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Synchronous and Asynchronous Communications

Synchronous Communication requires the sender and receiver to share the same clock. The sender provides a timing signal to the receiver so that the receiver knows when to “read” the data. Synchronous Communication generally has higher data rates and greater error-checking capability. A printer is a form of Synchronous Communication.

Asynchronous Communication has no timing signal or clock. Instead, it inserts Start/Stop bits into each byte of data to “synchronize” the communication. As it uses less wires for communication (no clock signals), Asynchronous Communication is simpler and more cost-effective. RS-232/RS-485/RS-422/TTL are the forms of Asynchronous Communications.

RS-232C, EIA RS-232, or simply RS-232, refers to the same standard defined by the Electronic Industries Association in 1969 for serial communication.

RS-232 Connections

A straight-through cable is used to connect a DTE (e.g. computer) to a DCE (e.g. modem), all signals in one side connected to the corresponding signals in the other side in a one-to-one basis. A crossover (null-modem) cable is used to connect two DTE directly, without a modem in between. They cross transmit and receive data signals between the two sides and there are many variations on how the other control signals are wired.

The data transmission starts with a Start bit, followed by the data bits (LSB sent first and MSB sent last), and ends with a “Stop” bit.

The voltage of Logic “1″ (Mark) is between -3VDC to -15VDC, while the Logic “0″ (Space) is between +3VDC to +15VDC.

RS-232 connects the Ground of 2 different devices together, which is the so-called “Unbalanced” connection. An unbalanced connection is more susceptible to noise, and has a distance limitation of 50 ft (which is around 15 meters).

Handshaking is also called “Flow Control”. The main purpose of Handshaking is to prevent receiver overloading. By using Handshaking signals, receivers will be able to tell the sending device to pause data transmission if the receiver is overloaded. There are three types of handshaking: Software handshaking, Hardware handshaking and Both.

Software handshaking uses two control characters: XON and XOFF. The receiver sends these control characters to pause transmitter during communication. XON is decimal 17 and XOFF is decimal 19 in the ASCII chart. The drawback of Software handshaking is that these two control characters can not be used in data. This is quite important when you are transmitting Binary data as you might need to use these two codes in your data.

Hardware handshaking makes use of actual hardware lines, such as RTS/CTS, DTR/DSR, and DCD/RI (for modem).

The DTE 1 uses the DTR (Data Terminal Ready) signal to indicate it is ready for information exchange, whereas the DTE 2 uses the DSR signal for the same purpose. Before sending a data, the DTE 1 asks permission by setting its RTS output to high. No data will be sent until the DTE 2 grants permission by using the CTS line.  RTS/CTS are ON or OFF for each data transmission. DCD (Data Carrier Ready) is used by the modem when a connection has been established with remote equipment, while RI (Ring Indicator) is used by the modem to indicate a ring signal from telephone line.

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@this is server implemented by socket and create usrname & password for server
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USB Device Driver

Introduction to USB:

These days we are using USB(Universal Serial Bus) devices like pen drive

,mouse,printer etc instead of using serial or parallel port devices. One of the original intentions of USB was to reduce the amount of cabling at the back of your PC. USB port can connect up to 127 peripheral devices by using hub. Averagely it gives us 12Mbps data transfer speed.

Basically host controller keep checking at the USB port to detect device. Whenever device came due to voltage difference it generate interrupt and inform to the system about the device.

Now to make USB communication between the host and the device there is need of the drivers on the both side.In our system there is core usb driver which load the appropriate interfacing driver (that is of our concern )for the usb device (mouse ,keyboard etc.) for service. In our system there are different different drivers for different type of devices to make core driver less bulky.

USB device basic:

USB device may have one or more configurations and each configuration consist of one or more interfaces and each interfaces consist of one or more endpoints. Generally there is only one configuration in more devices if more, then it will be select able by hardware switch. In our GSM modem there is facility of attaching storage device also so it has two interfaces.

Now for communication we have to send or receive data from the endpoints which is inside the interface ,these may be input endpoint or output endpoint for receiving and sending respectively.

Type of data transfer from endpoint:

Control :

Control endpoint is used to retrieve the device information at the time of enumeration of device

Enumeration is the process in which core driver take the information about the hardware from the device firmware to check the vendor id,product id ,serial number etc to load appropriate interfacing driver to serve.

Interrupt:

Interrupt endpoint is used to send the small data of in asynchronous manner like in keyboard. This endpoint is not used to transfer large amount of data.

Isochronous:

Isochronous endpoint is used to send large amount of data but it is generally used where we can accept some data loss like in video streaming where continuous stream of data is required.

Bulk:

Bulk endpoint is also used to send large amount of data and there is no loss in the data. It is used for the mass storage devices to copy the data to and from t he system. It split the data into small chunks if there is large amount of data according to the block size.

Now it is clear from above discussion that data is transferred from the endpoint. Data is generally transferred in URBs i.e USB request blocks.

Interfacing Driver:

Interfacing driver means that driver which is loaded by usb core driver for specific device to server. It communicate between the application layer and the usb core driver to serve the device request. If we want to use our interfacing driver in the system to provide service then we have to register our driver with USB subsystem first using usb_register() system call and then in our driver we also have to specify that our driver is made for which device (vendor id, product id etc.). All these information is provided in the linux/usb.h header file.

Now one important thing is that you have to remove the usbhid named driver from the system using rmmod usbhid if it is moduler otherwise recompile raw kernel by making hid(human interface device) driver moduler from static.

Thank you.

Client-server using socket(AF_INET) implemented. Also FTP, client-server netwroking based project implemented successfully.

Log file for server:

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@This is the soruce code file for server file for FTP, client server using sockets(domain AF_INET).
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Log file for client:

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->THIS IS SERVER

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@this is server which is asking for username and password from client to check its authentication
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@this is client which is requesting for connection to the server
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->THIS DATABASE OF SERVER

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