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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.
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Now that we have looked at how a frame is actually transmitted as an electrical signal on a shared bus, it is interesting to look in more detail at what happens when two hosts transmit a frame at almost the same time. This is illustrated in the figure below, where hosts A and B start their transmission at the same time (point `(1)`). At this time, if host C senses the channel, it will consider it to be free. This will not last a long time and at point `(2)` the electrical signals from both host A and host B reach host C. The combined electrical signal (shown graphically as the superposition of the two curves in the figure) cannot be decoded by host C. Host C detects a collision, as it receives a signal that it cannot decode. Since host C cannot decode the frames, it cannot determine which hosts are sending the colliding frames. Note that host A (and host B) will detect the collision after host C (point `(3)` in the figure below).
Frame collision on a shared bus
As shown above, hosts detect collisions when they receive an electrical signal that they cannot decode. In a wired network, a host is able to detect such a collision both while it is listening (e.g. like host C in the figure above) and also while it is sending its own frame. When a host transmits a frame, it can compare the electrical signal that it transmits with the electrical signal that it senses on the wire. At points `(1)` and `(2)` in the figure above, host A senses only its own signal. At point `(3)`, it senses an electrical signal that differs from its own signal and can thus detects the collision. At this point, its frame is corrupted and it can stop its transmission. The ability to detect collisions while transmitting is the starting point for the `Carrier Sense Multiple Access with Collision Detection (CSMA/CD)` Medium Access Control algorithm, which is used in Ethernet networks [Metcalfe1976]_ [IEEE802.3]_ . When an Ethernet host detects a collision while it is transmitting, it immediately stops its transmission. Compared with pure CSMA, CSMA/CD is an important improvement since when collisions occur, they only last until colliding hosts have detected it and stopped their transmission. In practice, when a host detects a collision, it sends a special jamming signal on the cable to ensure that all hosts have detected the collision.
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.
The worst collision on a shared bus
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.
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.
The short-frame collision problem
To solve this problem, networks using CSMA/CD require hosts to transmit for at least :math:`2\times\tau` seconds. Since the network transmission speed is fixed for a given network technology, this implies that a technology that uses CSMA/CD enforces a minimum frame size. In the most popular CSMA/CD technology, Ethernet, :math:`2\times\tau` is called the `slot time` [#fslottime]_.
The last innovation introduced by CSMA/CD is the computation of the retransmission timeout. As for ALOHA, this timeout cannot be fixed, otherwise hosts could become synchronized and always retransmit at the same time. Setting such a timeout is always a compromise between the network access delay and the amount of collisions. A short timeout would lead to a low network access delay but with a higher risk of collisions. On the other hand, a long timeout would cause a long network access delay but a lower risk of collisions. The `binary exponential back-off` algorithm was introduced in CSMA/CD networks to solve this problem.
To understand `binary exponential back-off`, let us consider a collision caused by exactly two hosts. Once it has detected the collision, a host can either retransmit its frame immediately or defer its transmission for some time. If each colliding host flips a coin to decide whether to retransmit immediately or to defer its retransmission, four cases are possible :
Both hosts retransmit immediately and a new collision occurs
The first host retransmits immediately and the second defers its retransmission
The second host retransmits immediately and the first defers its retransmission
Both hosts defer their retransmission and a new collision occurs
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.
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.
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.
This name should not be confused with the duration of a transmission slot in slotted ALOHA. In CSMA/CD networks, the slot time is the time during which a collision can occur at the beginning of the transmission of a frame. In slotted ALOHA, the duration of a slot is the transmission time of an entire fixed-size frame.
Carrier Sense Multiple Access with Collision Avoidance
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.
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.
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]_.
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.
Operation of a CSMA/CA device
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.
Collisions with CSMA/CA
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.
Detailed example with CSMA/CA
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]_
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.

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locale/cs/LC_MESSAGES/principles/sharing.po, string 106