Wireless LAN Basics

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The 802.3 family is for Ethernet LANs and the 802.11 family is for WLANs. Both standards define a frame format with a header and a trailer, with the header including a source and destination MAC address field, each 6 bytes in length. WLANs use radiated energy waves, generally called radio waves, to transmit data, whereas Ethernet uses electrical signal flowing over a cable (or light on optical cabling). If two wireless devices send radio waves in the same space as the same frequency, neither signal is intelligible, so a half – duplex (HDX) mechanism must be used. WLANs use the carrier sense multiple access with collision avoidance (CSMA/CA) algorithm to enforce HDX logic.

Wireless-capable devices communicate with a connector WLAN device called an access point (AP) to send and receive frames. APs connect WLAN clients (connecting devices) with Ethernet LAN or internet as implemented.

Various international organizations are involved in WLAN standardization as

Organization	Role
IEEE	Standardization of wireless LANs (802.11)
Wi-Fi Alliance	Industry body for interoperable products
ITU-R	Standardization of radiated energy communication

IEEE introduced WLAN standards with the creation of the 1997 ratification of the 802.11 standard which was replaced by more-advanced standards of 802.11b, 802.11a, and 802.11g as per order of ratification. The WLAN standards are described in the table below as

Feature	802.11 a	802.11 b	802.11 g
DSSS speed		11 Mbps	11 Mbps
OFDM speed	54 Mbps		54 Mbps
Non-overlapping Channels	12	3	3
Total Channels	23	11	11
Frequency Bands	5 GHz	2.4 GHz	2.4 GHz

WLANs work in either of two modes as

• Ad Hoc Mode – A wireless device communicates directly with another for a short period.
• Infrastructure Mode – Each device communicates with an AP, which connects to a wired Ethernet.

Devices in infrastructure WLAN sends frames to the AP and not directly to each other and hence, also called as the Independent Basic Service Set (IBSS). Infrastructure mode supports service sets

• Basic Service Set (BSS) – uses a single AP to create the WLAN and users move inside the coverage area.
• Extended Service Set (ESS) – uses more than one AP, with overlapping cells to allow roaming in a larger area.

Wireless Encoding and Nonoverlapping DSSS Channels

Signaling is important in WLANs as they use radio waves and transmit data at layer 1. The radio waves have a repeating signal that can be graphed over time. The graph shows characteristics as

• Frequency – Number of times the waveform repeats per second and measured in Hertz(Hz).
• Amplitude – the height of the waveform, representing signal strength.
• Phase – the particular point in the repeating waveform.

WLAN uses unlicensed frequencies. Various encoding techniques are used by WLANs as

• Frequency Hopping Spread Spectrum (FHSS) uses all frequencies in the band, hopping to different ones. By using slightly different frequencies for consecutive transmissions, a device can hopefully avoid interference from other devices that use the same unlicensed band, succeeding at sending data at some frequencies.
• Direct Sequence Spread Spectrum (DSSS) followed as the next general class of encoding type for WLANs. Designed for use in the 2.4 GHz unlicensed band, DSSS uses one of several separate channels or frequencies. This band has a bandwidth of 82 MHz, with a range from 2.402 GHz to 4.83 GHz.
• Orthogonal Frequency Division Multiplexing (OFDM) can use multiple nonoverlapping channels similar to DSSS or FHSS.

Wireless Interference and Coverage Area

With the nonoverlapping channels, each half-duplex BSS can run at 11 Mbps, for a cumulative bandwidth of 33 Mbps in this case. This cumulative bandwidth is called the WLAN’s capacity. Signal-to-Noise Ratio (SNR) measures interference by measuring the WLANs signal as compared to the other undesired signals (noise) in the same space. Higher SNR indicates better WLAN signal and less noise.

A WLAN coverage area refer to the physical space in which two WLAN devices can successfully send data but is dependent on various factors as

• Transmit power of an AP or WLAN NIC is limited to a particular level due to regulations.
• Materials and location of the materials placed near the AP impacts the coverage area due to reflections and scattering by metal objects which reduces the coverage area.
• Weaker wireless signals cannot pass data at higher speeds, but lower speeds.

Specialized antennas and increasing the transmitted signal power enhances the size of coverage area of AP.

Wireless Media Access (Layer 2)

WLAN devices uses modified CSMA/CD called as CSMA/CA algorithm as the sending WLAN device cannot tell if its transmitted frame collided with another frame or not thus, requiring an acknowledgement for every frame sent. Each WLAN device listens for the acknowledgement, which should occur immediately after the frame is sent. If no acknowledgement is received, the sending device assumes that the frame was lost or collided, and it resends the frame.

The CSMA/CA algorithm is summarized as

• Listen to ensure that the medium is not busy (no waves currently being received at the frequencies being used).
• Set a random wait timer before sending a frame to statistically reduce the chance of devices all trying to send at the same time.
• When the random timer has paused, listen again to ensure that the medium is not busy. If not busy, send the frame.
• After the frame is sent, wait for acknowledgement.
• If no acknowledgement is received, resend the frame, using CSMA/CA logic to wait for the appropriate time to send again

WLAN deployment is similar to LAN deployment but, has differences due to the media type being used. In an ESSS deployment due to multiple APs, all the Ethernet switch ports to which the APs attach should be in the same VLAN. The SSID is a 32-chracter ASCII text value to specify an AP. When configuring an ESS WLAN, each of the APs should be configured with the same SSID, which allows for roaming between APs, but within the same WLAN. Wireless APs operate at Layer 2 hence, does not need an IP address though, IP address makes easier management. APs use a straight-through Ethernet cable to connect to LAN switch.

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