Wireless Networks: Technology and Advantages

Wireless Network

Introduction What is a wireless network? A technology that enables two or more entities to communicate without network cabling

Differences between wireless and wired transmission Address is not equivalent to physical location Dynamic topology and restricted connectivity. Medium boundaries are not well defined. Error prone medium

Advantages of Wireless tech. Running additional wires or drilling new holes in a home or office could be prohibited and are too expensive Flexibility of location and data ports is required Roaming capability is desired; e.g., maintaining connectivity from almost anywhere Network access is desired outdoors; e.g., outside a home or office building

Wireless Network Electromagnetic radio waves are used for communication. These waves are characterized by their frequency(f),wavelength( ) Speed of propagation (c )=f x Speed of propagation varies from medium to medium except in vacuums. Radio waves are easy to generate and ability to pass through buildings and to travel long distance. Radio transmission are omnidirectional.

IEEE Standards subsets of Ethernet-based protocol standards: 802.11, 802.11a, 802.11b, 802.11g ,802.11n,802.11ac and 802.11ad

802.11 802.11a 802.11b 802.11g Operating Frequency 2.4 GHz 5 - 6 GHz 2.4 GHz 2.4 GHz Data rates 1 to 2 Mbps (PSK Modulation) 6 Mbps, 12 Mbps, or 24 Mbps (OFDM -54 Mbps) 11 Mbps complementary code keying (CCK) 54 Mbps Specification WLAN Wi-Fi 5 Wi-Fi offers the best features of both 802.11a and 802.11b No longer used backward compatible with the 802.11 backward compatible with the 802.11b Coverage Distance 400 feet 60 feet (need more access point)

802.11n 802.11n builds upon previous 802.11 standards by adding multiple-input multiple-output (MIMO). The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity through coding schemes like Alamouti coding. The real speed would be 100 Mbit/s (even 250 Mbit/s in PHY level), and so up to 4-5 times faster than 802.11g

802.11ac 802.11ac builds upon previous 802.11 standards, deliver data rates of 433Mbps per spatial stream, or 1.3Gbps in a three-antenna (three stream) design. The 802.11ac specification operates only in the 5 GHz frequency range . Support for wider channels (80MHz and 160MHz) and beam forming capabilities by default to help achieve its higher wireless speeds. 802.11ac Wave 2 :802.11ac Wave 2 is an update for the original 802.11ac spec that uses MU-MIMO technology other advancements to help increase theoretical maximum wireless speeds for the spec to 6.93 Gbps.

802.11ad 802.11ad is a wireless specification under development that will operate in the 60GHz frequency band Offer much higher transfer rates than previous 802.11 specs, with a theoretical maximum transfer rate of up to 7Gbps (Gigabits per second).

WLAN It is IEEE 802.11 standard. A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier. The last link with the users is wireless, to give a network connection to all users in a building or campus. The backbone network usually uses cables

IEEE 802.11 Components Access point (AP): A station that provides access to the DS. AP are interact with wireless as well wired node . It is connected at other network. Basic service set (BSS): A set of stations controlled by a single AP. Distribution system (DS): A system used to interconnect a set of BSSs to create an ESS. DS is implementation-independent. It can be a wired 802.3 Ethernet LAN, 802.4 token bus, 802.5 token ring or another 802.11 medium. Extended service set (ESS):Two or more BSS interconnected by DS Portal: Logical entity where 802.11 network integrates with a non 802.11 network.

Classification of WLANS Infrastructure network. Ad-hoc LANs

Infrastructure Infrastructure : it contains AP which are connected via existing n/w Mobile station(STA) communicate via APs. APs acts as a bridge with other network

Architecture Infrastructure : The wireless LAN connects to a wired LAN There is a need of an access point that bridges wireless LAN traffic into the wired LAN. The access point (AP) can also act as a repeater for wireless nodes.

Ad-hoc LAN Ad-hoc LAN: it do not need any fixed infrastructure. It can be setup at any place. Nodes communicate directly or forward messages through other node which is accessible.

Ad-Hoc Network Ad-Hoc Network: Group of computer can communicate directly without any access point. The physical size of the network is determined by the maximum reliable propagation range of the radio signals. Are self-organizing networks without any centralized control Suited for temporary situations such as meetings and conferences. image_1

Features of WLAN They use specialized physical and data link protocols. They integrate into existing networks through access points which provide a bridging function It allows to stay connected as you roam from one coverage area to another They have unique security considerations They have specific interoperability requirements They require different hardware They offer performance that differs from wired LANs.

