Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

16.2 PHYSICAL LAYER

The physical layer specifies the physical interface between devices. This layer describes the mechanical, electrical, functional, and procedural characteristics of the interface.

An example of the physical layer is Electronic Industries Association (EIA) RS232, which specifies the serial communication interface. We connect a modem to the PC through the RS232 interface. The modem is referred to as DCE (data circuit terminating equipment) and the PC as DTE (data terminal equipment). RS232 specifications are briefly described below to give an idea of the detail with which each layer in the protocol architecture is specified.

Mechanical specifications of RS232 specify that a 25-pin connector should be used with details of pin assignments. Pin 1 is for shield, pin 2 is for transmit data, pin 3 for receive data, pin 4 for request to send, pin 5 for clear to send, pin 6 for DCE ready, pin 7 for signal ground, and so on.

The physical layer specifies the mechanical, electrical, functional, and procedural characteristics of the interface. RS232 is an example of physical layer specifications.

Electrical specifications of RS232 give the details of voltage levels for transmitting binary one and zero, data rates, and so on. With respect to ground, a voltage more negative than –3 volts is interpreted as binary one and a voltage more positive than +3 volts is interpreted as binary zero. The data rate supported is less than 20kbps for a distance less than 15 meters. With good hardware design, higher data rate and higher distance can be supported; these are the minimum requirements given in the standard for data rate and distance.

Functional specifications of RS232 give the details of the data signals, control signals, and timing signals and how to carry out loop-back testing.

Procedural specifications of RS232 give the details of sequence of operations to be carried out for specific applications. For instance, if a PC is connected to the modem using RS232, the procedure is as follows:

• When modem (DCE) is ready, it gives the DCE ready signal.

• When PC (DTE) is ready to send data, it gives the request to send signal.

• DCE gives the clear to send signal indicating that data can be sent.

• DTE sends the data on the transmit data line.

A detailed procedure is specified to establish the connection, transfer the data, and then to disconnect. The procedural specifications also take care of the possible problems encountered during the operation such as modem failure, PC failure, link failure, and so forth.

When you connect two PCs using an RS232 cable, you need to specify the communication parameters. These parameters are the speed (300bps, 600bps, 19.2kbps, etc.), the number of data bits (seven or eight), the number of stop bits (one or two), and the parity (even or odd). Only when the same parameters are set on both the PCs can you establish the communication link and transfer the data.