SYN = Synchronise
STX = Start of text
ETX = End of text
Above is an example of a packet structure used with Asynchronous sending.
Bit and Baud Rate
When the term bit rate is used, it means the amount of bits per second that are sent. A single character is to be sent for example, the character is ASCII code is made of 7 digits and a parity bit and this is called the useful data. The standard speed for transmission sending is 8bits a second.
This number or ASCII code that is sent will not only comprise of the 7 bit and parity, it contains a start bit and an end bit to represent where the character starts and stops. The bit rate is then combined to include these two extra bits and this is called the baud rate, this would then be 10 bits a second (8+2).
Start bit Stop bit
Parity bit
Asynchronous and Synchronous
Asynchronous communications do not depend on the synchronisation of a transmitter or a clock. It can be said that they are never independent. When sending individual characters there is no need for synchronisation at all. Asynchronous has a very simple interface and the data rate is normally at 64kbps. It can be used for connecting Printer, Terminal, Modem and home connections to the internet. Asynchronous protocol was one of the first around and was popular with the early invention of the tele typewriters. It requires the use of start and stop bits as previously said and this provides byte timing and overhead. No clocks are needed and a parity bit is also sent to help with error detection. Asynchronous transmission will send single bits of data at a time. There is a standard for this form and it is called RSC323C. Asynchronous represents negative voltage as 1 and positive voltage as 0. Information is sent in a packet with the start and stop bit plus parity for error detection. This method of transmission is suitable for low speed. This is because for every character that is sent a further 2 bits (start and stop) must be added. For example, 1000 characters are to be sent, there are 8 bits per character plus a start and stop for each that’s a total of 10000 bits. This is very in efficient. Its packet structure is known as a packet frame.
Asynchronous Transmission
Start bit Stop bit
Parity bit
The synchronous transmission form occurs when bits are sent in the packet form. This form of data sending is normally sent through bigger Networks. To calculate the efficiency you must divide the bit rate by the baud rate and then multiply by 100.
Synchronous communication is far more efficient than its counterpart. To start, there is no need for a start and a stop bit. This time however the data is grouped, and then sent with characters and the start and the end of the packet. This can be seen above with the syn x 3 which stands for synchronisation idol, stx for start of etx and etx for end of text. Synchronous can achieve much higher speeds than asynchronous and is preferably used within large network structures.
It would be therefore worth bearing in mind that if large amounts of data were to be sent then the synchronous form would be far better. Less overhead means greater efficiency and asynchronous just cannot match this. However asynchronous does have its benefits and would be deemed worthy of smaller data packets. It’s the lack of timing information sent with the data which restricts it so greatly.
Synchronous Transmission
Packet of data
Effect Of Bandwidth Limitation And Noise
Bandwidth is the sum amount of data being sent to a recipient in a given time through a form of communicational media. This sum is normally represented on bit per second. Bandwidth also has an alternative meaning and can represent the differing frequency ranges.
All media types have their own forms of bandwidth limitation; this means we cannot transmit the same things on the differing types of media at the same speeds.
If sending something with 10bits it will be sent at 10bits a second. This should apply for the bandwidth also (10 bits a second) however; it can be affected by something that is known as cross talk. This is interference on the line, also known as noise. Noise can occur in many forms and this will always affect speed. The only method of transmitting data without any electrical noise affecting it is by using a fibre optic cable. Fibre optic cables uses the use of light and mirrors, this is why it can not be affected. However it is also very expensive.
