Text 27999, 106 rader
Skriven 2007-02-06 19:51:11 av mark lewis (1:3634/12.0)
  Kommentar till text 26525 av JEAN PARROT (1:123/140)
Ärende: Rates.
==============
 TW> Actualky internet speeds have so many factors involved it is almost
 TW> imposable to make a Flat Statement about them.

 JP>         I used that URL, Jay, to see the rate here and I kept
 JP> getting    about the same results as before, D/L at 2820, U/L at
 JP> 750. They    also show the rates in Bytes as 490 and 75, I see that
 JP> they show    bytes as an 8-bit block, no start nor stop bit.
 JP> Roughly ! 

this is because things are different in the networking world than they are in
the serial modem world... for simplicity, i'll use the following chart with a
note that the IP and TCP headers can contain additional bytes signifying
additional options and their settings... for now, this will work for the sake
of discussion...


    |        <-8 bits->       |        <-8 bits->       |
    +-------------------------+-------------------------+
    | IP version | IP length  |    IP service type      |  2 bytes
    +-------------------------+-------------------------+
    |                   Packet Length                   |  2 bytes
    +-------------------------+-------------------------+
 I  |                     Packet ID                     |  2 bytes
 P  +-------------------------+-------------------------+
    |0|DF|MF|            Fragment Offset                |  2 bytes
 H  +-------------------------+-------------------------+
 E  |      Time To Live       |         Protocol        |  2 bytes
 A  +-------------------------+-------------------------+
 D  |                  Header Checksum                  |  2 bytes
 E  +-------------------------+-------------------------+
 R  |                       IPv4                        |  4 bytes
    |                  Source Address                   |
    +-------------------------+-------------------------+
    |                       IPv4                        |  4 bytes
    |               Destination Address                 |
    +-------------------------+-------------------------+
    |  IP Options - 0 to 40 bytes - generally not used  |  0 bytes
====+=========================+=========================+==========
    |                   TCP Src Port #                  |  2 bytes
    +-------------------------+-------------------------+
    |                   TCP Dest Port #                 |  2 bytes
    +-------------------------+-------------------------+
 T  |                        TCP                        |  4 bytes
 C  |                     Sequence #                    |
 P  +-------------------------+-------------------------+
    |                        TCP                        |  4 bytes
 H  |                  Acknowledgement #                |
 E  +-------------------------+-------------------------+
 A  |*data offset|         reserved         |u|a|p|r|s|f|  2 bytes
 D  +-------------------------+-------------------------+
 E  |          TCP Receiver window size (RWIN)          |  2 bytes
 R  +-------------------------+-------------------------+
    |               TCP Message checksum                |  2 bytes
    +-------------------------+-------------------------+
    |         TCP Urgent Message-byte Position          |  2 bytes
    +-------------------------+-------------------------+
    | TCP Options - 0 to 40 bytes - generally not used  |  0 bytes
    +=========================+=========================+
    |                        User                       |  65535 bytes
    |                        Data                       |  minus all bytes
    |                        Here                       |  in above
    +-------------------------+-------------------------+

[* The header may have a variable size, so the data offset field points 
   to the beginning of the user data. To save header space, this field 
   acts like the IP header's "packet length" field: It gives the count 
   of 32-bit words that physically precede your data.]


so, if we take the normal TCP/IP MTU (Maximum Transmission Unit) size of 1500
and knock off 40 bytes for the IP and TCP headers and you can see that leaves
us with 1460 bytes for the data...

PPPoE users can only have a MTU of 1492 due to the additional 8 byte header
"wrapped" around the above... so that knocks another 8 bytes off of the 1460
leaving 1452 bytes for the data...


one might note that there is no indicator in the TCP header for the length of
the user data portion... that is taken care of by the IP header's Packet Length
field ;)

one might also note that the entire packet (all headers and data) can be a max
of 65535 bytes but in our discussion, we only use 1500 bytes per packet.
anything larger will be broken up into smaller packets (fragmented) for easier
control and processing.


To find your theoretical KByte download speed for your line, divide your
advertised speed by 8 (bits):           1500kbps /  8 = 187.5 KB/s

or by 10 (bits) to include overhead:    1500kbps / 10 = 150 KB/s

the above formula doesn't take off the header bytes to find the true(r)
transfer rate of the _data_ which is what /we/ are interested in.


much of the above info has been consolidated from experiance, dslreports FAQ,
and SAMS reference library...

hope this helps! O:)

)\/(ark


 * Origin:  (1:3634/12)