# Patent

## Title
Differential delay compensation

## Abstract

In one embodiment, a method includes recording, for each of a plurality of data frames, a virtual write address including a multiframe indicator and a byte number indicator; and reading a group of associated data frames identified by corresponding multiframe indicators and byte number indicators. The reading is based on determining a minimum write address from a plurality of physical write addresses in the group of associated data frames by comparing virtual write addresses of all members in the group.

## Background
PRIORITY APPLICATION
This application is a continuation of U.S. application Ser. No. 13/229,455, filed Sep. 9, 2011, now issued as U.S. Pat. No. 8,488,631, which is a continuation of U.S. application Ser. No. 11/093,907, filed Mar. 30, 2005, now issued as U.S. Pat. No. 8,018,926, all of which are incorporated herein by reference in their entireties.


BACKGROUND
The subject matter described herein relates generally to the field of electronic communication and more particularly to differential delay compensation.
Communication networks transmit data from an originator to a destination via a communication network that may include multiple transfer points, such as hardware routers, that receive data typically in the form of packets or data frames. Data transmission over fiber optics networks may conform to SONET and/or SDH standards. SONET and SDH are a set of related standards for synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical NETwork and SDH is an acronym for Synchronous Digital Hierarchy.
SONET/SDH networks may employ virtually concatenated payloads. Virtual concatenation partitions payload data into multiple virtual containers that may be assigned a single index, referred to as a Multiframe Indicator (MFI), and transmitted contemporaneously across different transmission media and/or different network paths. Because the payload data traverses different network paths, payload data transmitted contemporaneously can be received at different times, an effect referred to as differential delay. Differential delay can also result from pointer processing, or from other considerations.
Virtual concatenation compensates for differential delay at the receiving entity by reassembling the payload in an appropriate time-ordered sequence. Data from different members of a virtual concatenation group are stored in a memory at the receiver. Processing logic in the destination node reads payload data from members having the same MFI contemporaneously. To do this, the destination node may determine memory locations of members in the same group having the same MFI value at the same byte within the frame so that the data can be assembled correctly at the output of the data memory. Addressing data in this arrangement can be complex, as data is being received, assembled, and processed at varying points and may require data reading from or writing to the memory based on a variety of circumstances.



BRIEF DESCRIPTION OF THE DRAWINGS
The detailed description is described with reference to the accompanying figures.
FIG. 1A is a schematic illustration of a SONET/SDH communication system in accordance with one embodiment.
FIG. 1B is a schematic illustration of a suitable system in accordance with one embodiment.
FIG. 2A is a schematic illustration of write operations into a memory at a receiver.
FIG. 2B is a schematic illustration of a memory at a receiver in accordance with one embodiment.
FIG. 3 is a flowchart illustrating operations in one embodiment of a method for writing data frames into a memory.
FIG. 4 is a flowchart illustrating operations in one embodiment of a method for reading data frames from a memory.
FIG. 5 is a schematic illustration of an eight-byte wide memory with N words in accordance with one embodiment.
FIG. 6 is a schematic illustration of one embodiment of a memory architecture in which M bytes of SDRAM store 64 STS-3c members.



