Uploaded by Kartika Munir

4 Multipleksing

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DASAR
TELEKOMUNIKASI
4
Elfitrin Syahrul
Universitas Gunadarma
MULTIPLEXING
Multiplexing
"sharing” fasilitas komunikasi
Penggunaan satu fasilitas
komunikasi (spt saluran
transmisi) oleh beberapa user
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multiplexing…
1. Space Division Multiplex (SDM)
 Skema “original” memisahkan sinyal untuk transmisi dan
switching — contoh, analog telephone switching office 
crossbar menyambungkan saluran telpon yang terpisah
dengan wiring matrix
Fasilitas  didefinisikan sbg communication link akan tetapi
bisa juga didef. sbg pole line atau kabel pada SDM.
Contoh SDM dapat dilihat pada hal berikut :
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Contoh SPACE DIVISION MULTIPLEXING
2. Sharing same radio channels and cable
routes via separate rights of way
1. Sharing a “pole line” via separate
open wire transmission paths
3. Sharing same radio channels via
different satellite footprints
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4. Cellular telephone reuses same channels in non-adjacent cells. The smaller
the cells, the more participants per space can operate in a region.
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2. Frequency Division Multiplexing (FDM)
•Concept is of using sine waves as carriers
•Each carrier is modulated by a different signal using AM,
FM, PM, or combinations of AM and FM or AM and PM,
such as QAM
•Modulation schemes by their nature produce energy
within separate spectral ranges either centered on or near
the frequencies of each carrier frequency
•Band pass filters are used to separate out each
modulated signal
•Detection schemes are used to regenerate each
individual signal
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Contoh FDM …
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No Channel 1
Little gap 4-5
Big gap 6-7
Huge gap 13-14
Going 54 - 59
Going 60 - 64
Gone 64-68
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US (NTSC) TELEVISION BROADCAST SPECTRUM
Luminance (brightness contours), two colorado difference signals (hue and
saturation), and various sound signals are combined within each 6 MHz channel
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Wavelength Division Multiplexing (WDM) is a FDM scheme used in optical
communications where various wavelengths of infrared light are combined over
strands of fiber.
Each wavelength is amplitude modulated by a digital bit stream (usually NRZ
encoded, i.e. on/off pulses of light) so that as many as eighty 50 GHz wide
channels can be combined on each 5 micron diameter strand of glass in the
case of D (for dense) WDM. Of course, WDM is FDM. Wavelength is c/f, but
frequencies are in the THz range for "light.“
Optical communications with few exceptions are digital since light transmitters
and receivers are usually poorly suited for analog modulation.
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3. Time Division Multiplexing – four basic concepts
1.
LANs: Not discussed in this chapter is the contentious use of a shared facility by many
sources and destinations where no intervening multiplexer devices are used. Instead, a peer
to peer access scheme is built into each device attached to the shared facility. This is the
basis of Local Area Networks and Metropolitan Area Networks, and in a most general (but
not necessarily in a specific sense) the Internet. We will discuss LANs, MANs, and WANs
later in the course and in future courses.
2.
CIRCUIT SWITCHING: The facility--communications link--is seized by one user for the
length of communications session. Such circuit switched schemes dedicate the entire path
until the entire message is completed. This is the concept behind telephone trunks or party
lines. That is, the transmission path is dedicated until session is ended. Everyone else has
to wait or take another path if available.
3.
PACKET SWITCHING: The facility is shared for one packet at a time (packet switched) by
many users. The route the packet takes is either dedicated or can be dynamic. This is the
basis of X.25 and ATM circuit switched networks and the Internet “backbone”.
4.
PAM, PCM, etc.: Individual frames or even down to individual coded samples are
interspersed in “time slots” on single communications links. This is the basis of TDM
multiplexers studied in Stallings Chapter 8.
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Time Division Multiplexers
Synchronous & Statistical Mux’s
• Stallings doesn't mention the essentially analog technique of
interspersing analog samples from multiple analog signal sources
into a stream of amplitude modulated pulses on a single analog link
and separating these samples from each other to reconstruct the
originals. There is no quantizing or digital conversion. This “Pulse
Amplitude Modulation” (PAM) scheme was widely used in
telephone switching offices and PBXs into the 1970s and later.
• The TDM methods we will discuss in some length in Data
Communications 1 from Chapter 8 of the text involves combining
digital signals from various sources over a single link.
• The original uses of TDM mux’s was to allow low data rate multiple
“terminals” to share coaxial or twisted pair transmission paths
• Terminals did not necessarily generate full time data
• Control schemes define the two basic types of mux’s
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Time Division Multiplexers
• The topic of Stallings Chapter 8 is specifically dedicated to
discussing those devices inserted between end points to control and
format the sharing of signals
• These specific devices are called "Multiplexers”
• You will also hear the jargon term "mux“ referring to both
synchronous and statistical multiplexers
• The original use for “mux’s” was for many terminals to share a
communications link more efficiently.
• Synchronous mux’s provide dedicated time slots for each source and
minimal overhead and latency
• Stat mux’s share time slots as needed instead of permanently
assigning them to individual terminals.
