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23 Οκτ 2013 (πριν από 4 χρόνια και 2 μήνες)

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2
-
Tier

3/N
-
Tier

Fat Client

Fat Server

Adapted from Rob & Coronel (1997)

Client/Server Architectural
Styles

Middleware


Middleware software provides the
means through which clients and
servers communicate to perform
specific actions. Operates at 2 levels:


Physical level deals with communications
between client and server computers.


Logical level deals with communications
between client and server processes.

Types of Networks


Three types of networks transmit data
between computerized devices:


local area networks (LAN)



wide area networks (WAN)



value added networks (VAN)



LANs


A computer communication technology
is classified as a LAN if it provides a
way to interconnect multiple computers
and devices across short distances
-

e.g. floor of a building.


Computers and devices need additional
hardware to connect to a LAN.


A LAN is independent from the
computers it connects.

LANs


LANs support distributed computing in
small organizations and in departments
of larger organizations and offer the
following benefits.


Sharing equipment


sharing personal files


sharing databases


administering software


link to other networks

Network Topology


A network topology is the pattern of
connections between the devices on a
network (LAN or other types).


Three main network topologies


bus topology


ring topology


star topology

Bus Topology


Each node (computer, device) is attached
to a common cable called bus.


A device can access any other device
directly by using its address.


Message broadcast to all nodes at the
same time and only the recipient device
responds by accepting it.

Bus Topology


Addition/removal of a node does not
affect other nodes.



Network performance degrades as traffic
increases.



Ethernet uses a bus topology.

Ring Topology


Computers and devices are connected
directly in a continuous loop.


Messages must be re transmitted by all
the nodes between the source and the
destination.


IBM’s
token

ring network.


Nodes can be added or removed;
requires special effort while network is
running.

Star Topology


All messages go through a central node
-

computer or hub.


All network messages travel through the
central node.


Nodes can be added or removed without
affecting the operation of the rest of the
network.

Star Topology


Expansion is limited by processing power
of central node.



Reliability problems; network goes down
when the central node is down.


Networking Cabling


Most LANs use either twisted
-
pair wire or
coaxial cable.


Fiber optic cable is used rarely for LANs.
But it offers the highest data transmission
quality and allows greater distances
between cables.


Wireless communications media, such as
radio and satellite, are an alternative to
cables.

Data Transmission


Baseband transmission


Most LANs use this. Entire capacity of the
cable is used to transmit a single digitally
coded signal.


Ethernet using coaxial cable (at 10 or 100
Mbps) is a common baseband network.


Broadband transmission


capacity of the cable is divided into
separate frequencies to transmit voice,
data, and video signals simultaneously.

Connecting Networks


Links between LANs and between LANs
and other networks is accomplished using
bridges, routers, repeaters, and gateways.



A bridge is a device that connects two
LAN segments, which may be similar or
dissimilar.


Connecting Networks
(Contd.)


A
router

is a device that forwards data
packets from one LAN or WAN to
another. It is at a higher level than a
bridge and is sensitive to protocols.
Routers are the backbone of the
Internet.


A
repeater

is a device that is used in
LANs to regenerate signals to span
longer distances.

Connecting Networks
(Contd.)



A
gateway

is a computer that performs
protocol conversion between dissimilar
networks. Sometimes a “go
-
between” two
or more networks that use same
protocols.

Wide Area Networks (WANs)


A communications network that covers a
wide geographic area, such as a state or a
country.



Communications backbone for a large
distributed organization.

WANs (Contd.)


Often implemented in the form of a
virtual private network (VPN), a private
network configured within a public
network. Customers physically share
backbone trunks with other customers.
Trend is VPN over the Internet.


WAN can link to workstations or
terminals through LANs.

Value Added Networks
(VANs)


VANs are public data networks that “add
value” by transmitting data and providing
access to commercial databases and
software.



Customers subscribe to VANs and also
pay additional fees based on usage.

Value Added Networks
(VANs)


Used in EDI, where forms are collected in
an electronic mailbox, translated and
forwarded to recipients with guaranteed
delivery.



Internet is becoming an alternative to
VANs for many applications that do not
involve huge amounts of data.


OSI Reference Model

Open Systems Interconnection reference
model


Seven
-
layer network architecture model of
data communication protocols developed by
ISO and CCITT.




Each layer specifies particular network
functions.

OSI Reference Model


Layer 7, the application layer, the
highest layer of the model, defines the
way applications (client or server)
interact with the network.


Layer 6, the presentation layer, includes
protocols that are part of the operating
system, and defines how information is
formatted for display or printing and
how data is encrypted, and translation
of other character sets.


OSI Reference Model


Layer 5, the session layer, coordinates
communication between systems,
maintaining sessions for as long as needed
and performing security, logging, and
administrative functions.


Layer 4, the transport layer, controls the
movement of data between systems, defines
protocols for structuring messages, and
supervises the validity of transmissions by
performing error checking.


OSI Reference Model


Layer 3, the network layer, defines
protocols for routing data by opening and
maintaining a path on the network
between systems to ensure that data
arrives at the correct destination node.


Layer 2, the data
-
link layer, defines the
rules for sending and receiving
information from one node to another
between systems.


OSI Reference Model


Layer 1, the physical layer, governs
hardware connections and byte
-
stream
encoding for transmission. It is the only
layer that involves a physical transfer of
information between network nodes.