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Star-Star Architecture Fourth-Generation PBX.

For a wide range of business users, the much anticipated "fourth generation" of PBX technology exists today; not on paper, but in proven installations around the world.

This type of fourth-generation system has what might be called a star/star architecture (see Figure 1) in contrast to the token-passing ring-plus-switch (ring/star) architecture that is a characteristic of a number of systems expected to e available in one to five years. It is most significant that the star/star PBX has all the major features with which the industry describes the much-heralded fourth-generation of business communications systems: non-blocking capability; digital technology from end to end; implementation of voice and data in the same system; distributed processing; structured software architecture; and incorporation of local-area network capability.

The star/star PBX, unlike the token-passing ring/star, is based on pure circuit switching. The circuit transmission data rate is typically 64 kb/s, and with parallel circuit switching of up to 32 circuits, a maximum data rate of 2.048 Mb/s can be provided. This data rate is more than adequate for most user's data-switching requirements and can even be used for some video applications.

As will be seen below, the star/star architecture offers advantages over third-generation PBX systems and even over some high-performance ring/star systems. The greatest advantage of the star/star architecture is that it is able to solve users' needs today.

The present definition of fourth-generation PBX capability is drawn primarily from the characteristics of ring/star-type systems. These systems incorporate a token-passing local-area network with separate rings, one for circuit-switched voice traffic and another for packetized-data traffic. Connected to the LAN (the ring part) are circuit-switching modules (the star part), each with support for a number of stations ranging from 200 to 500 depending on the system. These modules are microprocessor-controlled and provide non-blocking circuit switching.

The ring/star systems, as the star/star systems, offer the advantage of modularity and distributed processing. It is expected that these characteristics will make system expansion, for either architecture, inexpensive relative to centralized PBXs. The ability to add modules as growth requirements demand increases the flexibility of the systems. The distributed-control structure protects against central processor failure. Similarities and Differences

while the ring/star and star/star systems are clearly different architectural approaches, the benefits they offer the user are more notable for their similarities than their differences.

The star/star systems, as the ring/star concepts, are distributed-processor-controlled systems. Rather than being based on a single intelligent central switch that is subject to central failure as are traditional PBXs, the star/star has as its basis intelligent modules that can be linked together in a highly flexible network. In the Ericsson star/star system (the MD110) this module is called a Line Interface Module (LIM).

The intelligent module of the star/star is itself a distributed processing system. Each module functions as an independent PBX and can have any combination of analog and digital trunks plus analog, digital and/or data extensions. Individual processors are employed for both high-level and low-level control. One processor serves as the module processor handling only high-level functions, while each line circuit unit has its own processor handling low-level functions. Within each module, communication among processors is accomplished using packet-switching techniques over a signaling bus (see Figure 2).

In the case of the MD110, the module contains a 32.8 Mb/s non-blocking digital switching unit providing 512 ports. Depending upon the configuration, up to 250 extensions with both voice and data are supported. The module also contains other special-purpose device cards for implementation of functions such as tone receiver, tone sender, conferencing and I/O ports. The system memory cards are housed within the module as well.

When the total system requirements exceed the capability of one module, two modules can be connected directly via a "system link" (called EriLink by Ericsson). The system link is based on a 32-channel PCM transmission standard with a digital rate of 2.048 Mb/s. The use of a standardized transmission method for the system link allows flexibility in choosing the transmission media. For collocated modules, coaxial cable serves as a convenient media. For off-campus locations, a converter to T1 is provided and the media can be either leased T1 carrier, microwave or optical fiber.

An important feature of the system link is that one channel is dedicated for signaling and is in fact an extension of the signaling bus. The signaling channel incorporates a link-level protocol and provides each module with the ability to send and receive signals to each other. Because the signals sent across the system link are identical to those on the signaling bus, only the destination is different; the two modules function as a single but distributed PBX system. If, for example, all tone receivers are occupied in one module, a call will activate a receiver in the other module. If, however, the link between modules should be severed for any reason, the two modules will still function. Likewise, if one module should be inoperable, the remaining module will operate independently.

Traffic capacity between the modules is controlled by the number of system links provided. A non-blocking system is achieved by equipping a module with as many system link channels as there are lines. Keeps Distributed Plan

The distributed nature of the star/star configuration is continued as the system expands. When larger systems are required, modules are connected to a space-time-space systems switch. (In the Ericsson MD110, it's called a group switch.) Modules are connected to the group switch by means of the system links. The group switch, which is also non-blocking, has the capability of switching any channel on one system link to any channel on another system link. It is important to note that the connections made by the group switch are controlled by the module from which the call is originated; the group switch itself is a functionally non-intelligent unit. The group switch also acts as a packet switch for distribution of the signals on the system links to appropriate modules (see Figure 3).

