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.
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