Transcript The job of moving data from one place to another is also a big job

Building a Network with OSI
Chapter 3
Contents
• Explain the major functions of network hardware
• Describe the functions of network software
• Define each of these functions as part of the OSI
seven-layer model
Overview
The Big Job
• Whenever you have a big job to do, it helps to
break it down into discrete chunks or functions
– For example, moving your family from one city to
another could be broken down into:
•
•
•
•
What to move
How to pack
How to load the van
How to unload the van
• The job of moving data from one place to
another is also a big job
OSI Seven-Layer Model
• The big job of moving data from one place to
another is broken down into functions defined
by the OSI Seven-Layer Model
Case Study Example
• Let’s look at the process
of networking from a
conceptual viewpoint
– Assume we are just
trying to move a file from
one PC to another in a
small office
– One of the workers has
just completed a new
employee handbook
– She needs to transfer the
Word document to the
other worker for review
Case Study Example
• The file could be copied to a diskette, USB, or
CD/DVD and handed over to the other worker –
called Sneakernet – but that’s not necessary
today
• The document may be transferred using the
network
• The next section examines the various hardware
required
Let’s Get Physical
Cables
• Most networks use a
cable like that shown as a
physical channel to move
the bits of data
Unshielded Twisted Pair
(UTP) cable
Uses 4 wires: 2 for
sending and 2 for
receiving data
Hubs
• Each computer system
has a cable leading to a
device called a hub
– Usually located in a
closet
• The hub sends the data
received from one system
to all the other systems
attached to it
Network Interface Card
• Network Interface Cards
(NICs – pronounced
“Nicks”) are installed in
PCs
• Network cables are
attached to the NICs
NIC to Hub Connections
• Cables run from the NIC
in the PC to a jack on the
wall
• Cables run through the
walls to the closet where
they connect to a hub
Network Cabling System
Network Interface Cards
• Since all the networked
systems are connected to
the same hub, each
system must have a
unique identifier
• Media Access Control
(MAC) address
– A unique address burned
into a ROM chip on the
network card
– Each MAC address is 12
hex characters or 48 bits
in length
MAC address printed on
surface of chip – it’s
burned inside the chip.
MAC Addresses
• MAC addresses are 48 bits long
• Usually represented using hexadecimal
characters (12 hex digits = 48 bits)
– Here’s a typical MAC address:
004005-607D49
Identifies the
manufacturer
Unique serial number determined
by the manufacturer
No two MAC addresses are ever the same!
WINIPCFG
• Used on Windows 98/Me systems to view network
configuration
MAC address
ipconfig /all
• Used on Windows NT/2000/XP systems to view network
configuration
MAC address
Bits
• A MAC address is a series of ones and zeros
called bits
• Data is sent using pulses of electricity, light, or
radio waves
Frames
• Data is sent across the network in frames
• Frames are discrete chunks of data
Fields
• Frames are made up of fields that contain
information
• Frames typically contain the recipient’s MAC
address, the sender’s MAC address, the data
itself, and a cyclic redundancy check (CRC) for
error checking
Data
• What is inside the data part of the frame?
– It could be part of a file, a print job, a web page,
anything
– NICs do not care what the data, or payload, is
Frame Size
• Different networks use different sizes of frames
• Many frames hold about 1500 bytes of data
• The sending software breaks up large amounts
of data into smaller chunks
• The receiving station must then put the chunks
back together in the proper order
Processing Framesq
• All devices on the network see the frame, but
only the device that it is addressed to will
process it
– Every frame
is received
by every NIC
– The MAC
address is used
to decide if the
belongs
given
frame
to a
device
Getting Data on the Cable
• Only one system may speak at a time since the
cable is shared
• Processes are used to keep two NICs from
talking at the same time
Getting To Know You
• How does the sending NIC know the MAC
address of the NIC to which it is sending data?
• Most of the time the two devices have talked
before, so the destination MAC address is
already known
• If the MAC address is not known, then a
broadcast message is sent over the network
– The destination device will respond by sending its
MAC address
– A broadcast MAC address is FF-FF-FF-FF-FF-FF
Moving Frames – Step 1
• The sending system’s network software hands
some data to the NIC
• The NIC begins building a frame
Moving Frames – Step 2
• After the NIC creates the frame, it adds the CRC
and data to it
• It also puts its MAC address and the
destination’s MAC address in the frame
Moving Frames – Step 3
• When no other NIC is using the cable, it sends
the frame through the cable
Moving Frames – Step 4
• The frame propagates down the wire to the hub
• The hub creates a copy of the frame to send to
every other
system
• The receiving
device processes
the frame
Moving Frames – Step 5
• The receiving station checks the CRC value in
the frame
– If the value matches what it should, then the NIC
sends the data portion to the network operating
system for processing
– If the value does not match, the frame has errors and
must be resent
Beyond the Single Wire – Network
Software
Beyond the Single Wire
• What if one system is using a modem to dial
into the network?
• What if one of the systems is a Macintosh?
