Upload
ariovaldojr
View
223
Download
0
Embed Size (px)
Citation preview
Data Communications and
Networking
Chapter 6:
Multiplexing
Multiplexing
Simultaneous transmission of multiple signals across a single data link.
Can utilize higher capacity links without adding additional lines for each device – better utilization of bandwidth.
Multiplexer (MUX) on sender’s end; Demultiplexer (DEMUX) separates transmission stream and directs signals to intended receiving devices.
Multiplexing
Breaking up a higher speed circuit into several slower (logical) circuits.
Several devices can use it at the same time
Requires two multiplexer: one to combine; one to separate
Main advantage: cost
Fewer network circuits needed
Multiplexing Vs. no Multiplexing
Multiplexing Techniques
3 basic techniques
Frequency-division multiplexing – analog
Wave-division multiplexing – analog
Time-division multiplexing – digital
6.1 Frequency-Division Multiplexing
Analog technique - when bandwidth of link is greater than combined bandwidth of signals to be transmitted.
Signals from each sending device modulate different frequencies.
Modulated signals are combined into a single composite signal.
Bandwidth ranges are channels through which the signals travel, separated by guard bands.
Figure 6.3 FDM
Figure 6.4 FDM Multiplexing Process
Figure 6.5 FDM Demultiplexing Process
Figure 6.6 Example 1
Guardband
Guardbands needed to separate channels
To prevent interference between channels
Unused frequency bands wasted capacity
Example 2 - Five channels, each with a 100-KHz
bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?
FDM Example 2
Figure 6.8 Analog Hierarchy of Phone Network
Other Applications of FDM
AM and FM radio broadcasting
Each station uses a different carrier frequency, shifting its signal and multiplexing
Receiver filters (tunes) to the frequency desired
Same concept for TV broadcasting and first generation cell phones
6.2 Wave-Division Multiplexing
Use light signals transmitted through fiber-optic channels.
Very narrow bands of light are combined from several sources to make a wider band of light.
A prism is used to bend the light beams based on the angle of incidence and frequency.
Receiver’s DEMUX separates signals.
Figure 6.9 WDM
Figure 6.10 Prisms in WDM multiplexing and demultiplexing
WDM Applications
Application: SONET network
Multiple optical fiber lines are muxed/demuxed.
DWDM (dense WDM) allows muxing of large numbers of channels by spacing channels closer to one another to achieve greater efficiency.
Over hundred channels per fiber
Each transmitting at a rate of 10 Gbps
Aggregate dat rates in terabit range
6.3 Time-Division Multiplexing (TDM)
Process of combining digital signals from several sources whereby each connection occupies a portion of time in the link.
Link is sectioned by time rather than frequency.
Time Slots and Frames
Data flow of each connection is divided into units
Link combines one unit of each connection to make a frame
n input connections n time slots
Data rate of link must be n times the duration of a time slot to guarantee flow of data
Time slots are grouped into frames; one complete cycle of time slots; each slot dedicated to one device
Figure 6.11 TDM Frames
Interleaving
Process of taking a specific amount of data from each device in a regular order.
May be done by bit, byte, or any other data unit.
Synchronizing
Framing bits are used to alleviate timing inconsistencies that may be introduced.
Usually one or two synchronization bits are added to beginning and end of each frame that allows the DEMUX to synchronize with the incoming stream so it can separate time slots accurately.
Figure 6.15 Framing bits
Comparison of TDM
Time on the circuit shared equally
Each channel getting a specified time slot, (whether it has any data to send or not )
More efficient than FDM
Since TDM doesn’t use guardbands, (entire capacity can be divided up between channels)
Digital Signal (DS) Service
Hierarchy of digital signals
DS-0 – single channel of 64 Kbps
DS-1 – single service or 24 DS-0 channels multiplexed to yield 1.544 Mbps
DS-2 – single service or 4 DS-1 channels or 96 DS-0 channels to yield 6.312 Mbps
DS-3 – single service, 7 DS-2 channels, 28 DS-1 channels, or 672 DS-0 channels to yield 44.376 Mbps
DS-4 – 6 DS-3 channels, 42 DS-2 channels, 168 DS-1 channels, 4032 DS-0 channels to yield 274.176 Mbps
Figure 6.16 DS hierarchy
Table 6.1 DS and T lines rates
Service Line Rate
(Mbps)
Voice
Channels
DS-1 T-1 1.544 24
DS-2 T-2 6.312 96
DS-3 T-3 44.736 672
DS-4 T-4 274.176 4032
T Lines
Digital lines designed for digital data, voice, or audio.
May be used for regular analog if sampled then multiplexed using TDM.
Figure 6.17 T-1 line for multiplexing telephone lines
Inverse TDM
Takes data stream from one high-speed line and breaks into portions and sends over several lower-speed lines simultaneously.
Used in bandwidth-on-demand where channels can be used for several applications requiring different transmission rates (i.e. voice, data, video).
Figure 6.21 Multiplexing and inverse multiplexing