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Lecture 10
Computer Communication Networks
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Bandwidth

• Band Width means the width of the band
  (channel)
• It is the maximum capacity of a link
• That means how much of data a link or medium can
  support
• It also refers to the maximum amount of data that
  can be pass from a medium
• Think of bandwidth as
• how wide the highway or pipe is
• The wider it is (10Mbps, 100Mbps or 1Gbps) the
  more bps can move through at a time
• A 6-meg satellite internet connection has much
  greater bandwidth than a 56k dial-up modem

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Bandwidth

• In analog circuits it is the difference between
  the maximum and minimum frequency signal
  component that it supports and is measured in
  hertz
• The digital bandwidth is related to bit width
  and measured in bits per second or bytes per
  second

1Mbps, 1µs/bit
1 sec
2Mbps, 0.5µs/bit
1 sec
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Data Rate

• The data rate is the amount of data that is moved
  from one place to another in a given time
• It can be viewed as the speed of travel of a
  given amount of data from one place to another
• So it is the transmission speed in a network
• In general, the greater the bandwidth of a given
  link, the higher the data rate

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Throughput

• Throughput is the average rate of successful
  message delivery over a communication channel
• Throughput is used to determine the maximum rate
  at which a computer user can expect data to
  transfer
• It is the actual data transfer rate
• It is used to measure the performance of a
  network
• Did you know that a perfect connection of 48kbps
  yields about 5.3kbps throughput?

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An Example

• Lets see an example consider an expressway
• Bandwidth refers to the total number of lanes on
  the expressway
• Data Rate is the number of vehicles that can
  possibly travel from one place to another in a
  given time
• Throughput refers to the actual number of
  vehicles that completed the journey
• A bigger expressway with many lanes will allow
  more vehicles to pass in lesser time, similarly a
  high capacity data link will allow a higher
  bandwidth and higher data rate
• Also, if there are greater number of vehicles on
  the road, the speed reduces overall, similarly,
  if a lot of data transfer is taking place
  simultaneously, the throughput will reduce

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Latency

• Latency means delay
• It refers to how long it takes to send data from
  one host to another
• In networking, it is the amount of time it takes
  a packet to travel from source to destination
• Latency is the same thing as seek time the
  minimum time between asking for a piece of data
  and getting it
• Once you have bad latency you're stuck with it
• Together, latency and bandwidth define the speed
  and capacity of a network

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Latency

• If you wanted to send ten characters (at eight
  bits per character) over your 33 Kbps modem link
  you might think it would take
• 80 bits / 33000 bits per second 2.4 ms
• Unfortunately, it doesn't. It takes 102.4 ms
  because of the 100 ms latency introduced by the
  modems at each end of the link
• If you want to send a large amount of data, say
  100KB, then that takes 25 seconds, and the 100 ms
  latency isn't very noticeable, but for smaller
  amounts of data, say 100 bytes, the latency
  overwhelms the transmission time

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Latency

• If you want to transfer a large file over your
  modem it might take several minutes
• The less data you send, the less time it takes,
  but there's a limit
• No matter how small the amount of data, for any
  particular network device there's always a
  minimum time that you can never beat
• That's called the latency
• For a typical Ethernet connection the latency is
  usually about 0.3 ms
• For a typical modem link its about 100 ms, about
  300 times worse than Ethernet

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• LATENCY
• 1)-In a network
• 1-latency, a synonym for delay
• 2-It is an expression of how much time it takes
  for a packet of data to get from one designated
  point to another.
• 3-In some usages (for example, ATT), latency is
  measured by sending a packet that is returned to
  the sender and the round-trip time is considered
  the latency.

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2)- Latency In a computer system 1-latency is
often used to mean any delay or waiting that
increases real or perceived response time beyond
the response time desired. 2-Specific
contributors to computer latency include
mismatches in data speed between the
microprocessor and input/output devices and
inadequate data buffers. 3-Within a computer,
latency can be removed or "hidden" by such
techniques as prefetching (anticipating the need
for data input requests) and multithreading, or
using parallelism across multiple execution
threads.
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4-Latency In 3D simulation, in describing a
helmet that provides stereoscopic vision and head
tracking. 5-latency is the time between the
computer detecting head motion to the time it
displays the appropriate image.
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3)-Latency of Satellite Internet
Service 1-Satellite Internet connections possess
both high bandwidth and high latency. 2-When
loading a Web page, for example, most satellite
users can observe a noticeable delay from the
time they enter a Web address to the time the
page begins loading. 3-This high latency is due
primarily to propagation delay as the request
message travels at the speed of light to the
distant satellite station and back to the home
network. 4-Once the messages arrive on Earth,
however, the page loads quickly like on other
high-bandwidth Internet connections (DSL or
cable). 5-Besides propagation delays, latency
also may also involve transmission delays
(properties of the physical medium) and
processing delays (such as passing through proxy
servers or making network hops on the Internet).
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Latency vs. Bandwidth Although the theoretical
peak bandwidth of a network connection is fixed
according to the technology used, the actual
bandwidth you will obtain varies over time and is
affected by high latencies. Excessive latency
creates bottlenecks that prevent data from
filling the network pipe, thus decreasing
effective bandwidth. The impact of latency on
network bandwidth can be temporary (lasting a few
seconds) or persistent (constant) depending on
the source of the delays.
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Round Trip Time

• RTT is the time required for a signal pulse or
  packet to travel from a source to a destination
  and back again
• RTT is the time it takes for a signal to be sent
  plus the length of time it takes for an
  acknowledgment of that signal to be received
• RTT is double the latency
• You can think of it as one complete loop

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Latency Factors
The contributors to network latency
include Propagation Delay This is simply the
time it takes for a packet to travel between one
place and another Transmission Delay The medium
itself introduces some delay The size of the
packet introduces delay in a round trip since a
larger packet will take longer to receive and
return than a short one. Processing Delay Each
intermediate node takes time to examine and
possibly change the header in a packet I/O and
Storage Delay Within networks at each end of the
journey, a packet may be subject to storage and
hard disk access delays or at intermediate
devices such as switches and bridges