Lecture 6
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Reminder: Homework 2, Programming Project
2 due on Thursday.
Questions?
Tuesday, September 13
CS 475 Networks - Lecture 6
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Outline
Chapter 2 - Getting Connected
2.1 Perspectives on Connecting
2.2 Encoding
2.3 Framing
2.4 Error Detection
2.5 Reliable Transmission
2.6 Ethernet
2.7 Wireless
2.8 Summary
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Ethernet
Ethernet is the overwhelmingly most successful
local area networking technology.
Ethernet is a working example of carrier sense,
multiple access with collision detection
(CSMA/CD) technology.
Ethernet was developed by Xerox and is based
on the Aloha packet-radio network once used at
the University of Hawaii. Ethernet was
standardized as IEEE 802.3
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Ethernet - Physical Properties
Original Ethernet (thick-net)
used 10Base5 (10
Mbps/Baseband/500 m link)
signaling.
Multiple hosts could be
connected to the same
cable and any signal was
received by all hosts.
Fig 2.22 Ethernet
transceiver and adaptor
Thin-net used 10Base2 (200 m). Twisted pair
uses 10/100BaseT (100 m) and CAT5 cable.
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Ethernet - Physical Properties
Multiple thick-net
segments could
be joined
together with (no
more than 4)
repeaters for a
total reach of
2500 m.
Fig. 2.23 Ethernet repeater
Thin-net segments could be joined similarly.
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Ethernet - Physical Properties
Fig. 2.24 Ethernet hub
With 10/100BaseT a hub (multiway repeater) is
used to connect hosts and segments.
All hosts on segments connected by hubs or
repeaters are in the same collision domain.
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Access Protocol - Frame Format
An Ethernet frame has a 64 bit preamble of
alternating 0s and 1s that are used for synch.
Ethernet (MAC) addresses are 48 bits. Each
adaptor has a unique address. The type field is
used for demuxing higher protocols.
Each frame contains at least 46 bytes of data (64
total) and no more than 1500 bytes. A 32 bit CRC
is included in each frame.
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Access Protocol - Addresses
Addresses are 48 bits (6 bytes). They are
typically printed in hex with colons separating the
bytes (8:0:2b:e4:b1:2). Manufacturers have their
own prefix to ensure that each adaptor has a
unique address.
A frame with a destination address of all 1s is a
broadcast frame. A destination address with a
leading 1 (but not a broadcast address) is a
multicast frame.
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Access Protocol Transmitter Algorithm
When an adaptor has a frame to send and it
senses that the line is idle, it transmits the frame
immediately. If the line is busy, it waits for the line
to go idle and then transmits the frame.
Due to propagation delays, collisions can occur.
When an adaptor detects that a frame it sent has
collided with another frame it stops transmitting
and immediately transmits a 32 bit jamming
sequence.
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Access Protocol Transmitter Algorithm
In the worst case, A and B are
widely separated and B starts
transmitting just as A's frame
reaches B.
A's frame must be at least 2 x
2500 m / 3 x 108 m/s = 16.67
μs long in order to detect the
collision. Ethernet specifies a
min. length of 51.2 μs (512 bits
or 64 bytes at 10 Mbps.)
Fig. 2.26 Worst-case collision scenario
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Access Protocol Transmitter Algorithm
Once a collision has been detected the adaptor
waits before transmitting the frame again.
Exponential backoff is used. It firsts waits either
(randomly) 0 or 51.2 μs. If there is another
collision it waits either 0, 51.2, 102.4 or 153.6 μs
and so on.
In general, a k value is randomly selected
between 0 and 2n – 1 (n is the number of
collisions). The adaptor waits k x 51.2 μs.
Adaptors usually give up after 16 tries and report
an
error to the host.CS 475 Networks - Lecture 6
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Experience with Ethernet
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Multi-access Ethernet has been used primarily
in situations where there are fewer than 200
hosts and distances in the 100s of meters,
since collisions waste network capacity. E.g. in
a room to a hub connected to switch in a wiring
closet.
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Experience with Ethernet
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Ethernet characteristics that made it successful
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Original Ethernet easy to administer and maintain no switches, no configuration tables, just add a tap
to the network wire.
Inexpensive - coaxial cable is cheap, one network
adapter per host.
Switched networks needed to similar. Resulted
in switched Ethernet. Most Ethernet links are
now point-to-point between hubs, switches, and
routers.
