Understanding Multiplexing Techniques

• Multiplexing is sharing a medium or bandwidth, combining multiple signals from various sources for transmission over a single line.
• A Multiplexer (MUX) combines n input lines into one output line (many-to-one).
• A Demultiplexer (DEMUX) separates a signal into its components at the receiving end (one-to-many).

Concept of Multiplexing

• Input lines are transmitted through a multiplexer to form a composite signal.
• This composite signal goes through a demultiplexer, separating the signal and transferring components to destinations.

Advantages of Multiplexing

• Allows sending more than one signal over a single medium.
• Effectively utilizes the bandwidth of a medium.

Types of Multiplexing

• Frequency Division Multiplexing (FDM).
• Time-Division Multiplexing (TDM).
• Wavelength Division Multiplexing (WDM).

Frequency Division Multiplexing

• FDM divides the bandwidth of a single physical medium into smaller, independent frequency channels.
• It is used in radio and television transmission.
• Inter-channel cross-talk can occur due to bandwidth division into frequency channels.
• Unused bandwidth strips, known as guard bands, are placed between each channel to prevent cross-talk.

Advantages of FDM

• Utilized for analogue signals.
• Modulation technique is relatively basic and straightforward.
• May send a large number of signals simultaneously.
• Synchronization is not required between the transmitter and recipient.

Disadvantages of FDM

• Suited to low-speed channels.
• Issue of crosstalk.
• Requires a large number of modulators.
• Needs a channel with a high bandwidth.

Time Division Multiplexing

• TDM shares time instead of bandwidth portions, unlike FDM.
• Signals operate at the same frequency (bandwidth) at different times.
• Primarily applied to digital signals.
• The shared channel is divided among users using time slots.
• Digital signals are divided into frames, equivalent to time slots.
• Multiplexer and Demultiplexer are synchronized to switch channels simultaneously.
• When a channel transmits its frame, the De-multiplexer provides media to that channel on the other end.
• De-multiplexer provides media to channels in synchronized manner.

Types of TDM

• Synchronous TDM.
• Asynchronous TDM.

Synchronous TDM

• A technique where time slots are preassigned to every device.
• Each device gets a time slot, regardless of whether it has data to send.
• If a device lacks data, the slot remains empty.
• Signals are sent in frames, and empty slots are transmitted.
• Popular examples include T-1 multiplexing, ISDN multiplexing, and SONET multiplexing. -If there are n devices, then there are n slots.
• Each device allocated with a time slot transmits data regardless of its need to send/not send data.

Disadvantages of Synchronous TDM

• The channel capacity is not fully utilized due to empty slots being transmitted.
• The transmission medium's speed should exceed the total speed of the input lines.

Asynchronous TDM

• Also known as Statistical TDM.
• Time slots are allocated to devices only when they have data to send.
• Dynamically allocates time slots to devices.
• The total speed of input lines can exceed the channel's capacity.
• Accepts incoming data streams and creates frames containing only data.
• Each slot includes an address part identifying the data source.
• Synchronous TDM has many unutilized slots whereas Asynchronous TDM slots are fully used. This leads to smaller transmission time.

Key differences between Asynchronous and Synchronous TDM

• In Synchronous TDM, if there are n sending devices, there are n time slots, where in Asynchronous TDM, if there are n sending devices, there are m time slots where m is less than n (m<n).
• The number of slots in a frame depends on the statistical analysis of the number of input lines.

Wavelength Division Multiplexing

• Used on fiber optics to increase single fiber capacity.
• Analog multiplexing technique.
• Optical signals from different sources are combined to form a wider light band using multiplexers.
• Demultiplexer separates signals to transmit them to destinations at the receiving end.
• Employs different wavelengths (colors), is done conceptually like FDM but uses light as signals.

Advantages of WDM

• Full-duplex transmission is achievable.
• Simple to reconfigure.
• Several signals may be sent at the same time.
• Less costly with simple system expansion.
• Highly secure.
• Optical components are dependable and provide more bandwidth.

Disadvantages of WDM

• High cost of optical equipment.
• Wavelength adjustment may be challenging because of poor bandwidth use.
• Major issue is scalability.

Telephone Network General Facts

• Used to provide voice communication using Circuit Switching.
• Originally referred to as a plain old telephone system (POTS) using analog signals.
• There's a feature to carry data in addition to voice, hence the network is now both analogous and digital.
• The bandwidth for voice purposes is 4000 Hz for the local loops.

