network speeds - kbps explained - how fast is a cable modem?
 

 

 

 

  
Explaining Network Speeds
How often have you wondered what the what kbps really means.  Ever wonder how fast a 10Mbps Ethernet is?  How long SHOULD it take to copy a file from one computer to another over different kinds of networks?  How long should a download take over a modem?  Is a 1mb network like PNA fast enough for your ? In this article I will attempt to clarify modem, boradband, and common networking speeds in a practical easy to follow way. 
Foreward:  To keep this article clear and simple, I have chosen to ignore the 1000/1024 'base 2' issues.
Definitions
BIT: abreviated to a small b (Kbps), unit of measurement in a computer representing a 1 or a 0
BYTE:  abbreviated to a capital B (KBps), another unit of measurement in a computer, 8 bits are in a single byte.  Other lengths are possible like 16, 32, 64, etc but network communications is based on 8 bit lengths.  
Kbps:   kilobits per second, or 1000 bits per second
Mbps:  Megabits per second or 1,000,000 bits per second
Is It Bytes or Bits? The small b in Kbps lets us know the unit of measurement is bits.  A capital B would let us know the measurement is in Bytes.
Even though the metric system uses a small k for kilo, the computer industry has chosen to use a capital K in most cases.
Definition Examples:
56Kbps      56 X 1000 bits per second = 56,000 ones and zero's
128Kbps      128 X 1000 bits per second = 128,000 ones or zero's
460Kbps     460 X 1,000 bits per second = 460,000 ones or zero's
1Mbps     1 X 1,000,000 bits per second = 1 million one's or zero's
10Mbps     10 X 1,000,000 bits per second = 10 million one's or zero's
Use:
So far so goood.  It would appear that you have enough information to start figuring out how long it takes to transfer files but you have to remember that these speeds are in BITS not BYTES.  When we look at a file size we look at the number of BYTES not the number of bits.  Using this information we can build the chart below.
Theoretical Maximum Speeds
Common Name Used With Bits per Second Bytes per Second
28.8Kbps Modems 28,800 3,600
56.6Kbps Modems 56,600 7,200
128Kbps ISDN 128,000 16,000
460Kbps Cable Modem 460,000 57,500
1Mbps PNA 1.0 1,000,000 125,000
1.6Mbps RF 1,600,000 200,000
10Mbps Ethernet 10,000,000 1,250,000
100Mbps Fast Ethernet 100,000,000 12,500,000
 
Aprox Theoretical Transfer Times:
Common Name Used With Time to Transfer 1 Megabyte Time to transfer 50 Megabytes
28.8Kbps Modems 4.5 Min 3.8 Hrs
56.6Kbps Modems 2.3 Min 2 Hrs
128Kbps ISDN 1 Min 50 Min
460Kbps Some Cable Modems 17 Sec 14.5 Min
1Mbps PNA 1.0, Cable Modes 8 Sec 6.5 Min
1.6Mbps RF 5 Sec 4 Min 
10Mbps Ethernet 1 Sec 40 Sec 
100Mbps Fast Ethernet 1 Sec 4 Sec
 
Theoretical vs Real times:
So many factors go into real transfer times that it is very difficult to wrap blanket numbers around them.  Once you get over ISDN speeds, network latency, protocol overhead, driver efficiency, and all kinds of other technical things get in the way.  In general, expect transfer speeds of 40 to 80% of the maximum speed. 
Aprox Real Transfer Times:
Common Name Used With Time to Transfer 1 Megabyte Time to transfer 50 Megabytes
28.8Kbps Modems 6-12 Min 4.8-9.6 Hrs
56.6Kbps Modems 3-6 Min 2.4-4.8 Hrs
128Kbps ISDN 1-3 Min 1-2.1 hrs
460Kbps Some Cable Modems 22-43 Sec 18-36 Min
1Mbps PNA 1.0**, Some Cable Modes 10-20 Sec 8-16  Min
1.6Mbps RF** 6-13 Sec 5-10 Min 
10Mbps Ethernet, PNA 2.0** 1-2 Sec 50-100 Sec
100Mbps Fast Ethernet 1 Sec 5-10 Sec*
 
*PCI Bus inefficiency can easly make this 9-14 seconds
**Often, these systems run at 1/2 speed because of interference

 


 

 

 


 
 
Site last updated: Monday, July 26, 2004 12:12 PM