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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.
the Internet checksum used by UDP, TCP and other Internet protocols which is defined in :rfc:`1071` and implemented in various libraries.
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.
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.
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
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.
hundred frames cent frames
Each acknowledgment contains `c` bytes of control information Chaque acknowledgment contient `c` bytes de information de contrôle
Each frame contains `D` bytes of data and `c` bytes of control information Chaque frame contient `D` bytes de donnée et `c` bytes d'information de contrôle
How would you set the duration of the retransmission timer in the alternating bit protocol ? Comment fixeriez-vous le délai du timer de retransmission dans le protocole du bit alterné ?
when there are no losses dans le cas où il n'y a pas de pertes
ten frames dix frames
Is it possible for a go-back-n receiver to inter-operate with a selective-repeat sender ? Justify your answer. Est-il possible pour un récepteur go-back-n de fonctionner avec un émetteur selective-repeat ? Justifiez.
Is it possible for a selective-repeat receiver to inter-operate with a go-back-n sender ? Justify your answer. Est-il possible pour un récepteur selective-repeat de fonctionner avec un émetteur go-back-n ? Justifiez.
Practice Exercices
there are no transmission errors il n'y a pas d'erreur de transmission
The bandwidth of the two directions of the link is set to `B` bits per second La bande passante dans les deux directions du câble est de `B` bits par seconde
The delay between the two hosts is `s` seconds in both directions Le délai entre deux hôtes est de `s` secondes dans les deux sens
The Cyclic Redundancy Checks (CRCs) are efficient error detection codes that are able to detect : Les contrôles de redondance cyclique (CRC) sont des outils efficaces de détection d'erreurs pouvant détecter :
Reliable protocols depend on error detection algorithms to detect transmission errors. The following questions will reinforce your understanding of these algorithms. Les protocoles fiables dépendent d'algorithmes de détection des erreurs de transmission. Les questions suivantes vont vous permettre d'approfondir votre compréhension de ces algorithmes.