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Advanced Computer
Architecture-CS501
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Advanced
Computer Architecture
Lecture
No. 44
Reading
Material
Patterson,
D.A. and Hennessy,
J.L.
Chapter
8
Computer
Architecture- A Quantitative
Approach
Summary
·
Physical
Media (Continued)
·
Shared
Medium
·
Switched
Medium
·
Connection
Oriented vs. Connectionless
Communication
·
Network
Topologies
·
Seven-layer
OSI Model
·
Internet
and Packet Switching
·
Fragmentation
·
Routing
Modem
To
interconnect different computers by
using twisted pair copper
wire, an interface is
used
which is called modem. Modem
stands for modulation/demodulation.
Modems are
very
useful to utilize the
telephone network (i.e. 4 KHz
bandwidth) for data and
voice
transmission.
Quality
of Telephone Line
Data
transfer rate depends upon
the quality of telephone
line. If telephone line is of
fine
quality,
then data transfer rate will be
sufficiently high. If the
phone line is noisy
then
data
transfer rate will be decreased.
Classification of
Fiber Optic
Cables
Fiber
optic cables can be classified
into the following
types.
Multimode
fiber
This
fiber has large diameter.
When light is injected, it
disperses, so the effective
data
rate
decreases.
Mono
mode Fiber
Its
diameter is very small. So
dispersion is small and data rate is
very high.
Wavelength
Division Multiplexing
(WDM)
Waves of
different wavelengths are
simultaneously sent through fiber. So as
a result,
throughput
increases.
Wireless
Transmission
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This is
another effective medium for
data transfer. Data is transferred in
the form of
electromagnetic
waves. It has the following
features:
·
Data rate is in
Mbits/Sec.
·
Very
effective because of
flexibility.
·
Band
width is much less than
fiber.
Example
1
Suppose
we have 20 magnetic tapes,
each containing 40GB. Assume
that there are
enough
tape readers to keep any
network busy. How long will
it take to transmit the
data
over a
distance of 5Km? The choices are
category 5 twisted-pair wires at
100Mbits/sec,
multimode
fiber at 1500Mbits/sec and single-mode
fiber at 3000Mbits/sec.
(Adapted
from
CA3: H&P)
Solution
The
total amount of data
= total
no. of mag. tapes x capacity
of each tape
= 20 x
40GB= 800GB
The
time for each
medium:
Twisted
pair = 800GB/100Mbits/sec
= 65536
sec = 18.2 hr
Multimode
Fiber = 800GB/1500Mbits/sec
=
4369.06sec = 1.213 hr
Single
mode Fiber = 800GB/3000Mbits/sec
=
2184.55sec
=
0.66hr
Car =
time to load car + transport
time + time to unload
car
= 250sec
+ 5Km/30Kph + 250sec
= 500.16
sec = 0.13hr
Shared/Switched
Medium
Shared
Medium
If a
number of computers are
connected with a single
physical medium (i.e.
coaxial or
fiber),
this situation is called
shared medium. Because of
many computers, collision
takes
place and
affects the data transfer
rate. As the number of
machines on a physical
medium
increases,
the data transfer rate
decreases.
Switched
Medium
To
increase the throughput, a
switched medium is
used.
Example
2
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Compare 20
nodes connected in three
different ways: a single
100Mbits/sec shared
medium; a
switch connected via cat5,
each segment running at 100Mbits/sec; and
a
switch
connected via optical fiber,
each segment running at 1500Mbits/sec.
The shared
medium is
700m long, and the average
length of each segment to a switch is
55m. Both
switches
can support full bandwidth. Assume
each switch adds 6µsec to
the latency, and
the
average message size is 200bytes.
Ignore the overhead of
sending or receiving a
message
and contention for the
network.