Contd There are two types of terminals. Portable terminal. Mobile terminal(MT). Portable terminals are accessed only when they are stationary. Mobile terminals are accessed when they are in motion. WLAN can satisfy the requirements like mobility , relocation of users , ad-hoc networking, coverage of locations which are difficult to connect using wire.

Design Goals of WLAN Operational simplicity: Power efficient operation: License-free operation: Tolerance to interference. Global usability. Security Safety requirement Quality of service requirement Compatibility with other technologies and application.

Wireless Lan 802.11 In presence of base station. In absence of base station.

Frequency Frequency band which is available world wide. Range to be covered. Battery life and power consumed by the device. Computer mobility User privacy should be maintained. System should have enough bandwidth.

DIFFERENCE WLAN The WLAN devices are based on IEEE 802.11 family of standards. WLAN devices use high energy radio frequency waves to transmit the data. Radio frequency waves travel in the space. Hence a physical connection is not needed between the devices which are connected to the WLANs. WLAN uses half duplex mechanism for communication. WLANs suffer from interference of various types during travel from source to the destination. WLANs use CSMA/CA to avoid collisions in the network. ETHERNET LAN The Ethernet LAN devices are based on IEEE 802.3 standards. Ethernet LAN devices use electric signals to transmit the data. Electric signals flow over the cables. Hence wired connection is needed between devices which are connected to the Ethernet LANs. Ethernet supports full duplex mechanism for communication when a switch connects using a single device rather than hub. LANs suffer less interference as electric signals travel using cables. Ethernet LANs use CSMA/CD to detect collisions in the network.

Services offered by 802.11 Association: Reassociation: Disassociation Distribution Integration Authentication Deauthentication Privacy Data delivery.

Applications of WLAN User would be able to surf internet , check email, and receive instant message on the move. In areas affected by earthquake, WLAN is used to set up networks on the fly. WLAN is set up in many historical building. LAN Extension. Cross building Interconnection. Nomadic Access. Ad-hoc Network.

Physical layer IEEE 802.11 supports three options for the medium to be used at the physical layer. One based on infrared Other two based on radio transmission.

Physical Layer Physical layer is divided into two parts: Physical medium dependent sub layer.(PMD) Physical layer convergence protocol.(PLCP)

PMD It handles encoding ,decoding and modulation of signal. Thus deals with unusual change of particular medium

PLCP It abstracts functionality that the physical layer has to offer to MAC layer. It offers service access point(SAP) which is independent of transmission technology. Clear channel assessment(CCA) which senses signal to MAC layer.

1. Physical layer has 3 choices: Frequency hopping spread spectrum. FHSS(2.4 GHz) Industrial , scientific and medical Band ISM Data Rate=1 Mbps using 2 level Gaussian frequency shift keying modulation. Data Rate=2 Mbps using 4level GFSK. 2. Direct sequence spread spectrum DSSS(2.4 GHz) Data rate=1 Mbps using differential binary phase shift keying.(DBPSK) Data rate=2 Mbps using differential quadrature phase shift keying. (DQPSK) 3. Infrared operating at wavelength 850-950 nm range At data rate 1 Mbps/2 Mbps using Pulse Position Modulation(PPM)

FHSS It uses frequency band of 2.4 GHz The process of switching from one channel to other using pseudorandom sequence known to both transmitter and receiver is known as frequency hopping. E.g.

Direct Sequence Spread Spectrum It uses same frequency band(2.4 GHz). Each bit is converted into group of bits called as chip code. The time required to send each chip code is same as original bit. DSSS uses the BPSK or QPSK modulation technique. E.g.

OFDM It is multi carrier transmission mechanism. It splits available bandwidth into a number of frequency channel. Data signal is split into multiple smaller sub-signal that are transmitted to receiver simultaneously at different carrier frequency. By choosing appropriate frequency spacing between subcarriers, the subcarriers are made orthogonal to each other. Orthogonality ensures error free reception at the receiver It is used by 802.11a All frequency subbands are used by one source at a time. E.g.OFDM uses the 5 GHz band for it s operation ,this band is divided into 52 subbands. 48 subbands are used for sending 48 group of bits and 4 subbands are used for sending control information. It uses BPSK and QAM (quadrature amplitude modulation)

HR-DSSS IEEE 802.11 B uses HR-DSSS It uses frequency band of 2.4 GHz It is different from DSSS by encoding method. It uses complementary code keying(CCK) which encodes 4 or 8 bits of original data into one CCK symbol. It uses four data rates 1,2,5.5 and 11 Mbps. It uses BPSK(5.5) and QPSK(11) for modulation.

