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Frames can be corrupted by transmission errors
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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 :
data frames carrying a SDU
These two types of frames can be distinguished by dividing the frame in two parts :
the `header` that contains one bit set to `0` in data frames and set to `1` in control frames
the payload that contains the SDU supplied by the application
The datalink entity can then be modeled as a finite state machine, containing two states for the receiver and two states for the sender. The figure below provides a graphical representation of this state machine with the sender above and the receiver below.
The above FSM shows that the sender has to wait for an acknowledgment from the receiver before being able to transmit the next SDU. The figure below illustrates the exchange of a few frames between two hosts.
Services and protocols
Reliable data transfer on top of an imperfect link
The datalink layer must deal with the transmission errors. In practice, we mainly have to deal with two types of errors in the datalink layer :
Frames can be corrupted by transmission errors
Frames can be lost or unexpected frames can appear
To deal with these types of imperfections, reliable protocols rely on different types of mechanisms. The first problem is transmission errors. Data transmission on a physical link can be affected by the following errors :
random isolated errors where the value of a single bit has been modified due to a transmission error
random burst errors where the values of `n` consecutive bits have been changed due to transmission errors
random bit creations and random bit removals where bits have been added or removed due to transmission errors
The only solution to protect against transmission errors is to add redundancy to the frames that are sent. `Information Theory` defines two mechanisms that can be used to transmit information over a transmission channel affected by random errors. These two mechanisms add redundancy to the transmitted information, to allow the receiver to detect or sometimes even correct transmission errors. A detailed discussion of these mechanisms is outside the scope of this chapter, but it is useful to consider a simple mechanism to understand its operation and its limitations.
`Information theory` defines `coding schemes`. There are different types of coding schemes, but let us focus on coding schemes that operate on binary strings. A coding scheme is a function that maps information encoded as a string of `m` bits into a string of `n` bits. The simplest coding scheme is the (even) parity coding. This coding scheme takes an `m` bits source string and produces an `m+1` bits coded string where the first `m` bits of the coded string are the bits of the source string and the last bit of the coded string is chosen such that the coded string will always contain an even number of bits set to `1`. For example :
`1001` is encoded as `10010`
`1101` is encoded as `11011`
This parity scheme has been used in some RAMs as well as to encode characters sent over a serial line. It is easy to show that this coding scheme allows the receiver to detect a single transmission error, but it cannot correct it. However, if two or more bits are in error, the receiver may not always be able to detect the error.
Some coding schemes allow the receiver to correct some transmission errors. For example, consider the coding scheme that encodes each source bit as follows :
`1` is encoded as `111`
`0` is encoded as `000`
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.

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Source string location
../../principles/reliability.rst:393
String age
4 years ago
Source string age
4 years ago
Translation file
locale/fr/LC_MESSAGES/principles/reliability.po, string 107