WO1997018649A1

WO1997018649A1 - Method and apparatus for multiplexing tdm and atm signals over a communications link

Info

Publication number: WO1997018649A1
Application number: PCT/US1996/018626
Authority: WO
Inventors: Farzad S. Nabavi; Thomas R. Eames
Original assignee: Dsc Communications Corporation
Priority date: 1995-11-14
Filing date: 1996-11-14
Publication date: 1997-05-22
Also published as: CA2237645A1; GB2322052A; AU1021297A; GB9810365D0

Abstract

A method and apparatus are disclosed for communicating between devices in a network. The invention allows the combination of TDM and ATM signals in a frame for communication over a communications link. In accordance with the method of the invention, a frame is formed in a first device. The frame comprises a predetermined number of bytes. Some of the bytes carry time division multiplexed time slot data while other bytes carry asynchronous transfer mode cell data. The frame is sent to a second device over a communications link.

Description

METHOD AND APPARATUS FOR MULTIPLEXING TDM AND ATM SIGNALS OVER A COMMUNICATIONS LINK

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to communication systems and more particularly to a method and apparatus for multiplexing TDM and ATM signals for transport over a communications link.

BACKGROUND OF THE INVENTION

Traditionally, separate networks have existed for different types of communications services such as telephone services and cable television services. In the future, both telephony signals and video signals will likely be carried over a single network. Video signals will be carried m a digital form and be switched within the network which will allow provision of such services as video on demand. Such a system is sometimes referred to as a switched digital video system.

The objective of a switched digital video system is to deliver both telephony and video signals to the home. To achieve this objective, telephony and video signals must first be carried over some type of communications link such as a fiber optic communications link. One of the main architectural considerations for such a system is the transport of video and telephony signals between devices in a communications network. The most common format for carrying telephony signals is a time division multiplexed (TDM) format. Digital video signals normally are carried m asynchronous transfer mode (ATM) format. Because ATM signals and TDM signals are inherently different, methods must be developed to carry telephony and digital video information over the same communications link.

SUMMARY OF THE INVENTION

The method and apparatus of the present invention allow ATM and TDM signals to be carried together over the same communications link. One aspect of the invention is a method of communicating between devices m a network. A frame is formed m a first device comprising a predetermined number of bytes. Some of the bytes are used to carry time division multiplexed time slot data while other bytes are used to carry asynchronous transfer mode cell data. This frame is then sent to a second device over a communications link.

The invention has several important technical advantages. Because the invention allows commanication of TDM and ATM signals m their native formats over a communications link, the invention saves the processing time and hardware necessary to convert data fram one format to another. This advantage also reduces the cost of devices in a communications network due to decreased hardware complexity. The invention also provides an optimized interface to a time slot mterchanger for TDM data and to an ATM multiplexer for ATM data. This maintains good performance of TDM signals in terms of delay and litter. The invention makes efficient use of the available bandwidth in a communications link. The a oun: of bandwidth used for TDM signals is flexible based upon the characteristics of the communication system. The bandwidth of the communications link devoted to TDM signals can be dynamically allocated, thus making efficient use of the available bandwidth.

The invention also allows use of the synchronous optical network (SONET) standards. The synchronous payload envelope of a SONET signal can be used as a frame for the TDM and ATM data. Use of SONET allows the invention to take advantage of the single-ended monitoring and maintenance capabilities of SONET. These capabilities allow for easy communications link performance monitoring and maintenance. In addition, because SONET is a standard interface, integrated circuits and other components are readily available at a low price to accomplish the physical signal transmission and receiving. Also, test equipment for SONET has been developed and can be used to diagnose and verify the performance of the communications link carrying the TDM and ATM signals.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken m conjunction with the accompanying drawings in which:

FIGURE 1 illustrates a block diagram of a fiber to the curb switched digital video communications network within which the present invention may be used; FIGURE 2 illustrates a block diagram of the connection between a host digital terminal and an optical network unit in the switched digital video system of FIGURE 1;

FIGURE 3 illustrates an example SONET synchronous payload envelope containing TDM and ATM signals and constructed in accordance with the teachings of the invention; 18649 PCI /US96/18626

FIGURE 4 illustrates a particular TDM frame format that can be combined with ATM signals m accordance with the invention; and

FIGURE 5 illustrates a SONET synchronous payload envelope containing both TDM signals extracted from the frame of FIGURE 4 and ATM signals.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present irvention and its advantages are best understood by referring to

FIGURES 1-5 of the drawings, like numerals beirg used for like and corresponding parts of the various drawings.