Routers
• A single
network with
single hub can
only support
up to 1,024
computers
before the
network
becomes
too slow
• Routers are
used to chop
large networks
up into smaller
ones
Network Protocols
• Network protocols define rules for how systems
are addressed, how to chop data up into chunks,
how to deal with routers, and so on
• As a network grows a more universal addressing
method than MAC addresses is needed
– TCP/IP are the most popular universal addressing
protocols
TCP/IP
• Transmission Control
Protocol (TCP)
• Internet Protocol (IP)
– Gives each device on the
network a unique
numeric identifier
– IP addresses consist of
four 8-bit numbers
– Each 8-bit number
ranges from 0 to 255
• 192.168.4.232
– No two systems on the
same network share the
same IP address
DHCP
• IP addresses must be manually configured on
each device
• Or they may be automatically configured using
Dynamic Host Configuration Protocol (DHCP)
• Each system in a network has two addresses
– MAC address burned into a chip on the NIC
– IP address configured through software
Sample Network with Addresses
Packets
• The network software creates a packet that
contains the sending and receiving IP addresses
along with the data
• The packet is enclosed within a frame that
contains the sending and receiving MAC
IP packet in a frame
addresses
IP packet
IP packet with frame added
Connecting to the Internet
• To connect the local network to the Internet a
router is needed
• The local hub is connected to the router
• The router is connected to the Internet through a
phone line
• The phone line uses a different kind of frame, so
the router strips the frame and creates a new
one
Connecting to the Internet
Router removing network frame
and adding one for telephone line
Adding a router to the network
Moving through the Internet
• The router strips off the MAC addresses and
uses the type of addressing the phone company
uses instead
• The frame uses the IP address to guide it to the
receiving system
• The receiving router strips off the telephone
frame and adds the MAC address for the
receiving system
• The NIC strips off the MAC header and hands the
frame off to the NOS
Assembly and Disassembly
• Most data is much larger than a single frame
• Network protocols chop up the data into smaller
packets and gives each one a sequence number
• The sequence numbers are used by the receiving
system to put the packets back in order and to
assemble them
• Transmission Control Protocol (TCP) is one such
protocol
Sessions
• If the receiving system also has a printer to
share on the network, then it may receive
packets for print jobs as well as packets for files
• The sending system must also contact the
receiving system to make sure it is ready to
handle a print job as well
• The software that handles these processes is
called session software
Multiple Sessions
Single session
Multiple sessions
Standardized Formats
• Macintoshes and PCs use very different formats
• Standardized formats have been created that
allow very different operating systems to
exchange data
VPNs
• Virtual Private Networks (VPNs) enable a user
away from the office to connect to the corporate
network via the Internet
Encryption
• Many networks encrypt data to prevent
unauthorized access
• Both the sending and receiving system must
know the encryption method used
Network Applications
• Users use network
applications to exchange
data on a network
– My Network Places in
Windows for files
– Internet Explorer or
Netscape Navigator for
web pages
– Outlook Express for email
Copy a File Across a
Network
• The next few slides illustrate a typical process
that takes place to copy a file from one machine
to another over the network
My Network Places
• The receiving station finds the remote file using
My Network Places
Moving the Word Document
• Drag and drop the Word document from My
Network Places to the Desktop
Disassembly
• The sending system chops the packet into
segments and assigns sequence numbers
Addressing
• Each packet is assigned addresses
Assembling the Frame
• The NIC adds a frame around each packet
containing the MAC addresses
Grabbing the Frame
• When the network cable is not busy, the frame is
sent down the wire
• Every NIC looks at the frame. Only the receiving
device grabs it
Checking the CRC
• The receiving station checks the CRC to make
sure the frame doesn’t have an error
• It then strips off the frame header and passes it
up to the next layer
Missing Packets
• If one of the packets is missing, the receiving
device asks for it to be resent
It’s Amazing!
• Once the document has been reassembled, it is
sent to the proper application
• It’s amazing but almost all of this process is not
seen by the user
The OSI Seven-Layer Model
Early Networking
• In the early days of networking each company
designed the networking process from the
ground up
• These proprietary systems could not talk to each
other
• For networking to grow a model needed to be
created to standardize the process
OSI Model
• The International Organization for
Standardization (ISO) proposed the Open
System Interconnection (OSI) Model
• The OSI Model is a seven-layer model that
describes the networking process
OSI’s Seven Layers
Layer 7
Application
Layer 6
Presentation
Layer 5
Session
Layer 4
Transport
Layer 3
Network
Layer 2
Data Link
Layer 1
Physical
Modular Design
• Each protocol is designed to deal with a specific
layer
• Each protocol needs to know how to interface
with the layer immediately above and below it
Layer 7 - Application
• Defines a set of tools that programs may use to
access the network
– HTTP
– FTP
– Microsoft APIs
Layer 6 - Presentation
• Presents data from the sending system in a form
that applications on the receiving system can
understand
– ASCII and Unicode text formats
– Encryption
– Compression
• Translates between different machine languages
Layer 5 - Session
• Manages the connections between machines on
the network
– Connections may be for file transfers, for print jobs,
for e-mail, or many other possibilities
• Allows machines to keep track of who they are
talking to
Layer 4 - Transport
• Breaks up data it receives from the upper layers
into smaller pieces for transport
• On the receiving side the packets are
reassembled from lower layers
• Provides for error checking
• Layer 4 is a pivotal layer in the process
– Lower layers are concerned with moving data from
point A to point B
– Upper layers deal with the data
Layer 3 - Network
• Helps to get packets from network to network by
adding unique identifiers (like IP addresses) to
the packets
Layer 2 – Data Link
• Defines the rules for accessing and using the
Physical layer
• Specifies the rules for identifying devices
• Determines which machine may use the network
at a given time
• Checks for errors
Layer 1 - Physical
• Defines the physical form taken by data when it
travels across a cable
– Defines how ones and zeros are turned into actual
electrical signals on a wire, light pulses on a fiber
optic cable, or radio waves in a wireless network
NICs and Layers
• Network Interface Cards work at both layer 2 and
layer 1
– When it is said that a NIC is a layer 2 device, keep in
mind that it also functions as a layer 1 device