Exit Zen
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MIME document
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->R3
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To deal with these types of imperfections, reliable protocols rely on different types of mechanisms. The first problem is transmission errors. Data transmission on a physical link can be affected by the following errors :
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Carrier Sense Multiple Access with Collision Avoidance
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A MIME document is a document, encoded by using the :term:`MIME` format.
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The figure below shows the path followed by the packets on the `R1->R3->R4->R2->R5` path in red with on each arrow the label used in the packets.
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random isolated errors where the value of a single bit has been modified due to a transmission error
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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.
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minicomputer
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MultiProtocol Label Switching (MPLS) is the example of a deployed networking technology that relies on label switching. MPLS is more complex than the above description because it has been designed to be easily integrated with datagram technologies. However, the principles remain. `Asynchronous Transfer Mode` (ATM) and Frame Relay are other examples of technologies that rely on `label switching`.
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random burst errors where the values of `n` consecutive bits have been changed due to transmission errors
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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.
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Unfortunately, this scheme is not sufficient to ensure the reliability of the transport service. Consider for example a short-lived transport connection where a single, but important transfer (e.g. money transfer from a bank account) is sent. Such a short-lived connection starts with a `CR` segment acknowledged by a `CA` segment, then the data segment is sent, acknowledged and the connection terminates. Unfortunately, as the network layer service is unreliable, delays combined to retransmissions may lead to the situation depicted in the figure below, where a delayed `CR` and data segments from a former connection are accepted by the receiving entity as valid segments, and the corresponding data is delivered to the user. Duplicating SDUs is not acceptable, and the transport protocol must solve this problem.
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A minicomputer is a multi-user system that was typically used in the 1960s/1970s to serve departments. See the corresponding Wikipedia article for additional information : https://en.wikipedia.org/wiki/Minicomputer
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random bit creations and random bit removals where bits have been added or removed due to transmission errors
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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]_.
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To avoid these duplicates, transport protocols require the network layer to bound the `Maximum Segment Lifetime (MSL)`. The organization of the network must guarantee that no segment remains in the network for longer than `MSL` seconds. For example, on today's Internet, `MSL` is expected to be 2 minutes. To avoid duplicate transport connections, transport protocol entities must be able to safely distinguish between a duplicate `CR` segment and a new `CR` segment, without forcing each transport entity to remember all the transport connections that it has established in the past.
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modem
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../../principles/linkstate.rst:128 • ../../principles/network.rst:505
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Footnotes
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The only solution to protect against transmission errors is to add redundancy to the frames that are sent. `Information Theory` defines two mechanisms that can be used to transmit information over a transmission channel affected by random errors. These two mechanisms add redundancy to the transmitted information, to allow the receiver to detect or sometimes even correct transmission errors. A detailed discussion of these mechanisms is outside the scope of this chapter, but it is useful to consider a simple mechanism to understand its operation and its limitations.
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