A Typical Modem Connection

written by: Leon Tufallo; article published: year 2007, month 09;


In: Root » Electronics and communication » Network security » A Typical Modem Connection

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The devices involved in a modem connection belong to one of two groups: data terminal equipment (DTE) or data communications equipment (DCE).

NOTE

Interestingly, the Electronic Industries Association (EIA) defines DCE as data communications equipment. However, the International Telecommunication Union-Telecommunications Standards Sector (ITU-TSS, or ITU-T) defines DCE as data circuit-terminating equipment.


Examples of the DTE devices are

  • PCs

  • Routers

  • Mainframe computers

DCE devices include

  • Modems

  • Channel service units/data service units (CSUs/DSUs)

Communication Between DTE Devices

Communication between DTE devices is accomplished through communication between DCE devices.

In other words, DTE-to-DTE communication involves three stages:

  • DTE(1)-to-DCE(1)

  • DCE(1)-to-DCE(2)

  • DCE(2)-to-DTE(2)

Each of the three stages requires different cabling and configuration. The next section describes how the DTE-to-DCE interface defined by the EIA/TIA-232 standard works. (TIA stands for Telecommunications Industries Association.)

DTE-to-DCE Communication

Out of the 25 pins available in a DB-25 connector, only eight are actually used for signaling to connect a DTE to a DCE. The remaining 17 signals are disregarded. In turn, the eight utilized signals can be divided into three categories. These categories and their corresponding signals are described below:

DTE-to-DCE Signals
Category Signal Function
Data transfer Transmit Data (TxD) The DTE transmits data to the DCE.
Receive Data (RxD) The DTE receives data from the DCE.
Ground (GRD) or pin 7 Provides the ground reference for voltage measurements.
Hardware flow control Request to Send (RTS) Indicates that the DTE has buffers available to receive from the DCE.
Clear to Send (CTS) Indicates that the DCE has buffers available to take data from the DTE.
Modem control Data Terminal Ready (DTR) The DTE tells the DCE that it can accept an incoming call.
Carrier Detect (CD) The DCE has established a carrier signal with the remote DCE.
Data Set Ready (DSR) or pin 6 The DCE is ready for use (a pin is not used on modem connections).


Modem Control Functions

The modem control category signals are sent between the DTE and the DCE to open or close the connection. They also check the connection status. An existing connection termination can be initiated by a DTE or a DCE.

When a termination is prompted by a DTE device, the access server drops the DTR signal. The modem must understand that the connection needs to end when a DTR signal is no longer present. In a DTE-initiated termination with an improperly configured modem control, the DTR signal might not be dropped or recognized, and the modem might not hang up as a result.

When a termination is initiated by a DCE device, the modem must correctly reflect the state of the carrier with the CD signal. The access server recognizes that the CD signal is low and therefore drops the connection. During a DCE-initiated termination with an improperly configured modem control, the CD signal might not be dropped or recognized, and you might get into someone else's modem session by mistake.

DCE-to-DCE Communication

When a modem has data to send, the following sequence of events takes place:When a modem has data to send, the following sequence of events takes place:

1. DTE data enters the sending modem via the TxD pin. When DTE sends data to a DCE and the sending modem's buffer is nearly full, a DCE can control flow (via hardware) by lowering the CTS signal. This way, the DTE knows not to use TxD.

2. Data is compressed. At the data compression stage, the sending and receiving modems agree on the compression algorithm. A standard MNP 5 or V.42bis algorithm is used.

3. Data is packetized. The following tasks are performed:

  • Windowing
  • Checksum
  • Error control
  • Retransmission

4. Data is modulated from digital into analog signals.

5. Data is sent over the telephone network.

When the receiving modem gets the data, it performs the same steps as just listed. Only this time, the order is reversed and is as follows:

  • The signal is demodulated.
  • The data is depacketized.
  • The data is decompressed.
  • The data is delivered to the destination DTE.

If the receiving DTE is unable to receive data on RxD, it can send an RTS signal.

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