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../../protocols/ethernet.rst:375 • ../../protocols/ethernet.rst:375 • ../../protocols/ethernet.rst:376
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Root
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../../protocols/ethernet.rst:377
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Designated
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../../protocols/ethernet.rst:382
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To illustrate the operation of the `Spanning Tree Protocol`, let us consider the simple network topology in the figure below.
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A simple Spanning tree computed in a switched Ethernet network
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../../protocols/ethernet.rst:391
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Assume that `Switch4` is the first to boot. It sends its own `BPDU = <4,0,4,1>` (resp. `BPDU = <4,0,4,2>`) on port 1 (resp. port 2). When `Switch1` boots, it sends `BPDU = <1,0,1,1>`. This `BPDU` is received by `Switch4`, which updates its `BPDU` and root priority vector tables and computes a new `BPDU = <1,3,4,1>` (resp. `BPDU = <1,3,4,2>`) on port 1 (resp. port 2). Indeed, there is only one root priority vector and hence, it is the best one. Port 1 of `Switch4` becomes the `Root` port while its second port is still in the `Designated` state.
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../../protocols/ethernet.rst:393
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Assume now that `Switch9` boots and immediately receives `Switch1` 's `BPDU` on port 1. `Switch9` computes its own `BPDU = <1,1,9,1>` (resp. `BPDU = <1,1,9,2>`) on port 1 (resp. port 2) and port 1 becomes the `Root` port of this switch. The `BPDU` is sent on port 2 of `Switch9` and reaches `Switch4`. `Switch4` compares the priority vectors. It notices that the last computed vector (i.e., `V[2] = <1,2,9,2,2>`) is better than `V[1] = <1,3,1,1,1>`. Thus, `Switch4`'s `BPDU` is recomputed and port 2 becomes the `Root` port of `Switch4`. `Switch4` compares its new `BPDU = <1,2,4,p>` with the last `BPDU` received on each port (except for the `Root` port). Port 1 becomes a `Blocked` port on `Switch4` because the `BPDU=<1,0,1,1>` received on this port is better.
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During the computation of the spanning tree, switches discard all received data frames, as at that time the network topology is not guaranteed to be loop-free. Once that topology has been stable for some time, the switches again start to use the MAC learning algorithm to forward data frames. Only the `Root` and `Designated` ports are used to forward data frames. Switches discard all the data frames received on their `Blocked` ports and never forward frames on these ports.
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../../protocols/ethernet.rst:397
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Switches, ports and links can fail in a switched Ethernet network. When a failure occurs, the switches must be able to recompute the spanning tree to recover from the failure. The `Spanning Tree Protocol` relies on regular transmissions of the `BPDUs` to detect these failures. A `BPDU` contains two additional fields : the `Age` of the `BPDU` and the `Maximum Age`. The `Age` contains the amount of time that has passed since the root switch initially originated the `BPDU`. The root switch sends its `BPDU` with an `Age` of zero and each switch that computes its own `BPDU` increments its `Age` by one. The `Age` of the `BPDUs` stored on a switch's table is also incremented every second. A `BPDU` expires when its `Age` reaches the `Maximum Age`. When the network is stable, this does not happen as `BPDU` s are regularly sent by the `root` switch and downstream switches. However, if the `root` fails or the network becomes partitioned, `BPDU` will expire and switches will recompute their own `BPDU` and restart the `Spanning Tree Protocol`. Once a topology change has been detected, the forwarding of the data frames stops as the topology is not guaranteed to be loop-free. Additional details about the reaction to failures may be found in [IEEE802.1d]_.
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Virtual LANs
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Another important advantage of Ethernet switches is the ability to create Virtual Local Area Networks (VLANs). A virtual LAN can be defined as a `set of ports attached to one or more Ethernet switches`. A switch can support several VLANs and it runs one MAC learning algorithm for each Virtual LAN. When a switch receives a frame with an unknown or a multicast destination, it forwards it over all the ports that belong to the same Virtual LAN but not over the ports that belong to other Virtual LANs. Similarly, when a switch learns a source address on a port, it associates it to the Virtual LAN of this port and uses this information only when forwarding frames on this Virtual LAN.
