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)
|
|