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In practice, as the frame header includes an `n` bits field to encode the sequence number, only the sequence numbers between :math:`0` and :math:`2^{n}-1` can be used. This implies that, during a long transfer, the same sequence number will be used for different frames and the sliding window will wrap. This is illustrated in the figure below assuming that `2` bits are used to encode the sequence number in the frame header. Note that upon reception of `OK1`, the sender slides its window and can use sequence number `0` again.
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Dealing with corrupted frames
 
The receiver FSM of the Alternating bit protocol discards all frames that contain an invalid CRC. This is the safest approach since the received frame can be completely different from the frame sent by the remote host. A receiver should not attempt at extracting information from a corrupted frame because it cannot know which portion of the frame has been affected by the error.
 
The figure below illustrates the operation of the alternating bit protocol.
 
The Alternating Bit Protocol can recover from transmission errors and frame losses. However, it has one important drawback. Consider two hosts that are directly connected by a 50 Kbits/sec satellite link that has a 250 milliseconds propagation delay. If these hosts send 1000 bits frames, then the maximum throughput that can be achieved by the alternating bit protocol is one frame every :math:`20+250+250=520` milliseconds if we ignore the transmission time of the acknowledgment. This is less than 2 Kbits/sec !
 
Go-back-n and selective repeat
 
Pipelining improves the performance of reliable protocols
 
`Pipelining` allows the sender to transmit frames at a higher rate. However this higher transmission rate may overload the receiver. In this case, the frames sent by the sender will not be correctly received by their final destination. The reliable protocols that rely on pipelining allow the sender to transmit `W` unacknowledged frames before being forced to wait for an acknowledgment from the receiving entity.
 
The sliding window
 
Sliding window example
 
In practice, as the frame header includes an `n` bits field to encode the sequence number, only the sequence numbers between :math:`0` and :math:`2^{n}-1` can be used. This implies that, during a long transfer, the same sequence number will be used for different frames and the sliding window will wrap. This is illustrated in the figure below assuming that `2` bits are used to encode the sequence number in the frame header. Note that upon reception of `OK1`, the sender slides its window and can use sequence number `0` again.
 
Utilisation of the sliding window with modulo arithmetic
 
Unfortunately, frame losses do not disappear because a reliable protocol uses a sliding window. To recover from losses, a sliding window protocol must define :
 
a heuristic to detect frame losses
 
a `retransmission strategy` to retransmit the lost frames
 
The figure below shows the FSM of a simple `go-back-n` receiver. This receiver uses two variables : `lastack` and `next`. `next` is the next expected sequence number and `lastack` the sequence number of the last data frame that has been acknowledged. The receiver only accepts the frame that are received in sequence. `maxseq` is the number of different sequence numbers (:math:`2^n`).
 
The operation of `go-back-n` is illustrated in the figure below. In this figure, note that upon reception of the out-of-sequence frame `D(2,c)`, the receiver returns a cumulative acknowledgment `C(OK,0)` that acknowledges all the frames that have been received in sequence. The lost frame is retransmitted upon the expiration of the retransmission timer.
 
Go-back-n : example
 
The main advantage of `go-back-n` is that it can be easily implemented, and it can also provide good performance when only a few frames are lost. However, when there are many losses, the performance of `go-back-n` quickly drops for two reasons :
 
the `go-back-n` receiver does not accept out-of-sequence frames
 
the `go-back-n` sender retransmits all unacknowledged frames once it has detected a loss
 
`Selective repeat` is a better strategy to recover from losses. Intuitively, `selective repeat` allows the receiver to accept out-of-sequence frames. Furthermore, when a `selective repeat` sender detects losses, it only retransmits the frames that have been lost and not the frames that have already been correctly received.
 

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In practice, as the frame header includes an `n` bits field to encode the sequence number, only the sequence numbers between :math:`0` and :math:`2^{n}-1` can be used. This implies that, during a long transfer, the same sequence number will be used for different frames and the sliding window will wrap. This is illustrated in the figure below assuming that `2` bits are used to encode the sequence number in the frame header. Note that upon reception of `OK1`, the sender slides its window and can use sequence number `0` again.
6 years ago
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18098 String information

Source string location
../../principles/reliability.rst:715
String age
6 years ago
Last updated
a month ago
Source string age
6 years ago
Translation file
locale/cs/LC_MESSAGES/principles/reliability.po, string 173