→ Wireless LANs are use high frequency radio waves instead of cables for connecting the devices in LAN. Users connected by WLANs can move around within the area of network coverage. Most WLANs are based upon the standard IEEE 802.11 or WiFi.
→ A wireless access point (WAP), or more generally just access point (AP), is a networking hardware device that allows other Wi-Fi devices to connect to a wired network.
→ Wireless access point in 802.11 WLAN services are association, association and Integration.

Given data,
-- 4 channels are multiplexed using TDM
-- Multiplex 1 byte per channel. So, total 4 bytes=4 channels.
-- Each channel sends 100 bytes/sec
-- Bit rate=?
Bit rate= 100 * 4 bytes(1 byte=8 bits)
= 100 * 4 *8 (we are converting into bits because we are finding bit rate)
= 3200 bps

-- Mobile phone system with hexagonal( equals to 6) cells
-- 840 frequencies are available.
-- How many can be used in a cell?
In this we need maximum 3 unique cells are required.

Total cells used with frequency is = 840/3
= 280 frequencies

Option-A:​ This is the maximum number of distinct tuples that you can form with p distinct values per dimensions.
Option-B:​ You need at least p tuples to contain p distinct values per dimension. In this case no tuple shares any value on any dimension.
Option-C:​ The minimum number of cells is when each cuboid contains only p cells, except for the apex, which contains a single cell.
Option-D:​ We have p+1 because in addition to the p distinct values of each dimension we can also choose ∗.

Multiplexing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. It can share a medium without interference.

The cluster size N = 7 and the frequency reuse factor is 1/7 since each cell contains one-seventh of the total number of available channels.
Total cells=7*3
=21
Frequency reuse concept, cells with the same letter use the same set of frequencies. A cell cluster is outline in blue color and replicated over the converge area.

→ Interim Standard 95 (IS-95) was the first ever CDMA-based digital cellular technology.
→ The IS-95 standards describe an air interface, a set of protocols used between mobile units and the network.
→ IS-95 is widely described as a three-layer stack, where L1 corresponds to the physical (PHY) layer, L2 refers to the Media Access Control (MAC) and Link-Access Control (LAC) sublayers, and L3 to the call-processing state machine.

→ In CDMA, multiple users share a 1.25 MHz channel by using orthogonal spreading codes (Walsh codes)
→ IS-95a standard designed for AMPS cellular band
→ Each cellular provider is allocated 25 MHz spectrum and ten 1.25-MHz CDMA duplex channels if A AMPS Band provider, 9 if B band provider

There are 2 popular GSM methods.
1. GSM-900
2. GSM-1800
GSM-900 uses 890 - 915 MHz to send information from the Mobile Station to the Base
Transceiver Station (uplink) and 935 - 960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used.

The transmission signal coding method of T1 carrier is called Bipolar. Bipolar encoding is a type of return-to-zero (RZ) line code, where two nonzero values are used, so that the three values are +, −, and zero. Such a signal is called a duobinary signal.

In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal. The most natural example of frequency-division multiplexing is radio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time.
Frequency Division Multiple Access(FDMA):
FDMA is simply another example of the familiar data and voice transmission technique called FDM. This technique is used to allocate small portions of a large bandwidth (500MHz for satellite transponders) to individual users. For instance, a telecommunications common carrier in a particular country like Brazil. It might want 132 voice grade channels for sending voice and analog coded data to various other countries. The bandwidth required on the current international satellite systems for this many channels is 10MHz. Because 1 transponder has a bandwidth of 500Mhz, it could accommodate 50 users, each requiring 132 channels. The Brazilian user might be allocated the frequency band between 5990 and 6000MHz for the uplink to the satellite and the corresponding downlink frequencies would be 3765 to 3775Mhz.

Propagation methods are 3 types
1. Ground
2. Sky
3. Line of sight

GPRS → An emerging wireless service that offers a mobile data
GSM → An integrated digital radio standard
UMTS → The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for networks based on the competing cdmaOne technology. UMTS uses wideband code division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators.
EDGE → Enhanced GPRS or EGPRS. It is a data system used on top of GSM networks. It provides nearly 3 times faster speeds than the outdated GPRS system. Nine different schemes for modulation and error correction.

GSM Architecture:

→Mobile Station (MS)
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
→Base Station Subsystem (BSS)
Base Transceiver Station (BTS)
Base Station Controller (BSC)
→Network Switching Subsystem (NSS)
Mobile Switching Center (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)

The VLF and LF bauds use ground propagation for communication.
Propagation methods are 3 types
1. Ground
2. Sky
3. Line of sight

→ Very low frequency or VLF is for radio frequencies (RF) in the range of 3 to 30 kilohertz (kHz), corresponding to wavelengths from 100 to 10 kilometers, respectively.
→ The low-frequency signals travel in all directions from the transmitting antenna.

Global Positioning System(GPS) is a satellite based tracking system that enables the determination of person’s position.