DETAILED DESCRIPTION
Described herein are exemplary systems and methods for differential delay compensation in a communication system. In the following description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments. However, it will be understood by those skilled in the art that the various embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the particular embodiments.
The methods described herein may be embodied as logic instructions on a computer-readable medium. When executed on a processor, the logic instructions cause a processor to be programmed as a special-purpose machine that implements the described methods. The processor, when configured by the logic instructions to execute the methods described herein, constitutes structure for performing the described methods.
FIG. 1A is a schematic illustration of a SONET/SDH communication switching system in accordance with one embodiment. Referring to FIG. 1A, SONET/SDH switching system 100 includes a transmitter 110 connected through a communication pathway 115 to a switching network 120. Switching network 120 is connected through a communication pathway 125 to a destination 130.
Transmitter 110 transmits data as a series of payloads/frames to the destination 130 through the switching network 120. Packets may pass through a variety of hardware and/or software components, such as servers, routers, switches, etc. in transmission across switching network 120. As each frame arrives at a hardware and/or software component, the component may store the frame briefly before transmitting the frame to the next component. The frames proceed through the network until they arrive at the destination 130. The destination 130 may contain one or more processors 135 and/or one or more memory modules 140.
FIG. 1B is a schematic illustration of a suitable system in accordance with one embodiment. The system 101 may include a line card 111, a line card 121, a switch fabric 141, and a backplane interface 131. Line card 111 may be implemented as a SONET/SDH add-drop multiplexer, a Fibre Channel compatible line input, an Ethernet line input, a SONET/SDH line input, or the like.
Line card 121 may be implemented as a transceiver capable of transmitting and receiving frames and/or packets to and from a network that is compatible with SONET/SDH as well as other protocols such as OTN, TFI-5, and Ethernet, although other standards may be used. For example, SONET/SDH and OTN are described for example in: ITU-T Recommendation G.709 Interfaces for the optical transport network (OTN) (2001); ANSI T1.105, Synchronous Optical Network (SONET) Basic Description Including Multiplex Structures, Rates, and Formats; Bellcore Generic Requirements, GR-253-CORE, Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria (A Module of TSGR, FR-440), Issue 1, December 1994; ITU Recommendation G.872, Architecture of Optical Transport Networks, 1999; ITU Recommendation G.825, “Control of Jitter and Wander within Digital Networks Based on SDH” March, 1993; ITU Recommendation G.957, “Optical Interfaces for Equipment and Systems Relating to SDH”, July, 1995; ITU Recommendation G.958, Digital Line Systems based on SDH for use on Optical Fibre Cables, November, 1994; and/or ITU-T Recommendation G.707, Network Node Interface for the Synchronous Digital Hierarchy (SDH) (1996). For example, an implementation of TFI-5 is described in TFI-5: TDM Fabric to Framer Interface Implementation Agreement (2003) available from the Optical Internetworking Forum (OIF). For example, IEEE 802.3 describes Ethernet standards.
Switching network 120 may be any network such as the Internet, an intranet, a local area network (LAN), storage area network (SAN), a wide area network (WAN). One embodiment of line card 121 may include physical layer processor 122, framer 124, network processor 126, and host-control plane controller 128.
Physical layer processor 122 may receive optical or electrical signals from the network and prepare the signals for processing by downstream elements such as framer 124. For example, for frames and/or packets received from the network, physical layer processor 122 may convert an optical signal to electrical format and/or remove jitter from signals from the network. For frames and/or packets to be transmitted to the network, physical layer processor 122 may remove jitter from signals provided by upstream devices such as framer 124 and prepare signals for transmission to the network, which may be optical or electrical format.
Framer 124 may utilize techniques described herein to process frames and/or packets received from a network. Framer 124 may transfer overhead from frames and/or packets to a higher layer level processor such as a network processor 126. For example, framer 124 and network processor 126 may communicate using an interface compatible for example with SPI-4 (described for example in the Optical Internetworking Forum (OIF Document) OIF-SPI4-02.1 and ITU-T G.707 2000, T1.105-2001 (draft), T1.105.02-1995, and ITU-T recommendations G.7042 and G.707), although interfaces compatible with other standards may be used.
Network processor 126 may perform layer 2 or layer 3 (as well as other higher layer level) processing on frames and/or packets provided by and to framer 124 in conformance with applicable link, network, transport and application protocols. Network processor 126 also may perform traffic management at the IP layer.
Host-control plane controller 128 may configure operation of framer 124 and network processor 126. For example, host-control plane controller 128 may program/provision framer 124 to control the content of frames. Host-control plane controller 128 may be implemented as separate from network processor 126 and communicate with the framer 124 and network processor 126 using an interface that complies with Peripheral Component Interconnect (PCI) Local Bus Specification, Revision 2.2, Dec. 18, 1998 available from the PCI Special Interest Group, Portland, Oreg., U.S.A. (as well as revisions thereof) or PCI-X Specification Rev. 1.0a, Jul. 24, 2000, available from the aforesaid PCI Special Interest Group, Portland, Oreg., U.S.A., although other standards may be used. Host-control plane controller 128 could be implemented as part of network processor 126, although other implementations may be used.
