Week3ia

Explain the differences between:

a. Synchronous and asynchronous communications

Both of these forms of communication are a means of transmitting data. The difference is in the format that the data is transmitted.

Synchronous Mode

Transmissions that are timed by a clocking signal and occur with equal time intervals between them. Synchronous mode does not require a start and stop codes as in asynchronous mode. See also Asynchronous Mode.

Asynchronous Mode

Not synchronous. A way to send transmissions by starting and stopping transmissions with a code rather than sending transmissions at specific time intervals as in synchronous mode. Asynchronous communication devices do not have to be synchronized with a clocking signal, which is required with synchronous transmission. Also frequently referred to as ATM or Asynchronous Transfer Mode. See also Synchronous Mode.

Asynchronous communications is the method of communications most widely used for PC communication and is commonly used for e-mail applications, Internet access, and asynchronous PC-to-PC communications. Through asynchronous communications, data is transmitted one byte at a time with each byte containing one start bit, eight data bits, and one stop bit, thus yielding a total of ten bits. With asynchronous communications, there is a high amount of overhead because every byte sent contains two extra bits (the start and stop bits) and therefore a substantial loss of performance.

Synchronous communications is the more efficient method of communications. CQ's connectivity solutions communicate through the synchronous method of communications.

Through synchronous communications, data is transmitted as frames of large data blocks rather than bulky individual bytes. One advantage of synchronous is that control information is easily inserted at the beginning and end of each block to ensure constant timing, or synchronization. Another advantage of synchronous is that it is more efficient than asynchronous. For example, a 56 Kbps dial-up synchronous line can carry 7000 bytes per second (56000/8) compared to a 56 Kbps dial-up asynchronous line which can only carry 5600 bytes per second (56000/10). When transmitting large amounts of information, this translates into a significant increase in speed and performance.

b. Analog and digital signals

Analog and Digital Transmission

There are a number of differences between analog and digital transmission, and it is important to understand how conversions between analog and digital occur. Let's look first at the older form of transmission, analog.

Analog Transmission

An analog wave form (or signal) is characterized by being continuously variable along amplitude and frequency. In the case of telephony, for instance, when you speak into a handset, there are changes in the air pressure around your mouth. Those changes in air pressure fall onto the handset, where they are amplified and then converted into current, or voltage fluctuations. Those fluctuations in current are an analog of the actual voice pattern--hence the use of the term analog to describe these signals (see Figure 2.9).

Analog transmission

When it comes to an analog circuit--what we also refer to as a voice-grade line--we need to also define the frequency band in which it operates. The human voice, for example, can typically generate frequencies from 100Hz to 10,000Hz, for a bandwidth of 9,900Hz. But the ear does not require a vast range of frequencies to elicit meaning from ordinary speech; the vast majority of sounds we make that constitute intelligible speech fall between 250Hz and 3,400Hz. So, the phone company typically allotted a total bandwidth of 4,000Hz for voice transmission. Remember that the total frequency spectrum of twisted-pair is 1MHz. To provision a voice-grade analog circuit, bandwidth-limiting filters are put on that circuit to filter out all frequencies above 4,000Hz. That's why analog circuits can conduct only fairly low-speed data communications. The maximum data rate over an analog facility is 33.6Kbps when there are analog loops at either end.

elicit meaning from ordinary speech; the vast majority of sounds we make that constitute intelligible speech fall between 250Hz and 3,400Hz. So, the phone company typically allotted a total bandwidth of 4,000Hz for voice transmission. Remember that the total frequency spectrum of twisted-pair is 1MHz. To provision a voice-grade analog circuit, bandwidth-limiting filters are put on that circuit to filter out all frequencies above 4,000Hz. That's why analog circuits can conduct only fairly low-speed data communications. The maximum data rate over an analog facility is 33.6Kbps when there are analog loops at either end.

How 56Kbps Modems Break the 33.6Kbps Barrier

With 56Kbps modems, only one end of the loop can be analog. The other end of the connection has to be digital. So, in other words, if you're using a 56Kbps modem to access your Internet service provider (ISP), you have an analog connection from your home to the local exchange. But the ISP has a digital subscriber line (DSL) or a digital termination facility from its location to its exchange.

Analog facilities have limited bandwidth, which means they cannot support high-speed data. Another characteristic of analog is that noise is accumulated as the signal traverses the network. As the signal moves across the distance, it loses power and becomes impaired by factors such as moisture in the cable, dirt on a contact, and critters chewing on the cable somewhere in the network. By the time the signal arrives at the amplifier, it is not only attenuated, it is also impaired and noisy. One of the problems with a basic amplifier is that it is a dumb device. All it knows how to do is to add power, so it takes a weak and impaired signal, adds power to it, and brings it back up to its original power level. But along with an increased signal, the amplifier passes along an increased noise level. So in an analog network, each time a signal goes through an amplifier, it accumulates noise. After you mix together coffee and cream, you can no longer separate them. The same concept applies in analog networks: After you mix the signal and the noise, you can no longer separate the two, and, as a result, you end up with very high error rates.

Digital Transmission

Digital transmission is quite different from analog transmission.