Module 3.3: Multiplexing

1
Module 3.3 Multiplexing

• WDM
• FDM
• TDM
• T-1
• ADSL

2
Wavelength Division Multiplexing

• Used for fiber optics
• Multiplexing and demultiplexing involve light
  signals
• Combining and splitting of light sources are done
  by prisms.

3
Frequency Division Multiplexing

• FDM
• Useful bandwidth of medium exceeds required
  bandwidth of channel
• Each signal is modulated to a different carrier
  frequency
• Carrier frequencies separated so signals do not
  overlap (guard bands)
• e.g. broadcast radio
• Channel allocated even if no data

4
FDM System
5
FDM Example

• Touch Tone Dialing
• When dialing 8, two bursts of analog signal with
  frequencies 852 and 1336 Hz are sent to the
  Central Office

6
TDM System
7
T-1 Frame
8
DS Hierarchy
9
Digital Carrier Systems

• Hierarchy of TDM
• 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
• For digital data
• Same format is used
• The effective data rate in general is 56k x 24
  1.344 Mbps

10
Leased T1

• A typical configuration scheme of a leased T1 WAN
  connection between two sites involves a V.35 link
  between a routers V.35 port and a CSU/DSU.
• The CSU/DSU provides the interface to the T1
  circuit. This circuit terminates at the telcos
  CO either directly or via a POP located near the
  customers premises. The CO then provides
  connectivity to the network.

11
Asynchronous TDM

• Called also statistical time-division
  multiplexing
• Synchronous TDM doesnt guarantee high link
  utilization because timeslots are pre-assigned
  and fixed. If a device is not transmitting, the
  corresponding timeslot is empty.
• 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.
• 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.

12
Synchronous vs. Asynchronous TDM
13
ISDN User Network Interface

• ISDN allows multiplexing of devices over single
  ISDN line
• Two interfaces
• Basic ISDN Interface
• Primary ISDN Interface

14
Basic ISDN Interface

• Digital data exchanged between subscriber and NTE
  - Full Duplex
• Separate physical line for each direction
• Pseudoternary coding scheme
• 1no voltage, 0positive or negative 750mV /-10
• 2B1Q or AMI digital baseband line encoding
• Data rate 192kbps
• Basic access is two 64kbps B channels and one
  16kbps D channel (2BD)
• 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

15
Primary ISDN

• Point to point
• Typically supporting PBX
• 1.544Mbps
• Based on US DS-1
• Used on T1 services
• 23 B plus one D channel
• 2.048Mbps
• Based on European standards
• 30 B plus one D channel
• Line coding is AMI using HDB3

16
Asymmetrical Digital Subscriber Line

• 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

17
DMT Transmitter

• 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.