8.4 Line Sharing

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Line sharing unbundles a portion of the frequencies conveyed by a telephone line. The frequency band above approximately 4 kHz is used by the data service provider while the frequency band below 4 kHz is used by the voice service provider. Thus, two different service providers simultaneously share the same line. A splitter device at the network end of the line combines the data and voice- band signals while preventing interference between the two transmission bands. The splitter consists of a high-pass filter leading to the DSLAM and a low-pass filter leading to the voice switch. The high-pass filter is most often implemented as a pair of dc (direct current) blocking capacitors whose principal function is to avoid disruption of the voice service if the wire pair was accidentally shorted at the DSLAM.

As shown in Figure 8.3, line sharing implies the sharing of a telephone line by an ILEC and a data service provider, and not the sharing of a phone line by multiple end- user customers such as an old-time party line. There is a configuration where multiple DSL customers do share one or more common telephone lines: a remote DSLAM located in a multiple dwelling unit (MDU). This scenario is not called line sharing, even though the multiple customers served by the remote DSLAM do share the same line(s) that connect the remote DSLAM to the network.

Figure 8.3. Line sharing splitter at network end of line.

In theory, there are many ways that multiple service providers could share a telephone line:

• Two fixed frequency bands : One band for traditional voice service, and one band for DSL data transmission. This is the method defined by the FCC for line sharing. This method works well for traditional voice services and for standard ADSL-based service. However its inflexible structure may be poorly suited to future technologies and services (e.g., voice-over-DSL).

• More than two fixed frequency bands : More than two service providers could share a line by dividing the line into multiple frequency bands. One example that could use three bands would be a customer subscribing to three service providers for voice, data, and alarm service. This method would suffer the inefficiencies of multiple frequency-guard-bands. Because some security/fire alarm services use signaling below the voice-band (below 200 Hz), the current practice of line sharing for voice and data can be considered a three-band method of line sharing.

• Flexible frequency bands : The frequency dividing the two bands could be variable depending on the needs of the two service providers. For example, one provider could use a frequency band based upon the number of derived voice channels provided, and the data service provider could use the remaining frequency band. In the extreme, the frequency band separation could change dynamically. This method would be complex to administer, could require a complex splitter, and would also complicate spectrum management.

• Time division sharing : Burst-mode transmission systems from multiple providers could be synchronized to allow more than one DSL modem on the line. The time division could be fixed or variable. This method trades the capacity loss of guard-time periods, and the complexity of modem synchronization for the capacity loss due to frequency guard-bands and splitting filters. This method could complicate spectrum management.

• Code division multiple access : The cost and technical feasibility of CDMA-type line sharing is uncertain .

• Higher-layer bit stream sharing : An access provider with a DSL modem at each end of the line would provide higher-layer multiplexing of packet/ cell streams from multiple service providers. The multiplexing could be performed at layer two via ATM virtual circuits (PVCs or SVCs), or at layer three via IP paths. This method has the following benefits:

  • Any number of service providers may have simultaneous access to the customer.

  • Service providers may be changed without wiring changes in the central office.

  • Full flexibility of the division of bandwidth between service providers.

  • Full flexibility for types of services (voice, data, alarm, and video).

  • Statistical multiplexing permits a bandwidth overbooking advantage.

  • No capacity loss due to frequency guard-bands.

  • Greater aggregate capacity for all DSL lines may be achieved by the mutual coordination of all transmitted signals; this is most feasible if all lines are controlled by the same entity.

  • DSL modem cost shared by multiplex providers.

  • Advances in DSL technology could be applied without changes to architecture or regulatory structures.

  • High reliability because a single entity has full administrative and diagnostic control of the line. There would be no need for multi-entity coordination of diagnostic testing.

• Hybrid of higher-layer bit stream sharing and fixed frequency division : One frequency band for traditional voice service, and DSL operation in the upper frequency band with multiplexed bit-streams from multiple service providers.

Line sharing is a boon to service competition. CLECs typically pay $5 to $7 per month to lease a line to carry data with line sharing, whereas non-shared, unbundled lines often cost at least twice as much. More important, line sharing permits the data service to customers where no spare wire pairs are available. With line sharing, the data service provider will almost never be unable to serve a customer due to a lack of spare pairs in a cable. Carrying two services on a line improves cable utilization and reduces cable plant exhaustion.

