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New Member

Tapping data as voice

Dear Experts,
It is very basic and fundamental concept. But many seniors were not able to exactly explain the concept .The thought is explained below:
We are able to take the telephone wire parallely by doing parallel connection
  PSTN --------------| |------------- TELEPHONE- 1
                         | |
                         | |
                  TELEPHONE -2
Thus in a single pair cable I am able to connect two phones and use the them
In Data environment, I know it is not possible. What is the reason. I sniffed the data when Ethernet NIC comes up. I didnt see anything new related to point to point connections
PC      --------------| |------------- PC-1
                         | |
                         | |
                      PC -2
Please excuse me if it is irrelevant to this forum . I just thought of sharing this
sairam
8 REPLIES

Re: Tapping data as voice

Hello Sairam

Everything related to data & packets are relevant to these forums. Its just a common place for sharing knowledge :)

By the way, with regards to Data, you can still sniff packets going to end users using the SPAN & RSPAN technologies. Im really not sure if your question dealt with sharing data between two different PCs or for you to enable two PCs to do the same functionality ! Can you please elaborate more?

Regards

Raj

Hall of Fame Super Silver

Re: Tapping data as voice

Hello Sairam,

the first case uses circuit switched technology and you are putting two phones on the same wire pair by putting them in parallel.

In IP telephony there is the concept of shared line: the same directory number can be associated to multiple phones and when someone calls that number all phones ring together at the same time we have it in office.

How this is done? Using a multicast address as destination for signalling traffic could be a way. Then when a specific phone is picked up the call state changes to answered and the multicast ring  is stopped.

This is what can be done with Cisco call manager and IP phones.

if you mean how to make a three or more parties conference in IP telephony there is the need for a so called conference bridge that gets all the input from involved phones and prepares the output for each phone summing all inputs minus the input of that specific participant to avoid echo.

So the RTP stream is between each single phone and the conference bridge.

Going more specifically some cisco IP phones have a built in conference bridge and allows to create a three party conference over a shared line.

Hope to help

Giuseppe

New Member

Re: Tapping data as voice

Hi Giuseppe,

Thanks for your response. Raj, its for you also I am describing the case further.I got the hint that Telephonly Network is using Circuit switching

Putting in simple words. But why data packet flow is not working like telephony. I know that RJ45 interface using 1,2,3 and 6 where Tx+,Tx- and Rx+ and Rx- is used. If two PCs are connected back to back Tx will be connected to Rx and thus data transfers. Please note I am not involving Switch/Hub inbetween.

Please find the attached diagramatic representation which may reveal the scenario better than my words

Thanks in advance

sairam

New Member

Re: Tapping data as voice

Hi Giuseppe,

Thanks for your response. Raj, its for you also I am describing the case further.I got the hint that Telephonly Network is using Circuit switching

Putting in simple words. But why data packet flow is not working like telephony. I know that RJ45 interface using 1,2,3 and 6 where Tx+,Tx- and Rx+ and Rx- is used. If two PCs are connected back to back Tx will be connected to Rx and thus data transfers. Please note I am not involving Switch/Hub inbetween.

Please find the attached diagramatic representation which may reveal the scenario better than my words

Thanks in advance

sairam

Re: Tapping data as voice

Hello Sairam

Saw your diagram, and i guess you are talking about data splitters, just like a splitter for voice ? You can have cat5 splitters which can split the same layer 1 cable to say 2 PCs. but again, this is only on physical layer, and does not involve higher layers ! Data traffic as you know takes a layered approach, and its only the physical layer which would be common between two end systems.. all other layers arent identical.. data splitter is more like connecting a very small hub, to connect all the PCs in the same broadcast domain.... again, im not sure if you want to connect 2 PCs using cross over cables and take a output out of that cable or to split the traffic from PC1 to PC2 & PC3 ?

http://www.accesscomms.com.au/products/P2315.htm

Hope this helps.. all the best..


Raj

Hall of Fame Super Silver

Re: Tapping data as voice

Hello Sairam,

as Raj has noted at physical layer you can setup this sort of mini hub, but already at OSI layer2 it fails :

each PC NIC has its own MAC address, so a frame with destination MAC = PC2 NIC can be physically received by PC3 NIC, but the low level packet filter discards the frame because dest MAC is different from PC3 NIC MAC Address and it is not a broadcast or a multicast.

When we use a software sniffer the NIC captures everything ( promiscous mode) but it is a special case

So wire replication leads to no result.

On this fact works hubs, there is a waste of bandwidth and a so called single collision domain.

Hope to help

Giuseppe

New Member

Re: Tapping data as voice

Hi Giuseppe/Raj,

Thanks for your feedback.

I also looked into some search and got this link. Just for your kind reference.

www.enigmacurry.com/category/diy/

regards,

sairam

Re: Tapping data as voice

Sairam

Thanks for the ratings, and the link.. It does tell us the logical & physical way of achieving taps ! Logical process is the one we described before (with regards to SPAN ports) with port mirroring... thats what most of us would do, but only on a "ON DEMAND" basis.. The cable layout given makes sense, but its a bit difficult for me to understand   Am sure this info would be useful for others who have similar queries.. Thanks again

Raj

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