en	Reliable transfer

en	Open questions

en	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.

en	How would you set the duration of the retransmission timer in the alternating bit protocol ?

en	A version of the Alternating Bit Protocol supporting variable length frames uses a header that contains the following fields :

en	a `number` (0 or 1)

en	a `length` field that indicates the length of the data

en	a Cyclic Redundancy Check (`CRC`)

en	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 :

en	Each frame contains `D` bytes of data and `c` bytes of control information

en	Each acknowledgment contains `c` bytes of control information

en	The bandwidth of the two directions of the link is set to `B` bits per second

en	The delay between the two hosts is `s` seconds in both directions

en	there are no transmission errors

en	when there are no losses

en	when every second acknowledgment is discarded due to transmission errors

en	Same question when using selective repeat instead of go-back-n. Note that the answer is not necessarily the same.

en	Practice

en	Reliable protocols depend on error detection algorithms to detect transmission errors. The following questions will reinforce your understanding of these algorithms.

en	Reliable protocols rely on different types of checksums to verify whether frames have been affected by transmission errors. The most frequently used checksums are :

en	the Internet checksum used by UDP, TCP and other Internet protocols which is defined in :rfc:`1071` and implemented in various libraries.

en	the 16 bits or the 32 bits Cyclical Redundancy Checks (CRC) that are often used on disks, in zip archives and in datalink layer protocols. See http://rosettacode.org/wiki/CRC-32 for CRC-32 implementations in various languages.

en	The Cyclic Redundancy Checks (CRCs) are efficient error detection codes that are able to detect :

en	all errors that affect an odd number of bits

en	all errors that affect a sequence of bits which is shorter than the length of the CRC

en	Implement a small software that computes the CRC-32 for a text file. Then, modify the contents of the file to change an even number of bits or an odd number of bits inside the file. When modifying the file, remember that an ASCII file is composed of 8 bits characters that are encoded by using the ASCII table that you can find at : http://en.wikipedia.org/wiki/ASCII . You can also write a small program that produces binary files that are a small variation of each other.

en	Discussion questions

en	Consider two high-end servers connected back-to-back by using a 10 Gbps interface. If the delay between the two servers is one millisecond, what is the throughput that can be achieved by a reliable protocol that is using 10,000 bits frames and a window of

en	one frame

en	ten frames

en	hundred frames

en	Is it possible for a go-back-n receiver to inter-operate with a selective-repeat sender ? Justify your answer.

en	Is it possible for a selective-repeat receiver to inter-operate with a go-back-n sender ? Justify your answer.

en	Same question as above, but assume now that both the sender and the receiver implement selective repeat. Note that the answer can be different from the above question.