Dr. Mark Humphrys

School of Computing. Dublin City University.

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Phone lines

Modems

DSL

Irish broadband speeds

bandwidth speed tests

2.2.2 Twisted pair (phone line)


(a) Older version. (b) Newer version.

2 insulated copper wires.
Twisted as helix - interference from different twists cancels out.
Can run several km before need amplification.

Change voltage (or current) on the line to give analog signal changing over time.
Analog signal may map directly to analog sound output (phone call).
Line frequency (voltage change over time) maps direct to sound frequency (audio change over time), since voltage changes in real time as audio changes.
Or analog signal can be quantised to carry information (modem).

The local loop

  1. Telephone network - The local loop ("the last mile" connecting your house to nearest phone company "end office")
    • Uses twisted pair.
    • Still analog.

  2. Telephone network - The backbone
    • Uses coax (not much any more), fiber optic (mostly now) and wireless (microwave, often useful alternative to fiber).
    • All-digital.

Twisted pair not going away any time soon

Twisted pair still has enormous installed base. Not going away any time soon.
Easy to upgrade telephone HQ. Not easy to upgrade every house.
So use methods to squeeze maximum possible performance out of these old lines.

Basic speed 2400 bps.
Clever coding used to get up to 56 kbps.
Unused frequencies used in DSL broadband to get even higher speeds.

2.5.3 Modems

The "last mile" is (usually) analog.
To send data, must convert digital to analog.
Done by modem (modulator - demodulator).

Telephone system originally designed for analog voice.
Now telephone system designed for digital. Except at local loop.
Telephone backbone - ISP - Internet backbone - remote web server, etc., all-digital.

Analog-digital conversion needed at local loop.
After that, usually all-digital (ISP 2).
ISP 1 (on analog local loop) doesn't exist any more.

Modulation


Forms of modulation.

(a) The binary signal we are trying to represent.
(b) Amplitude modulation. One amplitude is 0. One non-zero.
(c) Frequency modulation. 2 frequencies used.
(d) Phase modulation. Constant frequency. But the carrier wave is shifted by n degrees (e.g. 180 degrees). A phase shift indicates a change from 1 to 0, or 0 to 1. No phase shift indicates no change.

Modems on a standard telephone line

Telephone line

baud v. bps

baud - No. of samples per second.
Standard modem - 2400 baud.
Given the limited bandwidth (range of f's) of the phone line, it is actually pointless to sample much more often than this.

Clever coding may mean each sample (each "symbol") yields multiple bits.
So bits per second = some multiple (typically from 1 to up to 16) of baud.

Modulation coding

Example: Alter voltage to 0,1,2 or 3 volts. Then each symbol (voltage sample) contains 2 bits (00, 01, 10, 11). 2400 baud. 4800 bps.

Why not 1000 different voltage settings?
See problems of pure (continuous) analog transmission.

As we add more symbols, noise can more easily change from one symbol to another.
So as we add more symbols, modems introduce error-detection (see below).

In general, 2n possible symbols means each symbol can represent n bits, so:
bps = n . baud
  1. Phase

  2. Amplitude

  3. Frequency
    • Different frequencies commonly not used for modulation, but used to have 2-way transmission on same line (all modern modems).

Constellation diagram of the modem


Amplitude (distance of point from origin) and phase (angle of line from point to origin) modulation.

(a) QPSK - 4 phase shifts

(b) QAM-16. (c) QAM-64.

Error detection

As discussed, more symbols means more likely that error transforms into wrong symbol (compare with just 0 and 1).

So more advanced modems add error-detection.
Add extra bits to signal.

The V.32 modem standard uses 32 symbols, represent 5 bits.
Each is 4 data bits plus 1 parity bit.
9600 bps with error detection.

Error-detection: Parity bit

Even parity bit: Make the number of 1s even: Easy to detect 1-bit error (will get odd parity).

Can't detect 2-bit error.
All error-detection and correction methods only work below a certain error rate

V.32 and V.32 bis


(a) V.32. 4 data bits. 1 parity. 9600 bps.
(b) V.32 bis. 6 data bits. 1 parity. 14.4 kbps.
V.34 bis gets 14 data bits/symbol. 33.6 kbps.

Beyond that, modems compress before transmitting, to get effective data rate higher.
e.g. V.44 compression is based on the common LZW lossless data compression algorithm.

Sometimes, the telephone line isn't of sufficient quality to use higher standards.
Modems test the line (may take 15-30 sec), and cut back to simpler (slower) standard.

Limits (for ordinary phone line)

If machine at other end is also on analog local loop (e.g. some P2P sessions), theoretical limit is about 35 kbps (2 loops add noise rather than one).

If machine at other end is pure-digital (e.g. all Web sites), theoretical limit is about 70 kbps.

V.92 final standard 1999 (before DSL broadband took over) - 48 k upstream, 56 k downstream.
May not work for you - Many local loops are too noisy even for 33.6 k.

DSL - Broadband on phone line

Telephone designed for analog human voice.
No point transmitting frequencies that will be transformed by analog reader into sound outside human voice range. In fact, don't want to.
So local loop has filter (at end office) which deliberately attenuates frequencies below 300 Hz, above 3400 Hz.

What this means is that your telephone line has a filter put in to deliberately restrict its bandwidth.
Don't care about 0 to 300 Hz.
Care about 3400 Hz to 1.1 MHz.

DSL - Arrange with your end office to remove the filter.
Transmit those f's to your DSL modem, which can use them. 1.1 MHz spectrum now available.
Other existing equipment (e.g. your regular phone) can't deal with these f's, so you must add a filter in your house to all existing equipment (just plug it into filter, plug filter into telephone socket).
Can now self-install (compare with Internet on cable TV).

Theoretical DSL bandwidth


Theoretical bandwidth for DSL circa 2003.
Mpbs should read Mbps.
Higher bandwidth achieved since: VDSL2 has theoretical max speed 250 M bps.
Bandwidth falls off with distance from end office.

In practice, many problems. Speeds may be lower.

Broadband in Ireland

246 M - The fastest speed I've seen so far on my home Internet access. (Virgin Media TV cable.)
Tested here.



893 M - The fastest speed I've seen so far on my work Internet access. (DCU through HEAnet.)
Tested here.

ComReg report

Fastest broadband in Ireland

Broadband speeds in other countries
I first surfed the Internet from home 20 years ago at 2 k if I was lucky, but often 1 k.
This was fast enough for (entirely text-based) email, discussion lists, online chat, file downloads, online publishing and even remote editing.
I am now on at home at 100 M - 100,000 times faster.
Quite likely within the next 20 years I will be on at home at 10 G - 10 million times faster.

DSL phone line spectrum

1.1 MHz spectrum available:
Typically divided into 256 channels, some for upstream, some for downstream, one (0 to 4000 Hz) for old phone calls:


Phone and DSL co-exist on same line.
Many similar issues to old regular modem on phone line:

Future of the phone line

Old phone line use may become obsolete (same hardware, different use):
ancientbrain.com      w2mind.org      humphrysfamilytree.com

On the Internet since 1987.      New 250 G VPS server.

Note: Links on this site to user-generated content like Wikipedia are highlighted in red as possibly unreliable. My view is that such links are highly useful but flawed.