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4. THE DIGITAL LOSS PLAN
Transmission loss is required so that talker echo is minimized. Furthermore, transmission loss must
be kept low enough so that speech volume is perceived as adequately loud. Transmission loss is the
total of all losses and gains from one end of a connection to the other. Distributed transmission
losses as well as any connection loss inserted by the switch are included. Two different loss plans are
available. They are known as:
1.
2.
Via-net loss (VNL), which has traditionally been used to assign losses for analog circuits
terminating on an analog switch
Digital loss, which is used to assign losses for digital circuits terminating on a digital switch
The introduction of digital switching systems and digital trunk facilities permit more flexible
control of the switch connection loss (insertion loss) and thereby transmission performance.
From the network perspective, transmission performance becomes entirely a function of the
port-to-port losses (total loss) from all switches in the transmission link.
The digital loss plan is significantly different from the VNL. Since the two loss plans do differ, it
cannot be assumed that the port-to-port losses measured in an all-digital network or in a combined
digital and analog network are the same as the loss measured between the same ports in an all-analog
network.
Generic 1 and Generic 2 provide for implementing the digital loss plan specified by ANSI/EIA/TIA-
464-A-1989. There are two versions of the digital loss plan. The early version is called digital fixed
loss plan and the later version is called ISL digital low loss plan. Digital COs, such as AT&T 5ESSs
or Northern Telecom DMS 100s, and toll switches, such as AT&T 4ESSs, also implement parts of the
digital loss plan.
The digital loss plan requires a 6-dB loss (connection loss) from the switch port at one end to the
switch port at the other end for private digital networks. Transmission performance for private
networks using this 6-dB loss specification is very good. The public-network and digital COs have a
similar 6-dB requirement.
Private-network to public-network connections result in a total connection loss of 12 dB — possibly
more depending on the public network switch and factors such as whether a channel bank is used.
Transmission performance for this type of connection is generally considered marginal, and if another
private-network connection (another 6 dB of loss) is added, then the end-to-end transmission
performance becomes unacceptable. These types of connections and their unacceptable transmission
performance were the motivation for developing the ISL digital low loss plan.
For both Generic 1 and Generic 2, the administration options of the ISL low loss plan allows you to
optimize transmission performance for those private network to public network types of connections
at the planning and installation stage. With proper design and application, the ISL digital low loss
plan makes possible a significant improvement in transmission performance for call-forwarded calls
involving off-network connections.
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