The 7 Layers of the OSI
Model
The OSI, or Open System Interconnection, model defines a networking framework for
implementing protocols in seven layers. Control is passed from one layer to the next,
starting at the application layer in one station, proceeding to the bottom layer, over the
channel to the next station and back up the hierarchy.
Application
(Layer 7)
This layer supports application and end-user processes. Communication
partners are identified, quality of service is identified, user
authentication and privacy are considered, and any constraints on data
syntax are identified. Everything at this layer is application-specific.
This layer provides application services for file transfers, e-mail, and
other network software services. Telnet and FTP are applications that
exist entirely in the application level. Tiered application architectures
are part of this layer.
This layer provides independence from differences in data
representation (e.g., encryption) by translating from application to
network format, and vice versa. The presentation layer works to
Presentation
transform data into the form that the application layer can accept. This
(Layer 6)
layer formats and encrypts data to be sent across a network, providing
freedom from compatibility problems. It is sometimes called the syntax
layer.
Session
(Layer 5)
This layer establishes, manages and terminates connections between
applications. The session layer sets up, coordinates, and terminates
conversations, exchanges, and dialogues between the applications at
each end. It deals with session and connection coordination.
Transport
(Layer 4)
This layer provides transparent transfer of data between end systems, or
hosts, and is responsible for end-to-end error recovery and flow control.
It ensures complete data transfer.
Network
(Layer 3)
This layer provides switching and routing technologies, creating logical
paths, known as virtual circuits, for transmitting data from node to node.
Routing and forwarding are functions of this layer, as well as
addressing, internetworking, error handling, congestion control and
packet sequencing.
Data Link
(Layer 2)
At this layer, data packets are encoded and decoded into bits. It
furnishes transmission protocol knowledge and management and
handles errors in the physical layer, flow control and frame
synchronization. The data link layer is divided into two sublayers: The
Media Access Control (MAC) layer and the Logical Link Control (LLC)
layer. The MAC sublayer controls how a computer on the network gains
access to the data and permission to transmit it. The LLC layer controls
frame synchronization, flow control and error checking.
Physical
(Layer 1)
This layer conveys the bit stream - electrical impulse, light or radio
signal -- through the network at the electrical and mechanical level. It
provides the hardware means of sending and receiving data on a carrier,
including defining cables, cards and physical aspects. Fast Ethernet,
RS232, and ATM are protocols with physical layer components.
Purpose of networking is to share
Hardware
Printers
CD-ROMs
Scanners
Software
Network versions with user license
Communications
E-mailing with E-mail server
Types of LAN’s
1) Peer-to-Peer- A type of network in which each workstation has equivalent
capabilities and responsibilities. This differs from client/server architectures, in which some
computers are dedicated to serving the others. Peer-to-peer networks are generally simpler, but
disadvantage is
everyone has same permissions with no security
they usually do not offer the same performance under heavy loads..
2) Server Based - A computer or device on a network that manages
network resources. For example, a file server is a computer and storage device
dedicated to storing files. Any user on the network can store files on the server. A
print server is a computer that manages one or more printers, and a network
server is a computer that manages network traffic. A database server is a
computer system that processes database queries.
3) Client Server –( type of network our uses) The client part of a
client-server architecture. Typically, a client is an application that runs on a personal computer or
workstation and relies on a server to perform some operations. For example, an e-mail client is an
application that enables you to send and receive e-mail.
PACKETS - Refers to protocols in which messages are divided
into packets before they are sent. Each packet is then transmitted
individually and can even follow different routes to its destination. Once all
the packets forming a message arrive at the destination, they are
recompiled into the original message. Short for Carrier Sense Multiple Access
/ Collision Detection, a set of rules determining how network devices respond when two devices
attempt to use adata channel simultaneously (called a collision).
Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the
line at participating stations. If no transmission is taking place at the time, the
particular station can transmit. If two stations attempt to transmit simultaneously,
this causes a collision, which is detected by all participating stations. After a
random time interval, the stations that collided attempt to transmit again. If
another collision occurs, the time intervals from which the random waiting time is
selected are increased step by step. This is known as exponential back off.
Toplogies – This is the physical layout of a computer
network. There are 5-types of network patterns:
Point-To-Point – Each device is directly connected by a
cable. The worst network pattern for security(no login
full access)
Multi-Point – A network pattern with sharing of all
devices. Usually requires a login with permissions
coming from a server