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Data
Communications
Basic
Concepts
- Line
Configuration refers to the way two or more communication
devices attach to a link. A link is the physical
communication pathway that transfer data from one device to
another.
- A
point-to-point line configuration provides a dedicated
link between two devices.
- A
multipoint (also called multidrop) line
configuration is one in which more than two specific
devices share a single link.
- The
term topology refers to the way a network is laid out,
either physically or logically. A consideration when choosing
a topology is the relative status of the devices to be linked.
Peer-to-peer is where the devices share the line equally,
and primary-secondary is where one device controls
traffic and the others must transmit through it. Ring and
mesh topologies are more convenient for peer-to-peer transmission,
while star and tree are more convenient for primary-secondary.
A bus topology is equally convenient for either.
- In
a mesh topology every device has a dedicated point-to-point
link to every other device. The term dedicated
means that the link carries traffic only between the two
devices that it connects. A fully connected mesh network
therefore has n(n-1)/2 links. To accomodate that many
links, every device on the network must have n-1 input/output
(I/O) ports.
- In
a star topology, each device has a dedicated point-to-point
link only to a central controller, usually called a hub.
Unlike a mesh topology, a star topology does not allow
direct traffic between devices. There are n-1 links in
a star topology.
- A
tree topology is a variation of a star. As in a
star, nodes in a tree are linked to a central hub that
controls the traffic to the network. However, not every
device plugs directly into the central hub. The majority
of devices connect to a secondary hub that in turn is
connected to the central hub. The central hub in the tree
is an active hub. An active hub contains a repeater,
which is a hardware device that regenerates the received
bit patterns before sending them out. The secondary hubs
may be active or passive hubs. A passive hub provides
a simple physical connection between the attached devices.
- A
bus topology is multipoint. One long cable acts
as a backbone to link all the devices in the network.
- In
a ring topology (n links) each device has a dedicated
point-to-point line configuation only with 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.
- The
term transmission mode is used to define the direction
of signal flow between two linked devices.
- In
simplex mode the communication is unidirectional,
as on a one-way street.
- In
half duplex mode each station can both transmit
and receive, but not at the same time.
- In
full duplex mode both stations can transmit and
receive simultaneously.
- Categories
of networks
- A
local area network (LAN) is usually privately owned
and links the devices in a single office, building, or
campus. Currently, LAN size is limited to a few kilometers.
In general, a given LAN will use only one type of transmission
medium. The most common LAN topologies are bus, ring,
and star.
- A
metropolitan area network (MAN) is designed to
extend over an entire city. A MAN may be wholly owned
and operated by a private company, or it may be a service
provided by a public company such as a local telephone
company.
- A
wide area network (WAN) provides long-distance
transmission of data, voice, image, and video information
over large geographical areas that may comprise a country,
a continent, or even the whole world. In contrast to LANs,
WANs may utilize public, leased, or private communication
devices, usually in combinations, and can therefore span
an unlimited number of miles. A WAN that is wholly owned
and used by a single company is often referred to as an
enterprise network.
- When
two or more networks are connected, they become an internetwork,
or internet. Individual networks are joined into internetworks
by the use of internetworking devices.
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