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I connect a computer and an external device by a serial connection (RS 232). Then I send some data from the device to computer. Hyper terminal is accessing that data and show them. In this process what layers of the OSI model are used?

This is my understanding. RS-232 covers only Layer 1( Physical Layer). Hyper terminal runs in application layer. For this process only those two layers are used.

When I search stackoverflow I found something like “The serial protocol defines either the first layer or the first two layers of OSI model (depending on whether you consider the 7 or 8 bit serial packet a frame or not)”. See the full article.
What is meant by this?

Community
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user2900405
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  • *"When I search stackoverflow I found something like ..."* -- Then provide a link, so that the full content of what you quote can be read. I.E it's hard to interpret one sentence out of context, unless all you want are guesses. – sawdust Apr 23 '17 at 23:42
  • @sawdust. Noted with thanks and edited accordingly – user2900405 Apr 24 '17 at 03:18

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When I search stackoverflow I found something like “The serial protocol defines either the first layer or the first two layers of OSI model (depending on whether you consider the 7 or 8 bit serial packet a frame or not)”

That quote swaps the terms for "packet" and "frame".
(I adhere to the definition of "frame" as used in UART documentation.
This would also be consistent with the OSI hierarchy of packet->frame->bits.)

The frame in asynchronus serial communications consists of only one character (of 5 to 9 bits).
One char is not much of a message.
Whereas in synchronous links, the frame contains several bytes to compose a higher-level message unit.
So EliAlgranti in that answer seems to be pondering if this asynchronous frame qualifies as the OSI data-link layer, and if this is part of the "serial protocol".

But note that this UART framing does not seem to be part of the RS-232 standard (even though you can find some "RS-232 descriptions" on the web that do include the frame description as well as sites that omit framing.)
The actual TIA/EIA document costs US$156 from the TIA site.
The abstract for the standard makes no mention of framing.

The best confirmation that framing is not part of the RS-232 standard I found is in the 2nd to the last paragraph of "The RS232 Standard":

Note that neither the ASCII alphabet nor the asynchronous serial protocol that defines the start bit, number of data bits, parity bit, and stop bit, is part of the EIA232 specification.

So the RS-232 standard is likely only the Physical layer.

For this process only those two layers are used.

No, you've only identified the top and bottom layers.
The other layers in between those two do exist (at least conceptually), but you have not identified if they have actually been implemented and how.
Obviously there's framing per the Data Link layer.

sawdust
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  • Got the idea. So as an example, I am connecting a computer to an external device and communicate serially. At the computer vb 6 program is used to read data. MSComm control is used for serial communications at the program. Configuration of MSComm is, Bits per second 9600 Data bits : 8 Partity : odd Stop bits : 1 Flow control : None This is my understanding. Serial communication represents physical layer Configuration of MSComm (partiy,bits per second , stop bits etc.) represents data link layer And the VB 6 program i develop represents Application Layer. Is that right ? – user2900405 Apr 25 '17 at 04:05
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    Almost everybody would conflate the RS-232 spec and the async framing as *"serial communications"*. RS-232 is the Physical layer. The Data Link layer for serial communications is typically handled in hardware, i.e. the UART. MSComm seems to fit better at the Network layer. Your VB 6 program probably spans the Transport, Session, Presentation and Application layers. Apps that use the serial port typically "read" the bytes using a syscall, scan/parse the bytes to detect & verify a message packet, and then process that message. That's more than just an "Application Layer". – sawdust Apr 25 '17 at 07:08
  • that solved my problem. When communicating via serial cable and reading data from a vb 6 program, RS 232 is at physical layer. UART at Data link layer and the MSComm at Network layer. VB program handles all other layers. – user2900405 Apr 25 '17 at 07:47
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Maybe this image answers the question (it was taken from here)

serial protocol in the osi model

In this web page a found this explanation: "In the International Organization for Standardization Open Systems Interconnection (ISO/OSI) model for network communication, serial communications operates between the physical layer and the application layer. The RS–232–C standard describes the physical layer. Serial device drivers are stored in the next layer, the data-link layer. The Windows CE serial communications functions enable applications to exchange data by way of serial hardware. (...)."

Mauricio Tamayo
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