understanding bandwidth

7/31/2019 Understanding Bandwidth

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Understanding BandwidthWhat is bandwidth? There are two definitions, both of which are

important to Screencast.com account owners.1. Bandwidth is a measure of the amount of data that can be

transmitted through a connection over a given amount of time.2. Bandwidth, also called data transfer rate, is usually expressed in

bits per second (bps).

The Amount of Data Transferred Through a

Connection Over TimeBandwidth is simply a measure of the amount of data that can betransmitted through a connection over a given amount of time.Bandwidth, also called data transfer rate, is usually expressed in bits persecond (bps).

When a Screencast.com account owner shares content and that content isviewed, they are billed for the bandwidth required to transfer the content

throughout a particular billing cycle. Overall, this can be thought of asthe cost of sharing content. At the beginning of a billing cycle, the totalbandwidth amount starts over.For a rough calculation of how much bandwidth you might use, assumeyou have a 10 MB file and it is downloaded once, 10 MB of bandwidthis used. If that same 10 MB file is downloaded seven times, 70 MB of bandwidth is used. Partial downloads only use bandwidth for the part of the file downloaded, so a files views multiplied by its size rarely equalsits bandwidth consumed.While it may be difficult to figure out exactly how much bandwidth youwill need since you may not know how many people will view yourcontent or how much content you will make available for viewing,Screencast.com makes it easy for you to purchase extra blocks of rollover bandwidth. If all of the extra bandwidth is not consumed in one


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billing cycle, it will roll over to the next one until it is all used up.For more information on this subject, watch this helpful video .

Bandwidth or File Transfer Rate

Bandwidth is synonymous with capacity. Every machine on the Internetis connected by a cable or another type of connection. This cable has acapacity; it can carry a certain amount of information through it, similarto a hose. This cable may have a high or low capacity. If you compare agarden hose to a fire hose, more water can pass through the fire hose in aminute than a garden hose.

The slowest of these capacities is the bandwidth of the line; its the fastest you can communicatebetween the ends. Think of the cable as a long tubewith someone pouring water into it on one end, andsomeone draining it out the other. You cannot takeout more than was put in, and if you pour it in too fastthe water will spill (losing data).

For example, a 28.8 kbps dialup modem connection is much slower than

a cable modem connection. The cable modem connection can downloadmore data than the 28.8 kbps dialup modem connection can over thesame period of time. The cable modem has a higher bandwidthconnection than the 28.8 kbps dialup connection does. High bandwidthallows fast transmission of data or high volume transmission just like thefire hose.

File Size, Bit Rate, Bandwidth and Data

TransmissionFile size is measured in bytes. One kilobyte (K or KB) equals about1,024 bytes. For example, a small image file might be 20K or about20,000 bytes in size.
http://www.screencast.com/users/Training/folders/Screencast.com%20Training/media/6979b7b7-b32a-47a8-95a1-5d45ca040580http://www.screencast.com/users/Training/folders/Screencast.com%20Training/media/6979b7b7-b32a-47a8-95a1-5d45ca040580http://www.screencast.com/users/Training/folders/Screencast.com%20Training/media/6979b7b7-b32a-47a8-95a1-5d45ca040580http://www.screencast.com/users/Training/folders/Screencast.com%20Training/media/6979b7b7-b32a-47a8-95a1-5d45ca040580


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Measurements and Conversions

Data Transmission

Data transmission measured in bits per second (bps) is called the baud

rate or bit rate - it is the measure of bandwidth. It is commonly measuredin thousands of bits per second or kilobits per second. The abbreviationfor kilobits per second is kbps or simply k. One kilobit equals about1,000 bits.If a computer can receive 5KB in a second, it would take 4seconds for it to receive a file 20K in length.

But bandwidth is not measured in bytes it is measured in bits. 1 bytecontains 8 bits. So, if a computer can receive 5KB (5,000 bytes) in onesecond, it can receive 40,000 bits per second. 5,000 bytes x 8 bits =40,000 bits per second or 40 kbps.

If a computer connects to the Internet using a 56 kbps dialup modem, intheory, it means that the computer could receive 56,000 bits per second(56 kbps). That would mean that the computer could receive about 7,000bytes per second. Remember that a byte equals 8 bits, so 56,000 bps/ 8bits = 7,000 bytes.

So, to receive the 20K image file, the 56 kbps dialup connection wouldrequire slightly less than 3 seconds to receive the file (20K/7,000bytes=2.85 seconds).

Note : A 56 kbps modem cannot actually communicate at 56 kbps. Inreality its more like 35 -45 kbps. A 56 kbps modem connectiongenerally only provides 35-45 kbps of bandwidth.


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Bandwidth and Streaming Media FileTransmission

Computer video files are basically a number of still images called framescombined sequentially into one file. When the file is played on a playersuch as Windows Media Player, it goes through the video file anddisplays each frame sequentially in quick succession to create theillusion of motion similar to a video film rolling through a movieprojector.

When a file is transferred, frames are continuously delivered from thecomputer transferring the video to the computer playing it. Each frame isdisplayed as it is received.

