Computer and Communication by sofiaie

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									Computer and Communication
The following notes aims to explain the data communication pros and cons:
First the global communication and at the end a brief outline of the communication between
the microcomputer and its peripherals.
Note: This concept is better understood with the aid of diagrams.
        Diagrams will be given in the lecture and tutorial, as appropriate and necessary. You
        may yourself try to draw diagrams from the explanations!

In today's business world, reliable and efficient access to information has become an
important asset in the quest to achieve a competitive advantage. Modern offices no longer
have to depend on large libraries of filing cabinets containing accounting, personnel, or
marketing data to provide the information necessary to keep the organization running
smoothly and profitably. Mountains of papers have given way to computers that store and
manage information electronically. Coworkers thousands of miles apart can collaborate by
sharing information instantaneously, just as hundreds of workers in a single location can
simultaneously review research data maintained online. Falling computer prices and rapidly
improving technology have made computing technology much more powerful and affordable.

Computer networking technologies are the glue that bind these elements together. The public
Internet allows businesses around the world to share information with each other and their
customers. The global computer network known as the World Wide Web provides services
that let consumers buy books, clothes, and even cars online, or auction those same items off
when no longer wanted.

Networking, or attaching multiple electronic devices to facilitate information sharing, allows
one computer to send information to and receive information from another. We may not
always be aware of the numerous times we access information on computer networks.
Certainly the Internet is the most conspicuous example of computer networking, linking
millions of computers around the world, but smaller networks play a roll in information
access on a daily basis.
Many public libraries have replaced their card catalogs with computer terminals that allow
patrons to search for books far more quickly and easily. Airports have numerous screens
displaying information regarding arriving and departing flights. Many retail stores feature
specialized computers that handle point of sale transactions. In each of these cases,
networking allows many different devices in multiple locations to access a shared repository
of data, which could be anything from a searchable catalog of library books to a constantly
changing table of airline flights.

 The following, will take a very close look at networking, and in particular the Ethernet
networking standard, so you can understand the actual mechanics of how all of these
computers connect to one another. Before getting into the details of a networking standard,
we must first understand some basic terms and classifications that describe and differentiate
network technologies.

A global network connecting millions of computers. As of 1999, the Internet has more than
200 million users worldwide, and that number is growing rapidly. More than 100 countries
are linked into exchanges of data, news and opinions.
Unlike online services, which are centrally controlled, the Internet is decentralized by design.
Each Internet computer, called a host, is independent. Its operators can choose which Internet
services to use and which local services to make available to the global Internet community.
Remarkably, this anarchy by design works exceedingly well.

A group of two or more computer systems linked together. There are many types of computer
networks, including: local-area networks (LANs) : The computers are geographically close
together (that is, in the same building), wide-area networks (WANs) : The computers are
farther apart and are connected by telephone lines radio waves.
In addition to these types, the following characteristics are also used to categorize different
types of networks:
Topology : The geometric arrangement of a computer system. Common topologies include a
bus, star, and ring.
Bus topology: All devices are connected to a central cable, called the bus or backbone. Bus
networks are relatively inexpensive and easy to install for small networks. Ethernet systems
use a bus topology.
Ring topology : All devices are connected to one another in the shape of a closed loop, so that
each device is connected directly to two other devices, one on either side of it. Ring
topologies are relatively expensive and difficult to install, but they offer high bandwidth and
can span large distances.
Star topology: All devices are connected to a central hub. Star networks are relatively easy to
install and manage, but bottlenecks can occur because all data must pass through the hub.

These topologies can also be mixed. For example, a bus-star network consists of a high-
bandwidth bus, called the backbone, which connects a collections of slower-bandwidth star

Protocol : The protocol defines a common set of rules and signals that computers on the
network use to communicate. One of the most popular protocols for LANs is called Ethernet.
Another popular LAN protocol for PCs is the IBM token-ring network .
The protocol determines the following:
               the type of error checking to be used
              data compression method, if any
              how the sending device will indicate that it has finished sending a message
              how the receiving device will indicate that it has received a message

The protocol can be implemented either in hardware or in software.

         (1)   Objects on which data can be stored. These include hard disks, floppy disks,
               CD-ROMs, and tapes.

       (2)     In computer networks, media refers to the cables linking workstations
               together. There are many different types of transmission media, the most
               popular being twisted-pair wire (normal electrical wire), coaxial cable (the
               type of cable used for cable television), and fiber optic cable (cables made out
               of glass).

       (3)     The form and technology used to communicate information. Multimedia
               presentations, for example, combine sound, pictures, and videos, all of which
               are different types of media.

Architecture : Networks can be broadly classified as using either a peer-to-peer or
client/server architecture.
Computers on a network are sometimes called nodes. Computers and devices that allocate
resources for a network are called servers.

Leased line: A permanent telephone connection between two points set up by a
telecommunications common carrier. Typically, leased lines are used by businesses to
connect geographically distant offices. Unlike normal dial-up connections, a leased line is
always active. The fee for the connection is a fixed monthly rate. The primary factors
affecting the monthly fee are distance between end points and the speed of the circuit.
Because the connection doesn't carry anybody else's communications, the carrier can assure a
given level of quality.
For example, a T-1 channel is a type of leased line that provides a maximum transmission
speed of 1.544 Mbps.
You can divide the connection into different lines for data and voice communication or use
the channel for one high speed data circuit. Dividing the connection is called multiplexing.

