Chapter 1
The chapter starts with the broader picture of networking;
it compares a computer network to the network that the United States Postal
Service has to send out our mail. The sender does not need to know how the
letter arrives at the other end, just as the user of a computer does not need
to know how data is transferred across the network. It compares our local post office to Routers
in the networking world. If the letter does not belong to that post office it
forwards it to next post office depending on the rules set until it gets to the
local post office that takes care of the receivers address.
Just like each of our homes have unique physical addresses;
computers on a network must also have a unique address in order for the
information to be able to arrive correctly. The most important address in the
Internet world is our IP address this corresponds to our unique internet
connection.
We’ve seen TCP/IP all over the place, but it wasn’t like
this when networking started. It was a mess of sorts with different vendors
having different protocols. 1980’s was IBM, DEC as well as various other
smaller vendors. In the 90’s TCP/IP came in to play and by the 2000’s it took
over the networking world. OSI which
stands for Open Systems Interconnect was a direct competitor to TCP/IP but
because of its slowness was beaten by TCP/IP. Everyone though that OSI would
win the battle since it was a lot more detailed, because of this OSI is still
widely used for Documentation and Explanation purposes. Both TCP/IP and OSI are models that we can
divide into sections.
There are two ways to look at the TCP/IP model, the older
way and the newer way. I will only be focusing on the newer way which divides
the model into Five Sections.
1.
Application 2. Transport 3.Network 4. Data Link 5. Physical
The Application layer deals, with you guessed it
applications. Web Browsers, Email Clients, Phones. It uses protocols such as HTTP for browsers,
SMTP and POP for Email clients, SIP and VOIP for Phones, and alike. The transport layer Provides services to the application
layer, one of the most important service that was mentioned in chapter one is
Error recovery. Protocols that are used
at the transport layer are TCP and UDP. Network, provides services to Transport and it
does so by using the IP protocol specifically its Addressing and Routing
mechanisms. Data Link was a bit more
obscure for me, but it basically defines the protocols that are used to send
the date over the physical medium, in the case of most every network it is the Ethernet
protocol but it also includes Point-to-Point and T1 and I would assume frame
relay. Physical is exactly that, the
physical medium that the DATA will flow through.
The Information or packets that flow through the different
levels of the TCP/IP model have different names. The reason that they have
different names is because each layer of the model encapsulates the data with
headers and some of them add trailers. The easiest way I remember it is SPF as
it if was sunscreen. Segment, Packet, Frame and they only correspond to the
middle layers. Transport is segment, Network is Packet, and Frame is Data Link.
The only layer to add a trailer so far
is the Data Link Layer. Same Layer and
Adjacent layer interaction is mentioned, Same Layers can interact when dealing
with different computer communication, and adjacent layer interaction is used
when the top layer asks the bottom layer for help with something.
What about the OSI model, that one is a bit more complex
this is why it’s awesome. It has seven layers, they are (please, do, not,
throw, sausage, pizza, away) Physical, Data Link, Network, Transport, Session,
Presentation, Application, and that is in reverse order. You can see the bottom four layers of the OSI
model correspond to the bottom four of the TCP/IP model. You can see that we added Presentation and Session.
The application layer still provides an interface to application;
in this case it also defines the process for user authentication. Same Protocols
as before are used HTTP, FTP, POP, and VoIP.
Presentation Layer defines how things look J not only does it
define it also negotiates how they look. JPG, ASCII, binary. Encryption is also
under this layer.
Session, here is another weird one for me. The session layer
knows how to start control and end the communications between devices. IT controls multiple bidirectional messages,
and is able to have a seamless view.
Transport layer delivers data to another system using
protocols like TCP and UDP. It has error
recovery and flow control.
Network layer also uses the Internet Protocol (IP). This
includes Logical Addressing, Routing, and path Determination.
Data Link Layer in the OSI model defines when a device can
send and when to wait. DL layer defines what headers and trailers must conform
to. Ethernet, HDLC
Physical, the same as the TCP/IP model and this includes the
medium that the data will flow through UTP/STP, COAX, and Fiber.
Remember Segments, Packets and Frames. The OSI model has
something similar. But they are easier to remember L7PDU, L6PDU……, L2PDU, PDU
stands for Protocol Data Units.
In the Cisco World and probably other vendors different
devices work at different levels of the OSI model. Hosts and Firewalls work
both at the Application and Transport Levels (7,4), Routers are Level Three
devices because they look at IP addresses, Switches, WAP, Cable Modems and DSL
Modems work at Layer Two, Layer one includes Repeaters, Hubs and cabling.
I understand my writing is not great but I don’t think it’s
horrid. If you have negative comments I’d love to hear them, send them to my
editor /dev/null. If you have actual
comments go ahead and post them. I’m not sure If I will make a video for this
first chapter, but If I do make one I’ll go ahead and let you know.
