Transcript Chapter 2

Chapter 3
IrDA and Bar Code IR
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IrDA
• The Infrared Data Association (IrDA)
defines physical specifications
communications protocol standards for the
short-range exchange of data over infrared
light, for uses such as personal area
networks (PANs).
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IrDA
• IrDA is a very short-range example of free-space
optical communication. No government
regulation!
• IrDA interfaces are used in palmtop computers,
mobile phones, and laptop computers
• Many laptops no longer offer IrDA in favor of
Bluetooth or WiFi which don’t need line of sight.
• For the devices to communicate via IrDA they
must have a direct line of sight.
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Bluetooth
• Bluetooth is a short-range wireless communications
standard that utilizes radio-frequency transmissions in
the 2.4GHz
• RF – not line of sight
• Longer distance, so can be monitored
– Needs encryption
• Industrial-Scientific-Medical (ISM) band
– potentially subject to government regulation
• Devices like garage door openers, baby monitors, and
high-end cordless phones already inhabit the ISM band,
though, so it's a potentially noisy part of the spectrum.
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IrDA-Data
Bluetooth
Physical Media
Infrared
RF (2.4 GHz)
Communications
Range
Up to at least 1m
10cm to 100m
Connection Type,
Direction
Point-to-Point, Narrow Angle
(30 degrees)
Multipoint, Omni-directional
Maximum Data
Rate
4Mbps (16Mbps on the way)
1Mbps (aggregate)
Security
Physical limitations offer some
built-in protection
Authentication, encryption,
spread spectrum
Approximate
Cost
under $2
under $5
Table 1. IrDA and Bluetooth Feature Comparison
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• Can send document from PDA to printer
without any wires connected!
• Use software protocol stack
similar to TCP/IP
• IrDA-1.1 standard
– max data size is 2048 bytes
– maximum Xmit rate is 4 Mbps.
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• You never have to wonder which device you are
actually talking to.
• The infrared signal is emitted in a narrow cone
of light and with very little signal strength.
• You've generally got to be within one meter of
the other device and have your infrared
transceiver pointed in its approximate direction.
• So there's not much fear of interference from
other IrDA devices (except from fluorescent
lights and other background producers of
infrared radiation) or eavesdropping.
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4 Layers to IrDA
• IrPHY
Lowest Layer
– a physical layer that defines data rates, the encoding of
individual bits, and the way that bits are combined into frames
• IrLAP
2nd Layer
– Link Access Protocol
– Data Link Layer of the OSI model
– provides addressing, error detection, and retransmission
capabilities to ensure the reliable delivery of all packets
• IrLMP
3rd Layer
– Link Management Protocol layer that enables multiple
simultaneous conversations between the two connected systems
• Tiny TP
Top Layer
– Tiny Transport Protocol
– Segmentation and Reassembly of large messages.
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IrPHY
• Infrared Physical Layer Specification
• Lowest layer of the IrDA specifications
• Range (Standard: 1 m,
low-power to low-power: 0.2 m,
Standard to low power: 0.3 m)
• Angle (minimum cone +/-15°)
• Speed (2.4 kbit/s to 16 Mbit/s)
• Modulation (Base band, no carrier)
• half-duplex mode because while transmitting
receiver is blinded by the light of its own
transmitter, so full duplex not possible.
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SIR
• Serial Infrared (SIR) speeds cover those
transmission speeds normally supported
by an RS-232 port.
– 9600 bit/s,
– 19.2 kbit/s,
– 38.4 kbit/s,
– 57.6 kbit/s,
– 115.2 kbit/s.
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Bit Rates
• Serial Infrared (SIR)
<= 115.2 kbit/s
• MIR (Medium Infrared)
0.576 Mbit/s
and 1.152 Mbit/s
• Fast Infrared (FIR)
4 Mbit/s.
• Very Fast Infrared (VFIR)
• UFIR (Ultra Fast Infrared)
16 Mbit/s.
100 Mbit/s.
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Bar Code IR
• Modern bar code began in 1948.
