Exit Zen
161
../../principles/reliability.rst:584
English
The figure below illustrates the operation of the alternating bit protocol.
0/750 · 75
164
../../principles/reliability.rst:675
English
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 !
0/5140 · 514
165
../../principles/reliability.rst:685
English
Go-back-n and selective repeat
0/300 · 30
167
../../principles/reliability.rst:693
English
Pipelining improves the performance of reliable protocols
0/570 · 57
168
../../principles/reliability.rst:695
English
`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.
0/4140 · 414
170
../../principles/reliability.rst:703
English
The sliding window
0/180 · 18
172
../../principles/reliability.rst:712
English
Sliding window example
0/220 · 22
173
../../principles/reliability.rst:715
English
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.
0/5290 · 529
174
../../principles/reliability.rst:722
English
Utilisation of the sliding window with modulo arithmetic
0/560 · 56
175
../../principles/reliability.rst:727
English
Unfortunately, frame losses do not disappear because a reliable protocol uses a sliding window. To recover from losses, a sliding window protocol must define :
0/1590 · 159
176
../../principles/reliability.rst:729
English
a heuristic to detect frame losses
0/340 · 34
177
../../principles/reliability.rst:730
English
a `retransmission strategy` to retransmit the lost frames
0/570 · 57
179
../../principles/reliability.rst:737
English
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`).
0/3850 · 385
183
../../principles/reliability.rst:757
English
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.
0/3540 · 354
184
../../principles/reliability.rst:763
English
Go-back-n : example
0/190 · 19
185
../../principles/reliability.rst:766
English
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 :
0/2450 · 245
186
../../principles/reliability.rst:768
English
the `go-back-n` receiver does not accept out-of-sequence frames
0/630 · 63
187
../../principles/reliability.rst:769
English
the `go-back-n` sender retransmits all unacknowledged frames once it has detected a loss
0/880 · 88
188
../../principles/reliability.rst:776
English
`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.
0/3240 · 324
189
../../principles/reliability.rst:778
English
A `selective repeat` receiver maintains a sliding window of `W` frames and stores in a buffer the out-of-sequence frames that it receives. The figure below shows a five-frame receive window on a receiver that has already received frames `7` and `9`.
0/2490 · 249