L5 System : Signal Administration and Interconnection

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  • Copyright 1874 American Telephone and Telegraph Company T H S B I L L S m u TECHNICAL JOUSKAL

    Vol. S3, No. 10. December 1974 Printed in U.S.A.

    L5 SYSTEM:

    Signal Administration and Interconnection

    By R. K. B A T E S and D. J . Z O R N (Manuscript received October 11, 1973)

    Each L5 coaxial line is capable of carrying three 8600-channel basic jumbogroup signals that are translated to and from the L5 line spectrum (3.124 to 60.556 MHz) through jumbogroup multiplex equipment. The L5 line signal also includes several line pilots and switching, maintenance, and reference signals. Administration of all these signals is performed by the L5 line connecting circuits, which vary in complexity with the type of main station they serve.

    In addition to the circuits required to handle the various components of the L5 line spectrum, signal administration is also required at basic jumbogroup frequencies, before the jumbogroup multiplexing step, to allow interconnection to lower-order multiplex or other systems using the basic jumbogroup frequency format of 0.564 to 17.548 MHz. To perform this function, the basic jumbogroup trujik bay was developed and provides interconnection flexibility hitherto unavailable for direct connections to other long-haul systems, such as L4, LMD, TD, or TH radio, or L5 systems of another route.

    I. I N T R O D U C T I O N The L5 coaxial line is a transmission facility with a message capacity

    of three jumbogroups. These jumbogroups are placed on the line in the frequency format shown in Fig. 1. Each jumbogroup begins as a basic jumbogroup signal, formed by the mastergroup multiplex-2 ( M M X - 2 ) frequency-division multiplex or the basic jumbogroup trunk equipment. This basic jumbogroup consists of six 600-channel basic mastergroups and has a frequency assignment identical to that of the L4 line assignment.1 Each of three 3600-channel basic jumbogroups is translated to the L5 line spectrum through the jumbogroup multiplex


  • B A S I C J U M B O G R O U P

    M G 6

    M G 5

    M G 4

    M G 3

    M G 2

    M G 1

    L I N E - S W I T C H I N G S I G N A L S

    F A U L T - L O C A T I N G S I G N A L S

    E Q U A L I Z I N G P I L O T

    -17.548 M H Z

    . 5.888 J G P I L O T

    -0.584 )

    J U M B O G R O U P 3

    E Q U A L I Z I N G A N D

    T E M P E R A T U R E R E G U L A T I N G

    P I L O T

    J U M B O G R O U P 2

    E Q U A L I Z I N G P I L O T ~ v

    R E F E R E N C E F R E Q U E N C Y '

    S I G N A L

    J U M B O G R O U P 1

    E Q U A L I Z I N G P I L O T

    F A U L T - L O C A T I N G - S I G N A L

    . 68.78 M H Z 68.78 . 68.66 . 68-60 66.048 -60.656

    48.806 J G P I L O T

    - 43.672 42.880 -39.062

    27.392 J G P I L O T

    22.088 -20.982 -20.480 -20.108

    8448 J G P I L O T

    .3.124 2 J J 7 6 1.60 1.59

    Fig. 1Three-jumbogroup line frequency spectrum.

    ( J M X ) frequency-division multiplex equipment1 to form the 10,800-channel line signal.

    In addition to the message band, Fig. 1 also shows the placement of


  • several pilots used for the dynamic equalization and temperature regulation of the line,' switching signals required for the control of the line-protection switching system-3 ( L P S S - 3 ) , 4 fault-locating signals used for line maintenance tests in the transmission surveillance system ( T S S ) , 6 and a reference frequency signal used by the jumbogroup frequency supply ( J F S ) . 6 Several rules and administrative functions must be applied to the message and other signals forming the L5 line spectrum which vary under specific situations. The line-connecting circuits described in Section II perform all of these functions.

    In the evolution of the various long-haul systems such as the L-carrier and T D and T H radio systems, the need for more direct interconnection between these systems in large channel blocks became increasingly evident. Prior to the development of L5, the predominance of intersystem message interconnection was done on a basic mastergroup basis,* which requires the use of costly M M X terminals and mastergroup connectors. The need for a simpler and less expensive means of intersystem interconnection was recognized early in the L5 development, especially in view of the large 180-mastergroup capacity of this new system.

