Let us assume that a service provider wishes to limit the worst case data throughput per user at peak busy hour
to not less than 256 Kbps. Thus it means that for a given line card with a single one 27 Mbps usable
64QAMdownstream channel bandwidth, the total number of simultaneously active subscribers has to be
limited to 27000000/256000 ~= 100.
Since simultaneously active subscribers are assumed to be 10% of total subscriber base, we end up with a
number of around 1000 subscribers per line card. Cisco strongly recommends this number not be stretched
beyond 1500 subscribers per line card, since service will be severely degraded during the busy hour. This
could lead to disconnection, offline status, extremely erratic performance from the cable modem customer
point of view, longer than average ranging time for modems attempting to re−register, as well as other system
and performance anomalies.
Given a fairly uniform distribution of these subscribers across all 6 upstreams, assuming the use of a MC16c
card, the customer will end up with a total of about 200−250 subscribers per US port.
Another way to look at it is to try to limit the number of contending users per US port during peak activity
time. Cisco recommends the average number of simultaneously active/contending CMs per US be kept around
10−20. These recommendations are also based on how the multiplicity factor of colliding modems on each US
receiver can lead to saturation and clipping on the HFC network. Once we have a limit on max contending
CMs per US port, we can get total max CMs per US port by roughly multiplying the number by 10 (10% peak
demand assumption). Cisco has thousands of DOCSIS CMTS units operating world wide. Using engineering
data combined with real world field experience Cisco has proven that, based upon the DOCSIS protocol and
how it operates, deployments enjoy maximum success when subscribers per US does not exceed 250.
Of course every service offering is different and the customer must determine, based upon techniques
discussed here as well as other traffic engineering sources, what the proper number of modems for their
situation should be. Cisco can only make recommendations as determining the maximum or proper number of
cable modems per upstream/linecard is highly subjective based upon a multitude of factors.
从老哈那转CISCO的贴里面COPY过来的,但是都是英文的,只能看懂一小部分
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