Wireless Protocols

Wireless Protocols

Jason D. Knight

As with all data transmission technologies, protocols govern the manner in which information is transferred between stations. A protocol is simply an agreed upon standard that all parties use to ensure that different devices can communicate with each other. Wireless protocols can be discussed on their own, and where they fit in the OSI protocol suite. For the sake of clarity, we will limit our discussion to wireless network implementations.

Wireless protocols typically reside in layers 1 and 2, the application and presentation layers, respectively, of the OSI model. These two layers help direct how the data is gathered from the end station and prepared for transmission.

The main wireless protocol are: IEEE 802.11 covering wireless Ethernet; 802.15 dealing with wireless personal area networks (WPAN), including Bluetooth technology; and 802.16 for broadband wireless access.

802.11

The IEEE 802.11 specification family consists of four different, primary specifications:

* 802.11 - applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).

* 802.11a - an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.

* 802.11b (also referred to as 802.11 High Rate or Wi-Fi) - an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.

* 802.11g - applies to wireless LANs and provides 20+ Mbps in the 2.4 GHz band.

* 802.11n - applies to future standards for wireless data transmission that promises a maximum bandwidth of 108 Mbps through the use of multiple antennas (MIMO).

Strangely, the order of inception is not alphabetical. Rather, the specifications developed as follows: 802.11; 802.11b; 802.11a; 802,11g; and the projected 802.11n. The 802.11 family specifies the way in which wireless devices communicated with wired LAN base stations, or access points. Wireless-to-wired communication is known as infrastructure mode. A second mode, known as ad hoc mode, specifies the manner in which direct communication between wireless devices occurs.

Three of the four current specifications use the 2.4 GHz band. This is an unlicensed bandwidth, which means it is available free of charge to anyone with the technical to use it. The three specifications operating at 2.4 Ghz are also backwards compatible, although transmission speed between two devices with differing bandwidth is limited to the speed rating of the slowest appliance.

The remaining current protocol, 802.11a, operates in the 5 GHz spectrum and provides the greatest transmission rate, up to 54 Mbps. One other advantage provided by 802.11a comes as a result of the way in which the transmission spectrum is divided into channels: where b and g use overlapping channels resulting in only three interference-free channels out of eleven possible channels, a provides eleven channels. However, it is unable to communicate with any appliances using any of the other specifications.

Review of the 802.11 family reveals several methods by which more than one signal can be sent over each channel: FHSS, DSSS, and OFDM. FHSS, or frequency hopping spread spectrum, requires synchronized transmitting and receiving stations. The stations, though using many frequencies, maintains only one logical channel. Since the FCC regulations require no less than 75 frequencies per transmission channel, a single channel could have 75 or more discrete channels within a single transmission channel.

DSSS, or direct station spread spectrum, combines the signal with a higher data-rate bit sequence, called chipping. The chipping creates a redundant signal, which is much more resistant to data loss from interference.

OFDM, or orthogonal frequency division multiplexing, is used to transmit large amounts of data. A single carrier is split into many smaller sub-carriers that are transmitted simultaneously over different frequencies. This transmission method also reduces crosstalk, or electromagnetic interference.

Some other specifications in the 802.11 family are: