cnp3-ebook/principles/sharing — English @ Weblate

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97 ../../principles/sharing.rst:611

	English The second host retransmits immediately and the first defers its retransmission 79/10000 · 79
98 ../../principles/sharing.rst:612

	English Both hosts defer their retransmission and a new collision occurs 64/10000 · 64
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	English In the second and third cases, both hosts have flipped different coins. The delay chosen by the host that defers its retransmission should be long enough to ensure that its retransmission will not collide with the immediate retransmission of the other host. However the delay should not be longer than the time necessary to avoid the collision, because if both hosts decide to defer their transmission, the network will be idle during this delay. The `slot time` is the optimal delay since it is the shortest delay that ensures that the first host will be able to retransmit its frame completely without any collision. 618/10000 · 618
100 ../../principles/sharing.rst:616

	English If two hosts are competing, the algorithm above will avoid a second collision 50% of the time. However, if the network is heavily loaded, several hosts may be competing at the same time. In this case, the hosts should be able to automatically adapt their retransmission delay. The `binary exponential back-off` performs this adaptation based on the number of collisions that have affected a frame. After the first collision, the host flips a coin and waits 0 or 1 `slot time`. After the second collision, it generates a random number and waits 0, 1, 2 or 3 `slot times`, etc. The duration of the waiting time is doubled after each collision. The complete pseudo-code for the CSMA/CD algorithm is shown in the figure below. 722/10000 · 722
101 ../../principles/sharing.rst:641

	English The inter-frame delay used in this pseudo-code is a short delay corresponding to the time required by a network adapter to switch from transmit to receive mode. It is also used to prevent a host from sending a continuous stream of frames without leaving any transmission opportunities for other hosts on the network. This contributes to the fairness of CSMA/CD. Despite this delay, there are still conditions where CSMA/CD is not completely fair [RY1994]_. Consider for example a network with two hosts : a server sending long frames and a client sending acknowledgments. Measurements reported in [RY1994]_ have shown that there are situations where the client could suffer from repeated collisions that lead it to wait for long periods of time due to the exponential back-off algorithm. 787/10000 · 787
102 ../../principles/sharing.rst:647

	English Carrier Sense Multiple Access with Collision Avoidance 54/10000 · 54
103 ../../principles/sharing.rst:649

	English The `Carrier Sense Multiple Access with Collision Avoidance` (CSMA/CA) Medium Access Control algorithm was designed for the popular WiFi wireless network technology [IEEE802.11]_. CSMA/CA also senses the transmission channel before transmitting a frame. Furthermore, CSMA/CA tries to avoid collisions by carefully tuning the timers used by CSMA/CA devices. 356/10000 · 356
104 ../../principles/sharing.rst:654

	English CSMA/CA uses acknowledgments like CSMA. Each frame contains a sequence number and a CRC. The CRC is used to detect transmission errors while the sequence number is used to avoid frame duplication. When a device receives a correct frame, it returns a special acknowledgment frame to the sender. CSMA/CA introduces a small delay, named `Short Inter Frame Spacing` (SIFS), between the reception of a frame and the transmission of the acknowledgment frame. This delay corresponds to the time that is required to switch the radio of a device between the reception and transmission modes. 583/10000 · 583
105 ../../principles/sharing.rst:659

	English Compared to CSMA, CSMA/CA defines more precisely when a device is allowed to send a frame. First, CSMA/CA defines two delays : `DIFS` and `EIFS`. To send a frame, a device must first wait until the channel has been idle for at least the `Distributed Coordination Function Inter Frame Space` (DIFS) if the previous frame was received correctly. However, if the previously received frame was corrupted, this indicates that there are collisions and the device must sense the channel idle for at least the `Extended Inter Frame Space` (EIFS), with :math:`SIFS<DIFS<EIFS`. The exact values for SIFS, DIFS and EIFS depend on the underlying physical layer [IEEE802.11]_. 663/10000 · 663
106 ../../principles/sharing.rst:661