→ The amount of uncertainty in a system of symbol is called entropy. → Generally we can call entropy is disorder or uncertainty.

The frequency reuse factor is related to cluster size then the frequency reuse factor=1/N

Handoff is the mechanism that transfer an ongoing call from one base station to another.
→ Hard handover (or) Hard Handoff: Early systems used hard handoff. In a hard handoff, a mobile station only communicates with one base station. When the MS moves from one cell to another, communication must first be broken with the previous base station before communication can be established with the new one.
→ Soft handover (or) Soft Handoff: New systems use a soft Handoff. In this case, a mobile station can communicate with base stations at the same time. This means that, during handoff, a mobile station may continue with the new base station before breaking off from the old one.

A telephone conference call is an example of same time / different place type of communications.

We can easily differentiate by using name itself
→ Binary phase-shift keying (BPSK) possible phases are 00 and 1800.
→ Quadrature phase-shift keying (QPSK) possible phases are 00, 900, 1800 and 2700.
→ Minimum Shift keying(QSM), the two carrier waves of the same frequency are out of phase with each other by 90°, a condition known as orthogonality and as quadrature.
→ Quadrature Amplitude Modulation(QAM) mapping changes in continuous phase. Each bit time, the carrier phase changes by ±90°

A smart modem can dial, hangup and answer incoming calls automatically. Communication software provides the appropriate instructions to the modem for this purpose.

→ When a mobile telephone physically moves from one to another cell, the base station transfers ownership to the cell getting strongest signal. This process is known as handoff.
→ Hard handover (or) Hard Handoff: Early systems used hard handoff. In a hard handoff, a mobile station only communicates with one base station. When the MS moves from one cell to another, communication must first be broken with the previous base station before communication can be established with the new one.
→ Soft handover (or) Soft Handoff: New systems use a soft Handoff. In this case, a mobile station can communicate with base stations at the same time. This means that, during handoff, a mobile station may continue with the new base station before breaking off from the old one.

The technology is used for communication systems such as telemetry, weather balloon radiosondes, caller ID, garage door openers, and low frequency radio transmission in the VLF and ELF bands.

The threshold effect in demodulator is the rapid fall on output signal to noise ratio when the input signal to noise ratio fall below a particular value.

To employ multi-access in GSM, users are given different frequency bands.

→ GSM offers very low speed data service because mobility management in wide area network is the main focus in GSM.
→ GSM/CDMA systems are predominantly used for voice.

Cellular telephone systems can handle a multitude of users because cellular telephone systems permit extensive frequency reuse in a small local area.

In a mobile communication system, a geographic region is divided into cells. For each frequency set, there is a buffer two cells wide where that frequency is not used.

→ TDM:
1. TDM stands time division multiplexing.
2. Cross talk like problem not occurs in TDM.
3. TDM requires channel synchronization
4. TDM has greater noise immunity.
→ TDM is a non-priority peer/peer protocol.

→ TDMA sets cellular apart from all preceding mobile/radio systems.
→ TDM:
1. TDM stands time division multiplexing.
2. Cross talk like problem not occurs in TDM.
3. TDM requires channel synchronization
4. TDM has greater noise immunity.
→ TDM is a non-priority peer/peer protocol.

The Unlicensed National Information Infrastructure (U-NII) radio band is part of the radio frequency spectrum used by IEEE 802.11a devices and by many wireless ISPs. It operates over following ranges:

Cellular CDMA systems can use N=1(it means the same frequency is used in all cells)

Quadrature Amplitude Modulation means changing both amplitude and phase of the carrier.

Line coding→ Process of converting digital data to digital signal

Block coding→ Provide redundancy to ensure synchronization and inherits error detection

Scrambling→ Provides synchronization without increasing number of bits

Pulse code modulation→ A technique to change analog signal to digital data

Transmission rate over a noisy channel:

→ H(X) is the amount of information per symbol at the input of the channel.

→ H(Y) is the amount of information per symbol at the output of the channel.

→ H(X|Y) is the amount of uncertainty remaining on X knowing Y.

The information transmission is given by:

I(X;Y) = H(X)−H(X|Y) bits/channel use

For an ideal channel X = Y, there is no uncertainty over X when we observe Y. So all the information is transmitted for each channel use: I(X;Y) = H(X) If the channel is too noisy, X and Y are independent. So the uncertainty over X remains the same knowing or not Y, i.e. no information passes through the channel: I(X;Y) = 0.

Capacity of a noisy channel:

JPEG( Joint Photographic Experts Group) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality
MPEG (Moving Picture Experts Group): It is a method of lossy compression for coding audio-visual information (e.g., movies, video, music).
Arithmetic coding is a form of entropy encoding used in lossless data compression. Normally, a string of characters such as the words "hello there" is represented using a fixed number of bits per character, as in the ASCII code. When a string is converted to arithmetic encoding, frequently used characters will be stored with fewer bits and not-so-frequently occurring characters will be stored with more bits, resulting in fewer bits used in total.