Low/medium speed multi-casting device and method

#1. Field of the Invention

#This invention relates to asynchronous transfer mode (ATM) communication systems generally, and, more specifically, to multi-casting devices and processes for providing low/medium speed subscriber interfaces.

#2. Description of the Related Art

#A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which like reference symbols indicate the same or similar components, wherein:

#FIG. 1 is a block diagram of a hypothetical network drawn in an effort to illustrate the concept of point to multi-point connection as that concept is applied to a medium speed subscriber interface.

#FIG. 2 is a block diagram of low/medium speed subscriber multicasting devices of the type incorporated into the circuits detailed in FIGS. 7 and 8, for implementing broadcast calls from low/medium speed subscribers, in accordance with the principles of the present invention.

#Turning now to the drawings, FIG. 1 is a block diagram of a network illustrating the concept of a point to multi-point connection, which is called multi-casting, or broadcast calling from this point forward, as applied to a medium speed subscriber interface. A medium speed subscriber is a subscriber who connects to the system using a 45 Mbps transmission spaced. Medium speed interface module (MSIM) 4, is shown connected to three links, 8-1, 8-2, 8-3, which are individually connected to medium speed subscribers using the point to multi-point connection concept. Links 8-1, 8-2, 8-3 are connected to a port on medium speed interface module 4. The links and medium speed interface module 4 are connected using a 45 Mbps transmission speed. If medium speed interface module 4 is replaced by a low speed interface module (LSIM), seventy-two links are connected to low speed (2.048 Mbps) subscribers. Low speed subscribers are those subscribers who connect with the system using a 2.048 Mbps transmission speed. The network illustrated by FIG. 1 is divided into a lower layer block 7 with a physical layer function, shown as surrounded by a broken line in the drawing, and the remaining higher layer block with an asynchronous transfer mode layer function. Medium speed interface module 4 interfaces the higher layer block and lower layer block 7. In the higher layer block, interfacing is performed using a 155 Mbps transmission speed, medium speed interface module 4 and user interface subscriber assembler (UISA) 6 are connected to asynchronous transfer mode switch 2. UISA 6 has routing table 9 for enabling individual subscriber multicasting. Routing table R/T 9 contains virtual path identifier/virtual channel identifier (VPI/VCI) conversion values. The higher and lower layer blocks form the point to multi-point connection. Each port of lower layer block 14 within medium speed interface module 4 is connected to links 8-1, 8-2, 8-3. Each link 8-1, 8-2, 8-3 has an individual, discrete routing table R/T 10-1, 10-2, 10-3 respectively, containing VPI/VCI conversion tables that are used for multicasting. Routing tables R/T 10-1, 10-2, 10-3 are used in multicasting of each port. When medium speed interface module 4 receives a multicasting cell from asynchronous transfer mode switch 2, medium speed interface module 4 bypasses the multicasting cell to lower layer block 7. Lower layer block 7 performs virtual path/virtual channel conversion, using the port routing tables R/T 10-1, 10-2 and 10-3.

#As described above, when performing multicasting on low/medium speed subscriber modules, the links of each low/medium speed port must have routing tables that are only for multicasting . The routing tables of each port need a minimum 2.sup.11 .times.28 bits; eleven bits are used for multi-cast copy number (MCN) information, and twenty-eight bits are used for VPI/VCI information. Thus, each routing table has a thirty-two kilobyte memory requirement. As shown in FIG. 1, each link contains a thirty-two kilobyte routing table. We have noticed that this implementation of point to multipoint connection necessitates the use of multiple small memories, at a concomitant requirement of substantial board space, with the accompanying cost. Moreover, maintenance of this type of implementation incurs part supply problems. Furthermore, we have found that this type of implementation is not efficient because the number of subscriber links can not be flexibly handled.

#As shown in FIG. 7, a preferred embodiment of the present invention does not have separate routing tables 10-1 to 10-3 for each port as shown in FIG. 1, instead it has medium speed subscriber multicasting device 16 for performing a broadcast call for medium speed subscribers in lower layer block 14 within medium speed interface module 4.