Wireless Networks

Wireless networks are required when devices are mobile, users are nomadic and temporary connections are needed.

Terminology

Base Station

A wireless access point that provides connectivity to a wireless network.

BSS

Short for Basic Service Set. The smallest building block of a WLAN. A set of stations executing same MAC protocol and competing for same shared wireless medium.

DS

Short for Distribution System. A system used to interconnect a set of BSSs and integrated LANs to create an extended service set (ESS).

IBSS

Short for Independent BSS. Aka. ad hoc network. A BSS that forms a self-contained network, and in which no access to a DS is available. No infrastructure to connect to. Nodes themselves provide services.

Challenges

Wireless networks face more challenges compared to wired networks.

• Finding location of nodes
• Services (Addressing, Routing, etc.)
• Path loss, Fading, Interference
• Multipath propagation

Handoff

Change of attachment from one base station to another.

Hidden Terminal Problem

When two or more stations are within range of the base station, but not of each other. Causes them to transmit data in parallely.

Spread spectrum

A technique used in wireless networks to transmit signals over a wider frequency band than the required minimum.

Helps reduce interference, improve resistance to eavesdropping, and enhance reliability in environments with noise and multipath effects.

FHSS

Short for Frequency Hopping Spread Spectrum. A spread spectrum technique that uses a pseudo-random sequence of frequencies to transmit data. The transmitter hops between frequencies at a high rate, making it difficult for an attacker to intercept the signal. Spreading code defines the order of frequencies that are used.

Used in Bluetooth.

DSSS

Short for Direct Sequence Spread Spectrum. Each bit in the original signal is represented by multiple bits in the transmitted signal using a spreading code.

Spreading code is a pseudo-random sequence of bits. The data bits are XOR-ed by the spreading code to produce a chip sequence. Received signal is de-spread by XOR-ing with the same spreading code.

Used in Wi-fi.

CDMA

Short for Code Division Multiple Access. A channel access method used in wireless communication. Allows several transmitters to send information simultaneously over a single communication channel.

Each bit is encoded with a unique chipping code. Multiple signals are transmitted over the same frequency band. Receiver decodes the signal using the relevant chipping code.

Standard

Standardized by IEEE 802.11. There are many versions of this standard such as IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, and so on.

Frame Structure

Frame header includes 4 addresses:

• Address 1: MAC address of the wireless receiving station
• Address 2: MAC address of the wireless transmitting station
• Address 3: MAC address of the (router) interface via which the BSS connects to other networks
• Address 4: for Ad-hoc mode of operation

Media Access Control

DFWMAC

Short for Distributed Foundation Wireless MAC.

Uses Distributed Coordination Function (DCF) for distributed access control and Point Coordination Function (PCF) for centralized control.

Examples:

• DCF: CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)
• PCF: Polling by Centralized Polling Master