For example, consider a computer connected to the Internet using adialup modem. If the modem is connected at 40 kbps, it could receive5,000 bytes (5 KB) of data per second. If each frame of the video wasonly 5K then the modem could only receive 1 frame per second.

Commercial motion pictures are 24 fps (frames per second), television is30 frames per second. The more frames per second (fps), the smother thevideo playback appears to the viewer. So, a 1 fps video is a very slowand choppy video. The number of frames per second (fps) is also calledthe frame rate.

With a higher bandwidth connection, more frames per second could bereceived. With a 128 kbps ISDN connection for example, 32 5K framescould be delivered per second.

But, a 5K image or frame is not very big. A small 320x200, 16 bit JPEGfile can easily be 20K in size. So, for the modem connected with only 40kbps of bandwidth, it would take 4 seconds to receive only one frame of the video! At that rate, the video would degrade into a slide show, andnot be a video at all.


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This is the reason why many videos that you see online or on news sitesare very small, and why a dialup Internet connection just does not haveenough bandwidth to enjoy a very rich multimedia experience.

Video Compression and Key FramesAs static image files are compressed using various compressionalgorithms the video and audio data in transferred files is compressed.This reduces the number of bytes in each frame thus reducing thebandwidth requirement to deliver the video. While data compressionhelps considerably, another step is taken to reduce bandwidthrequirements.

Video files are a number of still images called frames combinedsequentially into one file. Each frame is displayed at some given numberof frames per second (fps) to create the illusion of movement. But manytimes there is no movement or change in the video between one frameand the next.For example, a video demonstration of an application may show theopening of a new window and then not change for several minutes whilethe audio narration explains the application. If nothing changes, there isno reason to send a new frame of video data. The player can just displaythe same frame. This reduces bandwidth requirements.

Consider a video that is a demonstration of an application. Perhaps, allthat is changing in the video is that the cursor moves around theapplication as the author of the video points out different areas in theapplication. Instead of sending the entire frame, only the changes to thenew frame are sent. The mouse pointer is very small and the number of

bytes of video data that represents it is minimal, so very little video dataneeds to be transmitted to reflect the change between frames. Sendingonly the part of the frame that has changed can also greatly reducebandwidth requirements.

Changes from one frame to the next, increases the bandwidthrequirements of the video. The more movement occurs, the more area of


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the screen is changed, resulting in larger amounts of video data that mustbe sent to update to the next frame. If the entire screen changed from oneframe to the next, the entire frame would have to be sent.

Key Frames versus Delta FramesThere are two types of video frames: key frames and delta frames. Keyframes contain all of the pixels that comprise the complete frame. Deltaframes only contain what changed from the previous frame. Key framesare placed in the video at regular intervals, either every so many secondsor so many frames. Windows Media Encoder, for example, defaults to 1key frame every 10 frames. It looks something like this:

Key frame | delta frame | delta | delta | delta | key | etc

If there is no change from one frame to the next, delta frames cancontain 0 bytes of data. If the only change from one frame to the next isthe movement of the mouse pointer, the delta frame would contain verylittle data. If the entire screen had changed, the delta frame would be aslarge as a key frame, as it would have to contain bytes of datarepresenting every pixel in the frame. Although high key frame rates

increase file size, they improve seeking.

Frame Rate and Bandwidth

Frame rate may or may not have much effect on the bandwidthrequirements of the video. If there is a lot of change between frames,then the size of each frame is larger and more data must be transmittedfor each frame. In this case, higher capture frame rates require increasedbandwidth. But, if there is little or no change between frames, then littleor no video data is transmitted for each frame. So, depending on thecontent of the video, increasing the frame rate may have little or greateffect on how much bandwidth is required.

Network Congestion, Bandwidth Spikes, and Buffering


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Transferred video is displayed as it is received so the video can start todisplay the data before the entire file has been transmitted. If for somereason the data is slowed or interrupted, the video will stop playing.Network congestion and other problems are fairly common, and to helpameliorate the interruption of the data transfer, buffering isimplemented.

Buffering works by storing a portion of the video locally, and thenplaying the video by retrieving data from the local buffer. Before thevideo starts playing, the player downloads some amount of the video and

stores it locally. Generally this is not a large portion of the video, usually10 seconds or so. It then plays the video from this local buffer whilecontinually downloading more of the video to keep the buffer full.If the network becomes congested, or if the transfer is interrupted forsome reason, the player can continue playing from the buffer, andhopefully the interruption will be corrected before the buffer is depletedand the video stops playing.

Buffering can also help with encodingvideos that contain spikes of highbandwidth. This can occur if something inthe video suddenly requires morebandwidth. For example, in a videodemonstrating an application, the onlymovement might be the mouse pointermoving across the screen for manyframes. If the author of the video clicks a

button in the application that causes a new window to open, the entireframe might change, requiring a large block of data needing to betransmitted to update the next frame. This causes a spike in the requiredbandwidth.


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Modern media encoders take into account the extra time afforded by thebuffer. While the extra data caused by the increase in bandwidth is beingdelivered, the video can be played from the buffer, thereby notinterrupting playback. Increasing the amount of buffering time can makethe difference between a failed or a successful encoding process.

understanding bandwidth

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