[wellylug] saturn cable connection with linux

[Mark]

Hi Don,

This is a little bit of info I am aware of about Hybrid Fibre Coax Networks:

Downspeed bandwidth  is greater than upspeed  about 30MB:10MB I believe
your bandwidth for your modem is rateshaped according to your contract
Network is partitioned and there is an effective broadcast network 
within the segment
the total bandwidth on the segment is shared between the users on that 
segment
there is filtering so that if you sniff traffic on the modem you see 
only your traffic and broadcast traffic (like a switch)
the network acts a layer 2 bridge with filtering, the modem has no ip 
address for the customer traffic
you cannot use a hub directly into the back of the modem since you only 
get 1 real ip

If you try to use a hub and use private IPs on the hub then those 
machines will not be able to use the internet because the source ip of 
the packets are in the RFC1912 address space and will be dropped by the 
internet

if you wish to use > 1 PC on the network configure 1 with Linux and then 
have 2 network cards and plug the hub into the second card as already 
described previously and then your setup will work for multiple PCS.

you canot bypass the modem - the cable is broadband (data modulated on a 
number of carrier frequencies) and ethernet is baseband where the data 
signals are written to the media without any modulation.  The cable 
modem does the modulation/demodulation.  An analogy would be connecting 
your pc serial port to the phoneline!!

any help

mark




Don Jones wrote:

>Hmmm
>
>Im getting a saturn cable modem installed on Friday
>
>>You plug your ethernet card into the cable modem 
>>
>using the ethernet 
>
>>cable they provide.
>>
>
>Is this a X-over do you know?
>
>>Saturn give you an IP address, a netmask, a 
>>
>gateway address and two dns 
>
>>servers. 
>>
>
>Whats the netmask?
>
>The description suggests that the saturn cable 
>"modem" is just a bridge/media converter/layer2 
>device. Has anyone investigated this? Essentially 
>the whole cable network is one big LAN. This 
>suggests that anyone who plugs the "modem" strait 
>into a hub with a number of hosts on it is 
>essentialy an extension of the lan. Except with a 
>bridge in between. So presumably it filters 
>incomming frames at the "modem" based on its 
>external MAC address. That will allow filtering of 
>inbound frames (ie you wont be able to sniff traffic 
>on the main LAN beyond the bridge). But what about 
>frames comming from internal hosts in the hubbed 
>setup described above, how does the bridge decide 
>which frames to forward? Either it forwards then all 
>and you have internal addresses out on the main 
>segment or does it do some kind of arp or deny any 
>ip addresses in the internal range? - whatever - 
>either would bump it up to a layer 3 device - anyone 
>know?? 
>
>I wonder what the specs of the segment are - is it 
>real ethernet (anyone know what type?) - 100Mbps or 
>Gigabit (its probably 1544Mbps or something)? Could 
>you hack together a connector to bypass the modem - 
>like a bnc 10base2 card running at whatever the 
>segments speed is? or some type of coax to cat5 
>media convertor running at the right speeds.
>
>Forgive my ramblings - but Im curious ;)
>
>Don Jones
>




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