Correction to MNP and PEP information (was Re: Why 3 and 7?)

Greg Andrews gandrews at netcom.COM
Sun Jun 9 16:01:52 AEST 1991

In article <2130 at> clewis at (Chris Lewis) writes:
>The main reason for this is that MNP is not full-duplex, and it takes time
>for the link to "turn around".  You see this even when terminal connected
>via your modem.  On slower modems and trailblazer PEP the link is full-duplex,
>so the ACKs can come thru simultaneous with transmissions of packets.

Uh, Chris, I'm sorry, but that's not the case.  The MNP error correction 
protocol is NOT half duplex.  It can handle data packets moving in both 
directions at the same time.  Acknowlegements for data packets can be carried 
in the headers of data packets, so you're not forced into only data packets in 
one direction and only acks in the other.  Data and acks can flow in both 
directions at the same time, provided the modem's modulation can do that also.

PEP is exactly the opposite.  PEP is a combined modulation and error correction
protocol.  The modulation is half duplex, and the error correction protocol
is structured to match the modulation.  PEP is half duplex.

PEP gets faster throughput with UUCP, Kermit, and X/Ymodem transfers because
of the special spoofing feature.  Spoofing converts the transfer protocol's
activity through the phone line from full duplex (data flowing one way and 
acks flowing the other way) into half duplex activity through the phone line
(just data flowing one way).

Your statement about slower modems being full duplex is correct, assuming you
mean modems that use the standard modulations:  V.32, V.22bis V.22/Bell 212,
and V.21/Bell 103.

>Chris Lewis, Phone: (613) 832-0541, Domain: clewis at

 |  Greg Andrews   |       UUCP: {apple,amdahl,claris}!netcom!gandrews    |
 |                 |   Internet: gandrews at netcom.COM                      |

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