FIGURE 1 illustrates an example of a switched digital video communications network 10 that may use the present invention. Switched digital video communications network 10 includes a fiber-to-the-curb access system comprising a host digital terminal 12, a plurality of optical network units 14, a plurality of network terminations 16 and a plurality of network interface units 18 located inside a plurality of set-top terminal units 40. The fiber-to-the-curb access system connects to the public switched telephone network 20 through one or more connections between host digital terminal 12 and TDM switch 22. In this example, TDM switch 22 comprises a class 5 switch such as a 5ESS or DMS-100. The switched digital video access system connects to ATM network 24 m two ways. Switched ATM services such as on-demand video are provided by ATM network 24 to host digital terminal 12 through ATM switch 26. Also, digital broadcast video signals in ATM format are provided by a direct connection between ATM network 24 and host digital terminal 12.

Host digital terminal 12 combines the ATM and TDM signals it receives and sends the appropriate combination of signals to each optical network unit 14 over a fiber optic link 28. For each fiber optic link 28, the transmission and receipt of the signal is handled by an HDT optical line unit 30 (located in host digital terminal 12) and an ONU optical line unit 32 (located m optical network unit 14) .

From the ONU, signals are transmitted to a home or a business over communications link 34. Communications link 34 may be any type of communications link such as a fiber optic link, twisted pair cable, or coaxial cable. In this example, communications link 34 is a twisted pair cable and connects optical network unit 14 to network termination 16.

Network termination 16 sends the signal received over communications link 34 to one or more devices connected to it. In this example, a telephone 36 and a television 38 is connected to network termination 16. Network interface unit 18 and set-top terminal unit 40 translate the signal received from ONU 14 into a proper format for television 38. Host digital terminal 12 is usually located in a telephone company central office. It may also be located in the outside plant away from the central office. In operation, host digital terminal 12 receives on demand video signals from ATM switch 26, broadcast digital video signals from ATM network 24 and telephony signals from

TDM switch 22. Host digital terminal 12 multiplexes the telephony (TDM) and video (ATM) signals and places them on the same fiber optic link 28 for transport to an optical network unit 14. An optical network unit 14 is usually located m a neighborhood, commonly on a curb side close to homes or businesses receiving telephony and video services. A typical optical network unit 14 will serve between 8 and 24 homes. In this example, the telephony and video signals are demultiplexed in optical network unt 14 and carried m different frequency bands over comm_mιcations link 34 to network termination 16. Alternatively, network termination 16, network interface unit 18, or set-top terminal unit 40 could demultiplex the telephony and video signals.

Network termination 16 may serve as the demarcation point between the telephone company and a residence or small business. In this example, network termination 16 performs limited functions such as placing video signals arriving over communications link 34 on the in-home distribution system so that they are sent to network interface unit 18. Network termination 16 also places telephone signals received over communications link 34 on the m-house telephone connections to telephone 36.

Network interface unit 18 is part of the set top terminal unit 40. Network interface unit 18 serves as the interface module between set top terminal unit 40 and network termination 16. It allows set top terminal unit 40 to be able to transmit and receive video signals from ATM network 24 and allows communication with ATM network 24.

FIGURE 2 illustrates a block diagram of the connection between host digital terminal 12 and optical network unit 14 in the switched digital video system of Figure 1. In this example, fiber optic link 28 uses the SONET standard and carries an STS-3c signal. This signal has a bit rate of 155.52 megabits per second.

