Prof. Michael P. Harris, CCNA CCAI
ITNW 1454 – Implementing & Supporting Servers
Chapter 2
The OSI Model
Implementing & Supporting Servers - Chapter 2
The OSI Model
Chapter Overview
In this chapter we will learn about Open Systems Interconnection Model (OSI Model) that
forms the framework for data communications and its importance in the field of networking.
We will also look at specific characteristics of each OSI layer.
Learning Objectives
 Objective 2.1: Given an example, identify a MAC address.
 Objective 2.2: Identify the seven layers of the OSI Model and their functions.
 Objective 2.4: Identify the OSI layers at which the following network components
operate: repeater, hub, network interface card (NIC), bridge, switch, and router.
Lecture Notes
The OSI Model
The OSI Model was designed by the International Organization for Standardization (ISO)
as a structural framework for the rules of data communications. 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.
The Physical Layer (OSI Layer 1) —repeaters & hubs —cabling
The Physical layer defines the way data (bits) are transmitted over the physical connections
of the network. Other component characteristics of this layer include physical topologies,
connection types, signal types, baseband or broadband, and bit synchronization. Two
common devices, repeaters and hubs, operate at the OSI Physical Layer.
1. Physical Topologies
Bus
The bus topology is the easiest to install and the most difficult to troubleshoot. Cables must
be terminated at each end. The bus topology has strict physical limitations to cable length
and distance from other nodes. Example connector types include vampire taps for ThickNet
(Ethernet) and BNC T-connectors for ThinNet (Ethernet). A bus network is a broadcast
domain —e.g. any message transmitted is received by all other nodes (like a party line).
Jerry K. Ainsworth, Kristine A. Kriegel, SYSTEM ADMINISTRATION
EMCParadigm ©2004
Page 1 of 3
ISBN: 0-7638-1972-7
Prof. Michael P. Harris, CCNA CCAI
ITNW 1454 – Implementing & Supporting Servers
Chapter 2
The OSI Model
Ring
The ring topology connects nodes (hosts) in a circle or ring, allowing only nodes directly
connected to talk to each other. Data is transmitted in a single direction only. A node can
accept and respond to packets addressed to it, and/or pass the packets on to the next node.
Star
The star topology is the most commonly used physical layout for networks today. When
building a star topology, a hub, switch, or MAU (multi-station access unit) will be the central
device connecting all nodes (hosts) on the segment. When a node fails, there are three
places where it can occur: the central hub, the cable connecting the node to the hub, or the
network interface card on the node. Star topologies are relatively easy to set up, and can be
expanded as long as the physical limitations of the networking technology are not exceeded.
Mesh
Mesh networks provide redundant links between many, most, or all nodes on the network,
thus creating a fault-tolerant data communications topology. Mesh networks are time
consuming and expensive to install because of the multiple connections between nodes.
Troubleshooting this network is relatively easy because failure points are easily identified.
Wireless
A wireless topology usually relies on either radio frequency (RF) or infrared (IR) channels, to
connect, or network, using either ad hoc or infrastructure networking.
2. Connection Types
 Point-to-point connections occur when two computers are connected and exchange
information. An example is a modem connection.
 Multipoint connections use many devices connected to the transmission media,
sharing the available bandwidth. The corporate network (enterprise) is and example.
3. Signal Types
There are two broad categories of signal types: digital and analog. Analog signals use
constantly varying voltages, or waves. Digital signals use electrical pulses. All signals are
subject to deterioration of the signal over distance called attenuation. Each category of
signal has a set of specific encoding schemes that allow for the efficient transmission of data.
4. Baseband versus Broadband
Baseband and broadband refer to the way the signals are passed across the media. In a
baseband transmission a signal transmission (channel) uses the entire bandwidth.
Broadband transmission allows many signals (channels) to share the same bandwidth,
therefore being more efficient in transmitting data.
Jerry K. Ainsworth, Kristine A. Kriegel, SYSTEM ADMINISTRATION
EMCParadigm ©2004
Page 2 of 3
ISBN: 0-7638-1972-7
Prof. Michael P. Harris, CCNA CCAI
ITNW 1454 – Implementing & Supporting Servers
Chapter 2
The OSI Model
5. Bit Synchronization
Bit Synchronization is the timing function that allows devices to detect the beginning and
end of a transmission frame. Asynchronous transmissions require that a start bit to begin
the signal transmission frame and a stop bit close the transmission. Synchronous
transmissions use a clocking signal to identify the beginning and end of a transmission frame.
The Data Link Layer (OSI Layer 2)
—bridges, switches, & NICs
The Data Link layer is subdivided into two sublayers known as the Logical Link Control (LLC)
layer and the Media Access Control (MAC) layer.
1) The LLC is responsible for the standards that govern how network communication will
take place. (e.g. communicates with OSI Layer 3, the Network Layer)
2) The MAC maintains the physical addressing scheme used by nodes connected on a
network, allowing each node to be uniquely identified as a host on the network.
The Network Layer (OSI Layer 3)
—routers
The network layer is responsible for two important functions: logical addressing of hosts on
the network or internetwork and routing of packets from source to destination.
1) The logical addressing scheme used most frequently is Internet Protocol (IP)
addressing, a binary method for building large numbers of unique addresses.
2) Routing is a “store and forward” action that allows the best path to be chosen when
sending a packet from one source host to a destination host.
The Transport Layer (OSI Layer 4)
The transport layer makes sure that data is transmitted reliably between nodes. This layer
also segments data streams based on the type of network. Each segment is given a sequence
number so that the receiving node can recreate the message correctly. It also controls the
flow or management of the rate of data transmission. To maintain reliable delivery of
segments, this layer will issue either an ACK (acknowledgment) or NAK (negative
acknowledgment) request for retransmission if errors are detected.
The Session Layer (OSI Layer 5)
Session layer functions include the management of sessions (conversations) between hosts.
The Presentation Layer (OSI Layer 6)
The presentation layer makes sure that both nodes understand in what format the data will
arrive. It functions as a translator when two different data formats are present. Encryption
and decryption are managed at this layer. Compression and decompression of data are
managed at this layer.
The Application Layer (OSI Layer 7)
The Application layer provides services to software applications. Some of these functions
include file access services, printing services, e-mail services, file transfer services, and file
management services.
Jerry K. Ainsworth, Kristine A. Kriegel, SYSTEM ADMINISTRATION
EMCParadigm ©2004
Page 3 of 3
ISBN: 0-7638-1972-7