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Time Division Multiplexed Insertion Ring Disclosure Number: IPCOM000036394D
Original Publication Date: 1989-Sep-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 32K

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This article describes an insertion ring device which provides a flexible interconnection mechanism in PBX and bandwidth manager applications. (Image Omitted)

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Time Division Multiplexed Insertion Ring

This article describes an insertion ring device which provides a flexible interconnection mechanism in PBX and bandwidth manager applications.

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Central office switches, PBXs, bandwidth managers, and other circuit switched systems have typically used one of two techniques or some combination of the two for controlling connectivity among large numbers of lines. These are time division multiplexed (TDM) busses or space division or cross- point switches. This disclosure describes a third technique for efficient circuit- switched systems, which avoids some of the problems of the other approaches.

Fig. 1 illustrates the topology of the TDM ring. Shown are multiple shelves of attachments, each with its own private TDM bus. The local TDM busses do not need to be identical in order to be connected via a common TDM ring, although design of interface adapter cards could be complicated if multiple local bus types coexist. In its simplest form, each shelf-bus is clocked at twice the speed of the external ring. This allows two adjacent time slots on the shelf bus to be used together (one for transmitting data and the other for receiving data from the same channel) to provide a full-duplex connection which maps into a single time slot on the global ring. This is possible because while data is flowing from shelf 1 to shelf 3 on one half of the ring, the other half of the ring can be used for the return path.

The key to making the ring work in a TDM environment is the propagation delay of the ring. To illustrate this point, Fig. 2 shows a simplified example of the TDM Ring, which is limited to four shelves and a ring with thirty-one time slots. It is further assumed that the delay from one shelf to the next shelf in the ring is the same duration as required to pass four time slots. This delay is basically the time required to get data from a local shelf bus onto the ring, and

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transfer the data from the ring to the next local shelf bus. This delay may consist of pipeline delay stages in both the transmitting and the receiving shelf interface circuits, as well as some propagation delay across the link. In fact, it is not critical how big this delay is, as long as the total of all such delays in the entire ring is equal to the time allocated for one frame. Because of the requirement to sample voice lines at 8 KHz in most PBX systems, 125 usec (1/8000) is usually chosen as the frame size. In the example of Fig. 2, shelf 1 is considered as a reference shelf, with time slots numbered 1 through 32. However, time slots on the second shelf are not numbered the same way. In fact, shelf 2 time slot numbers have been skewed by exactly the amount of time which is required to propagate data from shelf 1 to shelf 2. Time slots on each additional shelf likewise are skewed by the amount of time required for data to propagate there, so that as data propagates from a specific time slot in the fi...