1. The harm
Marking was introduced to suit the sending Host because it permits the text of a message to start on a word boundary, however, it does not suit the receiving Host with a different word length. Moreover,it introduces in the message useless bits. Let us illustrate this by the example of our Sigma 7, a 32 bit machine.
1.1 Inefficiency in Computation
Suppose we receive a message from an 18 bit machine (figure 1.1) coded in 8 bit ASCII characters which will eventually become standard on the network. In order to translate this message into our EBCDIC internal code, for instance.
0 17 0 31
-------------------------- ------------------------------
| leader | | leader |
-------------------------- ------------------------------
| | 0 0 0 1| | 0 0 0 1 | |
-------------------------- ----------- |
| | | |
| | | |
| | | |
| message | | message |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
Figure 1.1
we first have to shift the whole message. We must detect the firsl 1 following the leader, and from this determine that we must shift the message 4 bits to the left. This takes approximately 12 �sec per double word, which makes 1,5 msec per full regular message. This is not huge, but still it is about one-third of the time it will take to translate the message in internal code.
1.2 Inefficiency in transmission
More important is the inefficiency resulting from adding unnecessary bits to the message, especially if it turns out that one character messages are used. Figure 1.2 shows the example of a 1 character text sent by the sigma 7, which results in transmitting 112 bits to carry 8 bits of information, thus leading to an efficiency factor of 0.07. Supression of marking would
-----------------------------------
Sigma 7 | leader |
-----------------------------------
Message |00000000000000000000000000000001 |
-----------------------------------
| text | 000000000000000000000000 |
-----------------------------------
16 bits of padding | 1000000000000000 |
added by sending IMP --------------------
Figure 1.2
increase this efficiency to 0.10. For a 32 bit text (length of some control commands), it would increase the efficiency form 0.28 to 0.4. For one packet messages, the efficiency would still be increased by 3%.