NET0183 Networks and Communications
Lectures 1 and 2
THE ALOHA SYSTEM
Aloha in the Hawaiian language means affection, love, peace, compassion and mercy
http://en.wikipedia.org/wiki/Aloha
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Historical Case Study
http://portal.acm.org/citation.cfm?id=1478462.1478502
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1968
• In September 1968 the University of Hawaii
initiated research into the use of radio
communications to support computer-computer
and console-computer links.
• At that time, the University of Hawaii was
composed of a main campus in Manoa Valley
near Honolulu, a four-year college in Hilo, Hawaii,
and five two-year community colleges on the
islands of Oahu, Kauai, Maui, and Hawaii.
– ..plus various research institutes operating within a
200 mile radius of Honolulu.
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http://www.lonelyplanet.com/maps/north-america/usa/hawaii/
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http://www.williams.edu/astronomy/Course-Pages/111/Images/ems.jpg
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http://www.randomthink.net/pictures/emwave.gif
The speed of light (usually denoted c) is a physical constant. Its value is
exactly 299,792,458 metres per second,[1][2] often approximated as 300,000
kilometres per second or 186,000 miles per second.
c=fλ
http://en.wikipedia.org/wiki/Speed_of_light
f frequency
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λ wavelength
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IBM 360/65
• The computing centre on the main campus
operated an IBM 360/65 with 750 Kbytes of core
memory.
• A time sharing system UHTSS/2, written in XPL,
was developed as a joint project of the University
Computer Centre and THE ALOHA SYSTEM under
the direction of W. W. Peterson.
– University computing centres typically wrote their
own time sharing systems (early operating systems)
during the early days of computing.
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Goal/Markmið
• “The goal of THE ALOHA SYSTEM is to provide
another alternative for the system designer and
to determine those situations where radio
communications are preferable to conventional
wire communications.”
– Conventional wire communications provided dial-up
telephone services. It was possible to lease dedicated
lines, but this was usually an expensive option.
– In many parts of the world (~1970), a reliable, high
quality, wire communication network was not
available.
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Differences between data
communication and voice signals.
• The telephone system was designed for voice signals
where two parties hold a more or less continuous
conversation.
• In contrast, “Data transmitted in a time-shared
computer system comes in a sequence of bursts with
extremely long periods of silence between the bursts.”
• Using a leased line to connect an alphanumeric console
to a computer could mean that the communication
system was operated at less than 1 percent of its
capacity.
– Multiplexing can alleviate this problem.
multiplexing/fléttun
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http://www.columbia.edu/acis/history/teletype.jpg
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http://foldoc.org/
multiplexing
1. <communications> (Or "multiple access") Combining several signals for
transmission on some shared medium (e.g. a telephone wire). The signals are
combined at the transmitter by a multiplexor (a "mux") and split up at the
receiver by a demultiplexor. The communications channel may be shared
between the independent signals in one of several different ways: time division
multiplexing, frequency division multiplexing, or code division multiplexing.
If the inputs take turns to use the output channel (time division multiplexing)
then the output bandwidth need be no greater than the maximum bandwidth of
any input.
If many inputs may be active simultaneously then the output bandwidth must
be at least as great as the total bandwidth of all simultaneously active inputs. In
this case the multiplexor is also known as a concentrator.
(1995-03-02)
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Differences between data
communication and voice signals.
• “Statistical analyses of existing systems indicate
that the average amount of data transmitted
from the central system to the user may be as
much as an order of magnitude greater than the
amount transmitted from the user to the central
system.”
– This represented an additional factor in the inefficient
use of a wire communication channel.
• Addressing the asymmetry by providing different
capacity wire channels was not deemed possible.
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reliability/áreiðanleiki
Differences between data
communication and voice signals.
• Using a dial-up channel, errors in binary data were caused
by random and burst noise.
• Dial-up channels suffered from connection problems e.g.
busy signals, wrong numbers, and disconnects.
– Andy comments: I remember suffering all these problems.
• “... there is little doubt that in many locations the reliability
of wire communications is well below that of the remainder
of the computer-communication system.”
• Wire communications were usually obtained from the
telephone companies and so were not under the direct
control of the system designer.
