Transcript Physical Layer - The University of Vermont

Lecture 2

The OSI model

Chapter 2, specifically pages 42-58
Dave Novak
School of Business Administration, University of Vermont
Sources: 1) Network+ Guide to Networks, Dean 2013
2) Comer, Computer Networks and Internets, 2004
Lecture Outline
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Examine the seven layers of the OSI
model in detail
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What the OSI model is and how it is used in
networking
Understand how protocols at different layers
interact
OSI’s relationship to networking protocols
Difference between MAC and IP address
Define encapsulation
OSI model and networking

OSI model provides a universal
framework for network communication
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Predates popularity of TCP/IP
Defines relationships between various
protocols, the specific services provided by
protocols, and the layers of the model where
the protocols operate
The OSI model and networking
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Networked computers use many different
protocols simultaneously
Protocols are responsible for providing
different types of network services and
functions
The OSI model and networking
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Different layers of OSI are responsible
for doing different things by providing
different types of services and functions
The idea behind “layering” is to separate
functionality and services by individual
layer where there is no redundancy in
the services / functions between layers
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Each layer of the OSI has a specific set of
functions and services that are handled at
that layer
The OSI model and networking
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The OSI model is a standardized
framework for sub-dividing
communications system
functionality and services into
separate layers
OSI model
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The collection of networking protocols that
operate at the various OSI layers are referred
to as a protocol stack
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Protocols running on a networked computer work
together to provide all services required by a
particular application
Services provided by the protocols are not
redundant – if a protocol at one layer provides a
particular service, the protocols at the other layers
do not provide the same service
Protocols at different layers provide services to each
other – allowing interaction between adjoining
layers
OSI model
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Promotes open system communication
OSI is a theoretical representation or
framework for network services
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Does NOT prescribe hardware or software
Does NOT describe how software programs on
different computers interact, or how they interact
with humans
A Protocol Stack
The collection of protocols that operate at the
various layers of the OSI model are referred to
as a protocol stack
The protocols in the protocol stack work
together to provide all services required by an
application
Protocols at the different layers perform specific
functions that are NOT duplicated by other
protocols at other layers
Protocol Interaction
Services performed at a particular
layer of the OSI model at the sending
computer are also performed (or undone)
at the corresponding layer of the
receiving computer
The Session Layer (5) at the sending
computer does not communicate
directly with the Session Layer at the
Receiving computer  messages are
passed down from the Session Layer (5)
to the Transport Layer (4) and so on at
the sending computer
Messages are passed up from the
Transport Layer (4) to the Session Layer (5)
at the receiving computer
The OSI Reference Model
7
6
The Application Layer (7) is the
top most layer – it is NOT the same
thing as an application
5
MS Word is an example of an application
4
MS Word is NOT a protocol that operates
at the Application Layer (7) of the OSI
3
2
1
FTP is an example of an Application Layer
(7) protocol
The Physical Layer (1) is the bottom
most layer of the OSI model – it
addresses the transmission of bits over
a particular medium
Interaction Between OSI Layers
OSI model
OSI model
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Refer to the additional reading –
Webopedia definition of OSI
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http://webopedia.internet.com/quick_ref/OS
I_Layers.asp
The OSI model defines a framework for
implementing networking services via
specific PROTOCOLS (depending on the
protocol stack being used) in seven layers
Physical Layer (1)
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Concerned with transmitting signals
(representing raw bits) over a
communication channel
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Transmitting signals via energy of some
form or another
Data are not organized into frames or
packets at Layer 1
Physical Layer (1)
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Addresses the nature of the medium and
types of signals used
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Cable type: coaxial, twisted pair, fiber
Signal type: Light pulses, electrical voltage,
radio waves
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Why would one be concerned about the medium
that is used? What difference does it make?
