Transcript Cicso – Semester 1 – Chapter 2

Cisco – Semester 1 – Chapter 2
The OSI Model
International Standards
Organization
Interoperability
The ISO recognized that there was a need to
create a network model that would help
network builders implement networks that
could communicate and work together
Released model in 1984
OSI reference model is networking scheme
that supports networking standards
The 7 Layers
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5
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Application
Presentation
Session
Transport
Network
Data Layer
Physical
Why Layers?
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Reduces complexity
Standardizes interface
Facilitates modular engineering
Ensures interoperability
Accelerates evolution
Easier to teach and learn (or so they say)
Physical Layer – Layer 1
• Defines the electrical and functional
specifications for the link between end
systems
• Defines voltage levels, physical data rates,
maximum transmission distance, physical
connections
• THINK MEDIA AND SIGNALS
Layer 2 – Data Link
• Provides reliable transit of data across a
physical link
• Concerned with physical addressing (MAC)
address), network topology, and media
access, error notification, delivery of
frames, and flow control
• THINK FRAMES AND MEDIA ACCESS
CONTROL
Layer 3 – Network Layer
• Provides connectivity and path selection
between two host systems
• THINK PATH SELECTION, ROUTING,
AND ADDRESSING (logical or IP)
Layer 4 - Transport
• Provides a data transport service that shields the
upper layers from transport implementation details
• Concerned with reliable transport between two
hosts
• Establishes, maintains, and properly terminates
virtual circuits
• Uses error detection-and-recovery and information
flow control
• THINK QUALITY OF SERVICE AND
RELIABILITY
Layer 5 - Session
• Establishes, manages, and terminates sessions
between two communicating hosts.
• Provides its services to the presentation layer
• Synchronizes dialogue between the two hosts'
presentation layers and manages their data
exchange
• Provides data expedition, class of service, and
exception reporting
• THINK DIALOG AND CONVERSATIONS
Layer 6 - Presentation
• Ensures the information sent is readable by
receiving host
• Concerned with data structures & syntax
• Translates between multiple formats by
using a common format
• Encodes, encrypts, and compresses
• THINK COMMON DATA FORMAT
Layer 7 - Application
• Provides network services to user
applications
• DOES NOT provide services to any other
OSI layer
• Provides services to applications outside the
OSI model
• THINK BROWSERS
Layer Information
• In general each layer communicates with
three other layers
– The layer above
– The layer below
– The same layer in the peer host
• Know specifics of each layer
– E.g. NetBeui operates at layer 5
Encapsulation
• Wraps data with the necessary protocol
information before sending to next layer
and for network transit.
• Data receives headers (control information),
trailers, and other information
• Data – Segments – Packets – Frames – Bits
– THIS IS IMPORTANT (defines datagram at
each layer)
Headers
• Control information placed before the data
• Adds address information, et. al.
• Exact format depends on layer
Layers and Encapsulation
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Presentation – Data
Transport – Segments
Network – Packets
Data Link – Frames
Physical - Bits
Protocols
• Set of rules that determines format and
transmission of data; aka standards
• Different protocols for different layers
• PDU – Protocol Data Unit – exchange
between peer layers
TCP/IP
• Transmission Control Protocol/Internet
Protocol
• Makes data communication possible
between any two computers, anywhere in
the world
• Speed approaches speed of light
• WWW and Internet protocol – defacto
standard
TCP/IP Layers
• Application Layer
– Combines Application, Presentation, & Session
• Transport Layer
– Same as OSI Transport Layer
• Internet Layer
– Same as OSI Network Layer
• Network Access Layer
– Includes Physical & Data Link; Includes LAN and
WAN details
TCP/IP Protocols
• Application Layer
– FTP, TFTP, HTTP, SMPT, DNS, TFTP, Telnet,
Redirector
• Transport Layer
– UDP and TCP
• Internet Layer
– IP
• Network Access Layer
– LAN and WAN technology used
Model Similarities
• Network professionals need to know both
• Both use layers
• Both are packet switched (not circuit
switched)
• Both have similar transport and network
layers
Model Differences
• TCP/IP application layer includes OSI
presentation and session layers
• TCP/IP network access layer includes OSI
data link and physical layers
• TCP/IP protocols are standards used to
build Internet
• No networks are built around OSI protocols
• TCP/IP APPEARS simpler
Cisco Uses OSI Because
• Generic and protocol independent
• More details which makes it useful for
teaching and learning
• More details which makes it useful for
troubleshooting
• USE OSI MODEL but use TCP/IP
PROTOCOLS