Computer Communication & Networks

Lecture # 10

Bandwidth

Ϟ Band Width means the width of the band (channel)Ϟ It is the maximum capacity of a linkϞ 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 isϞ The wider it is (10Mbps, 100Mbps or 1Gbps) the more bps can

move through at a time

Ϟ A 6-meg satellite internet connection has much greater bandwidth than a 56k dial-up modem

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 bits per second or bytes per second

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

Ϟ In general, the greater the bandwidth of a given link, the higher the data rate

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

Ϟ Did you know that a perfect connection of 48kbps yields about 5.3kbps throughput?

An Example

Lets see an example; consider an expressway

ϞBandwidth refers to the total number of lanes on the expressway

ϞData Rate is the number of vehicles that can possibly travel from

one place to another in a given time

ϞThroughput refers to the actual number of vehicles that completed

the journey

ϞA 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

ϞAlso, 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

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 a network

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

overwhelms the transmission time

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 than Ethernet

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, AT&T), latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency.

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.

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.

3)-Latency of Satellite Internet Service1-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).

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.

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

Ϟ You can think of it as one complete loop

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