Communication methods

• Communication of data can take place between different devices, for example, when sending an email or an SMS message. Data is also communicated between the hardware components of a single device, for example, when a computer CPU exchanges data with a peripheral device such as a printer, or when data is fetched to the CPU from the main memory.
• In synchronous transmission, streams of bits are transferred over a communication channel at a constant rate. The transmitter and the receiver are synchronised using a common clock signal.
• In asynchronous transmission, there is no clock signal, so additional data (start and stop bits) is used to control the communication. Data is transmitted when it is available, rather than at specific intervals. This means that there can be periods of time when the transmission channel is idle.
• In asynchronous transmission, the beginning and end of a transmission are communicated using start and stop signals: a start bit is sent at the beginning of the transmission so that the receiver can prepare for the incoming data, and a stop bit marks the end of the transmission. The stop bit needs to be opposite to the start bit so that the receiver can recognise the next set of bits. Also, the stop signal is typically longer than one bit so that the receiver has time to get ready for the arrival of the next set of data, ensuring that the sender and receiver are in sync.
• In synchronous transmission, there is no need for a start or stop bit, so more information can be sent per unit of time than in asynchronous transmission. Synchronous transmission is commonly used with parallel transmission. For example, inside a computer, the clock signal is used to synchronise the signals of the control bus and of the data transmitted over the address and data buses.
• There are two main ways to transmit data using wires:

  • In serial transmission, data bits are sent in a sequence, one after the other, over a single wire.
  • In parallel transmission, several bits are sent at the same time over their own dedicated wires or printed circuit tracks. This type of transmission is only reliable over short distances, so it is mainly used inside a computer and in some early peripherals. The parallel connections used are known as 'buses' and are typically 8, 16, or 32 lines wide.
• Although parallel transmission sounds like the better option, it can only work efficiently over shorter distances and with lower bit rates than serial transmission. This is mainly due to the issues of skew and crosstalk, which become more prominent when using high bit rates and longer wires.
• Crosstalk occurs when electromagnetic interference between wires that are in proximity (such as parallel links) results in transmitting corrupted data that will need to be re-sent.
• Skew happens when the bits that are transmitted across parallel links travel at different speeds. In synchronous data transmission, this can result in data falling out of sync with the clock signal and therefore not being read correctly.