Chapter 22: Station Equipment
Overview
This chapter deals with station equipment including different types of telephones, station protection equipment, coin telephones, cordless telephones, and answering machines. These devices are positioned at the end of the telecommunications network yet they are very important in the design of the network. In fact much of the network is designed so that the telephone can remain a simple, rugged, and economical device. It is especially important for telecommunications professionals to know the types of station equipment available and how they function on a telecommunications network.
The fundamental principles of the telephone set have not changed much since it was invented by Alexander Graham Bell. Improvements have come in packaging, signaling, and transmission performance. The telephone typixcally uses a four-wire device interfaces with the two-wire loops of the network.
·
The
Transmitter converts voice to fluctuating direct current
·
Current
travels over metallic circuits to central office (may be converted to digital
signal)
·
The
terminating switch converts the current back into sound waves
·
Current
causes diaphragm to move with variations in current
·
Receiver
and transmitter are transducers meaning that they change sound waves to electric
current and current to sound waves (changes in sound pressure)
·
Switch
hook isolates all elements but ringer from network when telephone is idle or
on-hook
·
When
the telephone is off-hook the switch hook connects the line to the telephone
set
·
A
signal is then sent to central office to provide dial tone to the line
·
Two
types of dial circuits: rotary dial and tone dial (Dual-tone multi-frequency
or DTMF)
·
Rotary
interrupts flow of line current. When a dial is released contacts close and
open to produce pulses which operates switches
·
Tone
dial sends a combination of frequencies over the line
·
At
least 23mA of current needed if < 23mA poor transmission. If > 60mA too
loud
·
Service
where central office sends name and number of the calling party over an analog
line
·
An
analog display services interface or ADSI is a protocol for converting
the signal into a display either on the telephone itself or on an external caller
ID box
·
Telephone
circuits must be protected against surges, lightning, etc.
·
Air
gap or gas tube devices conduct high voltage from either side of the line to
a ground
·
LEC
places protectors (often forms demarcation point with customer wiring)
·
Protector
connects to telephone set with jacketed wiring which is called inside wiring
·
FCC
designated outlet RJ-11 is used on a protector for a single line. RJ21X for
multiple lines
·
After
divestiture customer-owned coin-operated telephones (COCOT) became
wide spread
·
COCOT
industry is expensive and very high risk. Fraud and vandalism are biggest problems
·
Many
COCOTs use alternate operator service providers (OSP) who charge more for services
and long distance than major IXCs. This is a problem for users who may be unaware
of this
Coin telephones
have several components:
·
Communication
Circuitry - essentially the same as regular telephone
·
Totalizer
- identifies coin denominations. Counts or relays value of money received
·
Coin
Chute - directs coins from coin slot (through totalizer) into coin box
·
Coin
Collect and Control Apparatus - controls coins - sends to coin box or refund
chute
·
Coin
Box - receptacle that receives and stores coins
COCOTs and LEC coin telephones
have some typical features:
·
Coin
Box Accounting - enables owner to determine amount of money in box without counting
manually. Helps prevent theft. Allows owner to determine when box is full
·
Alarming
- reporting system alarms when vandalism or tampering occurs or when box almost
full. Sounds local alarm, dials number (owner or police) when tampering
·
Remote
Diagnostics – ability to dial into coin telephone (manual or automatic center)
and run diagnostic tests to determine if telephone is functioning properly
·
Call
Timing - LEC charges usage on public access lines (charges COCOT). This feature
times call, requests additional coin deposit and cuts off caller when time limit
is exceeded. Optional readout allows callers to see when more coins needed
·
Call
Restrictions - blocks certain codes
·
Voice
Store and Forward - caller can leave voice message and coin phone will attempt
to deliver at certain intervals. Ex: traveler leaves message to be delivered
to busy telephone and therefore does not have to re-call
·
Database
Access - intelligent coin phones retain database of numbers on speed dial. COCOT
can collect fee from called party. Ex: store, taxi service, hotel, hospital.
User selects entry and phone speed dials
·
Facsimile
Capabilities - available in many public locations. COCOT can charge for facsimile
service plus normal long-distance charge
·
Keypad
Volume Control - adjust receiver volume using buttons on dial pad
·
Dialing
Instruction Display - Display and help keys assist user
·
RJ-11
Jack - accommodates modem and laptop computers
·
Cordless
phones have a base station that is connected by wire to a central office line
·
The
cordless feature allows the user to carry the device to any location within
range of the base (usually a few hundred feet). It is designed to cover an average
residential lot
·
The
primary drawbacks are lack of assured privacy and reliable transmission quality
·
More
worrisome is that someone else can use a phone on the same frequency to make
unauthorized long-distance calls or eavesdrop
·
New
generation of cordless phones contain safeguards against unauthorized calls.