Increasingly, leased lines are being used by companies, and even individuals, for Internet
access because they afford faster data transfer rates and are cost-effective if the Internet is
used heavily.

Data transfer rate: The speed with which data can be transmitted from one device to
another. Data rates are often measured in megabits (million bits) or megabytes (million bytes)
per second. These are usually abbreviated as Mbps and MBps, respectively.
Another term for data transfer rate is throughput. Data transfer rates for disk drives and
networks are measured in terms of throughput.

Dial-up access: Refers to connecting a device to a network via a modem and a public
telephone network. Dial-up access is really just like a phone connection, except that the
parties at the two ends are computer devices rather than people.
Because dial-up access uses normal telephone lines, the quality of the connection is not
always good and data rates are limited. In the past, the maximum data rate with dial-up access
was 56 Kbps (56,000 bits per second), but new technologies such as ISDN are providing
faster rates.
An alternative way to connect two computers is through a leased line, which is a permanent
connection between two devices. Leased lines provide faster throughput and better quality
connections, but they are also more expensive.

Local Area vs. Wide Area
We can classify network technologies as belonging to one of two basic groups. Local area
network (LAN), a computer network that spans a relatively small area. Most LANs are
confined to a single building or group of buildings. However, one LAN can be connected to
other LANs over any distance via telephone lines and radio waves. A system of LANs
connected in this way is called a wide-area network (WAN).
Most LANs connect workstations and personal computers. Each node (individual computer )
in a LAN has its own CPU with which it executes programs, but it is also able to access data
and devices anywhere on the LAN. This means that many users can share expensive devices,
such as laser printers, as well as data. Users can also use the LAN to communicate with each
other, by sending e-mail or engaging in chat sessions.
There are many different types of LANs Ethernets being the most common for PCs. Most
Apple Macintosh networks are based on Apple's AppleTalk network system, which is built
into Macintosh computers. LANs are capable of transmitting data at very fast rates, much
faster than data can be transmitted over a telephone line; but the distances are limited, and
there is also a limit on the number of computers that can be attached to a single LAN.

Wide area network (WAN) technologies connect a smaller number of devices that can be
many kilometers apart. For example, if two libraries at the opposite ends of a city wanted to
share their book catalog information, they would most likely make use of a wide area
network technology, which could be a dedicated line leased from the local telephone
company, intended solely to carry their data. In comparison to WANs, LANs are faster and
more reliable, but improvements in technology continue to blur the line of demarcation. Fiber
optic cables have allowed LAN technologies to connect devices tens of kilometers apart
while at the same time greatly improving the speed and reliability of WANs.

 Local-area wireless network (LAWN). A type of local-area network that uses high-frequency
radio waves rather than wires to communicate between nodes.

The Ethernet
In 1973, at Xerox Corporation’s Palo Alto Research Center (more commonly known as
PARC), researcher Bob Metcalfe designed and tested the first Ethernet network. While
working on a way to link Xerox’s "Alto" computer to a printer, Metcalfe developed the
physical method of cabling that connected devices on the Ethernet as well as the standards
that governed communication on the cable. Ethernet has since become the most popular and
most widely deployed network technology in the world. Many of the issues involved with
Ethernet are common to many network technologies, and understanding how Ethernet
addressed these issues can provide a foundation that will improve your understanding of
networking in general.

The Ethernet standard has grown to encompass new technologies as computer networking has
matured, but the mechanics of operation for every Ethernet network today stem from
Metcalfe’s original design. The original Ethernet described communication over a single
cable shared by all devices on the network. Once a device attached to this cable, it had the
ability to communicate with any other attached device. This allows the network to expand to
accommodate new devices without requiring any modification to those already on the

Ethernet is a local area technology, with networks traditionally operating within a single
building, connecting devices in close proximity. At most, Ethernet devices could have only a
few hundred meters of cable between them, making it impractical to connect geographically
dispersed locations. Modern advancements have increased these distance limitations
considerably, allowing Ethernet networks to span tens of kilometers.

Ethernet Terminology
Ethernet follows a simple set of rules that govern its basic operation. To better understand
these rules, it is important to understand the basics of Ethernet terminology. Ethernet devices
attach to a common medium that provides a path along which the electronic signals will
Historically this medium has been coaxial copper cable, but today it is more commonly
twisted pair or fiber optic cabling. In any case, we refer to a single shared medium as an
Ethernet segment, and devices that attach to that segment are stations or nodes. The nodes
communicate in short messages called frames, which are variable-sized chunks of
information. Frames are analogous to sentences in human language. In English, we have rules
for constructing our sentences. We know that each sentence must contain a subject and a
predicate. The Ethernet protocol specifies a set of rules for constructing frames. There are
explicit minimum and maximum lengths for frames, and a set of required pieces of
information that must appear in the frame. Each frame must include, for example, both a
destination address and a source address, which identify the recipient and the sender of the
message. The address uniquely identifies the node, just as a name identifies a particular
person. No two Ethernet devices should ever have the same address.