Host digital terminal 12 comprises ATM cell multiplexer 42, control circuitry 44, time slot mterchanger 46, and HDT optical line unit 30. Control circuitry 44 connects to ATM cell multiplexer 42, time slot mterchanger 46, and HDT optical line unit 30 and controls each of these circuits. Telephony signals are received from and transmitted to a TDM switch through time slot mterchanger 46. In this example, the signals received from the TDM switch by time slot mterchanger 46 are placed in an internal host digital terminal system signal format known as Subscriber Bus Interface. Each Subscriber Bus Interface signal comprises a 2.048 Mb/s signal and carries the user information, signaling, and internal system controls for twenty-four DSOs or one VT1.5 signal. The Subscriber Bus Interface signals from the time slot mterchanger 46 are made in groups of up to eight and sent to the HDT optical line unit 30. Thus, time slot mterchanger 46 both transmits TDM data to and receives TDM data from the TDM switch as well as to and from HDT optical line unit 30. At HDT optical line unit 30, these TDM signals are multiplexed with ATM signals.

ATM video traffic is both transmitted to and received from one or more ATM switches through ATM cell multiplexer 42. ATM cell multiplexer 42 also serves as an ATM cell demultiplexer. ATM cell multiplexer 42 transmits and receives ATM cells to and from HDT optical line unit 30. In this example, the cells for a particular optical network unit 14 are passed to HDT optical line unit 30 with a maximum cell rate of ECR-3, which is equal to 149.76 Mb/s. Other cell rates could also be used. ATM cell multiplexer 42 communicates with HDT optical line unit 30 using an ATM interface that uses the UTOPIA interface protocol.

HDT optical line unit 30 comprises transmission convergence circuit 50, multiplexer and formatter 52 and physical media circuitry 54. Transmission convergence circuit 50 provides cell header error control, identifies cell boundaries (cell delineation function) , and provides cell scrambling and descrambling. It addition, it divides ATM cells into individual bytes for multiplexing with TDM data and also combines individual bytes back into ATM cells when data is received from an optical network unit 14. Multiplexer and formatter 52 builds frames for transmission over fiber optic link 28 and multiplexes TDM and ATM signals together using the methods of the invention described more fully connection with FIGURES 3-5 below. When receiving data from an optical network unit 14, multiplexer and formatter 52 separates the incoming SONET payload into separate ATM and TDM signals. Physical media circuitry 54 serves as a physical interface between multiplexer and formatter 52 and fiber optic link 28. It converts electrical signals to optical signals and optical signals to electrical signals.

Each optical network unit 14, as illustrated, has similar circuitry. Here, however, because the ATM and TDM signals are demultiplexed by optical network unit 14, they must then be frequency multiplexed by frequency multiplexer circuitry 56 for communication with network termination 16. It should be understood that each optical network unit 14 may be connected to one or more network terminations 16.

FIGURE 3 illustrates a data frame for carrying both TDM and ATM signals over a communications link. In FIGURE 3, this frame comprises a SONET synchronous payload envelope 58 having an STS-3c format. Payload envelope 58 comprises nme rows and 261 columns of byte slots wherem each byte slot of the payload envelope represents one byte. The first column of payload envelope 58 comprises path overhead data, while the remaining 260 columns of payload envelope 58 co-nprises ATM and TDM signals being sent over a communications link. Payload envelope 58 carries both TDM and ATM signals their native formats. Some of the bytes m payload envelope 58 are used for TDM time slot data while other bytes m payload envelope 58 are used for asynchronous transfer mode cell data. Each byte of payload envelope 58 may be allocated to carry TDM signals or ATM signals on a byte by byte basis.

Allocation of particular bytes of payload envelope 58 is controlled by provisioning of the communications network. For example, when a telephone is connected to switched digital video communications network 10, a TDM time slot is assigned to that telephone. When this TDM time slot is assigned to the telephone, a TDM time slot s also reserved m synchronous payload envelope 58 for that telephone. Because these time slots are assigned when the telephone line is first activated, multiplexer and formatter 52 may receive signals from control circuitry 44 indicating which bytes of payload envelope 58 contain TDM time slot data and which bytes contain ATM cell data. Those bytes that are not provisioned for TDM time slot data are used for ATM cell data.

