Intro to xdsl part 1

xDSL

Alternatives for data services

• Fiber, coax, HFC
• COST: $10K-$20K / mile
• TIME: months to install
• T1/E1
• COST: >$5K/mile for conditioning
• TIME: weeks to install
• DSL
• COST: 0 (just equipment price)
• TIME:  0 (just setup time)

xDSL

• Need higher speed digital connection to subscribers
• Not feasible to replace UTP in the last mile
• Older voice grade modems assume 4KHz analog line
• Newer (V.90) modems assume 64Kbps digital line
• DSL modems don’t assume anything
• Use whatever the physics of the UTP allows

xDSL System Reference Model

• POTS

• SPLITTER

• UTP

• CO SWITCH

• DSLAM

• xTU-C

• network/

• ISP

• router

• xTU-R

• POTS

• SPLITTER

• PSTN

• PDN

• POTS-R

• POTS-C

• WAN

• x = H, A, V, ...

• Analog modem

Splitter

• Splitter separates POTS from DSL signals
• Must guarantee lifeline POTS services!
• Hence usually passive filter
• Must block impulse noise (e.g. ring) from phone into DSL
• ADSLforum/T1E1.4 specify that splitter be separate from modem
• No interface specification yet (can’t buy splitter and modem from different vendors)
• Splitter requires installation
• Costly technician visit is the major impediment to deployment
• G.lite is splitterless ADSL

Why is DSL better than a voice-grade modem?

• Analog telephony modems are limited to 4 KHz bandwidth
• Shannon’s theorem tells us that the maximum transfer rate
• for SNR >> 1
• C = BW log2 ( SNR + 1 ) C(bits/Hz) = SNR(dB) / 3
• So by using more BW we can get higher transfer rates!
• But what is the BW of UTP?

• S

• N