8.4 Public and Private Network Interoperability

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The need to mix private and public TDM-based network elements into a hybrid arrangement and still implement network management and control requires an intelligent networking multiplexer with a sophisticated transport management system (TMS). The purpose of the TMS is to extend intelligence to the backbone network where the performance requirements of particular applications can be monitored. The TMS goes well beyond mere bandwidth allocation to ensure the integrity of applications for each circuit on the network.

A T1 multiplexer, for example, should support aggregate rates from 4.8 Kbps to 1.544 Mbps. Locations should be able to adjust to new transmission rates by software selection. When a new interface is required, the architecture of the T1 multiplexer should require that only a plug-in module be changed on the aggregate card. This facilitates network growth very economically, while providing maximum protection of existing hardware investments.

An integral subrate data multiplex (SDM) capability is desirable to support the transport of bandwidth in increments of 9.6 Kbps in standard DDS formats. The carrier’s DCS can then switch these increments into 64-Kbps channels for DS1 transport to a private network hub, at which the channels can be unbundled by the T1 multiplexer and directed to their destinations at the proper speed. This arrangement allows users to tie low-traffic locations into the backbone very economically. Likewise, the T1 multiplexer must have the capability to support FT1 services, which offer incremental bandwidth from 64 to 768 Kbps, plus voice compression. With a T1 multiplexer that is equipped with ISDN PRI, disaster recovery can be facilitated. If the private line goes down, a carrier-provided ISDN service can be used to offload the traffic.

The proprietary frame structure of a bit-oriented multiplexer can ensure low nodal delay for the best response time. All aggregates can be used to support data transfer rather than losing some aggregates to supervisory channel overhead.

However, building a hybrid network comprised of both private and public elements requires that the T1 multiplexer interconnect with the byte-oriented public network. This would seem to pose a dilemma; after all, T1 multiplexers are either bit or byte oriented at the bus level. When a bit-oriented multiplexer performs a bit-to-byte conversion at the interface, it can selectively interconnect with the public network without sacrificing the efficiency of private network elements.

By being able to interconnect selectively with the public network (including the ISDN service nodes of interexchange carriers), no sacrifice is made in end-to-end network management and control. Supervisory channel information is embedded into each 64-Kbps channel of a bundle of channels so that utilization of public network services is available without the loss of network management. Users can derive the full benefit of the public network’s interconnectivity without depriving themselves of the means to exercise control of remote locations.

By providing aggregate interfacing into the byte-oriented public network, the multiplexer not only facilitates access to services provided by the DCS network, it also accepts direct PBX connection in support of voice applications. Voice support is provided with an interface card that supports supervisory information within each N x 64-Kbps bundle of a T-carrier link. This interface connects to the DCS in the public network or a digital PBX or D4 channel bank service. Within the same multiplexer, however, there is the choice of using private aggregate interfaces for bandwidth efficiency and an aggregate interface for public network ( N x DS0) interconnectivity.

The multiplexer accepts the 1.544-Mbps line rate (DSX-1) directly from the digital PBX. Pulse density, needed to keep line regenerators synchronized, is met by placing a mark bit in the seventh bit position of the 8-bit DS0s. In this way, dense compaction transport efficiencies are combined with the interconnectivity of public standard support for maximum network flexibility. This allows organizations to design and implement high-capacity digital networks based on the needs of the applications, rather than on the limitations of the network technology.

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