Abstract:
Systems and methods for point-to-multipoint communications with CAS are provided. In one embodiment, a line unit comprises: an interface to communicate with a node using a first frame structure comprising timeslots, the frame structure transporting signaling bits in an x&#39;th timeslot; the unit coupled another line unit via a communication link, wherein the other unit communicates with another network node using a second frame structure comprising timeslots, the second frame structure transporting signaling bits via its x&#39;th timeslot. Payload timeslots for a first frame received from the other line unit are mapped from the second frame structure to payload timeslots of a second frame using the first frame structure and transmitted via the interface. The x&#39;th timeslot for each of the first and second frame structures are formatted in a multiframe structure comprising a schedule of signaling bit locations allocated to signaling bits corresponding to a designated payload timeslot.

Description:
CROSS-REFERENCE TO RELATED CASES 
     This application is a continuation of and claims benefit of application Ser. No. 11/566,493 filed on Dec. 4, 2006, entitled “POINT-TO-MULTIPOINT DATA COMMUNICATIONS WITH CHANNEL ASSOCIATED SIGNALING” (currently pending), which is incorporated herein by reference and to which this application claims priority. 
    
    
     BACKGROUND 
     Digital Subscriber Line (DSL) networks, such as G.SHDSL, allow flexible, high-speed data transmission rates over standard copper interfaces. Normally a single line unit at a central office location is connected to a single remote unit using one or two G.SHDSL pairs for increased bandwidth. A very useful enhancement to this standard configuration is point-to-multipoint, where a single line unit at a central office is connected to two remote units, using a single G.SHDSL pair to each remote unit. A point-to-multipoint network implementation provides cost advantages over point to point because it eliminates the need to install separate DSL line units at a central office for each remote unit served by the central office. Further, point-to-multipoint allows service providers to offer fractional G.703/E1 lines to customers who may not require the entire bandwidth capacity provided by a full G.703/E1 line. 
     One useful feature of a full G.703/E1 interface line is the Channel Associated Signaling (CAS) signaling bits carried in timeslot 16 of each E1 frame. CAS signaling bits use routing information to direct voice/data payloads carried by a G.703/E1 interface line to its destination. Because G.703/E1 signal channels are dedicated to single interface applications, one problem with point-to-multipoint applications in the art today is that there is no means for communicating CAS signaling bits between a central office line unit and the line units of one or more remote units having fractional G.703/E1 lines that share the central office line unit&#39;s single G.703/E1 interface. 
     For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved systems and methods for point-to-multipoint data communications with CAS signaling. 
     SUMMARY 
     The Embodiments of the present invention provide methods and systems for point-to-multipoint data communications with CAS signaling, and will be understood by reading and studying the following specification. 
     In one embodiment, a line unit comprises: a first network interface to communicate with a first network node using a first frame structure comprising a plurality of timeslots, the first frame structure transporting signaling bits in an x&#39;th timeslot reserved for signaling; the first line unit further coupled to at least one other line unit via a communication link, wherein the at least one other second line unit is configured to communicate with a second network node using a second frame structure comprising a plurality of timeslots, the second frame structure transporting signaling bits via its x&#39;th timeslot; wherein payload timeslots for a first frame received from the at least one other line unit are mapped from the second frame structure to payload timeslots of a second frame using the first frame structure and transmitted via the first network interface; wherein the x&#39;th timeslot for each of the first frame structure and the second frame structure is formatted in a multiframe structure, wherein the multiframe structure comprises a schedule of signaling bit locations each allocated to signaling bits corresponding to a designated payload timeslot; and wherein when the first line unit receives from the at least one other line unit a first set of signaling bits for a first payload timeslot within the second frame structure, based on where in the first frame structure the first payload timeslot is mapped to a second payload timeslot, the first line unit schedules retransmission of the first set of signaling bits for the x&#39;th timeslot of a frame scheduled for signaling bit locations corresponding to the second payload timeslot as scheduled according to the multiframe structure. 
    
    
     
       DRAWINGS 
       Embodiments of the present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIG. 1  is a block diagram illustrating a communication network of one embodiment of the present invention; 
         FIG. 2  is a diagram illustrating timeslot mapping for a communication network of one embodiment of the present invention; 
         FIGS. 3A and 3B  are diagrams illustrating CAS signaling bit mapping for a communication network of one embodiment of the present invention; 
         FIG. 4  is a diagram illustrating timeslot mapping onto a DSL framework for a communication network of one embodiment of the present invention; and 
         FIG. 5  is a flow chart illustrating a method of one embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present invention. Reference characters denote like elements throughout figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustrating specific embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims and equivalents thereof. 
