, and, therefore, propagation delay may be calculated in a relatively short time consistent with the present disclosure.

FIG. 7 b illustrates a simplified view of a G.709 compliant frame 750 , which includes header 752 and payload 754 . Fields TCM 1 to TCM 6 are shown in header 752 , which may also be referred to as the overhead portion of frame 750 . Frame 300 shown in FIG. 3 may also be G.709 compliant and have a structure similar to that shown in FIG. 7 b.

The length of each byte discussed above may be 8 bits. It is understood, however, that bytes having any number of bits or even a single bit may be provided in accordance with the present disclosure. Preferably, however, each byte should include the same number of bits.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the present disclosure. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.