Communications
Computer Connections
Data Communications
- Merger of communications and computers is called telecommunications .
- Data communications systems have been evolving since the
mid-1960s.
- Early data communications systems were teleprocessing
systems - terminals connected to a central computer via communications
lines.
- Distributed data processing (DDP) accommodates not only remote
access but remote processing.
- Networks use communications equipment to connect two or more
computers and their resources.
Communications System Components
- A sending device
- A communications link
- A receiving device
Digital and Analog Transmission
- Digital transmission sends data as distinct pulses, either
on or off.
- Analog transmission is a continuous electric signal in the
form of a wave.
- To be sent over analog lines, a digital signal must first be converted
by altering an analog signal, called a carrier
wave.
Voice Grade Circuits
- Much of the telephone network consists of analog, voice
grade circuits.
- The telephone network was originally designed to handle voice communication
only.
Amplitude Modulation (AM)
- The amplitude of the carrier signal is varied to carry the transmitted
signal.
- More susceptible to noise than frequency modulation.
Frequency Modulation (FM)
- The frequency of the carrier signal is varied to carry the transmitted
signal.
- Different tones are used.
Modems
- Acronym for modulate-demodulate.
- Convert between digital pulses from computer or terminal and analog signal
for voice grade circuit.
- Can be external or internal.
Asynchronous and Synchronous Transmission
- Asynchronous Transmission
- Each character is transmitted independently of all other characters.
- A start and stop signal separate characters
from each other.
- Synchronous Transmission
- Both the sending and receiving devices operate simultaneously.
- Don't need start and stop signals.
- "Sync" bits are used to synchronize the devices
between blocks of data.
Simplex, Half-Duplex, and Full-Duplex Transmission
- Simplex transmission sends data in one direction only (example:
TV).
- Half-duplex transmission sends data in either direction,
but only one way at a time (example: CB radio).
- Full-duplex transmission allows transmission in both directions
at once (example: telephone).
Communication Media
- Wire Cables
- Insulated pairs of copper wires bundled in cables.
- Usually twisted to minimize electrical interference.
- Coaxial Cable
- Copper conductor in the center and outer shield.
- Can transmit a wider bandwidth (range of frequencies)
than twisted pair.
- Microwave Transmission
- High frequency radio communication transmitted over a line-of-site
path between two points.
- Carries hundreds of simultaneous messages - common for long distance
transmission.
Communication Media
- Satellite Transmission
- Wide bandwidth transmission to satellites in space.
- Fiber Optic Cable
- High speed streams of data are sent via light pulses over very high
bandwidth glass fibers.
- Much more difficult to tap optical cables than electrical.
- Not affected by electrical interference.
- Cellular Radio
- High frequency system in which service areas are divided into
cells.
- As you move from one cell to the next, transmission is switched to
the closest antenna.
Protocols
- A line protocol is a set of rules for the exchange of data between a terminal
and a computer or between two computers.
- Standard protocols are agreed upon for interconnection of communications
equipment from different companies.
- Standards organizations such as the International Standards Organization
(ISO) and the International Telecommunications
Union (ITU) define and/or endorse standards.
Line Configurations
- A point-to-point line is a direct connection between each
terminal and the central computer.
- A multipoint line contains several terminals connected on
the same line to the central computer, but only one terminal can transmit
at a time.
- In a packet-switched network, the data is broken into units
called packets, that also contain instructions about the destination of the
packet. Packets are directed from node
to node through the network.
Carriers and Regulation
- Communications in the US are regulated by the Federal Communications
Commission (FCC) and by state regulatory agencies, such as the
New York Public Service Commission
(PSC).
- Communications services are offered through a list of services and rates
called tariffs, which must be approved
by regulatory agencies.
- An organization that has been approved to offer communications services
to the public is called a common carrier.
Carrier Lines
- Switched lines connect through switching centers to a variety
of destinations.
- Private lines offer communications to fixed destinations
and are dedicated to one customer.
- Integrated Services Digital Network (ISDN) service provides
channels for both voice and data communication. It can also be used to transmit
video for desktop video conferencing .
Divestiture of AT&T
- Prior to divestiture, AT&T controlled 90% of the telephone system in
the United States.
- On January 1, 1984, AT&T divested its local telephone service companies
in return for more regulatory freedom in the long distance and computer manufacturing
areas.
Local Telephone Service
- Bell Operating Companies
- 22 individual telephone companies owned by AT&T were consolidated
into 7 Bell Operating Companies at divestiture.
- Independent Telephone Companies
- There are hundreds of independent local telephone companies throughout
the country that were never owned by AT&T.
Star Topology
- All nodes are connected to a central host that routes traffic to the appropriate
place.
- If the central node fails, the entire network fails.
Ring Topology
- Messages pass to each station in turn.
- Data is transmitted in one direction only, but often double
rings are employed.
- Most common ring network is the Token Ring network.
Bus Topology
- All nodes connect to a cable that runs the length of the network.
- Most common bus network is the Ethernet network.
Wide Area Network (WAN)
- A network of geographically distant computers and terminals.
- Uses the communication circuits of a common carrier.
- Usually uses packet switching.
Metropolitan Area Network (MAN)
- Interconnects buildings in a a geographic area that encompasses a city
or county.
Local Area Network (LAN)
- A group of microcomputers or other workstations connected in the same general
area for the exchange of information.
- Generally, one or more computers acts as a server for the
purpose of sharing peripherals such as hard disks, printers, tape backup,
etc.
Telecommuting
- Telecommuting is the substitution of telecommunications and
computers for the commute to work.
- Most telecommuters work at home two to three days a week.
- Benefits include savings in fuel costs and commuting time, increased productivity,
and an opportunity to work in an undisturbed
environment
Commercial Communications Services
- On-line services such as Prodigy, CompuServe, and America Online provide
e-mail, shopping, discussions, magazines, banking, etc. via a modem and phone
line.
- These services are also starting to provide access to the
Internet.
The Internet
- A "network of networks", with thousands of computer networks
connecting millions of computers to one another.
- A decentralized collection of different networks, each managed by separate
groups who have agreed on a common set of protocols to link their networks
together.
The World Wide Web (WWW)
- Enables hypertext based point and click access to information throughout
the Internet
- Developed at the CERN research center in Switzerland to allow physicists
to easily share their work
- Browsers such as Netscape and NCSA Mosaic are used to access
the Web