Patent Application
20100189440
The present invention relates generally to telecommunications systems and in particular to methods and systems for transmitting data in passive optical networks.
Communications technologies and uses have greatly changed over the last few decades. In the fairly recent past, copper wire technologies were the primary mechanism used for transmitting voice communications over long distances. As computers were introduced the desire to exchange data between remote sites became desirable for many purposes such as those of businesses, individual users and educational institutions. The introduction of cable television provided additional options for increasing communications and data delivery from businesses to the public. As technology continued to move forward, digital subscriber line (DSL) transmission equipment was introduced which allowed for faster data transmissions over the existing copper phone wire infrastructure. Additionally, two way exchanges of information over the cable infrastructure became available to businesses and the public. These advances have promoted growth in service options available for use, which in turn increases the need to continue to improve the available bandwidth for delivering these services, particularly as the quality of video and overall amount of content available for delivery increases.
One promising technology that has been introduced is the use of optical fibers for telecommunication purposes. Optical fiber network standards, such as synchronous optical networks (SONET) and the synchronous digital hierarchy (SDH) over optical transport (OTN), have been in existence since the 1980s and allow for the possibility to use the high capacity and low attenuation of optical fibers for long haul transport of aggregated network traffic. These standards have been improved upon and today, using OC-768/STM-256 (versions of the SONET and SDH standards respectively), a line rate of 40 gigabits/second is achievable using dense wave division multiplexing (DWDM) on standard optical fibers.
In the access domain, information regarding optical networking can be found in Ethernet in the First Mile (EFM) standards (IEEE 802.3ah which can be found at www.ieee802.org and is included herein by reference) supporting data transport over point-to-point (p2p) and point-to-multipoint (p2 mp) optical fiber based access network structures. Additionally the International Telecommunications Union (ITU) has standards for p2 mp relating to the use of optical access networking. Networks of particular interest for this specification are passive optical networks (PONs). For example, three PONs of interest are, e.g., Ethernet PONs (EPONs), broadband PONs (BPONs) and gigabit capable PONs (GPONs), various exemplary characteristics of which are displayed below for comparison in Table 1.
With these ongoing improvements in optical networks, many telecommunication companies are choosing to upgrade their copper centric access networks with fiber optic access networks. Some such upgrades include, for example, using one of the above described PON networks combined with fiber to the home (FTTH), and/or hybrid networks, e.g., fiber to the cabinet (FTTC) combining optical EFM and/or PON for data backhaul with very high speed digital subscriber line (VDSL2) by reusing the last hundred meters or so of copper wire. These upgrades allow an increase in the types and quality of services delivered by companies to end users. As such services are deployed, there will be more demand for these networks and it will be desirable to be able to deploy PONs which are, for example, both flexible, e.g., from a manufacturing and supplier perspective, and scalable to meet this demand.
Systems and methods according to the present invention address this need and others by facilitating scalability in passive optical networks (PONs).
According to one exemplary embodiment a method for optical communications an optical communication system includes a first printed circuit board including a medium access control (MAC) unit for generating MAC frames to be transmitted over the optical communication system, a second printed circuit board including elements of an optical transceiver module for receiving the MAC frames and transmitting the MAC frames; an interconnect which connects the first printed circuit board to the second printed circuit board to enable the MAC frames to be communicated from said MAC unit to the optical transceiver module via a predetermined communication protocol, wherein the first printed circuit board further includes logic for processing the MAC frames in accordance with the predetermined communication protocol into other frames for communication over the interconnect and the second printed circuit board further includes logic for processing the other frames to extract said MAC frames for transmission.
According to another exemplary embodiment, a method for optical communications includes the steps of: generating MAC frames to be transmitted over the optical communication system, processing the MAC frames in accordance with a predetermined communication protocol into other frames, transmitting the other frames over an interconnect, receiving the other frames, extracting the MAC frames from the other frames, and transmitting the MAC frames over the optical communication system.
