Data Communications Technical Brief

Comment: Several years ago, I was the leader of an operationalized telecommunication cell. The purpose of the cell was to monitor the effectiveness and readiness of the telecommunications in support of the ongoing operations. The staff regularly turned over due to the operational tempo and I had to train new staff quickly. I did so by preparing a series of technical briefs on topics the cell dealt with. This brief was dealing with telecommunication transmission and compression. 

Data Communications Technical Brief

Data communications are complex and there are many methodologies available in the marketplace. Choosing the correct service is extremely important not only in terms of financail performance but also in terms of a business’s operational performance. The correct selection of a data communication service is a balance between the cost and technology.

The current communication technology standards include the familiar phone line often referred to as a 1FB, Integrated Services Data Network (ISDN), frame relay also known as a fractional T1, T1, and T3. Regardless of the service, the fundamental communication technology each service utilizes is the same. All service levels transmit asynchronously. This means that the data is transmitted at irregular intervals.

Figure 1: Attenuating Parallel Busses

The first step to transmitting data is to convert a computers digital bus to a stream of sequential information, Figure 1. In this example, a single byte of information consist of eight bits that simultaneously occur at a instance in time on the data bus. As shown to the left a single byte is then converted to a sequential stream of bits in which each bits occurs over several identically spaced instances in time during transmission. For instance, a byte of eight bits may occur in 1 second but after converting the byte to a stream of bits it occurs over 8 seconds. The rate at which data is transmitted in known as the B.A.U.D. rate or the Bit Attenuated Uniform Delay rate.  

Next transmission control information is added before and after the stream of data, Figure 2. The minimum transmission control information added includes a line idle prior to transmission for a minimum specified time interval. A start bit indicates the beginning of the data transmission. Then the data follows at standard time intervals. Finally, a stop bits indicates the end of the transmission and the line returns to line idle.

Figure 2:  Asynchronous Communication Signaling

In order to achieve higher transmission rates over communication lines, data compression is used. Instead of transmitting a single bit stream bit as indicated by ±5 volt signals; varying signal amplitude or frequency can indicate a specified bit pattern. Additionally, by increasing the bandwidth of the transmission signal more information may be sent in the same instance of time. Bandwidth is the difference between the highest and lowest frequency used to transmit data. However, signaling standards are set by the United Nations Communications Committee for International Telegraph and Telephone standards, CCITT, in which the bandwidth limits are set for each type of communications standard. In the United States the Federal Communications Committee, FCC, regulates each service and limits the signaling amplitude or line power rating. The regulations can be significantly below the physical limitations of the transmission media.

Figure 3: Data Compresion