1. Module 2.2: ADSL, ISDN, SONET
WDM
FDM
TDM
T-1
ADSL
ISDN
SONET
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2. Wavelength Division Multiplexing
Used for fiber optics
Multiplexing and demultiplexing involve light signals
Combining and splitting of light sources are done by prisms.
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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
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4. FDM System
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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
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6. TDM System
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7. T-1 Frame
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8. DS Hierarchy
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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 = Mbps
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10. Leased T1
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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11. Asynchronous TDM Called also statistical time-division multiplexing
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.
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.
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12. Synchronous vs. Asynchronous TDM
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13. ISDN User Network Interface
ISDN allows multiplexing of devices over single ISDN line
Two interfaces
Basic ISDN Interface
Primary ISDN Interface
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14. Basic ISDN Interface
Digital data exchanged between subscriber and NTE - Full Duplex
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
Data rate 192kbps
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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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
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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
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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 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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18. ADSL Terminology ATU-C: ADSL Transmission Unit, Central
The ADSL point of termination in the central office
An ADSL modem
ATU-R: ADSL Transmission Unit, Remote
The remote user’s ADSL modem
The CPE
DSLAM: DSL Access Multiplexer
Central office device that concentrates many ADSL connections into one
DSLAM contains ATU-Cs
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19. End-to-End ADSL Architecture
PSTN
DSLAM
ATU-R IP IP
ISP
ATM/ Frame Relay
Network
ISP
L2/L3 Services
Aggregator
Corporate
UCP
ATU-R
Content
Providers
Local Loop
Central Office
Broadband Network
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