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Module 3.3: Multiplexing
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WDM
FDM
TDM
T-1
ADSL
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Wavelength Division Multiplexing
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Used for fiber optics
Multiplexing and demultiplexing involve light signals
Combining and splitting of light sources are done by prisms.
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Frequency Division Multiplexing
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FDM
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Each signal is modulated to a
different carrier frequency
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Useful bandwidth of medium
exceeds required bandwidth of
channel
Carrier frequencies separated so
signals do not overlap (guard
bands)
e.g. broadcast radio
Channel allocated even if no data
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FDM System
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FDM Example
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Touch Tone Dialing
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When dialing 8,
two bursts of
analog signal with
frequencies 852
and 1336 Hz are
sent to the
Central Office
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TDM System
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T-1 Frame
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DS Hierarchy
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Digital Carrier Systems
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Hierarchy of TDM
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For digital data
– Same format is used
– The effective data rate in general is 56k x 24 = 1.344 Mbps
US/Canada/Japan use one system, Europe uses different.
US system is based on DS-1 format.
For voice each channel contains one word of digitized data (PCM, 8000
samples per sec)
– Data rate 8000x193 = 1.544Mbps
– Signaling bits form stream for each channel containing control and
routing info
– Data: 56,000 bps per channel at 24 channels = 1,344,000 bps
– Control: 8000 bps per channel at 24 channels = 192,000 bps
– Framing: 8000 bps for frame synchronization = 8000 bps
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Leased T1
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A typical configuration
scheme of a leased T1
WAN connection
between two sites
involves a V.35 link
between a router’s V.35
port and a CSU/DSU.
The CSU/DSU provides
the interface to the T1
circuit. This circuit
terminates at the telco’s
CO either directly or via
a POP located near the
customer’s premises. The
CO then provides
connectivity to the
network.
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Asynchronous TDM
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Called also statistical time-division multiplexing
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Suppose you have multiplexed the output of 20 computers to a single line.
– In synchronous TDM, the speed of the line must be at least 20 times the
speed of each input line.
– Half of this capacity is wasted if we have 10 computers only in use at a
time.
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Synchronous TDM doesn’t guarantee high link utilization because timeslots are
pre-assigned and fixed. If a device is not transmitting, the corresponding
timeslot is empty.
Asynchronous TDM is designed to avoid this type of waste by filling up all
timeslots. Timeslot is not fixed per device. Multiple devices could share the
same timeslot.
Statistical TDM allocates time slots dynamically based on demand
Multiplexer scans input lines and collects data until frame full
In asynchronous TDM, the total speed of the input lines can be greater than the
capacity of the path.
– In synchronous TDM, if we have n input lines, the frame contains at least n
timeslots
– In asynchronous TDM, if we have n input lines, the frame contains m slots,
with m less than n.
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Synchronous vs. Asynchronous TDM
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ISDN User Network Interface
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ISDN allows multiplexing of devices over single ISDN line
Two interfaces
– Basic ISDN Interface
– Primary ISDN Interface
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Basic ISDN Interface
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Digital data exchanged between subscriber and NTE - Full Duplex
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Data rate 192kbps
Separate physical line for each direction
Pseudoternary coding scheme
– 1=no voltage, 0=positive or negative 750mV +/-10%
– 2B1Q or AMI digital baseband line encoding
Basic access is two 64kbps B channels and one 16kbps D channel (2B+D)
This gives 144kbps multiplexed over 192kbps
Remaining capacity used for framing and sync
B channel is basic isdn channel
Data
PCM voice
Separate logical 64kbps connections to different destinations
D channel used for control or data: LAPD frames
Each frame 48 bits long
One frame every 250s
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Primary ISDN
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Point to point
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2.048Mbps
– Based on European standards
– 30 B plus one D channel
– Line coding is AMI using HDB3
Typically supporting PBX
1.544Mbps
– Based on US DS-1
– Used on T1 services
– 23 B plus one D channel
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Asymmetrical Digital Subscriber Line
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ADSL uses Analog signaling (DMT or
Discrete Multitone)
Link between subscriber and network
– Local loop
Uses currently installed twisted pair
cable
– Can carry broader spectrum
– 1 MHz or more
Asymmetric
– Greater capacity downstream
than upstream
Frequency division multiplexing
– Lowest 25kHz for voice
 Plain old telephone service
(POTS)
– Use FDM to give two bands
– Use FDM within bands
Range 5.5km
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DMT Transmitter
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ADSL uses Discrete Multitone
(DMT)
Upstream and downstream
bands are divided into 4 khz
channel, each capable of
transfering 60 kbps.
With 256 downstream
subchannels, we can transmit
up to 15.36 Mbps, but
transmission impairments
prevent this.
Current rates go from 1.5 to 9
Mbps.
HDSL and SDSL use digital
signaling, AMI and 2B1Q
line coding.
ADSL and VDSL use analog
signaling, DMT modulation.
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