Since a signal on the Ethernet medium reaches every attached node, the destination address is
critical to identify the intended recipient of the frame. For example, in the figure below, when
computer B transmits to printer C, computers A and D will still receive and examine the
However, when a station first receives a frame, it checks the destination address to see if the
frame is intended for itself. If it is not (e.g. computers A and D), the station discards the
frame without even examining its contents. One interesting thing about Ethernet addressing is
the implementation of a broadcast address. A frame with a destination address equal to the
broadcast address (simply called a broadcast, for short) is intended for every node on the
network, and every node will both receive and process this type of frame.

The acronym CSMA/CD signifies Carrier Sense Multiple Access with Collision Detection
and describes how the Ethernet protocol regulates communication among nodes. While the
term may seem intimidating, if we break it apart into its component concepts we will see that
it describes rules very similar to those people use in polite conversation. To help illustrate the
operation of Ethernet, we will use an analogy of a dinner table conversation. Let’s represent
our Ethernet segment as a dinner table, and let several people engaged in polite conversation
at the table represent the nodes. The term Multiple Access covers what we already discussed
above. When one Ethernet station transmits, all the stations on the medium hear the

transmission, just as when one person at the table talks, everyone present is able to hear him
or her.

Now let's imagine that you are at the table and you have something you would like to say. At
the moment, however, I am talking. Since this is a polite conversation, rather that
immediately speak up and interrupt, you would wait until I finished talking before making
your statement. This is the same concept described in the Ethernet protocol as Carrier Sense.
Before a station transmits, it "listens" to the medium to determine if another station is
transmitting. If the medium is quiet, we say that the station can sense the carrier, and it
recognizes that this is an appropriate time to transmit.

Carrier Sense Multiple Access gives us a good start in regulating our conversation, but there
is one scenario we still need to address. Let’s go back to our dinner table analogy and
imagine that there is a momentary lull in the conversation. You and I both have something we
would like to add, and we both "sense the carrier" based on the silence, so we begin speaking
at approximately the same time. In Ethernet terminology, a collision occurs when we both
spoke at once. In our conversation, we can handle this situation gracefully. We will both hear
the other speak at the same time we are speaking. Then we can stop to give the other person a
chance to go on. Ethernet nodes also listen to the medium while they transmit to ensure that
they are the only station transmitting at that time. If the stations hear their own transmission
returning in a garbled form, as would happen if some other station had begun to transmit its
own message at the same time, then they know that a collision occurred. A single Ethernet
segment is sometimes called a collision domain because no two stations on the segment can
transmit at the same time without causing a collision. When stations detect a collision, they
cease transmission, wait a random amount of time, and attempt to transmit when they again
detect silence on the medium.

The random pause and retry is an important part of the protocol. If two stations collide when
transmitting once, then both will need to transmit again. At the next appropriate chance to
transmit, both stations involved with the previous collision will have data ready to transmit. If
they transmitted again at the first opportunity, they would most likely collide again and again
indefinitely. Instead, the random delay makes it unlikely that any two stations will collide
more than a few times in a row.

Limitations of Ethernet
A single shared cable can serve as the basis for a complete Ethernet network, which is what
we discussed above. However, there are practical limits to the size of our Ethernet network in
this case. A primary concern is the length of the shared cable. Electrical signals propagate
along a cable very quickly, but they weaken as they travel, and electrical interference from
neighboring devices (fluorescent lights, for example) can scramble the signal. A network
cable must be short enough that devices at opposite ends can receive each other's signals
clearly and with minimal delay. This places a distance limitation on the maximum separation
between two devices (called the network diameter) on an Ethernet network. Additionally,
since in CSMA/CD only a single device can transmit at a given time, there are practical limits
to the number of devices that can coexist in a single network. Attach too many devices to one
shared segment and contention for the medium will increase. Every device may have to wait
an inordinately long time before getting a chance to transmit.

Engineers have developed a number of network devices that alleviate these difficulties. Many
of these devices are not specific to Ethernet, but play roles in other network technologies as

The first popular Ethernet medium was a copper coaxial cable known as "thicknet." The
maximum length of a thicknet cable was 500 meters. In large building or campus
environments, a 500-meter cable could not always reach every network device. A repeater
addresses this problem.
Repeaters connect multiple Ethernet segments, listening to each segment and repeating the
signal heard on one segment onto every other segment connected to the repeater. By running
multiple cables and joining them with repeaters, you can significantly increase your network

Bridges and Segmentation
In our dinner table analogy, we had only a few people at a table carrying out the
conversation, so restricting ourselves to a single speaker at any given time was not a
significant barrier to communication. But what if there were many people at the table and
only one were allowed to speak at any given time? In practice, we know that the analogy
breaks down in circumstances such as these. With larger groups of people, it is common
for several different conversations to occur simultaneously. If only one person in a crowded
room or at a banquet dinner were able to speak at any time, many people would get frustrated
waiting for a chance to talk. For humans, the problem is self-correcting: voices only carry so
far, and the ear is adept at picking out a particular conversation from the surrounding noise.
This makes it easy for us to have many small groups at a party converse in the same room,
but network cables carry signals quickly and efficiently over long distances, so this natural
segregation of conversations does not occur.

