8.6 Unbundling and Network Operations
| Line sharing posed a huge challenge to telephone company methods and operations systems. It had never been imagined that two service providers might share a line. Revisions were necessary to keep track of services from different providers on the same line, and new methods were needed so that the entire line was not disconnected when a customer asked for one service to be disconnected. Also, new provisions were needed to deal with the line sharing splitter in the central office. Nevertheless, the changes proved to be feasible . CLECs are provided direct access to the telephone company operations systems that keep track of telephone lines (such as LFACS, loop facilities assignment system) and test the lines (such as LMOS-MLT, loop maintenance operations system ”mechanized loop test). LFACS contains information about what services are assigned to each line, and length of the line. When a CLEC orders an unbundled line, the ILEC assigns a spare wire pair, and places the necessary jumper wires at the MDF and cross-box at the FDI (feeder distribution interface). The ILEC will remove loading coils and bridged taps if the CLEC requests and pays for the special conditioning. Usually, the ILEC does not perform preservice tests of the line unless requested by the CLEC. One benefit of line sharing is that more than 95 percent of lines work the first time because the line was already working for voice service, whereas about one-third of dedicated unbundled lines (not line shared) do not work the first time due to bad wire-pairs, and various line provisioning errors. The early stipulations from the FCC indicated that a DSL transmission system was acceptable for general use if the system had been successfully used for service without reported harm to other services. This policy was motivated by the desire to spur competition and innovation. However, it highlighted the urgent need for a technically sound basis for determining spectral compatibility. "Harm" in the original policy was not well defined, and a system that caused no harm on short lines, or in small numbers , could easily cause a total loss-of-service for other systems within the same cable where the conditions were different. By the time such problems were widely reported and the fault proven, corrective measures could be difficult and expensive. The situation would be like the Food and Drug Administration approving a new drug for general use after reviewing only one clinical trial involving two persons. Realizing this in 1998, the FCC held an industry forum on spectrum management of DSL systems, and then requested T1E1.4 to develop a standard to define the technical specifications for DSL spectrum management. The T1.417 spectrum management standard was completed by T1E1.4 at the end of the year 2000. It provides a set of predefined spectrum management classes that are suitable for DSL systems operating over smaller and larger bandwidths. Each spectrum management class has specified limits on total transmitted power, power spectral density (PSD), output voltage, transverse balance, and a deployment guideline. The deployment guideline specifies the maximum length of line the system may be used on. The spectrum management standard also stated technology-specific criteria for SDSL and SHDSL type transmissions systems, as well as providing an open -ended Method B that permits the demonstration of spectral compatibility for future technologies. Chapters 10 and 11 provide a more detailed discussion of spectrum management. A carrier (ILEC or CLEC) using a line should identify the class of signal being transmitted on the line. This may be the T1.417 spectrum management class, or some other equivalent indication of the type of signal bandwidth, power, and deployment restriction. This information is needed for the administration of the deployment restrictions, and to assist trouble resolution. DSL equipment at the customer's premises must be certified to not harm the network by the FCC Part 68 process or a new DSL specific process that may be developed in the future. Similarly, equipment at the network-end should also be addressed by DSL certification in the future. It is expected that this certification will require that signals transmitted by sample equipment be measured to verify that the signals are within the limits specified in the T1.417 spectrum management standard. These measurements and the corresponding interpretation of the measurement results should be verified by an entity with the necessary test systems, expertise, and impartiality. It is unlikely that spectrum management will be proactively enforced. Carriers will be contractually bound to adhere to the requirements for the type of signal being sent, and to use certified equipment. Corrective action will be taken only if service trouble is detected . The FCC rules do not specify the process for resolving interference troubles. When excessive crosstalk is suspected to be the cause of service trouble, the carrier of the victim system may request the ILEC to investigate the trouble. If the ILEC finds that a transmission system is transmitting a signal in excess of the contractual terms for the use of the line, the ILEC has the right to disconnect that line. If all carriers adhere to the T1.417 spectrum management standard, this should not occur. The diagnosis of trouble resulting from excessive crosstalk will likely require lengthy efforts by highly qualified technicians. The degree of investigation will depend on the contractual agreements between the ILEC and CLEC. If the source of the trouble is not agreed to, then the dispute between carriers may need to be resolved by the state PUC. Because there is 10 dB less crosstalk coupling between pairs in different binder groups, better DSL performance can be achieved when certain types of transmission systems are segregated into dedicated binder groups. This is true for systems whose performance is not limited by self near-end crosstalk, such as ADSL. Regardless of the number of ADSLs within a binder group, ADSL will perform better if there are no HDSL, SDSL, SHDSL, or T1 carries in the same binder group . Proposals to segregate ADSL from other services were rejected by the FCC because the spare pairs in these binder groups would be unavailable for other types of DSL service. The FCC line sharing order does permit the segregation of T1 carrier into dedicated binder groups because the FCC identified T1 carrier as a known disturber . T1 carrier is the only known disturber at this time, and indeed, T1 carrier is one of the worst crosstalkers found in telephone cables. The T1 carrier can even cause substantial crosstalk into the lines in adjacent binder groups. However, the T1 carrier does not always cause trouble for DSLs in the same binder group. ADSL, for example can operate reliably at 1.5 Mb/s on lines less than 9 kft long with a T1 carrier disturber in the same binder group. Nevertheless, ILECs have realized that removing T1 carrier will greatly expand the capacity for other services. As a result, new T1 carrier lines were rarely installed after the year 2000, and some ILECs are proactively replacing a large portion of their existing T1 carrier lines by moving the DS1 service to fiber-to-the-business, or HDSL2 lines. A 4-wire 1.5 Mb/s transmission system using TC-PAM modulation is expected to assure spectral compatibility for all lines, including long lines with repeaters. It is tempting to try to expand the bit rate or line length permitted for a certain DSL technology by placing a restriction on the number such systems within a binder group, for example, extending the spectrum management deployment guidelines for SDSL technology by an extra 1000 feet, if only five SDSLs were permitted within a binder group. Such methods for binder group management are impractical for several reasons:
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