WildBlue Yonder

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How the Upstream Cap can affect Downstream Speed

Although downstream speeds are usually high (typically in the range of 768
Kbps to 1.5 Mbps), consumer-grade Cable or DSL service often has an upstream cap
(artificial limit) of 128 Kbps, which is only about 4 times faster than a V.90
(56K) dial-up modem (limited to about 31 Kbps upstream), and a fraction of the downstream speed.

What is not generally well-known is that the upstream cap can also affect
the downstream speed -- if the upstream is saturated by uploading (e.g.,
sending a large PowerPoint file to the boss, or running a Napster
or other public service), the downstream will drop to about the same speed.
This is due to a weakness in the basic TCP Internet protocol, not
Cable or DSL per se, and not the service provider.

Cable Internet is more vulnerable to this problem than DSL. Unlike DSL,
where each subscriber has a dedicated connection to the head-end (DSLAM),
the Cable Internet upstream path to the head-end (CMTS)
is shared by all subscribers on a given cable segment. If that upstream gets
saturated, which might be caused by only a relatively few subscribers, downstream speeds
take a big drop for all subscribers on that segment.

As an illustrative example, consider a DOCSIS
cable segment with 4 upstream channels per downstream channel, and 1000
subscribers (a recommended maximum).

• The upstream channels can be anywhere from 160 Kbps (200 kHz
  
  QPSK) to 10 Mbps (3.2 MHz QAM
  16), with 800 Khz QPSK perhaps the most common in practice, giving an upstream channel capacity of 640
  Kbps.


• The downstream channel can be 27 Mbps (QAM 64) or 36 Mbps (QAM
  256), with 27 Mbps (QAM 64) perhaps the most common in practice.

The aggregate upstream capacity of 4 channels would be about 2.5 Mbps, as compared to downstream capacity of 27
Mbps. If the upstream saturates, the downstream rate will drop to about the same speed, a dramatic slowdown of about

90% (2.5 Mbps as compared to 27 Mbps).

Even with cable modems capped to 128 Kbps upstream, 2.5 Mbps
upstream capacity can handle only 20 (2.5 Mbps / 128 Kbps) simultaneously active modems before saturation. That's
generally not a problem if cable modem usage is typically (1)
infrequent, (2) downstream [e.g., web surfing], and (3) interactive [e.g., fetch-use]. The system can break down if those conditions are
not met.

This makes it easier to see why certain Cable Internet
providers condemn continuous
use of upstream (e.g., running a popular public service) as "abuse" -- each such
subscriber consumes capacity normally allocated for 1000 / 20 = 50
subscribers. Worse, there's a threshold effect: If the upstream is running at (say) 80% of capacity with typical subscribers, it takes only
4 (out of 1000) heavy upstream users at 128 Kbps to drive the upstream into saturation, thereby slowing downstream to a crawl for
all
subscribers on that segment. (Exact numbers, of course, depend on actual channel
numbers and speeds.)

For more information, see RFC
3449 "TCP
Performance Implications of Network Asymmetry".

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