The OSI Model layers by jianghongl


									                                         QuickTime™ and a
                                     TIFF (LZW) decompresso r
                                  are neede d to see this picture.

Figure 1: Encapsulation of Data (Cisco)

Figure 2: Encapsulation of Data (Microsoft)
The diagrams in Figure 1and Figure 2 show that most instructional material us very
similar diagrams for this concept.
Historical points
Points from week ending 22-1-05
      Have found the audio of the first time the above and am now transcribing.
      Looked at other conceptually different
Points from week ending 29-1-05
      Met with Helen Armstrong
             o 2 more papers accepted, both will be presented by Helen.
                     The Forth World Conference on Information Security
                        Education, Moscow, May 18-20, 2005
                     EduCause Australasia, Auckland, April 5th -8th 2005-01-30
      Continued audio transcription
      CCNA instructor courses running from 24th Feb to 11th Jan kept things busy
         along with enrolments.
Points from week ending 5-02-05
      First draft of the modified description.
      Tried on Sue Murray.
             o Diagram attached.
The information below is the first run through the students will get. In the table, the
first column relates to the text in the on line material, (only the figure numbers are
altered so the description is still relevant. The second column notes alteration that
may be necessary for the vision impaired students.

Included is a brief background that I believe is necessary to be able to understand
Figure 1. Without some conceptual framework the diagram is meaningless to

 I have tried this out on my brother, who is legally blind and has very little technical
understanding of the subject matter. (he is a political scientist)
Background for the VI students.

NetBEUI, X.25, cabling, topology, error detection, NICs, all those cables, routers and
hubs –there’s a lot to learn and there are lots of protocols. There is an easier way to
manage this information. The use of “layered reference models” is a way to simplify
the design, and for us, the understanding of how communication occurs between a
sending computer and the receiving computer. There are 2 reference models in use
today, the Department of Defense or DoD 4 layer model and the one we are going to
look at now, the 7 layer OSI model. There are several benefits in using a standarised
layered model. Firstly Complex network concepts are divided into more manageable,
more easily understood layers. Secondly, one part of the model can be changed
without changing all the other parts of the model and finally defining a universally
accepted standard interface for multivendor integration aids with interoperability.
These are the key points to remember, however there are others that come into play -
which we will discuss later.

The OSI Model layers

When a tech refers to “Layer 2” Networking or “Layer 3 devices,” they almost always
are referring to the Open Systems Interconnect (OSI) Model. The OSI Model was
developed in the later 1970s by the International Organization for Standardization
(and that’s ISO, so it’s the OSI Model developed by ISO) to deal with a vendor issue
that threatened the growth of the IT industry. When networks were first created,
computers talked only with computers with which it shared a manufacturer. For
instance, DECnet didn’t talk to IBM, Primenet only spoke to other Primes etc. This is
obviously a very bad situation, if you bought your first lot of equipment from one
vendor you were trapped into buying all your other equipment from that same vendor.
(A bit like Microsoft nowadays)

The OSI Model’s job is to divide the complex task of host-to-host networking or
communication ,(often called internetworking), into a 7-layer vertical stack that looks
like this:2

Is above the
Is above the
Is above the
Is above the
Is above the
Is above the
    This is the ideal time to bring the blocks into play.

There are several memory tricks used to remember these layers. The most popular is
to take the first letter of each layer, Layer 7 to Layer 1, and remember “All People
Seem To Need Data Processing.” Another popular method is to take the first letter of
each layer, Layer 1 to Layer 7, and remember “Please Do Not Throw Sausage Pizza
Away.” Or make up your own. Whatever you do, learn and know the layer order.

Now how (and why) do the layers communicate?

Always read these layers from bottom to top. For instance, the Physical Layer is
Layer 1, Data Link Layer is Layer 2 on up to the Application layer, which is Layer 7.
However, Application, Presentation and Session are called the “upper” layers while
Transport, Network, Data Link and Physical are called the lower layers. One thing
you should get from that is data flows both ways. When data is leaving a host, it goes
from the Application layer down. When data enters a host, it goes from the Physical
Layer up. Each layer can communicate with the layer immediately above it and can
pass information onto the next layer. This is important, it may not seem so here but
later it really does matter that the layer talks to the layer immediately above it and
only passes data to the one below it. This is the key factor in encapsulation.

When two hosts communicate on a network, the software at each layer on one host
talks to the same layer on the other host. For instance, the Transport layer of one host
talks to the Transport layer on the other host. The Transport layer on the first host
doesn’t care one bit about how the data passes to the lower layers and over the
network, then up through the lower layers of the second host. As far as the Transport
layer of the first host is concerned, it is in direct communication with the Transport
layer of the second host. This can be likened to making a phone call. You don’t care
that your voice is converted to an electrical signal passed to a telephone exchange,
possibly digitized, passed thought to the receivers exchange, demodulated, sent as an
analogue signal to the receivers phone and then converted to mechanical energy
which creates the sound the receiver hears. You are just talking “directly” with the
other person.

