***      Of Line noise, The Phone Company, and Your Modem or FAXModem    ***

TEL_LINE.TXT  rev. 1.2  (PHM 6/30/93)  from Supra BBS (503-967-2444)

* NOTICE to users on PBX, "Lease-Line", and other non-PSTN phone connections *

  Your modem is designed to function only on standard phone service (the
  common analog 2-wire Hybrid single subscriber line).

  PBX or similar use on a phone jack connected to a digital phone port (as with
  many AT&T, Northern Telecom, and similar multi-line office phone systems) you
  will need to do one of the following things to use your modem:

        1. Have your phone system administrator install an "analog port" card
           into the PBX to allow standard telephone devices (like your modem)
           to be connected.
        2. Talk with your phone system administrator about getting a digital
           to analog jack converter box, or a digital telephone with an analog
           modem jack, to allow your modem to be connected via the existing
           type of telephone service.
        3. Have a separate analog 2-wire hybrid phone line installed by the
           local phone company for use by your modem.

  "Leased-Line"
        2-Wire Hybrid:  Not truly supported, but can be forced to connect by
                        simultaneously issuing ATA on one modem and ATXD on the
                        opposite modem.  (This method is also usable with cases
                        of "simulated" phone lines.  Like connecting a terminal
                        or other device via two modems connected with twisted
                        pair wiring with a 12VDC source connected across the
                        line.)
        4-Wire:         Not supported via any method.

  Other phone system using non-standard telephone connections should be avoided
  unless it has been demonstrated that common 2-wire analog telephone devices
  are correctly supported.



"Line noise" is commonly used as a general term to describe all types of
signal loss and signal damage that occur in the phone system, however, this
is not the correct terminology to use in a discussion of phone system
problems.  According to the Standards set by the Telecommunications Industry
Association, "line noise" refers to a single specific impairment, properly
labeled "1004Hz Signal to 3KHz Noise".  (cf: EIA/TIA-496-A)  To the
layman, "line noise" also refers to the white or pink thermal or diode noise
injected into a channel by an amplifier (or amplifiers) with limited negative
feedback.  With this in mind a more correct phrases would be "telephone line
defects", "transmission media problems", or  "unfaithful signal transfer".
This broader scope of problems related to correctly passing the original
signal from one modem to another via the telephone network is the major topic
of this document.

In reading the following information on your subscriber line, you must
remember that you can only address the type of phone service you pay for and
the long-distance carrier your call is routed through.  You normally have NO
control over conditions that may be present in local office switches or calls
placed in your "LOCAL LONG DISTANCE" area.  This means that regardless of how
much the phone company may do to your line, there will still be places you
can't get a good connection to.  Also, if the whole problem turns out to be
the local switching equipment, you are just plain out of luck (unless you can
get the local phone company to replace equipment that runs between tens of
thousands to hundreds of thousands of dollars).




POTS (Plain Old Telephone Service)

This is the type of phone service most people have.

This is what it you get:
                        -most 300hz to 3000hz sounds are passed.  (Although
                         the system is not required fully pass this range.)
                        -You aren't likely to get any more than -10db loss
                         from the local telephone office to your phone jack.
                         (Measured from a 1mW 1khz tone source.)
                        -VOICE use (you should be able to hear another
                         person's voice using normal, FCC approved, telephone
                         devices via this service).  (No implied ability to
                         use Modems, Faxes, or other "non-voice" devices
                         over this service.)

  The legal requirements for voice lines aren't even this good, but this is
  an average service.



DATA-GRADE / ASSURED QUALITY (PAC BELL) / or other "BETTER" service offered.

These types of service are offered as an "upgrade" from POTS (often at a
much higher installation fee, and an additional monthly service fee).
The services supported and/or promised by such upgrades will vary widely
from one telephone service provider to the next and are not supported at
all in some cases.  In general, they will provide a "cleaner" telephone
connection that is able to support fax and data communication to a specific
carrier rate, and normally have no more than a 5db loss.  These services may
also provide a specially installed line with no LOADS and no BRIDGE-TAPS.
Ideally, these lines will be able to supply a 600 ohm balanced line, and
should respond very well when devices (like FCC approved Telephones, Modems,
Faxes, etc.) are connected.



Items and Conditions that commonly degrade telephone lines:

BRIDGE-TAPS & HALF-TAPS

These are "extra" wires connected to the wire pair coming to your home from
the local telephone office.  Often these are the result of normal service
and repair procedures that occur over the years.  These don't normally
create a problem for VOICE use (although they have been traced to things
like radio stations or buzz-type noises in some cases).  Devices using
complex signaling tones and patterns, however, can be greatly affected by
these "extra" signals on the phone line.  Theses extra wires act like
antennas and pick up noises from a wide range of sources.

LOADS (Step-Up Transformers)

These are commonly used to increase the volume of voices on longer phone
lines.  They cause distortion of the rated 300hz to 3000hz band pass, and
can cause the frequencies to be shifted up or down from their original
values.  The volume of these frequencies is also not increased equally.
In some cases, the line can become poor enough that TOUCH-TONE dialing
may not be supported, and only PULSE dialing is possible.  LOADS are most
commonly used in older, rural settings, but still do exist in urban
installations.  LOADS also serve a means of introducing high levels of
noise onto the line, both by increasing the level of noise on the line
along with the voice level and by picking up environmental noise from the
area directly around the transformer itself (other transformers, power
supplies, high voltage power lines, etc.).

