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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. Objective • Understand various definition of standards related to this layer • Identify the characteristics and role of physical layer • Understand 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 media • 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. • Electrical specification such as the electronic signals for certain pins. • Functional 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. EIA-232-D • 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. – 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. • Not 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 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 Return Null Modem Transmit Receive Signal Ground Transmit DTE-DCE (Synchronous) • 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. Serial communications • 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: – 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. Serial communication controller • 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 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. • The modem is always either in the command mode, or the online 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. Basic Commands • 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 characters • +++, 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 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. An example: ATD 27888639 • Below shows the connection to CityU's Link Plus to access the Internet service. • The AT commands include ATZ, AT10, ATTD2xxxxxx etc. X.20 and X.21 Recommendation • For digital networks such as X.25 packet switching network, • 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. 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. • 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. Current Loop • 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. • 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 connection