Specifications

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Cisco Cable Modem Termination System Feature Guide
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Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Feature Overview
Upstream Signal Channel Overview
The upstream channel is characterized by many cable modems transmitting to the CMTS. These signals
operate in a burst mode of transmission. Time in the upstream channel is slotted. The CMTS provides
time slots and controls the usage for each upstream interval. The CMTS periodically broadcasts
Upstream Channel Descriptor (UCD) messages to all cable modems. The UCD message contains the
upstream frequency and transmission parameters associated with an upstream channel. These messages
define upstream channel characteristics including the upstream frequencies, symbol rates and
modulation schemes, forward error correction (FEC) parameters, and other physical layer values.
Cisco supports all DOCSIS error-correction encoding and modulation types and formats. Upstream
signals are demodulated using QPSK or QAM. QPSK carries information in the phase of the signal
carrier, whereas QAM uses both phase and amplitude to carry information.
Sending data reliably in the upstream direction is an issue. Because upstream spectrum varies greatly
between cable plants, select upstream parameters based on your cable plant’s return paths. Select or
customize upstream profiles for maximum trade-off between bandwidth efficiency and upstream channel
robustness. For example, QAM-16 requires approximately 7 dB higher CNR to achieve the same bit error
rate as QPSK, but it transfers information at twice the rate of QPSK.
Note The above specifications are based on predetermined sets of frequencies that may or may not have an
adequate CNR at any given time.
Upstream frequencies can be assigned as follows:
• Fixed—Configuring a spectrum group disables the fixed upstream frequency setting.
• Single subband—The CMTS administrator can define a center frequency and symbol rate such that the
boundaries of the upstream carrier stay within the subband. The frequency and symbol rate can change
within the boundary in response to noisy line conditions, based on the defined upstream parameters.
• Multiple subbands—The data carrier can remain in a particular subband for a duration of time and
then hop to another subband based on the defined upstream parameters.
Tip Measurement of noise power levels with a spectrum analyzer should be part of the procedure in initially
selecting and setting up frequency allocations. Cisco recommends having fixed frequency settings
during early deployment, at least until amplifier cascade adjustments or plant repair have become
infrequent enough that they no longer significantly affect the nodes connected to the upstream port.
Upstream Frequency Changes
As stated in the DOCSIS radio frequency interface (RFI) specification, RF channel migration or upstream
frequency change occurs when a change in the UCD message is broadcast to all cable interfaces.
The speed of channel migration via the UCD message is typically less than 20 milliseconds (ms). During
this time, upstream transmission is interrupted until the cable interface transmitter adjusts to its new
frequency. Data is stored in the cable interface's buffers during this time and is sent when the frequency
hop is complete.
Station maintenance intervals are used to perform per-modem keepalive polling. The CMTS polls each cable
modem at least once every 30 seconds, with the default being once every 25 seconds. When ingress noise
causes loss of keepalive messages from a configurable percentage of all cable interfaces, resulting in missed
polls, a new frequency is selected from the allocation table and a UCD update is performed. The migration
time is 2 msec for any upstream UCD update. After the UCD is updated, the hop occurs. The system must
wait until a hop-threshold time interval has elapsed before it can change the UCD a second time.