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The transport and datalink layers usually chose different strategies to place their CRCs or checksums. Transport layer protocols usually place their CRCs or checksums in the segment header. Datalink layer protocols sometimes place their CRC in the frame header, but often in a trailer at the end of the frame. This choice reflects implementation assumptions, but also influences performance :rfc:`893`. When the CRC is placed in the trailer, as in Ethernet, the datalink layer can compute it while transmitting the frame and insert it at the end of the transmission. All Ethernet interfaces use this optimization today. When the checksum is placed in the header, as in a TCP segment, it is impossible for the network interface to compute it while transmitting the segment. Some network interfaces provide hardware assistance to compute the TCP checksum, but this is more complex than if the TCP checksum were placed in the trailer [#ftso]_.
Thick coaxial cable, 500m
Thin coaxial cable, 185m
Today, WiFi is a very popular wireless networking technology. There are more than several hundreds of millions of WiFi devices. The development of this technology started in the late 1980s with the `WaveLAN <http://en.wikipedia.org/wiki/WaveLAN>`_ proprietary wireless network. WaveLAN operated at 2 Mbps and used different frequency bands in different regions of the world. In the early 1990s, the IEEE_ created the `802.11 working group <http://www.ieee802.org/11/>`_ to standardize a family of wireless network technologies. This working group was very prolific and produced several wireless networking standards that use different frequency ranges and different physical layers. The table below provides a summary of the main 802.11 standards.
To determine the state of its other ports, the switch compares its own `BPDU` with the last `BPDU` received on each port. Note that the comparison is done by using the `BPDUs` and not the `root priority vectors`. If the switch's `BPDU` is better than the last `BPDU` of this port, the port becomes a `Designated` port. Otherwise, the port becomes a `Blocked` port.
To illustrate the operation of the `Spanning Tree Protocol`, let us consider the simple network topology in the figure below.
Two multi-mode optical fiber, 2 km maximum
Two multi-mode or single mode optical fibers with lasers
Two pairs of category 3+ UTP
Two pairs shielded twisted pair, 25m maximum
Two types of encapsulation schemes were defined to support IP in Ethernet networks : the original encapsulation scheme, built above the Ethernet DIX format is defined in :rfc:`894` and a second encapsulation :rfc:`1042` scheme, built above the LLC/SNAP protocol [IEEE802.2]_. In 802.11 networks, the situation is simpler and only the :rfc:`1042` encapsulation is used. In practice, this encapsulation adds 6 bytes to the 802.11 header. The first four bytes correspond to the LLC/SNAP header. They are followed by the two bytes Ethernet Type field (`0x800` for IP and `0x806` for ARP). The figure below shows an IP packet encapsulated in an 802.11 frame.
Typical throughput
Virtual LANs
Virtual Local Area Networks in a switched Ethernet network
What is the Ethernet service ?
When a frame is sent from a WiFi device to a server attached to the same LAN as the access point, the first address of the frame is set to the MAC address of the access point, the second address is set to the MAC address of the source WiFi device and the third address is the address of the final destination on the LAN. When the server replies, it sends an Ethernet frame whose source address is its MAC address and the destination address is the MAC address of the WiFi device. This frame is captured by the access point that converts the Ethernet header into an 802.11 frame header. The 802.11 frame sent by the access point contains three addresses : the first address is the MAC address of the destination WiFi device, the second address is the MAC address of the access point and the third address the MAC address of the server that sent the frame.
When all switches boot, their `MAC address table` is empty. Assume that host `A` sends a frame towards host `C`. Upon reception of this frame, switch1 updates its `MAC address table` to remember that address `A` is reachable via its West port. As there is no entry for address `C` in switch1's `MAC address table`, the frame is forwarded to both switch2 and switch3. When switch2 receives the frame, its updates its `MAC address table` for address `A` and forwards the frame to host `C` as well as to switch3. switch3 has thus received two copies of the same frame. As switch3 does not know how to reach the destination address, it forwards the frame received from switch1 to switch2 and the frame received from switch2 to switch1... The single frame sent by host `A` will be continuously duplicated by the switches until their `MAC address table` contains an entry for address `C`. Quickly, all the available link bandwidth will be used to forward all the copies of this frame. As Ethernet does not contain any `TTL` or `HopLimit`, this loop will never stop.
When developing its family of standards, the `IEEE 802.11 working group <http://www.ieee802.org/11/>`_ took a similar approach as the `IEEE 802.3 working group <http://www.ieee802.org/3/>`_ that developed various types of physical layers for Ethernet networks. 802.11 networks use the CSMA/CA Medium Access Control technique described earlier and they all assume the same architecture and use the same frame format.
Where should the CRC be located in a frame ?
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.