– Andy comments: Today´s system designers do not have direct
control of the Internet either.
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MENEHUNE is a legendary Hawaiian elf.
Figure 1. THE ALOHA SYSTEM
• The MENEHUNE is a small interface computer. “Much of the design of
this multiplexor is based on the design of the Interface Message
Processors (IMP´s) used in the ARPA computer net.”
• The HP 2115A had a 16-bit word size, a cycle time of 2 microseconds
and an 8K-word core storage capacity.
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THE ALOHA SYSTEM
• Two 100 KHz channels at 407.350 MHz and 413.475
MHz were assigned to ALOHA.
– Operating at a rate of 24,000 baud.
• One of the channels was dedicated to data from the
MENEHUNE to the remote consoles.
– Outgoing messages were queued and prioritised (to any
given scheme) before being transmitted sequentially by
the MENEHUNE.
• The other channel was dedicated to data from the
remote consoles to the MENEHUE.
– “Message from the remote consoles to the MENEHUNE
however are not capable of being multiplexed in such a
direct manner.”
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Why no frequency multiplexing?
• A different radio frequency could have been used for
each of the remote consoles.
• However, this would have considerably increased the
costs of the remote consoles and the receiving station.
• Also, when a remote console was not transmitting, its
frequency channel would be unused, meaning
inefficiency at the receiving station.
• Also, the ALOHA project team expected to have many
remote consoles and they wanted to minimize the
complexity of the communications equipment at each
console.
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http://foldoc.org/
baud
<communications, unit> /bawd/ (plural "baud") The unit in which the
information carrying capacity or "signalling rate" of a communication channel is
measured. One baud is one symbol (state-transition or level-transition) per
second. This coincides with bits per second only for two-level modulation with
no framing or stop bits.
The term "baud" was originally a unit of telegraph signalling speed, set at one
Morse code dot per second. It was proposed at the International Telegraph
Conference of 1927, and named after J.M.E. Baudot (1845-1903), the French
engineer who constructed the first successful teleprinter.
Where data is transmitted as packets, e.g. characters, the actual "data rate" of a
channel is R D / P where R is the "raw" rate in bits per second, D is the number
of data bits in a packet and P is the total number of bits in a packet (including
packet overhead). The term "baud" causes much confusion and is usually best
avoided. Use "bits per second" (bps), "bytes per second" or "characters per
second" (cps) if that's what you mean.
(1998-02-14)
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THE ALOHA SYSTEM
• Information was transmitted in “packets”.
• Packets had a fixed-length of 80 8-bit characters plus
32 identification and control bits and 32 parity bits.
– 704 bits
– At 24,000 baud, each packet transmission lasted for 29
milliseconds.
• The parity bits were used for a “cyclic error detecting
code”.
– “The possibility of using the same code for error
correction at the MENEHUNE will be considered for a later
version of the ALOHA system.”
• Andy comments: Should computers automatically correct errors in
data transmissions?
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http://foldoc.org/
parity
<storage, communications> An extra bit added to a byte or word to reveal errors
in storage (in RAM or disk) or transmission. Even (odd) parity means that the
parity bit is set so that there are an even (odd) number of one bits in the word,
including the parity bit. A single parity bit can only reveal single bit errors since if
an even number of bits are wrong then the parity bit will not change.
Moreover, it is not possible to tell which bit is wrong, as it is with more
sophisticated error detection and correction systems.
See also longitudinal parity, checksum, cyclic redundancy check.
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THE ALOHA SYSTEM
• Consoles transmitted packets to the MENEHUNE
over the same channel in a “completely
unsynchronised manner”.
• If a packet was received without error it was
acknowledged by the MENEHUNE.
• The transmitting console waited a certain amount
of time for an acknowledgement. If no
acknowledgement was received, the packet was
retransmitted.
– The process repeated until a succesful transmission
took place.
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Two different types of errors
• A transmitted packet could be received
incorrectly because of random noise.
– “The first type of error is not expected to be a
serious problem.”
• A transmitted packet could be received
incorrectly because of interference with a
packet transmitted by another console.
– “The second type of error... will be of importance
only when a large number of users are trying to
use the channel at the same time.”
