Chapter 6 - Physical Layer It is about the characteristics of Physical Layer, the lowest layer in the ISO/OSI reference model. Most of the network problems such as noise interference, incompatible cable connection and line disconnection come from this layer. It is about the characteristics of Physical Layer, the lowest layer in the ISO/OSI reference model. Most of the network problems such as noise interference, incompatible cable connection and line disconnection come from this layer.

Objective Understand various definition of standards related to this layerUnderstand various definition of standards related to this layer Identify the characteristics and role of physical layerIdentify the characteristics and role of physical layer Understand the PC communications using 8250 UART communication chipUnderstand the PC communications using 8250 UART communication chip

Physical Layer The lowest layer in ISO/OSI reference model and is about the transmission of data from one point to another over the communications mediaThe lowest layer in ISO/OSI reference model and is about the transmission of data from one point to another over the communications media This layer is dealt with the physical data transmission and is related to:This layer is dealt with the physical data transmission and is related to: –Type of transmission medium (Copper wire, air, optical fiber etc.) –The transmission media such as telephone cable are sometimes regarded as Layer 0. –Modulation and demodulation scheme (FSK, PSK, digital data). –Message switching techniques

Four Specification Mechanical specification such as socket layout, pin size etc.Mechanical specification such as socket layout, pin size etc. Electrical specification such as the electronic signals for certain pins.Electrical specification such as the electronic signals for certain pins. Functional specification such as timing or clocking signalsFunctional specification such as timing or clocking signals Procedural specification such as the sequence of events required to effect the data transfer across a pair of modems.Procedural specification such as the sequence of events required to effect the data transfer across a pair of modems.

EIA-232-D The EIA standard RS-232C (RS refers to Recommended Standard) was developed in 1969 is still widely used.The EIA standard RS-232C (RS refers to Recommended Standard) was developed in 1969 is still widely used. The latest version is called RS-232D.The latest version is called RS-232D. –Electrical, ITU-T V.28 –Functional, ITU-T V.28 –Mechanical, ISO 2110 –Procedural, CCITT V.24

Characteristics This is a 25-pin connector with data, control, timing and ground signal pins.This is a 25-pin connector with data, control, timing and ground signal pins. Not all of them are used for data transmissionNot all of them are used for data transmission

DTE-DTE (Computer to Computer port) When terminals are connected directly to a computer port, it is necessary to cross over some of the wires in order to communicate properly.When terminals are connected directly to a computer port, it is necessary to cross over some of the wires in order to communicate properly. When two computer ports are connected running at synchronous protocol, either one should provide the correct timing or a null modem with proper timing signals is required.When two computer ports are connected running at synchronous protocol, either one should provide the correct timing or a null modem with proper timing signals is required.

PC to PC Three Pins, Transmit, Receive and Signal ReturnThree Pins, Transmit, Receive and Signal Return

DTE-DCE (Synchronous) This is a normal configuration of the V24 interface between the computer and modem.This is a normal configuration of the V24 interface between the computer and modem. It therefore uses the straight through cable. That is to say there is no cable cross.It therefore uses the straight through cable. That is to say there is no cable cross.

Serial communications It takes place by transmitting and receiving data in a stream of consecutive electrical pulses that represents bits.It takes place by transmitting and receiving data in a stream of consecutive electrical pulses that represents bits. The EIA (Electronic Industries Association) has recommended several standards for serial communication such as:The EIA (Electronic Industries Association) has recommended several standards for serial communication such as: –RS-232-D (Similar to ITU-T V.24) –RS-422 –RS-423 –RS-449

Wiring Diagram Apart from the conventional wiring diagrams between computer & modem and computer & computer.Apart from the conventional wiring diagrams between computer & modem and computer & computer.

Serial communication controller The fundamental element of the serial port is an IC called 8250, universal asynchronous receiver and transmitter (UART).The fundamental element of the serial port is an IC called 8250, universal asynchronous receiver and transmitter (UART).

