Transcript OSI Data Link Layer

Chapter 2
(handout 1– only sections 2.1, 2.2 and 2.3)
Dr. Clincy
Professor of CS
Exam #3 Monday (3/14/16): Opened Book, No Computer, No Phone: All
Chapter 4 (except 4.14) and sections 5.1 and 5.2 (and all related lectures)
Lab 8 Posted on website
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Lecture
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In understanding the “full scope” of data communications and
networking is VERY COMPLEX (and convoluted)
Open Systems Interconnection
(OSI) Reference Model
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Why Study OSI?
An excellent model for conceptualizing and understanding data communications
More granularity in functionality - more functional delineation
Key points:
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Modular
Hierarchical (chain of command, pecking order)
Boundaries between layers (called interfaces)
Packet Switch Versus Circuit Switch
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Lecture
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Internet – Underlying Technologies
• The Internet is an interconnection of “backbone” networks
JOINED together via routers, gateways and switches
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Internet is comprised of LANs, Point-to-Point WANs and Switched
WANs
LAN Examples: Ethernet, Token Ring (not in book), Wireless and
FDDI Ring (not in book)
Pt-to-Pt WAN Examples: Telephony Modem, DSL, Cable/Modem,
T-Lines and SONET
Switched WAN Examples: X.25, Frame Relay and ATM
Domain also includes
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Tx Issues
Rx Issues
Networked Applications
Servers
Storage Systems
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Routers, Gateways, Bridges,
Brouters, Modems, Switches
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OSI Model
• Open Systems Interconnection
• Developed by ISO
(International Organization for Standardization)
• Contains seven layers
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Application
Presentation
Session
Transport
Network
Data Link
Physical
Note 1: CS3501 focuses on layer 1 (Physical Layer)
Note 2: CS4622 focuses on layers 2 – 4 (TCP/IP)
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OSI Reference Model ?
• Bottom 3 layers
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Responsible for getting data or info to destination
Routing and switching occurs
Define the electrical and physical standards
Performs bit ordering, transmission of the bits, and error
detecting and correcting
• Top 4 layers
• Creation and interpretation of the data
• Protocols for data formatting, message syntax, dialogue
management, message sequences and info presentation are
used
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OSI Physical Layer
• Responsible for transmission of bits
• Always implemented through hardware
• Encompasses mechanical, electrical, and functional
interfaces
• Encoding and Decoding issues: how 0’s and 1’s are
converted to signals
• Signal translation (ie. electrical to optical)
• Signal Multiplexing and Demultiplexing
• Signal Modulation and Demodulation
• Transport medium: Coaxial, Twisted Pair, Optical, etc..
• Transmission Rate/Data Rate – how fast to send bits
• Transmission mode: transmission direction (simplex,
duplex)
• Dr.Physical
Topology: networkLecture
layout
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OSI Data Link Layer
• Responsible for error-free, reliable transmission of data
• Framing, Flow control, Error control
(detection/correction), Access Methods
• Makes use of physical address because with in the same
network
Network Layer
Actually sends the packets (groups of
frames) from node to node using a routing
algorithm
Data Link Layer
Takes raw data (bits) and transform
them into frames, error control, etc.
Physical Layer
Transmit and receive the raw data
(bits)
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OSI Network Layer
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Responsible for routing of messages through networks
Concerned with type of switching used (circuit v. packet)
Handles routing among different networks
NOTE: with in the same network, only the DATA LINK
layer is needed – amongst multiple networks, the
NETWORK LAYER is needed
No need for routing with in the same network (LAN)
Routing across “internetworks”
Makes use of logical address vs physical address because
not with in the same network
Fragmentation occurs at this layer - explain
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OSI Network Layer
High Level View of Network Layer’s Functions:
1. Transmitting data packets through a network in a timely
manner
2. There are more than one route between the source and
destination, the network layer chooses the best route (next
hop) based on some criteria.
Examples
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Node Cost
Link Cost
Distance
Spare Cap.
Low Util.
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3. Makes sure the network does not become congested when
link or node failures occur.
Passes data between two networks (differing networks)
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OSI Transport Layer
• Isolates messages from lower and upper layers
• Breaks down message size (segmentation) (down) and
performs re-assembly (up)
• Monitors quality of communications channel (oversee
all hops)
• Selects most efficient communication service
necessary for a given transmission (could change over
hops)
• Flow and Error control for Source and Sink
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OSI Session Layer
• Establishes logical connections between systems
(up/down)
• Manages log-ons, password exchange, log-offs
(up/down)
• Terminates connection at end of session (up/down)
• Three-way handshake
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OSI Presentation Layer
• Provides format and code conversion services
• Examples
– File conversion from ASCII to EBDIC
– Invoking character sequences to generate bold, italics, etc on
a printer
• The source and sink could operate using different
encoding schemes – the presentation layer makes the
translations
• Security
• Compression
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OSI Application Layer
• Provides access to network for end-user (end-user
being a human being or software application)
• User’s capabilities are determined by what items are
available on this layer (ie. remote log-in, file transfer,
email service, directory service, etc.)
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Lecture
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