Connect ethernet interface to a wireless network

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Going Wireless

Sometimes you don’t want to use cables at all. This book’s focus is Ethernet, which by definition is a wired interface. It’s possible, however, to connect a system with an Ethernet interface to a wireless network. The term wireless Ethernet usually refers to a network that follows one of the IEEE 802.11 standards. The main 802.11 standard, like Ethernet’s 802.3 standard, specifies a physical layer and a method of media-access control for networking. The physical layer may use radio-frequency (RF) transmissions in the 2.4 Gigahertz frequency band or infrared transmissions. Both allow transmitting data at 1 or 2 Mb/s. A variety of supplements to the standard describe additional options for the physical layer at higher speeds. A popular standard for wireless networks has been the 802-11b supplement, which describes the interface known as Wi-Fi, for wireless fidelity. An 802-11b interface transmits at up to 11 Mb/s in the 2.4 Gigahertz band. The 802-11g supplement approved in 2003 enables transmitting at 54 Mb/s in the same band and can fall back to the 802-11b rate when needed. Networks that use the physical layer described in the 802-11a supplement transmit at up to 54 Mb/s in the 5 Gigahertz band. The easiest way to connect a device with an Ethernet interface to a wireless network is to use a wireless access point. The access point has an 802-11b or other wireless interface and connects via a cable to an interface that wants to communicate over the wireless network. Initial configuration of the access point typically requires a PC, but once the access point is configured, the network administrator can usually change the configuration via a Web page hosted by the access point.

Media Systems

The media systems defined in the IEEE 802.3 standard each use a particular cable type at a specific network speed. So for example, a 10-Mb/s twisted-pair network uses a different media system than a Fast Ethernet twisted-pair network or a 10-Mb/s fiber-optic network. For each media system, the standard specifies the electrical characteristics, signaling protocol, and methods of connecting to an interface. Some of the media systems defined in the standard are rarely used. This networking tutorial focuses on the more popular ones, which Table 2-4 lists. The standard uses a system of identifiers to distinguish the media systems. Each identifier has three elements. The first number is the network speed in Megabits per second (10, 100, 1000) or bits per second (10G). Then the word BASE or BROAD indicates the type of signaling used, baseband or broadband. All of the popular media systems use baseband signaling, which means that the cable carries only Ethernet data and signaling. A broadband media system carries multiple types of data and signaling. The final value identifies either the cable type or the maximum length of a cable segment. In more recently defined media systems, the value indicates the cable type. For example, in the identifier 10BASE-T, the T signifies twisted-pair cable. The maximum cable length per segment as specified by the standard is 100 meters, but the identifier doesn’t contain this information. In identifiers defined earlier in Ethernet’s history, the third value indicates maximum cable length. For example, in the identifier 10BASE-5, the maximum segment length is 500 meters. The media type is coaxial cable, but the identifier doesn’t contain this information.

Encoding

One of the characteristics specified by the media system is how the data is encoded for transmitting. The encoding method helps to ensure that the data reaches its destination without errors. The encoding defines what voltages or light levels correspond to different values. In addition, for some media systems, the encoding defines code symbols to represent groups of bits. The controller chip’s hardware handles the encoding and decoding. You don’t need to understand how the encoding works to design and program a network, so this networking tutorial includes only brief descriptions of the methods and the reasons for their use.

Block Encoding

Fast Ethernet and Gigabit Ethernet media systems use methods of block encoding. Block encoding groups bits to be transmitted into blocks that are typically 4 or 8 bits each, then converts each block into a set of bits called a code symbol. Block encoding can ensure frequent transitions in transmitted data, to help in keeping the transmitting and receiving interfaces synchronized. Another advantage is the availability of additional code symbols. After assigning code symbols to all of the possible groups of bits, extra symbols remain. A protocol can specify any use for these, and typically uses the symbols to provide status or control information.

A code symbol is longer than the bits it represents. In the 4B/5B block encoding used in Fast Ethernet, a 5-bit code symbol represents 4 data bits. Each code symbol contains one or more transitions. In the 8B/10B block encoding used in fiber-optic Gigabit Ethernet, a 10-bit code symbol represents 8 data bits. In both cases, the extra bits increase the maximum transition rate in the cable by 25 percent. In addition to ensuring that there are sufficient transitions, 8B/10B encoding attempts to maintain a DC balance by ensuring that the number of transmitted zeros is roughly the same as the number of transmitted ones over time. The encoding accomplishes this by assigning two possible code symbols to some 8-bit values, with the two symbols containing different numbers of zeros and ones. The transmitter maintains a disparity value that is a measure of the number of transmitted zeros versus ones over time. When the ratio of transmitted zeros to ones gets out of balance, the transmitter switches to the set of code symbols that will restore the balance. Twisted-pair Gigabit Ethernet uses block encoding along with pulse amplitude modulation and transmitting on all four wire pairs at once. The result is an interface that can transmit at a very high bit rate without requiring a very high bit rate in the cable. The code symbols are 5 bits, with each symbol representing 2 bits.