Sec. 68.310  Longitudinal balance limitations.

    (a) Technical description and application. The metallic-to-
longitudinal balance coefficient, BALANCEm-1, is expressed 
as:
[GRAPHIC] [TIFF OMITTED] TC13NO91.007

where eL is the longitudinal voltage produced across a 500-
ohm longitudinal termination and eM is the metallic voltage 
across the tip-ring or tip 1 and ring 1 interface of the input port when 
a voltage (at any frequency 200< f<4000 Hertz) is applied from a 
balanced 600-ohm metallic source. The source voltage should be set such 
that eM=0.775 volts rms (0dBm) when a 600 ohm termination is 
substituted for the terminal equipment. The minimum balance coefficient 
specified in this section (as appropriate) shall be equalled or exceeded 
for all 2-wire network ports, OPS line ports and the transmit pair (tip 
and ring) and receive pair (tip 1 and ring 1) of all 4-wire network 
ports at all values of dc loop current that the port under test is 
capable of drawing when attached to the appropriate loop simulator 
circuit (see Sec. 68.3). An illustrative test circuit that satisfies the 
above conditions is shown in Figure 68.310(a); other means may be used 
to determine the balance coefficient specified herein, provided that 
adequate documentation of the appropriateness, precision, and accuracy 
of the alternative means is provided by the applicant. The minimum 
balance requirements specified below shall be equalled or exceeded under 
all reasonable conditions of the application of earth ground to the 
equipment or protective circuitry under test:

                                                                        
------------------------------------------------------------------------
                                                   Minimum    Frequency 
        Paragraph             Equipment state      balance      range   
------------------------------------------------------------------------
(b).....................  On-hook...............         60     200-1000
                          On-hook...............         40    1000-4000
                          Off-hook..............         40     200-4000
(c).....................  On-hook...............         60     200-1000
                          On-hook...............         40    1000-4000
                          Off-hook..............         40     200-4000
(d).....................  Off-hook..............         40     200-4000
(e).....................  On-hook...............         60     200-1000
Voice Equipment.........  On-hook...............         40    1000-4000
                          Off-hook..............         40     200-4000
(e).....................  On-hook...............         60     200-1000
Data Equipment..........  On-hook...............         40    1000-4000
                          Off-hook..............         40     200-4000
(f).....................  Off-hook..............         40     200-4000
(g).....................  On-hook...............         60     200-1000
                          On-hook...............         40    1000-4000
                          Off-hook..............         40     200-4000
(h).....................  Off-hook..............         40     200-1000
(i).....................  On-hook...............         60     200-1000
                          On-hook...............         40    1000-4000
                          Off-hook..............         40     200-4000
(j).....................  Off-hook..............         40     200-4000
------------------------------------------------------------------------


    (b) Registered one-port terminal equipment for 2-wire non-data 
applications with loop start, ringdown, inband signaling or voiceband 
metallic channels. The one-port shall be driven from a 600-ohm metallic 
source having a 500-ohm longtitudinal impedance.
    (c) Registered one-port terminal equipment for 2-wire data 
applications with loop start, ringdown, inband signaling or voiceband 
metallic channels. The one-port shall be driven from a 600-ohm metallic 
source having a 500-ohm longitudinal impedance.
    (d) Registered one-port equipment for ground-start and reverse-
battery applications. The one-port shall be driven from a 600-ohm 
metallic source having a 500-ohm longitudinal impedance.
    (e) Registered protective circuitry for 2-wire applications with 
loop start, ringdown, inband signaling or voiceband metallic channels. 
These criteria shall be met with either terminal of the interface to 
other equipment connected to earth ground. The interface to other 
equipment shall be terminated in an impedance which will be reflected to 
the telephone connection as 600-ohms in the off-hook state of the 
registered protective circuit, and the interface should not be 
terminated in the on-hook state. Figure 68.310(e) shows the interface of 
the protective circuitry being tested and the required arrangement at 
the interface to other equipment.

