Chapter 5 - Making Connections Efficient

A)     Introduction

a.      Multiplexing

B)     Frequency Division Multiplexing

a.      Frequency division multiplexing (FDM) – the assignment of non-overlapping frequency ranges to each “user” of a medium

                                                              i.     Channel – an assigned set of frequencies that is used to transmit the user’s signal. In frequency division multiplexing, this signal is either discrete analog (digital) or analog

                                                             ii.     Multiplexor – the device that accepts input from one or more users

1.      Device is attached to the receiving end of the medium that splits off each signal to deliver it to the appropriate receiver called the second multiplexor, or demultiplexor

2.      Guard band – set of unused frequencies usually inserted between two signals to provide a form of insulation

C)     Time division multiplexing

a.      Time division multiplexing (TDM) – allows only one user at a time to transmit, and the sharing of the medium is accomplished by dividing available transmission time among users

b.      Synchronous time division multiplexing – gives each incoming source signal a turn to be transmitted, proceeding through the sources in a round-robin fashion

c.      Statistical time division multiplexing (Stat TDM) – transmits data only from active users and does not transmit empty time slots

D)     Wavelength Division Multiplexing

a.       Fiber exhaust – inability of a single fiber-optic line to meet users’ needs

b.      Wavelength division multiplexing (WDM) – multiplexes multiple data streams onto a single fiber-optic line. It is, in essence, a frequency division multiplexing technique that assigns input sources to separate sets of frequencies

                                                              i.     Wave division multiplexing uses different wavelengths (frequency) lasers to transmit multiple signals at the same time over a single medium. The wave length of each differently colored laser is called the lambda

                                                             ii.     Dense wavelength division multiplexing (DWDM) – when WDM can support a large number of lambdas

                                                            iii.     Coarse wavelength division multiplexing (CWDM) – less expensive technology because it is designed for short-distance connections and has only a few lambdas, with a greater space between lambdas

                                 

E)      Code division multiplexing (CDM) – allows multiple users to share a common set of frequencies by assigning a unique digital code to each user

a.      Chip spreading codes – codes that are used to tell each cell phone apart

F)      Discrete multitone (DMT) – a multiplexing technique commonly found in DSL systems

G)     Comparison of Multiplexing Techniques

a.      Disadvantages of Frequency division multiplexing

                                                              i.     First disadvantage found in systems that multiplex multiple channels over a single medium. Because the frequencies are usually statically assigned, devices that do not have anything to transmit are still assigned frequencies, and thus bandwidth is wasted

                                                             ii.     Second disadvantage is due to the fact that the technique can use non-discrete analog signals, and these analog signals are more susceptible to noise disruption than digital signals

b.      Disadvantage/advantages of time division multiplexing

                                                              i.     Same as frequency division where some bandwidth is wasted

                                                             ii.     Has less noise than frequency division due to digital transmissions over analog

H)     Compression – Lossless vs Lossy

a.      Compression – the process of taking data and somehow packing more of it into the same space

                                                              i.     Lossless compression technique – when a compression technique compresses data and then decompresses it back into the original data. No data is lost due to compression

                                                             ii.     Lossy compression – if a compression technique does lose some of the data as a result of the compression process

b.      Lossless compression

                                                              i.     Run-length encoding – compression technique that replaces any repetitions of the same bit or byte that occur in a sequence of data with a single occurrence of the bit/byte and a run count, or simply with a run count

c.      Lossy compression

                                                              i.     Perceptual encoding – the compressed version of an audio stream sounds fairly close to the uncompressed version even though some of the original data has been removed

                                                             ii.     MP3 – MPEG (Moving Picture Experts Group) Audio Layer-3 is a common form of audio compression

                                                            iii.     JPEG – Joint Photographic Experts Group – used to compress video images

1.      Involves discrete cosine transformation, quantization, and run-length encoding