e Data Terminal Equipment Equipment (DTE) (DCE) DCE-line side interface DTE typically: work DCE typically: modem DTE-DCE Interface station, computer or or channel service PC, other peripherals units (CSU)/data service unit (DSU) Figure 10.9 Data circuit interfaces, physical layer. The most well-known DTE-DCE standard was developed by EIA/TIA (Electronic Industries Association/Telecommunication Industry Association) and identified by the familiar EIA-232 (Ref. 5). At this writing the F version is current. We suggest drop- ping the letter after 232 and just add latest version. EIA-232 is essentially equivalent to ITU-T Recs. V.24 and V.28 and ISO IS2110. EIA-232 is applicable to the DTE-DCE interface employing serial binary data inter- change. It defines signal characteristics, the mechanical interface and gives functional descriptions of the interchange circuits. This standard is the old workhorse of data commu- nications and is applicable to data rates up to 20 kbps and for synchronous/asynchronous serial binary data systems. EIA/TIA has subsequently published many other standards for the DTE/DCE interface. Unlike EIA-232, these standards only provide the mechanical and functional interfaces. The electrical interface is left to still other documents, typically EIA-422 and EIA-423. We think the following concept is crucial for the reader's appreciation of data-link physical layer. It is from Section 2.1.3 of EIA-232 (Ref. 5). For data interchange circuits, the signal shall be considered in the marking condition when the voltage (V1) on the interchange circuit, measured at the interface point, is more negative than minus three volts with respect to circuit AB (signal ground). The signal shall be considered in the spacing condition when the voltage V1 is more positive than plus three volts with respect to circuit AB. The region between plus three volts and minus three volts is defined as the transition region. The signal state is not uniquely defined when the voltage (V1) is in this transition region. During the tra