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Chapter 8 : Conventional Public Data Networks (PDNs) (Page Seven)

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Channel Service Units (CSUs) and Digital Service Units (DSUs)

Discussed at length in Chapters 2 and 7, DSUs are devices which, in combination, interface the user environment to the digital network at the physical (mechanical) and electrical level, corresponding with Layer 1 of the OSI model. In contemporary systems, they generally are combined into a single device, known as a CDSU or an ISU (Integrated Service Unit), which may be under the skin of another device, such as a multiplexer (MUX). They are used in a wide variety of digital data networks, including DDS and T-carrier.

Multiplexers (MUXs)

MUXs are a significant step up from channel banks in terms of intelligence, capability, and cost. Originally based on channel banks and containing CSUs and DSUs, contemporary Statistical Time Division Multiplexers (Stat MUXs) offer a tremendous range of flexibility and capability. MUXs typically offer capabilities that include support for both channelized and non-channelized service, support for multiple medium interfaces (e.g., twisted-pair, coax, and fiber), support for multiple trunk types (e.g., DID and combination), support for superrate transmission (>64 Kbps channels), and support for subrate transmission (<64 Kbps channels). Additionally, they offer the advantage of user-definable configuration, internal diagnostics, voice compression, and T-carrier to-E-carrier protocol conversion. Intelligent MUXs also have the ability to allocate bandwidth on a priority basis for specified users and applications, and even to reserve bandwidth, perhaps for a scheduled videoconference. Finally, intelligent MUXs can allocate bandwidth on a dynamic basis, assigning channel capacity as required to meet the demands of traffic. For instance, a videoconference may require superrate capacity for a short period of time, multiple low-speed data communications may require subrate channels for a brief moment and, at other times, the entire capacity of the circuit may be in support of 32 Kbps voice conversations.

Nodal Multiplexers

Nodal MUXs, a further step up the MUX food chain, act as T-carrier network switches. In addition to serving as a traditional MUX for the resident site, they also serve as true networking devices, much as does a combined CO/Tandem switch in the voice world. Nodal MUXs provide the additional function of dynamic alternate routing (Figure 8.5), which allows them to switch traffic over an alternate path in the event of a condition of blockage or failure in the primary circuit.

Digital Access Cross-Connect Systems (DACS or DCCS)

These are nonblocking, electronic common control switches which serve to cross-connect digital carrier bit streams on a buffered basis by redirecting individual channels or frames from one circuit to another. Effectively, they provide an electronic common control means of cross-connection which replaces the traditional manual method of physical cross-connection of wires. A DAC can redirect traffic in order to better manage the capacity and performance of the T-carrier network [8-10]. Although originally developed for carrier use, DACS also are deployed in large user organizations to support private digital carrier networks. Smaller versions, residing on a PC, are available for less communications-intensive environments. Typically of significant port capacity, DACS provide support for DS-0, DS-1, and DS-3 [8-3].

Variations on the Theme: E-carrier and J-carrier

While the theme for digital carrier was set in the United States, the concept was quickly adopted by the CEPT (Committee on European Post and Telegraph). The resulting E-carrier standard is quite different in its implementation. The Japanese version, J-carrier, is similar to T-carrier, but with differences sufficient to cause incompatibility.

E-carrier is characterized by an entirely different digital hierarchy, (Table 8.2) beginning with E-1, at 2.048 Mbps. E-1 supports 30 clear information channels; 2 channels are set aside for signaling and control. This non-intrusive signaling and control convention, plus the fact that there are no 1s density rules which apply, results in E-carrier’s providing clear channel communications of a full 64 Kbps per channel. The E-carrier multiframe (16 Frames) corresponds to the T-carrier superframe (12-24 Frames).

J-carrier is quite similar to T-carrier, although the hierarchy is slightly different. Line coding and framing also vary from the U.S. ANSI approach. The advantages of these differences are questionable; incompatibility is assured, which fact should not be surprising. The J-carrier digital hierarchy begins at 1.544 Mbps, proceeding to 6.313 Mbps, 32.064 Mbps (J1), 97.728 Mbps (J3), and 397.20 Mbps [8-3] and [8-6].

Fractional T1

Fractional T1 (FT1), previously offered only in Canada, first was tariffed in the United States in 1987 by Cable and Wireless. Now offered by many LECs and IXCs, FT1 provides T1 functions and features, but involving fewer DS-0s. It is offered in fractions of T1 channel capacity, generally at 1, 2, 4, 6, 8 or 12 DS-0 channels. Subrate transmission also is available at speeds of 9.6 Kbps. FT1 is particularly applicable where remote locations are connected to a more significant location such as a regional office. At that point, they connect to a full T1 MUX or nodal processor, which aggregates the traffic with that of the larger site over a full T1/T3 backbone network. FT1 also serves videoconferencing, data communications, and other applications that require more than 56/64 Kbps, but less than a full T1 [8-4].

T-carrier Applications

The applications for Digital Carrier are many. Large user organizations find Digital Carrier services to be highly cost-effective for local loop access, typically replacing multiple, individual PBX trunks. Large corporations find T-carrier effective for private, leased-line networks or access to virtual private networks (VPNs). The ability of T-carrier to accommodate voice, facsimile, data, video, and image information on an unbiased basis and, therefore, to eliminate or reduce the number and variety of specialized circuits, offers great advantage.

Internet service providers commonly use T-carrier for access to the LEC CO, as well as access to an Internet backbone provider. Given current US tariffs, a T1 clearly becomes cost-effective in replacement of 8-10 or more individual circuits; a T3 becomes cost-effective at a level of 3-4 T1s. Notably, the relationship is far different in Europe and parts of Asia, where leased line costs are considerably higher.


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