Solution
First we
will calculate the aggregate
bandwidth:
For
shared medium
Aggregate
bandwidth = 100Mbits/sec
For
switched twisted pair
Aggregate
bandwidth = 20 x 100Mbits/sec
=
2000Mbits/sec
For
switched optical
fiber
Aggregate
bandwidth = 20 x 1500Mbit/sec
=
30,000Mbits/sec
Transport
time = Time of flight +
(message size/BW)
(700/1000)Km
Transport
time shared = ---------------------- x
106µsec
(2/3 x
300,000)Km
+ (200 x
8bits / 100Mbits/sec)
= 3.5µsec
+ 16µsec = 19.5µsec
For
the switches, the distance is
twice the average segment. We
must also add latency
for
the
switch.
(55/1000)Km
Transport
time switch = 2x ---------------------- x
106µs
(2/3 x
300,000)Km
+
6µsec
+ (200 x
8bits / 100Mbits/sec)
=
0.55µsec + 6µsec +16µsec
=
22.55µsec
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(55/1000)Km
= 2x
---------------------- x 106µs
Transport
time fiber
(2/3 x
300,000)Km
+
6µsec
+ (200 x
8bits / 1500Mbits/sec)
=
0.55µsec + 6µsec +1.06µsec
=
7.61µsec
Although
the bandwidth of the switch
is many times that of the
shared medium, the
latency
for unloaded networks is
comparable.
Connection
Oriented vs. Connection less
Communication
Connection
Oriented Communication
· In
this method, same path is
always taken for the
transfer of messages.
· It
reserves the bandwidth until
the transfer is complete. So no
other server could
use
that path until it becomes
free.
· Telephone
exchange and circuit switching is
the example of connection
oriented
communication.
Connection
less Communication
· Here
message is divided into
packets with each packet
having destination
address.
· Each packet can
take different path and reach the
destination from any route
by
looking
at its address.
· Postal
system and packet switching are
examples of connection
less
communication.
Network
Topologies
Computers
in a network can be connected together in
different ways. The
following three
topologies
are commonly used:
· Bus
topology
· Star
topology
· Ring
topology
Bus
Topology
In this
arrangement, computers are
connected via a single
shared physical
medium.
Star
topology
Computers
are connected through a hub.
All messages are broad cast
because the hub is
not an
intelligent device.
Ring
Topology
All
computers are connected
through a ring. Only one
computer can transmit data at
one
time,
having a pass called
"Token".
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Seven
Layer OSI Model
There
are seven layers in this
model.
1.
Physical Layer
2. Data
Layer
3.
Network Layer
4.
Transport Layer
5.
Session Layer
6.
Presentation Layer
7.
Application Layer
OSI
Model Characteristics
· An
interface is present between any
two layers.
· A
layer may use the
data present in another
layer.
· Each
layer is abstracted from
other layers.
· The
service provided by one layer can be
used by the other
layer.
· Two
layers can provide same
service e.g. Check Sum
calculated at different
layers.
· On
two machines, six layers
are logically connected
except the physical
layer.
The
physical layers of two
machines are physically
connected.
Internet
and Packet
Switching
Internet
works on the concept of packet
switching. Application layer
passes data to the
lower
layer and that lower layer
passes data to the next
lower layer and on so on. In
this
data
passing process through
different layers, different
headers are attached with
the data
which
shows the source and destination
addresses, number of data
bytes in packet, type
of
message etc. At physical
layer, this packet is transmitted
into the network. At
reception,
reverse procedure is adopted.
Fragmentation
When a
packet is lost in the network, it is
re-transmitted. If the size of the packet
is large
then
retransmission of packet is wastage of
resources and it also increases the
delay in the
network.
To minimize this delay, a
large packet is divided into
small fragments. Each
fragment
contains a separate header
having destination address and
fragment number.
This
fragmentation effectively reduces
the queuing delay. At
destination, these
fragments
are
re-assembled and data is sent to the
application layer.
Routing
Routing
works on store-and-forward policy.
There are three methods
used for routing:
· Source-based
routing
· Virtual
Circuit
· Destination-based
routing
TCP/IP
Internet
uses TCP/IP protocol. In the
TCP/IP model, session and
presentation layers
are
not
present, so Store-Forward routing is
used.
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