The invention thus allows dynamic provisioning of payload envelope 58. A byte of payload envelope 58 is assigned for TDM time slot data only when a TDM device, such as a telephone or ISDN device, requires a time slot. Otherwise, the bytes m the payload envelope may be used for ATM cell data. Therefore, the invention makes efficient use of bandwidth and dynamically allocates bandwidth between TDM devices and ATM devices as that bandwidth is needed. TDM traffic is given priority and a byte is provisioned in payload envelope 58 for each TDM device. Thus, the invention does not require controlling the rate of flow of TDM time slot data. However, because the number of bytes available to ATM traffic m synchronous payload envelope may vary, the flow of ATM cells should preferably be controlled. The UTOPIA interface protocol allows control of the flow rate of cells between the ATM cell multiplexer 42 and HDT optical

Referring again to the example payload envelope 58 illustrated FIGURE 3, the path overhead byte H4 can be used as an indicator for ultiframe. The use of multiframe should not affect allocation of synchronous payload envelope 58 in accordance with the invention. Payload envelope 58 has M bytes allocated for TDM time slot data. These bytes are numbered TS1, TS2, TS3 TSM-2, TSM-1, TSM. Control circuitry 44 keeps track of the provisioning of switched digital video communications network 10 and provides control information to optical line unit 48 so that optical line unit 48 can determine which bytes m payload envelope 58 have been provisioned to carry TDM time slot data. FIGURE 4 illustrates a specific time division multiplexed signal 60 that can be carried in a SONET synchronous payload envelope using the teachings of the invention. Time division multiplexed signal 60 comprises eight Subscriber Bus Interfaces (SBI0-SBI7) . Each Subscriber Bus Interface comprises 32 DS0 time slots. With a 125 microsecond frame, all 32 time slots from eight Subscriber Bus Interfaces are carried over a communications link. A one millisecond superframe comprises eight of these 125 microsecond frames. Depending upon the provisioning of switched digital video communications network 10, one or more of the time slots withm t me division multiplexed signal 60 may not be used.

FIGURE 5 illustrates a SONET synchronous oayload envelope 62. Payload envelope 62 has been constructed m accordance with the mvention to carry the time division multiplexed signal 60 of FIGURE 4 along with ATM cell data. In the embodiment illustrated FIGURE 5, it is assumed that all time slots of time division multiplexed signal 60 are needed and that these time slots have all been provisioned to devices connected to switched digital video communications network 10. Thus, a byte withm payload envelope 62 has been provisioned for each time slot m time division multiplexed signal 60 to carry the corresponding time slot data.

In this example, time division multiplexed signal 60 is mapped mto an STS-3c synchronous payload envelope. Time slot data is conveniently arranged withm payload envelope 62 according to the following method. Each small box withm payload envelope 62 illustrated in

FIGURE 5 and labeled "TS " represents a byte allocated to TDM time slot data. The remainder of the bytes of payload envelope 62 are allocated to ATM cell data.

A byte m the Nth column where 1 < N < 8 and in of the first row of payload envelope 62 is used to carry time slot 0 (TS0) of Subscriber Bus Interface 0. The followmg seven bytes of the first row of payload envelope 62 are used to carry ATM cell data. Followmg the ATM cell data, the next byte is used to carry time slot 0 of Subscriber Bus Interface 1. This process repeats itself until time slot 0 of Subscriber Bus Interface 7 is carried by the byte column N+56 of row 1.

Followmg the allocation of this byte to time slot 0 of Subscriber Bus Interface 7, seven bytes are again allocated to ATM cells. Then, the next byte row 1 will be used to carry time slot 1 of Subscriber Bus Interface 0. Following the completion of the first row, no bytes will be allocated to TDM until the Nth column of 12 the second row. This process continues until the last TDM time slot (time slot 31 of Subscriber Bus Interface 7) is carried in byte N+248 of row 8 of payload envelope 62 as illustrated in FIGURE 5. The remainder of row 8 of payload envelope 62 and the entire row 9 of payload envelope 62 is used for ATM cell data.

