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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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data frames carrying a SDU
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These two types of frames can be distinguished by dividing the frame in two parts :
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the `header` that contains one bit set to `0` in data frames and set to `1` in control frames
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the payload that contains the SDU supplied by the application
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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.
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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.
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Services and protocols
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Reliable data transfer on top of an imperfect link
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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 :
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Frames can be corrupted by transmission errors
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Frames can be lost or unexpected frames can appear
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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 :
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random isolated errors where the value of a single bit has been modified due to a transmission error
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random burst errors where the values of `n` consecutive bits have been changed due to transmission errors
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