Speed

Speed in data communications can be a quite complex and confusing issue.

• Introduction
• Bits, Bytes, and Speeds
• Access Speed, Capacity, 'Gateway Speed'
• 'Application Speed' v. 'Physical Speed'

Introduction

From in which units we're talking - (e.g.) bits per second, bytes per second - including too often different, simply sloppy, use, intention, and intepretation of symbols such as 'b', 'B', 'k', 'K', 'M', and so on.

Example:

	Max speed (bits per second, bps)
1. Your Internet Service Provider (ISP) offers 'Gigabit service'	1,000,000,000 bps	raw capacity
2. You're using an iPhone 5s, w/ 802.11a/b/g/n Wi-Fi, max 300 Gbps	300,000,000 bps	raw capacity
3. You're checking speed using tool in a web browser	'235 Mbps DL' 235,000,000 bps or, maybe: '10.2 Mbps DL' 10,200,000 bps	as presented

Also, in between your computer, the local network, and your ISPs network, there is the whole rest of the Internet, and how the remote (e.g. testing) server you're connecting to is connected to the Internet, how fast this system is, how many others are using it at the same time, and more. I.e., there are many, many, factors impacting the speed you experience.

Explanation to the vastly different speeds in example above: any Wi-Fi device can be quite sensitive to a number of factors, like

• Which standard actually used (802.11a, 802.11b, 802.11g, or 802.11n in case of example) - which depend on what network - local area network - components you have in your home or office.
• How these are configured. Worst case scenario is when configuration actually forces all devices ending up using an older standard with low speed.
• Interference of all sorts of devices in the environment (microwave ovens, cables, speakers, ... - see e.g. Potential sources of Wi-Fi and Bluetooth interference - Apple HT201542 for much more on this topic)
• Distance, number of and types of physical barriers like walls etc, between devices in you network.

No Chain is Stronger Faster than it's Weakest Slowest Link.

Bits, Bytes, and Speeds

(More also in Bits, Bytes, and Prefixes, in Data > Numeric.)

Units - Bits ('b') and Bytes ('B')

Name	Symbol	What
bit	b	Unit, data, one bit is either a zero or a one - lowest component in computers
byte	B	Unit, 8 bits forms a byte. Always.

Unfortunately, people are NOT as stringent using 'b' for bits and 'B' for bytes and sometimes this is also mixed up - be careful.

Prefixes - kilo ('k'), mega ('M'), ...

 

To be very clear, as abbreviations in this field can be very confusing, absolutely also for professionals who work with this daily and are not clear on what they absolutely mean (especially kilo, mega, etc when talking bytes).

For instance, in general (the whole wide world, based on the SI system):

• Base 10, Decimal system
  • a 'k' means a 'kilo' and a factor of 1,000 (10³),
  • a 'M' means a 'mega' and a factor of 1,000,000 (10⁶), and so on.

But, in the digital world it gets a bit messier, including on which datacom layer someone is talking...

• In some cases, prefixes are used as per above (the international SI standard) but in other cases:
• Base 2, Binary system:
• a 'k' is still interpreted as 'kilo' but using a factor of 1,024 (2¹⁰),
• a 'M' is still interpreted as 'mega' but using a factor of 1,048,576 (2²⁰), and so on.
• When discussion transmission speeds on the lowest level, Physical, people almost always think and mean using SI (base 10), a kilo is 1000.
• When discussion transfer speeds on highest level, Application, it very much depend on who's talking, what they are looking at. Some examples:
  • Someone talks about transfering content of a CD, then they are implying Base 2, 700 MB means 700x1024² bytes.
  • Someone talks about transfering content of a DVD then they are more likely implying Base 10, 4.7 GB means 4.7 x 1000³ bytes.

Access Speed, Capacity, 'Gateway Speed'

What your network service provider offers and what is potentially available for real use.

		1. Internet service
Ex.		Some 'Giga-service'
		Physical connection to service: optical fiber,
NB/WAN	Theory	1 Gbps, 1,000,000,000 bits per scond in from service provider
SB/LAN		Wired	Wi-Fi
		1000-BaseT (1999)	802.11g (Jun 2003)	802.11n (Oct 2009)		802.11ac (Dec 2013)
	Freq.	(n/a)	2.4 GHz	2.4 GHz	5 GHz	5 GHz
	Theory	max 1 Gbps	max 54 Mbps	max 300 Mbps		max 1 Gbps
Use		In reality, the most likely way of actually seeing any 'Giga speed'.	The 2.4 GHz band is quite busy with lots of different technologies that may interfere and it's LESS likely to reach higher speeds, at any distances.		The 5 GHz band is far less busy and it's MORE likely to reach higher speeds, at least at shorter distances.

Legend:

• NB/WAN: North-bound / Wide Area Network - side;
  • Towards the Internet, "Internet Providers" like AT&T, Verizon, T-Mobile, Orange, Telefonica...
  • This is the speed your internet provider talks about, and what you pay for, like 6 Mbps, 24 Mbps, 100 Mbps, 1 Gbps ('Giga'...)
• SB/LAN: South-bound / Local Area Network - side;
  • Your own local equipment/networking in house or office.
  • Typically different technologies are avaialable depending on the 'modem' (= your Internet gateway) your service provider is offering but typically include
    • one or more wired Ethernet port(s), and
    • one or more wireless Wi-Fi radios handling different generations/standards of 802.11*.

That is:

• Access Speed, ('Gateway Speed') - no official and specified definition could be interpreted
  • as the [highest possible or currently available or experienced] speed of the WAN side,
  • or the [highest possible or currently available or experienced] speed you have when you access the Internet from some particular device.
• Capacity could mean both the access speed above as is (speed alone) and/or including how many megabytes or gigabytes per month you have as part of purchased package. I.e. both
  • A speed metric (e.g. 6 Mbps - megabits per second), and/or
  • A volume metric (e.g. 200 MB - megabytes - per month, 1 TB - terabyte - per month)

'Application Speed' v. 'Physical Speed'

Data communications is a complex matter and using the OSI 7 layer model highlights layers and functions involved:

OSI Model
Layer		Data unit	Who uses, talks about speed at which level
Host layers	7. Application	'Data', in bytes or other unit	End-users, real use like downloading files, loading web pages, streaming audio, music, video
	6. Presentation		(Techies)
	5. Session		(Techies)
	4. Transport	Segment (TCP) / Datagram (UDP)	(Techies)
Media layers	3. Network	Packet	(Techies)
	2. Data link	Frame	(Techies)
	1. Physical	Bit	Internet service providers, and often theoretical max speed possible by a specific technology.

That is, from the lowest layer - 1. Physical - to highest - 7. Application - many things happens, and each layer adds some small overhead, for packaging data, handling resending of lost packages, etc down to physically moving bits as (e.g.) electrical voltages or electromagnetic waves over some physical medium.

Different technologies at different layers have different features - and more or less overhead - but for general considerations regarding downloading files etc over TCP/IP-Internet networks, best case is about 90% efficiency - i.e. of potential physical speed at layer 1, max 90% is available at the application level.

Examples:

	max practical
7. Application	5.4 Mbps	21.6 Mbps	90 Mbps	270 Mbps	0.9 Gbps
6. Presentation
5. Session
4. Transport
3. Network
2. Data link
1. Physical	6 Mbps	24 Mbps	100 Mbps	300 Mbps	1 Gbps
	max theoretical, raw speed