MAC Layer Primary function is to arbitrate and statistically multiplex the transmission requests of various wireless stations that are operating in an area. To avoid or reduce collisions, MAC layer supports for roaming , authentication and taking care of power consumption Basic services are Asynchronous data services: used for unicast or multicast packet Real time services: used in only in infrastructure based network where Aps control access to shared medium.

Distributed Foundation Wireless Medium Access Control(DFWMAC) Primary access method is through distributed coordination function(DCF). It is based on CSMA/CA. To avoid hidden terminal problem additional RTS- CTS mechanism is implemented. Point coordination function is implemented to provide real time services. When PCF is in operation , the AP controls medium access and avoids simultaneous transmission by the nodes.

Inter Frame Spacing(IFS) IFS refers to the time interval between the transmission of two successive frames by any station. Types of IFS Short Inter Frame spacing(SIFS) PCF Inter Frame Spacing(PIFS) DCF Inter Frame Spacing(DIFS) Extended Inter Frame Spacing(EIFS)

SIFS It is shortest of all IFS and denotes highest priority to access the medium. It is defined for short control message such as acknowledgement and polling responses. Transmission of any packet begin only after channel is sensed to be ideal for a minimum time period of at least SIFS.

PIFS It is waiting time whose value lies between SIFS and DIFS This is used for real time services.

DIFS It is used by station that are operating under DCF mode to transmit packet. This is for asynchronous data transfer within contention period.

EIFS It is longest of all the IFS and denotes the least priority to access the medium. It is used resynchronization whenever physical layer detects incorrect MAC frame reception.

CSMA/CD Mechanism It is well studied technique in IEEE 802.x wired Lan. But it can not be used in context of WLAN effectively because the error rate in WLAN is much higher and allowing collision will lead to a drastic reduction in throughput. Detecting collision in wireless medium is not always possible so in WLAN collision avoidance is used.

Distributed Coordination Function (DCF). This is a random access scheme, based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. Retransmission of collided packets is managed according to binary exponential backoff rules Point Coordination Function (PCF), which is a centralized MAC protocol able to support collision free and time bounded services

DCF Techniques DCF describes two techniques to employ for packet transmission. A two-way handshaking technique called Basic Access mechanism. A four way handshaking technique, known as Request-To-Send/Clear-To-Send (RTS/CTS) mechanism.

Contd A station with a new packet to transmit monitors the channel activity If the channel is idle for a period of time equal to a Distributed InterFrame Space (DIFS), the station transmits. Otherwise, if the channel is sensed busy (either immediately or during the DIFS), the station persists to monitor the channel until it is measured idle for a DIFS. At this point, the station generates a random backoff interval before transmitting (this is the Collision Avoidance feature of the protocol), to minimize the probability of collision with packets being transmitted by other stations. In addition, to avoid channel capture, a station must wait a random backoff time between two consecutive new packet transmissions, even if the medium is sensed idle in the DIFS time.

Contd For efficiency reasons, DCF employs a discrete-time backoff scale. The time immediately following an idle DIFS is slotted, and a station is allowed to transmit only at the beginning of each Slot Time. The Slot Time size is set equal to the time needed at any station to detect the transmission of a packet from any other station.

DCF adopts an exponential backoff scheme. At each packet transmission, the backoff time is uniformly chosen in the range (0, -1). The value is called Contention Window, and depends on the number of transmissions failed for the packet. At the first transmission attempt, is set equal to a value Cwmincalled minimum contention window. After each unsuccessful transmission, is doubled, up to a maximum value CWmax = 2Cwmin

Basic Access method This mechanism is characterized by the immediate transmission of a positive acknowledgement (ACK) by the destination station, upon successful reception of a packet transmitted by the sender station. Explicit transmission of an ACK is required since, in the wireless medium, a transmitter cannot determine if a packet is successfully received by listening to its own transmission. It is two-way handshaking technique to be optionally used for a packet transmission.

Two stations A and B share the same wireless channel. At the end of the packet transmission, station B waits for a DIFS and then chooses a backoff time equal to 8, before transmitting the next packet. We assume that the first packet of station A arrives at the time indicated with an arrow in the figure. After a DIFS, the packet is transmitted. Note that the transmission of packet A occurs in the middle of the Slot .Time corresponding to a backoff value, for station B, equal to 5. As a consequence of the channel sensed busy, the backoff time is frozen to its value 5, and the backoff counter decrements again only when the channel is sensed idle for a DIFS. Since the CSMA/CA does not rely on the capability of the stations to detect a collision by hearing their own transmission, a positive acknowledgement (ACK) is transmitted by the destination station to signal the successful packet reception.