Networks
(03/18/2004)
Introduction: A network is a collection
of computers and computer peripherals/devices (e.g. printers etc) that
are connnected via some sort of transmission medium that allows the sharing
of resources.
Protocols: a set of rules that defines how two devices interact.
Client/Server Model: one device (the client) makes a request of a
second device (server). The "work" is perfomed by the server and the results
passed back to the client. A good example of this is a database query. The
datebase is maintained on the server. A client makes a database request which
is executed by the server on the resident database and the results sent back
to the client. Your h: drive is another example of the client/server model
in action. Physcially your h:drive is on a file server. When you access a
file on your h: drive, the file server locates the file and "returns" it
to the client (you).
Types of Networks
- LAN - Local Area Neworks
- MAN - Metropolitan Area Networks
- WAN - Wide Area Networks
Network Configurations (topologies): ways to physically connect the
nodes of a LAN
Bus: All nodes are connected to a common line (or "bus"-
a common carrier). All nodes listen to the "traffic" but only respond to
messages which are addressed to them. A common type of bus network system
is EtherNet which uses a "carrier sense multiple access with collision
detection (CSMA/CD) protocol. What this means is each nodes "listens" to
the bus and if a node wants to transmit something it waits until the bus
is free. However if two nodes try to transmit at once (a collision), this
is detected and both nodes bake-off for a randomly chosenp period of time,
Star: All nodes are connected to a common hub; communication
is made through the hub.
Ring:: nodes are connected in a loop. A type of ring topology
is called Token Ring where an electronic token is endlessly circulated. When
a node wishes to transmit, it grabs the token, attaches its message to it
and sends it on its way. When it reaches the receiving node, the receiver
extracts the message and send the token on to be used by the next node that
wishes to transmit.
OSI (Open Systems Interconnection) Model:
The OSI is a seven layer model for decsribing/implementing a network. The
idea is to implement the higher more advanced layers on top of the lower
layers - sort of like constructing a class out of member variables and functions.
These are the 7 layers in order from bottom to top.
- Physical: responsible for transmitting the bits over the
physical medium
- Data Link: organizes bits into logical frames. Adds header and
trailer information (includes addresses) which is used to transmit the frame
from one node to the next adjacent node.
- Network: organizes information into packets; responsible
for deliving the packet from source to final destination
- Transport : responsible for delivering the entire message from
source to destination
- Session: controls dislog between end users; may add synchronization
points so that when part of the message is lost you only need backup to the
last synchronization point
- Presentation: deals with miscellaneous functions like converting
between different coding methods (ASCII vs Unicode); some encryption/decryption
can be done here
- Application: interface with users
There is a simple way to remember this: A Powered-down System
Transmits No Data! Period!
Internet Model
This is a four layer model which uses the Transmission Control Protocol/Internet
Protocol (TCP/IP) protocol.
- Physical & Data Link: (from OSI model)
- Network Level : Internet Protocol (IP) : the Internet Protocol
deals with routing packets from source to destination.
- Transport Level: TCP/UDP Level : TCP (Transmission Control Protocol)
breaks the message down into packets which are handed off to the IP level.
At the other end TCP reassembles the packets. UDP (User Datagram Protocol)
is an alternate to TCP which is less reliable but faster.
- Application Level (OSI Session thru Application): Applications
like FTP(File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), POP
(Post-Office Protocol), TELNET (Terminal Network), HTTP (HyperTest Transfer
Protocol)
Network Addresses
- Hostname: unique alpha-numeric identifier that specifics a particular
ocmputer on the Internet (e.g. userpages.wittenberg.edu).
- IP address: a four byte numerical address that an hostname translates
to. An IP address is partitioned into an netwokk address which identifies
a particular network and a host number which identifies a computer on that
network. There are 3 classes of IP addresses which are used to specify various
sized networks
- Class A: first byte specifies the network address; the three remaining
bytes specify the host number; these are for very large networks
- Class B: first two bytes specifies the network; the remaining two are
the host number
- Class C: first three bytes specify the network; the remainly
byte can only specify 255 hosts.
The Domain Name System (DNS) is a distributed database used
to translate Hostnames into IP addresses
The Internet is NOT the World Wide Web
The Internet is a network of networks; the World Wide Web is an infrastructure
of web-pages combined with an method that facilites the transmission and
displaying of those web pages. The web pages may contain text, images, video,
graphics, and programs.
Devices
Repeaters: refreshes signals; extends range of networks
Bridges: connects two LAN's allowing local traffic on each to be isolated
from other LAN
Routers: connects a LAN to another network like a WAN
Gatesway: connecting device that allows protocol conversions
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