     Embodiments of the present invention allow multiple remote units to share a single G.703/E1 interface of a central office while providing CAS signaling with the multiple remote units. 
       FIG. 1  shows a communications network  100  of one embodiment of the present invention. The network  100  includes a central office  105  in communication with a first remote unit  106  and in communication with a second remote unit  107 . The central office  105  includes a DSL line unit (STU-C)  120  that is coupled to communicate with a DSL line unit (STU-R)  126  located at the remote unit  106  over a first G.SHDSL loop  116 . DSL line unit (STU-C)  120  is also coupled to communicate with a DSL line unit (STU-R)  127  located at the remote unit  107  over a second G.SHDSL loop  117 . 
     In the embodiment shown in  FIG. 1 , STU-C  120  includes a single G.703/E1 interface  121  coupled to communicate with a network node ( 130 ) in the upstream direction through G.703/E1 communication link  122 . STU-R  126  and STU-R  127  each include a G.703/E1 interface (shown as  123  and  124 , respectively) coupled to communicate via a fractional G.703/E1 line with respective network nodes  131  and  132  in the downstream direction through G.703/E1 communication links  128  and  129 , respectively. 
     Embodiments of the present invention enable G.703/E1 timeslots and resources of STU-C  120  to be shared between the two remote STU-R&#39;s  126  and  127 . When viewed from node  130  looking into the G.703/E1 interface  121  of STU-C  120 , the G.703/E1 interface  123  of STU-R  126  and the G.703/E1 interface  124  of STU-R  127  are not individually visible, but appear as a single G.703/E1 interface. With embodiments of the present invention, each of STU-R  126 &#39;s and STU-R  127 &#39;s respective G.703/E1 interfaces are allocated a proportion of STU-C  120 &#39;s G.703/E1 interface, up to 100% of the bandwidth capacity of STU-C  120 &#39;s G.703/E1 interface. In addition, timeslot (TS) 16, the CAS signaling channel, is mapped between the G.703/E1 interfaces of STU-R  126  and STU-R  127 . 
     As would be appreciated by one of ordinary skill in the art upon reading this specification, a standard E1 frame consists of up to 32 time slots. Timeslot 0 is reserved for carrying framing information, optional cyclic redundancy check bits, and other overhead related information. Timeslot 16 is reserved to carry CAS signaling bits. Minus these two reserved time slots, a full E1 frame can carry up to 30 timeslots of user-data. Time slots used to carry user-data are hereinafter referred to as “user-data timeslots”. The 30 user-data timeslots available in a full E1 frame (i.e., timeslots 1-15 and 17 to 31 of the E1 frame) are also frequently referred to as telephone channels 1 to 30. Point-to-multipoint with CAS is accomplished by embodiments of the present invention by applying timeslot remapping and CAS signaling bits remapping. 
     As used in this specification, when a pair of timeslots are said to be “mapped to” or “mapped with” each other, it means that data received by one of the timeslot pairs via a G.703/E1 interface is communicated to the other of the pairs for transmission via another G.703/E1 interface. For example, assuming that an STU-C&#39;s timeslot 17 is mapped with an STU-R&#39;s timeslot 1, then any data received by the STU-C&#39;s E1 interface on timeslot 17 is retransmitted via the STU-R&#39;s G.703/E1 interface on timeslot 1. In the same way, any data received by the STU-R&#39;s E1 interface on timeslot 1 is retransmitted via the STU-C&#39;s G.703/E1 interface on timeslot 17. 
       FIG. 2  is a chart illustrating timeslot mapping of one embodiment of the present invention. In one embodiment,  FIG. 2  illustrates timeslot mapping for a communications network such as network  100 , where a first STU-R and a second STU-R share the bandwidth provided via an STU-C. 
     As illustrated in  FIG. 2 , an E1 frame  200  carried by STU-C includes two reserved timeslots (i.e., non-user-data timeslots) at timeslot 0 and timeslot 16 (shown at  202 ). E1 frame  200  also includes up to 30 user-data timeslots (shown at  204 ) which occupy timeslot 1 to timeslot 15 and timeslot 17 to timeslot 31, as needed. 