Telephone Network Components

• Local loops.
• Trunks.
• Switching Offices.
• The office is defined by the first three digits and the local loop number is defined by the next four digits defines.
• Transmission is performed through optical fibers or satellite links.

Advantages of Telephone Network

• It is a circuit-switched network.
• There is no transmission delay as any receiver can be selected.
• It is cheap in price because it is a widely spread network.

Disadvantages of Telephone Network

• It requires a large time for connection.
• It has a low transmission speed.

Applications of Telephone Network

• It helps to connect people.
• It is used by business organizations to advertise their products.
• It is also used around the world for recreational purposes.

Cellular System

• Designed to provide communications between two moving units, called mobile stations (MSs), or between one mobile unit and one stationary unit, often called a land unit.
• It is divided into small regions called cells, each with an antenna controlled by a base station (BS).
• Base stations controlled by a mobile switching center (MSC).
• The MSC coordinates communication between all the base stations and the telephone central office.
• Cell radius typically 1 to 12 mi.
• Cell size is optimized to prevent interference of adjacent cell signals.

Frequency-Reuse Principle

• Neighbouring cells cannot use the same set of frequencies, and frequencies need to be reused.
• A frequency reuse pattern is a configuration ofN cells, N being the reuse factor, in which each cell uses a unique set of frequencies.

Transmitting a call from a mobile station

• The caller enters a code of 7 or 10 digits (a phone number) and presses the send button.
• The mobile station then scans the band, seeking a setup channel with a strong signal, and sends the data (phone number) to the closest base station using that channel.
• The base station relays the data to the MSC. The MSCsends the data on to the telephone central office. If the called party is available, a connection is made and the result is relayed back to the MSC. At this point, the MSC assigns an unused voice channel to the call, and a connection is established. The mobile station automatically adjusts its tuning to the new channel, and communication can begin.

Receiving a call from a mobile phone.

• The telephone central office sends the number to the MSC.
• The MSC searches for the location of the mobile station by sending query signals to each cell in a process called paging.
• Once the mobile station is found, the MSC transmits a ringing signal and, when the mobile station answers, assigns a voice channel to the call, allowing voice communication to begin.

Hand Off

• The MSC monitors the level of the signal every few seconds. If the strength of the signal diminishes, the MSC seeks a new cell that can better accommodate the communication. The MSC then changes the channel carrying the call (hands the signal off from the old channel to a new one).
• 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 HandoffNew systems use a soft handoff. In this case, a mobile station can communicate with two base stations at the same time.

Roaming

• One feature of cellular telephony is called roaming.
• Roaming means, in principle, that a user can have access to communication or can be reached where there is coverage.

Advantages of Cellular Systems

• Higher capacity: Frequency can be reused when one transmitter is far away from another or outside the interference range.
• Less transmission power: A receiver or mobile handheld devices far away from a base station needs more power than the current few Watts. Hence the small cell transmission reduces this need.
• Local interference only: With small cells, mobile stations and base stations have to deal with 'local' interference only unlike long distance interference.
• Robustness: Cellular systems are decentralized, hence, if one antenna fails, this only influences communication within a small area.

Disadvantages of Cellular Systems

• Infrastructure needed: Need many antennas, switches, location registers.
• Handover needed: Handover to another cell or base station is needed when changing from one cell to another.
• Frequency planning: To avoid interference frequencies have to be distributed carefully.

MOBILE COMMUNICATION-GSM

• The Global System for Mobile Communication (GSM) is a European standard that was developed to provide a common second-generation technology for all Europe.
• The aim was to replace a number of incompatible first-generation technologies. Bands GSM uses two bands for duplex communication. Each band is 25 MHz in width, shifted toward 900 MHz. Each band is divided Into 124 channels of 200 kHz separated by guard bands.
• It is the most successful digital mobile telecommunication system As a part of building fully digital system, the groupe spéciale mobile (GSM) was founded in 1982, later named as global system for mobile communications (GSM) The primary goal of GSM.
• A GSM system that has been introduced in several European countries for railroad systems is GSM-Rail. This system gives 19 exclusive channels for railroad operators for voice and data traffic.

GSM Mobile services

• GSM permits the integration of different voice and data services and the interworking with existing networks. GSM has defined three different categories of services: bearer, tele, and supplementary services. A mobile station (MS) is connected to the GSM public land mobile network (PLMN) via the Um interface.
• The mobile termination (MT) performs all network specific tasks making the MS independent and unaware of network details. R, S, U, Um are all interfaces.

