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../../principles/sharing.rst:436
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To better understand these collisions, it is useful to analyze what would be the worst collision on a shared bus network. Let us consider a wire with two hosts attached at both ends, as shown in the figure below. Host A starts to transmit its frame and its electrical signal is propagated on the cable. Its propagation time depends on the physical length of the cable and the speed of the electrical signal. Let us use :math:`\tau` to represent this propagation delay in seconds. Slightly less than :math:`\tau` seconds after the beginning of the transmission of A's frame, B decides to start transmitting its own frame. After :math:`\epsilon` seconds, B senses A's frame, detects the collision and stops transmitting. The beginning of B's frame travels on the cable until it reaches host A. Host A can thus detect the collision at time :math:`\tau-\epsilon+\tau \approx 2\times\tau`. An important point to note is that a collision can only occur during the first :math:`2\times\tau` seconds of its transmission. If a collision did not occur during this period, it cannot occur afterwards since the transmission channel is busy after :math:`\tau` seconds and CSMA/CD hosts sense the transmission channel before transmitting their frame.
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../../principles/transport.rst:536
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To deal with these issues, the transport layer includes several mechanisms that depend on the service that it provides. It interacts with both the applications and the underlying network layer.
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../../glossary.rst:291
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An :term:`ISO` standard that defines codes to represent countries and their subdivisions. See http://www.iso.org/iso/country_codes.htm
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../../principles/network.rst:373 • ../../principles/network.rst:412 • ../../principles/network.rst:414 • ../../principles/network.rst:424 • ../../principles/network.rst:434 • ../../principles/network.rst:444
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../../principles/reliability.rst:336
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data frames carrying a SDU
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../../principles/sharing.rst:443
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The worst collision on a shared bus
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../../principles/transport.rst:542
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Interactions between the transport layer, its user, and its network layer provider
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../../glossary.rst:295
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ISP
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../../principles/network.rst:373 • ../../principles/network.rst:422 • ../../principles/network.rst:444
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none
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../../principles/sharing.rst:446
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Furthermore, on the wired networks where CSMA/CD is used, collisions are almost the only cause of transmission errors that affect frames. Transmission errors that only affect a few bits inside a frame seldom occur in these wired networks. For this reason, the designers of CSMA/CD chose to completely remove the acknowledgment frames in the datalink layer. When a host transmits a frame, it verifies whether its transmission has been affected by a collision. If not, given the negligible Bit Error Ratio of the underlying network, it assumes that the frame was received correctly by its destination. Otherwise the frame is retransmitted after some delay.
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../../principles/transport.rst:544
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We have already described in the datalink layers mechanisms to deal with data losses and transmission errors. These techniques are also used in the transport layer.
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../../glossary.rst:297
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An Internet Service Provider, i.e. a network that provides Internet access to its clients.
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../../principles/network.rst:375 • ../../principles/network.rst:377
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../../principles/reliability.rst:339
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These two types of frames can be distinguished by dividing the frame in two parts :
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../../principles/sharing.rst:449
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Removing acknowledgments is an interesting optimization as it reduces the number of frames that are exchanged on the network and the number of frames that need to be processed by the hosts. However, to use this optimization, we must ensure that all hosts will be able to detect all the collisions that affect their frames. The problem is important for short frames. Let us consider two hosts, A and B, that are sending a small frame to host C as illustrated in the figure below. If the frames sent by A and B are very short, the situation illustrated below may occur. Hosts A and B send their frame and stop transmitting (point `(1)`). When the two short frames arrive at the location of host C, they collide and host C cannot decode them (point `(2)`). The two frames are absorbed by the ends of the wire. Neither host A nor host B have detected the collision. They both consider their frame to have been received correctly by its destination.
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../../principles/transport.rst:548
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Connectionless transport
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../../glossary.rst:39
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ITU
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../../principles/network.rst:375
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West
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../../principles/reliability.rst:341
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the `header` that contains one bit set to `0` in data frames and set to `1` in control frames
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../../principles/sharing.rst:456
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The short-frame collision problem
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