It is the group switch in combination with the system links that forms a star-architecture LAN. In a traditional LAN the ring functions to tie together intelligent units in cooperative operation. In the star/star system the group switch performs this function. In fact, if the group switch/system link element of the star/star were lifted out of the system and a ring with its interfaces were put in its place, the resulting system would be largely interchangeable with a ring/star system. Standardized Medium

In the star/star architecture, the system link takes the place of the LAN protocol. The star/star's advantage is that the physical medium of the system links is standardized, whereas existing ring LAN standards are very new and consequently proprietary to each system implementation.

The star-architecture LAN provides circuit switching with digital transport capacity in the hundreds of Mb/s range. The group switch is a matrix switch that can be built up in modular units. Thus as the total PBX system size increases, the group switch need only be increased proportionately.

Unlike a traditional PBX with an intelligent central processor switch connecting non-intelligent modules, the star/star has a non-intelligent group switch interconnecting a number of intelligent modules, each of which is itself distributed. Each module functions independently, but when interconnected becomes part of a distributed PBX system.

In either the ring/star or the star/star architectures, the individual modules have stand-alone capability since they contain all functions necessary for autonomous operation. Normally, inherent system functions such as the man-machine interface control are implemented in one module only. In such cases, the function is generally duplicated in a second module for backup purposes. Handles Large Data Load

Like the ring/star configurations, the star/star architecture is virtually non-blocking, a necessity in systems with increasing data traffic. The switches within the modules and the group switch are fully non-blocking. Between the modules and the group switch some blocking can conceivably occur; however, any degree of intermodule traffic can be accommodated by providing sufficient system links between the module and the group switch. For the MD110 it is possible to configure a completely non-blocking system of 7,440 lines.

Because of the cost of the system links and the group switch represents only a small percentage of the total system cost, it is economically possible to design the star/star system for any grade of service (blocking probability) required. Typical system dimensioning for voice traffic in these systems gives a blocking factor significantly under 0.1 percent.

The modularity and distributed architecture of the star/star systems enable development of highly customized, flexible and low-cost networks. Of particular benefit in networking is the ability to connect remote modules to the system in a similar manner as local modules are linked. This in one area in which the star/star topology excels over the ring/star systems. The rings cannot be extended over long distances without the use of a gateway. The gateway technique is considerably more expensive than the star/star system link technique and usually creates significant blocking conditions in the system. The ring/star architecture does not lend itself to providing solutioins for wide-area network problems.

The system link can be used in the star/star architecture to connect remote modules. The distasnce limitation is constrained only by the propagation time for the system link signal. (In the MD110 this limitation is usually 600 miles.) Since the remote module is connected with the same system link as a local module, the remote module has all the features and characteristics as the locally connected modules.

The system link concept also enables larger networks to be built up as is shown in Figure 4. In most current network applications, the network system links are the lines. While tie lines provide most commonly required capabilities, the concept of network-system links lends itself to other signaling implementations. Common-channel signaling can be used with the advantage of providing transportability of system features between network modes. As network transmission standards develop, for example, ISDN, these signaling schemes can be easily accommodated. Review of the Benefits

The benefits of the fourth-generation architecture to the PBX user are quite evident. First, the traffic-handling capability of the system increases as the size of the system increases, since the addition of each module adds more system capacity; that is, each new module adds processors, non-blocking switching and transmission capacity. This feature gives a near-constant cost-per-line factor independent of system size and feature level. Ultimately, the limit on size is determined by the system signaling capacity, and current technology is expanding this capability. This is in direct contrast, of course, to any third-generation PBX without distributed architecture, which of necessity has an upper limit on call-handling capability because of the limitations of the central switch.

The distributed star/star architecture has a direct impact on system cost. Because the group switch is non-intelligent it is very inexpensive to manufacture. In addition, its modularity significantly minimizes the cost of capacity expansion. Additions to the star/star system, then, can always be done at virtually the same cost per extension.

It is also inexpensive to interface to the star/star system because first-order PCM technology (T1 carrier) is low in cost. In this case, the same cannot be said for the ring/star, as interface into the token-passing ring system requires more-costly technology.

The low cost of the star/star system also makes it possible to increase the system's reliability and security in a cost-effective manner. A user can choose to duplicate the group switch in a large system in order to eliminate any possibility of losing communication between modules. The inexpensive group switch makes this a highly feasible and recommended option. Likewise, any of the system links may be duplicated.
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Author:Barnicoat, G.; Geyer, A.
Publication:Communications News
Article Type:evaluation
Date:May 1, 1984
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