Now we need to examine what is added and why. Why don’t we send our message 3
all in one go? Two main reasons. First if we wanted to send a large email and
attachment for example 1Mb, it would take approximately 19- 32 seconds over a
modem link. During this time no other information could be sent or received. Imagine
if you were sharing this link with others, this would be “unfair”. Note that fairness is a
real term in networking, we look at it in the Ethernet modules. Secondly, the longer
you continuously transmit, the greater the chance of error. Say we have a chance of 1
in 10 of a bit being in error. We transmit one bit and it all goes OK. This means
(stochastically) the chance of the next bit being in error is 1 in 9, then if that’s ok 1 in
8 and so on. So if we transmit our large file and our error rate is 1 in 10^^8 (pretty
standard for a modem) it is highly probable that at least 1 bit will be in error and with
digital information that means that the entire file is corrupt and unusable. (we will

  The difference between information, messages, sessions, data etc should have
already been covered.
look at error detection and correction later) - so we must retransmit it all. If we break
it up into smaller pieces and something goes wrong we will only need to retransmit
the small chunk that was corrupted. So the first step in our message being transmitted
is to have the application, in this example our email client (outlook, Eudora etc) “talk”
to the application layer and pass the message into our protocol stack (the osi model as
implemented on this platform)

Application Layer
This layer

PDUS and maybe SDU
CCNA Curriculum Explanation                     Modified Description                            Justification of Method
All communications on a network                 A network consists of 2 or more devices         Do I need to name the sender and receiver
originate at a source, and are sent to a        (may be computers or some other network         or does that just complicate matters?
destination. The information sent on a          device such as printers, routers etc)           Something along these lines may be
network is referred to as data or data          connected by some type of transmission          needed at this point “Broadly speaking, we
packets. If one computer (host A) wants to      media, for example wires or fibre optic         start with and idea, for example “I want to
send data to another computer (host B), the     cable. All communication between devices        tell Bill something” this “something” is the
data must first be packaged through a           originate at a source, (the sending device)     information and we cannot send an idea
process called encapsulation.                   and are sent to a destination device (the       over a piece of wire. The idea must be
                                                receiver). If one computer (Host A) wishes      encoded (eg written note) into symbols that
                                                to send information to another computer         can be transmitted”
                                                (Host B) the data must be packaged              Also the section labelled The OSI Model
                                                (formatted) through a process termed            layers should be inserted here to give an
                                                encapsulation. Information sent on a            overview of the figure. Most would look
                                                network is termed data. This data               briefly at the figure prior to reading the
                                                undergoes several transformations as it         text, so I feel it should be included here.
                                                traverses the network. This transformation
                                                or packaging of the data is termed
Encapsulation wraps data with the               Encapsulation wraps data with the                  1. Student must know what a protocol
necessary protocol information before           necessary protocol information needed to               is. “A protocol is an agreed method
network transit. Therefore, as the data         facilitate the communication, much like                of communication” In the case of a
packet moves down through the layers of         that of placing a letter into an envelope and          class we use the protocol “English”
the OSI model, it receives headers, trailers,   writing the address on the front. To                   so we can understand each other.
and other information.                          simplify the implementation and                        The same applies to computers,
                                                understanding of this process, a model is              they must speak the same language
                                                used to break down the many operations                 to be understood. Eg TCP/IP
                                                performed on the data. This model is the           2. Need to explain what a model is.
                                                “7 layer” OSI model. As should be               *Use a pancake or in tray analogy here?
                                                obvious, it consists of 7 layers, with each     There is a need to use some examples at
layer “stacked” on top of the next. (This is    this point. We have an email message that
where the term protocol stack comes             we type into our email client, outlook for
from.) *The information, once encoded           example. The act of typing converts the
into an electronic form, is passed into the     information, our idea we need to
topmost layer, as data, where it is             communicate, to and electronic form, the
processed and passed down the stack to the      text in the mail message. This message is
next layer. Each of these layers is a process   the data we want to send. The text is
that can perform some service on the block      passed into the top level layer (called the
of data that is passed to it from the           application layer) which needs to know
adjacent layer. To perform the correct          what sort of data it is. So a label is placed
operation on the data, the layer needs to       on the front of this block of data (our email
add some information and request that           text) to say we are sending it as an email
some action be taken. This is called a label.   message and will use SMTP to get it to the
Labels are prepended to the data so that the    receiver.
layers below know what actions to               There is a need to bring in serialisation of
