1.

Broadband local access technology

(B.A.Forouzan, Business Data CommunicationsA.S.Tanenbaum, Computer Networks 4/e)

2.

Figure 8-1

A telephone system

3.

Major Components of the Telephone System

• Local loops Analog twisted pairs going to houses and

businesses

• Trunks Digital fiber optics connecting the switching

offices

• Switching offices Where calls are moved from one trunk to

another

4.

Figure 8-5 Internet access via the local loop

Modems are used to transmit digital signal via analog local loop.

5.

Figure 8-6 Modem concept

6.

Figure 8-7 56K modem

7.

Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.

How does the xDSL concept extend the date rate to the Mbps?

8.

xDSL Summary TableDSL Type Description

Data RateDownstream;

Upstream

Distance Limit Application

DSL Lite (same as G.Lite)

"Splitterless" DSL without the "truck roll"

From 1.544 Mbps to 6 Mbps downstream, depending on the subscribed service

18,000 feet on 24 gauge wire

The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business

HDSLHigh bit-rate Digital Subscriber Line

1.544 Mbps duplex on two twisted-pair lines;2.048 Mbps duplex on three twisted-pair lines

12,000 feet on 24 gauge wire

T1/E1 service between server and phone company or within a company;WAN, LAN, server access

SDSL Symmetric DSL1.544 Mbps duplex (U.S. and Canada); 2.048 Mbps (Europe) on a single duplex line downstream and upstream

12,000 feet on 24 gauge wire

Same as for HDSL but requiring only one line of twisted-pair

ADSLAsymmetric Digital Subscriber Line

1.544 to 6.1 Mbps downstream;16 to 640 Kbps upstream

1.544 Mbps at 18,000 feet;2.048 Mbps at 16,000 feet;6.312 Mpbs at 12,000 feet;8.448 Mbps at 9,000 feet

Used for Internet and Web access, motion video, video on demand, remote LAN access

RADSLRate-Adaptive DSL from Westell

Adapted to the line, 640 Kbps to 2.2 Mbps downstream; 272 Kbps to 1.088 Mbps upstream

Not provided Similar to ADSL

VDSLVery high Digital Subscriber Line

12.9 to 52.8 Mbps downstream;1.5 to 2.3 Mbps upstream;1.6 Mbps to 2.3 Mbps downstream

4,500 feet at 12.96 Mbps;3,000 feet at 25.82 Mbps; 1,000 feet at 51.84 Mbps

ATM networks;Fiber to the Neighborhood

9.

ADSL Design

• Asymmetric– Greater capacity downstream than upstream

• Frequency division multiplexing– Lowest 25kHz for voice

• Plain old telephone service (POTS)

– Use echo cancellation or FDM to give two bands

– Use FDM within bands

• Range 5.5km

10.

Figure 8-8 Bands for ADSL

11.

Digital Subscriber Lines (2)

12.

Discrete Multitone• DMT: Discrete Multitone

• Multiple carrier signals at different frequencies

• Some bits on each channel

• 4kHz subchannels

• Send test signal and use subchannels with better signal to noise ratio

13.

DMT Transmitter

14.

Typical ADSL configuration

15.

Community Antenna Television

16.

Internet over Cable (2)

17.

Spectrum Allocation

18.

Modulation and data rate Downstream Upstream

Bandwidth 750-550=200MHz 42-5=37M

channels 200/6=33 37/6=6

Modulation 64-QAM

or 256-QAM

QPSK

Theoretical Data rate

56M = 30 Mbps

(1bit for error con)

26M = 12 Mbps

Limited data rate

10 Mbps as 10base-T interface to PC

Less than 12 Mbps

Sharing broadcast timesharing

19.

Cable ModemsCable modems are devices that allow high-speed access to the Internet via a cable television network.

20.

Cable vs. ADSL

• effective capacity unpredictable

• Seriously affect

• If you have a cable

• Encryption

•Specific bandwidth for every one user

• Increase numbers of user has little affect

•Max coverage 5.5km

•More secure, more reliable

21.

Comparison With Other Broadband Technologies

• DSL vs. T1

• DSL vs. ISDN

• DSL vs. 56K Modem

22.

An ADSL system using DMT allocates 3/4 of the available data channels to the downstream link. It uses QAM-64 modulation on each channel. What is the capacity of the

downstream link? (Assuming ADSL modulate at 4000 baud).

• There are 256 channels in all, minus 6 for POTS and 2 for control, leaving 248 for data.

• If 3/4 of these are for downstream, that gives 186 channels for downstream.

• ADSL modulation is at 4000 baud, so with QAM-64 (6 bits/baud) we have 24,000 bps in each of the 186 channels.

• The total bandwidth is then 186*24kbps=4.464 Mbps downstream.