How can hdlc and tcp/ip machines talk?

[KillKata77400]

Let's assume I have a machine that takes bit-wise HDLC connection and another machine that takes tcp/ip(such as a router or a computer). If i wanted these two machines to talk how will I be able to make it happen?

I know there are devices available but being a software student with somewhat knowledge of networking and access to a good lab in campus I was wondering if I could make the magic happen myself.

Very Thankful,

Ribash

[Richard Burts]

I do not fully understand your question. hdlc is a layer 2 protocol. For a connection to communicate successfully using hdlc it must connect to another device which is using hdlc on its connecting interface. As we generally understand the term tcp/ip it is communicating at layers 3 and higher layers. Are you asking how a layer 2 device can communicate with a layer 3 device?

HTH

Rick

[KillKata77400]

Yes sir. That is exactly what I was wondering. I apologies for my poor wording of the question. I know FarSite does sell a device that can make a layer 2 and layer 3 machines talk but I was wondering with some cables and configuration if I could make it happen.

thank you.

[Richard Burts]

In networking layer 2 talks only to layer 2 and layer 3 talks only to layer 3. It is not possible for a layer 2 device to communicate directly with a layer 3 device. There are multiple issues in trying to do that. The first example of the issue would be that the addressing is not compatible. For a layer 2 device the address is typically a mac address. For a layer 3 device the address is typically an IP address. If 2 devices attempt to communicate but neither device can understand the address of the peer then how can communication occur?

HTH

Rick

[KillKata77400]

Rick,

Thanks for your answer. For a better understanding, I've attached a poorly illustrated image of what i am trying to do. Any insights is appreciated.

[Richard Burts · Accepted solution]

Ribash

Thank you for the drawing. I believe that it does provide some clarification. From my perspective here is what I see in it:

- comp 1 has an interface that might well be an Ethernet of some type.

- comp 1 sends a frame out its interface and over the Ethernet. This frame is using layer 2 Ethernet and its payload is some layer 3 IP packet. 

- the frame gets to some converter. The converter has one interface that is Ethernet and one interface that is rs232 and uses hdlc as its layer 2 protocol over rs232. 

- the converter receives the Ethernet frame and strips off the layer 2 Ethernet header, retaining the layer 3 IP payload.

- the converter builds a new layer 2 header for hdlc, places the layer 3 IP payload into the new frame, and transmits the new frame over its rs232 interface.

- the hdlc frame is received by the other converter.

- the other converter strips off the hdlc header retaining the IP layer 3 payload.

- the other converter builds a new Ethernet header, places the layer 3 IP payload into the new frame, and transmits the new frame over its Ethernet interface. 

- the frame is received by com 2 and processed. 

You certainly could build something to function as the converter in this. Any Cisco router with an Ethernet interface and an appropriate serial interface has this capability built in.

In thinking about your question it seems to me that it is based on an assumption that hdlc and tcp/ip are either/or. Something going through the network is either hdlc or it is tcp/ip. But in current network technology it is really both/and. Any data moving through the network has some layer 2 header (hdlc, Ethernet, frame relay, or something else) and a layer 3 payload. The basic function of routers in the network is to receive a data packet on one interface, strip off the layer 2 header retaining the layer 3 payload, build a new layer 2 header, insert the layer 3 payload into the new frame, and send it out another interface.

HTH

Rick

[KillKata77400]

Rick,

Thank you for your input. I cannot tell you how grateful I am. Thanks for helping me out.

[Richard Burts]

Ribash

You are welcome. I am glad that our discussion has been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick

[Joseph W. Doherty]

As Rick explains, HDLC and TCP/IP are different logical layers. Your question is like asking how do Ethernet and TCP/IP intercommunicate.

If one device has an HDLC interface, such as a host, the other device, such as a router would usually need an HDLC interface too. Both devices would then need a common L3 protocol, such as IP.

BTW, what a router often does is move L3 traffic from one L2 media to another. For example, you might have a host with an HDLC interface connected to a router's HDLC interface. Additionally, the router has an Ethernet interface which in turn connects to another host with an Ethernet interface. The two hosts, and router, would all support IP. The two hosts then could intercommunicate using IP over their local L2 media. Further, the two hosts, if both supported TCP, could intercommunicate using that too (NB: the router would not need TCP).