Upstream Admission Control
Admission control is an example of a CMTS vendor-specific option, which adds capabilities to a DOCSIS 1.0 or 1.1 network. With admission control the cable modems receive additional information in their configuration files in vendor-specific TLV fields. Cable modems are unaware of the meaning of these TLVs but the CMTS makes use of them to provide added benefits, control, and capabilities to the system.
In a scenario where several upstream channels are in use and many cable modems are connected with throughput guarantees provisioned through the DOCSIS configuration file, it is possible that all cable modems might connect to a single upstream channel that does not have the capacity to meet all bandwidth guarantees. Although not a part of the DOCSIS specification, some vendors have implemented upstream admission control features to limit the overuse of upstream bandwidth and to help load-share modems across multiple upstream channels.
Upstream admission control monitors the aggregate amount of guaranteed upstream bandwidth expected by connected cable modem devices. The parameter is set relative to upstream channel width and is typically expressed as an over-subscription percentage. After the sum of traffic guaranteed to connected modems exceeds the set parameter, cable modems are denied registration and forced to connect to some other upstream channel. If no upstream bandwidth is available on any upstream channel, modems are denied registration completely. Upstream admission control and guaranteed bandwidth together are effective means to ensure that no single customer will ever be prevented from accessing the network, and it also allows the cable service provider to guarantee that no single upstream channel can become so congested that service levels reach unacceptable levels.
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