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GET /api/translations/cnp3-ebook/principlesreliability/en/changes/?format=api
{ "count": 33, "next": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/changes/?format=api&page=2", "previous": null, "results": [ { "unit": "https://weblate.info.ucl.ac.be/api/units/37669/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.865899+02:00", "action": 59, "target": "`optical fiber`. Optical fibers are frequently used in public and enterprise networks when the distance between the communication devices is larger than one kilometer. There are two main types of optical fibers : multi-mode and single-mode. Multi-mode is much cheaper than single-mode fiber because a LED can be used to send a signal over a multi-mode fiber while a single-mode fiber must be driven by a laser. Due to the different modes of propagation of light, multi-mode fibers are limited to distances of a few kilometers while single-mode fibers can be used over distances greater than several tens of kilometers. In both cases, repeaters can be used to regenerate the optical signal at one endpoint of a fiber to send it over another fiber.", "id": 14924, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14924/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37670/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.865962+02:00", "action": 59, "target": "These physical media can be used to exchange information once this information has been converted into a suitable electrical signal. Entire telecommunication courses and textbooks are devoted to the problem of converting analog or digital information into an electrical signal so that it can be transmitted over a given physical `link`. In this book, we only consider two very simple schemes that allow to transmit information over an electrical cable. This enables us to highlight the key problems when transmitting information over a physical link. We are only interested in techniques that allow transmitting digital information through the wire. Here, we will focus on the transmission of bits, i.e. either `0` or `1`.", "id": 14925, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14925/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37671/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866006+02:00", "action": 59, "target": "In computer networks, the bit rate of the physical layer is always expressed in bits per second. One Mbps is one million bits per second and one Gbps is one billion bits per second. This is in contrast with memory specifications that are usually expressed in bytes (8 bits), KiloBytes (1024 bytes) or MegaBytes (1048576 bytes). Transferring one MByte through a 1 Mbps link lasts 8.39 seconds.", "id": 14926, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14926/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37672/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866045+02:00", "action": 59, "target": "A `time-sequence diagram` describes the interactions between communicating hosts. By convention, the communicating hosts are represented in the left and right parts of the diagram while the electrical link occupies the middle of the diagram. In such a time-sequence diagram, time flows from the top to the bottom of the diagram. The transmission of one bit of information is represented by three arrows. Starting from the left, the first horizontal arrow represents the request to transmit one bit of information. This request is represented by a `primitive` which can be considered as a kind of procedure call. This primitive has one parameter (the bit being transmitted) and a name (`DATA.request` in this example). By convention, all primitives that are named `something.request` correspond to a request to transmit some information. The dashed arrow indicates the transmission of the corresponding electrical signal on the wire. Electrical and optical signals do not travel instantaneously. The diagonal dashed arrow indicates that it takes some time for the electrical signal to be transmitted from `Host A` to `Host B`. Upon reception of the electrical signal, the electronics on `Host B`'s network interface detects the voltage and converts it into a bit. This bit is delivered as a `DATA.indication` primitive. All primitives that are named `something.indication` correspond to the reception of some information. The dashed lines also represents the relationship between two (or more) primitives. Such a time-sequence diagram provides information about the ordering of the different primitives, but the distance between two primitives does not represent a precise amount of time.", "id": 14927, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14927/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37673/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866086+02:00", "action": 59, "target": "Time-sequence diagrams are useful when trying to understand the characteristics of a given communication scheme. When considering the above transmission scheme, it is useful to evaluate whether this scheme allows the two communicating hosts to reliably exchange information. A digital transmission is considered as reliable when a sequence of bits that is transmitted by a host is received correctly at the other end of the wire. In practice, achieving perfect reliability when transmitting information using the above scheme is difficult. Several problems can occur with such a transmission scheme.", "id": 14928, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14928/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37674/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866122+02:00", "action": 59, "target": "The first problem is that electrical transmission can be affected by electromagnetic interference. Interference can have various sources including natural phenomenons (like thunderstorms, variations of the magnetic field,...) but also other electrical signals (such as interference from neighboring cables, interference from neighboring antennas,...). Due to these various types of interference, there is unfortunately no guarantee that when a host transmit one bit on a wire, the same bit is received at the other end. This is illustrated in the figure below where a `DATA.request(0)` on the left host leads to a `Data.indication(1)` on the right host.", "id": 14929, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14929/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37675/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866159+02:00", "action": 59, "target": "From a Computer Science viewpoint, the physical transmission of information through a wire is often considered as a black box that allows transmitting bits. This black box is commonly referred to as the `physical layer service` and is represented by using the `DATA.request` and `DATA.indication` primitives introduced earlier. This physical layer service facilitates the sending and receiving of bits, by abstracting the technological details that are involved in the actual transmission of the bits as an electromagnetic signal. However, it is important to remember that the `physical layer service` is imperfect and has the following characteristics :", "id": 14930, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14930/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37676/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866195+02:00", "action": 59, "target": "Computer scientists are usually not interested in exchanging bits between two hosts. They prefer to write software that deals with larger blocks of data in order to transmit messages or complete files. Thanks to the physical layer service, it is possible to send a continuous stream of bits between two hosts. This stream of bits can include logical blocks of data, but we need to be able to extract each block of data from the bit stream despite the imperfections of the physical layer. In many networks, the basic unit of information exchanged between two directly connected hosts is often called a `frame`. A `frame` can be defined as a sequence of bits that has a particular syntax or structure. We will see examples of such frames later in this chapter.", "id": 