Ethernet networks faced congestion problems as they increased in size. If a large number of
stations connected to the same segment and each generated a sizable amount of traffic, many
stations may attempt to transmit whenever there was an opportunity. Under these
circumstances, collisions would become more frequent and could begin to choke out
successful transmissions, which could take inordinately large amounts of time to complete.
One way to reduce congestion would be to split a single segment into multiple segments, thus
creating multiple collision domains. This solution creates a different problem, as now these
now separate segments are not able to share information with each other.

To alleviate these problems, Ethernet networks implemented bridges. Bridges connect two or
more network segments, increasing the network diameter as a repeater does, but bridges also
help regulate traffic. They can send and receive transmission just like any other node, but
they do not function the same as a normal node. First, the bridge does not originate any traffic
of its own; like a repeater, it only echoes what it hears from other stations. (That last
statement is not completely accurate. Bridges do create a special Ethernet frame that allows
them to communicate with other bridges, but that is outside the scope of this article.)

Remember how the multiple access and shared medium of Ethernet meant that every station
on the wire received every transmission, whether it was the intended recipient or not? Bridges
make use of this feature to relay traffic between segments. In the figure below, the bridge
connects segments 1 and 2. If station A or B were to transmit, the bridge would also receive
the transmission on segment 1. How should the bridge respond to this traffic? It could
automatically transmit the frame onto segment 2, like a repeater, but that would not relieve
congestion, as the network would behave like one long segment. One goal of the bridge is to
reduce unnecessary traffic on both segments. It does this by examining the destination
address of the frame before deciding how to handle it. If the destination address is that of
station A or B, then there is no need for the frame to appear on segment 2. In this case, the
bridge does nothing. We can say that the bridge filters or drops the frame. If the destination
address is that of station C or D, or if it is the broadcast address, then the bridge will transmit,
or forward the frame on to segment 2. By forwarding packets, the bridge allows any of the
four devices in the figure to communicate. Additionally, by filtering packets when
appropriate, the bridge makes it possible for station A to transmit to station B at the same
time station C transmits to station D, allowing two conversations to occur simultaneously!
Switches are the modern counterparts of bridges, functionally equivalent but offering a
dedicated segment for every node on the network. We discuss switches further in the section
titled "Ethernet Today."

Routers: Logical Segmentation Bridges can reduce congestion by allowing multiple
conversations to occur on different segments simultaneously, but they have their limits in
segmenting traffic as well. An important characteristic of bridges is that they forward
Ethernet broadcasts to all connected segments. This behavior is necessary, as Ethernet
broadcasts are destined for every node on the network, but it can pose problems for bridged
networks that grow too large. When a large number of stations broadcast on a bridged
network, congestion can be as bad as if all those devices were on a single segment.

Routers are advanced networking components that can divide a single network into two
logically separate networks. While Ethernet broadcasts cross bridges in their search to find
every node on the network, they do not cross routers, because the router forms a logical
boundary for the network. Routers operate based on protocols that are independent of the
specific networking technology, like Ethernet or Token Ring (see below). This allows routers
to easily interconnect various network technologies, both local and wide area, and has led to
their widespread deployment in connecting devices around the world as part of the global

Ethernet Today
Modern Ethernet implementations often look nothing like their historical counterparts. Where
long runs of coaxial cable provided attachments for multiple stations in legacy Ethernet,
modern Ethernet networks use twisted pair wiring or fiber optics to connect stations in a
radial pattern. Where legacy Ethernet networks transmitted data at 10 Mbps (10 million bits
per second), modern version of Ethernet, called 100Base-T (or Fast Ethernet), supports data
transfer rates of 100 Mbps. And the newest version, Gigabit Ethernet supports data rates of 1
gigabit (1,000 megabits) per second.

Perhaps the most striking advancement in contemporary Ethernet networks is the use of
Switched Ethernet (see below for definition). Switched networks replace the shared medium

of legacy Ethernet with a dedicated segment for each station. These segments connect to a
switch, which acts much like an Ethernet bridge, but can connect many of these single station
segments. Some switches today can support hundreds of dedicated segments. Since the only
devices on the segments are the switch and the end station, the switch picks up every
transmission before it reaches another node. The switch then forwards the frame over the
appropriate segment, just like a bridge, but since any segment contains only a single node, the
frame only reaches the intended recipient. This allows many conversations to occur
simultaneously on a switched network.

Ethernet switching gave rise to another advancement, full-duplex Ethernet. Full-duplex is a
data communications term that refers to the ability to both send and receive data at the same
time. Legacy Ethernet is half-duplex, as only one device on the network can transmit at any
given time. In a totally switched network, nodes only communicate with the switch and never
directly with each other. Switched networks also employ either twisted pair or fiber optic
cabling, both of which use separate conductors for sending and receiving data. In this type of
environment, Ethernet stations can forgo the collision detection process and transmit at will,
since they are the only potential devices that can access the medium. This allows end stations
to transmit to the switch at the same time the switch transmits to them, achieving a collision
free environment.

Switched Ethernet

An Ethernet LAN that uses switches to connect individual hosts or segments. In the case of
individual hosts, the switch replaces the repeater and effectively gives the device full 10
Mbps bandwidth (or 100 Mbps for Fast Ethernet) to the rest of the network. This type of
network is sometimes called a desktop switched Ethernet. In the case of segments, the hub is
replaced with a switching hub.
Traditional Ethernets, in which all hosts compete for the same bandwidth, are called shared
Ethernets. Switched Ethernets are becoming very popular because they are an effective and
convenient way to extend the bandwidth of existing Ethernets.

