Also covered by Section 255 is telecommunications equipment (TE), defined as “equipment, other than customer premises equipment, used by a carrier to provide telecommunications services, and includes software integral to such equipment (including upgrades).” Thus TE includes all those elements of the network not touched by end users, whether they be wireline or wireless.
Central office (CO) local switches and long distance switches provide between them the basic backbone and fabric of the wireline public switched voice telephone network. A customer’s local CO switch is responsible for all basic features in placing a call, such as dial tone, recognizing touchtones, signaling other TE about the call, and maintaining a record of the call. Long distance switches receive call information about long distance calls, route them over the appropriate facilities, and deliver them to the local switch that serves the number being called. At the same time, information about the call itself is transmitted and prepared for delivery to an end party, such as ringing or Caller ID information.
Each switch is a complicated computer whose hardware connects tens of thousands of individual telephone lines to transmission facilities that link to other switches, which link to other individual telephone lines, etc. Switch software controls how each element in telephone service works, from the sound of the dial tone to what happens when the user begins dialing with a “1”.
In the wireless environment there are also central switches, with local functionality performed by the radio ports, the base stations located within each cell. Wireless is also different because wireless handsets have so much embedded intelligence; many of the features in wireless are not controlled by the network but by the handset itself.
Both switch hardware and software are continually being upgraded by the switch manufacturers. There are some signs that “mass customization” is entering the telephone industry so that each subscriber will be able to configure the service on his or her line or wireless account. For more information on customized services, see the section on AIN below.
Since many features of telecommunications services are controlled by the switch, it can play an important role in making telephone service more accessible. Here are some of those features and the accessibility issues they are connected with:
Telecommunications networks are engineered to perform well in statistically
average conditions, allocating resources according to typical usage.
For example, at any given time only a small percentage of users are in
the act of dialing. A limited number
of touch tone decoders shuttle among the lines to serve the users that are currently
dialing. Switches must be able
to limit the amount of time they will wait for a user to complete dialing a
number, or they will run out of decoders.
If this were not the case, a few hundred people could “bring down”
a switch by picking up their phones, dialing one digit only, and then waiting. Once all the decoders were allocated, no one else would be
able to dial. Instead what happens
is after a pre-set timeout, the switch frees the decoder from that line and
plays an announcement: “If you’d like to place a call, please hang
up and try again.”
All network resources are allocated this way, and some of their timeouts may interfere with their use by people with manipulation or cognitive limitations. Some of these timeouts are hard-coded by the switch software manufacturer, and some are adjustable by the telecommunications carrier.
The tones used on the telephone network were selected for transmissibility and reception by people with average hearing or better. This causes some access problems. For example, the call waiting tone is difficult for many hard of hearing people to hear.
For engineering reasons, telephone equipment limits the frequency response of the network to approximately 300 - 3400 Hz. Any signals much above or below these frequencies will be very weak. As better sound quality became common for other media (radio, television, recorded music), the low fidelity of the telephone has become more obvious. Hard of hearing people often mention that they can hear other electronic media better and wonder why the frequency response of the telephone cannot be improved. There is no technical reason, and using special equipment on digital lines, CD-quality transmissions are possible. For now, the common equipment used on analog lines retains the original limitation.
Although all wireline and some wireless switches carry and pass through TTY signals accurately enough, many of the features mounted on those switches are not TTY compatible. That is, any time the switch offers an audible tone or voice prompt and expects a touch-tone reply or similar user action in return, there is a likelihood that TTY users will not be able to interact. Examples of these incompatible switch-based features may include Call Waiting, Call Forwarding (in some cases), and Call Trace.
For reasons of network reliability, there are strict regulations concerning what can and cannot be plugged into the telephone network. These especially cover voltage and current limits, for connecting the phone line to the house wiring might damage the telephone line and other lines in the same cable, the telephone line card in the central office, and other equipment. These regulations complicate connecting assistive devices to the network; see Recommendation F2.
More and more local areas can provide digital service, such as ISDN, to both residences and business. Digital service offers greater speeds for data communication, which is what most residential customers use it for. At speeds of 128 kilobits per second for Basic Rate ISDN, videotelephony becomes quite realistic. Some users have found it acceptable for the transmission of sign language.
Digital subscriber line (DSL) offers even greater speeds, and is being deployed
in many parts of the U.S. DSL is
installed “on top of” an existing telephone line.
That is, no changes are required to the installed voice telephones, but
an additional adapter allows direct connection to computers.
