Inter-networking
models
(System
fundamentals continuation)
The word inter
according to word web dictionary means between or among. Network as
already defined is a system which permits devices to share common
resources together. Inter-networking simply means sharing resources
between different networks. Initially when networking started
devices can only interconnect between devices from the same vendors.
Meaning HP machines can never interconnect with Dell machines.
Example is take for example your HP machine cannot work on any
network that is not a HP network! Meaning all the network must be
made up of HP devices. This can be tedious, it only promote monopoly.
The most popular system will always dominate the market, end users
(consumers) have no choice than to stick with the popular device so
as to share resources. To solve this problem the ISO (international
standard organization) came up with a solution. The hero was born to
save the networking industry from monopoly, by enhancing competition.
The OSI (Open system interconnection) reference model was born.
OSI Reference Model
OSI reference model emergence in the late 1970s change the way devices interconnect!
It becomes standard which makes devices from different vendors
communicate! This sound great! No need to disturb your self
with vendor restriction! You can use devices from different vendors
to set up your own network. OSI model is grouped into 7-layers which
makes it easy for system interconnection! As we all know division of
labour leads to specialization! Laughing.....
OSI Reference Model 7 layers
Application layer Layer 7
Presentation layer Layer 6
Session layer Layer 5
Transport layer Layer 4
Network layer Layer 3
Data link layer Layer 2
Physical layer Layer1
You have to always remember
the 7layers the way they are arranged. It seems hard to but there is
a shortcut (mind you shortcuts in IT is always permitted lol! ).
The Mnemonics used are All
Peoples Seems To Need Data Processing.
That's
when you are using top down approach.
Please Do Not Teach Stupid
Peoples Acronyms
When
you are using the down to up approach (from the receiving end).
All you need to do is to take
the first letter of the sentence,
for example All( you take the
A=Application.)
Application layer: This
layer serves as
interface between the user
and the system. When using system or trying to access the network
like browsing the internet, we directly use the browser. Computer
systems only understand the binary digits 1 and 0, thus making it
impossible for the user to interact directly with those boring codes.
When browsing the internet everything from data entered, addresses
must be converted to that boring code before presenting it to the
network and vice versa. Protocols like HTTP which is used in
accessing hypertext(web-pages) resides here. The user interact with
the HTTP protocols to access any resources that deals with the
websites. Although one can access such files without the internet nor
network, this can only happens when the HTML documents are locally
resided on ones computer hard-drive.
Presentation Layer:
Guess what? Presentation layer gets its name from its function! It
presents data to the application layer. Presentation layer doesn't
only presents data, it also make sure data sent across the network is
can be read by the target devices without problem. Presentation layer
translate data into codes like ASCII code(American standard code for
information interchange), EBCDIC (extended binary coded decimal
interchange code) etc and vice versa. Data compression(reducing
the size of data bandwidth without affection the actual data. Take
for example squeezing lot of items into a box for easy
transportation.) decompression
(This is the opposite of
compression. When the squeezed items are brought back to their actual
size!), encryption(This
is the process of converting data into code, this time not for
bandwidth management but for security purpose. Take for example, you
traveled are together with friends, you and one of your friend
decided to create a code to discuss a private message without the
rest understanding what you are saying. The act of coding the message
into unique way that the rest wont understand is encryption.)
and decryption( The act of
understanding or translating the code is decryption)
is done here!
Session Layer: The
word session according to word web dictionary (sequence
of interactions over a network, or time connected to a certain
computer). This layer
creates, manages and tear down sessions between transport layer and
the presentation layer. Each
time you want to make call or send data across the network, session
is created, the devices uses the session created to send and receive
data across the network. On
finishing the session has to he tear down (destroyed). Session is
governed by rules for a successful transmission of data.
Modes
are used as dialog control for successful data delivery.
Session
modes
- Half duplex
- full duplex
- simplex mode
Half
duplex: This session mode let the systems or devices
to interact in a walkie-talkie manner, one device has to wait for the
other device to finish sending data before it can send it own data.
Just like walkie-talkie. You cant speak together at a time, one has
to finish. After finish talking he signals the other user with “over”
meaning he is though
with the session you can now talk.
Full
duplex:
Full duplex mode allow devices to send and receive data
simultaneously on the network. Just like how we interact on the GSM
networks, sender and the receiver can talk at the same time and even
argue.
Simplex:
This
is a one way transmission of data. Where one is identified as the
sender and the other as the receiver. Example is the text
message(SMS), in this mode one only sends data and the other receives
the message.
NB:Mind you English is not my tongue, you have to be patient with my rough English! After-all the purpose of communication is interconnection! hahahaha understanding.
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to be continued
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