MSc in Industrial Computing Systems Computer Communications Lecture 3.

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MSc in Industrial Computing SystemsComputer
CommunicationsLecture 3.

• Lecturer Dr. David Al-Dabass
• Room N315
• Tel. 6015
• email david.al-dabass_at_doc.ntu.ac.uk

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Multiplexing techniques.
Distributed devices used for data collection and
control communicate with central central
processor through 2 basic network arrangements

• Star Network

• Remote Multiplexer Network

Star Network
Central Processor
Each device has its own physical connection to
the central processor
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Remote Multiplexer Network
Central Processor
Each device has connection to the nearest remote
multiplexer and shares a connection to the
computer with other devices.Choice between star
remote multiplexer depends on cost
performance, i.e. getting the highest data rate
at the lowest possible cost.Data transfer rate
should be high enough to satisfy the devices
data communication needs.
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Star Network
Remote device
I/O Bus
Communication Medium
V24 Interface
V24 Interface
- UART does not provide V24 signals (/- 12 v),
needs level converters, which is done by another
chip.Multiple Devices when 100s of devices are
present is there a more economical way of
connecting them to the computer?
Local Multiplexers
What is multiplexer - a circuit capable of
switching several inputs into one output.BUT,
to get back these individual circuits at the
other end we need DE-MULTIPLEXER.
12n

...
Outputs
Inputs
De-multiplexer
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A De-multiplexer is a circuit capable of
switching one input to several outputs.A 2-way
Multiplexer is a circuit for 2-way communication
between a single computer interface and several
devices (usually up to 16 for local
multiplexers).If each device has a data
transfer rate of 1200 b/s, the computer must work
at 1.2 Kb/s x 16 19.2 Kb/sFor comparison the
computer bus works at millions b/s, so no problem
in handling 19.2 Kb/s.
Local multiplexer
UART 1
Serial Line 1
High Speed interface
MULTIPLEXER
Computer I/o bus parallel lines
UART 2
Serial Line 2
UART 3
Serial Line 3
Level converters
...
UART 16
Serial Line 16
Multiplexer uses a scanning register to address
each UART on a round robin sequence.Each UART
has a unique address within the
multiplexer.Multiplexer tests UART addressed by
scanning register for character waiting in
receive buffer (from that device).
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If there is a char, then multiplexer removes the
character from buffer and joins it to uart number
to form a line-no/char pair ready for
processing by the high speed processor.Multiplex
er test a flag for a char to go out to port if
there is one, char is removed from interface
buffer and placed in uart buffer.Scanning
register is then incremented to deal with next
port uart
port, or line number
Input from multiplexer to high speed interface,
ready to be read by the i/o software.
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Remote Multiplexer
Remote multiplexer is connected by high speed
line to the computer.Multiplexer is remote
from computer, PSTN is often used to connect
multiplexer to computer.Remote multiplexer
differs from local multiplexer it uses a several
high speed links (not parallel bus lines) to the
computer.
fast serial lines
Computer UART
modem
modem
Telephone Network
Capacities of Fast and Slow serial lines1. To
ensure fast line capacity is enough when ALL slow
lines are busy at the same time Cf ? C1 C2
C3.. Cne.g. for 3 devices, each set to 300
b/s Cf ? 900 b/s(note that 900 b/s is not an
international standard capacity, so it get
rounded up to 1200 b/s)
slow serial lines
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2). Not all devices are busy at exactly the same
time. If storage is provided in the multiplexer,
then if fast line is busy store data until line
is free.In this case total averages of slow
line rates must be lt fast line capacirty. C1
C2 C3.. Cn ? Cf where indicates
time average
Circuits used to do this temporary storage while
fast line is busy are called concentrators.i.e.
it is a multiplexer storage control circuitry
to even out fluctuations in actual slow line data
transfer rates.
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Questions
1. Explain with the aid of diagrams the two
arrangements used in character networks.2. What
is a multiplexer? Explain how a local multiplexer
performs its function.3. Comment on the
relationship between the device baud rate, the
multiplexer baud rate and a computer bus speed
within the context of local multiplexing and
illustrate your discussion with numerical
examples.4. What is UART/char pair?5. In
what way does a remote multiplexer differ from a
local multiplexer? Use a diagram to discuss a
typical example where a remote de-multiplexer is
used.6. What relationship exists between slow
and fast line data transfer rates in a remote
multiplexer?7. Comment on the differences
between actual and average data rates of the
slow lines in remote multiplexing use equations
to illustrate your discussion.8. What is a
concentrator? What differences are there between
multiplexers and concentrators?
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Multiplexing Techniques
The bandwidth of channel may be divided among
slower channels according to either 1.
Frequency or 2. Time
Frequency
Frequency/time space to be divided among slow
channels.
Time
Sharing the frequency is called Frequency
Division Multiplexing (FDM)Time sharing the
fast channel among the slow channels is
called Time Division Multiplexing
(TDM)Frequency and time are two orthogonal
characteristics of channel bandwidth and can
co-exist in the channel simultaneously.
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Channel 4
Channel 5
Frequency Division Multiplexing (FDM)
Channel
n
Frequency
n -1
2
1
0
Time
Guard Bands

• Frequency range is divided into a number of
  narrower bands, each has the same bandwidth as a
  slow channel.- A guard band is used to separate
  the narrow bands to avoid interference between
  them.Characteristics of FDM1. Each slow
  channel has a permanently allocated (narrow)
  channel in frequency range.2. If slow channel
  is not being used then its slot is wasted.3.
  Slow channels have to be modulated up to the
  frequency slot allocated to them and after that
  demodulated at the other end. e.g. a fast
  channel having a bandwidth of 100 kHz can be
  divided among 30 slow channels of approximately 3
  kHz each (and, theoretically, a guard bands of
  344.8 Hz).