• In both kinds of mux’s, time slots are assigned to digital
representations of individual samples or frames of each signal
source in a rotating manner
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TDM MULTIPLEXERS :
A. Synchronous Multiplexers
- Each device is assigned a time slot in a fixed rotating manner
- Assigned time slots are unused when device is not active
- Latency is low since there is no processing other than delay
for placing sample into appropriate slot and recovering it
- Identification of user is by physical connection or equivalent
- Pulse Stuffing is necessary to account for (slightly) different
clocking of sources
- The concept is fully developed in T-carrier systems and
SONET which are good examples to study
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T-1 SCHEME BASICS
• Sample each of 24 3,000 Hz voice telephone lines at 8000 samples/second
•Binary encode samples at 128 levels, either linear or non-linear quantizing. Digital bit
stream for each signal is called a “DS-Ø” thus providing 7 bits per sample
•Use an 8th bit per source channel for signaling and supervision and to ensure that “one’s
density” is sufficient for receiver to regenerate clock signal
•D-4 “Super Frame” T-1 uses a framing word to ensure receiver can identify location of
each sample in digital stream. The framing word in interspersed into data one bit at a
time after each set of 24 signals are sent (every 193 bits in the data stream) and
retrieved via a cleaver correlation technique invented 50 years ago!
•In “Extended Super Frame” T-1 some of the framing word’s bits can be used
alternatively for other purposes such as a CRC calculation and an additional low rate
data link which is usually used for maintenance purposes
•T-1 uses the AMI encoding format over 4-wire twisted pairs with roughly one mile (“6 kft”)
spacing between repeaters.
•AMI polarity violations can be used to free up 8th bit when data is sent through so “clear
channel” schemes (e.g. B8ZS) which use bit substitution similar to that used for HDLC
flags but in this case to prevent too many zeros in a row.
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T-Carrier hierarchy
•Definition: DS-x is data rate and T-x is scheme
•Much involved combining of “T-carriers” into higher data rate systems
•Difficult to separate out individual channels other than with basic T-1 requiring a
“Digital Access and Cross-Connect System” (DACS)
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SONET and SONET hierarchy
•Optical fiber based system
•Starts at basic 51.84 Mbps rate called OC-1 with OC-n multiples of that rate
•Much simpler than T-carrier for separating out individual OC channels
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B. Statistical Multiplexer Basics
-
More attached devices than time slots
-
Dynamic assignment of time slots based on need with numerous possible
schemes of deciding how to do this
-
Identification of device connections (called, “tail circuits”) needs to be added
to data since slot allocations are not permanent. Implies additional overhead
bits. This adds overhead bits
-
Originally for many dumb terminals—with low occupancy—to communicate
with main frames.
-
Now used for Cable Modems which has a similar characteristic.
-
Usually all devices can not be sending simultaneously for any length of time
-
The Service Ratio defines potential aggregate data rate need versus actual
capacity of communications link. This can be large number for dumb terminals
-
Stat mux schemes can employ data buffers to handle bursts of more data
being generated than can be put on communication channel
-
New data can not be handled when buffers fill
-
Latency is an issue since individual users may have to wait for available time
slot(s) directly or for the buffers to be emptied
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Contoh MULTIPLEXING
•STAT MUX: CABLE MODEMS (DOCSIS)
•FDM: xDSL
•TDM: TOKEN RING AND ARCNET LOCAL AREA NETWORKS
(peer to peer controlled time slot schemes)
•TDM: EITHERNET LOCAL AREA NETWORK (peers contend for
access to send packets)
•FDM and TDM: IEEE 802.11 WIRELESS LANS
•FDM, TDM, SPREAD SPECTRUM: CELLULAR AND
CORDLESS TELEPHONES (channelized FDM, TDMA, Frequency
Hopping Spread Spectrum, Direct Sequence Spread Spectrum,
CDMA)
You will get the chance to study all of these. Let’s look at a few
interesting ones now…
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Konsep CABLE MODEM (DOCSIS)
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Perbandingan modem DOCSIS-1
•DOCSIS-2 and other systems to provide much greater data rates
•DOSCIS systems are usually “asymmetrical”
•Transmission encoding is 64-QAM, 128-QAM, or 256-QAM
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DIGITAL
SUBSCRIBER
LINKS
(xDSL)
•Uses Discrete
Multi-tone (DMT)
with 4 kHz spacing
•Each “tone” uses
QAM
•Can be
“symmetrical” or
“asymmetrical”
•Data rates up to
multiple Mbps per
twisted pair lines
plus traditional
telephone
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4. A Time Division Multiplex Variation
TIME DIVISION MULTIPLE ACCESS (TDMA)
TDMA is really just a kind of STATISTICAL multiplexing used for cell
phones. Channels are assigned for individual calls and reassigned as
users moves between cells. The American cellular TDMA networks and
American and international GSM networks use variations of TDMA.
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5. Some Frequency Division Multiplex Variations
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
OFDM is a newer encoding scheme used in IEEE 802.11a and IEEE
802.11g wireless LANs. 802.11a incorporating 8 non-overlapping 20 MHz
channels where each channel is divided in 52 sub carriers, each
approximately 300 KHz wide. 802.11g is similar but shares channel with
802.11b. This is “straight” channel based WDM but like the next one, the
use of each channel is interesting.
CODED ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
COFDM fills the transmission channel with 2,000 or even 8,000 carriers
(raw value, actual number of carriers is less), each modulated by QPSK,
16-QAM, or 64-QAM used for digital TV broadcasting but not in the US
which uses 8-VSB, a 3 bit/baud digital version of analog TV broadcasting.
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6. A Horse of a Different Color
CODE DIVISION MULTIPLE ACCESS
(CDMA)
CDMA is a time correlation scheme which
does not use the time slot method of TDM
to combine and retrieve signals.
Instead digital code words are used to spread each bit into a higher
bit rate unique pattern. The signals combined by this method can be
separated by the receivers using the same code word used to
encode each signal.
We will address CDMA as part of our upcoming study of SPREAD
SPECTRUM.
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