Computer Communication &
Networks
Muhammad Waseem Iqbal
Bandwidth
Ϟ Band Width means the width of the band (channel) Ϟ It is the maximum capacity of a link
Ϟ That means how much of data a link or medium can support
Ϟ It also refers to the maximum amount of data that can be pass from a medium
Think of bandwidth as
Ϟ how wide the highway or pipe is
Ϟ The wider it is (10Mbps, 100Mbps or 1Gbps) the more bps can move through at a time
Bandwidth
1 sec
1 sec
1Mbps, 1µs/bit
2Mbps, 0.5µs/bit
Ϟ In analog circuits it is the difference between the maximum and minimum frequency signal component that it supports and is measured in hertz
Ϟ The digital bandwidth is related to “bit width” and measured in
Data Rate
Ϟ The data rate is the amount of data that is moved from one place to another in a given time
Ϟ It can be viewed as the speed of travel of a given amount of data from one place to another
Ϟ So it is the transmission speed in a network
Throughput
Ϟ Throughput is the average rate of successful message delivery over a communication channel
Ϟ Throughput is used to determine the maximum rate at which a computer user can expect data to transfer
Ϟ It is the actual data transfer rate
Ϟ It is used to measure the performance of a network
An Example
Lets see an example; consider an expressway
Ϟ Bandwidth refers to the total number of lanes on the expressway Ϟ Data Rate is the number of vehicles that can possibly travel from
one place to another in a given time
Ϟ Throughput refers to the actual number of vehicles that
completed the journey
Ϟ 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
Ϟ Also, if there are greater number of vehicles on the road, the
speed reduces overall, similarly, if a lot of data transfer is taking
Latency
Ϟ Latency means delay
Ϟ It refers to how long it takes to send data from one host to
another
Ϟ In networking, it is the amount of time it takes a packet to travel
from source to destination
Ϟ Latency is the same thing as seek time; the minimum time
between asking for a piece of data and getting it
Ϟ Once you have bad latency you're stuck with it
Ϟ Together, latency and bandwidth define the speed and capacity of
Latency
Ϟ 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
Ϟ 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
Latency
Ϟ If you want to transfer a large file over your modem it might take
several minutes
Ϟ The less data you send, the less time it takes, but there's a limit Ϟ No matter how small the amount of data, for any particular
network device there's always a minimum time that you can
never beat
Ϟ That's called the latency
Ϟ For a typical Ethernet connection the latency is usually about 0.3
ms
Ϟ For a typical modem link its about 100 ms, about 300 times worse
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.
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
buffer
s.
3-Within a computer, latency can be removed or
"hidden" by such techniques as
prefetching
(anticipating the need for data input requests) and
4-Latency In 3D simulation, in describing a helmet
that provides stereoscopic vision and head
tracking.
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).
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.
Round Trip Time
Ϟ RTT is the time required for a signal pulse or packet to travel from
a source to a destination and back again
Ϟ 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
Ϟ RTT is double the latency
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