Ethernet or 802.3?
You may have heard the term 802.3 used in place of or in conjunction with the term Ethernet.
"Ethernet" originally referred to a networking implementation standardized by Digital, Intel,
and Xerox. (For this reason it is also known as the DIX standard.)

In February of 1980, the Institute of Electrical and Electronics Engineers, or IEEE
(pronounced I triple E) created a committee to standardize network technologies. The IEEE
titled this the 802 working group, named after the year and month of its formation.
Subcommittees of the 802 working group separately addressed different aspects of
networking. The IEEE distinguished each subcommittee by numbering it 802.X, with X
representing a unique number for each subcommittee. The 802.3 group standardized the
operation of a CSMA/CD network that was functionally equivalent to the DIX Ethernet.

Ethernet and 802.3 differ slightly in their terminology and the data format for their frames,
but are in most respects identical. Today, the term Ethernet refers generically to both the DIX
Ethernet implementation and the IEEE 802.3 standard.

Alternative Network Technologies
The most common local area network alternative to Ethernet is a network technology
developed by IBM, called Token Ring. Where Ethernet relies on the random gaps between
transmissions to regulate access to the medium, Token ring implements a strict, orderly
access method. A token ring network arranges nodes in a logical ring, as shown below. The
nodes forward frames in one direction around the ring, removing a frame when it has circled
the ring once. The ring initializes by creating a token, which is a special type of frame that
gives a station permission to transmit. The token circles the ring like any frame until it
encounters a station that wishes to transmit data. This station then "captures" the token by
replacing the token frame with a data-carrying frame, which encircles the network. Once that
data frame returns to the transmitting station, that station removes the data frame, creates a
new token, and forwards that token on to the next node in the ring. Token ring nodes do not
look for a carrier signal or listen for collisions; the presence of the token frame provides
assurance that the station can transmit a data frame without fear of another station
interrupting. Because a station transmits only a single data frame before passing the token
along, each station on the ring will get a turn to communicate in a deterministic and fair
manner. Token ring networks typically transmit data at either 4 or 16 Mbps.

Fiber Distributed Data Interface (FDDI) is another token passing technology that operates
over a pair of fiber optic rings, with each ring passing a token in opposite directions. FDDI
networks offered transmission speeds of 100 Mbps, which initially made them quite popular
for high-speed networking. With the advent of 100 Mbps Ethernet, which is cheaper and
easier to administer, FDDI has waned in popularity.

A final network technology that bears mentioning is Asynchronous Transfer Mode, or ATM.
ATM networks blur the line between local and wide area networking, being able to attach
many different devices with high reliability and at high speeds, even across the country. ATM
networks are suitable for carrying not only data, but voice and video traffic as well, making
them versatile and expandable. While ATM has not gained acceptance as rapidly as originally
predicted, it is nonetheless a solid network technology for the future.

A computer communicates to the external world through its Input/Output devices, interfacing
the peripherals.
Parallel interface chips, such as Motorola MC6821 peripheral interface adapter (PIA) or Intel
8255A programmable parallel interface (PPI), are designed to connect directly to the
computer’s bus structure, and provide both input and output ports with which the CPU can
interface to the outside world. Outside world means peripheral devices such as printer,
keyboard, modem, etc. which are said to be either parallel or serial.
The advantage of parallel interfacing is speed; all eight bits of an 8-bit data byte can be
transferred simultaneously. The disadvantage of parallel interfacing is space; 8 (16 or 32)

parallel wires running across the surface of a PCB constitutes a significant percentage of the
available board real estate (cabling to external devices even more so).

Serial interfacing refers to transmitting data one bit at a time across a single line. The
advantage of serial I/O is lower cost (in terms of number of wires connecting the
microcomputer to the peripheral device), while the disadvantage is much slower data transfer
Since communication within a microcomputer take place over the system bus in parallel
form, there is obviously a need for parallel-to-serial (and serial-to-parallel) conversion when
interfacing to serial I/O devices. There are available several programmable devices which
contain most of the circuitry needed for serial communication. A device such as Motorola
MC6850 asynchronous communication interface adapter (ACIA) and National INS8250,
which can only do asynchronous communication, is often referred to as a universal
asynchronous receiver-transmitter or UART. A device such as Intel 8251A, which can be
programmed to do either asynchronous or synchronous communication, is often called a
universal synchronous-asynchronous receiver-transmitter or USART.
Serial data can be sent synchronously or asynchronously. For synchronous transmission, data
is sent in blocks at a constant rate. The start and end of a block are identified with specific
bytes or bit patterns. For asynchronous transmission each data character has a bit which
identifies its start and 1 or 2 bits which identify its end. Since each character is individually
identified, characters can be sent at any time (asynchronously), in the same way that a person
types on a keyboard. Also see definition below.
For sending serial data over long distances, the standard telephone system is a convenient
path, because the wiring and connections are already in place. Standard phone lines, often
referred to as switched lines because any two points can be connected together through a
series of switches. Digital signals from the computer can not be sent directly over standard
telephone lines. The solution to this problem is to convert the digital signals to audio-
frequency tones (using a modem – see below), which are in the frequency range that the
phone lines can transmit.
Note: Phone lines capable of carrying digital data directly can be leased, but these are
somewhat costly and are limited to the specific destination of the line.
See your previous notes for diagrams.