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Time Division Multiplexing (TDM)
Data is sent one character at a time, with some
idle time in between. Use this idle time to
transmit characters from other channels, i.e.
Divide the time axis into slots, each wide
enough to send one character. e.g. for 4 slow
channels, every 4th slot contains a character
form the same speed channel.
1 2 3 4 1 2 3 4 1 2
3 4 1 2 3 4
Time
4 characters being sent along channel 2 in a
4-channel time-multiplexed link.
Sender Receiver must synchronise to ensure the
right character goes to the right channel round
robin way is the simplest.TDM has the
disadvantage of slot remaining empty if slow
channel has no char to send.There are ways of
overcoming this problem. How? The answer is
Intelligent (Statistical) Time Division
Multiplexing.
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Intelligent (Statistical) Time Division
Multiplexing
Instead of round robin (i.e. prefixed time slot
for each channel) we can employ more flexible
approach first come (channel) first serve
(next empty slot)i.e. put latest character to
be sent in the next free slot, regardless which
channel the char comes from.BUTProblem 1. How
does the multiplexer at the other end know which
slow channel has the character just received come
from?Solution add channel identification
number to each character sent.
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8 7
0
Control Bit can be used to indicate if content
(in remaining 15 bits) is data or some special
control signal/messages between multiplexer s at
the opposite ends of the channel.Example using
variable size slots, a number of characters from
same slow channel are sent together as a
unit.gives much improved rate as channel
identification number with every character is
avoided however, there is need for
microprocessor processing to provide this type of
intelligence.Second Problem what if there
are no free slots?Solution need to store
characters (with their channel identification
numbers) until there is a free slot i.e.
concentrator.
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Must ensure that the sum of average transfer
rates for slow channels is lt capacity of fast
channel, hence the name statistical as it
relies on averages.Microprocessor provides
functions such as error checking and
control/correction using parity bit(s).Most
current remote multiplexers are of the
intelligent/statistical/concentrator type due to
the sharp drop in the price of the
microelectronics hardware.
MULTIPLEXER NETWORKS
For large (mainframe) computers, we need an
interface device at the computer end toa)
send/receive characters to the remote
multiplexers.b) exchange this data with the
host machine.Usually they have front end
processor (FEP) to1) Handle all character
echoes.2) Edit lines before passing them to main
computer, e.g. rubout/deletions and line
editing.3) When main computer goes down FEP
outputs messages to let user know what is
happening.4) Monitors the network performance,
e.g. log in/out ratio, idle/busy ratio etc. and
compiles appropriate statistics.
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Communication between FEP and the main computer
is done bya) direct memory access (DMA) to a
shared memory.b) shared disk.c) high speed I/O
channel.
mainframe computer
DMA or other (see above)
FEP
CASCADED MULTIPLEXER
With front end processor dedicated to network
control, complex networks can be designed and
implemented. One way to do it is through using
Cascaded Remote Multiplexers
i.e. connecting multiplexers in series. This very
efficient use of the line capacity. The more
devices connected through the same high speed
line the more averaging can be done maximise the
use of idle time.
A
B
C
mainframe computer
FEP
High speed lines between multiplexers and the
computer must get faster the nearer the
multiplexer gets to the computer
Cascaded Remote Multiplexers
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Design Criteria for Asynchronous Networks.1)
Local Devices connect using a star type network
cheap gives good performace.2) Remote Devices
needing PSTN a) few devices use individual
telephone lines in a star-like arrangement. b)
large number of devices use remote
multiplexer.Cross-over between star remote
multiplexer depends on individual cases and must
consider - PSTN availability. - Other
facilities offered by the de/multiplexer at the
computer end, e.g. if the same multiplexer can be
used for local devices, then only need a single
computer interface (FEP). - Locality of the
remote devices (price).
An Asynchronous Device Network
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Questions1) Explain the terms FDM and TDM and
with the aid of a diagram discuss the phrase
frequency/time space of a channel.2) What are
guard bands? Use a diagram to illustrate the
relationship between a) the number of slow
channels, b) the bandwidth of each slow channel
and c) the bandwidth of the fast channel.3)
Explain how synchronisation is achieved in the
simple Time Division Multiplexing technique and
comment on the efficiency of this technique when
there are no characters ready to be sent.4) Why
is the Intelligent TDM so called? Explain its
operation using diagrams and discuss the use of
the control bit to improve efficiency.5)
Explain the function of FEP.6) What are
cascaded multiplexers? Explain using diagrams.