Dr. Mark Humphrys

School of Computing. Dublin City University.

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Coaxial cable


Internet on cable TV


2.2.3 Coaxial cable

Still copper.
Again, change voltage to get analog signal.
Frequency = rate of change of voltage.

Compared to twisted pair phone line:
Longer distances. Low noise. Higher speeds (historically, though advanced DSL may challenge this).

Used to be used in telephone backbone. Now replaced by fiber optic.
Main surviving installed base of coax is cable TV.



2.7 Internet on cable TV

Main surviving installed base of coax is cable TV local loop.
Internet can be provided over this too.

Like telephone network, cable TV network backbone has gone fiber and wireless.
Local loop coax.



2.7.2 Shared local loop

Unlike telephone, local loops are shared among multiple houses.
Sharing is no problem for broadcasting.
For Internet, though, sharing means:
  1. Contention problems.
  2. Privacy/security problems.

As Internet access taken up, the cable companies have been progressively reducing sharing to smaller numbers of houses per cable (still shared, but not as much contention).



The difference between:
(a) the (shared, designed for 1-way broadcast) cable TV local loop
and:
(b) the (non-shared, designed for 2-way comms) telephone local loop.

Telephone network would be simply better for Internet if it were not for the fact that coax is faster than phone line.


2.7.3 Spectrum Allocation

TV and Internet coexist on same cable, using different frequency bands.


Note that TV is all downstream.
Can use upstream amplifiers that work only below 42 MHz,
and downstream amplifiers that work only above 54 MHz.


2.7.4 Cable modems

With coax, still need analog modulation to send digital data.
QAM-64 or higher used downstream.
Upstream not as good a line. Simpler scheme like QPSK needed to get through.

Contention is on upstream (multiple senders).
Aloha / Ethernet-like system: If contention, back off, wait random time, try to send again.

No contention on downstream - only one sender, the broadcaster.
Everyone can see all traffic. Only note what is addressed to you.
For privacy, need encryption.



2.7.5 DSL v. Cable



Feeds      w2mind.org

On Internet since 1987.