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    (f) Registered protective circuitry for ground-start and reverse-
battery applications. These criteria shall be met with either terminal 
of the interface to other equipment connected to earth ground. The 
interface to other equipment shall be terminated in an impedance which 
will be reflected to the telephone connection as 600-ohms in the off-
hook state of the protective circuit. Figure 68.310(e) shows the 
interface of the protective circuitry under test and the required 
arrangement at the interface to the other equipment.
    (g) Registered multi-port equipment for loop-start applications. 
These criteria shall be satisfied for all ports when the ports are 
terminated in their appropriate networks, as will be identified below, 
and when interface connections other than the ports are terminated in 
circuits appropriate to that interface. The minimum balance coefficients 
shall also be satisfied for all values of dc loop current that the 
registered equipment is capable of drawing through each of its ports 
when these ports are attached to the loop simulator circuit specified in 
these rules. The port under test shall be driven from a 600-ohm metallic 
source having a 500-ohm longitudinal impedance. The termination for all 
ports other than the particular one whose balance coefficient is being 
measured shall have a metallic impedance of 600 ohms and a longitudinal 
impedance of 500 ohms. Figure 68.310(g) shows this termination.
    (h) Registered multi-port equipment for ground start and reverse 
battery applications. These criteria shall be satisfied for all ports 
when all ports not under test are terminated in their appropriate 
networks as will be identified below, and when interface connections 
other than the ports are terminated in circuits appropriate to that 
interface. The minimum balance coefficients shall be satisfied for all 
values of dc loop current that the registered equipment is capable of 
drawing through each of its ports when these ports are attached to the 
loop simulator circuit specified in these Rules. The port under test 
shall be driven from a 600-ohm metallic source having a 500-ohm 
longitudinal impedance. The terminations for all ports other than the 
particular one whose balance coefficient is being measured shall have a 
metallic impedance of 600 ohms and a longitudinal impedance of 500 ohms. 
Figure 68.310(g) shows this termination.
    (i) Registered terminal equipment and registered protective 
circuitry for 4-wire network ports. The pair under test shall be driven 
from a 600-ohm metallic source having a 500-ohm longitudinal impedance. 
The pair not under test shall be terminated in a metallic impedance of 
600-ohms.
    (1) Registered protective circuitry for loop start, ground start, 
reverse, battery, ringdown, inband signaling or voiceband metallic 
channel applications. These criteria shall be met with either terminal 
of the interface to other equipment connected to earth ground. The 
interface to other equipment shall be terminated in an impedance that 
will result in 600-ohms at each of the transmit and receive pairs of the 
4-wire telephone connection in the off-hook state of the registered 
protective circuit, and the interface should not be terminated in the 
on-hook state. Figure 68.310(j) shows the interface of the protective 
circuitry being tested and the required arrangement at the interface to 
other equipment.
    (2) Registered multiport equipment for loop start, ground start, and 
reverse battery, ringdown, inband signaling, or voiceband metallic 
channel applications. These criteria shall be satisfied for all network 
ports when the ports are terminated as defined below, and when interface 
connections other than network ports are terminated in circuits 
appropriate to the interface. The criteria shall also be satisfied for 
all values of dc loop current that the registered equipment is capable 
of drawing through each port when the port is connected to the 
appropriate 4-wire loop simulator circuit, Figure 68.3(c) or 68.3(d). 
The terminations for both pairs of all network ports not under test 
shall have a metallic impedance of 600-ohms and a longitudinal impedance 
of 500-ohms. Figures 68.310(g) shows this termination.
    (j) Registered PBX equipment (or similar systems) with Class B or 
Class C off-premises interfaces. These criteria shall be satisfied for 
all off-premises station interface ports when these ports are

[[Page 300]]

terminated in their appropriate networks for their off-hook state, and 
when all other interface connections are terminated in circuits 
appropriate to that interface. The minimum balance coefficients shall 
also be satisfied for all values of dc loop current that the registered 
PBX is capable of providing through off-premises station ports when 
these ports are attached to the off-premises line simulator circuit 
specified in these rules. The port under test shall be driven from a 
600-ohm metallic source having a 500-ohm longitudinal impedance.
    (k) Ringing type Z equipment for loop-start applications. Equipment 
which has on-hook impedance characteristics which do not conform to the 
requirements of Sec. 68.312 (e.g., ``Type Z''), need comply with a 
minimum balance requirement of 40 dB in the frequency range 200 to 400 
Hertz, under the applicable subparagraph above. See Sec. 68.312(f) for 
conditions upon registration of ``Type Z'' equipment.
    (l) The maximum balance requirement for registered terminal 
equipment connected to digital services specified in Figure 68.310(k) 
shall be equaled or exceeded for the range of frequencies applicable for 
the equipment under test and under all reasonable conditions of the 
application of earth ground to the equipment. All such terminal 
equipment shall have a longitudinal balance in the acceptable region of 
Figure 68.310(k). The metallic termination used for the longitudinal 
balance measurements for 2.4, 4.8, 9.6, and 56 Kbps shall be 135 Ohms 
plus or minus one percent. The metallic termination used for the 
longitudinal balance measurements (M-L balance) for subrate, ISDN (BRA) 
and PSDS shall be 135 ohms +/- 1% and for 1.544 Mbps and ISDA (PRA) 
shall be 100 ohms +/- 1%. The longitudinal termination for these 
measurements (L-M balance) shall be 90 ohms in all cases.

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[45 02 FR 20853 , Mar. 31, 1980, as amended at  45 FR 54343 , Aug. 15, 1980; 
 45 FR 61632 , Sept. 17, 1980;  47 FR 39687 , Sept. 9, 1982;  49 FR 48724 , 
Dec. 14, 1984;  51 FR 950 , Jan. 9, 1986;  51 FR 16690 , May 6, 1986;  61 FR 42393 , Aug. 15, 1996]