3. • A router is a device that forwards data packets between computer networks. This
creates an overlayinternetwork, as a router is connected to two or more data lines
from different networks. When a data packet comes in one of the lines, the router
reads the address information in the packet to determine its ultimate destination.
Then, using information in its routing table or routing policy, it directs the packet to
the next network on its journey. Routers perform the "traffic directing" functions
on theInternet. A data packet is typically forwarded from one router to another
through the networks that constitute the internetwork until it reaches its
destination node.
• The most familiar type of routers are home and small office routers that simply
pass data, such as web pages, email, IM, and videos between the home computers
and the Internet. An example of a router would be the owner's cable or DSL router,
which connects to the Internet through an ISP. More sophisticated routers, such as
enterprise routers, connect large business or ISP networks up to the powerful core
routers that forward data at high speed along the optical fiber lines of the Internet
backbone. Though routers are typically dedicated hardware devices, use of
software-based routers has grown increasingly common.
6. A modem (modulator-demodulator) is a device that modulates an analog carrier
signal to encodedigital information and demodulates the signal to decode the
transmitted information. The goal is to produce a signal that can be transmitted
easily and decoded to reproduce the original digital data. Modems can be used
with any means of transmitting analog signals, from light emitting diodes toradio.
A common type of modem is one that turns the digital data of a computer into
modulatedelectrical signal for transmission over telephone lines and demodulated
by another modem at the receiver side to recover the digital data.
Modems are generally classified by the amount of data they can send in a given unit of
time, usually expressed in bits per second (bit/s or bps), or bytes per second (B/s).
Modems can also be classified by their symbol rate, measured in baud. The baud
unit denotes symbols per second, or the number of times per second the modem
sends a new signal. For example, the ITU V.21 standard used audio frequency shift
keying with two possible frequencies, corresponding to two distinct symbols (or
one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the
original ITU V.22 standard, which could transmit and receive four distinct symbols
(two bits per symbol), transmitted 1,200 bits by sending 600 symbols per second
(600 baud) using phase shift keying.
9. • An Ethernet hub, active hub, network hub, repeater
hub, multiport repeater or hub is a device for connecting
multiple Ethernet devices together and making them act as a
single network segment. It has multiple input/output (I/O) ports, in
which a signalintroduced at the input of any port appears at the
output of every port except the original incoming. A hub works at
the physical layer (layer 1) of the OSI model.Repeater hubs also
participate in collision detection, forwarding a jam signal to all ports
if it detects a collision.
• Some hubs may also come with a BNC and/or Attachment Unit
Interface (AUI) connector to allow connection to
legacy 10BASE2 or 10BASE5 network segments. The availability of
low-priced network switches has largely rendered hubs obsolete
but they are still seen in 20th century installations and more
specialized applications.
