Chapter 2: The OSI Model
Chapter 2 Outline
1. On the Test
2.1. Given an example, identify a MAC address.
2.2. Identify the seven layers of the OSI Model and their functions
2.4. Identify the OSI layers at which the following network components
operate: hubs; switches; bridges; routers; network interface cards.
2. The OSI Model
a. The OSI Model was designed by the International Standards Organization
(ISO) as a structural framework for the rules of data communication.
b. This model allows any vendor who adheres to the framework to design a
component for data communications with the assurance that it can be used
with products from any other vendor and provide seamless
interoperability.
3. The Seven Layers
a. Each layer of the OSI Model adds information to the data packet so that it
can be passed to the next layer in the model
b. Every layer is responsible for a part of the communications process.
c. To remember the seven layers, there are two mnemonics:
i. “All People Seem To Need Data Processing”
ii. “Please Do Not Throw Sausage Pizza Away”
4. The Physical Layer
a. The Physical layer defines the way data is transmitted over the physical
connections of the network.
b. Other component characteristics of this layer include physical topology,
connection types, signal type, baseband and broadband, and bit
synchronization.
5. Physical Topology
a. Bus: The bus topology is the easiest to install because all computers
connect to the same piece of cable.
b. Star
i. The most common physical setup
ii. Requires a hub or switch and is easy to troubleshoot and repair
c. Ring
i. Connects nodes in a circle where the nodes can only “talk” to two
other nodes – the one immediately preceding on the ring and the
one immediately succeeding on the ring.
ii. Data moves in a single direction.
iii. A node can only “talk” if it has a token in its possession.
d. Mesh
i. All nodes are physically connected to all other nodes on the
network.
ii. This is 100% fault tolerant.
e. Wireless
i. Wireless is rapidly emerging in the network communications
market.
ii. Wireless systems rely on either radio frequency (RF) or infrared
(IR) frequencies to connect directly to each other or to access
points (AP).
iii. Wireless devices connect using either ad hoc mode or
infrastructure (multipoint networking).
f. Ad Hoc Mode
i. In ad hoc mode, devices send out radio waves that can be
intercepted by other wireless devices whenever they move within a
certain distance range of each other.
ii. The network is created as needed. Network association can be
changed by moving the location of the device closer to another
wireless device.
g. Infrastructure (Multipoint networking)
i. Devices connect to a fixed network through a wireless bridge
functioning as an AP.
ii. Usually the AP is wired to the network backbone and serves
essentially the same purpose as a switch or hub in a wired network.
6. Connection Types
a. Point-to-point connections occur when two computers are connected to
each other.
b. Multipoint connections occur when many devices are connected to the
network or transmission media.
7. Signal Types
a. Analog signaling uses a continuously varying voltage or signal. Signaling
types include Amplitude Shift Keying, Frequency Shift Keying, and Phase
Shift Keying.
b. Digital signaling uses an electrical pulse to transmit data. Signaling types
include current-state encoding and state-transition encoding.
8. Baseband versus Broadband
a. Baseband and broadband refer to the way the signals are passed across the
media.
b. Baseband transmission allows a single transmission to use the entire
bandwidth, while broadband transmission requires that many signals share
the same bandwidth.
9. Bit Synchronization
a. Bit synchronization is the timing function that allows devices to
understand the beginning and end of a transmission.
b. Asynchronous transmissions require that a start bit begin the transmission
and a stop bit close the transmission.
c. Synchronous transmissions use a clocking signal to identify the beginning
and ending of a transmission.
10. The Data Link Layer
a. The Data Link layer is subdivided into two sublayers known as the logical
link control layer (LLC) and the media access control layer (MAC).
b. The LLC is responsible for the standards that govern how network
communication will take place.
c. The MAC layer maintains the physical addressing scheme used by
network-connected nodes, allowing each node to be uniquely identified as
a participant on the network.
11. The Network Layer
a. The Network layer is responsible for two important functions: logical
addressing of nodes on the network or internetwork and routing of packets
from source to destination.
b. The logical addressing scheme used most frequently is IP addressing, a
binary method for building large numbers of unique addresses.
c. Routing is a store and forward action that allows the best path to be chosen
when sending a packet from source node to a destination node.
12. The Transport Layer
a. The Transport layer makes sure that the data is transmitted reliably
between nodes.
b. This layer also segments large packets based on the type of network.
c. Each segment is given a sequence number so that the receiving node can
recreate the message correctly.
d. Transport layer function also includes flow control, or the management of
the rate of data transmission.
e. To maintain reliable delivery of segments, this layer will issue either an
ACK (acknowledgement) or a request for retransmission if errors are
detected.
13. The Session Layer: Session layer functions include the management of sessions or
conversations between nodes.
14. The Presentation Layer
a. The Presentation layer makes sure that both nodes understand in what
format the data will arrive.
b. Its function is that of translator when two different data formats are
present.
c. Encryption/decryption are managed at this layer.
d. Compression/decompression of data is managed at this layer.
15. The Application Layer
a. The Application layer provides services to software applications used by a
user.
b. Some of the services include file access services, printing services, e-mail
services, file transfer services, and file management services.