K. Salah 1

Module 2.2: ADSL, ISDN, SONET

• WDM

• FDM

• TDM

• T-1

• ADSL

• ISDN

• SONET

K. Salah 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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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

K. Salah 4

FDM System

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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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TDM System

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T-1 Frame

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DS Hierarchy

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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

K. Salah 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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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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Synchronous vs. Asynchronous TDM

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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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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 250s

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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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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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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.

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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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End-to-End ADSL Architecture

ATU-RATU-R

ATU-RATU-R

IP

UCP UCP

Central Office

IPIP

Content ProvidersBroadband NetworkLocal Loop

PSTN

ISP

ISP

Corporate

DSLAMDSLAM

L2/L3 L2/L3 ServicesServices

AggregatorAggregator

ATM/Frame Relay

Network