From: clemon@lemsys.UUCP (Craig Lemon VE3XCL)
Newsgroups: comp.dcom.modems
Subject: Re:  Serial and Parallel interface ?????
Distribution: world
Date: 25 Jul 92 22:57:36 EDT
Organization: Private UUCP "Twig" Node

In article <1992Jul18.135932.3904@qiclab.scn> leonard@qiclab.scn.rain.com writes:
>
>jvmg9796@uxa.cso.uiuc.edu (Doc) writes:
>
>>tnixon@hayes.com writes:
>
>>>Parallel communications sends several bits at a time.  In the PC 

>
>>To take the original question a step further, why cant a modem be
>>designed that uses the parallel port for increased throughput?
>
>Perhaps because the telephone line is *effectively* serial. It's
>a single wire pair. The signal can only have a single value at any point
>in time.

        Well, unless I have misconceptions about TCM (Trellis Coded Mod.)
that is used in virtually all of the high speed standards, there is more
than one bit encoded at any one time.  A document on the ZyXEL 1496S that I
have says it all.  Here is a portion of a speed/compatibility chart seen on
this 'glossy' :

Operation Mode          Bit Rate        Baud Rate       Modulation
---------------         -----------     ------------    --------------
V.33 14400              14400           2400            128-TCM
V.33 12000              12000           2400            64-TCM
V.32bis 14400           14400           2400            128-TCM
V.32bis 12000           12000           2400            64-TCM
V.32bis 7200            7200            2400            16-TCM
V.32 9600               9600            2400            32-TCM
V.32 9600 uncoded       9600            2400            16-QAM
V.32 4800               4800            2400            4-DPSK
V.29 9600               9600            2400            16-QAM
V.29 7200               7200            2400            8-QAM
V.29 4800               4800            2400            4-DPSK
V.27ter 4800            4800            1600            8-PSK
V.27ter 2400            2400            1200            4-DPSK
V.22bis 2400            2400             600            16-QAM
V.22/BELL 212A 1200     1200             600            4-DPSK
V.21/BELL 103 300        300             300            FSK
V.23 1200/75            1200/75         1200/75         FSK

        Although they didn't say so on the sheet, here are the modulation
short form dfinitions :
                                                |   o
                                                |  /
TCM = Trellis Coded Modulation                  | / Phase
QAM = Quadrature Amplitude Modulation           |/______________Amplitude
DPSK = Dual Phase-Shift Keying                  |
PSK = Phase-Shift Keying                        |
FSK = Frequency Shift Keying                    |

        The baud rate does not equal the bit rate, obviously.  The baud
rate indicates the rate at which the modulation updates, or changes.  In
other words, the baud rate indicates how much information is transferred. 
This information is encoded and will be decoded to produce more information
(hence the bit rate that is a multiple of the baud rate).  TCM encoding can
be represented on polar axes with signal amplitude represented linearly and
the phase represented by the angle (see above).  Bits are encoded at
multiple phase-amplitude "grid locations" resembling a grid or matrix where
the locations are optimized to give the greatest distance between bits so
that there is little chance of one bit being confused with another.  As
better algorithms are developed, more bits can be packed into the same
domain.  The difference between some v.fast modulation schemes and
present-day v.32bis is simply the number of bits that they could manage to
encode given the bandwidth of the phone connection.  Calculations give 6
'bits/baud' for v.32bis 14.4k and 12 'bits/baud' for v.fast 28.8k.  This is
of course an incomplete, simplistic view of things.  It is more complicated
in the real world dealing with changing phone lines etc...  I may have some
of the details wrong here but I believe that I'm at least on the right
track :-)

>
>In the old days, some leased line modems increased speed by using
>*two* pairs of wires.

        This is still done today, Telebit Netblazers can be configured this
way.  It's referred to as 4-wire leased line.  The ZyXEL U-1496S also has
some special 4-wire features.

>-- 
>Leonard Erickson                     leonard@qiclab.scn.rain.com
>CIS: [70465,203]                        70465.203@compuserve.com
>FIDO:   1:105/56        Leonard.Erickson@f56.n105.z1.fidonet.org
>(The CIS address is checked daily. The others infrequently)
>

--
 Craig Lemon VE3XCL - Kitchener, Ontario. Amiga B2000  OS 2.04  UUCPv1.15D.
 +1 519 741 0297    +1 519 578 7817              | 1A Electrical Engineering
 clemon@lemsys.UUCP clemon%lemsys@xenitec.on.ca  |  University of Waterloo
 IP/Packet: ve3xcl@ve3xcl.ampr.org [44.135.84.51]|        Fall 1992