	English The figure below shows the basic operation of CSMA/CA devices. Before transmitting, host `A` verifies that the channel is empty for a long enough period. Then, its sends its data frame. After checking the validity of the received frame, the recipient sends an acknowledgment frame after a short SIFS delay. Host `C`, which does not participate in the frame exchange, senses the channel to be busy at the beginning of the data frame. Host `C` can use this information to determine how long the channel will be busy for. Note that as :math:`SIFS<DIFS<EIFS`, even a device that would start to sense the channel immediately after the last bit of the data frame could not decide to transmit its own frame during the transmission of the acknowledgment frame. 752/10000 · 752
107 ../../principles/sharing.rst:668

	English Operation of a CSMA/CA device 29/10000 · 29
108 ../../principles/sharing.rst:674

	English The main difficulty with CSMA/CA is when two or more devices transmit at the same time and cause collisions. This is illustrated in the figure below, assuming a fixed timeout after the transmission of a data frame. With CSMA/CA, the timeout after the transmission of a data frame is very small, since it corresponds to the SIFS plus the time required to transmit the acknowledgment frame. 388/10000 · 388
109 ../../principles/sharing.rst:680

	English Collisions with CSMA/CA 23/10000 · 23
110 ../../principles/sharing.rst:686

	English To deal with this problem, CSMA/CA relies on a backoff timer. This backoff timer is a random delay that is chosen by each device in a range that depends on the number of retransmissions for the current frame. The range grows exponentially with the retransmissions as in CSMA/CD. The minimum range for the backoff timer is :math:`[0,7*slotTime]` where the `slotTime` is a parameter that depends on the underlying physical layer. Compared to CSMA/CD's exponential backoff, there are two important differences to notice. First, the initial range for the backoff timer is seven times larger. This is because it is impossible in CSMA/CA to detect collisions as they happen. With CSMA/CA, a collision may affect the entire frame while with CSMA/CD it can only affect the beginning of the frame. Second, a CSMA/CA device must regularly sense the transmission channel during its back off timer. If the channel becomes busy (i.e. because another device is transmitting), then the back off timer must be frozen until the channel becomes free again. Once the channel becomes free, the back off timer is restarted. This is in contrast with CSMA/CD where the back off is recomputed after each collision. This is illustrated in the figure below. Host `A` chooses a smaller backoff than host `C`. When `C` senses the channel to be busy, it freezes its backoff timer and only restarts it once the channel is free again. 1403/10000 · 1403
111 ../../principles/sharing.rst:693

	English Detailed example with CSMA/CA 29/10000 · 29
112 ../../principles/sharing.rst:696

	English The pseudo-code below summarizes the operation of a CSMA/CA device. The values of the SIFS, DIFS, EIFS and :math:`slotTime` depend on the underlying physical layer technology [IEEE802.11]_ 188/10000 · 188
113 ../../principles/sharing.rst:725

	English Another problem faced by wireless networks is often called the `hidden station problem`. In a wireless network, radio signals are not always propagated same way in all directions. For example, two devices separated by a wall may not be able to receive each other's signal while they could both be receiving the signal produced by a third host. This is illustrated in the figure below, but it can happen in other environments. For example, two devices that are on different sides of a hill may not be able to receive each other's signal while they are both able to receive the signal sent by a station at the top of the hill. Furthermore, the radio propagation conditions may change with time. For example, a truck may temporarily block the communication between two nearby devices. 781/10000 · 781
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	English The hidden station problem 26/10000 · 26
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	English To avoid collisions in these situations, CSMA/CA allows devices to reserve the transmission channel for some time. This is done by using two control frames : `Request To Send` (RTS) and `Clear To Send` (CTS). Both are very short frames to minimize the risk of collisions. To reserve the transmission channel, a device sends a RTS frame to the intended recipient of the data frame. The RTS frame contains the duration of the requested reservation. The recipient replies, after a SIFS delay, with a CTS frame which also contains the duration of the reservation. As the duration of the reservation has been sent in both RTS and CTS, all hosts that could collide with either the sender or the reception of the data frame are informed of the reservation. They can compute the total duration of the transmission and defer their access to the transmission channel until then. This is illustrated in the figure below where host `A` reserves the transmission channel to send a data frame to host `B`. Host `C` notices the reservation and defers its transmission. 1053/10000 · 1053
116 ../../principles/sharing.rst:744

	English Reservations with CSMA/CA 25/10000 · 25

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