14931, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14931/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37677/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866232+02:00", "action": 59, "target": "0111110", "id": 14932, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14932/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37678/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866266+02:00", "action": 59, "target": "011111100111110001111110", "id": 14933, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14933/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37679/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866300+02:00", "action": 59, "target": "`Character stuffing`, like bit stuffing, increases the length of the transmitted frames. For `character stuffing`, the worst frame is a frame containing many `DLE` characters. When transmission errors occur, the receiver may incorrectly decode one or two frames (e.g. if the errors occur in the markers). However, it will be able to resynchronize itself with the next correctly received markers.", "id": 14934, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14934/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37680/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866335+02:00", "action": 59, "target": "Bit stuffing and character stuffing allow recovering frames from a stream of bits or bytes. This framing mechanism provides a richer service than the physical layer. Through the framing service, one can send and receive complete frames. This framing service can also be represented by using the `DATA.request` and `DATA.indication` primitives. This is illustrated in the figure below, assuming hypothetical frames containing four useful bits and one bit of framing for graphical reasons.", "id": 14935, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14935/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37681/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866371+02:00", "action": 59, "target": "We can now build upon the framing mechanism to allow the hosts to exchange frames containing an integer number of bits or bytes. Once the framing problem has been solved, we can focus on designing a technique that allows reliably exchanging frames.", "id": 14936, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14936/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37682/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866415+02:00", "action": 59, "target": "When running on top of a perfect framing sub-layer, a datalink entity can simply issue a `send(SDU)` upon arrival of a `DATA.req(SDU)` [#fsdu]_. Similarly, the receiver issues a `DATA.ind(SDU)` upon receipt of a `recvd(SDU)`. Such a simple protocol is sufficient when a single SDU is sent. This is illustrated in the figure below.", "id": 14937, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14937/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37683/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866449+02:00", "action": 59, "target": "Unfortunately, this is not always sufficient to ensure a reliable delivery of the SDUs. Consider the case where a client sends tens of SDUs to a server. If the server is faster than the client, it will be able to receive and process all the frames sent by the client and deliver their content to its user. However, if the server is slower than the client, problems may arise. The datalink entity contains buffers to store SDUs that have been received as a `Data.request` but have not yet been sent. If the application is faster than the physical link, the buffer may become full. At this point, the operating system suspends the application to let the datalink entity empty its transmission queue. The datalink entity also uses a buffer to store the received frames that have not yet been processed by the application. If the application is slow to process the data, this buffer may overflow and the datalink entity will not able to accept any additional frame. The buffers of the datalink entity have a limited size and if they overflow, the arriving frames will be discarded, even if they are correct.", "id": 14938, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14938/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37684/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866507+02:00", "action": 59, "target": "control frames carrying an acknowledgment indicating that the previous frames was correctly processed", "id": 14939, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14939/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37685/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866545+02:00", "action": 59, "target": "An important aspect to understand before studying computer networks is the difference between a *service* and a *protocol*. For this, it is useful to start with real world examples. The traditional Post provides a service where a postman delivers letters to recipients. The Post precisely defines which types of letters (size, weight, etc) can be delivered by using the Standard Mail service. Furthermore, the format of the envelope is specified (position of the sender and recipient addresses, position of the stamp). Someone who wants to send a letter must either place the letter at a Post Office or inside one of the dedicated mailboxes. The letter will then be collected and delivered to its final recipient. Note that for the regular service the Post usually does not guarantee the delivery of each particular letter. Some letters may be lost, and some letters are delivered to the wrong mailbox. If a letter is important, then the sender can use the registered service to ensure that the letter will be delivered to its recipient. Some Post services also provide an acknowledged service or an express mail service that is faster than the regular service.", "id": 14940, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14940/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37686/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866585+02:00", "action": 59, "target": "A first glance, loosing frames might seem strange on a single link. However, if we take framing into account, transmission errors can affect the frame delineation mechanism and make the frame unreadable. For the same reason, a receiver could receive two (likely invalid) frames after a sender has transmitted a single frame.", "id": 14941, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14941/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37687/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866621+02:00", "action": 59, "target": "This simple coding scheme forces the sender to transmit three bits for each source bit. However, it allows the receiver to correct single bit errors. More advanced coding systems that allow recovering from errors are used in several types of physical layers.", "id": 14942, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14942/?format=api" }, { "unit": "https://weblate.info.ucl.ac.be/api/units/37688/?format=api", "component": "https://weblate.info.ucl.ac.be/api/components/cnp3-ebook/principlesreliability/?format=api", "translation": "https://weblate.info.ucl.ac.be/api/translations/cnp3-ebook/principlesreliability/en/?format=api", "user": null, "author": null, "timestamp": "2022-09-17T01:14:35.866657+02:00", "action": 59, "target": "Besides framing, datalink layers also include mechanisms to detect and sometimes even recover from transmission errors. To allow a receiver to notice transmission errors, a sender must add some redundant information as an `error detection` code to the frame sent. This `error detection` code is computed by the sender on the frame that it transmits. When the receiver receives a frame with an error detection code, it recomputes it and verifies whether the received `error detection code` matches the computed `error detection code`. If they match, the frame is considered to be valid. Many error detection schemes exist and entire books have been written on the subject. A detailed discussion of these techniques is outside the scope of this book, and we will only discuss some examples to illustrate the key principles.", "id": 14943, "action_name": "String updated in the repository", "url": "https://weblate.info.ucl.ac.be/api/changes/14943/?format=api" } ] }