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I am new here and hope I can get some help!

Recently I have been learning the Physical Layer PHY specifications of IEEE.802.11 i.e the WLAN standards. A tutorial I have used defines the general PHY packet structure that has a Long training field, short training field, signal field, data field.

It also defines three different types of frames: Management, Control and Data frames. And that each frame consists of MAC header, payload and frame check sequence. One example of frame is beacon frame.

Confusion is, what is the difference between packet and frame? I am totally confused. Does anyone have a clear explanation and understand of each.

Thanks

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The term packet can be ambiguous, so the standards often use 'acronym soup' to avoid such ambiguity. In particular, often 'packet' is taken to be the layer above 'frame' (i.e. level 3 of the OSI model).

However, as you have found out, that's not always so. Here the discussion is about physical (layer 1) IEEE 802.11 packets. So the frame is encapsulated within the data field of the packet, not the other way round.

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  • thank you very much. Do you mean that packet that people refer to in PHY standards in IEEE 802.11 is not the best term as it often refers to layer 3? also do you mean to say that the frame for example beacon frame is part of the data field of the packet description? thanks again – Tyrone May 17 '15 at 19:59
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The terms frame and packet are essentially agnostic of the communication medium (physical layer/PHY - wireless in this case). Referring to the OSI 7-layer model, a frame is a collection of data that is used to communicate on layer 2 of the OSI model, while packets perform the same function on layer 3. The layer 3 packet, is encapsulated within a layer 2 frame as the payload. Take a look at the first table here: http://en.wikipedia.org/wiki/OSI_model .

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  • thanks do you mean that frame and packet are essentially the same. then why do they have different parts? usually any phy layer description desribes the packet structure for example it has stf ltf sig data. How is this different to beacon frame? – Tyrone May 17 '15 at 2:21
  • @Tyrone Packet and Frame are essentially just names used to describe the data at different stages of encapsulation. The term Packet is used when the data has an IP header and is traversing an IP network and Frame is used when it only has a Mac address and is traversing an Ethernet link e.g. between layer 2 switches or AP and client. – Jimmy May 17 '15 at 7:53
  • Thanks, so if I need to simulate the PHY layer, do I simulate a packet or do I simulate a frame? They have difference structures @Jimmy – Tyrone May 17 '15 at 15:17
  • @Tyrone, Layer-1 (the physical layer) is independent of, and doesn't care about, frames or packets. Layer-3 packets get encapsulated into a Layer-2 frames. The frames gets converted to "bits on the wire" at Layer-1. The Layer-1 medium just transports the bits. Layer-1 doesn't know if the bits resolve to frames or random garbage; that is up to Layer-2. Neither does Layer-2 know if the Layer-3 packet it gets is valid; that is the problem of Layer-3. – Ron Maupin May 17 '15 at 19:15
  • thanks, but please check answer below. in phy specifications layer 1 packet term is used. the relationship between phy 1 packet and frame is ambiguous – Tyrone May 17 '15 at 22:37
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I'll assume that you are understanding the layered structure of the network.

+--------------------------------------+
|Layer      |Protocol Data Unit (PDU)  |
+--------------------------------------+
|Application|Message (application data)|
|Transport  |Segment                   |
|Network    |Datagram                  |
|Link       |Frame                     |
|Physical   |....                      |
+--------------------------------------+

The term "packet" refers to the data unit of somewhere between the Network and the Link layer. Some sources do explain that "packet" is the pdu of the network layer, and in most cases it does mean the Network layer pdu (used more often than the word "datagram").

It's natural that it is ambiguous to you. Look more on protocol data unit (pdu) and service data unit (sdu). Hope it helped!

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One big difference is that on the physical layer, you require a means to provide bit-level and word-level synchronization. With wireless transmission, the radio requires some more functions like level training, access control and channel aggregation.

On the data link layer all that is taken for granted and you can just use words.

802.11 is a bit more complicated due to the radio stuff but on Ethernet the distinction is easy to see.

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packet --- when data is in layer 3 we call it as packet

frame ---- when data sent from layer 3 to layer 2 (data link) we call it as frame

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