Networking - Tripod

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Networking
Terms:



Computer Networking



OSI Reference model for networking



Kinds of Computer Networking


LANs (Example: Ethernet), WANs
(Example:Internet), MANs



Network Protocols: STP; TCP/IP
, BGP



Routers
, Switches



Load balancing servers



Voice over IP (VoIP
)


A
computer network

is a system for communication among two or more computers.
These networks may be fixed (cabled, permanent) or temporary (as via modems).

(
Note:
A
modem

(word constructed from
mod
ulator and
dem
odulator) is a device that
modulates an an
alog carrier signal (sound), to encode digital information, and that also
demodulates such a carrier signal to decode the transmitted information. The goal is to
produce a signal that can be transmitted easily and decoded to reproduce the original
digital
d
ata.)


The
Open Systems Interconnection Reference Model

(
OSI Model

or
OSI Reference
Model

for short) was
developed as part of the Open System Interconnect initiative

and

also called the
OSI seven layer model
.

It is a 7 layered abstract description of netw
ork
protocol design.

This OSI model is roughly adhered to in the computing and networking industry. Its main
feature is in the junction between layers which dictates the specifications on how one
layer interacts with another. This means that a layer writte
n by one manufacturer can
operate with a layer from another (assuming that the specification is interpreted
correctly.) These specifications are typically known as Request for Comments or "RFC"s
in the TCP/IP community. They are ISO standards in the OSI co
mmunity.


Control is passed from one layer to the next, starting at the application layer in one
station, proceeding to the bottom layer, over the channel to the next station and back up
the hierarchy.

7 layers:

7. Application Layer

Functions:

S
upports a
pplications and end user processes
;
determines the communication
;
partners
;
performs user authentication
;
ensures privacy
;
determines constraints on data syntax
;
provides application services for file transfers (Email, Telnet, FTP)

6.

Pr
e
sentation L
ayer

F
unction:

Formats and encrypts data to be sent across a network, providing freedom from
compatibility problems. It is sometimes called the
syntax layer
.


5. Session Layer

Function:

The session layer sets up, coordinates, and terminates conversations, excha
nges, and
dialogues between the applications at each end. It deals with session and connection
coordination.

4. Transport Layer

Function:

Ensures complete data transfer.

3. Network layer

Function:

This layer provides switching and routing technologies, cr
eating logical paths, known as
virtual circuits, for transmitting data from node to node. Routing and forwarding are
functions of this layer, as well as addressing, internetworking, error handling, congestion
control and packet sequencing.

2. Data Link Lay
er

Function:

At this layer, data packets are encoded and decoded into bits.

1. Physical Layer

Function:

This layer conveys the bit stream
-

electrical impulse, light or radio signal
--

through the
network at the electrical and mechanical level.

A
local ar
ea network

(
LAN
) is a
computer network
covering a local area, like a home,
office or small group of buildings such as a college. E.g. the Ethernet.

Ethernet is a frame based computer networking technology

for LAN’s
,

widely used

during the 1990s to the

pre
sent, and has largely replaced all other LAN standards such
as

token ring
,
FDDI
, and

ARCNET
.
Ethernet was originally developed as one of the
many pioneering projects at
XeroxPARC
. by
Robert Metcalfe

in 1973.
Token
-
Ring

(LAN) technology, developed and promo
ted by
IBM
in the early
1980s.

Network Protocols:

A layout known as a
spanning tree
is often used to maintain a loop
free network topology within a LAN, particularly with ethernet.

The
spanning tree

network protocol
provides a loop free
topology
for any
LA
N
or
bridged
network. The Spanning Tree Protocol, which is also referred to as
STP
.

Its structure corresponds to that of the
spanning tree
in
graph theory
. Networks must have
only one path to any destination active at any one point in time to avoid the same

frame
arriving at the destination multiple times, causing dysfunction. The
minimum spanning
tree
algorithm ensures that if multiple paths exist to the same destination then all but one
will be blocked.

The Internet protocol suite

is the set of
communicati
on protocols
that implement the
protocol stack
on which the
Internet
runs. It is sometimes called the
TCP/IP

protocol
suite, after the two most important protocols in it: the
Transmission Control Protocol
(TCP) and the
Internet Protocol
(IP), which were al
so the first two defined.

TCP/IP is built into the Unix OS and is used by
the Internet
, making it the de facto
standard for transmittingdata over networks. Even network operating syste
ms that have
their own
protocols
, such as Netware, also support TCP/IP.