Acronym for modulator-demodulator. A modem is a device or program that enables a
computer to transmit data over telephone lines. Computer information is stored digitally,
whereas information transmitted over telephone lines is transmitted in the form of analog
waves. A modem converts between these two forms.

Fortunately, there is one standard interface for connecting external modems to computers
called RS-232. Consequently, any external modem can be attached to any computer that has
an RS-232 port, which almost all personal computers have. There are also modems that come
as an expansion board that you can insert into a vacant expansion slot. These are sometimes
called onboard or internal modems.

While the modem interfaces are standardized, a number of different protocols for formatting
data to be transmitted over telephone lines exist. Some, like CCITT V.34, are official
standards, while others have been developed by private companies. Most modems have built-

in support for the more common protocols -- at slow data transmission speeds at least, most
modems can communicate with each other. At high transmission speeds, however, the
protocols are less standardized.
Aside from the transmission protocols that they support, the following characteristics
distinguish one modem from another:

bps : How fast the modem can transmit and receive data. At slow rates, modems are
measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher
speeds, modems are measured in terms of bits per second (bps). The fastest modems run at
57,600 bps, although they can achieve even higher data transfer rates by compressing the
data. Obviously, the faster the transmission rate, the faster you can send and receive data.
Note, however, that you cannot receive data any faster than it is being sent. If, for example,
the device sending data to your computer is sending it at 2,400 bps, you must receive it at
2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some
telephone lines are unable to transmit data reliably at very high rates voice/data: Many
modems support a switch to change between voice and data modes. In data mode, the modem
acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems
that support a voice/data switch have a built-in loudspeaker and microphone for voice
auto-answer : An auto-answer modem enables your computer to receive calls in your
absence. This is only necessary if you are offering some type of computer service that people
can call in to use.
data compression : Some modems perform data compression, which enables them to send
data at faster rates. However, the modem at the receiving end must be able to decompress the
data using the same compression technique.
 flash memory : Some modems come with flash memory rather than conventional ROM,
which means that the communications protocols can be easily updated if necessary.
Fax capability: Most modern modems are fax modems, which means that they can send and
receive faxes.
To get the most out of a modem, you should have a communications software package, a
program that simplifies the task of transferring data.

                             Communications Protocols

       Protocol               Maximum               Duplex
                              Transmission          Mode
       Bell 103               300 bps               Full
       CCITT V.21             300 bps               Full
       Bell 212A              1,200 bps             Full
       ITU V.22               1,200 bps             Half
       ITU V.22bis            2,400 bps             Full
       ITU V.29               9,600 bps             Half
       ITU V.32               9,600 bps             Full
       ITU V.32bis            14,400 bps            Full
       ITU V.34               36,600 bps            Full
       ITU V.90               56,000 bps            Full

Wireless modem, now in use. A modem that accesses a private wireless data network or a
wireless telephone system, such as the Cellular Digital Packet Data (CDPD) system.

CDPD, a data transmission technology developed for use on cellular phone frequencies.
CDPD uses unused cellular channels (in the 800- to 900-MHz range) to transmit data in
packets. This technology offers data transfer rates of up to 19.2 Kbps, quicker call set up, and
better error correction than using modems on an analog cellular channel.
Abbreviation of integrated services digital network, an international communications
standard for sending voice, video, and data over digital telephone lines or normal telephone
wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second). Most ISDN
lines offered by telephone companies give you two lines at once, called B channels. You can
use one line for voice and the other for data, or you can use both lines for data to give you
data rates of 128 Kbps, three times the data rate provided by today's fastest modems.

The original version of ISDN employs base-band transmission. Another version, called B-
ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mbps. B-
ISDN requires fiber optic cables and is not widely available.

USB (Universal Serial Bus) is a plug-and-play interface between a computer and add-on
devices (such as audio players, joysticks, keyboards, telephones, scanners, and printers). With
USB, a new device can be added to your computer without having to add an adapter card or
even having to turn the computer off. The USB peripheral bus standard was developed by
Compaq, IBM, DEC, Intel, Microsoft, NEC, and Northern Telecom and the technology is
available without charge for all computer and device vendors.
USB supports a data speed of 12 megabits per second. This speed will accommodate a wide
range of devices, including MPEG video devices, data gloves, and digitizers. It is anticipated
that USB will easily accommodate plug-in telephones that use ISDN and digital PBX.
Since October, 1996, the Windows operating systems have been equipped with USB drivers
or special software designed to work with specific I/O device types. USB is integrated into
Windows 98 and later versions. Today, most new computers and peripheral devices are
equipped with USB.
A different plug-and-play standard, IEEE 1394, supports much higher data rates and devices
such as video camcorders and digital video disk (DVD) players. However, USB and IEEE
1394 serve different device types.

Serial port

A port, or interface, that can be used for serial communication, in which only 1 bit is
transmitted at a time.

