Legend:
Functions of DTE in the test network are provided by an IBM-PC compatible computer and the terminal emulator program running on it (e.g. Norton Term90, Procomm Plus). Functions of DSO and E are integrated in this program. Functions of DCE are provided by a terminal adapter (modem) fitted into the digital telephone equipment or connected to it. Data transmission channel C is one B (with 64 kbit/s capacity) of the 2B+D channels built up between the digital devices.
Blocks of the model can be connected on standard connection surfaces (interfaces). In the test network I3 and I1 interfaces are inner connection points, which are not available for users. I2 (DTE-DCE) interface is a serial duplex asymmetric interface , which is used primarily for connecting the data transmission terminal to a modem. (Serial interfaces of the IBM-PC compatible computers (COM1: COM2:) are like this, too.) This interface is specified by more organizations dealing with international standardization (ITU-T, EIA, ISO) According to it, this interface is usually named also ITU-T V.24/V.28 or EIA RS232C interface.
Specification of an interface generally consists of the following information:
Overview the specifications of I2 DTE-DCE interface according to these aspects:
Data changing cables:
This recommendation specifies numerous data changing cables, user can use from it the amount necessary for the given task. Here are the data changing cables used in the introduced system:
| ITU-T Id | RS232 Id | ISO 2110 pin number | Function | DTE-DCE direction | Type |
|---|---|---|---|---|---|
| 101 | FGND | 1 | Protective ground | -- | |
| 102 | SGND | 7 | Common ground | -- | |
| 103 | TD | 2 | Data transmission | -> | D |
| 104 | RD | 3 | Data reception | <- | D |
| 105 | RTS | 4 | Data request | -> | C |
| 106 | CTS | 5 | Ready to transmission | <- | C |
| 107 | DSR | 6 | DCE ready | <- | C |
| 108 | DTR | 20 | DTE ready | -> | C |
| 109 | DCD | 8 | Data channel received signal detector | <- | C |
| 113 | TTC | 24 | Transmitter signal clock ( from DTE ) | -> | CK |
| 114 | TC | 15 | Transmitter signal clock ( from DCE ) | <- | CK |
| 115 | RC | 17 | Receiver signal clock | <- | CK |
Electrical characteristics:
Based on the recommendation of ISO 2110: Connection: 25 pin D-type connection
Frame formats:
Format of signals going through the interface - framing - is not the specification of the interface, but the DTE-DCE. For framing of data we usually use the following three simple methods:
In the test network terminals and modems operates in an asynchronous mode.
In asynchronous (start-stop) transmission mode frame consists of only one character and its build-up is the following:
Name of the asynchronous mode derives from that the clock signal is not forwarded on the transmission way. Start bit indicates the beginning of the frame, and stop bit indicates the end of the frame. A parity bit is applied for error detection. Parameters of the frame can be programmed:
For reception of the short frame it is not necessary to have the transmission of timing information, it is enough, if the bit rate of the transmitter and the receiver is the same within a certain endurance. Receiver identifies the start of the frame with a pattern receiving transmission detection, which is indicated by the running down edge of the start bit.
The character-based asynchronous transmission can be realized with simple devices, but the applied synchronization solution can only allow realizations with a small rate.
In case of the character-based synchronous transmission we transmit several characters in one frame. General format of the frame:
+--------------------------------------------+
? SYN ? data field /characters/ ? BCC ? SYN?
+--------------------------------------------+
Start and end of the frame are indicated with special /SYN/ characters, these are must not appear in the data field. Error protection of the frame is executed with a cyclic control code (BCC). The long frame makes necessary the transmission of the timing information, too. The character-based synchronous transmission provides a bigger transmission rate, than the asynchronous one. Its drawback, that there are several limits to the forwarded messages (character organization, exclusion of certain characters).
The bit-organized synchronous transmission (HDLC) eliminates this imperfections. Start and end of the frame is indicated with a special character: 01111110. HDLC process provides the character-independent ? transparent ? transmission, that in the data field the transmitter after five bits with value 1 following each other automatically inserts a zero bit. Receiver removes automatically these inserted zero bits from the received bit series. Error protection of the frame is executed with a cyclic control code.
Connection set up processes
Before the beginning of the data transmission we have to set up the connection between the
terminals in the line-switched network. Terminal initiates the automatic set up of the
connection, and there is an active role of the modem, too, which provides also telecommunication functions (dialling, call detection). During the time of connecting and dissociation there is a dialogue between DTE and DCE. For the protocol of the dialogue there are several recommendations. However, users use in practice ? and in the introduced system, too ? the connection process developed by an American firm named HAYES. This process controls the modem in a dialogue operation mode in different operation states with messages, commands sent through the data controllers. Set of commands consists of a basic set, and an optional set, which can be developed optionally. Every command sent to the modem by the terminal (except A/ and +++) begins with the AT prefix, which is followed by the other characters of the command. The
Example:
command instructs the modem to initiate a call for the calling number 5044 in the dialling mode of DTMF. Modem answers to the command with a message in numeric, or text format ( OK, RING, ERROR, CONNECT etc.).