BGP
-

The
Border Gateway Protocol (BGP)

is an interautonomous system routing
protocol. An autonomous system is a

network or group of networks under a common
administration and with common routing policies. BGP is used to exchange routing
information for the Internet and is the protocol used between Internet service providers
(ISP). Customer networks, such as univers
ities and corporations, usually employ an
Interior Gateway Protocol (IGP) such as RIP or OSPF for the exchange of routing
information within their networks. Customers connect to ISPs, and ISPs use BGP to
exchange customer and ISP routes. When BGP is used b
etween autonomous systems
(AS), the protocol is referred to as External BGP (EBGP). If a service provider is using
BGP to exchange routes within an AS, then the protocol is referred to as Interior BGP
(IBGP). Figure 39
-
1 illustrates this distinction.


Fig
ure

39
-
1



External and Interior BGP


BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is
the routing protocol employed on t
he Internet. At the time of this writing, the Internet
BGP routing tables number more than 90,000 routes. To achieve scalability at this level,
BGP uses many route parameters, called attributes, to define routing policies and
maintain a stable routing envi
ronment.


LANs can be interlinked by connections to form a Wide area network. A
router
is used to
make the connection between LANs.

LANs are distinguished from other kinds of networks by three characteristics:

1.

their size

2.

their transmission technology and


3.

their
topology

The
Home Phoneline Networking Alliance

is an incorporated non
-
profit association of
more than 150 companies. The Home Phoneline Networking Alliance seeks to
establish
standards

among telecom, computer and network products such that they ar
e
compatible

for HomePNA.

Power line communication

(
PLC
), also called
Broadband over Power Lines

(
BPL
) or
Power Line Telecoms

(
PLT
), is a
wireline
technology that is able to use the current
electricity networks for data and voice transmission.

Metropolitan

Area Networks

or
MANs

are large
computer networks
usually spanning a
campus
or a

city
. They typically use
wireless infrastructure
or
optical fiber
connections to
link their sites.

For instance a
university
or
college
may have a MAN that joins together man
y of their
local area networks
(LANs) situated around site of a fraction of a square kilometer. Then
from their MAN they could have several
wide area network
(WAN) links to other
universities or the

Internet
.

Some technologies used for this purpose are
ATM
,
FDDI
and

SMDS
. These older
technologies are in the process of being displaced by
Gigabit
-
Ethernet
based MANs in
most areas. MAN links between LANs have been built without cables using either
microwave, radio, or infra
-
red
free
-
space optical communication
l
inks.

DQDB
, Distributed Queue Dual Bus, is the Metropolitan Area Network standard for data
communication.

A
wide area network

or
WAN

is a
computer network
covering a wide geographical
area, involving a vast array of computers. This is different from (PANs)
, (MANs) or
(LANs) that are usually limited to a room, building or campus. The best example of a
WAN is the
Internet.

WANs are used to connect (LANs) together, so that users and computers in one location
can communicate with users and computers in other lo
cations. Many WANs are built for
one particular organization and are private. Others, built by

Internet Service Providers
,
provide connections from an organization's LAN to the Internet. WANs are most often
built using
leased lines
. At each end of the lease
d line, a
router
connects to the LAN on
one side and a hub within the WAN on the other. Network
protocols
including
TCP/IP
deliver transport and addressing functions.

A
router

is a
computer networking device
that forwards data packets toward their
destinat
ions through a process known as

routing
. Routing occurs at layer 3 (
Netw
or
k
Layer
) of the
OSI model.

In non
-
technical terms, a router acts as a connecting bridge between two networks to
transfer data packets among them. A router is essentially different fr
om a
switch
which
connects devices to a local network. One easy illustration for the different functions of
routers and switches is to think of switches as roads connecting all homes in a city and
routers as highways connecting the cities in a country.

Rou
ting is most commonly associated with the

Internet Protocol
, although other less
-
popular routed protocols remain in use.

Network Switches
-

A
network switch

is a computer networking device that connects
network segments. It uses the logic of a network brid
ge but allows a physical and logical
star topology. It is often used to replace network hubs. A switch is also often referred to
as an intelligent hub.

(Note:
A network hub is a

common connection point fo
r devices in a network
. Hubs are
commonly used to co
nnect
segments

of a

LAN
. A
hub

contains multiple
ports
. When a
packet
arrives at one port, it is copied to the other ports so that all segments of the
LAN

can see all packets.