According to another exemplary embodiment, a communications node includes a medium access control (MAC) unit for generating Gigabit Passive Optical Network (GPON) transmission convergence layer (GTC) frames to be transmitted by an optical transceiver module, and logic for processing the GTC frames into other frames in accordance with a predetermined communication protocol and for transmitting the other frames toward the optical transceiver module.
According to yet another exemplary embodiment, a communications node includes logic for receiving frames formatted in accordance with a predetermined communication protocol and processing said frames into Gigabit Passive Optical Network (GPON) transmission convergence (GTC) layer frames, and an optical transceiver module for receiving the GTC frames and for processing and optically transmitting the GTC frames.
The accompanying drawings illustrate exemplary embodiments, wherein:
a) shows a conventional GPON protocol stack;
b) illustrates a conventional GPON transmission convergence (GTC) frame;
a)-6(c) show GTC frames being carried via other frames according to exemplary embodiments; and
The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.
According to exemplary embodiments it is desirable to provide mechanisms and methods that will, among other things, facilitate scalability of a passive optical network (PON). In order to provide some context for this discussion, an exemplary Gigabit-capable PON (GPON) is shown in
According to exemplary embodiments, GPON 100 in
It will be understood by those skilled in the art that this purely illustrative GPON 100 can be implemented in various ways, e.g., with modifications where different functions are combined or performed in a different manner. For example the multiplexers (108, 110 and 116) typically are duplexers, but if an additional signal is being transmitted, e.g., a cable-television signal in a GPON 100, they can act as triplexers. Additionally in the upstream direction, the optical signal would typically have a different wavelength from the downstream signal and use the same multiplexers 106, 110 and 116, which have bidirectional capabilities.
In the upstream direction, a TDMA scheme (e.g., as shown in
Although it is shown in
As seen in
More specifically, as shown in
Optical transceivers used to transmit GTC frames 312 in GPONs may be implemented on a printed circuit board (PCB) 400, e.g., disposed in the OLTs 104 of a GPON, including various modules as shown in
According to exemplary embodiments, in order to, for example, support scalability of GPONs, it may be desirable to separate the GTC frame processing module 404 (or, more generally, the medium access control (MAC) frame processing module) from the optical transceiver module(s) 402 which it supports as shown in
As shown in
In addition to these modifications, exemplary embodiments also need to employ a transport mechanism over which the GTC frames can be carried from the GTC frame processing module 404 over the backplane (or other interconnect) 500 and to the desired one of the optical transceiver modules 402. Conventionally, when the optical transceiver module 402 and the GTC frame processing module 404 were disposed on the same PCB 400, no such transport mechanism was necessary. Selecting a suitable transport mechanism for this purpose requires consideration of the transmission requirements of the GPON system itself, since the selected transport mechanism must permit the GPON system to transmit its GTC frames as set forth in the standard, i.e., the use of the intermediate transport mechanism according to these exemplary embodiments to transport the GTC frames from the now remotely located GTC frame processing module 404 to the appropriate optical transceiver module 402 via the backplane/switch 500 should not impact the transmission requirements of the GPON system. In part, this can be accommodated by the provision of the GTC receive buffers 502. However the overhead and other characteristics of the transport protocol should also be considered to ensure that they do not conflict with GPON requirements.