     Bandwidth provided by the STU-C that is allocated to a first fractional E1 frame  210  of the first STU-R is allocated the first N user-data timeslots on E1 frame  200 . As illustrated, the first N user-data timeslots of E1 frame  200  are mapped with the first N user-data timeslots of the first fractional E1 frame  210 . Timeslot 16 of the STU-C frame  200  is also mapped to timeslot 16 of the fractional E1 frame  210 , however the STU-C also further perform signaling bit remapping for timeslot 16, as described in greater detail below. 
     Bandwidth provided by the STU-C that is allocated to a second fractional E1 frame  220  of the second STU-R is allocated the next M user-data timeslots on E1 frame  200  that are appended to the N user-data timeslots already allocated to the first fractional E1 frame  210 . The M user-data timeslots allocated to the second STU-R&#39;s fractional E1 frame  220  are mapped to the first M user-data timeslots of the second fractional E1 frame  220 . Timeslot 16 of the STU-C frame  200  is also mapped to timeslot 16 of the fractional E1 frame  210 , however the STU-C also further perform signaling bit remapping for timeslot 16, as described in greater detail below. 
     To illustrate timeslot mapping more specifically, when N+M is less than or equal to 15, the timeslots of the first fractional E1 frame  210  map to timeslots 1 to N on E1 frame  200 , while timeslots of the second fractional E1 frame  220  map to timeslots N+1 to N+M on the E1 frame  200 . 
     When N is greater than 15, the timeslots of the first fractional E1 frame  210  map to timeslots 1 to 15 and 17 to N on E1 frame  200 , while timeslots of the second fractional E1 frame  220  map to timeslots N+2 to N+M+1 on the E1 frame  200 . 
     When N is less than or equal to 15, and N+M is greater than 15, the timeslots of the first fractional E1 frame  210  map to timeslots 1 to N on E1 frame  200 , while timeslots of the second fractional E1 frame  220  map to timeslots N+1 to 15, and timeslots 17 to N+M+1 on the E1 frame  200 . 
     With respect to any of the above three cases, timeslot 16 for the STU-C is always mapped to timeslot 16 of both the first and second STU-R based on the CAS signaling bit remapping described in greater detail below. 
     In addition to timeslot remapping, embodiments of the present invention provide CAS signaling bit remapping to enable the communication of signaling bits between multiple remote units and the central office. As previously mentioned, time slot 16 of a standard E1 frame is reserved to carry CAS signaling bits. 
       FIG. 3A  is a chart illustrating the location of CAS signaling bits within each timeslot 16 of frames 0 to 15 of an G.703/E1 interface line. Generally, time slot 16 across Frames 0 to 15 is structured to include up to four signaling bits (shown as A, B, C, D) to carry routing information for up to 30 user-data timeslots of an E1 frame. As illustrated in  FIG. 3A , timeslot 16 for each frame of E1 data can include signaling bits for up to two timeslots in what are referred to in this application as “CAS bit locations” within timeslot 16. The CAS signaling bits for up to the first 15 user-data timeslots are stored in CAS bit locations 1 to 15, which as shown in  FIG. 3A , occupy the first four bits of timeslot 16 in frames 1 to 15. Assuming the E1 line is configured to carry user-data in more than the first 15 user-data timeslots, CAS signaling bits for up to 15 additional user-data timeslots are stored in CAS bit locations 16 to 30, which occupy the second four bits of timeslot 16 in frames 1 to 15. 
     Shown in  FIG. 3B , embodiments of the present invention map CAS signaling between an STU-C and corresponding CAS bit locations for a first STU-R and a second STU-R, based on the STU-C timeslots mapped with the first STU-R and the second STU-R. As shown in  FIG. 3B , the concept behind mapping CAS bit locations is comparable to the mapping of user-data timeslots, as described with respect to  FIG. 2 . Assuming that the first N user-data timeslots of an STU-C G.703/E1 interface are mapped with a first fractional E1 interface of a first STU-R, the CAS signaling bits associated with those N STU-C user-data timeslots would be located in CAS bit locations 1 to N. Therefore, the first N CAS bit locations for the STU-C interface are mapped to the first N CAS bit locations for the first fractional E1 interface. Assuming that the next M user-data timeslots of the STU-C are mapped with a second fractional E1 interface of a second STU-R, the CAS signaling bits associated with those M STU-C user-data timeslots would be located in CAS bit locations N+1 to M. Therefore, the next M CAS bit locations for the STU-C E1 interface (after the first N CAS bit locations) are mapped to the first M CAS bit locations for the second fractional E1 interface. As would be appreciated by one of ordinary skill in the art upon reading this specification, time slot 16 of frame 0 is not used to communicate CAS signaling bits. 