Most serial ports on personal computers conform to the RS-232C or RS-422 standards. A
serial port is a general-purpose interface that can be used for almost any type of device,
including modems, mice, and printers (although most printers are connected to a parallel

Parallel port
A parallel interface for connecting an external device such as a printer. Most personal
computers have both a parallel port and at least one serial port.

On PCs, the parallel port uses a 25-pin connector (type DB-25) and is used to connect
printers, computers and other devices that need relatively high bandwidth. It is often called a
Centronics interface after the company that designed the original standard for parallel
communication between a computer and printer. (The modern parallel interface is based on a
design by Epson.)

A newer type of parallel port, which supports the same connectors as the Centronics
interface, is the EPP (Enhanced Parallel Port) or ECP (Extended Capabilities Port). Both of
these parallel ports support bi-directional communication and transfer rates ten times as fast
as the Centronics port.

Macintoshes have a SCSI port, which is parallel, but more flexible.

Occurring at regular intervals. The opposite of synchronous is asynchronous. Most
communication between computers and devices is asynchronous -- it can occur at any time
and at irregular intervals. Communication within a computer, however, is usually
synchronous and is governed by the microprocessor clock. Signals along the bus, for
example, can occur only at specific points in the clock cycle.

 Not synchronized; that is, not occurring at predetermined or regular intervals. The term
asynchronous is usually used to describe communications in which data can be transmitted
intermittently rather than in a steady stream. For example, a telephone conversation is
asynchronous because both parties can talk whenever they like. If the communication were
synchronous, each party would be required to wait a specified interval before speaking.

The difficulty with asynchronous communications is that the receiver must have a way to
distinguish between valid data and noise. In computer communications, this is usually
accomplished through a special start bit and stop bit at the beginning and end of each piece of
data. For this reason, asynchronous communication is sometimes called start-stop
Most communications between computers and devices are asynchronous.


Pronounced u-art, and short for universal asynchronous receiver-transmitter, the UART is a
computer component that handles asynchronous serial communication. Every computer
contains a UART to manage the serial ports, and all internal modems have their own UART.
Motorola MC6850 Asynchronous Communication Interface Adapter (ACIA) is a prime
example of a UART.
As modems have become increasingly fast, the UART has come under greater scrutiny as the
cause of transmission bottlenecks. If you are purchasing a fast external modem, make sure
that the computer's UART can handle the modem's maximum transmission rate. The newer
16550 UART contains a 16-byte buffer, enabling it to support higher transmission rates than
the older 8250 UART.

Universal synchronous/asynchronous receiver-transmitter, the USART is a serial
communication interface, which is capable of being programmed for either synchronous or
asynchronous operation.

Short for recommended standard-232C, a standard interface approved by the Electronic
Industries Association (EIA) for connecting serial devices. In 1987, the EIA released a new
version of the standard and changed the name to EIA-232-D. And in 1991, the EIA teamed
up with Telecommunications Industry association (TIA) and issued a new version of the
standard called EIA/TIA-232-E. Many people, however, still refer to the standard as RS-
232C, or just RS-232.

Almost all modems conform to the EIA-232 standard and most personal computers have an
EIA-232 port for connecting a modem or other device. In addition to modems, many display
screens, mice, and serial printers are designed to connect to a EIA-232 port. In EIA-232
parlance, the device that connects to the interface is called a Data Communications
Equipment (DCE) and the device to which it connects (e.g., the computer) is called a
Data Terminal Equipment (DTE).
The EIA-232 standard supports two types of connectors -- a 25-pin D-type connector (DB-
25) and a 9-pin D-type connector (DB-9). The type of serial communications used by PCs
requires only 9 pins so either type of connector will work equally well.

Although EIA-232 is still the most common standard for serial communication, the EIA has
recently defined successors to EIA-232 called RS-422 and RS-423. The new standards are
backward compatible so that RS-232 devices can connect to an RS-422 port.

RS-422 and RS-423
Standard interfaces approved by the Electronic Industries Association (EIA) for connecting
serial devices. The RS-422 and RS-423 standards are designed to replace the older RS-232
standard because they support higher data rates and greater immunity to electrical
interference. All Apple Macintosh computers contain an RS-422 port that can also be used
for RS-232C communication. RS-422 supports multipoint connections whereas RS-423
supports only point-to-point connections.

Refers to transmission in only one direction. Note the difference between simplex and half-
duplex. Half-duplex refers to two-way communications where only one party can transmit at
a time. Simplex refers to one-way communications where one party is the transmitter and the
other is the receiver. An example of simplex communications is a simple radio, which you
can receive data from stations but can't transmit data.

Half duplex
Refers to the transmission of data in just one direction at a time. For example, a walkie-talkie
is a half-duplex device because only one party can talk at a time. In contrast, a telephone is a
full-duplex device because both parties can talk simultaneously.
Most modems contain a switch that lets you select between half-duplex and full-duplex
modes. The correct choice depends on which program you are using to transmit data through
the modem.
In half-duplex mode, each character transmitted is immediately displayed on your screen.
(For this reason, it is sometimes called local echo -- characters are echoed by the local
device). In full-duplex mode, transmitted data is not displayed on your monitor until it has
been received and returned (remotely echoed) by the other device. If you are running a
communications program and every character appears twice, it probably means that your
modem is in half-duplex mode when it should be in full-duplex mode, and every character is
being both locally and remotely echoed.