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The figure below illustrates a switched Ethernet network with three Virtual LANs. `VLAN2` and `VLAN3` only require a local configuration of switch `S1`. Host `C` can exchange frames with host `D`, but not with hosts that are outside of its VLAN. `VLAN1` is more complex as there are ports of this VLAN on several switches. To support such VLANs, local configuration is not sufficient anymore. When a switch receives a frame from another switch, it must be able to determine the VLAN in which the frame originated to use the correct MAC table to forward the frame. This is done by assigning an identifier to each Virtual LAN and placing this identifier inside the headers of the frames that are exchanged between switches.
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Virtual Local Area Networks in a switched Ethernet network
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IEEE defined in the [IEEE802.1q]_ standard a special header to encode the VLAN identifiers. This 32 bit header includes a 12 bit VLAN field that contains the VLAN identifier of each frame. The format of the [IEEE802.1q]_ header is described below.
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Format of the 802.1q header
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The [IEEE802.1q]_ header is inserted immediately after the source MAC address in the Ethernet frame (i.e. before the EtherType field). The maximum frame size is increased by 4 bytes. It is encoded in 32 bits and contains four fields. The Tag Protocol Identifier is set to `0x8100` to allow the receiver to detect the presence of this additional header. The `Priority Code Point` (PCP) is a three bit field that is used to support different transmission priorities for the frame. Value `0` is the lowest priority and value `7` the highest. Frames with a higher priority can expect to be forwarded earlier than frames having a lower priority. The `C` bit is used for compatibility between Ethernet and Token Ring networks. The last 12 bits of the 802.1q header contain the VLAN identifier. Value `0` indicates that the frame does not belong to any VLAN while value `0xFFF` is reserved. This implies that 4094 different VLAN identifiers can be used in an Ethernet network.
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802.11 wireless networks
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The radio spectrum is a limited resource that must be shared by everyone. During most of the twentieth century, governments and international organizations have regulated most of the radio spectrum. This regulation controls the utilization of the radio spectrum, in order to prevent interference among different users. A company that wants to use a frequency range in a given region must apply for a license from the regulator. Most regulators charge a fee for the utilization of the radio spectrum and some governments have encouraged competition among companies bidding for the same frequency to increase the license fees.
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../../protocols/wifi.rst:9
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In the 1970s, after the first experiments with ALOHANet, interest in wireless networks grew. Many experiments were done on and outside the ARPANet. One of these experiments was the `first mobile phone <http://news.bbc.co.uk/2/hi/programmes/click_online/8639590.stm>`_ , which was developed and tested in 1973. This experimental mobile phone was the starting point for the first generation analog mobile phones. Given the growing demand for mobile phones, it was clear that the analog mobile phone technology was not sufficient to support a large number of users. To support more users and new services, researchers in several countries worked on the development of digital mobile telephones. In 1987, several European countries decided to develop the standards for a common cellular telephone system across Europe : the `Global System for Mobile Communications` (GSM). Since then, the standards have evolved and more than three billion users are connected to GSM networks today.
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While most of the frequency ranges of the radio spectrum are reserved for specific applications and require a special license, there are a few exceptions. These exceptions are known as the `Industrial, Scientific and Medical <http://en.wikipedia.org/wiki/ISM_band>`_ (ISM) radio bands. These bands can be used for industrial, scientific and medical applications without requiring a license from the regulator. For example, some radio-controlled models use the 27 MHz ISM band and some cordless telephones operate in the 915 MHz ISM. In 1985, the 2.400-2.500 GHz band was added to the list of ISM bands. This frequency range corresponds to the frequencies that are emitted by microwave ovens. Sharing this band with licensed applications would have likely caused interference, given the large number of microwave ovens that are used. Despite the risk of interference with microwave ovens, the opening of the 2.400-2.500 GHz allowed the networking industry to develop several wireless network techniques to allow computers to exchange data without using cables. In this section, we discuss in more detail the most popular one, i.e. the WiFi [IEEE802.11]_ family of wireless networks. Other wireless networking techniques such as `BlueTooth <http://en.wikipedia.org/wiki/BlueTooth>`_ or `HiperLAN <http://en.wikipedia.org/wiki/HiperLAN>`_ use the same frequency range.
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