A
passive hub

serves simply as a conduit for the data, enabling it to go from one device
(or segment) to another. So
-
called
intelligent hubs

include additional features that
enables an administrato
r to
monitor

the traffic passing through the hub and to configure
each port in the hub. Intelligent hubs are also called
manageable hubs.


A third type of hub, called a

switching hub
, act
ually reads the destination address of each
packet and then forwards the packet to the correct port.
)

Different Networks:

1.

Bus Network

2.

Star Network

3.

Ring Network

4.

Mesh Network

5.

Star
-
Bus Network

Load Balancing Servers
-

Distributing processing and
communications

activity evenly
across a
computer

network so that no single device is overwhelmed.
Load balancing

is
especially important for
networks

where it's difficult to predict the number of requests

that will be issued to a server. Busy websites typically employ two or more web serversin
a load balancing scheme. If one
server

starts to get swamped, requests are forwarded
to
another server with more capacity. Load balancing can also refer to the communications
channels themselves.

Voice over Internet Protocol
, is a method for taking analog audio signals, like the kind
you hear when you talk on the phone, and turning them in
to digital data that can be
transmitted over the Internet.

VoIP can turn a standard Internet connection into a way to place
free phone calls
. The
practical upshot of this is that by using some of the free VoIP software that is available to
make Internet p
hone calls, you are bypassing the phone company (and its charges)
entirely.
It has the potential to completely rework the traditional phone systems. VoIP
providers like Vonage have already been around for a little while and are growing
steadily. Major carr
iers like AT&T are already setting up VoIP calling plans in several
markets around the United States, and the FCC is looking seriously at the potential
ramifications of VoIP service.

There are VoIP
-
enabled video phones that use broadband
connectivity to pl
ace calls over the Internet.

There are three different "flavors" of VoIP service in common use today:

1.

ATA

-

The simplest and most common way is through the use of a device called
an ATA (analog telephone adaptor). The ATA allows you to connect a standard
phone to your computer or your Internet connection for use with VoIP. The ATA
is an analog
-
to
-
digital converter. It takes the analog signal from your traditional
phone and converts it into digital data for transmission over the Internet.
Providers like Von
age and AT&T CallVantage

2.

IP Phones

-

These specialized phones look just like normal phones with a
handset, cradle and buttons. But instead of having the standard RJ
-
11 phone
connectors, IP phones have an RJ
-
45
Ethernet

connector. IP phones connect
directly to your
router

and have all the hardware and software necessary right
onboard to handle the IP call. Soon,
Wi
-
Fi

IP phones will be available, allowing
subscribing callers to make VoIP calls from any Wi
-
Fi hot spot.

3.

Computer
-
to
-
computer

-

This is certainly the easiest way to use V
oIP. You don't
even have to pay for long
-
distance calls. There are several companies offering
free or very low
-
cost software that you can use for this type of VoIP. All you need
is the software, a
microphone
,
speakers
, a
sound card

and an Internet connection,
preferably a fast on
e like you would get through a
cable

or
DSL modem
. Except
for your normal monthly ISP fee, there is usually
no charge for computer
-
to
-
computer calls, no matter the distance.

Technology behind VoIP:


It turns out that everything you do on the Internet involves
packets
. For example, every
Web page that you receive comes as a series of packets, and every e
-
mail you

send leaves
as a series of packets. Networks that ship data around in small packets are called
packet
switched networks
.

On the Internet, the network breaks an e
-
mail message into parts of a certain size in
bytes
.
These are the packets. Each packet carries the information that will help it get to its
destination
--

the sender's IP address, the intended receiver's IP address, something that
tells the network how many packets this e
-
mail message ha
s been broken into and the
number of this particular packet. The packets carry the data in the protocols that the
Internet uses: Transmission Control Protocol/Internet Protocol (TCP/IP). Each packet
contains part of the body of your message. A typical pack
et contains perhaps 1,000 or
1,500 bytes.

Each packet is then sent off to its destination by the best available route
--

a route that
might be taken by all the other packets in the message or by none of the other packets in
the message. This makes the net
work more efficient. First, the network can balance the
load across various pieces of equipment on a millisecond
-
by
-
millisecond basis. Second, if
there is a problem with one piece of equipment in the network while a message is being
transferred, packets ca
n be routed around the problem, ensuring the delivery of the entire
message.

