10.2 The Concept of Spectrum Management

   


As we have mentioned earlier, spectrum management is the administration of the loop plant in such a way that it assures spectral compatibility between the signals, services, and technologies that use the wire pairs in the same cable. In order to provide spectrum management, we need an understanding or a definition of what it means for different signals to be spectrally compatible with one another. To set this definition, we need an agreed-upon criteria or basis for defining the levels and conditions of spectral compatibility.

Prior to the Telecom Act of 1996 and the associated FCC rules requiring local loop unbundling and collocation of equipment, the network already had numerous types of signals deployed in the cable. The most common legacy signal is analog voice, which uses the lower 4 kHz of the frequency spectrum for provisioning POTS. Other types of signals and services deployed in the loop plant at that time include T1 AMI, DDS, ISDN, ADSL, and HDSL. These signals and others already present in the network would be the first to consider as a basis for determining spectral compatibility criteria.

The spectrum management standard (T1.417) defines a list of systems as the basis systems list, which forms the basis for quantifying the acceptable levels of disturbance for declaring spectral compatibility. Once these conditions are defined, any new system to be deployed in the network would be measured according to these conditions. In an unbundled loop environment, systems can only be deployed according to the guidelines for which they are declared to be spectrally compatible. In summary, any system deployed in the cable in unbundled loop environment must be spectrally compatible with the basis systems, and the spectrum management standard provides the compatibility criteria.

The purpose of the spectrum management standard is to prevent service trouble due to excessive crosstalk from other systems in the same cable. In effect the, spectrum management standard protects the legacy transmission systems, as well as future transmission systems. In fact, early drafts of the spectrum management standard used the term protected systems that was later changed to basis systems . The basis systems act as a surrogate for all loop transmission system. The spectrum management standard defines spectral compatibility as not interfering with any of the basis systems more than a specified amount.

Spectral compatibility applies to both directions of transmission, that is, the signals transmitted by equipment at both ends of the line.

T1.417 contains two methods for determining the spectral compatibility of a transmit signal. Method A defines a set of signal power limitations that specify the maximum amplitude, frequency distribution, and total power of the electrical signals at the point where the signal enters the cable. In T1.417, there are nine spectrum management classes that define such signal limitation. Any signal that falls within the specified limits of any spectrum management class is declared to be spectrally compatible within the deployment guidelines of that class. Method B defines an analytical method for determining spectral compatibility; this analytical method is an alternative method to that of signal power limitations. The driving need for defining the alternative analytical method is that future (and existing) technologies may use transmit signals whose power spectral densities do not conform to any of the signal power limitations of the spectrum management classes, but would still be spectrally compatible with the basis systems. Applying the power spectral density of any signal to the analytical method will determine the deployment guideline assuring spectral compatibility.

The first issue of spectrum management standard T1.417 has the following key assumptions: the cable plant is modeled as a 50 pair cable with all equipment at each of the loop respectively collocated. There are some DSL systems that are deployed with midspan repeaters. There are also DSL systems deployed from a remote terminal (RT) or a digital loop carrier (DLC). In general, any DSL signal that is regenerated in the mid-span of a cable such as a repeater or originates in a remote terminal is called an intermediate termination unit (or intermediate TU). The first issue of T1.417 does not define the spectral compatibility of intermediate TU systems with those deployed directly from the central office. The spectral compatibility of these systems is a subject of high priority for the second issue of T1.417. Figure 10.1 shows the different deployment scenarios mentioned: (a) direct from the central office, (b) repeatered service, and (c) RT or DLC.

Figure 10.1. Deployment scenarios in the cable plant.

graphics/10fig01.gif

Because there is about 10 dB of additional crosstalk loss between binder groups, one possible method of assuring spectral compatibility is to restrict certain DSL types from occupying selected binder groups (e.g., binder group management). Binder group integrity is the assurance of the same set of wire pairs remaining together throughout the loop plant. Binder group integrity is often not maintained when the subscriber loop is provisioned through multiple cross-connect boxes. Because binder group integrity cannot be reliably administered in a public network, the spectral compatibility guidelines in T1.417 do not consider binder group management. However, binder group separation might be a useful method of assuring spectral compatibility in situations where binder group integrity can be assured. However, with the legacy exception of T1 carrier, the FCC prohibits the restriction of services to selected binder groups because this could favor certain service providers. Furthermore, such restrictions would result in stranding wire pairs that could have provided service.

T1.417 defines spectral compatibility on the basis of allowing 49 disturbers in a 50 pair binder group. It has been suggested that special allowances be made for systems that are restricted to a small number of systems in any binder group. This method is not practical because it is not possible to reliably enforce such a restriction. Furthermore, restricting a system to a number such as 12 disturbers provides relatively little benefit compared with the 49-out-of-50 disturber assumption.


   
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DSL Advances
DSL Advances
ISBN: 0130938106
EAN: 2147483647
Year: 2002
Pages: 154

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