The FCC released the line sharing order December 9, 1999, as the combined Third Report and Order in CC Docket 98-147 and Fourth Report and Order in CC Docket 96-98. The line sharing order required ILECs to provide a new UNE to certified CLECs. The new UNE was termed the high-frequency portion loop (HFPL), and the order describes it as residing above the traditional voice band. To avoid excessive constraints on technology, the FCC purposely did not define the dividing line between the high- and low-frequency portions of the line, though the order does mention that the voice-band is commonly considered to reside from 300 Hz to 3.4 kHz and that the HFPL should not interfere with voice band services. This would suggest that ILECs might not be required to provide line sharing on lines with P-phone type service that incorporates signaling at 8 kHz. ^[2] The FCC order does not require that any specific type of DSL technology be used on the high frequency portion of the line, but mention is made of ADSL and MVL ^[3] type technologies. ILECs are required to provide line sharing only on lines that are already used for traditional voice service. Thus, the ILEC is not required to provide line sharing on currently unused lines or nonvoice lines. However, the CLEC still can lease dedicated lines for nonshared use. The price for the line sharing UNE is set by the states using a process called TELRIC; this price is approximately equal to the telephone company's cost. The line sharing UNE consists of the high-frequency portion of a line from the MDF in a CO to the point of network demarcation at the customer's premises, or from a DLC-RT to the network demarcation at the customer's premises. The copper line from the DLC-RT to the customer is sometimes called a subloop.

^[2] Germany, and possibly some other countries in Europe, may implement line sharing of ADSL over basic rate ISDN using 4B3T line coding with approximately 0 to 100 kHz for ISDN and ADSL occupying the band above this.

^[3] MVL is multiple virtual line, a proprietary DSL technology.

As with dedicated unbundled lines, line sharing uses a telephone line that is owned by the ILEC and leased to the CLEC. The ILEC retains maintenance responsibility for the line. If requested by the CLEC, the ILEC must perform line conditioning provided that the line conditioning would not degrade the voice-band service. Line conditioning generally consists of the removal of loading coils or removal of bridged taps. Loading coils are necessary for good voice-band performance on lines longer than 18 kft. Loading coils must be removed for DSL operation. Bridge tap remove is necessary for less than 5 percent of lines. The ILEC may charge a reasonable fee for line conditioning.

The line sharing order requires that the DSL line sharing transmission not interfere with voice service. The two services are combined by use of a splitting filter (see description earlier in this section) that may be owned by either the ILEC or the CLEC. The splitter ownership, physical location, technical characteristics, installation process, and equipment cabling arrangements are negotiated jointly by the ILEC and CLEC. Both ILEC- and CLEC-owned splitters are in practice (Figures 8.4 and 8.5). Factors in the positioning of the splitter are the amount of central office cabling required, the number of main distributing frame (MDF) wiring jumpers , and the provisions for direct access to the line for testing. As shown below, the CLEC owned splitters could eliminate one MDF jumper and shorten some CO cabling. Whereas, the ILEC-owned splitters could permit a pool of splitters to be shared by many CLECs, the CLEC-owned splitter would also allow the splitter to be integrated with the DSLAM.

Figure 8.4. Central office configuration with CLEC-owned line sharing splitter.

Figure 8.5. Central office configuration with ILEC-owned line sharing splitter.

The line sharing splitter at the central office restricts the bandwidth available to the DSLAM (low frequencies eliminated) and to the voice switch (high frequencies eliminated). Neither the DSLAM nor the voice switch could test the full bandwidth of the shared line. This restricts the ability to fully diagnose the line characteristics. Figure 8.6 shows four alternative locations for line test systems:

1. Inside or in front of the DSLAM . This position could be used for a line testing system owned by the data carrier. Testing would be limited to frequencies above 4 kHz unless a bypass function was included with the splitter. Only lines that had already been connected to the data carrier's equipment could be tested ; thus this configuration is of little value for preservice line qualification.

2. Before the voice switch . This position could be used for a line testing system owned by the voice carrier. Testing would be limited to frequencies below 4 kHz unless a bypass function was included with the splitter. Pre-service line qualification could be performed on any line with voice service.

3. Behind the voice switch . This position could be used for a line testing system owned by the voice carrier. This takes advantage of the existing metallic test access network within the voice switch. However, the bandwidth of this test bus is less than 100 kHz on some voice switches, and a splitter bypass function would be needed for testing beyond about 4 kHz. Testing within the 4 kHz voice band, such as performed by MLT (mechanized loop test system), is sufficient to detect gross line faults (e.g. line short, open , foreign voltage) and to make a gross estimate of line length. A much greater bandwidth measurement is necessary to generate a moderately accurate estimate of DSL bit-rate capacity. ILECs are providing CLEC access to MLT to perform tests on lines used for line sharing.

4. Line side of the splitter . This position could be used by the ILEC or the CLEC. This test access location avoids the need to bypass the splitter for full-band line testing. It is unlikely that this could be used for pre-service line qualification since it would be too expensive to connect all voice lines to the test access system. Integration of the splitter and the test access would help the economics of this configuration.

5. Manual test access (not shown in the figure) . A technician could connect a portable test set at the MDF or possibly at the splitter.

Figure 8.6. Potential locations for line testing systems.

A combination of methods 1 and 3 above, could be used to test the high band via functions built into the DSLAM, and the low band via automated access to the ILEC's voice-band line test system.

The FCC line sharing order also expanded the charter of the NRIC to advise the FCC on policy for spectrum management of local lines based on technical standards developed by the T1E1.4 national standards working group .

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