Q. Dear Mr. Know-It-All,
When the technical people at work talk about the Internet, they sometimes use
the terms T1, T3, SLIP and PPP.
What are these things?
A.
T1 and T3 are kinds of telephone service.
A T1 line runs at 1.544 megabits per second (Mbps), and a T3 runs at 45
Mbps.
For comparison, a “56K” modem runs at 0.056 Mbps (actually, it runs
slower than that during typical usage), a “cable modem” runs at
somewhere around 0.5 Mbps (this is a “ballpark” figure since
there’s lots of different cable modems and their speeds are all
different), and an Ethernet network connection runs at 10 Mbps (although the
actual data throughput may be lower).
So, a T1 is a lot faster than your “56K” modem, and a lot slower
than your office LAN connection.
(There are also T2 and T4 lines which run at 6.312 Mbps and 274.176 Mbps,
respectively.)
The last time I priced a local T1 line, it was about $1,500 a month.
SLIP and PPP are methods of sending data over (typically) a “plain old
voice grade” telephone line to your Internet Service Provider
(ISP).
SLIP (Serial Line Internet Protocol) is pretty stupid; it just breaks the data
into blocks called “frames”, sticks an “END” byte on the
end of each frame, and sends the data out over the wires.
At the other end, the “END” byte is removed and the frames are stuck
back together to recreate the file being sent.
(The “END” byte has the value 192 decimal, which is C0 hexadecimal
or 300 octal.
If any bytes with that value are in the actual data being sent, they’re
converted into a special “ESCAPE” two-byte sequence so the
receiving computer doesn’t think the data has ended, and the two-byte
“ESCAPE” sequence is converted back to the proper value once
it’s in the other computer.)
PPP is Point-to-Point Protocol, and it’s as smart as SLIP is
stupid.
The computers at both ends of a PPP connection talk to each other before any
data is sent and decide on what (if any) special transmission options they
want to use.
In addition, PPP connections talk to each other while data is being
transferred between them.
SLIP and PPP both send data at about the same speed (PPP has slightly more
overhead), but PPP is more robust and can help find transmission problems
while SLIP can’t.
SLIP wasn’t originally an Internet standard and still isn’t, but it
is now documented (see ftp://ds.internic.net/rfc/rfc1055.txt).
PPP has always been an Internet standard (see
ftp://ds.internic.net/rfc/rfc1661.txt).
Curious or in doubt, ask Mr. Know-It-All.
He gets email at MrKIA@SCOUG.COM.
Mr. Know-It-All lives in Southern California.
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