Download Networks

Networks
CS105
What is a computer network?
•
A computer network is a collection of computing devices
that are connected in various ways so that they can
communicate and share resources
Purpose
Computer networks are used for:
1. Facilitating communications: Email, instant messaging, chat rooms,
telephone, video telephone calls, and video conferencing.
2. Sharing hardware: Hardware resources on the network –
printing
3. Sharing files, data, and information: Access data and
information stored on other computers on the network.
4. Sharing software: Users connected to a network may run
application programs on remote computers.
Terminology
•
•
Any device/computer connected to a network is
called a node or host
Data transfer rate or bandwidth: The speed
with which data travels from one node to
another
–
•
•
Number of bits per unit time
Protocols: Set of rules that defines how data is
formatted and processed on a network
Client-Server models: A distributed approach in
which a client makes requests of a server and
Response
the server responds
–
File servers, Web Servers
Server
Client
Request
Types of networks
The connections between computers in a network are either
made using physical wires/cables or they can be wireless
Networks can be classified into:
• Local-Area network (LAN): A network connecting a small
number of computers + devices in a close geographic area
• Wide-Area network (WAN): A network connecting two or
more LANS
Types of networks
LAN
LAN
Gateway
Gateway
Wide Area Network (WAN)
Internet
• An infrastructure for communication
• Internet backbone: High capacity data routes that carry
internet traffic
– High performance computers whose only purpose is to route data
– Provided by commercial & government agencies
False!
– No one owns the internet, no central administration
– Anybody who connects to the net owns a slice of the net
– Protocols to communicate
Why can’t I connect to the backbone directly? Why do I have to
pay for internet?
In theory, you can. But you need a few million (perhaps billion) dollars to make a
global network of cables and routers and get other ‘backbone’ agencies to network
with you.
How do we get internet in our computers?
• Through an ISP (internet service provider): a company that
provides other companies or individuals with access to the
internet
– ISPs connect directly to the internet backbone or to a larger ISP
Internet
Backbones
ISPs
Users
Different colors represent
different users
How do we get internet in our computers?
• Three most popular ways to connect to the net is:
– Phone modem: A device converts computer data into an analog audio
signal for transfer over a telephone line
• Upper boundary on the amount of data transmitted per second (64K/sec)
• Data is treated as a phone conversation
• Phone cannot be used while you are on the net
– Digital subscriber line: Uses regular phones to transmit digital data at a
higher frequency
• Phone line can be used for voice conversations and browsing the net at the same
time
– Cable modem: Data is transferred on the lines through cable TV signals
are transmitted
• Both DSL and cable modem are called Broadband because
data speeds are greater than 128Kbits per second
Data transmission
Connectionless transmission
• Data is sent over a network in packets: unit of data sent across
a network
Sent
Message
Message is divided
into packets
Packet 1
Packet 2
Packet 2
Packet 3
Packet 3
Packet 1
Packets are sent over the internet
by the most expedient route
Packet Switching
Received
Message
Packets are reordered
then assembled
Packet Switching
• Router: A network device that directs a packet between
networks towards its final destination
Host C
Host D
Host A
Router 1
2
Router 3
Router 2
1
Router 4
1
9
6
5
7
Router 5
Host B
4
10
Host E
3
11
Router 7
Router 6
Packet Switching
• Advantages:
– More efficient
– Data is sent immediately
• Disadvantages:
– Packets are rerouted so there might be delay
– Packets may be lost or ‘dropped’ while en route