Physical Layer (1)
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Hardware dealing with transmission of
signals is defined at layer 1 of the OSI
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Cables, hubs, repeaters
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Do not “understand” packets or frames only
signals
The NIC provides a physical connection to
the network and bridges layers 1 and 2
Physical Layer (1)
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Networking technologies may use a variety of
physical layer options
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Ethernet (the most popular wired networking
technology) supports a number of different physical
layer options
Designation
Cable Type
Topology
Speed
Max Segment
Length
10Base5
RG-8 coaxial
Bus
10 Mbps
500 meters
10Base2
RG-58 coaxial
Bus
10 Mbps
185 meters
10BaseT
CAT 3 UTP
Star
10 Mbps
100 meters
100BaseFX
62.5 / 125
multimode fiber
Star
100 Mbps
412 meters
100BaseTX
CAT 5 UTP
Star
100 Mbps
100 meters
Physical Layer (1)
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Different types of media may be used to
implement a given LAN technology
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For example, depending on the standard being
followed, an Ethernet LAN may require coaxial,
fiber, or twisted pair wiring
Each standard has different topology and
medium requirements
Physical Layer (1)
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Max length of cable
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What happens if you exceed max cable length
standards?
Type of connectors
Bit rate (data transmission rate)
Monitor data error rates
Data Link Layer (2)
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Converts signals and streams of bits into
frames and vice versa
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Creates and recognizes frame boundaries
What is a frame and why is it
important?
Data Link Layer (2)
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The frame format is different for various
networking technologies
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Ethernet
Token Ring
ATM
Different Frame Formats
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Ethernet frame (IEEE 802.3)
Preamble
8 bytes
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SFD
1
Dest.
MAC
Source
MAC
Length
6
6
2
v2 length >= 1536 B
DATA
46 – 1500 bytes
Token Ring frame (IEEE 802.5)
Start
Del.
1
Access Frame
Control Control
1
1
Dest.
MAC
6
Source
MAC
6
FCS
DATA
4500 >= 0
4
length > 4500 B
FCS
End
Del.
Frame
Status
4
1
1
Data Link Layer (2)
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Access control technique of various
technologies defined at Layer 2
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The way in which networked devices “gain
access to the medium”, communicate with
other devices, and transfer data differs
based on the technology being used
For example, the process two devices use to
communicate over wireless is different from
Ethernet
Data Link Layer (2)
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Error detection in the bit to frame
conversion process
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Identifies and corrects frame errors
Errors related to LAN communication
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Between 2 hosts
The physical address or MAC address
is contained in the frame header
Data Link Layer (2)
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Provides conduit or link between the
hardware and software on the computer
and the physical network medium
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This is done via the NIC
Data Link Layer (2) Sublayers
LLC – Interface to layer 3. Controls frame synchronization, flow control,
and error checking
MAC – Interface to layer 1. Controls how PC accesses and transmits data
specifies the Media Access Technique used
Data Link Layer (2)
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In terms of network design – Data Link
Layer (2) is single most important
layer in determining what hardware
is used
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LAN technology and topology requires
certain physical layer options and vice versa
Implies access technique used
Data Link Layer (2)
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Data link layer protocols designed to
work locally (LAN or subnet centric)
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Other higher-layer protocols (at layers 3 and
4) are required for error detection and flow
control in communicating remotely and over
larger distances
Hardware that recognizes frames
operates at layer 2 of the OSI
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Switch, bridge
Network Layer (3)
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Packages data/frames into IP datagrams
Higher level, routable network addresses (like
the IP address) are recognized and managed
Hardware that recognizes network addresses
(like IP addresses) works at Layer 3
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Routers
Network Layer (3)
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Responsible for end-to-end
communication
Network Layer (3)
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Responsible for end-to-end
communication
How is end-to-end functionality different
from functionality provided by Data Link
Layer (2) protocols?
Network Layer (3)
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Example protocol: IP
Example services: network addressing,
fragmentation/reassembly, routing
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What does routing mean?
Frames, Packets, and IP Datagrams
MAC and IP Address Comparison
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MAC address (physical address)
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IP address (network address)
Transport Layer (4)
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Network layer (3) and Transport layer (4)
protocols designed to work together as a
pair
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TCP/IP – Internet protocol stack
SPX/IPX
Layer 4 protocols provide services to
complement Layer (3)
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TCP provides very specific services that IP
does not – designed to work together with
each protocol providing unique services
Transport Layer (4)
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Provides messaging service for Session
layer (5) and hides the underlying
network from the upper layers
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Example protocols: TCP, UDP
Example of Transport Layer services: flow
control, multiplexing, retransmission,
message sequencing
Transport Layer (4)
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Provides end-to-end error control on the
network
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How is this different from Layer 2 error
control?
