Short answer: it's a billion bits. Thank you. Y'all hurry back.
Honestly, when most people encounter a gigabit it's in the description of their network or internet speed. Usually written Gbps, it means one billion bits per second, which is equivalent to 1000 Mbps or 1000 million (or Mega) bits per second.
The "per second" identifies the distinction between the quantity of information and the rate at which it can be transmitted. Transmission rate is usually expressed in bits while quantity is counted as bytes. Although a byte (the common unit of file size) is equal to 8 bits, communication overhead means the byte rate is approximately 1/10 the bit rate. Thus, to a first approximation, 1 Gbps might move data at about 100 MBps (note that bits are abbreviated with a lower case “b” while bytes are a capital “B”).
So, how fast is a Gbps?
Internet Service Providers often describe their speed in terms like “download a movie (or song) in x number of seconds.” Except that most people don’t care too much how long it takes to download a file. They want the movie they’re watching right now to play smoothly. With streaming media, they download a movie over two hours. One second of HD streaming video might require 4-5 megabits of data (cutting edge “4K UHD” video might be 25 megabits). Even doubling the numbers for a 100% reserve, that’s a long way from 1000 Mbps.
“Your mileage may vary…”
Despite the sales pitch, a promotion for 1 Gbps might not give 1 Gbps. Just about every internet package is described as “Up to xxx Mbps.” This implies they’re not actually guaranteeing any particular minimum speed. There are many factors more or less beyond the ISP’s control that affect the actual internet speed delivered to an end device.
Alert: tech talk coming up. You may want to jump to “What else is on the line?”
Obviously, the ISP has no control over the connections or devices inside the house. Until recently most computers shipped with wired network cards that wouldn’t communicate faster than 100 Mbps. While the computers’ connections got faster, upstream devices such as home routers may not have been upgraded. Some older routers were limited to only 10 Mbps input from the internet so internal computers could communicate much faster between themselves than with the internet. Ramping up speed with WiFi technology has come later and at much higher cost than for wired connections. Also, most WiFi specifications share a single total capacity with all devices connected to it so any one device’s speed is limited by what others may be doing, even if they're not using the internet.
Before it gets to the subscriber’s equipment, the ISP’s local modem may not support their maximum speed. Outside the house, there are mitigating factors for some neighborhood transmission technologies. On some systems all subscribers may share a single cable from a local hub. While the cable may be capable of more than the advertised speed, it may not be so if everyone is using it at the same time. Other systems may give a single wire to a single subscriber, but the potential speed falls off with distance from the hub. And at every hub from the user to the ISP’s connection to the internet more users are vying for a finite amount of capacity.
Leaving the ISP does not mean clear sailing for maximum speed. The web page still has to go through up to a couple dozen routers; any of which could be technologically limited, failing, or overloaded and slowing the connection. At the ultimate server the same failings on that end could slow down communications.
Another fly in the mud is the complexity of what’s in the content. With the proliferation of rich web page advertising some pages may contain content from scores of servers all over the world. This material has to be requested individually by the computer, each one going through its own gauntlet mentioned above. Some pages downloaded over excellent connections can take up to 45 seconds to complete even before the streaming services that are the goal of the connection start.
What else is on the line?
If the carrier really delivers a consistent 1 Gbps to the doorstep and all the subscriber equipment is up to the speed, there’s more overhead that could nibble away at the best rate. Services such as telephone or security systems may be constantly consuming capacity. Computers and other devices (is the refrigerator talking to the supermarket yet?) may not be friendly about when they request their updates (corporate internet services have been slowed to a crawl on days that smart phones got an update – without anyone requesting or expecting it). Online data backup and synchronization services need to move large amounts of data and they don’t want to wait until overnight in case it’s needed before then.
But the real bandwidth hogs are the subscription services many assume are separate from the internet connection. Streaming media – audio, and especially, video delivered on demand – consume capacity immediately and continuously. And, the higher quality delivered, the more capacity needed.
Television, even traditional television channels, is decreasingly being viewed via broadcast – one signal delivered to every viewer. Instead, the “tuner” is located at the provider’s offices and each subscriber receives a dedicated stream of the program; even if everyone on the block is watching the same football game. “Digital recording” works the same way with all the recordings and stop points stored in a database and generated from a central server on demand.
How much is enough?
Visualize 3 televisions or computers in the house in use at once. Add another stream for each simultaneous channel a “traditional,” on premise DVR is recording. That’s 5 Mbps each. Audio streaming is about one-half Mbps per channel. Online gaming is indeterminate, but allow 1 Mbps because it demands immediate response. Add up to a couple more Mbps for incidental services, mail, and web surfing as these demands are typically intermittent.
Total everything up and double it for reserve and future demands to get a conservative number of need. Now match need to affordability. Currently, depending on package and promotion, 50 Mbps may cost $35-$120. If that cost is too high, most streaming services will automatically adjust their quality to the available bandwidth. Most people probably won’t notice the first couple steps back from ultimate quality in most cases. Also, ask the provider if the extra cost TV package consumes bandwidth already billed for. Then ask them again and write down their name.
If the cost for 50 Mbps is not excessive, then consider the upgrade. In some areas 1 Gbps is only $20 more than 50 Mbps. In most areas, 1 Gbps may be promoted but is actually pie-in-the-sky.
And still more gotchas.
While packages may be sold as 50 Mbps or 1 Gbps, these are download speeds. Most residential plans only offer 1 Mbps upload. Web surfing and media streaming have minimal and intermittent upload demands so 1 Mbps is sufficient. Online backups, synchronization, and media sharing may take longer to complete, but are rarely timely. However, more consumers are using two-way audio and video communications which may quickly saturate this capacity; especially if they originate a conference call. Unfortunately, greater upload speeds are mostly available only with business class packages, which are often much more expensive.
All of these discussions have only been concerned with rate of connection with no mention of total quantity of data moved. While most US wired ISPs have not (yet) started metering quantity, most cellular plans do. Cellular may offer up to 25 Mbps, a continuous download at that rate will burn through a 2 GB (gigabyte) plan in 10 minutes. A standard DVD movie (not Blu-ray) runs about 4 GB or an hour of good quality audio is 30 MB.
References:
Mega - Giga, etc.: https://en.wikipedia.org/wiki/Gigabit
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