• Bernard Silver, a grad student at Drexel Institute of
Technology in Philadelphia,
• Overheard president of a local food chain asking one of
the deans to undertake research to develop a system to
automatically read product information during checkout.
• Silver told his friend Norman Joseph Woodland about
the food chain president's request.
• Woodland was a twenty seven year old graduate student
and teacher at Drexel. The problem fascinated
Woodland and he began to work on the problem.
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UPC
•
Universal Product Code.
•
http://electronics.howstuffworks.com/upc.htm
• UPC symbol printed on a package has two parts:
– The machine-readable bar code
– The human-readable 12-digit UPC number
• BYG Publishing's manufacturer identification number is the first six
digits of the UPC number -- 639382.
• The next five digits -- 00039 -- are the item number.
• The last digit, 3, of the UPC code is called a check digit.
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Check Digit
• Here is how the check digit is calculated for the other 11 digits, using
the code 63938200039 from "The Teenager's Guide to the Real
World" example shown above:
• Step 1 - Add together the value of all of the digits in odd positions
(digits 1, 3, 5, 7, 9 and 11).
6 + 9 + 8 + 0 + 0 + 9 = 32
• Step 2 - Multiply that number by 3.
32 * 3 = 96
• Step 3 - Add together the value of all of the digits in even positions
(digits 2, 4, 6, 8 and 10).
3 + 3 + 2 + 0 + 3 = 11
• Step 4 - Add this sum to the value in step 2.
96 + 11 = 107
• Step 5 - Take the number in Step 4. To create the check digit,
determine the number that, when added to the number in step 4, is a
multiple of 10.
107 + 3 = 110 The check digit is therefore 3.
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UPC
• One thing you will notice if you start
looking at UPC codes in detail is that the
big manufactures have manufacturer IDs
with lots of zeros in them. Here are a few:
• Post - 043000
• General Mills - 016000
• Del Monte - 024000
• Quaker Oats - 030000
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Decoding the Bars
• First of all, look at any 12-digit bar code. It
is made up of black bars and white spaces
between the bars. Assume that the
thinnest bar or space that you see can be
called "one unit wide." The bars and
spaces can therefore be seen to have
proportional widths of one, two, three or
four units. If you look at any bar code you
can see examples of these four widths.
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Start Code
• The start of any bar code is "1-1-1." That
is, starting at the left you find a one-unitwide black bar followed by a one-unit-wide
white space followed by a one-unit-wide
black bar (bar-space-bar).
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Bar Code
Following the start code, the digits are encoded as follows:
0 = 3-2-1-1
1 = 2-2-2-1
2 = 2-1-2-2
3 = 1-4-1-1
4 = 1-1-3-2
5 = 1-2-3-1
6 = 1-1-1-4
7 = 1-3-1-2
8 = 1-2-1-3
9 = 3-1-1-2
Notice that all of these encodings add up to 7.
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The code embedded in the bars is
043000181706
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Decoding the Bar Code
The code embedded in the bars is 043000181706
• The bar code starts with the standard start code of 1-1-1 (bar-space-bar).
• The zero is 3-2-1-1 (space-bar-space-bar).
• The four is 1-1-3-2 (space-bar-space-bar).
• The three is 1-4-1-1 (space-bar-space-bar).
• The next three zeros are 3-2-1-1 (space-bar-space-bar).
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• In the middle there is a standard 1-1-1-1-1 (spacebar-space-bar-space), which is important because it
means the numbers on the right are optically
inverted!
• So now we have bar-space-bar-space,
rather than space-bar-space-bar
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The one is 2-2-2-1 (bar-space-bar-space).
The eight is 1-2-1-3 (bar-space-bar-space).
The one is 2-2-2-1 (bar-space-bar-space).
The seven is 1-3-1-2 (bar-space-bar-space).
The zero is 3-2-1-1 (bar-space-bar-space).
The six is 1-1-1-4 (bar-space-bar-space).
The stop character is a 1-1-1 (bar-space-bar).
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