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Figure 2. Packets being transmitted by k active consoles
in the ALOHA random access communication system.
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Figure 2.
• T (tau) is the duration of a packet – about 34
milliseconds.
– 29 milliseconds for transmission of 704 bits.
– plus 5 milliseconds for “receiver synchronization”
• “For analysis purposes we make the pessimistic
assumption that when an overlap occurs neither
packet is received without error and both packets
are therefore retransmitted.”
– “... we must make sure the retransmission delays in
the two consoles are different.”
• Andy comments: exactly how this is achieved is not stated.
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Analysis
• Assume there are k active users.
• Packets transmitted for the first time are
called message packets.
• Packets retransmitted are called repetitions.
• Let λ be the average rate of message packets
from a single active user and assume this rate
is identical for all users.
• So r, the average number of message packets
per unit time from k active users, is kλ.
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r=kλ
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Analysis
• τ (tau) is the duration of a packet.
• If messages were packed perfectly into the
available channel space (with no space between
messages) then we would have rτ =1.
• rτ is referred to as the channel utilization.
• “Our objective in this section is to determine the
maximum value of the channel utilization, and
thus the maximum value of k, which this random
access data communication channel can
support.”
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Analysis
• Let R be the average number of message packets
plus retransmissions per unit time from the k
active users.
• If there are some retransmissions, then R>r.
• Rτ is defined as the channel traffic.
– The average number of message packets plus
retransmissions per unit time multiplied by the
duration of each packet or retransmission.
• “In this section we shall calculate Rτ as a function
of the channel utilization, rτ.”
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Analysis
• Assume the interarrival times (defined by the start times of
all the message packets plus retransmissions) are
independent and exponential.
• The true interarrival time distribution may not be
exponential and because of the retransmissions, the
assumption “is strictly speaking not even mathematically
consistent.”
• “If the retransmission delay is large compared to τ... and
the number of retransmissions is not too large this
assumption will be reasonably close to the true
distribution.”
– Andy comments: it would be useful to have some real
measurements showing an exponential distribution of
interarrival times.
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Interarrival time
• The interarrival time is the amount of time
between the arrival of one message (or
retransmission) and the arrival of the next.
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Internet Data Analysis for the Undergraduate Statistics Curriculum
Juana Sanchez and Yan He (2005)
http://www.amstat.org/publications/jse/v13n3/datasets.sanchez.html
A visual inspection of the
histogram might suggest an
exponential, but a detailed
analysis by Sanchez and He
suggests that interarrival
times of Internet packets are
not distributed exponentially.
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Analysis
• Under the exponential assumption, the
probability that there will be no events (starts of
message packets or retransmissions) in a time
interval T is exp(-RT).
• “The first packet will overlap with another packet
if there exists at least one other start point τ or
less seconds before or τ or less seconds after the
start of the given packet. Hence the probability
that a given message packet or retransmission
will be repeated is [1-exp(-2Rτ)].”
probability of a collision happening
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Analysis
• The average number of retransmissions per
unit time is R[1-exp(-2Rτ)].
• R = r + R[1-exp(-2Rτ)].
• rτ = Rτe-2Rτ
– This is the relationship between channel
uitilization (rτ) and channel traffic (Rτ).
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Figure 3. Channel utilization vs. channel traffic.
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Figure 3.
• Channel utilization reaches a maximum value of
1/2e = 0.186.
– Channel traffic is equal to 0.5 at this point
– Mathematical constant e ≈ 2.718
• “The traffic on the channel becomes unstable at
rτ=1/2e and the average number of
retransmissions becomes unbounded.”
• “Because of the random access feature the
channel capacity is reduced to roughly one sixth
of its value if we were able to fill the channel with
a continuous stream of uninterrupted data.”
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Maximum number of interactive users kmax.
•
•
•
•
rτ = kλτ = 1/2e
kmax = (2eλτ)-1
τ = 34 milliseconds
Suppose each active user sends a message
packet at an average rate of one every 60
seconds. (λ = 1/60)
• kmax = 324
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Analysis checked by simulations
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December 17, 1972
• An IMP (Interface Message Processor) was
delivered and installed which allowed ALOHA
to connect to the ARPANET by means of the
first satellite channel.
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