In one embodiment, one or more of physical layer processor 122, framer 124, or network processor 126 may be coupled to volatile and/or nonvolatile memory module 127. For example, memory module 127 may include one or more of the following: read-only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically EPROM (EEPROM), a disk drive, a floppy disk, a compact disk ROM (CD-ROM), a digital video disk (DVD), flash memory, a magneto-optical disk, or other types of nonvolatile machine-readable media suitable for storing electronic instructions and/or data.
In one embodiment, components of line card 121 may be implemented among the same integrated circuit. In another embodiment, components of line card 121 may be implemented among several integrated circuits that communicate using, for example, a bus or conductive leads of a printed circuit board.
Backplane interfaces 131 may be implemented as a single or multi-pin interface and may be used by line cards to interface with system fabric 141. For example, backplane interfaces 131 may be compatible with TFI-5 or CSIX (described in CSIX-L1: Common Switch Interface Specification-L1 (2000)), although other standards may be used. Switch fabric 141 may transfer IP packets or Ethernet packets (as well as other information) between line cards based on relevant address and header information. Switch fabric 141 can be implemented as a packet switch fabric or a TDM cross connect. Switch fabric 141 can be any device (or devices) that interconnect numerous dataplanes of subsystems (i.e., linecards) together.
SONET/SDH defines optical carrier levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic based hierarchy. In SONET, any type of service, ranging from voice to high speed data and video, can be accepted by various types of service adapters. A service adapter maps the signal into the payload envelope of the STS-1 or virtual tributary. All inputs received are converted to a base format of a synchronous signal, referred to as STS-1, which transmits at 51.84 Mbps (or higher). Several synchronous STS-1s may be multiplexed together to form a higher-level STS-n signal, which are integer multiples of an STS-1 signal.
SONET networks transmit data in frames, which include a transport overhead and a synchronous payload envelope (SPE). An SPE includes an STS path overhead section and a payload section, which holds the data being transported over the SONET network. Once the payload is multiplexed into the SPE and transmitted, the payload is not examined at intermediate nodes.
SONET/SDH architecture supports virtual concatenation. In virtual concatenation a large payload may be divided into a group of smaller payloads, which may be transmitted contemporaneously across different communication channels. Each SPE within a concatenated group contains an identifier, called a Multi-Frame Identifier, or MFI. The MFI forms part of the SONET/SDH path overhead information in the SPE and indicates the SPE's sequence and position within the group.
Virtual concatenation does not require intermediate node support. To compensate for differential delay, a receiver at the destination temporarily stores frames in a memory so that the payload data can be properly realigned. Applying an address calculation method that allows continuous data storage from members (i. e., time slots) in virtually concatenated groups permits the efficient use of the memory, thereby increasing the differential delay range which may be compensated for a given memory size. In one embodiment, a 1-byte wide on-chip memory is used as an example. The scheme can be extended for wider memory or for external memory modules. On-chip memory or external RAM modules can be used as a memory.
Exemplary operations for writing received data frames into a memory are explained with reference to FIG. 2A and FIG. 3. FIG. 2A is a schematic illustration of a memory at a receiver, and FIG. 3 is a flowchart illustrating operations in one embodiment of a method for writing data frames into a memory such as the buffer illustrated in FIG. 2A. Referring to FIG. 3, at operation 310 data frames are received at a destination node in a communication network. At operation 315 the payload data from the received data frames are stored in a memory. At operation 320 the physical write address at which a received frame is written in memory is recorded in a suitable memory, for example a flip-flop or other memory device. In one embodiment the physical write address corresponds to a physical location in the memory. At operation 325 a virtual write address is recorded in a suitable memory, for example a flip-flop or other memory device. In one embodiment the virtual write address includes the MFI value associated with the received data frame and the byte number for the last received byte.
As illustrated in FIG. 2A, the payload data frames received at the receiver may be written into the memory continuously. The physical write address is incremented continuously as frames are received. FIG. 2A illustrates an embodiment in which a 2.5 KB (2560 bytes) memory is used to store STS-3c payloads, which include 2349 bytes per frame. In this example the J1 byte for a given frame i is stored at address 130, and the J1 byte for frame i+1 and frame i+2 are stored at address 2479 and 2268, respectively. Writing data to the memory in a continuous fashion makes efficient use of the memory.
To compensate for differential delay, data frames from different members having the same MFI and same byte number may be read at the same time. Exemplary operations for reading received data frames into a memory are explained with reference to FIG. 2B and FIG. 4. FIG. 2B is a schematic illustration of a memory at a receiver, and FIG. 4 is a flowchart illustrating operations in one embodiment of a method for reading data frames from a memory such as the buffer illustrated in FIG. 2B.
Referring to FIG. 4, at operation 410 a minimum write address for a group is determined from the virtual write addresses of all members belong to this group. In one embodiment the minimum write address may be determined by comparing the virtual write addresses of all members in the same group, and selecting the minimum write address. The minimum write address may be expressed using a multiframe indicator and a byte number.
At operation 415 a virtual read address is determined using the minimum write address. In one embodiment the virtual read address is determined by subtracting a threshold value equal to the write to read delay in memory from the minimum write address.
At operation 420 a physical read address is determined using the virtual read address. In one embodiment the physical read address for each member in a group may be determined using the relationship:

RAPhy(i)=WAPhy(i)−(WAVir(i)−RAVir)

where RAPhy (i) is the physical read address for member i, WAPhy (i) is the physical write address for member i, WAVir (i) is the virtual write address for member i, and RAVir is the virtual read address for the whole group.

Although the virtual read address is the same for the whole group, due to the address conversion, each member may have a different physical read address. From the virtual read address, the actual byte number and MFI value can be identified easily for any follow up processing.
The addressing scheme is illustrated schematically in FIG. 2B. Referring briefly to FIG. 2B, a group of three members 240-244 are shown. The J1 byte for member 240 is at byte 130, the J1 byte for member 242 is at byte 2103, and the J1 byte for member 2 is at byte 1027.
The current physical write addresses for each of members 240, 242, 244 is at byte 2105, which is indicated by hash marks in FIG. 2B. While in this example the physical write address for members 240, 242, 244 is the same, the physical write address may differ for members in the same group, e.g., due to different SONET pointer movements from different members. In this example:

WAPhy(0)=2105

WAPhy(1)=2105

WAPhy(2)=2105

The virtual write addresses for the respective members may be expressed in the format (MFI, byte number), as follows:

WAVir(0)=(i,1975)

WAVir(1)=(i,2)

WAVir(2)=(i,1078)

For member 240, the byte at the current write address is byte number 1975 in frame i, so it is denoted (i, 1975). Similarly, for member 242, the byte at current write address is the second byte in frame i, so it is denoted as (i, 2). And for member 244, the byte at the current write address is byte number 1078 in frame i, so it is denoted as (i, 1078). Among the members 240, 242, 244, member 242 has the minimum virtual write address.
In the example presented in FIG. 2B the threshold between the group read address and the minimum write address of the group is 2. Thus, the current read address is at J1 byte of frame i, that is:

RAVir=(i,0)

A common virtual read address may be assigned to the whole group. However, the values of physical read address are different for different members. Applying the relationship RAPhy (i)=WAPhy (i) (WAVir (i)−RAVir), the physical read addresses of the respective member may be determined, as follows:

RAPhy(0)=130

RAPhy(1)=2103

RAPhy(2)=1027

The described addressing method can be applied to memory wider than one byte, and also to external memory (e.g., SDRAM). FIG. 5 is a schematic illustration of an eight-byte wide memory 500 with N words in accordance with an embodiment. Received data frames may be written continuously into the memory. Hence, any byte of the memory may store a J1 byte of a received data frame. The address translation described above for one-byte wide memory can be applied to an eight-byte wide memory. Further, the memory may be treated as a circular memory. Thus, when a write operation reaches the final byte of word N−1, the write operation can continue with the first byte of word zero.
FIG. 6 is a schematic illustration of one embodiment of a memory architecture in which M bytes of SDRAM store 64 STS-3c members (e.g., for an OC-192 device). Hence, an M-byte block of SDRAM 610 is divided into four banks identified by bank 0 (612), bank 1 (614), bank 2 (616), and bank 3 (618). Each of the four banks of SDRAM 610 stores data from sixteen members. In the embodiment depicted in FIG. 6, bank 0 (612) stores data from the set of members (0, 4, 8 . . . 60), bank 1 (614) stores data from the set of members (1, 5, 9 . . . 61), bank 2 (616) stores data from the set of members (2, 6, 10, . . . 62), and bank 4 (618) stores data from the set of members (3, 7, 11, . . . 63). Separating members into different banks in the SDRAM 610 improves the access efficiency when reading from and writing to SDRAM 610.
In one embodiment each bank 612, 614, 616, 618 may be divided into chunks of 1024 bytes, illustrated in table 630. This allocates 64 bytes of memory to each of the sixteen members assigned to the bank, as illustrated in table 640. In turn, each 64 byte memory allocation is divided into eight words of memory, each of which is 8 bits in width, as illustrated in table 650. The memory may be treated as a circular memory. Thus, when a write operation reaches the final byte of word M/512-1, the write operation can continue with the first byte of word zero.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be all referring to the same embodiment.
Thus, although embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing the claimed subject matter.