    This need was met with a new bay, designated the basic jumbogroup trunk bay ( B J G T ) , which allows interconnection of single- or multi-mastergroup signals in the basic jumbogroup spectrum; i.e., before the jumbogroup multiplexing step to the L5 spectrum through J M X equipment. Interconnections may be made directly to radio systems (using 3A wire line entrance links), LA systems, L-carrier mastergroup digital ( L M D ) terminals, M M X - 2 terminals, or other L5 systems. The B J G T circuits are covered in detail in Section III .

    II. U N E - C O N N E C T I N G CIRCUITS As mentioned in the introduction, the function of the line-connecting

    circuit is to process the L5 line signal in accordance with circuit requirements and certain administrative rules. Although many options are required to handle the various conditions that arise, there are only three basic line-connecting arrangements, one for power-feed main ( P F M ) stations, another for switching power-feed main ( S P F M ) stations and, finally, one to cover terminal stations or terminal main ( T M ) stations.*

    All line-connecting equipment is located in the line transmit-receive bay. With the exception of line-connecting equipment, transmit-

    " The basic mastergroup is the " U 6 0 0 " output of the L-type multiplex terminal (Ref. 7 ) .

    * The distinction among the four types of main stations is covered in Ref. 8.



  • - 8 0 dB

    3 . 1 2 4

    20.108 22.068

    O ) JG1 B L O C K I N G


    3.124 20.108 22.068 I I

    39.052 43.572 70.0

    lb) J G 2 B L O C K I N G

    -80 d B


    I I 3.124 39.052 43.672

    ICI J G 3 B L O C K I N G

    -80 dB

    3.124 39.052 43.572

    (dl JG1 A N D J G 2 B L O C K I N G

    - 8 0 dB ,

    I 70.0

    3.124 20.108 22.068

    let J G 2 A N D J G 3 B L O C K I N G


    Fig. 3Jumbogroup blocking filter requirements (frequency in MHz).

    receive bays are virtually identical for all types of stations. 3 , 9 The line-connecting circuits are simplest at P F M stations where no signal processing is performed, and all message continues through the office. A pad and hybrid, for fault-location oscillator ( F L O ) access to the line, are the only apparatus in the line-connecting circuit. At S P F M stations, all message is again connected through ; however, switching apparatus, filtering, and additional access to the transmission surveillance center ( T S C ) are added to the line-connecting circuits for syetem administration.


    70.0 M H z

    - B O d B

  • Terminal or T M stations require similar basic equipment units in the line-connecting circuits, but the terminal station options within that equipment are simpler. Each T M station transmit-receive bay requires six basic circuits to perform the line-connecting functions : the receiving line-connecting circuit, transmitting line-connecting circuit, pilot generator circuit, line-interconnecting circuits, line-interconnecting detector circuit, and, optionally, the lme-branching circuit. These circuits are shown in the block diagram found in Fig. 2, and are discussed more thoroughly below.

    The signal administration role of the line-connecting circuits in T M stations may be reduced to six major functions :

    (i) Message Administration: Provides routing of the message signal for any combination of the three jumbogroups.

    (it) Pilot Administration: Introduces equalizing and temperature pilots to the Line and provides pilot blocking when required,

    (tit) Switching Administration: Provides line switches, introduces switching signals to the line, and provides detection access and signal blocking when required.

    (iv) Transmission Surveillance Administration: Provides access to the line at strategic points for T S C analysis, and furnishes signal blocking when required.

    (v) Reference Frequency Signal Administration: Introduces a reference frequency signal to the line, and provides signal blocking when required and a distribution network for circuits requiring the use of a reference frequency.

    (vi) Restoration Access: Provides access to the regular transmitting and receiving lines for restoring L5 over other facilities and access to the standby transmitting and receiving lines for restoring other facilities over L5.

    2.1 Message administration The simplest of the T M station line-connecting circuits is required

    when all message passes through the office; i.e., no jumbogroups are branched to other L5 lines or are dropped to J M X equipment. In such a case, the line signal entering the line-connecting circuit from the receiving equalizer (Fig. 2) would only connect (via splitting hybrids) to the A and T H R U C K T modules at the output of the receiving line-connecting circuit. No message blocking niters are required in the T H R U C K T modules for this application.*

    ' Although no message blocking filters are required, other circuitry may be required for other line-connecting functions at T M stations, as described below.

    2134 T H E BELL S Y S T E M T E C H N I C A L J O U R N A L , DECEMBER 1974

  • In the example shown in Fig. 2, the message administration is arranged to split the jumbogroup signals into three directions: one jumbogroup connected through, one branched, and one dropped to J M X . Assuming J O I is connected through, J O 2 branched, and J O 3 dropped, blocking filters would be assigned to block J G 2 and J Q 3 in the T H R U C K T and J G I and J G 3 in the B R A N C H C K T . N O blocking niters are required in the drop circuit, since the J M X circuitry selects that jumbogroup for which it is equipped.

