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Cisco CVA122/CVA122E Cable Voice Adapter Description
The CMTS receives the downstream data from its Internet or other WAN connections. It addresses the
data to the appropriate Cisco CVA122/CVA122E Cable Voice Adapter and modulates it for
transmission on the cable network. When the data arrives at the subscriber's site, the cable voice adapter
modulates it for transmission over the Ethernet or USB connection to the appropriate CPE device.
Upstream Transmissions
The data transmitted in the upstream direction (from the user to the network) is typically much less than
that on the downstream direction, so a smaller bandwidth is allocated to it. The upstream transmissions
share a 200 kHz-wide to 3.2 MHz-wide channel in the 5 to 42 MHz range, providing a bandwidth of up
to 10 Mbps.
Depending on the quality of the physical plant and the CMTS used at the headend, users on a single
downstream can be allocated across several upstreams to ensure a responsive network. Service
providers can allocate different upstream bandwidths depending on the services purchased by a
subscriber. For example, a subscriber purchasing basic home service might be allocated a 128 kbps
upstream, while businesses purchasing premium services might be allocated a 384 kbps upstream.
The Cisco CVA122/CVA122E Cable Voice Adapter receives the upstream data from the CPE devices
to which it is connected. It modulates this data for transmission on the coaxial cable system to the
CMTS. The CMTS then routes the data to the appropriate destination (local server, Internet, and so
forth) through its WAN interfaces.
All DOCSIS cable modems use a request/grant mechanism to obtain bandwidth on the upstream. The
CMTS grants the bandwidth according to the requestor's level of service, ensuring that the cable modem
is not exceeding the maximum bandwidth for upstream transmissions that has been specified by the
subscriber's service agreement.
Note
End-to-end throughput varies based on the design and loading of network components, the
mix of traffic, the processing speed and interface of the host server(s), the processing
speed and network performance of the subscriber's computer, as well as other parameters.
Because the network can be configured to support multiple levels of service with different
performance requirements, the subscriber's service level agreement also affects
throughput. DOCSIS also specifies fundamental performance limitations to ensure that the
majority of subscribers experience good performance, rather than permitting a few users
to consume the entire capacity.
Voice Operations
The Cisco CVA122/CVA122E Cable Voice Adapter supports Voice over IP (VoIP), which transmits
voice, modem, and FAX calls over a TCP/IP network such as the Internet. Depending on the services
purchased from the cable service provider, subscribers can place and receive calls without using the
local exchange carrier.
The cable voice adapter contains two voice ports, which support two simultaneous voice, modem, and
FAX calls. You can connect a single-line telephone or FAX device to each voice port, or you can connect
a dual-line telephone or FAX device to the first voice port.
Subscribers can connect standard analog telephones, modems, and FAX machines to the
Cisco CVA122/CVA122E Cable Voice Adapter; IP telephones are not required. Depending on the voice
network set up by the service provider, subscribers can place calls to numbers that are in the existing
telco network; the called party does not have to be using VoIP telephone service.
Cisco CVA122/CVA122E Hardware Installation Guide
1-10
Chapter 1
Product Overview
OL-0799-02 (05/2001)
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Troubleshooting
