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Reliable transfer
Open questions
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`)
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
when there are no losses
when every second acknowledgment is discarded due to transmission errors
Same question when using selective repeat instead of go-back-n. Note that the answer is not necessarily the same.
Reliable protocols depend on error detection algorithms to detect transmission errors. The following questions will reinforce your understanding of these algorithms.
Reliable protocols rely on different types of checksums to verify whether frames have been affected by transmission errors. The most frequently used checksums are :