## Claims

1. A method, comprising:
receiving a plurality of data frames at a destination node in a communications system;
storing payload data from each of the received plurality of data frames in a memory, the plurality of data frames representing at least one virtually concatenated data stream;
recording a physical write address at which each of the plurality of received data frames is stored in the memory;
recording a virtual write address for each of the plurality of received data frames to a memory location, the virtual write address including a corresponding multiframe indicator value for each of the received data frames and a byte number for the last received byte from the received data frames; and
reading a group of associated data frames identified by corresponding ones of the multiframe indicators and the byte number indicators, the reading being based on determining a minimum write address from a plurality of physical write addresses in the group of associated data frames by comparing virtual write addresses of all members in the group.

2. The method of claim 1, wherein the plurality of data frames is received contemporaneously from a plurality of different communication channels coupled to the destination node.

3. A method comprising:
determining a minimum write address for a group of data from each of the virtual write addresses of each of the plurality of members within the group;
determining a virtual read address for the group using the determined minimum write address and subtracting a threshold value equal to a write-to-read delay in memory from the minimum write address;
determining a physical read address for the group using the virtual read address for the group; and
reading frame data based on a determination of the physical read addresses for each of the plurality of members of the group.

4. The method of claim 3, further comprising determining the minimum read address by operations comprising:
comparing each of the virtual write addresses of each of the plurality of members within the group; and
selecting an address having the lowest value of each of the virtual write addresses.

5. The method of claim 3, further comprising expressing the minimum write address using a multiframe indicator and a byte number.

6. The method of claim 3, further comprising determining the physical read address for each of the plurality of members within the group by subtracting the difference between a virtual write address for a selected one of the plurality of members and the virtual read address for the group from the physical write address of the selected one of the plurality of members.

7. The method of claim 3, further comprising determining multiframe indicator values and actual byte numbers for each of the plurality of members within the group from physical read addresses for each of the plurality of members.

8. A method, comprising:
recording, for each of a plurality of data frames, a virtual write address including a multiframe indicator and a byte number indicator; and
reading a group of associated data frames identified by corresponding ones of the multiframe indicators and the byte number indicators, the reading being based on determining a minimum write address from a plurality of physical write addresses in the group of associated data frames by comparing virtual write addresses of all members in the group.

9. A computer-readable storage medium having no transitory signals, the computer-readable storage medium comprising a plurality of instructions that, when executed by one or more processors configures the one or more processors to perform operations for compensating for differential delays in communications, the operations comprising:
receiving a plurality of data frames at a destination node in a communications system;
storing payload data from each of the received plurality of data frames in a memory, the plurality of data frames representing at least one virtually concatenated data stream;
recording a physical write address at which each of the plurality of received data frames is stored;
reading contemporaneously from the memory a group of the received data frames, each of the received data frames being identified by corresponding multiframe indicators and byte number indicators, the reading being based at least partially on determining a minimum write address from a plurality of physical write addresses in the group of the received data frames by comparing virtual write addresses of all members in the group; and
recording a virtual write address for each of the plurality of received data frames to a memory location.

10. The computer-readable storage medium of claim 9, further comprising one or more instructions that, when executed by the one or more processors, further configures the one or more processors to record, for each of the plurality of data frames, the plurality of physical write addresses that each indicate a position in the memory.

11. The computer-readable storage medium of claim 9, wherein reading contemporaneously from the memory comprises:
determining a virtual read address based on the minimum write address; and
determining a physical read address for each member in the group of associated data frames based on the virtual read address.

12. A method comprising:
determining a minimum write address for a group of data from each of the virtual write addresses of each of the plurality of members within the group;
determining a virtual read address for the group using the determined minimum write address;
determining a physical read address for the group using the virtual read address for the group and subtracting the difference between a virtual write address for a selected one of the plurality of members and the virtual read address for the group from the physical write address of the selected one of the plurality of members; and
reading frame data based on a determination of the physical read addresses for each of the plurality of members of the group.