TRANSMISSION THROUGHPUT COMPARISONS
-----------------------------------

I recently observed the transmission speed of binary files over normal
dialed-up phone lines and PC Pursuit. This was done for the 128-byte
and the 1-KByte block sizes. The differences are quite startling.

Some Fundamental Assumptions & Precepts:

  a. There is a total of 133 bytes transmitted for each block in the
     128-byte mode:  SOH, block number, 1's complement of the block
     number, 128 data bytes, CRC or checksum, and an ACK or NAK in the
     other direction.

  b. Similarly, in the 1K mode, there's a total of 1024 + 5 = 1029 bytes
     for each block.

  c. Each byte transmitted consists of a basic 8-bit byte plus 1 start
     bit and 1 stop bit, for a total of 10 bits. The start and stop bits
     are stripped off at the receiving terminal.

  d. Net useful information throughput is calculated in terms of bits per
     second on an 8-bit per byte basis, not for the 10-bits per byte
     actually transmitted on the line. The theoretical maximum useful
     information throughput rates for 1200 baud transmission are 924 bps
     for 128-byte protocol and 955 bps for 1K protocol. (These numbers are
     calculated by setting overhead time to zero.)

  e. Records are defined as 128 bytes, in order to maintain equivalency.

The following tabulation shows the results of my observations, based upon
averages derived by stopwatch timing methods. A TRS-80 Model 4 was used,
running at 5.9 MHz clock speed, and equipped with a hard disk. Results will
probably vary slightly for different clock speeds, and somewhat more for
floppy disk drive operation.


          BAUD      BLOCK         Sec per    Records    NET INFO      Percent
 LINE     RATE      SIZE          Record     per Min.   THRUPUT       OH Time
 ----     ----      ----          ------     -------    --------      -------

Dialup    1200B     128-byte       1.207      49.7       848 bps       8.2%

Dialup    1200B     1K             1.083      55.4       946 bps       1.0%

PCP       1200B     128-byte       3.220      18.6       318 bps      65.6%

PCP       1200B     1K             1.600      37.5       640 bps      33.0%

Dialup     300B     128-byte       4.510      13.3       227 bps       1.7%


Note that the use of 1K protocol increases the effective throughput by only
11.5% with standard dialup phone lines. With PC Pursuit, however, throughput
with 128-byte protocol is very low, but can be doubled by using 1K protocol.


                                                Larry Lavins
                                                Philadelphia, PA
                                                April 6, 1987
.