Hi,
1. So incase of PPP ethernet header will not be available Right ?
Correct. With PPP framing, there will be no Ethernet frame present. Check the attached PPP.pcap file (unzip the ZIP attachment first) with Wireshark - it shows a simple ping between two routers that was captured on a serial link running pure PPP. Notice that there is no Ethernet, simply PPP, then IP, then ICMP.
2. Similar is the case if I use Framerelay as well that ethernet header will not be there ?
Correct, just like with PPP. Check the attached FR.pcap file with Wireshark.
3. Can I use encapsulation framerelay or encapsulation PPP in ethernet interfaces ?
No. As opposed to serial interfaces that are "naked" as I called them and that have a number of applicable Layer1 and Layer2 versions, Ethernet is a technology with a fixed frame format and a very specific set of alternative Layer1 versions (different for each speed and medium) and it is not possible to change the framing on Ethernet interfaces to anything else. Ethernet network cards are only prepared to handle Ethernet frames, and they would not understand any other frame format.
There is a particular technology called PPP over Ethernet, or PPPoE, in which a software client on a device (PC or a router) generates a PPP frame and sends it over an Ethernet card to a neighboring device. This is often used in DSL access scenarios. However, in that case, the PPP frame is put into an Ethernet frame. The entire chain of encapsulations is Data -> TCP/UDP -> IP -> PPP -> PPPoE session header -> Ethernet. Check the attached PPPoE.pcap file for an example. Even here, however, the outer frame format is Ethernet because the frame was sent out from an Ethernet network card. An Ethernet network card can never send out anything different that an Ethernet frame. There may be a PPP frame inserted into the Ethernet frame but to every Ethernet device, it is just data and they do not care - it's a payload.
To keep things simple: Serial interfaces can process frame formats such as HDLC, PPP, Frame Relay, but they do not generally support Ethernet formats, mostly because nobody really needed that. On the other hand, Ethernet network cards can process only Ethernet frame format because, again, nobody ever wants an Ethernet network card to process any other frame types. Once a particular frame type is used, it is not combined with Ethernet or any other frame type. The only exception to this rule is the PPPoE in which a PPP frame can be inserted into an Ethernet frame for special purposes.
Best regards,
Peter