Authenticate signals between portable and base using a code
·
Cordless
phones operate in 1 of 3 frequency bands using radio signals
·
Older
phones use 2 channels in the 46- to 49-MHz range - one frequency for base-to-portable
- one frequency for portable-to-base
·
Newer
models use the 900-MHz band
·
Newer
models also use the unlicensed spectrum of in the 1900-MHz PCS band
·
Anything
from a simple tape recording answering set to voice mail can be attached or
integrated into a telephone station
·
Some
use cassette tapes. Newer models use digital recording
·
Other
features are
·
Multiple
outgoing messages
·
2
line capacity
·
Selective
message save and delete
·
Remote
message retrieval
·
Message
time/date stamp
·
An
ordinary speakerphone is unsatisfactory for use in larger conference rooms because
it may lack the sensitivity needed for voice pickup from all parts of the room
or it may clip parts of the conversation
·
Ordinary
speaker phones use half duplex - loudest talker gets signal. Conference room
speaker phone is full duplex to allow normal conversation
·
When
using multiple devices on a single line it is helpful to have a line transfer
device
·
Generally
there are 2 categories of devices. One recognizes a distinctive ring (from central
office). The other recognizes the actual incoming signal
·
LECs
offer separate ringing pattern for telephone, fax, and modem
·
Some
terminal equipment can be programmed to operate off distinctive ring
·
Fax/modem
switches have ports for a fax machine, modem, and telephone
·
Answers
call coming in on line and listens for modem signal, fax signal, or no signal.
Switches to appropriate station (no signal gets transferred to telephone)
· There are two categories of analog telephone sets. General purpose (also referred to as 2500 sets) and special purpose telephones like ADSI phones, cordless phones, etc.
· The specialty phones still contain the basic inner workings of the 2500 set
· Telephones and auxiliary equipment do not have any specific standard. FCC registration just requires that devices are designed to protect the network
· The lines owned by the LECs are surge protected to protect a telephone but not necessarily other electric devices connected to the network
Intended
use and the following are important considerations when selecting a telephone:
·
Durability
and reliability: can it withstand frequent dropping?
·
Type
of dial: is it a DTMF? Will it be required to interface with telephone-related
services (i.e. companies that use queues, ACD, AA, voice mail, IVR, database)?
·
Number
of telephone lines served: will it be on a single line? multi-line? key telephone?
·
Transmission
performance: how reliable is transmission? how clear?
·
Additional
features: does it have last number redial? multi-number storage? speed dial?
The following features are important
when selecting a special-feature telephone:
·
Dialers:
stores a list of numbers accessed by pushing single button
·
Speaker
telephones: hands free operation using speaker and microphone
·
Cordless
telephones: authorization circuitry, extended range, multi-line operation
·
Memory:
store digits, last-number redial, correction of dialed digits before sending
signal
·
Calling
Party Display: caller ID shows alphanumeric information about caller
·
Telephones
for disabled: special dialing, amplified handsets, visual ringing, hearing aid
compatible, keyboards and single line readout for communication by the deaf
The following features are important
when selecting a special-feature telephone:
·
Battery
backup for continued operation during power failure
·
Call
counter to display number of messages recorded
·
Call
monitoring capability (incoming calls heard over speaker for screening)
·
Dual
tape. 1 for announcement and 1 for recording to prevent having to hear announcement
when retrieving messages
·
Digital
vs. recording tapes
·
Remote
access for retrieving, administering, recording announcement
·
Selective
erase/save options
·
Multiple
line capacity to answer multiple lines
·
Time/Date
Stamp
The following
is important when evaluating coin telephones
·
Is
the phone durable enough to withstand extremes in temperature/elements?
·
Is
the coin box resistant to tampering or lock picking?
·
Are
the transmitter and receiver covers cemented on?
·
How
much intelligence does the phone have? Does it return coins for incomplete calls?
Does it “slave” off the central office for power?
·
Alarming,
diagnostic and coin metering features?
·
Can
it read credit cards or prepaid calling cards?