cnp3-ebook/principles/reliability

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98 ../../principles/reliability.rst:349

	English These two types of frames can be distinguished by dividing the frame in two parts : 83/10000 · 83
99 ../../principles/reliability.rst:351

	English the `header` that contains one bit set to `0` in data frames and set to `1` in control frames 93/10000 · 93
100 ../../principles/reliability.rst:352

	English the payload that contains the SDU supplied by the application 61/10000 · 61
101 ../../principles/reliability.rst:354

	English The datalink entity can then be modeled as a finite state machine, containing two states for the receiver and two states for the sender. The figure below provides a graphical representation of this state machine with the sender above and the receiver below. 257/10000 · 257
102 ../../principles/reliability.rst:388

	English The above FSM shows that the sender has to wait for an acknowledgment from the receiver before being able to transmit the next SDU. The figure below illustrates the exchange of a few frames between two hosts. 209/10000 · 209
103 ../../principles/reliability.rst:407

	English Services and protocols 22/10000 · 22
104 ../../principles/reliability.rst:409

	English 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. 1161/10000 · 1161
105 ../../principles/reliability.rst:424

	English Reliable data transfer on top of an imperfect link 50/10000 · 50
106 ../../principles/reliability.rst:426

	English The datalink layer must deal with the transmission errors. In practice, we mainly have to deal with two types of errors in the datalink layer : 143/10000 · 143
107 ../../principles/reliability.rst:428

	English Frames can be corrupted by transmission errors 46/10000 · 46
108 ../../principles/reliability.rst:429

	English Frames can be lost or unexpected frames can appear 50/10000 · 50
109 ../../principles/reliability.rst:432

	English 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. 324/10000 · 324
110 ../../principles/reliability.rst:434

	English To deal with these types of imperfections, reliable protocols rely on different types of mechanisms. The first problem is transmission errors. Data transmission on a physical link can be affected by the following errors : 221/10000 · 221
111 ../../principles/reliability.rst:436

	English random isolated errors where the value of a single bit has been modified due to a transmission error 100/10000 · 100
112 ../../principles/reliability.rst:437

	English random burst errors where the values of `n` consecutive bits have been changed due to transmission errors 105/10000 · 105
113 ../../principles/reliability.rst:438

	English random bit creations and random bit removals where bits have been added or removed due to transmission errors 109/10000 · 109
114 ../../principles/reliability.rst:440

	English The only solution to protect against transmission errors is to add redundancy to the frames that are sent. `Information Theory` defines two mechanisms that can be used to transmit information over a transmission channel affected by random errors. These two mechanisms add redundancy to the transmitted information, to allow the receiver to detect or sometimes even correct transmission errors. A detailed discussion of these mechanisms is outside the scope of this chapter, but it is useful to consider a simple mechanism to understand its operation and its limitations. 570/10000 · 570
115 ../../principles/reliability.rst:442

	English `Information theory` defines `coding schemes`. There are different types of coding schemes, but let us focus on coding schemes that operate on binary strings. A coding scheme is a function that maps information encoded as a string of `m` bits into a string of `n` bits. The simplest coding scheme is the (even) parity coding. This coding scheme takes an `m` bits source string and produces an `m+1` bits coded string where the first `m` bits of the coded string are the bits of the source string and the last bit of the coded string is chosen such that the coded string will always contain an even number of bits set to `1`. For example : 638/10000 · 638
116 ../../principles/reliability.rst:444

	English `1001` is encoded as `10010` 28/10000 · 28
117 ../../principles/reliability.rst:445

	English `1101` is encoded as `11011` 28/10000 · 28

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