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Cisco IP Telephony Solution Reference Network Design
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Chapter 1 IP Telephony Deployment Models
Design Considerations for Section 508 Conformance
Design Considerations for Section 508 Conformance
Regardless of which deployment model you choose, you should consider designing your IP telephony
network to make the telephony features more accessible to users with disabilities, in conformance with
Section 255 of the Telecommunications Act and U.S. Section 508.
Observe the following basic design guidelines when configuring your IP telephony network to conform
to Section 508:
·
Enable Quality of Service (QoS) on the network.
·
Configure only the G.711 codec for phones that will be connected to a terminal teletype (TTY)
device or a Telephone Device for the Deaf (TDD). Although low bit-rate codecs such as G.729 are
acceptable for audio transmissions, they do not work well for TTY/TDD devices if they have an
error rate higher than 1% Total Character Error Rate (TCER).
·
Configure TTY/TDD devices for G.711 across the WAN, if necessary.
·
Enable (turn ON) Echo Cancellation for optimal performance.
·
Voice Activity Detection (VAD) does not appear to have an effect on the quality of the TTY/TDD
connection, so it may be disabled or enabled.
·
Configure the appropriate regions and device pools in Cisco CallManager to ensure that the
TTY/TDD devices always use G.711 codes.
·
Connect the TTY/TDD to the IP telephony network in either of the following ways:
Direct connection (Recommended method)
Plug a TTY/TDD with an RJ-11 analog line option directly into a Cisco FXS port. Any FXS
port will work, such as the one on a Cisco IP Phone 7900 Series or on the Cisco VG 248,
Catalyst 6000, Cisco ATA 186/188 modules, or any other Cisco voice gateway with an FXS
port. Cisco recommends this method of connection.
Acoustic coupling
Place the IP phone handset into a coupling device on the TTY/TDD. Acoustic coupling is less
reliable than an RJ-11 connection because the coupling device is generally more susceptible to
transmission errors caused by ambient room noise and other factors.
·
If stutter dial tone is required, use an analog phone in conjunction with an FXS port on the
Cisco VG 248 or ATA 186/188.