Depending on the type of network, packets may be referred to by another name:



frame



block



cell



segment

Most packets are split into three parts:



header

-

The header contains

instructions about the data carried by the packet.
These instructions may include:

o

Length of packet (some networks have fixed
-
length packets, while others
rely on the header to contain this information)

o

Synchronization (a few
bits

that help the packet match up to the network)

o

Packet number (which packet this is in a sequence of packets)

o

Protocol (on networks that carry multiple types of information, the
protocol defines what type of packet is
being transmitted: e
-
mail, Web
page, streaming video)

o

Destination address (where the packet is going)

o

Originating address (where the packet came from)



payload

-

Also called the
body

or
data

of a packet. This is the actual data that the
packet is deliver
ing to the destination. If a packet is fixed
-
length, then the payload
may be
padded

with blank information to make it the right size.



trailer

-

The trailer, sometimes called the
footer
, typically contains a couple of
bits that tell the receiving device th
at it has reached the end of the packet. It may
also have some type of error checking. The most common error checking used in
packets is
Cyclic Redundancy Check (CRC)
. CRC is pretty neat. Here is how it
works in certain computer networks: It takes the sum
of all the 1s in the payload
and adds them together. The result is stored as a hexadecimal value in the trailer.
The receiving device adds up the 1s in the payload and compares the result to the
value stored in the trailer. If the values match, the packet
is good. But if the values
do not match, the receiving device sends a request to the originating device to
resend the packet.

It turns out that everything you do on the Internet involves
packets
. For example, every
Web page that you receive comes as a ser
ies of packets, and every e
-
mail you send leaves
as a series of packets. Networks that ship data around in small packets are called
packet
switched networks
.

On the Internet, the network breaks an e
-
mail message into parts of a certain size in
bytes
.
These are the packets. Each packet carries the information that will help it get to its
destination
--

the sender's IP address, the intended receiver's IP address, something that
tells the network how

many packets this e
-
mail message has been broken into and the
number of this particular packet. The packets carry the data in the protocols that the
Internet uses: Transmission Control Protocol/Internet Protocol (TCP/IP). Each packet
contains part of the
body of your message. A typical packet contains perhaps 1,000 or
1,500 bytes.

Each packet is then sent off to its destination by the best available route
--

a route that
might be taken by all the other packets in the message or by none of the other packet
s in
the message. This makes the network more efficient. First, the network can balance the
load across various pieces of equipment on a millisecond
-
by
-
millisecond basis. Second, if
there is a problem with one piece of equipment in the network while a mess
age is being
transferred, packets can be routed around the problem, ensuring the delivery of the entire
message.

Depending on the type of network, packets may be referred to by another name:



frame



block



cell



segment

Most packets are split into three
parts:



header

-

The header contains instructions about the data carried by the packet.
These instructions may include:

o

Length of packet (some networks have fixed
-
length packets, while others
rely on the header to contain this information)

o

Synchronizatio
n (a few
bits

that help the packet match up to the network)

o

Packet number (which packet this is in a sequence of packets)

o

Protocol (on networks that carry multiple types of information, the
prot
ocol defines what type of packet is being transmitted: e
-
mail, Web
page, streaming video)

o

Destination address (where the packet is going)

o

Originating address (where the packet came from)



payload

-

Also called the
body

or
data

of a packet. This is the ac
tual data that the
packet is delivering to the destination. If a packet is fixed
-
length, then the payload
may be
padded

with blank information to make it the right size.



trailer

-

The trailer, sometimes called the
footer
, typically contains a couple of
bi
ts that tell the receiving device that it has reached the end of the packet. It may
also have some type of error checking. The most common error checking used in
packets is
Cyclic Redundancy Check (CRC)
. CRC is pretty neat. Here is how it
works in certain
computer networks: It takes the sum of all the 1s in the payload
and adds them together. The result is stored as a hexadecimal value in the trailer.
The receiving device adds up the 1s in the payload and compares the result to the
value stored in the trail
er. If the values match, the packet is good. But if the values
do not match, the receiving device sends a request to the originating device to
resend the packet.

Each packet's header will contain the proper protocols, the originating address (the IP
addre
ss of your computer), the destination address (the IP address of the computer
where you are sending the e
-
mail) and the packet number (1, 2, 3 or 4 since there are
4 packets). Routers in the network will look at the destination address in the header
and co
mpare it to their lookup table to find out where to send the packet. Once the
packet arrives at its destination, your friend's computer will strip the header and
trailer off each packet and reassemble the e
-
mail based on the numbered sequence of
the packet
s.