perform.                                        the data but I don’t think that this is the
The model divides the functions of a            correct place, may jus confuse matters as
protocol into a series of layers. Each layer    the data at this point is a discrete block of
has the property that it only uses the          memory and is passed as an object.
functions of the layer below, and only
exports functionality to the layer above. A
system that implements protocol behaviour
consisting of a series of these layers is
known as a 'protocol stack' or 'stack'.
Protocol stacks can be implemented either
in hardware or software, or a mixture of
both. Typically, only the lower layers are
implemented in hardware, with the higher
layers being implemented in
To see how encapsulation occurs, examine       To see how encapsulation occurs, we need
the manner in which data travels through       to examine the manner in which data
the layers as illustrated in Figure 1. Once    travels through the layers. Once the data is
the data is sent from the source, it travels   sent from the source, it travels through the
through the application layer down through     application layer down through the other
the other layers. The packaging and flow       layers. The packaging and flow of the data
of the data that is exchanged goes through     that is exchanged goes through changes as
changes as the layers perform their            the layers perform their services for end
services for end users. As illustrated in      users. Networks must perform the
Figure 1 , networks must perform the           following five conversion steps in order to
following five conversion steps in order to    encapsulate data:
encapsulate data:                              Note that the first 3 layers deal with the
                                               first step.
1.      Build the data – As a user sends            1.      Build the data – As a user          The bolded text relates directly to the
an e-mail message, its alphanumeric                         sends an e-mail message, its        labels within the diagram itself.
characters are converted to data that can                   alphanumeric characters are         Protocols are the method of
travel across the internetwork.                             converted to data that can travel   communication as explained earlier
                                                            across the internetwork. The        Need to bring in the fact that layers cannot
                                                            first 3 layers deal with this       look inside the data handed to it. This is
                                                            conversion. The message is          indirectly illustrated in the diagram by the
                                                            now termed a data stream. This      darker shaded blocks labelled data. And is
                                                            relates to the fact it (later) is   shown much better by Figure 2:
                                                            passed as a sequential stream of    Encapsulation of Data (Microsoft)
                                                            bits with the header being sent     Stressing the addition of headers as this is
                                                            first, followed by the data.        a part that is difficult to convey. The
                                                    The message is first passed into the        example of http is inferred in the diagram
                                                    Application Layer. This is the              so must be explicitly explained in the
                                                    topmost layer in the model. At this         textual version. See Figure 2 AH
                                                    point the data is treated as a complete
                                                    block. Ie the entire message is passed      It is necessary to explain briefly what each
                                                    to the layer. The application layer has     of the layers do so as to ensure that they
    the protocols that allow applications to    are seen as separate layers. This is not
    talk to the network. Eg HTTP which          done in the original version but the
    sets the rules on how your browser          diagram shows clearly that there are 3
    asks to get a webpage from a remote         upper layers involved.
    server. A header is added at the front of   PDU is also not shown in the Cisco
    this data containing information such       diagram but is a necessary term in the
    as this data relates to SMTP, its           explanation. It is just the term used to
    version number (of the protocol) and        describe the data with its header as a single
    many other “fields”                         entity.
    Presentation layer receives the             If asked, it would be pointed out that
    application layer PDU (protocol data        instructions on what to do with the data is
    unit), that is the data with the attached   handled by SDUs (service points) which is
    header information. The presentation        a separate issue and not covered here.
    layer is responsible for taking care of
    the syntactical differences in unlike
    An example of a presentation service
    would be the conversion of an
    EBCDIC-coded text file to an ASCII-
    coded file. So the header added at this
    point may specify the data is encoded
    in EBSDIC rather than ASCII format.
    Note that this layer is rarely used and
    may not add a header at all.
    Session Layer The Session layer
    provides the mechanism for managing
    the dialogue between end-user
    application processes. Again it is rarely
    used and may or may not add a header.
So at this point we have passed our data
through 3 layer. Each has had the data
                                             passed to it from the layer above but has
                                             no knowledge of what that data contains. It
                                             treats the entire package it was passed as a
                                             “closed envelope” ie it does not examine
                                             the contents of what is passed. A key point
                                             in encapsulation. All 3 layers are often
                                             referred to as the “upper layers” as they are
                                             often implemented as a single layer.

