Re: Why isn't ADSL rate adaptive

On Tue, 15 Jul 2008 13:45:53 +0100, The Natural Philosopher passed an
empty day by writing:

> Andy Furniss wrote:
>> The Natural Philosopher wrote:
>>> Andy Furniss wrote:
>>>> ato_zee@hotmail.com wrote:
>>>>>> They are really handy if you want to run a packet tap on the
>>>>>> traffic going into and coming out of another computer, and
that's
>>>>>> useful to help figure out why some network protocol is
>>>>>> malfunctioning or how a computer is being attacked.
>>>>>
>>>>> Plus getting a handle on the effect of errors and retransmissions.
>>>>
>>>> Not the same as loss on a dsl line though, as ethernet stores and
>>>> resends the frame so the tcp layer never sees a loss.
>>>>
>>> DSL does its own error correction and retransmision too..or something
>>> does...could be at the ATM level I guess.
>>
>> Error correction yes, when interleaving is on, retransmission no, not
>> at any level below TCP.
>
> I did the research, and it appears you are essentially correct. Well
> nice to learn new stuff!
>
>
>> TCP assumes loss means congestion and backs off so you really don't
>> want
>>   link level loss. DSl IIRC defines a 0db SNR margin as that giving a
>> BER of 1E-7.
>>
>> There are TCPs with congestion control algorithms that rely less on
>> loss - you have some if you use linux, but they don't get used on
>> internet servers, they are more for wireless I think.
>>
>>
> Most TCP implementations carry some awareness of congestion, but it
> certainly helps t tune things up a bit.
>
>
> Not sure what my bit error rate is, but I am getting as I type about one
> CRC every 10k frames.
>
>
>
>> Andy.

But the key here still remains that if the link is iffy, there will be a
loss which can affect the throughput - so upping the speed of the link
does not necessarily mean a massive increase in throughput if you get a
frame full of packets lost every so often. Imagine reading a 10,000 word
speech as fast as you can only to find the audience saying 'I didn't get
that, can you say it again' every couple of minutes.

--
begin  oefixed_in_2005.exe
[ Auf dieses Posting antworten ]
Antworten

Frank t2 (16.07.2008 09:17)
Mark (16.07.2008 10:09)
Dennis Ferguson (17.07.2008 06:22)