How is this different from end-to-end
communications provided at Layer 3?
Encapsulation
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Each protocol adds headers to information it
receives from the layer above it
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When a datagram is encapsulated, the entire
datagram is placed into the payload area of a
specific frame format
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Transport Layer (4) adds its header to message and passes
down to Network Layer (3)  Network Layer adds its
header in front of Transport Layer header and so on
Encapsulation
Upper Layers
4
3
2
Lower Layers (1 – 4)
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Layers 1 – 4 are called the LOWER
layers
Lower layers are concerned with proper
transmission of data across the
network
Upper Layers (5 – 7)
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Layers 5 – 7 called the UPPER layers
Upper layers are concerned with how
different application communicate
between different hosts
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Difficult to identify and separate upper layer
protocols in some cases – many applications bundle
services provided at layers 5, 6, and 7
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Example: WS FTP not only uses Layer 7 protocol FTP, but
manages syntax and compression issues at Layer 6, and
session management issues at Layer 5
Upper layer protocols/services know nothing about,
or understand networking or addressing
Session Layer (5)
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Responsible for establishing, maintaining,
and ending communication
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There are no separate Session Layer
protocols
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Session Layer functions are integrated into
other protocols that also include
Presentation and Application Layer functions
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Set up, manage, and tear down “sessions” or
“connections” between Presentation Layer (6)
entities
Session Layer (5)
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Coordinates communication and
organizes into one of three categories:
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1) Simplex – only one node can transmit
2) Half duplex – both nodes can transmit, but
only one at a time. Once one node is finished
transmitting data, the other node can transmit
3) Full duplex – both nodes can transmit
simultaneously without disrupting the other node
Session Layer (5)
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Traffic cop for communications between two
nodes on a network
For Internet applications – mapping between
logical ports and sessions
Presentation Layer (6)
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Primary role is to preserve meaning of
information transmitted between systems
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Computers communicating on a network often use
different syntax
Ensures communication between entities is of a
form both can understand
Computers must negotiate a common syntax so
they can choose a transfer syntax that they both
have in common and both understand
Semantics?
Presentation Layer (6)
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Different applications and programming
languages use different data types and
different syntax
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Text-based languages based on sequence while
visual languages are based on spatial layout and
relationships between symbols
Which statements in a program are acceptable to
the compiler?
Presentation Layer (6)
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Network perspective: applications send
messages to each other
Application perspective: messages contain
specific types of data
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Many types of data use very standard (universal)
formats
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MPEG for video
JPEG for still images
ASCII for text
Not all data types have universal formats
Not all computers format data types the same way
Presentation Layer (6)
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If needed, systems can select transfer
syntax that provides additional services
such as:
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Data compression and decompression
Encryption and decryption
Application Layer (7)
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Entrance point for applications to access
the OSI model (structured networking
framework) and use network resources
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NOT an application, but access to protocols
that provide network services
Coordinates network services
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Identify parties and make sure each can be
reached
Ensures communication resources exist (For
example, is there are modem at the sender’s computer?)
Application Layer (7)
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Most Application Layer protocols provide
services (such as mail, print, network
management services) that applications (such
as WS FTP, Internet Explorer, Outlook) use to
access the network
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Application Layer protocols often include Session
and Presentation Layer functions
Typical protocol stack consists of 4 separate
protocols that run at the application, transport,
network, and data-link layers
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FTP, TCP, IP, specific Ethernet protocols
Application Layer (7)
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FTP, TCP, IP, specific Ethernet protocols
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Example: WS_FTP
uses FTP (a layer 7
protocol that ALSO performs layer 6 and 5 functions) running over
TCP/IP (the layer 4 and 3 protocols – Internet communication) and
Ethernet LAN technology (layer 2 protocols specifying frame
(application software)
formats, error detection, addressing on the LAN)
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The upper layers using FTP are unaware of what
type of transport protocol (4), what type of network
protocol (3), and what type of LAN technology or
data-link protocol (2) is being used
Different lower layer protocols CAN be used – the
application is not aware of this, and doesn’t need to
deal with any lower layer functionality
Lecture Summary
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Details of the OSI model
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KNOW the layers (A, P, S, T, N, D, P) and be
able to describe basic services provided at
each layer
Using TCP/IP stack as example, on what
layer does IP operate? What about TCP?