2.      Package the data for end-to-end         2.       Package the data for end-to-        No mention of why data is broken up into
transport – The data is packaged for                     end transport – The data is         segments is made here. This is a complex
internetwork transport. By using segments,               packaged for internetwork           layer and is covered in detail in later
the transport function ensures that the                  transport. By using segments,       modules.
message hosts at both ends of the e-mail                 the transport function ensures      Stress the fact that headers are added to
system can reliably communicate.                         that the message hosts at both      every block of data at each level of the
                                                         ends of the e-mail system can       stack.
                                                         reliably communicate.
                                             The Transport Layer is passed the data
                                             from the presentation layer. This layer
                                             firstly breaks the data up into more
                                             manageable pieces termed segments. A
                                             great deal of information is added at this
                                             point. (covered later) This is where we first
                                             add addresses to the information we want
                                             to send. We need to know what application
                                             this data belongs to and because it is now
                                             broken into smaller pieces, it is numbered
                                             so it can be put back together in the correct
                                             order. The addresses and sequence
                                             numbers make up the header that is added
                                             to each segment.
3. Add the network IP address to the          3. Add the network IP address to the        We see in the diagram (Figure 1) that each
header – The data is put into a packet or     header. At the Network Layer each of the data block is passed separately and has its
datagram that contains a packet header        segments is passed to the network layer,    own header added.
with source and destination logical           one at a time, and each is treated as a
addresses. These addresses help network       separate entity. Ie there is no connection
devices send the packets across the           between the segments passed from the
network along a chosen path.                  transport layer as far as the network layer
                                              is concerned. The data now termed a
                                              packet or datagram that contains a packet
                                              header with source and destination logical
                                              addresses. These addresses help network
                                              devices send the packets across the
                                              network along a chosen path.

4.      Add the data link layer header        4.      Add the data link layer header             Framing marks the start and end of the
and trailer – Each network device must        and trailer Each datagram or packet is             information being sent so needs to know
put the packet into a frame. The frame        passed to the Data Link Layer. Each                when to start reading the bits and also
allows connection to the next directly-       network device must put the packet into a          when to stop. It is the first place a trailer is
connected network device on the link.         frame. The frame allows connection to the          added.
Each device in the chosen network path        next directly-connected network device on
requires framing in order for it to connect   the link. Each device in the chosen
to the next device.                           network path requires framing in order for
                                              it to connect to the next device. The packet
                                              may also be broken into smaller pieces at
                                              this time. So a single packet may become
                                              several frames, each with its own header
                                              and, for the first time in the stack, a trailer.
                                              The trailer is similar to a header, except its
                                              appended to the frame ie put on the end of
                                              the information. This often contains
                                              information relating to the end of the
                                              packet so the receiver knows that all the
                                              data for this frame has arrived.

5.       Convert to bits for transmission –   5.      Convert to bits for transmission –
The frame must be converted into a pattern    The data link Layer passes each frame to
of 1s and 0s (bits) for transmission on the   the Physical Layer. The frame must be
medium. A clocking function enables the       converted into a pattern of 1s and 0s (bits)
devices to distinguish these bits as they     for transmission on the medium. A
travel across the medium. The medium on       clocking function enables the devices to
the physical internetwork can vary along      distinguish these bits as they travel across
the path used. For example, the e-mail        the medium. The medium on the physical
message can originate on a LAN, cross a       internetwork can vary along the path used.
campus backbone, and go out a WAN link        For example, the e-mail message can
until it reaches its destination on another   originate on a LAN, cross a campus
remote LAN.                                   backbone, and go out a WAN link until it
                                              reaches its destination on another remote
                                              LAN. The purpose of this layer is to take
                                              each frame, add some timing information
                                              and encoding, convert this bit pattern into
                                              and electrical or optical signal that can
                                              traverse the medium and finally arrive at
                                              the receiver.
The Lab Activity will provide an in depth     There is a need to create a separate
review of the OSI model.                      document to describe the lab activities and
                                              interactive media activity
The Interactive Media Activity requires
students to complete an encapsulation
process flowchart.
Once the signal, be it electrical or optical    Points are just notes to me, not fully
reaches the destination, the reverse must       explained yet.
occur to that described above. Each header
is read and removed from the data as it
travels from layer 1 up to layer 7, where
the data (it has been fully reconstructed) is
passed out of the application layer to the
client application for processing or display.
Eg the web server was asked for a
particular web page.
Each layer in the model do not know what        Missing from the textual description. This
function the layers below it perform.           is shown clearly in the diagrams by the
Layers act as if they are communicating         connection between layer from source to
directly with their peer layer at the           destination at each of the 7 layers.
destination (or source). Eg the transport
layer at the source “thinks” it is talking
directly to the transport layer at the
destination. It does not know, or care what
layers below it are doing. Think of it like
an interpreter, You wish to speak to
someone who only speaks German, You
only speak English. However you know
some one that speaks French and English
(so you can talk to them) they know
someone who speaks French and German.
Communication can occur, you speak
English to the first person who translates it
to French for the second person, who then
translates it to German for the person to
whom you wish to converse. Does it matter
that you do not know what the
                                     conversation is between the interpreters?
                                     No you are only concerned that you are
                                     talking to the “peer” you intended to. In
                                     the same way peer layers go through
                                     interpreters (other layers) and talk directly
                                     to their peer layer at the destination.
Comment Colour Legend
Relates to why a part is necessary
Notes that need to be checked

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