    Any combination of jumbogroups can be passed in the through, branched, or dropped path. The blocking is accomplished through the use of five arrangements of filter designs developed for L5 use. The requirements for jumbogroup blocking are outlined in Fig. 3.* In all but jumbogroup 2 blocking, single high-pass or low-pass filter designs are used ; however, in the case of jumbogroup 2, filters d and e are paralleled with a "split-apart" filter at the input and output of the paralleled pair. The advantage of split-apart filters, as compared to hybrids, allows better return loss with lower in-band loss.

    A minimum of 80-dB out-of-band discrimination for each blocking arrangement is required. Return loss is approximately 26 dB (75 ohms). Maximum insertion loss is less than 4.0 dB.

    2.2 Pilot admlnl$traUon

    Four full-time pilots are required for the administration of the L5 line: 2.976, 20.992, 42.880, and 66.048 MHz. The 42.880-MHz pilot is used for dynamic equalization and temperature regulation as well as the switch initiation described below, whereas the remaining pilots are used for dynamic equalization only (see Fig. 1).

    Ideally, it is desirable to maintain continuous-line pilot continuity throughout a frogging8 section (approximately 800 miles). Two factors, however, preclude this possibility. When jumbogroup blocking filters are required at T.M stations, one or more line pilots are attenuated to some degree because they fall within the attenuation region of the filters. In addition, the stability of each line pilot is adversely affected as the number increases of regulating repeaters or dynamic equalizers through which they must pass. Therefore, the pilots are blocked and reinserted at specific intervals.'

    Since the administration of individual pilot blocking and reinsertion would be unwieldy, all pilots are blocked when any pilot or jumbogroup signal is blocked. Thus, rules for pilot blocking were established such

    * A more detailed description of filter design techniques for L5 is given in Ref. 10.


  • that all pilots are blocked when either (t) jumbogroup blocking is provided or (it) pilots will otherwise pass through more than four E 3 equalizers.

    Minimum requirements for the pilot band-elimination filters are as follows :

    Return loss requirements are 26 dB (75 ohms) and insertion loss is less than 4 dB.

    2.3 Switching administration

    The line-connecting circuits provide several functions for the administration of the L P S S - 3 . A bridged access point is provided at two locations, (i) on the line side of the receiving switch and (it) on the line side of the transmitting-line switch hybrid (Fig. 2). The switch-initiator circuit monitors the 42.880-MHz pilot at these points for L P S S - 3 operations.

    Unlike L3 and L4, the L5 line switches are located within the line-connecting equipment to reduce the cable length in the through signal path. This was made possible through the use of parallel switch cabling, as opposed to the series arrangement in L3 and L4 systems.4

    Two switches are provided in the receiving line-connecting circuits, designated the R C V G S W and the T E R M S W . The R C V G S W furnishes access from the standby line when the associated regular line is out of service for any reason. This switch also provides a termination to the out-of-service line. The T E R M S W is either automatically operated as a result of a line overload condition, thereby preventing noise propagation to other systems, or manually operated under certain abnormal line conditions.4 The transmitting line-connecting circuit also includes two switches, the T B M T G S W and the M C O (message cutoff) sw. The T R M T G S W provides access to the standby line when the associated regular line is out of service. The M C O S W , also controlled by the L P S S - 3 , provides a means for opening the transmitting end of a regular line, primarily as an access for line equalization.

    As indicated in Fig. 1, the line-switching signals occupy the bandwidth between 68.76 and 68.78 MHz. These signals are blocked in the

    Frequency ( 1 / 1 0

    from nominal) Discrimination


    50 50 80 50

    2.976 20.992 42.880 66.048

    2136 T H E BELL S Y S T E M T E C H N I C A L J O U R N A L , DECEMBER 1974

  • line-interconnecting circuit at every switching station to prevent interference with switching functions between switching sections.

    In addition to those switches described for the regular bay, the standby transmit-receive bay line-connecting circuits use a receiving and transmitting line director switch (see Fig. 4).* These switches provide inputs and outputs to the 10 regular lines for switched access to the standby line.

    2.4 Transmission surveillance system administration

    Each transmit-receive bay at T M and S P F M stations has six measurement points which can be accessed by the T S C for automated remote measurements of the L5 line status. 5 In addition to the measurement points, an input access is also provided in the transmitting line-connecting circuit so that out-of-service frequency characteristic measureme...