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Reliable transfer
Open questions
Consider a `b` bits per second link between two hosts that has a propagation delay of `t` seconds. Derive a formula that computes the time elapsed between the transmission of the first bit of a `d` bytes frame from a sending host and the reception of the last bit of this frame on the receiving host.
Transmission links have sometimes different upstream and downstream bandwidths. A typical example are access networks that use ADSL (Asymmetric Digital Subscriber Lines). Consider two hosts connected via an ADSL link having an upstream bandwidth of 1 Mbps and a downstream bandwidth of 50 Mbps. The propagation delay between the two hosts is 10 milliseconds. What is the maximum throughput, expressed in frames/second, that the alternating bit protocol can obtain on this link if each data frame has a length of 125 bytes and acknowledgments are 25 bytes long. Same question if the protocol is modified to support 1500 bytes long data frames.
How would you set the duration of the retransmission timer in the alternating bit protocol ?
A version of the Alternating Bit Protocol supporting variable length frames uses a header that contains the following fields :
a `number` (0 or 1)
a `length` field that indicates the length of the data
a Cyclic Redundancy Check (`CRC`)
To speedup the transmission of the frames, a student proposes to compute the CRC over the data part of the frame but not over the header. What do you think of this proposed solution ?
Derive a mathematical expression that provides the `goodput`, i.e. the amount of payload bytes that have been transmitted during a period of time, achieved by the Alternating Bit Protocol assuming that :
Each frame contains `D` bytes of data and `c` bytes of control information
Each acknowledgment contains `c` bytes of control information
The bandwidth of the two directions of the link is set to `B` bits per second
The delay between the two hosts is `s` seconds in both directions
there are no transmission errors


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4 years ago
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locale/pot/exercises/reliability.pot, string 1