Bit robbing ATM channels

1. Field of the Invention

The present invention is directed to a method and apparatus for robbing bits in an ATM cell stream, and more particularly, to inserting information into the robbed bits to form a new robbed bit stream.

2. Description of the Related Art

A standard format for digital video transmission has not yet been adopted. A standard for ATM access has also not yet been adopted. In the United States, a 6 MHz channel plan for National Television Standards Committee (NTSC) video broadcast has been the accepted standard. The desirability of incorporating other information streams, e.g., teletext, into the digital video stream has been recognized, but has not been realized. However, whatever the standards to be adopted in the future, maximum use of an ATM cell will be required.

ATM access makes use of ATM cells to transmit information from the headend of a system to, for example, individual homes in a serving area. The ATM standard provides a fifty-three (53) byte cell including a payload of 48 bytes and a five byte header. The five byte header includes one byte for header error control (HEC), a status half-byte, including payload type bits (PT) and cell loss priority bits (CLP). The remaining three and one-half (3½) bytes are apportioned in two different ways depending upon where the signals are in the overall network. These two interfaces are known as a user to network interface (UNI) and a network to network interface (NNI). According to the ATM standard, all 3½ bytes (28 bits) are used for addresses at the NNI while according to the ATM standard 3 bytes (24 bits) are used for addresses at the UNI and the remaining half-byte is reserved for generic flow control (GFC). However, in an access environment, only 10-12 bits are needed for addresses. The remaining 12 to 18 bits need not be used for ATM addressing.

The present invention provides a method and system for robbing bits from a header in an ATM cell. An ATM switch at an office assigns and places information into the header portion of an ATM cell stream. Not all of the bits in the header portion of the ATM cell stream are used. A circuit “robs” the unused bits from the header portion and then inserts information into the unused bits in the header portion. A terminal unit at a subscriber end is provided for extracting the information from the unused bits and inputting the information to various information appliances.

The above-mentioned features and advantages of the present invention are achieved by employing a method providing an ATM cell stream, identifying unused portions of the ATM cell stream, robbing bits from the unused portions of the ATM cell stream to form a robbed bit stream, inserting information into the robbed bits, and extracting the information from the robbed bit stream at a subscriber end.

In addition, a system is provided including an ATM switch for outputting an ATM cell stream, a unit for robbing bits from the cell stream, a unit for inserting information into the robbed bits of the cell stream and a unit for extracting the inserted information.

The above features provide the advantages that additional information can be carried, such as, for example, a program guide or an additional broadcast video channel.

These objects, together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully described and claimed, reference being had to the accompanying drawings forming a part hereof, like reference numerals referring to like parts throughout.

FIG. 1 is a diagram of a bit robbing architecture; and

FIG. 2 is a diagram of a system using the bit robbing architecture shown in FIG. 1 .

In general, ATM specifies packet format and routing information. In some access networks, all information in an ATM cell stream 20 , including a payload 22 and a header 24 , is sent to all subscribers on a network. Each terminal filters the packets out of the ATM cell stream 20 and passes them to a specific subscriber based on the addressing in the header 24 . As noted above, there are unused bits in the header address. The present invention makes use of the unused bits in the header address.

As shown in FIG. 1, the present invention robs unused address bits 26 from the ATM cell stream 20 . These robbed bits 26 then make up a new robbed bit stream (channel) 28 . Because bits from the address are robbed and present ATM standards are violated, standard ATM addressing cannot be easily applied within the robbed bit stream 28 . Therefore, the present invention is useful in applications in which every subscriber in the system receives the ATM cell stream 20 and uses the information transmitted in the robbed bit stream 28 . Such an application would be, for example, a program guide or an entire additional broadcast video channel depending on the number of the unused bits. For example, if the ATM cell stream 20 is transmitted at 30 Mb/s, then the robbed bit stream 28 could be as much as 1.27 Mb/s.

FIG. 2 is a diagram of a system using the present invention. An ATM switch 30 outputs the ATM cell stream 20 including the payload 22 and header 24 . The ATM cell stream 20 , including header error control signals (HEC), is then input to a “robbing” circuit 32 which operates under conventional conditions. The robbing circuit 32 overwrites the unused (robbed) bits 26 of the ATM cell stream 20 which will form the robbed bit stream 28 . For example, of the 3 bytes of address, 10 bits are used for addressing, and the remaining 14 bits are overwritten with additional information to form a modified bit stream 34 . The modified bit stream 34 , which includes the overwritten bits and the “normal” bits, is then sent to, for example, individual homes 36 in a serving area. In or near the individual homes 36 , an extractor circuit 38 , which is, for example, a conventional digital circuit, is provided which receives the modified bit stream 34 , extracts the information in the overwritten bits to form a robbed bit stream 28 , replaces the overwritten bits in the modified bit stream 34 with, for example, all zeroes, calculates a new HEC, passes the extracted information in the robbed bit stream 28 to information appliances 40 within the individual homes 36 , and passes the ATM cell stream 20 to an ATM switch 42 to appropriate devices, such as, as shown in FIG. 2, selected information appliances 40 . The information appliances 40 can be, for example, a computer, TV, Web TV, etc., but are not limited to these devices.

Thus, the present invention can carry an additional video channel or other information, albeit in violation of protocol layering, and other formal tools which are used in the development of protocols,. only over the link between the headend and subscriber terminal. That is, the present invention only violates protocol over one link in the system, it does not corrupt the data stream.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications and equivalents may resorted to, falling within the scope of the invention and the appended claims and their equivalents.