Full duplex
Refers to the transmission of data in two directions simultaneously. For example, a telephone
is a full-duplex device because both parties can talk at once. In contrast, a walkie-talkie is a
half-duplex device because only one party can transmit at a time.
Most modems have a switch that lets you choose between full-duplex and half-duplex modes.
The choice depends on which communications program you are running.
In full-duplex mode, data you transmit does not appear on your screen until it has been
received and sent back by the other party. This enables you to validate that the data has been
accurately transmitted. If your display screen shows two of each character, it probably means
that your modem is set to half-duplex mode when it should be in full-duplex mode.

 In computer software, any symbol that requires one byte of storage. This includes all the
ASCII and extended ASCII characters, including the space character. In character-based
software, everything that appears on the screen, including graphics symbols, is considered to
be a character. In graphics-based applications, the term character is generally reserved for
letters, numbers, and punctuation.

Acronym for the American Standard Code for Information Interchange. Pronounced ask-ee,
ASCII is a code for representing English characters as numbers, with each letter assigned a
number from 0 to 127. For example, the ASCII code for uppercase M is 77. Most computers

use ASCII codes to represent text, which makes it possible to transfer data from one
computer to another.

Text files stored in ASCII format are sometimes called ASCII files. Text editors and word
processors are usually capable of storing data in ASCII format, although ASCII format is not
always the default storage format. Most data files, particularly if they contain numeric data,
are not stored in ASCII format. Executable programs are never stored in ASCII format.

The standard ASCII character set uses just 7 bits for each character. There are several larger
character sets that use 8 bits, which gives them 128 additional characters. The extra
characters are used to represent non-English characters, graphics symbols, and mathematical
symbols. Several companies and organizations have proposed extensions for these 128
characters. The DOS operating system uses a superset of ASCII called extended ASCII
or high ASCII. A more universal standard is the ISO Latin 1 set of characters, which is used
by many operating systems, as well as Web browsers.

Another set of codes that is used on large IBM computers is EBCDIC.

Standard ASCII (Alphanumeric Characters)

33 !    49 1      65 A    81 Q     97 a     113    q
34 "    50 2      66 B   82 R      98 b     114     r
 35 #    51 3     67 C   83 S      99 c     115    s
36 $     52 4     68 D   84 T      100 d     116    t
37 %     53 5     69 E  85 U      101 e     117    u
38 &    54 6     70 F   86 V      102 f     118    v
39 '    55 7     71 G   87 W      103 g     119    w
40 (     56 8     72 H   88 X     104 h     120    x
41 )     57 9     73 I  89 Y       105 i    121    y
42 *    58 :      74 J  90 Z      106 j     122    z
43 +     59 ;     75 K 91 [       107 k     123    {
 44 ,    60 <     76 L 92 \       108 l      124    |
45 -     61 =     77 M 93 ]       109 m     125    }
 46 .     62 >     78 N 94 ^      110 n     126    ~
 47 /     63 ?     79 O 95 _      111 o     127    _
48 0     64 @     80 P  96 `      112 p

IEEE 802 standards
A set of network standards developed by the IEEE. They include:

IEEE 802.1: Standards related to network management.
IEEE 802.2: General standard for the data link layer in the OSI Reference Model. The IEEE
divides this layer into two sublayers -- the data link control (DLC) layer and the media access
control (MAC) layer.
The MAC layer varies for different network types and is defined by standards IEEE 802.3
through IEEE 802.5.
IEEE 802.3: Defines the MAC layer for bus networks that use CSMA/CD. This is the basis
of the Ethernet standard.
IEEE 802.4: Defines the MAC layer for bus networks that use a token-passing mechanism
(token bus networks).
IEEE 802.5: Defines the MAC layer for token-ring networks.
IEEE 802.6: Standard for Metropolitan Area Networks (MANs).

IEEE 1394
A new, very fast external bus standard that supports data transfer rates of up to 400 Mbps
(400 million bits per second). Products supporting the 1394 standard go under different
names, depending on the company. Apple, which originally developed the technology, uses
the trademarked name FireWire. Other companies use other names, such as I-link and Lynx,
to describe their 1394 products.
A single 1394 port can be used to connect up 63 external devices. In addition to its high
speed, 1394 also supports isochronous data -- delivering data at a guaranteed rate. This
makes it ideal for devices that need to transfer high levels of data in real-time, such as video
devices. Although extremely fast and flexible, 1394 is also expensive. Like USB, 1394
supports both Plug-and-Play and hot plugging, and also provides power to peripheral devices.
The main difference between 1394 and USB is that 1394 supports faster data transfer rates
and is more expensive. For this reason, it is expected to be used mostly for devices that
require large throughputs, such as video cameras, whereas USB will be used to connect most
other peripheral devices.

Network operating system
An operating system that includes special functions for connecting computers and devices
into a local-area network (LAN). Some operating systems, such as UNIX and the Mac OS,
have networking functions built in. The term network operating system, however, is generally
reserved for software that enhances a basic operating system by adding networking features.
For example, some popular NOS's for DOS and Windows systems include Novell Netware,
Artisoft's LANtastic, Microsoft LAN Manager, and Windows NT.

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