Specifications

4-10
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 4 Verifying Modem Performance
Initiating a Modem Loopback Test Call
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Exactly one digital to analog conversion must exist in the circuit. The digital line must connect into
a digital switch,
not
a channel bank. V.90 requires PRI (64k clear-channel DS0s). Channel banks
destroy V.90 by adding additional analog-to-digital conversions. Telcos occasionally refer to
channel banks as line-side services. Digital switches are sometimes referred to as trunk-side
services. Figure 4-5 shows this.
Figure 4-5 No Channel Banks for V.90
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In the local loop, less than three miles of twisted-pair copper line with no load coils is ideal. Load
coils limit frequencies (passband). V.90 requires a 3000 Hz passband. A circuit that does not deliver
a 3200 Hz passband will most likely not deliver V.90. Load coils are common in long loops in
North America (at the 3.5 mile mark).
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Sometimes the PSTN switch fabric is extended by a digital carrier. It is then converted to analog by
a SLC. This setup complies with V.90. The digital-to-analog conversion is moved closer to the
subscriber. However, non-integrated or universal SLCs do not comply to V.90.
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Use a recommended V.90 client modem.
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Electrical house wiring sometimes causes V.90 trainup to fail. For details, see the section “V.34
Basic Rules.”
Task 1. Initiating a Modem Loopback Test Call
Test the access server’s ability to initiate and terminate a modem call. Similar to sending a ping to the
next-hop router, this test verifies basic connectivity for modem operations. Successfully performing this
test gives you a strong indication that remote clients should be able to dial into the NAS. Figure 4-6
shows this test.
After completing this test, dial into the EXEC from a client PC and a client modem (no PPP).
Figure 4-6 Initiating and Terminating a Modem Call on the Same NAS
Note
When calling between two digital modems, you will not achieve V.90.
V.90 requires one digital and one analog modem.
Line side service
Channel bank
Trunk side service
Digital switch
Digital switch NAS
NAS
27172
27169
NAS
Modem B receive
Modem A transmit
Digital switch
Call path
PSTN