Upstream Transmissions - Cisco uBR905 Installation Manual

Chapter 1 Product Overview
Functional Overview
The CMTS receives the downstream data from its Internet or other WAN connections. It addresses the
data to the appropriate Cisco uBR905 router and modulates it for transmission on the cable network.
When the data arrives at the subscriber's site, the router modulates it for transmission over the Ethernet
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.
Typically, service providers allocate different upstream bandwidths depending on the services
purchased by a subscriber. For example, a business purchasing basic service might be allocated a
128 kbps upstream, while businesses purchasing premium services might be allocated a 384 kbps
upstream. The CMTS enforces this maximum bandwidth on a per cable modem basis, so that each user
receives the services purchased.
The CMTS allocates an upstream frequency to the Cisco uBR905 cable access router when it first
registers on the system. The CMTS can then move the router to a new upstream or a new upstream
frequency as needed to respond to current network conditions and noise levels.
Depending on the bandwidths allocated to each user, the quality of the physical plant and the CMTS
used at the headend, users on a single downstream can be allocated across several upstreams. This
ensures a responsive network for all users and simplifies bandwidth management when new users are
added, or when a user wants to upgrade to a higher bandwidth service.
The Cisco uBR905 cable access router 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.
The Cisco uBR905 cable access router uses a request/grant mechanism to obtain upstream bandwidth,
where each user on the upstream must make a specific request before being allowed to transmit on the
upstream. DOCSIS rules and the maximum bandwidth cap prevent any single user from monopolizing
the upstream bandwidth. If supported by the CMTS, the Cisco uBR905 cable access router supports
DOCSIS 1.1 concatenation, whereby multiple transmissions can be combined into one bandwidth
request, reducing overhead and maximizing bandwidth efficiency.
The Cisco uBR905 cable access router also supports unsolicited grants, in which part of the upstream
bandwidth is reserved for a particular use. This allows service providers to sell services with both a
maximum bandwidth and a guaranteed bandwidth. (The CMTS must also support this feature and be
configured on a per-user basis.)
End-to-end throughput varies based on the design and loading of network components, the
Note
mix of traffic, the processing speed and interface of the host servers, the processing speed
and local Ethernet 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.
Cisco uBR905 Router Hardware Installation Guide
1-16
OL-0598-01 (9/2000)