b. Types of Circuits. Types of circuits in a military telephone network
are illustrated in figure 2-5. Voice power of the speaker at X in local area
network 1 will be dissipated by the amount of loss in all circuits connecting
with listener Y in local area network 3. By tracing the call from station X to
station Y, you can see that the total loss is the sum of all losses distributed
throughout the network.
Tracing the call discloses the inclusion of the
following network sections.
(1) Loops to the two tributary switchboards (SWBD).
(2) Two tributary trunks.
(3) Two
tributary
switchboards
and
three
long-distance
(LD)
switchboards.
(4) Two long-distance four-wire trunks between LD switchboards.
(5) Supplemental LD trunks can be used for alternate routing
if
necessary. The supplemental trunk bypasses one LD switchboard.
c. Maximum Allowable Loss.
Although maximum permissible loss from one
telephone to another must not exceed 36 db, the preferred maximum is 20 db.
The losses may be distributed in a number of ways. As a general rule, no more
than two tributary trunks should be switched in tandem. Limitations of losses
in a telephone system are as follows:
(1) Loops connected to a switchboard must not exceed 18 db.
(2) Loop plus tributary trunks must not exceed 9 db.
(3) LD trunk net loss must not exceed 6 db, and nominally should be 3
db.
(4) If maximum loss of an LD trunk is greater than 6 db but less than 12
db, it is called terminal grade.
Terminal grade trunks may be
connected to telephone loops, switchboard, or to tributary trunks
only. They may not be connected to other LD grade facilities.
d. Long-Distance Trunks.
Long-distance trunks extend between switching
centers which may be separated by many hundreds of miles. Since losses over
these great distances probably will exceed the limitation placed on LD trunks,
amplifying equipment (repeaters) become essential.
Such amplifying equipment
raises the level of noise as it raises the level of signal. The transmission
level diagram in figure 2-6 shows three repeaters in an LD trunk.
The
combination provides 3-db net loss; that is, 0 dbm input to the trunk results
in -3-dbm output. To say it another way, receive level equals the send level
minus the net loss. If the amplifying repeaters were not used, the total loss
would be at least 45 db (25 db + 23 db), thus exceeding the maximum permissible
loss.
(1) Input signal level of 0 dbm is raised to +6 dbm by 6-db gain in
repeater 1.
(2) Line loss of 22 db drops the signal level to -16 dbm at the repeater
2 input.
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