DCOF FULL NOTES (MODULE 2)

MODULE 2: MULTIPLEXING

• Definition: The process of transmitting multiple signals simultaneously through a single path by combining them.

• Path: Refers to the physical link between devices, containing multiple channels that carry transmissions between pairs of devices.

• Multiplexer (MUX): Combines multiple data streams into a single stream (many to one).

• Demultiplexer (DEMUX): Splits the combined stream into different signals or streams (one to many).

Three Multiplexing Techniques

1. Frequency-Division Multiplexing (FDM)

2. Wave-Division Multiplexing (WDM)

3. Time-Division Multiplexing (TDM)

   • Synchronous TDM

   • Asynchronous TDM (Statistical TDM or Concentrator)

FREQUENCY-DIVISION MULTIPLEXING (FDM)

• An analog technique where the available bandwidth of a single transmission medium is divided into multiple channels.

• Each frequency channel is assigned to different devices, modulating signals to different carrier frequencies.

• Signals are combined and transmitted through a single link, with guard bands to prevent overlapping of channels.

WAVE-DIVISION MULTIPLEXING (WDM)

• Similar to FDM but specifically for optical signals transmitted through fiber optic cables.

• Utilizes prisms for multiplexing and demultiplexing.

• Input beams of light are combined into a wider band of light.

• The demultiplexer separates the signals and sends them to their respective devices.

TIME DIVISION MULTIPLEXING (TDM)

• Definition: Digital technique where different devices transmit data at different allotted time intervals known as time slots.

• Signals are transmitted in the form of frames, with each frame containing a cycle of time slots dedicated to each user.

Synchronous TDM

• Allocates the same time slot to each device at all times.

• If a device has no data to send, the slot is left empty.

Asynchronous TDM (Statistical TDM)

• Involves a frame with a varying number of slots that aren't fixed.

• Time slots are allocated only to devices that have data to send, containing an address part for source identification.

MULTIPLEXING APPLICATION: THE TELEPHONE SYSTEM

• The North American telephone system includes multiple common carriers/telephone companies providing local and long-distance services (e.g., Pacific Bell, AT&T, MCI, Sprint).

Telephone Service Categories:

1. Analog Services

   • Analog Switched Services

     • Dial-up service using twisted-pair cables to connect homes to the local office of the telephone company (local loop).

     • Part of the public switched telephone network (PSTN).

   • Analog Leased Services

     • Customers lease a dedicated line, permanently connected to another customer, appearing like a single line.

     • The connection still passes through switches but does not require dialing.

   • Conditioned Lines: Quality service improving line conditions by reducing attenuation and distortion, usable for digital communication when connected to modems.

Analog Hierarchy

• Utilizes FDM to multiplex signals from lower bandwidth lines onto higher bandwidth lines.

  • Groups: 12 voice channels multiplexed onto a higher bandwidth to create a group (48 KHz).

  • Supergroups: 5 groups multiplexed for 240 KHz bandwidth supporting 60 channels.

  • Master Groups: 10 supergroups combined require 2.52 MHz (upport 600 channels).

  • Jumbo Groups: 6 master groups combined for 16.984 MHz, supporting up to 3600 channels.

Digital Services

• Less sensitive to noise due to noise being analog in nature. Includes:

  • Switched/56 Service: Switched digital service with data rates up to 56 Kbps.

  • Digital Data Service (DDS): Digital leased line with a maximum data rate of 64 Kbps.

  • Digital Signal (DS) Service: Hierarchy of digital signals with different capacities and multiplexing capabilities.

CELLULAR SYSTEM

• Comprises base stations covering designated areas (cells) with varying radii based on environmental factors.

• Implements Space Division Multiplexing (SDM) allowing different senders for separate communication channels.

Advantages

• Higher capacity with frequency reuse based on distance.

• Reduced transmission power requirements.

• Localized interference management.

• Robustness due to decentralized infrastructure.

Disadvantages

• Significant infrastructure requirements (antennas, switches, etc.).

• Handover processes needed when moving between cells.

• Careful frequency planning to avoid interference.

• Borrowing channel allocation (BCA) for managing traffic loads.

GSM (Global System for Mobile Communications)

• Most successful digital mobile telecommunication system established in 1982.

• Aims for seamless mobility and voice service compatibility with PSTN.

GSM Mobile Services

• Integration of voice and data services with distinct bearer, tele, and supplementary services.

• Bearer Services: Facilitate transparent/non-transparent, synchronous/asynchronous data transmission.

  • Transparent requires only physical layer functions; error correction managed by FEC.

• Tele Services: Focus on encrypted voice transmission and SMS/MMS capabilities.

• Supplementary Services: Enhancements like user identification and call forwarding.

System Architecture

• Consists of Radio Subsystem (RSS), Network and Switching Subsystem (NSS), and Operation Subsystem (OSS).

• Various interfaces link components.

• Mobile Station (MS) contains user equipment and SIM for user-specific data.

• NSS connects wireless networks to public networks and manage user mobility.

• OSS handles operational functions like network monitoring and user security.