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`NUL`: `0000000` b
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`BEL`: `0000111` b
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`CR` : `0001101` b
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`LF` : `0001010` b
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`DLE`: `0010000` b
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`STX`: `0000010` b
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`ETX`: `0000011` b
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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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**1** **2** **3** **4**
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`DLE STX` **1** **2** **3** **4** `DLE ETX`
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**1** **2** **3** **DLE** **STX** **4**
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`DLE STX` **1** **2** **3** **DLE** `DLE` **STX** `4` `DLE ETX`
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**DLE STX DLE ETX**
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`DLE STX` **DLE** `DLE` **STX** **DLE** `DLE` ETX** `DLE ETX`
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Recovering from transmission errors
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In this section, we develop a reliable datalink protocol running above the physical layer service. To design this protocol, we first assume that the physical layer provides a perfect service. We will then develop solutions to recover from the transmission errors.
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The datalink layer is designed to send and receive frames on behalf of a user. We model these interactions by using the `DATA.req` and `DATA.ind` primitives. However, to simplify the presentation and to avoid confusion between a `DATA.req` primitive issued by the user of the datalink layer entity, and a `DATA.req` issued by the datalink layer entity itself, we will use the following terminology :
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the interactions between the user and the datalink layer entity are represented by using the classical `DATA.req` and the `DATA.ind` primitives
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the interactions between the datalink layer entity and the framing sub-layer are represented by using `send` instead of `DATA.req` and `recvd` instead of `DATA.ind`
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To solve this problem, a reliable protocol must include a feedback mechanism that allows the receiver to inform the sender that it has processed a frame and that another one can be sent. This feedback is required even though there are no transmission errors. To include such a feedback, our reliable protocol must process two types of frames :
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