Bearer services

• Bearer services allow transparent and non-transparent, synchronous or asynchronous data transmission. Transparent bearer services only use the functions of the physical layer (layer 1) to transmit data. The delay and performance is constant if there is no error in transmission.
• Non-transparent bearer services use protocols of layers two and three to implement error correction and flow control. These services use the transparent bearer services along with radio link protocol (RLP). This protocol includes special mechanisms to retransmit error prone data.
• Using transparent and non-transparent services, GSM specifies several bearer services for interworking with PSTN, ISDN, and packet switched public data networks (PSPDN).

Tele services

• GSM mainly focuses on voice-oriented tele services that include encrypted voice transmission.
• GSM provides the service of emergency number. Another service is the short message service (SMS), which allows messages of size 160 characters. Enhanced message service (EMS), offers a larger message size (760 characters), supported animated pictures, small images and ring tones.
• Multimedia message service (MMS) supported larger pictures (GIF, JPG), short video clips etc.
• Another non-voice tele service is group 3 fax, where fax data is transmitted as digital data over analog telephone network.

Supplementary services

• These services offer various enhancements for the standard telephony service, and may vary from provider to provider.
• Typical services are user identification, call redirection, or forwarding of ongoing calls, closed user groups and multiparty communication are also available.

System architecture

• A GSM system consists of three subsystems, the radio sub system (RSS), the network and switching subsystem (NSS), and the operation subsystem (OSS).

Radio subsystem (RSS)

• Contains all radio specific devices which are the mobile stations (MS) and the base station subsystem (BSS).
• Interface between MS and BTS is Um (air or radio interface).
• Interface linking BSC and BTS is Abis.
• Interface that provides communication between BSS and MSC is A.
• OMC monitors all other entities through O interface

Base station subsystem (BSS)

• A GSM network has many BSSs, each controlled by a base station controller (BSC). BSS contains several BTSs.
• Functions include coding/decoding of voice and data rate adaptation to/from the wireless network etc.
• Base transceiver station (BTS) contains radio equipment such as antennas, amplifiers etc. A BTS can form a radio cell and is connected to MS.
• Base station controller (BSC): The BSC controls BTSs. It reserves radio frequencies and handles the handover of device from one BTS to another within the BSS.
• Mobile station (MS): The MS comprises all user equipment and software needed for communication with a GSM network. An MS consists of user independent hard- and software and of the subscriber identity module (SIM), which stores all user-specific data that is relevant to GSM. MS is identified through international mobile equipment identity (IMEI) and is used for theft protection.
• SIM card stores identifiers such as card-type, serial number, a list of subscribed services, a personal identity number (PIN), a PIN unblocking key (PUK), an authentication key Ki, and the international mobile subscriber identity (IMSI).
• The MS stores dynamic information (location information) while logged onto the GSM system.

Network and switching subsystem (NSS)

• Connects the wireless network with standard public networks, performs handovers between different BSSs, supports roaming of users between different providers in different countries.
• Mobile services switching center (MSC): MSCs are high-performance digital ISDN switches. They setup connections to other MSCs and to the BSCs.
• Gateway MSC (GMSC) has additional connections to other fixed networks, such as PSTN and ISDN. Using Inter Working Functions (IWF), an MSC can also connect to public data networks (PDN)
• Home location register (HLR): It is the database in a GSM system that stores all user-based information, comprising static information, Dynamic information like the current location of MS, VLR and MSC is also stored, the mobile subscriber ISDN number (MSISDN) and the international mobile subscriber identity (IMSI).

Operation subsystem (OSS)

• Contains the necessary functions for network operation and maintenance.
• Operation and maintenance center (OMC) - monitors and controls all other network entities which includes traffic monitoring, subscriber and security management, accounting and billing.
• Authentication centre (AuC): Protects user identity and data transmission. The AuC contains the algorithms for authentication and keys for encryption. It generates the values needed for user authentication in the HLR.
• Equipment identity register (EIR): The EIR is a database for all IMEIs. The EIR has a blacklist of stolen (or locked) devices. It also contains a list of valid IMEIs (white list), and a list of malfunctioning devices (gray list).
• Visitor location register (VLR): Dynamic database which temporarily stores all important information needed for the MS users currently in the location that is associated to the MSC (IMSI, MSISDN, HLR address). If a new MS comes into the location associated with VLR, it copies all relevant information for this user from the HLR.