Modem Commands The modem supports the standard and extended Hayes* AT command set.The modem supports the standard and extended Hayes* AT command set. The AT prefix (also known as the Attention Code), signals the modem that one or more commands are to follow.The AT prefix (also known as the Attention Code), signals the modem that one or more commands are to follow. These commands are industry standard language used to communicate with the modem.These commands are industry standard language used to communicate with the modem. The modem is always either in the command mode, or the on-line mode.The modem is always either in the command mode, or the on-line mode. Commands are only accepted by the modem when it is in command mode.Commands are only accepted by the modem when it is in command mode. Commands may be entered from the terminal mode of most communications software packages.Commands may be entered from the terminal mode of most communications software packages.

Basic Commands AT, This prefix begins all but two commands you issue to the modem locally,, and tells the modem "ATtention! commands to follow".AT, This prefix begins all but two commands you issue to the modem locally,, and tells the modem "ATtention! commands to follow". D, Dial. Use the D command to dial a telephone number from the command line. The format of the command is ATD and the string parameter can contain up to 45 charactersD, Dial. Use the D command to dial a telephone number from the command line. The format of the command is ATD and the string parameter can contain up to 45 characters +++, Escape Character Sequence. After the users have connected to another modem+++, Escape Character Sequence. After the users have connected to another modem A/, Repeat. This command does not use the AT prefix nor does it require a carriage return to enter. Typing this command causes the modem to repeat the last command line enteredA/, Repeat. This command does not use the AT prefix nor does it require a carriage return to enter. Typing this command causes the modem to repeat the last command line entered Zn, Reset Modem. This command (in which n=0 or 1) resets the modem to the configuration profile stored in non-volatile memory location 0 or 1.Zn, Reset Modem. This command (in which n=0 or 1) resets the modem to the configuration profile stored in non-volatile memory location 0 or 1.

An example: ATD Below shows the connection to CityU's Link Plus to access the Internet service.Below shows the connection to CityU's Link Plus to access the Internet service. The AT commands include ATZ, AT10, ATTD2xxxxxx etc.The AT commands include ATZ, AT10, ATTD2xxxxxx etc.

X.20 and X.21 Recommendation For digital networks such as X.25 packet switching network,For digital networks such as X.25 packet switching network, X.20 and X.21 are another interface standards.X.20 and X.21 are another interface standards. X.21 is a 15-pin connector. However, only eight lines as shown below are required.X.21 is a 15-pin connector. However, only eight lines as shown below are required.

EIA RS-449/RS-422/RS-423 RS449 corresponding to CCITT (ITU-T) V.35 is a 37-pin connector and supports transmission rates up to 2Mbps with cable length to 60 meters.RS449 corresponding to CCITT (ITU-T) V.35 is a 37-pin connector and supports transmission rates up to 2Mbps with cable length to 60 meters. It defines both mechanical (pin and plug configuration) and procedural (signal descriptions) aspects similar to RS232.It defines both mechanical (pin and plug configuration) and procedural (signal descriptions) aspects similar to RS232. The electrical specifications (balanced or unbalanced circuit) are defined by RS422 and RS423 respectively. The electrical specifications (balanced or unbalanced circuit) are defined by RS422 and RS423 respectively.

Current Loop An obsolete technology to extend the physical distance up to 300 meters.An obsolete technology to extend the physical distance up to 300 meters. It uses the current signal, presence of current signal or absence (no current), to represent binary 0 or 1 (mark or space) instead of voltage level in RS232D.It uses the current signal, presence of current signal or absence (no current), to represent binary 0 or 1 (mark or space) instead of voltage level in RS232D. The most common current values are 20 mA and 60 mA.The most common current values are 20 mA and 60 mA. This transmission method could support half or full duplex, which requires 2 or 4 wire connectionThis transmission method could support half or full duplex, which requires 2 or 4 wire connection