1. The five components that
make up a data communications system are the message, sender, receiver, medium, and protocol.
2. Data flow between two
devices can occur in one of three ways: simplex, half-duplex, or full-duplex.
a. In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive.
b. In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa
c. In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously.
3. A protocol is a set of rules that governs data communication; the key
elements of a protocol are syntax, semantics, and timing.
a.
The term syntax
refers to the structure or format of the data, meaning the order in which they
are presented.
b.
The word semantics
refers to the meaning of each section of bits. How are a particular pattern to
be interpreted, and what action is to be taken based on that interpretation?
c.
The term timing
refers to two characteristics: when data should be sent and how fast they can
be sent.
4.
All communications technology is subject to
regulation by government agencies suchas the Federal Communications Commission (FCC) in the United States. The
purpose of these agencies is to protect the public interest by regulating
radio, television,and wire/cable communications. The FCC has authority over
interstate and internationalcommerce as it relates to communications.
5. A point-to-point connection provides a dedicated link between two
devices. The entire capacity of the link is reserved for transmission between
those two devices.
6. A multipoint (also called multidrop)
connection is one in which more than two specific devices share a single link.In
a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it
is a spatially shared connection. If
users must take turns, it is a time shared connection.
7. Transit time is the amount of time required for a message to travel from
one device to another. Response time
is the elapsed time between an inquiry and a response.
Topology refers to the physical or logical arrangement of a network.
8. Mesh Topology: In a mesh topology, every device has a dedicated
point-to-point link to every other device.
a.
In a fully connected
mesh network with n nodes. We
need n(n - 1) physical links.
b.
In a mesh topology, we need n (n -1) /2 duplex-mode links. To accommodate that many links,
every device on the network must have n
– 1 input/output (VO) ports to be connected to the other n - 1 station.
Advantages:
c.
The use of dedicated links guarantees that each connection
can carry its own data load, thus eliminating the traffic problems that can
occur when links must be shared by multiple devices.
d.
A mesh topology is robust. If one link becomes unusable, it
does not incapacitatethe entire system.
e.
There is the advantage of privacy or security.
f.
Finally, point-to-point links make fault identification and
fault isolation easy.
Disadvantages:
g.
The main disadvantages of a mesh are related to the amount of
cabling and the number of I/O ports required.
h.
Because every device must be connected to every other device,
installation and reconnection are difficult.
i.
The sheer bulk of the wiring can be greater than the
available space (in walls, ceilings, or floors) can accommodate.
j.
Finally, the hardware required to connect each link (I/O
ports and cable) can be prohibitively expensive.
9. In a star topology, each device has a dedicated point-to-point link only
to a central controller, usually called a hub.
a.
It is less expensive than a mesh topology.
b.
In a star, each device needs only one link and one I/O port
to connect it to any number of others. This factor also makes it easy to
install and reconfigure.
c.
Far less cabling needs to be housed, and additions,moves, and
deletions involve only one connection: between that device and the hub.
d.
Other advantages include robustness. If one link fails, only
that link is affected. All other links remain active. This factor also lends
itself to easy fault identification and fault isolation.
e.
One big disadvantage of a star topology is the dependency of
the whole topology on one single point, the hub. If the hub goes down, the
whole system is dead.
10. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network. Nodes are connected
to the bus cable by drop lines and taps.
a.
A drop line is a
connection running between the device and the main cable.
b.
A tap is a
connector that either splices into the main cable or punctures the sheathing of
a cable to create a contact with the metallic core. As a signal travels along
the backbone, some of its energy is transformed into heat. Therefore, it
becomes weaker and weaker as it travels farther and farther. For this reason
there is a limit on the number of taps a bus can support and on the distance
between those taps.
c.
Difficult reconnection and fault isolation.
d.
In addition, a fault or break in the bus cable stops all
transmission, even between devices on the same side of the problem. The damaged
area reflects signals back in the direction of origin, creating noise in both
directions.
11. In a ring topology, each device has a dedicated point-to-point
connection with only the two devices on either side of it. A signal is passed
along the ring in one direction, from device to device, until it reaches its
destination. Each device in the ring incorporates a repeater. When a device
receives a signal intended for another device, its repeater regenerates the
bits and passes them along.
a.
To add or delete a device requires changing only two
connections. The only constraints are media and traffic considerations (maximum
ring length and number of devices).
b.
In addition, fault isolation is simplified.Generally in a
ring, a signal is circulating at all times. If one device does not receive a
signal within a specified period, it can issue an alarm. The alarm alerts the
network operator to the problem and its location.
c.
However, unidirectional traffic can be a disadvantage. In a
simple ring, a break in the ring (such as a disabled station) can disable the
entire network. This weakness can be solved by using a dual ring or a switch
capable of closing off the break.
12. Categories of Network: The category into which a network falls
is determined by its size. Networks of a size in between are normally referred
to as metropolitan area networks and span tens of miles.
13. A local area network (LAN) is usually
privately owned and links the devices in a single office, building, or campus
(see Figure 1.10). Depending on the needs of an organization and the type of
technology used, a LAN can be as simple as two PCs and a printer in someone's
home office; or it can extend throughout a company and include audio and video
peripherals. Currently, LAN size is limited to a few kilometers.
14. A WAN can be as complex as the backbones that connect the Internet or
as simple as a dial-up line that connects a home computer to the Internet. We
normally refer to the first as a switched
WAN and to the second as a point-to-point
WAN.
a.
The switched WAN
connects the end systems, which usually comprise a router (internetworking
connecting device) that connects to another LAN or WAN.
b.
The point-to-point WAN
is normally a line leased from a telephone or cable TV provider that
connects a home computer or a small LAN to an Internet service provider (ISP).
15.
A metropolitan
area network (MAN) is a network with a size between a LAN and a WAN. It
normally covers the area inside a town or a city. It is designed for customers who
need a high-speed connectivity, normally to the Internet, and have endpoints spread
over a city or part of city. A good example of a MAN is the part of the
telephone company network that can provide a high-speed DSL line to the
customer. Another example is the cable TV network that originally was designed
for cable TV.
16. In 1967, at an Association for Computing Machinery (ACM)
meeting, ARPA presented its ideas for ARPANET
(first network), a small network of connected computers. The idea was that
each host computer (not necessarily from the same manufacturer) would be
attached to a specialized computer, called an interface message processor (IMP). The IMPs, in turn, would be
connected to one another. Each IMP had to be able to communicate with other
IMPs as well as with its own attached host.
By 1969, ARPANET was a
reality. Four nodes, at the University of California at Los Angeles (UCLA), the
University of California at Santa Barbara (UCSB), Stanford Research Institute
(SRI), and the University of Utah, were connected via the IMPs to form a
network. Software called the Network
Control Protocol (NCP) provided communication between the hosts.
17. Frequencies of failure and
network recovery time after a failure are measures of the reliability of a network.
18. Communication between a
computer and a keyboard involves simplex
transmission.
19. A television broadcast is an
example of simplex transmission.
20. In the original ARPANET, IMP’s were directly connected together.
21. FCC organization has authority over interstate and international
commerce in the communications field.
22. Forums are special-interest groups that quickly test, evaluate, and
standardize new technologies.
23. EIA agency developed standards for physical connection interfaces
and electronic signaling specifications.
24. RFC (Request for comment) is an idea or concept that is a precursor to an
Internet standard.
25. A backbone is a network that connects to smaller networks.
26. An Internet Draft is a working internet document (a work in progress)
with no official status and a six-month life-time.
27. RFC is a formal internet document concerning an internet issue.
Comments
Post a Comment