The most commonly available residential DSL service is asymetrical (ADSL),
with a faster speed downstream than upstream.
This accommodates typical applications like web browsing, but can be
a disadvantage where high upstream speed is needed, as in videotelephony.
The speeds are dependent on the distance between the point of usage and
the switch providing DSL service.
Digital service poses a problem for users of analog equipment, such as TTYs. Analog equipment cannot be plugged into digital line jacks. Adapters that convert digital lines into analog are available for this purpose.
One blind ISDN user commented:
“Once I got ISDN my Internet connection was much faster, but I lost
something too. I didn’t get
to hear the tones of the modem dialing and handshaking.
So I was not always sure I was connected.
I have to keep repeating the dial-up window in [screen reader name] to
see if I’m on yet.”
Centrex is a special wireline service offered customers needing many lines, such as businesses and public institutions. It can provide a range of features similar to PBXs, such as call hold, call transfer, conference calling, restricted lines, voice mail, etc. In some states it is possible for residential customers to get Centrex features.
Public voice mail systems (VMS), which can serve residential or business customers, are connected to the switch. The service is configured so that calls directed to a voice mail subscriber’s line that reach a busy signal or get no answer within a certain number of rings are forwarded to the VMS along with the subscriber’s number. The VMS plays that customer’s outgoing message and records the caller’s message. In wireline the VMS then signals the CO switch to apply stutter dial tone to the subscriber’s line.
A user who is blind:
“One reason I switched to [company name] voice mail is that I don’t
need to see the machine and I can use the voice [menus] for changing how the
voice mail works.”
There are devices that convert this stutter dial tone into a visible signal. Some TTY users can recognize stutter dial tone on their TTY’s LED line indicator.
When the subscriber accesses the VMS and clears all the pending messages, the VMS signals the CO switch to restore regular dial tone to that line.
In wireless the VMS tells the wireless switch to signal the presence of a new message to the handset, using the base station in contact with that handset to deliver the message. If the handset is turned off, the message may be stored in the VMS until the handset is activated.
Voice mail can be useful to people who cannot use Call Waiting. Because messages can be left while the line is busy, the subscriber can check for messages after any call to see if someone called during it.
There are two compatibility problems between TTYs and voice mail. First, the menu for accessing messages and making changes to the service is all in voice. Second, some public voice mail platforms do not seem to record or play back TTY messages well, possibly because they use compression technologies to save disk space that distort the Baudot signal. Both telephone companies and voice mail platform manufacturers are aware of this problem. For more details on public voice mail and TTYs, see the Appendix on TTY Access to Voice Mail.
These are several wireline network messages that are currently delivered in voice. They include:
These messages are provided by a switch adjunct when the CO switch is triggered by whatever condition caused the “error.” Some of these adjuncts are available with TTY capability for the same reason that they can play messages in languages other than English. They typically play the TTY version of the message immediately after the voice version. Note that the selection of the adjunct as well as whether and how it plays the TTY version of the message is the responsibility of the telephone company providing service to the customer’s line.
At several points the content of a call may be converted between analog and
digital forms, or between several digital forms.
When the voice signal is converted from analog to digital or when one
digital form is converted to another, the network element involved may unintentionally
introduce errors. These are usually
unimportant for voice communication, but may interfere with TTY communication.
(See Guideline 37.)
The Advanced Intelligent Network (AIN) is a new telephone technology that moves service feature functionality out of the central office (CO) switch and onto a separate element. This new element, as well as all the attendant hardware and software, are manufactured by the same companies that manufacture CO switches and develop CO switch software.
In AIN the CO switch is no longer responsible for service features like call waiting or three-way calling. This allows different customers in the same CO to have different services and “flavors”: call waiting can work one way for one customer and a different way for another customer.
One of the purposes of implementing AIN is to allow for mass customization of telephone service. Customers will be able to arrange their telephone service features as they choose, based on individual preferences rather than on platform constraints. AIN architecture is designed to permit the rapid implementation of new features, at lower cost. AIN promises to reduce costs to the point where a relatively small customer base can support almost any desired feature. Stockbrokers can hear stock quotes instead of dial tone when they pick up the phone, sports fans can have scores delivered to them on a scheduled basis, etc.
Persons with disabilities may be able to benefit from an AIN approach.
Both individuals with disabilities and their advocates have proposed
a number of features that could improve telephone service for people whose hearing,
vision, mobility, speech, or cognition are limited.
Note: Switches and adjuncts operate only in conjunction with CPE. See the specific CPE sections for more details.