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Chapter 5
Communication Systems

Questions Answered in This Chapter:

What is a communication system?

What are LANs, MANs, WANS, and GANS?

What do ATM and ADSL have to do with LANs, MANs, WANS,and GANs?

What are network topologies?

Communication Systems

Why do we need to know anything about communication systems to understand ATM and ADSL? Mainly because ATM and ADSL are communication technologies. Because ATM and ADSL will replace, work with, modify, and/or unify our existing communication systems. If there is no understanding of what a communication system is, there is no understanding of the import of the technologies. Opportunity comes with understanding the impact ATM and ADSL have on our communication systems.

A communication system is a collection of sometimes diverse elements such as computers, routers, hubs, bridges, gateways, switches, radios, modems, software, and infrastructure working together to achieve a common purpose: getting information to and from the systems’ users. There are public communication systems, such as the Public Switched Telephone Network, there are Department of Defense communication systems, and there are private communication systems, such as local area networks. LANs may be connected through public or private transmission mediums to form MANs, WANs, and GANs. Regardless of the ownership (public, Department of Defense, private, etc.) of a communication system, there exist some fundamental building blocks that all communication systems have in common.


Figure 5-1  Typical digital data terminal communications network

In Figure 5-1, a digital data terminal communication network is shown. The figure shows two Digital Terminal Interfaces (DTIs) arranged in duplex communications mode. Duplex, in this instance, refers to the ability to simultaneously transmit and receive data. Usage determines if the two DTIs actually transmit and receive simultaneously. However, the ability to transmit while receiving, whether or not the users choose to, defines duplex operation. A cursory glance at the figure will reveal each DTI possesses both a receive path and a separate transmit path.

Let’s look at the transmit path of the left DTI. Digital data, or information already in digital format, is applied directly to the DTI. Audio frequency information, also called analog information, must be converted first to a digital signal, hence the analog input is processed by an A/D converter before being applied to the DTI. The DTI is responsible for converting the information into whatever particular format is applicable based upon the network specifications. The digital and/or audio information is then conveyed through the transmission medium to the DTI on the right side. The right side DTI receives and processes the digital data. The DTI then transmits the data out to the D/A converter and the digital data channel. The D/A converter converts the digital data signal into an analog signal that closely matches the original analog signal. If we followed the transmit path of the right DTI, it would be exactly the same as that described for the left DTI.

The DTI may be a computer and all its component parts, such as the Universal Asynchronous Receiver Transmitter (UART) and analog modem and any additional components such as hubs, routers, and gateways to form the particular network of interest, or it may be a special device designed and built for the express purpose of transmitting and receiving digital data directly to and from another device of the exact type. While the figure shows just two DTIs interconnected, the number of DTIs that can be interconnected is virtually unlimited. Notice the transmission medium physically connects the DTIs.

The figure represents a general digital data terminal representing any one of several types of protocols. The digital data can be encoded in various ways such as Ethernet, frame relay, or ATM. Lets assume the encoding scheme is ATM. The DTI is responsible for framing the data bits into ATM cells and adding the control information to the headers. Also, the DTI includes the ADSL modem functions, if ADSL is utilized.


Figure 5-2  Typical digital radio data communications network

Figure 5-2 represents a digital radio data communication network. The digital radio network works identically to the data terminal interface network with the exception that the digital radio must modulate for transmission and demodulate for reception the digital data signal before presenting the signal to an antenna for propagation into the atmosphere. Modulation consists of combining the digital data signal with an analog carrier suitable for transmission at the frequency desired. Demodulation consists of removing the transmitted carrier from the received signal and recovering the original digital data.

The digital radio is responsible for framing the data bits into ATM cells and adding the control information to the headers. The transmission medium between radios is the atmosphere. Since ADSL is intended as a twisted wire pair technology, the digital radios do not convert the ATM cells to ADSL. However, the input to the digital radio could be an ADSL input (after all, the ADSL signal is an analog signal) that is transmitted to another digital radio which provides the appropriate ADSL output. Such an ATM/ADSL application exists for remote video and remote Internet access.

A modern communication system typically consists of many hardware and software elements working together to achieve reliable communications from end to end (between users). The Public Switched Telephone Network includes all of the previously mentioned elements in various flavors and in a multitude of locations interconnected in seemingly incomprehensible complexity. An example of a Public Switched Telephone Network inter-regional communication system is shown in Figure 5-3. The Local Access and Transport Area (LATA) is the geographical area the local telephone company is confined by regulation to operate and offer local and long-distance telecommunications services in.

Local Exchange Carriers (LECs) are typically the Regional Bell Operating Companies (RBOCs or Baby Bells) and other suppliers of local service such as GTE. The Interexchange Carriers (IECs) are typically AT&T (Granny Bell), Sprint, MCI, etc.


Figure 5-3  Typical LATA Network


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