Wireless technology has developed a large way from its infancy and is fast becoming the latest trend in communication. Wireless LANs have added an entirely new dimension to the communication sector. The advent of cellular technology, infrared and spread spectrum radio services have bought about a huge change to our world of communication and wireless technology has materialized our dream of a mobile workforce. Today there is no single standard in the industry and there are different wireless technologies available. Let us have a brief outlook of the 'IEEE 802.11' and 'Bluetooth', two of the important wireless LAN technologies.
IEEE 802.11 Protocol
The IEEE 802.11 (First adapted in 1997) is primarily a standard developed to provide time bound and asynchronous wireless services. The MAC layer is meant to handle different physical layers with distinct medium and transmission characteristics. The standard uses 2.4 GHz ISM band and a minimum data transfer rate of 1 Mbits / s. IEEE 802.11 specifies the physical and the medium access layer standards in particular to the wireless LAN's. IEEE 802.11 has seamless integration with other standards for wired networks. The Data link layer and the Logical Link layer control other aspects pertaining to different forms of the media. The MAC layer is concerned with three main tasks namely medium access, data fragmentation and encryption while the Physical layer takes care of the modulation, encoding and decoding of the signals. Let us study these two layers in a little detail.
The protocol supports three types of physical layers based on two kinds of radio and one infrared transmission. The protocol includes channel detection provision, which (CCA) is used for medium control.
Frequency Hopping Spread Spectrum
IEEE 802.11 includes Frequency hopping spread spectrum technique whereby communication on one channel of radio spectrum is transferred to another channel as data is transferred through the network. [Raymond P. Wenig, 57] Currently for a 1 Mbit data transfer a 2 level FSK (Frequency Shift Keying) is used as the standard. Each data frame in the physical layer consists of two main parts namely the physical layer convergence protocol (preamble and the header) and the payload. The PLCP is always transmitted at a standard speed of 1 Mbits/s while data part can vary from 1 Mbit/s to 2 Mbit/s. An 80-bit Synchronization pattern is used to synchronize the receivers with the transmitter once the receiver gets a line clear signal from the CCA. (Clear channel assessment signal). This is followed by a start frame delimiter (16 bits), which helps to synchronize the individual frames of data. The next part PLW (PLCP_PDU length word) contains information regarding the size of the payload. Next part of the frame is the PSF field (PCLP signaling Field), which indicates the actual data speed of the payload. (Either 1 or 2 Mbit/s). Furthermore data is scrambled using the polynomial equation s (z) = z^7 +Z^4+1. The field immediately before the actual payload is the HEC (Header Error Check) [Jochen Schiller, 171]
Direct Sequence Spread Spectrum
This is another form of spectrum wherein modulation is achieved by codes and not by frequency. Instead of the frequency shift keying the DSSS system uses the binary phase shift keying system for 1 Mbit/s data transfer and differential quadrature phase shift keying for 2 Mbit/s data transfer. Each data frame consists of seven parts with the first part being a 128 bit-synchronizing pattern followed by a 16-bit frame delimiter and an 8-bit stream indicating the data rate. (1 Mbit/s or 2 Mbit/s). The other fields are the Service, Length (length of the data or payload) and the HEC. [Jochen Schiller, 172]
The third type of transmission based on infrared light uses visible light in the range of 850 to 950 nm. The infrared transmission is of very short-range (10 m) and is primarily used for transmission within a confined area.
MAC Layer of IEEE 802.11
The MAC layer is responsible not only for medium access but also for managing power control, roaming and authentication services. In 802.11 there are three main types of medium access methods namely the CSMA/CA, a modified form of CDMA/CA with RTS and CTS, and a contention free polling method. In the CSMA / CA method if the medium is sensed busy the nodes have to wait for the longest waiting time (DIFS) before entering the contention phase. Each of the nodes wait for a random backoff time within its contention window. If a node does not have access to the medium during a particular cycle it stops its backoff timer and waits for the channel to be idle once gain. Once the channel is sensed to be idle for entire DIFS, the node starts the backoff counter again (as left from the previous cycle) and accesses the medium once the counter expires.
This way the nodes, which are waiting the longest duration enjoy high priority over other nodes, which are just entering the contention. If there is any data collision transmission is started again with a new randomly selected backoff time. Based on the collision the contention window doubles up. However the size of the contention window and access delays are interdependent and the smaller the contention window the lesser the access delays. [Daniel L. Lough]
CSMA with RTS and CTS
This is an improved form of the original CSMA / CA with special 'Request to send' and 'Clear to send' message control packets. Including these control functions improves the overall efficiency of the protocol because it eliminates the problem of hidden terminals. (A condition wherein two nodes start sending to a particular receiver without any idea about transmission state of each other leading to data collision at the receiving node). To overcome this problem each node, after waiting for the DIFS and the random backoff period, starts transmission with a RTS (request to send) signal to the intended recipient node. Actual data transmission occurs only after receiving the CTS ('Clear To Send' transmitted to all nodes within the range of the sender) from the receiver. In this way the transmission is collision free except for the possible collisions of the RTS packets.
Bluetooth (1998) was the outcome of the joint effort of five companies namely IBM, Ericsson, Intel, Nokia and Toshiba. Bluetooth represents a single chip, radio-based wireless network, which considerably reduces the cost factor involved in a typical wireless infrastructure. Bluetooth is the latest wireless technology that gives us total freedom from the system tethers associated with wired networks and infrastructure-based wireless networks. It is expected that 'Bluetooth technology' will in the near future eliminate cable connectivity and enable wireless link between the different communication gadgets such as computers, cell phones, printers, etc. By using the radio communication bluetooth technology enables communication between the different portable communication devices thereby totally eliminating the use of cables. This is made possible by embedding small radio chips called 'Bluetooth chips' inside the gadgets. This way, data which is transmitted by the blue chip in the transmitting gadget, can be deciphered back by the blue chip at the receiving gadget. [Bluetooth Solutions]
Bluetooth uses frequency band in the range of 2.4 GHZ and uses frequency hopping as well as time-division duplex scheme of data transmission. Typically a hopping rate of 1,600 hops per second is used. Bluetooth protocol uses 79 equally spaced hop carriers. Since Bluetooth devices are entirely battery dependent they cannot remain in the active transmit mode all the time. Thus each device goes into a standby mode and checks for messages periodically. (Every 1.28s). Devices listen to wake up carriers and visit the hop carriers corresponding to the wake up carrier. If there is correlation with the incoming signal the device is automatically activated. At any given time a bluetooth device can be in either the SNIFF, HOLD or the PARK states respectively. (with different levels of power consumption) [Jochen Schiller, 206]
Bluetooth uses the polling and reservation scheme of controlling the medium access within a piconet. (Devices using the same hopping sequence). In a piconet there can be only one master device and all others are slave devices. Bluetooth offers both synchronous and asynchronous wireless links. Each packet of data has three parts. The first part is the 72-bit access code, which is based on the master device identity. Since the access code is common for the particular piconet only those messages that carry the access code are accepted by the devices and the rest are just ignored. The packet header (54 bit) follows the access code and has a 3-bit MAC address. Each packet header also contains a FEC (Forward Error Correction code) which ensures data integrity.
To handle any transmission problems there is also a provision for requesting retransmission. An automatic request scheme (ARQ) makes sure that data is transmitted back if there is a negative acknowledgement from the receiver. By allowing individual devices to be part of different piconets (scatternet) at any given instance of time the overall efficiency…
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