TOPIC: data on OSI layerz

OSI layers
if a data is from application layer paasing to presentation layer then session and then transport and n/w layer then datalink layer then to physical layer ...
all layer add it head etc to the data to make it possible to reach it 2 the destination correctli and safli
my question iz ... all the thing will added to data only or somthing might be else which are not enveloped with data ?
for example if message signal is sent to the destination that either it would be existed , connection may be established , or ready to accept the data, etc ?
anyone will guide me how data passing through the 7 layerz and which which controlz are sent b4 data sending to destination and how data then preparing from all the layerz upto the physical layer ? with example?

Let’s take an example of 2 Computers A and B, where A is sending an email to B:-
1. Since as a user we have access to Application layer services, so we can use some email client for this.

2. As soon as email is sent, all the contents in the email are included in data to be sent, (Remember: At Application layer there are no Packets). Package of data that moves through the OSI layers is called a [color=yelloProtocol Data Unit (PDU).[/color]

3. PDU’s at Application layer along with AH (Application header) will pass it to next layer (Presentation).

4. Presentation layer will consider “PDU’s + AH = Actual Data to be sent”.

5. Header will be added from layer 6 which will eventually passed to Session layer, where again it will consider all the PDU’s with headers as data to be sent.

6. Data at[color=yellow Layer 7,6 &5 [/b][/color]is called as PDU

7. Once layer 5 passes PDU’s along with its header to Layer 6, PDU’s will reformatted into Segments and again whatever that has been handed by session layer will be constituted as data to be sent. Each layer passes the data with its header attached and next layer consider that header as part of data.

8. Segments [/b]from layer 5 with its header will then sent to Network layer where these segments are reformatted as Packets. Which means a packet contains all the information which is passed from upper layers.

9. Layer 4 will add its own header with the packet and sent it over to Data-Link layer where packets will reformatted into Frames with Layer4 header.

10. Once it reaches to physical layer only actual data is sent over to the physical medium.

11. On the receiving end all peer layers strip off there header information and pass it to upper layer just like the way header was added, all the layers would be able to strip off the header and finally displaying actual email on Computer B in browser(As a user we have access to layer7).

:wink: Hope this will answer your query

brother u right
but my asking waz not dat as given description
becox v know dat when data will pass from each layer //// from appl layer tp physical adding header etc ..(some dividing & coverzion)

i waz asking dat either control sent to destination b4 data arrival to tiz destination ,,, (i-e either destination existz,how much capacity has to hold data, be ready data has being coming etc...)?

or everything iz sent along the data when data sending to destination
?

if example attach with description then more appreciable

checkout these links from our tutorial section

www.firewall.cx/osi-encap-decap.php
www.firewall.cx/tcp-intro.php

thanx SOSE

i had read that but there i couldnt find my answer :roll:

Okay "truesdq" what I understand now that you wanna know if any control is sent to destination that data is coming before sending data..!!!

[code:1]-Well answer for this question is bit tricky as when Application layer sends data to its next layer it has no idea whether destination is up or down. No communication send to destination before sending data till Transport layer.

As you know transport layer is known for its reliable transportation.

At this layer TCP/UDP protocols ensures destination is up and ready to receive data whereas TCP is connection oriented and UDP is connection less but still control bits are send to destination to inform that data is coming.
Handshaking, Flow control, error detection are commonly used techniques.
[/code:1]
[code:1]
So, to summarize

Layer 7 - Layer 5 -> No info about destination

Layer 4 - Destination would be checked (Handshake) to deliver data

Layer 3 - Route the packets to destination

Layer 2 - MAC will be traced

Layer 1 - Physical transmission to Destination with start and stop bits.[/code:1]