The format of network communications

Every data transmission has three fundamental characteristics. Format (analog vs. digital), synchronization (synchronous vs. asynchronous), and how communication sessions are handled (baseband vs. broadband). These characteristics are discussed next.

Analog vs. Digital

To transfer data from one location to another, a signal must be created. This signal serves as the medium by which data is transported. There are two signaling formats; analog and digital. Analog signals are used by technologies such as radios and have a wave shape. The wave shape enables an analog signal to express an infinite amount of values that flows continuously. Digital signals, on the contrary, are discreet and a voltage within a certain range will represent either a 0 or a 1. As a comparison, think of how an analog watch is different than a digital watch. With an analog watch, hands are used to convey the current time, and, if the watch has a second hand, the time is constantly flowing. A digital watch simply read the time at that moment. There is no movement or flow involved.

For the purposes of networking, digital signals are the preferred format. Computers have always processed data in 0's and 1's. When telecommunications networks carried only analog signals computers required a modem to modulate and demodulate analog signals. With new advances this is no longer needed. Digital signals are also able to travel for much further distances before they degrade. These are a couple of the reasons why telecommunications and other networks are moving to all digital formats.

Asynchronous and Synchronous

When two machines need to share data over a network, this is much like two people having a conversation. As people speak with each other, there are natural pauses between sentences and thoughts. Theses gaps allow the other to process what has been said, and it helps form a natural rhythm to the conversations. We also have rules for written language. Periods and commas can show when a thought ends, and spaces seperate words into individual units. These rules allow us to synchronize our communications. Just as we have grammar rules to synchronize our conversations, so do computers. Asynchronous and synchronous describe two different sets of rules for how computers communicate with one another.

In asynchronous mode, start and stop bits are used to distinguish when a character starts and ends. This is done for the whole message so that the receiving system is sure to correctly interpret the message. This is just like the earlier example of inserting spaces between words so that a person can easily read them. In synchronous communication, there are no start and stop bits used. Instead, the data is transmitted in a continuous stream. To synchronize the transmission, a clock pulse is used. This is similar to when we verbally talk and I use pauses to form a natural rhythm. For synchronous transmission to be used, both systems must be using a synchronous protocol such as high-level data link control (HDLC). This protocol allows the system to interpret the information it's sent. Asynchronous transmission happens through a protocol known as asynchronous transmission mode (ATM).

Baseband and Broadband

Baseband and broadband refer to how communication sessions are handled in the physical transmission media. A baseband technology uses all of the communication channel for its transmission. A broadband technology divides the channel into sub-channels so that multiple transmissions can occur simultaneously. For instance, a coaxial cable TV is a broadband technology that delivers multiple television channels over the same cable.

There is an important distinction that just because a technology could transmit multiple signals on one channel, doesn't mean it is broadband. Unless there are specific rules for how the channel will be divided, it is still a baseband technology. As an analogy, think of a large one lane highway. Because there is only one lane through which to travel it is baseband. But if we go and paint white lines down the middle (put in rules for dividing the channel) the highway now supports more traffic and is broadband.

All of these characteristics come together to make up a transmission technology. For instance, WiFi is an analog transmission that uses ATM for synchronization and is broadband because it divides set frequencies into channels. These characteristics exist for every transmission media and are important to understanding how data is transmitted.