Transcript Data Communications - Home (www.dginter.net)

Data Communications is
the Real World
OSI Layers 1 & 2
a.k.a
TCP/IP Network Interface Layer
What is a Local Area Network
 WikiPedia:
a computer network covering a
small physical area….
 WhatIs.com: a group of computers and
associated devices that share a common
communications line or wireless link.
 Most verbal definitions follow these two
main definitions: close in area, or on a
common link.
Ginter Defines LANS:
 Any
group of computer that can
communicate at layers 1 & 2 of the OSI
reference model.
 This means that all of these computer
share a common broadcast domain.
 This also means that all of the systems are
on the “inside” of a router if the LAN
connects to a greater network.
 What does all of this mean?
LAN Addressing
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All Network Interface Cards have a unique
address burned into them by the manufacturer.
 This
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address has a lot of names:
Hardware Address
Physical Address
Ethernet Address
Media Access Control (MAC) Address
LAN Communication Uses Frames
Bytes Name
Description
7
Preamble
Bit-pattern to ID Frame Start
1
FDS
IDs Frame Start
6
6
Destination MAC Address of the
Destination MAC
MAC
Source MAC My MAC
2
Length/Type
IDs number of bytes/Frame Type
461500
Data
The Payload
4
FCS
Frame Check Sum
What about Errors
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1.
2.
As the data is added to the frame a
formula is calculated using the data.
At the receiving end, the same formula is
used.
If there is an error: one of the following:
An automatic repeat request (ARQ)
Cancel the error-packet (ignore or pass
on)
LAN Communication
 Host A sends
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Media Access Control: Can Host A Send?
Addressing: Who am I, Who Gets this.
Queuing of Data Packets
 Host
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message to Host B.
B receives a Packet
Checks to see if Destination Address is mine?
If so, Check the frame type and pass it up.
If not, it goes to the bit-bucket.
LAN Equipment
 LAN
Equipment works at Layers 1&2
 Hubs: Wired as a Star, but acts like Bus.
 Switches: Wired as a Star and acts it.
 Wireless Access Points
 Bridges
They all Look the Same
Hubs
 Connect
as Star, Looks Like Bus.
 When a host transmits (on transmit lines),
Hub sends out on ALL ports Receive Lines
 This behavior is just like a BUS
connection.
 Hubs have Contention.
 Access Control is Carrier Sense, Multiple
Access, Collision Detect (CSMA/CD)
CSMA/CD
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Carrier Sense
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Multiple Access
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Just means many can connect
Collision Detect
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Listen First, then talk
Listen to the receive lines.
If the signal is different than what I sent then there is a
collision (contention)
This is like being at a Party where only two
people are allowed to talk at one.
 Similar to have a Phone on a “Party Line”
CDMA Logic Diagram
Switches
 Like
Phone Switches (after 1964).
 Everyone gets a “Private Line”
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Switches only send received data to the port that
has the destination address attached.
 Forwarding
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Methods:
Store and Forward (buffer, err chk, forward)
Cut Through (read destination addr, forward)
Fragment Free (errchk first 64B, forward)
Adaptive: select between the previous three
Switching is Cool!!!!!
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Virtually Eliminates Contention.
 Allows Full-Duplex Connections
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Allows for management of ports
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Can Send and Receive at the same time
Quality of service
Traffic monitoring
Virtual LAN
Use of redundant connections for reliability (Spanning
Tree Protocol).
This is a Party where Everyone can talk at once.
Wireless Access Points
 Wireless Access
Points are like Hubs
without Wires.
 WAPs use CSMA/CA
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Collision Avoidance disables the ability of a
node to transmit if another one already is.
 WAPs
have speeds of 11 Mbs, 54 Mbs,
and (about) 300 Mbs on 802.11 b, g, and n
respectively.
Review
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Data Communication in the Real World (OSI
Layers 1 & 2, TCP/IP Layer 1)
What is a LAN?
LAN Addressing
Frames
Error Detection and Handling
Hubs
Switches
Wireless Access Points