Transmission convergence circuit 50 handles mapping of ATM cells mto payload envelope 62. Transmission convergence circuit 50 transmits and receives ATM cells to and from the cell multiplexer over an ATM interface using the UTOPIA interface protocol. Transmission convergence circuit 50 performs HEC generation/verification to detect/correct byte errors in the ATM cells. It also provides ATM cell data, byte by byte, to multiplexer and formatter 52 which builds payload envelope 62. Cell boundaries are detected using the cell delineation function.

It should be understood that the invention s not limited to the illustrated structures and that a number of substitutions can be made without departing from the scope and teachings of the present invention. For example, TDM signal 60 in FIGURE 4 can be mapped mto a SONET OC-3c synchronous payload envelope by the followmg alternate method. A byte m the Nth column of the first row of the OC-3c synchronous payload envelope is allocated to TDM time slot data. The following eight bytes are then allocated to ATM cell data. Next, another smgle byte is allocated to TDM time slot data, followed by eight more bytes allocated to ATM cell data. This process of allocating one byte for TDM time slot data followed by eight bytes for ATM data continues until all TDM time slot data m TDM signal 60 are mapped mto the SONET synchronous payload envelope. All remaining bytes m the SONET synchronous payload envelope are allocated to ATM cell data. This alternate method of mapping TDM and ATM signals mto a SONET synchronous payload envelope is only additional method that could be chosen. Other methods can also be used in accordance with the mvention.

Although frames are sent over fiber optic link 28 using a SONET frame format, any other frame format could be used. Also, the invention has been illustrated in the context of a communication system having telephony TDM signals and video ATM signals. Other types of TDM signals and ATM signals could be sent over a communications link using the invention. For example, Integrated Services Digital Network signals (ISDN signals) may be sent in a time division multiplexed format. Similarly, data from computer networks may be sent an ATM format. Also, voice signals can be sent as ATM adapted voice signals. The invention can be used to combine any type of ATM signals with TDM signals for transmission over a communications link. The mvention has been illustrated the context of a specific switched digital video communications network 10. The invention could be used, however, for transmission of ATM signals and TDM signals over a communications link between two devices any type of communications network. Moreover, the network does not necessarily have to be a communications network. The invention could be used with any type of network.

The invention has been described in the context of a general and more specific example of how TDM signals can be mapped mto a payload envelope for transmission along with ATM cell data. Any type of mapping could be used. The illustrated examples used an STS-3c synchronous payload envelope format. Other formats and other bit rates could also be used without departing froir the scope and teachings of the present invention.

Although the present invention has been described m detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the mvention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of communicating between devices m a network, comprising: formmg a frame in a first device, the frame comprising a predetermmed number of bytes; using some of the bytes to carry time division multiplexed time slot data; using some of the bytes to carry asynchronous transfer mode cell data; and sending the frame to a second device over a communications link.

2. The method of Claim 1 wherein the time division multiplexed time slot data comprises data from telephony signals .

3. The method of Claim 1 wherein the time division multiplexed time slot data comprises data from integrated services digital network signals.

4. The method of Claim 1 wherem the asynchronous transfer mode cell data comprises data from video signals.

5. The method of Claim 1 wherein the asynchronous transfer mode cell data comprises computer data.

6. The method of Claim 1 wherem the number of bytes used carry time division multiplexed time slot data can be allocated based upon provisioning of the network. 18649 PCI US96/18626

1 6

7. The method of Claim 1 wherein the frame comprises a synchronous optical network synchronous payload envelope.

8. The method of Claim 1, further comprising: using some of the bytes to carry overhead data.

9. The method of Claim 1 wherein the first device comprises a host digital terminal and the secord device comprises an optical network unit.

10. The method of Claim 1 wherein the communications link comprises a fiber optic link.

11. The method of Claim 10 wherein the frame comprises a synchronous optical network synchronous payload envelope; and wherein the method further comprises using some of the bytes to carry overhead data.

12. The method of Claim 11 wherein the number of bytes used to carry time division multiplexed time slot data can be allocated based upon provisioning of the network.

