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../../principles/reliability.rst:160
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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.
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../../exercises/reliability.rst:81
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hundred frames
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../../exercises/network.rst:360
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Destination
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../../exercises/sockets.rst:301
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Now, we have built everything we need to send a message to the remote program. The ``create_socket_and_send_message`` function below assembles all the building blocks we created until now in order to send the message ``"hello"`` to the program running on port ``55555`` on the computer identified by the ``::1`` IPv6 address.
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../../principles/naming.rst:152
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It is interesting to note that to prevent any homograph attack, Google Inc. registered the `g00gle.com` domain name but does not apparently use it.
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../../principles/referencemodels.rst:139
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the Link layer which combines the functions of the physical and datalink layers of our five-layer reference model
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../../principles/reliability.rst:167
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Manchester encoding
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../../exercises/reliability.rst:83
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Is it possible for a go-back-n receiver to inter-operate with a selective-repeat sender ? Justify your answer.
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../../exercises/network.rst:360
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Routes on A
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../../exercises/sockets.rst:322
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Note that we can reuse our ``send_hello_to_peer`` function without any modification as we wrote it to handle any kind of sockets, including sockets using the IPv6 network protocol.
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../../principles/referencemodels.rst:141
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Besides this difference in the lower layers, the TCP/IP reference model is very close to the five layers that we use throughout this document.
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../../exercises/reliability.rst:85
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Is it possible for a selective-repeat receiver to inter-operate with a go-back-n sender ? Justify your answer.
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../../exercises/network.rst:360
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Routes on B
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../../exercises/sockets.rst:326
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Endianness: exchanging integers between different computers
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../../principles/referencemodels.rst:146
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The OSI reference model
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../../principles/network.rst:143
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Computing forwarding tables
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../../principles/reliability.rst:180
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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.
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../../principles/transport.rst:86
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Besides character strings, some applications also need to exchange 16 bits and 32 bits fields such as integers. A naive solution would have been to send the 16- or 32-bits field as it is encoded in the host's memory. Unfortunately, there are different methods to store 16- or 32-bits fields in memory. Some CPUs store the most significant byte of a 16-bits field in the first address of the field while others store the least significant byte at this location. When networked applications running on different CPUs exchange 16 bits fields, there are two possibilities to transfer them over the transport service :
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../../exercises/network.rst:360
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Routes on C
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../../exercises/sockets.rst:328
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Besides character strings, some applications also need to exchange 16 bits and 32 bits fields such as integers. A naive solution would have been to send the 16- or 32-bits field as it is encoded in the host's memory. Unfortunately, there are different methods to store 16- or 32-bits fields in memory. Some CPUs store the most significant byte of a 16-bits field in the first address of the field while others store the least significant byte at this location. When networked applications running on different CPUs exchange 16 bits fields, there are two possibilities to transfer them over the transport service :
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