From: brian@SJF.Novell.COM (Brian Meek)
Newsgroups: comp.sys.novell
Subject: Re: ODI and dialup, PPP, SLIP
Date: 23 Dec 92 01:46:58 GMT
Organization: Novell, Inc.

saint@sun2 (Ming-Jer Lee) writes:
>  I doubt that the speed of ODI for dialup (SLIP or PPP) is very slow.
>Can anyone give some related report?

I suppose the question is "how fast is a network connection over
SLIP or PPP?".

This of course, depends upon many variables...  The primary limitation
is the speed of the modem connection, but other factors such as line 
quality, error correction and data compression (at various levels), and
the type of serial port hardware in your PC all play a role.  

Two V.32bis modems can negotiate a maximum modulation speed of 14400 bits
per second.  This data transmission speed, coupled with V.42 error control 
and V.42bis data compression make it possible to attain data throughput 
speeds of nearly the DTE speed at which your PC serial port communicates 
with your modem.  The following picture might help to illustrate this:

   FTP client                                     FTP Server
   TCPIP.EXE                                      TCPIP.EXE
   LSL.COM                                        LSL.COM
   SLIP_PPP.COM <-VJ TCP Header compression here  SLIP_PPP.COM
   -------------                                  -------------
   Serial Port                                    Serial Port
   Hardware                                       Hardware
   (16550A UART)                                  (16550A UART)
   -------------                                  -------------
        |                                              |
     57600bps                                       57600bps
        |                                              |
   V.32bis/V.42bis                                V.32bis/V.42bis
   Modem <--------14.4Kbps V.42bis compressed data--------> Modem

In the above picture, SLIP_PPP.COM feeds TCP/IP packets in PPP or SLIP
frames to the V.32bis/V.42bis modem at 57600bps.  The modem buffers the
data and applies V.42bis compression (unless the data is already
compressed, in which case V.42bis recognizes this and doesn't try to
compress it any further) and jams said compressed data across the phone
line to the other end at 14400bps.  The inverse happens on the other end.

In real terms?  I can FTP an ASCII file across such a connection and see
file transfer speeds of about 4KB/second on average.  Compressed ZIP
files seem to move at about half that speed (a bit less).  Consider that
14400bps = 1.7578KB/second theoretical maximum throughput of solid data...
the extent to which your network traffic can be compressed is the extent to 
which you'll see better than this.  V.42bis can supposedly obtain 4 to 1 
compression ratios, which pretty well explains 4KB/second FTP speeds of
ASCII data.

Other technologies come in to make the above picture yet more efficient.
Van Jacobson TCP/IP header compression is used to reduce the size of TCP
headers by a factor of 10 -- very significant when using Telnet, less
noticeable when doing FTP transfers.  VJ TCP header compression over a line 
where the entire data stream undergoes V.42bis compression still helps, but 
to a lesser degree than when using lesser modem technology.

If running NetWare IPX connections over the link, I use the latest
BNETX.EXE at home, and PBURST.NLM on my server which helps make the idea 
of reading of data files from NetWare volumes a reasonable thing to do.  
My primary use of this is Trumpet's NEWS.* files which are stored in my
NetWare email directory, so that articles read from home are not reread
at the office (and viz-versa).  

Anyway, I'm a happy camper with an Internet/Novell-Corporate-Net node at 
my home office :-).

-- brian
___________________________________________________________________________
Brian Meek                            Novell, Inc. -- Desktop Systems Group
                                      Internet mail:  brian@Novell.COM