13. The method of Claim 12 wherem the TDM time slot data comprises data from telephony signals; and wherem the asynchronous transfer mode cell data comprises data from video signals.

14. The method of Claim 13 wherein the first device comprises a host digital terminal and the second device comprises an optical network unit.

15. The method of Claim 1 wherein the asynchronous transfer mode cell data comprises ATM adapted voice signals .

18

16. A method of communicating between devices m a network, comprising: formmg a frame in a first device, the frame comprising a predetermmed number of bytes; using some of the bytes to carry time div,sion multiplexed time slot data; usmg some of the bytes to carry asynchronous transfer mode cell data; and sending the frame to a second device over a communications link.

17. The method of Claim 16 wherein the time division multiplexed time slot data comprises data from telephony signals.

18. The method of Claim 16 wherem the time division multiplexed time slot data comprises data from integrated services digital network signals.

19. The method of Claim 16 wherein the asynchronous transfer mode cell data comprises data from video signals .

20. The method of Claim 16 wherein the asynchronous transfer mode cell data comprises computer data .

21. The method of Claim 16 wherein the number of bytes used carry time division multiplexed time slot data can be allocated based upon provisioning of the network.

22. The method of Claim 16 wherein the frame comprises a synchronous optical network synchronous payload envelope.

23. The method of Claim 16, further comprising: using some of the bytes to carry overhead data.

24. The method of Claim 16 wherein the first device comprises a host digital terminal and the second device comprises an optical network unit.

25. The method of Claim 16 wherein the communications link comprises a fiber optic link.

26. The method of Claim 25 wherein the frame comprises a synchronous optical network synchronous payload envelope; and wherein the method further comprises using some of the bytes to carry overhead data.

27. The method of Claim 26 wherein the number of bytes used to carry time division multiplexed time slot data can be allocated based upon provisioning of the network.

28. The method of Claim 27 wherem the TDM time slot data comprises data from telephony signals; and wherem the asynchronous transfer mode cell data comprises data from video signals.

29. The method of Claim 28 wherein the first device comprises a host digital terminal and the second device comprises an optical network unit.

30. The method of Claim 16 wherein the asynchronous transfer mode cell data comprises ATM adapted voice signals .

31. A communications system, comprising: an asynchronous transfer mode signal source; a time division multiplexed signal source; a multiplexer coupled to the asynchronous transfer mode and time division multiplexed signal sources and operable to form a frame comprising a predetermmed number of bytes where some of the bytes are used to carry time division multiplexed time slot data received from the time division multiplexed signal source and wherein some of the bytes are used to carry asynchronous transfer mode cell data received from the asynchronous transfer mode signal source.

32. The communications system of Claim 31 wherein the asynchronous transfer mode signal source further comprises a source of video signals in asynchronous transfer mode format.

33. The communications system of Claim 31 wherein the time division multiplexed signal source further comprises a source of telephony signals time division multiplexed format.

34. The method of Claim 31 wherein the number of bytes used to carry time division multiplexed time slot data can be allocated based upon provisioning of the network.

35. The communications system of Claim 31 where the frame comprises a synchronous optical network synchronous payload envelope.

36. The communications system of Claim 31 wherein some of the bytes are used to carry overhead data.

37. The communications system of Claim 31 wherein the multiplexer forms a part of a host digital terminal.

38. The communications system of Claim 31 wherein the multiplexer forms a part of an optical network unit.

39. The communications system of Claim 31, further comprising: a fiber optic link connected to the multiplexer and operable to carry the frame to a communications device.

40. The communications system of Claim 31 wherein the frame comprises a synchronous optical network synchronous payload envelope and wherein some of the bytes are used to carry overhead data.

41. The communications system of Claim 40 where the number of bytes used to carry time division multiplexed time slot data can be allocated based upon provisioning of the network.

42. The communications system of Claim 41 where the asynchronous transfer mode signal source further comprises a source of video signals asynchronous transfer mode format and wherein the time division multiplexed signal source further comprises a source of telephony signals m time division multiplexed format.