Exit Zen
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../../principles/reliability.rst:231
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`Bit stuffing` increases the number of bits required to transmit each frame. The worst case for bit stuffing is of course a long sequence of bits set to `1` inside the frame. If transmission errors occur, stuffed bits or markers can be in error. In these cases, the frame affected by the error and possibly the next frame will not be correctly decoded by the receiver, but it will be able to resynchronize itself at the next valid marker.
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../../principles/reliability.rst:236
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`Bit stuffing` can be easily implemented in hardware. However, implementing it in software is difficult given the complexity of performing bit manipulations in software. Software implementations prefer to process characters than bits, software-based datalink layers usually use `character stuffing`. This technique operates on frames that contain an integer number of characters. In computer networks, characters are usually encoded by relying on the :term:`ASCII` table. This table defines the encoding of various alphanumeric characters as a sequence of bits. :rfc:`20` provides the ASCII table that is used by many protocols on the Internet. For example, the table defines the following binary representations :
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../../principles/reliability.rst:238
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`A` : `1000011` b
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../../principles/reliability.rst:239
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`0` : `0110000` b
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../../principles/reliability.rst:240
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`z` : `1111010` b
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../../principles/reliability.rst:241
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`@` : `1000000` b
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../../principles/reliability.rst:242
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`space` : `0100000` b
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../../principles/reliability.rst:244
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In addition, the :term:`ASCII` table also defines several non-printable or control characters. These characters were designed to allow an application to control a printer or a terminal. These control characters include `CR` and `LF`, that are used to terminate a line, and the `BEL` character which causes the terminal to emit a sound.
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../../principles/reliability.rst:246
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`NUL`: `0000000` b
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../../principles/reliability.rst:247
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`BEL`: `0000111` b
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../../principles/reliability.rst:248
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`CR` : `0001101` b
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../../principles/reliability.rst:249
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`LF` : `0001010` b
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../../principles/reliability.rst:250
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`DLE`: `0010000` b
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../../principles/reliability.rst:251
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`STX`: `0000010` b
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../../principles/reliability.rst:252
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`ETX`: `0000011` b
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../../principles/reliability.rst:254
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Some characters are used as markers to delineate the frame boundaries. Many `character stuffing` techniques use the `DLE`, `STX` and `ETX` characters of the ASCII character set. `DLE STX` (resp. `DLE ETX`) is used to mark the beginning (end) of a frame. When transmitting a frame, the sender adds a `DLE` character after each transmitted `DLE` character. This ensures that none of the markers can appear inside the transmitted frame. The receiver detects the frame boundaries and removes the second `DLE` when it receives two consecutive `DLE` characters. For example, to transmit frame `1 2 3 DLE STX 4`, a sender will first send `DLE STX` as a marker, followed by `1 2 3 DLE`. Then, the sender transmits an additional `DLE` character followed by `STX 4` and the `DLE ETX` marker.
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../../principles/reliability.rst:260
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**1** **2** **3** **4**
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../../principles/reliability.rst:260
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`DLE STX` **1** **2** **3** **4** `DLE ETX`
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../../principles/reliability.rst:261
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**1** **2** **3** **DLE** **STX** **4**
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../../principles/reliability.rst:261
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`DLE STX` **1** **2** **3** **DLE** `DLE` **STX** `4` `DLE ETX`
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