The document discusses the seven layers of the OSI model: Physical, Data Link, Network, Transport, Session, Presentation, and Application. It provides details on the functions of the Physical, Data Link, and Network layers, including framing bits, error control, addressing, encapsulation, and routing. The Transport layer establishes connections between applications on different hosts. The top three layers (Session, Presentation, Application) deal with coding/decoding data and establishing communication between applications.

1. OSI
Physical Layer: http://learn-networking.com/network-
design/the-difference-between-straight-through-
crossover-and-rollover-cables
The Physical Layer is the copper, fiber, or wireless that
connects network devices. The bits that arrive here are
encoded according to the protocols that govern their
type of end device. The control code is added to
indicate the beginning and ending of each frame of bits.
The encoded bits are transformed into signals
(voltage, light, or radio waves) and sent to the
destination device.

2. The Data-Link layer arranges the bits it receives into
frames and provides synchronization, error control and
flow control. It has two sub layers that are the logical link
control and media access control. It begins the process of
encapsulation and adds a trailer and header. The source
and destination addresses are included along with frame
check sequence values, quality of service, and type of
protocol for whatever media is present. It links the
hardware and software layers together.

3. Network Layer
Translates addresses from logical to physical and visa-
versa the other way. This is where addressing the
networks and subnetting comes into play.
Encapsulation of frames: IP header, source IP
address, destination IP address.
All available common network layer protocols.
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4. Type of service determines the priority of the packet
and looks for the Quality of Service to decide who goes
first. EX: Voice transmits faster than e-mail.
Time to Live ensures a packet can’t get stuck looping in
to infinity.
Protocol determines which type of data packet it is:
ICMP, TCP, UDP
The header checksum provides a security measure to
make sure the data is exactly how it was from
origination to destination.
Packet Length is between

5. Routing:
Carrying Data end to end:

9. Transport Layer: Visual Version

15. Source Port
Address or name of remote host
destination Port
Connection State

30. The top three layers
Application Layer: Is the first step to getting data onto
the network.
Three primary functions:
1. Coding and conversion of application layer data to
ensure that data from the source device can be
interpreted by the appropriate application on the
destination device.
2. Compression of the data so that it can be
decompressed by the destination device.
3. Encryption of the data for transmission and
decryption of data by the destination device.
Applications are computer programs with which the user
interacts and that initiate the data transfer process at
the user’s request.
Services are background programs that provide the
connection between the application layer and the lower
layers of the networking model.
The most widely-known TCP/IP Application layer
protocols are those that provide for the exchange of
user information. These protocols specify the format
and control information necessary for many of the
common Internet communication functions. Among
these TCP/IP protocols are:

31. DNS: TCP/UDP port 53
Telnet: TCP port 23
SMTP:TCP port 25, POP:UDP port 110
DHDHCP: UDP port 67
port 80a
TCP ports 20 and 21