     To further illustrate this mapping, when N+M is less than or equal to 15, the STU-C maps the CAS signaling bits for the first N user-data timeslots from the first four bits of frame 1, time slot 16 to the first four bits of the first STU-R&#39;s frame 1, time slot 16. STU-C further maps the CAS signaling bits from the first four bits of frame 2, time slot 16 of its G.703/E1 interface to the first four bits of the first STU-R&#39;s frame 2, time slot 16, and so on up through the N&#39;th frame. For the next M timeslots of user-data, STU-C maps the CAS signaling bits from the first four bits of frame N+1, time slot 16 to the first four bits of the second STU-R&#39;s frame 1, time slot 16, the first four bits of frame N+2, time slot 16 of its G.703/E1 interface to the first four bits of the second STU-R&#39;s frame 2, time slot 16, and so on up through the M&#39;th frame. 
     When N is greater than or equal to 15, timeslots allocated to the second STU-R&#39;s G.703/E1 interface include timeslots N+2 to N+M+1 on the STU-C G.703 interface. In that case, signaling bits for the first 15 user-data time slots for the STU-C interface are respectively mapped into the first four bits of time slot 16 of frames 1 to 15 of the first STU-R. Signaling bits for the next N-15 user-data time slots for the STU-C interface are respectively mapped from the second four bits of timeslot 16 of frames 1 to N-15 of the STU-C to the second four bits of time slot 16 of frames 1 to N-15 of the first STU-R. Signaling bits for the remaining M time slots of user-data for the STU-C interface are mapped from the second four bits from timeslot 16 of frames N-14 to M+N-15 of the STU-C to the first four bits of time slot 16 of frames 1 to M of the second STU-R. 
     When N is less than 15 and N+M is greater than 15, the second STU-R&#39;s G.703/E1 interface is allocated timeslots N+1 to 15, and timeslots 17 to N+M+1 on the STU-C G.703 interface. In that case, signaling bits for the first N user-data time slots for the STU-C G.703/E1 interface are respectively mapped to the first four bits of time slot 16 of frames 1 to N of the first STU-R. Signaling bits for the next 15-N time slots of user-data for the STU-C G.703/E1 interface are mapped from the first four bits of timeslot 16 of frames N+1 to 15 of the STU-C to the first four bits of time slot 16 of frames 1 to 15-N of the second STU-R. When M is less than 16, signaling bits for the remaining time slots of user-data for the STU-C G.703/E1 interface are mapped from the second four bits of timeslot 16 of frames 1 to M+N−15 of the STU-C to the first four bits of time slot 16 of frames 16−N to M of the second STU-R. When M is not less than 16, the first 15 signaling bits are mapped as described in the preceding sentence, while any remaining signaling bits are mapped to the second four bits of time slot 16 of frames 1 to M−15 of the second STU-R. 
     As taught above, a single G.703/E1 interface at an STU-C with an active CAS signaling channel can be used to create two fractional G.703/E1 interfaces at a first STU-R and a second STU-R, each with a dedicated CAS signaling channel  FIG. 4  is a diagram (shown generally at  400 ) illustrating timeslot mapping onto DSL communications links used to carry a fractional G.703/E1 line between an STU-C and an STU-R of one embodiment of the present invention. A DSL frame structure (shown generally at  420 ) comprises a frame sync data block  422  followed by a plurality of alternating overhead data blocks  424  and payload blocks  426  until the end of the DSL frame  420  (indicated by the Stb block at  428 ). Each payload block  426  further comprises 12 sub-blocks (shown generally at  430 ) each carrying one frame of G.703/E1 timeslots (shown generally at  410 ). Assuming each frame carries N user-data timeslots (shown at  412 ) within each sub-block  430 , the N user-data timeslots  412  are rearranged so that the CAS signaling bits carried in timeslot 16 occupy the last timeslot (shown at  414 ) of the payload block  426 . 