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Wireless
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Wireless networks share some common
characteristics with wired networks
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Bit errors are of great concern
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Media is inherently multi-access
Wireless networks differ from wired networks in
several ways
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Low power, especially small mobile devices
Limits on transmission power and frequency, uses
spread spectrum to minimize interference from
other devices
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Wireless
Table 2.4 Overview of Leading Wireless Technologies
Bluetooth
802.15.1
Typical link 10 m
length
Typical
2.1 Mbps
data rate
(shared)
Typical
Peripherals to
use
computer
(PAN)
Wired
USB
analogy
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WiFi
802.11
100 m
3G Cellular
54 Mbps
(shared)
Laptop to
wired network
384+ kbps
(per conn.)
Mobile
device to
wired tower
DSL
Ethernet
CS 475 Networks - Lecture 6
Tens of km
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Wireless
Most common
wireless links are
asymmetric. One
node (the base
station) is not mobile
and has a wired (or
high BW) connection
to another network.
Fig 2.28 Wireless network with base station
Even though radio waves transmitted by one
client may be received by another, all node-tonode comm. is routed through the base station.
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Wireless
The base station topology is associated with three
levels of mobility: (1) no mobility (original WiMAX),
(2) mobility within the range of a base (Bluetooth),
and (3) mobility between bases (Wi-Fi, cell
phones).
An alternative topology is the mesh or ad hoc
network in which there is no base station. All
nodes are peers and communicate with
neighbors. Messages may be forwarded along a
chain of peer nodes.
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Wireless
Fig 2.29 A wireless mesh or ad hoc network
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802.11 / Wi-Fi –
Physical Properties
Wi-Fi runs over six different physical layer
protocols. The original 802.11 defined spread
spectrum and direct sequence channels at 2
Mbps. 802.11b defined a new channel operating
at 11 Mbps. All operated at 2.4 GHz. 802.11a
delivers up to 54 Mbps and operates at 5 GHz.
802.11g went back to 2.4 GHz and delivers up to
54 Mbps.
Most commercial products support 802.11a,
802.11b, and 802.11g to ensure compatibility with
all WiFi devices.
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802.11 / Wi-Fi Collision Avoidance
Due to hidden nodes
collision avoidance is
used instead of collision
detection.
The sender sends a
Request to Send (RTS)
packet waiting for a
Clear to Send (CTS)
from the receiver. Any
adjacent nodes that see
the CTS do not transmit.
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Fig. 2.30/2.31
Hidden and Exposed Nodes
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802.11 / Wi-Fi - Scanning
To enable initial connection
and mobility between access
points (APs):
1) A node sends a Probe.
2) All APs within range send
a Probe Response.
3) The node sends an
Assoc. Request to one AP
4) The AP responds with an
Assoc. Response frame
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Fig. 2.32/2.33 Node Mobility
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802.11 / Wi-Fi - Frame Format
A WiFi frame can carry up to 2,312 data bytes.
The Control field indicates if the frame is an RTS
or CTS and also contains the ToDS and FromDS
bits.
If two nodes are directly comm. both DS bits are
0, Addr1 ids the destination and Addr2 the source.
In the more complex case where two nodes use
two different APs, both bits are 1 and the AP
addresses are included in the frame.
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Bluetooth (802.15.1)
Bluetooth operates in the license exempt band of
2.45 GHz. It offers speeds up to 2.1 Mbps and
low power consumption. It has a short range of
only 10 m.
A Bluetooth piconet consists of a master device
and up to seven active slave devices (up to 255
devices can be inactive or parked). All comm. is
through the master.
Bluetooth uses spread spectrum across 79
channels to prevent interference with other
devices operating at 2.45 GHz.
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Bluetooth (802.15.1)
Fig 2.35 A Bluetooth piconet
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Cell Phone Technologies
The area served by a cell phone base station is
known as a cell. Cells overlap, but at any time a
phone is communication with only one base. As
the phone moves a handoff may be used to
transfer the phone to a new base.
There are several competing cell phone
technologies. 2G (and higher) technologies are
digital and optimized for voice. 3G and 4G have
higher BW and simultaneous voice and data
transmission. There are several (incompatible)
technologies.
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Summary
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Chapter 2 introduced the many and varied
types of links, and looked at five key problems
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How to encode bits into a signal at a source node
and recover the bits at the receiving node
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How to package bits into frames
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How to detect bit errors
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How to make a link appear reliable
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How to mediate access to a shared link so that all
nodes eventually have a chance to transmit data
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