According to exemplary embodiments, Applicants have determined that one protocol which can be used to transport GTC frames as described above is the XAUI standard, specified in the IEEE 802.3ae standards document, the disclosure of which is incorporated here by reference. Briefly, XAUI is conventionally used as a mechanism for connecting a 10 Gigabit Ethernet physical layer to an Ethernet MAC layer on a PCB, however these exemplary embodiments propose using XAUI to encapsulate GTC frames for transport over an architecture such as that shown in
Selection of XAUI as a transport protocol for GTC frames in the exemplary embodiment of
Regarding overhead, and based on a data rate of 10 Gbps and a constant frame length of 125 us, every frame on XAUI has a constant frame size of 156250 bytes, i.e., 10 Gbps/8 bits*125 us=156250 bytes. This means that a 10G GPON GTC frame can be carried over XAUI including a maximum of 730 bytes of overhead, i.e., 156250 B−155520 B=730 bytes, where 155520 B is the downstream GTC frame size calculated as 9.95328 Gbps/8 bits*125 us=155520 bytes. A XAUI data frame is delimited by a /S/—“SOP—Start of Packet” control character (which must be sent on lane-0) and a /T/—“EOP—End of Packet” control character and, therefore, XAUI data frames have only 2 bytes of overhead. This, among other things, makes XAUI a good choice as a protocol to carry the GTC frames between decoupled GTC frame processing units 404 and the optical transceiver modules 402 which they are supplying with GTC frames. Depending on the vendor of the XAUI silicon, the XAUI data frames can be sent back-to-back, over short electrical distances, without an inter-frame gap. Again, depending on the vendor of the XAUI silicon, handling of 64K+ bytes of XAUI data frames is typical. Other specific issues associated with overhead in the context of using various protocols or combinations of protocols to carry the GTC frames between the GTC frame processing unit 404 and the optical transceiver module(s) 402 according to these exemplary embodiments are discussed below.
Regarding fragmentation issues, taking into account that typical XAUI implementations support a maximum frame size of approximately 64 kB, and that a 10G GPON frame size is 155520 bytes, fragmentation will be required in order to transport a GPON frame over XAUI. In order to help in providing a constant data rate for fragmented GPON frames, the prime factorization of the 10G GPON frame size is:
155520=2×2×2×2×2×2×2×3×3×3×3×3×5
This prime factorization can be useful in order to control the amount of overhead used when fragmenting the GTC frames, as it is easier to control the maximum allowed overhead by fragmenting GTC frames with constant sized fragments. Based on the foregoing analysis, it can be seen that GTC frames can be transported over XAUI between the non-colocated GTC frame processing module 404 and the optical transceiver modules 402 according to these exemplary embodiments. The resulting frames will be, for example, stripped of their XAUI overhead and have their payloads concatenated to reform the GTC frames by the XAUI processing units or logic 504, prior to being loaded into their respective buffers 502 for transmission.
Numerous variations and modifications to the above-described exemplary embodiments are contemplated and within the scope of the present invention. For example, a scalable GPON which decouples the optical transceiver module 402 from the GTC frame processing module 404 can be implemented with a receive buffer. Such a buffer implementation can use an adaptive rate limiting scheme wherein one or more IDLE patterns are injected into the output stream of the GTC frame processing module 404 to compensate for what would otherwise be a potential buffer over-flow condition at the optical transceiver module 402. Similarly, fewer or no IDLE patterns can be injected to avoid a potential buffer under-flow condition. This technique for selectively throttling the bandwidth when transmitting GTC frames over XAUI takes advantage of the capability provided in XAUI to tune the interframe gap which is provided between XAUI data frames. For example, according to one exemplary embodiment, FIFO “high/low” watermark settings trigger a rate limiting message (e.g., a single bit), toggling high/low data rates between the GTC frame processing module 404 and the optical transceiver module 402. More specifically, a receive buffer reacts to a “high/low” watermark threshold, and forwards a rate limiting message towards the remote sender. The remote sender, upon reception of a rate limiting message, throttles its transmission rate by inserting IDLE patterns between data frames. The rate limiting message can be sent as a standalone data frame, or optionally embedded in the GTC frame at intervals of N-bytes (which could be useful for symmetrical 10G/10G).
Although a considered selection of the transport protocol to be used for carrying GTC frames is suggested by these exemplary embodiments, and the XAUI protocol described above is one candidate, the present invention is not limited to the use of XAUI. For example, the 10 Gigabit Media Independent Interface (XGMII) can be used as an alternative protocol. XGMII is specified for a 32-bit wide data bus (4 byte wide bus), whereas XAUI is specified for a 4 lane bus, such that one XGMII byte of data is equivalent to 1 XAUI lane of serialized data of 10-bits. Additionally, GTC over another protocol over XAUI can be used according to other exemplary embodiments to provide connectivity between one GTC frame processing module 404 to many optical transceiver modules 402, where the other protocol is a switching protocol such as Ethernet, Serial-Rapid-IO, SPI-4.2 or HyperTransport.