       FIG. 5  is a flow chart illustrating a method for providing point-to-multipoint communication with CAS signaling. The method begins at  510  with mapping a first set of user-data timeslots of a first G.703/E1 interface to timeslots of a first fractional G.703/E1 interface, wherein the first set of user-data timeslots includes a first N user-data timeslots of the first G.703/E1 interface. Then method proceeds to  520  with mapping a second set of user-data timeslots of the first G.703/E1 interface to timeslots of a second fractional G.703/E1 interface, wherein the second set of user-data timeslots includes a next M user-data timeslots of the first G.703/E1 interface after the first N user-data timeslots. 
     As previously discussed, a G.703/E1 frame of data includes up to a total of 32 timeslots, wherein timeslots 0 and 16 are reserved. This leaves timeslots 1 to 15 and 17 to 31 of an G.703/E1 interface available for use to carry user-data. Timeslots 1 to 15 and 17 to 31 are therefore referred to in this specification as the “user-data timeslots”. The 30 user-data timeslots available in a full E1 frame are also frequently referred to as telephone channels 1 to 30. Mapping the first N user-data timeslots to a first fractional G.703/E1 interface means that user-data carried in the first N user-data timeslots of the first G.703/E1 interface is carried in the N timeslots of the first fractional G.703/E1 data link Similarly, the next M user-data timeslots of the first G.703/E1 interface are carried in the M timeslots of the second fractional G.703/E1 interface 
     In one embodiment, when N is less than 16, the method maps timeslots 1 to N of the first fractional G.703/E1 interface with respective timeslots 1 to N of the first G.703/E1 interface. In an alternate embodiment, when N is greater than 16, the method maps user-data timeslots 1 to N of the first fractional G.703/E1 interface with respective timeslots 1 to 15 and 17 to N+1 of the first G.703/E1 interface. User-data is not mapped to or from timeslot 16 because timeslot 16 is reserved for carrying CAS signaling bits rather than user-data. 
     In one embodiment, when N+M is less than 16, the method maps timeslots 1 to M of the second fractional G.703/E1 interface with respective timeslots N+1 to N+M of the first G.703/E1 interface. In an alternate embodiment, when N is greater than 15, the method maps timeslots 1 to M of the second fractional G.703/E1 interface with respective timeslots N+2 to N+M+1 of the first G.703/E1 interface. In one embodiment, when N is less than 15 and N+M is greater than 15, the method maps the first M user-data timeslots of the second fractional G.703/E1 interface with respective timeslots N+1 to 15 and 17 to N+M+1 of the first G.703/E1 interface. Again, user-data is not mapped to or from timeslot 16. 
     To provide the first and second fractional G.703/E1 interfaces each with a dedicated CAS signaling channel, the method further maps CAS signaling bits between timeslot 16 of frames 1-15 of the first 0.703/E1 interface and timeslot 16 of frames 1-15 for the first and second fractional 0.703/E1 interfaces. The method thus proceeds to  530  with mapping a first set of signaling bits associated with the first set of user-data timeslots with the first fractional G.703/E1 interface. The method further proceeds to  540  with mapping a second set of signaling bits associated with the second set of user-data timeslots with the second fractional G.703/E1 interface. 
     In one embodiment, CAS bit locations 1 to N of the first G.703/E1 interface are mapped to the first N CAS bit locations for the first fractional G.703/E1 interface. Similarly, to map the CAS signaling bits of the next M user-data timeslots from the first G.703/E1 interface to the second fractional G.703/E1 interface, CAS bit locations N+1 to M of the first G.703/E1 interface are mapped to the first M CAS bit locations for the second fractional G.703/E1 interface. 
     Although embodiments described in this specification discuss the present invention in terms of network interfaces that utilize G.703/E1 standards, one of ordinary skill in the art would appreciate that embodiments of the present invention are not so limited, but also apply to other network interface standards. 
     Several means are available to implement the systems and methods of the current invention as discussed in this specification. These means include, but are not limited to, digital computer systems, microprocessors, programmable controllers and field programmable gate arrays. Therefore other embodiments of the present invention are program instructions resident on computer readable media which when implemented by such controllers, enable the controllers to implement embodiments of the present invention. Computer readable media include any form of computer memory, including but not limited to punch cards, magnetic disk or tape, any optical data storage system, flash read only memory (ROM), non-volatile ROM, programmable ROM (PROM), erasable-programmable ROM (E-PROM), random access memory (RAM), or any other form of permanent, semi-permanent, or temporary memory storage system or device. Program instructions include, but are not limited to computer-executable instructions executed by computer system processors and hardware description languages such as Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL). 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.