LINE LOSS

This is the composite of all of the types of loss on your phone line which
include the following:

-Pure Loss

 This natural loss in the phone line caused by the resistance of the wire
 and connections between the local office and your home.

-Return Loss

 This is a measurement of the signal loss on the line.  This is affected
 by the line itself (see above) and the devices you have connected to it.
 It is basically a measurement of the impedance match between the local
 office and the line and the devices you have connected (problems in any
 part will affect the over-all measurement).

-Frequency Distortion and Attenuation.

 This is basically the loss of volume and shift in frequency on the line.
 Problems often arise in high-speed data transfers because only part (or
 in very poor conditions none) of the original signal gets transferred
 faithfully to the receiving end.  In some cases many frequencies have been
 "reflected" back on to each other, further damaging the signal.

SOURCES OF SIGNAL DAMAGE

-Quanitazation noise

 Digital links have quanitazation noise which is inherent in A/D converters.

-Thermal Noise

 This is naturally occurring noise sourced from normal electrical decay and
 energy transformation.  Local loops, if properly engineered, have a thermal
 noise of 22 dBrn.  But it is not uncommon to find it at higher levels.
 (22 dBrn in "reference noise" is equal to -68 dBm.)

-Digital Compression

 Compressing the signal to get a higher number of voice signals on one
 carrier.  (This is just fine for voice, as the human ear does not notice
 the small gaps in the signal this creates, but modems can't deal well with
 gaps in the data received.)

-Power line hum

 Low frequency noise conducted into the phone line as it passes power lines.
 (Also it is possible for power hum to be passed into the phone line from
 power sources in the phone system itself.)

-Phase Jitter

 The shifting or bouncing of the carrier signal in relation to time.

-Crosstalk

 Noise or sounds from other lines in the phone system.  (In very bad
 conditions this can be loud enough you can hear other lines ringing and
 even hear other voice or data signals.)

-RFI (Radio Frequency Interference)

 High frequency electrical signals conducted into the phone lines.  (In very
 bad cases radio stations or other radio transmissions can be heard.)  If you
 can figure out the source of the signal (like the name of the radio station),
 then you should contact them in regard to curing this problem.  In the US,
 FCC regulations require the person in charge on the transmission to fix this
 problem at his expense. (In the case of a radio station, contact the station
 engineer.  He can normally construct a filter to remove the signal from your
 phone line.)


Common Tests to ask to have done by the phone company:

-Frequency Test (also called Frequency "Slope" or Frequency "Twist" test)

 This test shows both total loss in signal, and the amount of difference in
 signal loss at different frequencies.  A 1mW 1Khz signal is measured for
 total  loss (in dBm) and then compared to readings taken from signals at
 400hz and 2800Hz.  For a VOICE line, the high and low frequency values
 should be within a +3dBm to -5dBm range of the value of the center frequency
 (a DATA grade line should be closer to the reading of the center value).
 In conditions where this problem is quite bad, you will hear the voices as
 being "tinny" and it may be more difficult to recognize voices over those
 lines.

-Loss and/or Return Loss Test

 This will test the loss of a signal transmitted from one end of the line
 to the other.  (In the case of a Return Loss Test, it tests the loss of a
 signal transmitted into the line and reflected back from the other end.
 -45dBm is a reasonable level, -60dBm is not acceptable.


Common Problems, Tests, and Repairs for end users to address:

-Try the modem/fax with ALL other devices removed from the line.

 This tests for two common problems:

 1. NOISE -from devices like cordless phone and "neon-display" phones most
           commonly, but also can be sourced by any telephone line device.

 2. Signal Loss -from devices that "pull" power off the phone line.  Phones
           and devices that take power from the line when not actively in
           use for dialing memory, status lights, and lighted displays
           (although most lighted displays are only taking power when the
           handset is picked up) will lower the power level on the line,
           and will also lower the signal levels being passed on the line.

-Check the condition of the phone wiring in your home.

 "High Twist" or "Data Grade" phone cable, rather than standard "low twist"
        or non-twisted phone cable, wire is better for high-speed data
        transfer.

 "Silk" cable (that flat cable that runs between the phone device and the
        phone jack) should be kept short (under 8 feet).  It is better to
        put in a new phone jack (preferably wired with "High Twist" cable),
        then to connect the modem via a long "silk" type extension cable.

 Wires should be at least 26 gauge, 22 gauge is better.

 Insulation should be in good condition.

 Wiring runs should be kept out of direct sunlight and weathering.  (If
        wiring must be done outside, use phone wire rated for outdoor use.)

 Remove wiring runs no longer in use (this does not mean to rip out any
        phone outlet you aren't using, but it is a good idea to remove
        wiring that has already been disabled or covered up by remodeling
        -many older homes and rentals have phone lines that go nowhere!)

       *WARNING* If you find equipment that you don't understand connected
        to the phone lines in your home, or "extra" wiring to transformers
        or grounding rods/water lines, do not disturb it without talking to
        the phone company about it.

*** This document will be updated at random points in time, so please check
    the Supra BBS {503-967-2444} to assure the most current information.

This document is a combination of data from engineers at Supra, a telephone
call with an engineer who works for AT&T, electronic mail from other people
involved with the modem industry, and my own knowledge and education
relating to data communication and transmission lines.  Please feel free to
post this document in its original and unedited form.

Patrick Moore
Supra Technical Support
SysOp Supra BBS.
Feb. 25, 1993
Last Update:
June 30, 1993