Analog phone lines and devices Flashcards
What does POTS stand for?
plain old telephone
What does PSTN stand for?
publicly switched telephone network
Describe analog lines.
Analog lines are copper twisted-pair cables that provide bidirectional communications on a two-wire loop. They use a narrow frequency range and can also carry specific tones at specific frequencies within the voiceband frequency range. This provides everything needed for fax and analog lines have very high reliability.
Analog takes audio and translate it into electronic signaling. Other things were developed to assist with analog faxing, such as fax class and ITU standards. ITU standards for faxing deal with fax class and data communication over the telephone network as well as compression of the fax image.
Analog lines work with analog devices and only analog devices. Their voltage levels and frequencies are made for just this and can damage other equipment if attempted to be used in a digital network.
Some of the commonly used configuration settings for analog devices are listed below. All of the options listed below are in the FaxMaker Configuration. This information is retrieved from the modems.dev file in the FaxMaker installation directory, C:\Program Files (x86)\GFI\FaxMaker. All supported modems and their configuration specifics are listed there. This can be opened from Programs in the start menu.
Describe flow control.
In most cases, the Hardware (RTS/CTS) will be selected as the hardware of the modem itself controls the flow of the data.
Hardware flow control is called so because the control is handled by the hardware. That is, the circuitry design is such that when the UART or the CPU is ready to receive data, it sends a Clear To Send signal to the device on the other end. On the other hand, when a device has something it wants to send, it sends a Ready To Send, and waits for a CTS to come back its way. These signals are sent apart from the data itself on separate wires.
The earlier standard in flow control was via software, in which the signal to start and stop the flow of traffic is sent as part of the data. The front of the data stream has an “Xon” signal, and the “Xoff” is the end. This type of flow control has a large margin of error. If an Xon signal is lost or corrupt, all the data sent will be lost, since the receiving device has to wait for the next Xon character to even start paying attention to the signal.
Describe fax class.
Computer modems are often designated by a particular fax class, which indicates how much processing is offloaded from the computer’s CPU to the fax modem.
Class 1 (also known as Class 1.0) fax devices do fax data transfer, while the T.4/T.6 data compression and T.30 session management are performed by software on a controlling computer. This is described in ITU-T recommendation T.31. What is commonly known as "Class 2" is an unofficial class of fax devices that perform T.30 session management themselves, but the T.4/T.6 data compression is performed by software on a controlling computer. Implementations of this "class" are based on draft versions of the standard that eventually significantly evolved to become Class 2.0. All implementations of "Class 2" are manufacturer-specific. Class 2.0 is the official ITU-T version of Class 2 and is commonly known as Class 2.0 to differentiate it from many manufacturer-specific implementations of what is commonly known as "Class 2". It uses a different but standardized command set than the various manufacturer-specific implementations of "Class 2". The relevant ITU-T recommendation is T.32. Class 2.1 is an improvement of Class 2.0 that implements faxing over V.34 (33.6 kbit/s), which boosts faxing speed from fax classes "2" and 2.0, which are limited to 14.4 kbit/s. The relevant ITU-T recommendation is T.32 Amendment 1. Class 2.1 fax devices are referred to as "super G3".
Describe voice chipset.
If a modem has a voice chipset, for fax purposes, this means the modem can detect DTMF tones. This will be used with inbound fax routing. Most modems do not have a voice chipset and therefore are not capable of supporting capturing DTMF tones.
Describe DLE codes method (DTE->DCE).
To understand DLE (Data Link Escape) message we must first understand modem modes. Most modems have two modes: command and data.
Command mode is the mode the modem is in when receiving commands from the DTE (data terminal equipment)
Data mode is the mode the modem is in when transmitting data, for our use, through the fax line.
DLE is not used for standard modems in an internet connection, for example, as the commands are given in command mode and the data is transmitted in data mode and data mode ends when the connection is terminated. DLE messages are only used by a modem for fax and voice applications since the modems rapidly switch between command and data modes during a call. It is not just handing data from point A to point B, but it is negotiating parameters and pages with another fax machine or switching audio modes or transmitting or receiving data, again and again depending on the number of pages in the fax. A DLE message can be sent from the modem to the computer followed by the number 2 which would specify the caller pressed the number 2 on their phone keypad. This could happen at any time as long as the call is connected. These are also known as out of band messages.
The DTE in our case is the PC or server that is interacting with the modem and the DCE is the modem itself. The settings here will vary by modem type and manufacturer.
Describe analog logging.
The logs for the fax lines for analog devices will vary by device.
Devices that use COM ports will produce line logs (logs which show the interactions on the line used for transmission error troubleshooting) called fax_COMx.gfi_log.txt, where x = the COM port. This will apply to all analog modems and Mainpine devices.
Brooktrout devices, all physical cards, will produce line logging in log files called ltbtrtxx.txt, where xx = the line number in the FaxMaker Configuration > Lines and Devices.
