Title - Introduction to Data Communications

Introduction

Data communications is an important part of the world in which we do business.  While most people do not need to know the details on how the data is transported, it is helpful to become familiar with some of the basic concepts.  This seminar is provides a background of some of the more important aspects of data communications.

There has been a shift over the last few years in transporting data through public networks.   Previously, voice communications dominated the types of traffic that were carried by the Regional Bell Operating Companies (RBOCs) such as BellSouth, Verizon or Southwest Bell.  This type of networking is known as Telecommunications.   More recently, there is more computer data being carried by the RBOCs than voice traffic.  The transport of data between computers is known as Data Communications.

The basic elements involved in data communications can be seen in Figure 1.


Figure 1. Mixture of Data Networking Components

 

In the bottom left of the above picture, there is a standard phone connected to the telephone network.   Generically, this is known as a Plain Old Telephone (POTs ) line.   In the upper left is a standard business that connects its computers together using a standard Local Area Network (LAN).   The business is connected to the rest of world using a high speed link known as a T1.

 

The RBOCs use switches to get the data to the proper destination.  The interconnections between their switches generally use fiber optic cables.  The Optical Carrier (OC) links may be many different speeds but is indicated in the figure as an OC-24.

 

The Internet generally uses a different kind of network than the RBOCs and they use Routers to connect their main sites together.   There is a major difference between routers and switches and this will be explained in more detail in the next seminar.

 

Also shown are wireless networks and the Cable networks.   These networks also have vastly different types of technologies to provide services to you.  Cable networks are very efficient at providing the same information to many different homes simultaneously using a single cable.  Wireless networks, such as cell phones, are constantly in touch with your phone as it moves into and out of the many different cell sites within a city.

 

Digital Loop Carrier equipment helps the RBOCs transport data to homes and businesses that are generally too far for a standard wire to be run from the RBOCs Central Office (CO).  This type of equipment is generically known as Digital Loop Carrier (DLC) equipment and it physically sits out in neighborhoods on small concrete pads.  The DLC equipment concentrates many POTs lines together from a neighborhood and puts them on a single higher speed link to the CO.

LANs, MANs, & WAN Technologies

As we can see from the picture, there are wide varieties of elements in a data network.  It is useful to think of data networks in terms of the geographic area that they serve. Within a small business, it LANs are used.  Connecting machines together across a city requires a Metropolitan Network (MAN).  Connecting cities together requires Wide Area Networks (WANs).  There is specialized equipment for each of these three networks.

 

Within each of these three major geographies, there are many different types of networks today.   Speed, cost, and evolving technologies are all reasons that there are several different types of LAN, WAN, and MAN networks.  Figure 2 shows some of the more common networks in each of these geographic networks.

 

Figure 2. LANs, MANs, & WANs

 

There is an important distinction to be made between the Public Networks and private networks. For private networks, once the network is installed there is very little ongoing costs for the usage of the network. Public networks need to recover the cost of providing a service and hence charge for the distance traveled and the link speed. As a result of this distinction, Local Area Networks used within a building tend to run as fast as possible while external connections are kept at the minimum speed that people will accept.

 

A way to reduce the cost of a network is to share the cost of the links amongst more people and thus spread out the fixed costs associated with the network. For a cable modem network, one wire may serve 2,000 homes. For an ADSL network, the data from many users is combined at the central office onto a single higher speed link. This sharing of data links works efficiently because in most circumstances, you only transmit & receive data for a small fraction of the time.

 

Different types of data traffic have different ratios of how often they are idle and how much information is transferred occurs during each data burst. The technique of dealing with optimizing the utilization of these links is called Statistical Multiplexing (stat-muxing).


T - Carrier Links

The basic unit of the phone system is the channel required to transport a single POTs line.   As shown in Figure 3, an analog signal is converted into a digital signal by an Analog to Digital converter (ADC).  To accurately reconstruct the spoken voice, it is only necessary to take periodic samples of the changing voice.  This occurs about 8,000 times a second.  This basic unit of data, one byte for each 8,000 samples produces 64 K bits each second (64 Kb/s).  This is known as a DS0 channel.

 

A link that carries 24 of these voice channels simultaneously is known as a T1 link.  It speed is 1.544 Mbits per second (64 Kbits second times 24 channels).   Another link commonly used by the RBOCs is a T3 link.   It carries 28 T1 signals simultaneously and has a capacity of 43 Mbits per second.  As was previously mentioned, fiber optic links provides the transportation of large amounts of data that occurs between two sites.  An OC-12 can provide transport of twelve (12) T3 links simultaneously.

Figure 3. Framing of the T - Carrier Links

 

To summarize the capability of the various T Carrier links:

 

Table 1. North America - Voice

Service

Channels

Speed

DS-0

1

64 Kb/s

DS-1

24 DS-0s

1.544 Mb/s (T1)

DS-2

96 DS-0s

6.312 Mb/s (T2)

DS-3

672 DS-0s

44.736 Mb/s (T3)


Just to add some additional information, the US and Europe have different speed links.  The same basic unit of 64 Kbits per second are still used but the number of channels in the links may vary.  Table 2 shows the European standard links.

 

Table 2. EUROPE (ITU) - Voice

Service

Channels

Speed

E0

1

64 Kb/s

E1

30

2.048 Mb/s

E2

120

8.448 Mb/s

E3

480

34.368 Mb/s

E4

1920

139.264 Mb/s

E5

7680

565.148 Mb/s


Above T1 speeds in a metropolitan area, fiber cables are generally used.  T1s are put into DS-3 frames and these operate at T3 speeds.   These are than padded together in an OC-1 frame that has 51.84 Mbits per second of data bandwidth.  OC-1s are combined together into OC-3, OC-12, or even higher speeds.

 

Table 3. SONET Circuits

Service

Speed (Mb/s)

Description

STS-1 / OC1

51.84

28 DS1s or 1 DS3

STS-3 / OC3

155.52

3 STS-1s

STS-3c  / OC3c

155.52

concatenated

STS-12 / OC12

622.08

12 STS-1s, 4 STS-3s

STS-48 / OC48

2488.32

32 (48 STS-1s, 16 STS-3s)



 

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