Networks Types
Data Transfer
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During the ’70s:
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Minicomputers became affordable;
Need to communicate information;
Data transferred using removable storage
device---tapes, floppy.
Drawback:
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(very) slow
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Point-to-Point (Direct)
Connection
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Dedicated circuit boards connected by cable;
To transfer data from A to B:
 A writes on its circuit board;
 A’s board transfers data to B’s board;
 B reads data from its board.
A
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B
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Point-to-Point Connection
(cont.)
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Pros:
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Cons:
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High transfer speed;
Secure connection;
Difficult to add a new computer to a set of communicating
computers.
Difficult if communication is between heterogeneous
computers;
Expensive: circuit boards needed for every pair of
communicating computers;
How many boards would you need to connect 5
computers via this method?
Spring 2002
Computer Network Applications
Local Area Network (LAN)
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A single shared medium, usually a
cable, to which computers can attach.
PC 1
Printer
PC 2
PC n
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File Server
Computer Network Applications
Local Area Networks (cont.)
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Designed and developed for communications and
resource sharing in a local work environment (room,
campus, building).
Because sharing occurs:
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Cost decreases
Computers have to coordinate the use of the network;
Overall, LANs connect more computers than any type
of network.
Spring 2002
Computer Network Applications
Connecting a Computer to a
LAN
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Requires a network interface;
A network interface consists of:
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a circuit board that plugs into the computer
A cable that connects the circuit board to the LAN;
The network interface isolates a computer
from the LAN  heterogeneous computers
can communicate across the LAN.
Spring 2002
Computer Network Applications
LAN Topologies
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Star Topology
Bus
Ring
Spring 2002
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Star Topology
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Connections are made from all connected machines to one central place,
called a hub.
The hub: accepts messages from the sending computer, and sends data to
recipient.
Spring 2002
Computer Network Applications
Star Topology (cont.)
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Pros:
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Some degree of fault tolerance: the failure
of any link does not affect the other
computers;
Cons:
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Expensive (the hub is a dedicated
computer);
If the hub fails, no connection is possible.
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Ring Topology
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Messages in the network are passing from machine to machine.
This gives controlled and stable data traffic in the network.
No central control or configuration of the traffic.
RING
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Computer Network Applications
Ring Topology (cont.)
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Sending a message:
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There is a special message, called token;
Exactly one token exists on the ring at any time which is
passed along the ring;
To send data, a computer waits for the token to arrive, and
then transmits one message;
The message is transmitted to the next computer, and then
to the next, until it arrives back to the sender.
After the message is transmitted, the computer holding the
token, passes the token to the next computer in the ring.
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Ring Topology (cont.)
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Pros:
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Requires less wire than star;
Less expensive;
Cons:
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If any cable is cut, the entire network is
disabled.
May incur delays: A computer has to pass
the token even if it has more messages to
transmit and nobody else does.
Computer Network Applications
Bus Topology
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Each unit is connected to a cable.
Ex: Ethernet network; original transmission rate: 10 Megabits/s; now:
1000Megabit/s.
BUS
Spring 2002
Computer Network Applications
Bus Topology (cont.)
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Sending a message:
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The sender sends a message only when the cable is not in
use;
The message propagates to both ends of the cable;
The receiver process the message (all computers have to
check if they are the intended destination)
Collisions:
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A collision occurs when two computers try to send in the
same time  garbled transmission;
When a collision is detected; each computer have to wait a
randomly chosen time before retransmitting.
Computer Network Applications
Bus Topology (cont.)
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Pros:
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Cons:
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No delays when only one computer wants to
transmit;
If the bus fails, no transmission possible.
Limited number of communicating parties (the bus
< 500 m, >3m between two connections)
Generally: each topology has adv. and
disadv.;
Spring 2002
Computer Network Applications
Wide Area Networks (WANs)
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Span a large geographic area, cross public
property;
Scalable: allow many computers to send data
simultaneously;
Often based on services provided by 3rd
party companies, like telephone networks,
for transmission from one node to another;
Can be used to connect several LANs
together;
Spring 2002
Computer Network Applications
Packet Switches
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A WAN is constructed from many switches;
A switch moves message fragments called packets
from one connection to the other;
A switch is a dedicated computer, with two types of
connections:
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High-speed connections with other switches; they can be:
leased phone lines, optical fibers, microwave, satellite.
Low-speed connection: used to connect with an individual
computer, or a LAN.
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Switched Network
Switch
High speed
connection
Switch
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Computer Network Applications
Switch
Transmitting messages across
a WAN
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Store and forward technique:
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When a packet arrives at a switch:
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The processor examines the packet and
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it is stored into its internal memory;
the processor is informed of its arrival;
if the destination is idle, then the packet is
forwarded to the destination.
Otherwise, it places the packet in a queue---it
will be sent when the destination is idle.
Computer Network Applications
WAN (cont.)
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Pros:
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Scalable
Many messages can be sent simultaneously;
A message passes only through switches
Fault tolerant: if a switch fails, another route can
be found;
Cons:
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Spring 2002
expensive
Computer Network Applications