For example, according to one exemplary embodiment, a GPON's GTC frame can be carried over XAUI encapsulated in an Ethernet packet 600 between the GTC frame processing module 404 and the optical transceiver module 402 as shown conceptually in
730 bytes/(1+1+8+14+4+12) bytes=18.25 frames (1)
155520 bytes/18 frames=8640 bytes per frame (2)
Note that in this particular example, which assumes that an Ethernet switch is interconnecting the different modules, an IFG of 12 bytes is specified. However it will be appreciated by those skilled in the art that an implementation which uses different equipment vendors or system configurations could lead to much lower values of overhead associated with the IFG, which would make the available bandwidth utilization even more efficient. The IFG depends on, for example, how much time each user of the communication link requires between frames, also considering that for XAUI, a SOP is required to be on lane 0.
According to another exemplary embodiment, illustrated in
730 bytes/(1+1+14+4+12) bytes=22.81 frames (3)
155520 bytes/20 frames=7776 bytes per frame (4)
According to still another exemplary embodiment, the FCS 606 can be omitted resulting in the exemplary frame structure shown in
730 bytes/(1+1+14+12) bytes=26.07 frames (5)
155520 bytes/24 frames=6480 bytes per frame (6)
According to still other embodiments, it should be appreciated that protocols other than Ethernet may be used to encapsulate the GTC fragments. For example, the frame structures of
Specifying a mechanism for GTC over XAUI, or another selected protocol, according to these exemplary embodiments allows for greater scalability of PONs. Among other things, this permits separation of the control and MAC functions of the GPON standard, as described above, where the control functions associated with the optical transceiver are provided on a separate processor than that which performs the MAC functions. According to one exemplary embodiment, a method for optical communications includes the steps shown in the flowchart of
Various other modifications and details may be included according to exemplary embodiments. For example, the optical transceiver module 402 and GTC frame processing module 404, in addition to the above-described payload data protocol (e.g., XAUI) can also have other interfaces to enable them to communicate with each other (e.g., SERDES Framer Interface (SFI), low voltage differential signaling (LVDS), or the like). The GTC frame processing module 404 can also have some form of memory to buffer the frames to send via the interconnect 500, as well as an interface for configuring GTC framer software (e.g., in accordance with the Joint Test Action Group (JTAG) interface standard). The receiving mechanism used by the optical transceiver module 402 should be a burst mode mechanism, as opposed to a continuous mode mechanism, since the ONUs 118 transmit in bursts and the OLT 104 transmits in continuous mode. In addition to an optical transceiver itself, the optical transceiver modules 402 may also include electronic components such as limiting amplifiers (LIAs) and clock and data recovery (CDR) mechanisms.
Additionally, as mentioned above, although the foregoing exemplary embodiments are described primarily in the context of downstream (OLT to ONU) transmissions, they are equally applicable to upstream (ONU to OLT) transmissions. Although current specifications for upstream transmission require less bandwidth than downstream transmissions, it is possible that future PON specifications will also provide for higher bandwidth on the upstream transmissions and the corresponding challenges to avoid oversubscription of the upstream traffic. Moreover, although one of the foregoing exemplary embodiments refers to the transmission of GTC frames using Ethernet over XAUI, it will be appreciated that the present invention is not so limited. For example, other protocols could be used over XAUI which provide for the specification of a source/destination for directing frames over the interconnect. In the foregoing exemplary embodiments, a complete GTC frame is produced on a first printed circuit board and then transferred to an optical transceiver module on another printed circuit board. However, according to another exemplary embodiment, instead only part of the GTC frame is generated in the MAC module and transmitted to the optical module. For example, GTC headers and GEM headers/payload could be sent independently to optical modules, which would then assemble them into a complete GTC frame.
The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.
