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29 ../../principles/reliability.rst:88
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 599/10000 · 599
30 ../../principles/reliability.rst:91
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 653/10000 · 653
31 ../../principles/reliability.rst:106
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — With the above transmission scheme, a bit is transmitted by setting the voltage on the electrical cable to a specific value during some period of time. We have seen that due to electromagnetic interference, the voltage measured by the receiver can differ from the voltage set by the transmitter. This is the main cause of transmission errors. However, this is not the only type of problem that can occur. Besides defining the voltages for bits `0` and `1`, the above transmission scheme also specifies the duration of each bit. If one million bits are sent every second, then each bit lasts 1 microsecond. On each host, the transmission (resp. the reception) of each bit is triggered by a local clock having a 1 MHz frequency. These clocks are the second source of problems when transmitting bits over a wire. Although the two clocks have the same specification, they run on different hosts, possibly at a different temperature and with a different source of energy. In practice, it is possible that the two clocks do not operate at exactly the same frequency. Assume that the clock of the transmitting host operates at exactly 1000000 Hz while the receiving clock operates at 999999 Hz. This is a very small difference between the two clocks. However, when using the clock to transmit bits, this difference is important. With its 1000000 Hz clock, the transmitting host will generate one million bits during a period of one second. During the same period, the receiving host will sense the wire 999999 times and thus will receive one bit less than the bits originally transmitted. This small difference in clock frequencies implies that bits can "disappear" during their transmission on an electrical cable. This is illustrated in the figure below. 1749/10000 · 1749
32 ../../principles/reliability.rst:128
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A similar reasoning applies when the clock of the sending host is slower than the clock of the receiving host. In this case, the receiver will sense more bits than the bits that have been transmitted by the sender. This is illustrated in the figure below where the second bit received on the right was not transmitted by the left host. 335/10000 · 335
33 ../../principles/reliability.rst:150
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 : 654/10000 · 654
34 ../../principles/reliability.rst:152
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `Physical layer service` may change, e.g. due to electromagnetic interference, the value of a bit being transmitted 119/10000 · 119
35 ../../principles/reliability.rst:153
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `Physical layer service` may deliver `more` bits to the receiver than the bits sent by the sender 101/10000 · 101
36 ../../principles/reliability.rst:154
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `Physical layer service` may deliver `fewer` bits to the receiver than the bits sent by the sender 102/10000 · 102
37 ../../principles/reliability.rst:160
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Many other types of encodings have been defined to transmit information over an electrical cable. All physical layers are able to send and receive physical symbols that represent values `0` and `1`. However, for various reasons that are outside the scope of this chapter, several physical layers exchange other physical symbols as well. For example, the Manchester encoding used in several physical layers can send four different symbols. The Manchester encoding is a differential encoding scheme in which time is divided into fixed-length periods. Each period is divided in two halves and two different voltage levels can be applied. To send a symbol, the sender must set one of these two voltage levels during each half period. To send a `1` (resp. `0`), the sender must set a high (resp. low) voltage during the first half of the period and a low (resp. high) voltage during the second half. This encoding ensures that there will be a transition at the middle of each period and allows the receiver to synchronize its clock to the sender's clock. Apart from the encodings for `0` and `1`, the Manchester encoding also supports two additional symbols : `InvH` and `InvB` where the same voltage level is used for the two half periods. By definition, these two symbols cannot appear inside a frame which is only composed of `0` and `1`. Some technologies use these special symbols as markers for the beginning or end of frames. 1429/10000 · 1429
38 ../../principles/reliability.rst:167
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Manchester encoding 19/10000 · 19
39 ../../principles/reliability.rst:189
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — All the functions related to the physical transmission or information through a wire (or a wireless link) are usually known as the `physical layer`. The physical layer allows thus two or more entities that are directly attached to the same transmission medium to exchange bits. Being able to exchange bits is important as virtually any information can be encoded as a sequence of bits. Electrical engineers are used to processing streams of bits, but computer scientists usually prefer to deal with higher level concepts. A similar issue arises with file storage. Storage devices such as hard-disks also store streams of bits. There are hardware devices that process the bit stream produced by a hard-disk, but computer scientists have designed filesystems to allow applications to easily access such storage devices. These filesystems are typically divided into several layers as well. Hard-disks store sectors of 512 bytes or more. Unix filesystems group sectors in larger blocks that can contain data or `inodes` representing the structure of the filesystem. Finally, applications manipulate files and directories that are translated in blocks, sectors and eventually bits by the operating system. 1200/10000 · 1200
40 ../../principles/reliability.rst:193
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Computer networks use a similar approach. Each layer provides a service that is built above the underlying layer and is closer to the needs of the applications. The datalink layer builds upon the service provided by the physical layer. We will see that it also contains several functions. 288/10000 · 288
41 ../../principles/reliability.rst:197
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The datalink layer 18/10000 · 18
42 ../../principles/reliability.rst:201
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 758/10000 · 758
43 ../../principles/reliability.rst:203
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — To enable the transmission/reception of frames, the first problem to be solved is how to encode a frame as a sequence of bits, so that the receiver can easily recover the received frame despite the limitations of the physical layer. 232/10000 · 232
44 ../../principles/reliability.rst:208
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — If the physical layer were perfect, the problem would be very simple. We would simply need to define how to encode each frame as a sequence of consecutive bits. The receiver would then easily be able to extract the frames from the received bits. Unfortunately, the imperfections of the physical layer make this framing problem slightly more complex. Several solutions have been proposed and are used in practice in different network technologies. 446/10000 · 446
45 ../../principles/reliability.rst:211
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Framing 7/10000 · 7
46 ../../principles/reliability.rst:213
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The `framing` problem can be defined as : "`How does a sender encode frames so that the receiver can efficiently extract them from the stream of bits that it receives from the physical layer`". 193/10000 · 193
47 ../../principles/reliability.rst:215
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A first solution to this problem is to require the physical layer to remain idle for some time after the transmission of each frame. These idle periods can be detected by the receiver and serve as a marker to delineate frame boundaries. Unfortunately, this solution is not acceptable for two reasons. First, some physical layers cannot remain idle and always need to transmit bits. Second, inserting an idle period between frames decreases the maximum bit rate that can be achieved. 482/10000 · 482
48 ../../principles/reliability.rst:217
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Bit rate and bandwidth 22/10000 · 22