|
data frames carrying a SDU
|
|
|
Dealing with corrupted frames
|
|
|
Decoded bit
|
|
|
`DLE`: `0010000` b
|
|
|
`DLE STX` **1** **2** **3** **4** `DLE ETX`
|
|
|
`DLE STX` **1** **2** **3** **DLE** `DLE` **STX** `4` `DLE ETX`
|
|
|
`DLE STX` **DLE** `DLE` **STX** **DLE** `DLE` ETX** `DLE ETX`
|
|
|
**DLE STX DLE ETX**
|
|
|
`ETX`: `0000011` b
|
|
|
Even parity
|
|
|
For example, consider a sender that sends `111`. If there is one bit in error, the receiver could receive `011` or `101` or `110`. In these three cases, the receiver will decode the received bit pattern as a `1` since it contains a majority of bits set to `1`. If there are two bits in error, the receiver will not be able anymore to recover from the transmission error.
|
|
|
For example, consider the transmission of `0110111111111111111110010`. The sender will first send the `01111110` marker followed by `011011111`. After these five consecutive bits set to `1`, it inserts a bit set to `0` followed by `11111`. A new `0` is inserted, followed by `11111`. A new `0` is inserted followed by the end of the frame `110010` and the `01111110` marker.
|
|
|
Frames can be corrupted by transmission errors
|
|
|
Frames can be lost or unexpected frames can appear
|
|
|
Framing
|
|
|
Given that multi-symbol encodings cannot be used by all physical layers, a generic solution which can be used with any physical layer that is able to transmit and receive only bits `0` and `1` is required. This generic solution is called `stuffing` and two variants exist : `bit stuffing` and `character stuffing`. To enable a receiver to easily delineate the frame boundaries, these two techniques reserve special bit strings as frame boundary markers and encode the frames so that these special bit strings do not appear inside the frames.
|
|
|
Go-back-n and selective repeat
|
|
|
Go-back-n : example
|
|
|
If the physical layer were perfect, the problem would be very simple. We would simply need to define how to encode each frame as a sequence of consecutive bits. The receiver would then easily be able to extract the frames from the received bits. Unfortunately, the imperfections of the physical layer make this framing problem slightly more complex. Several solutions have been proposed and are used in practice in different network technologies.
|
|
|
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.
|
|
