Transcript Networking

Networking
Access methods &
Protocols including TCP/IP
Carl Smith - National Certificate
(parts adapted from HNC lectures by Steve Leggett)
Access Methods
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On a busy network protocols have
been devised to control access to the
medium
These include Token passing for Token
ring & bus networks
CSMS/CD for Ethernet based
networks
AppleTalk (MAC) Networks use
CSMA/CA
Collisions
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All methods have one aim in mind…
To avoid collisions between packets of
data using the network
Collisions still occur, which slow down
the network, but the protocols discussed
are in place to manage them
If collisions occur the packet(s) are
corrupted and have to be re-transmitted
CSMA/CD
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We have discussed Token Passing as
an access method
CSMA/CD is used on Ethernet
networks – IEEE 802.3
It is an Acronym for “Carrier Sense
with Multiple Access and Collision
Detection”
It operates at Layer 2 of the OSI model
The Dinner Party Analogy
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The CSMA/CD protocol functions
somewhat like a dinner party in a dark
room, no one interrupts each other.
If two computers start talking at the
same time, they both stop, wait a bit
and then one of them starts talking
again.
CSMA/CD - Dinner Party - 2
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That is, everyone around the table must
listen for a period of quiet before speaking
(Carrier Sense).
Once a space occurs everyone has an
equal chance to say something (Multiple
Access).
If two people start talking at the same
instant they detect that fact, and quit
speaking (Collision Detection.)
CSMA/CD – Carrier Sense
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To translate this into Ethernet terms, each
interface must wait until there is no signal on
the channel, then it can begin transmitting.
If some other interface is transmitting there
will be a signal on the channel, which is
called the carrier.
All other interfaces must wait until carrier
ceases before trying to transmit, and this
process is called Carrier Sense
CSMA/CD – Multiple Access
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All Ethernet interfaces are equal in
their ability to send frames onto the
network.
No one gets a higher priority than
anyone else, and democracy reigns.
This is what is meant by Multiple
Access.
CSMA/CD – Collisions
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Since signals take a finite time to travel
from one end of an Ethernet system to
the other, the first bits of a transmitted
frame do not reach all parts of the
network simultaneously.
Therefore, it's possible for two
interfaces to sense that the network is
idle and to start transmitting their frames
simultaneously.
CSMA/CD – Collision Detect
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When this happens, the Ethernet
system has a way to sense the
"collision" of signals and to stop the
transmission and resend the frames.
This is called Collision Detect
CSMA/CD - Summary
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The CSMA/CD protocol is designed to provide fair
access to the shared channel so that all stations get
a chance to use the network. After every packet
transmission all stations use the CSMA/CD protocol
to determine which station gets to use the Ethernet
channel next.
CSMA/CD is a type of contention protocol which is
a competition for resources.
The term is in networking to describe the situation
where two or more nodes attempt to transmit a
message across the same wire at the same time.
Higher Level Protocols
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However, the use of protocol suites at
the higher levels of the seven layer
model has gathered pace over the
years, as networking has become
more prevalent.
Today’s typical protocols include,
TCP/IP, OSI, SNA, NETBEUI,
IPX/SPX, etc
TCP/IP v IPX
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TCP/IP was developed as the protocol suite
for the Internet in America as part of the
DARPA project, (Defence Advanced
Research Projects Agency), with the USA
having supplied funding into communications
research for quite some time now.
Novell went for a proprietary version
(SPX/IPX) (Sequenced Packet Exchange /
Internetworked Packet Exchange) of a Xerox
protocol (XNS).
Evolving Network Protocols
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TCP/IP is generally a heavyweight protocol
suite, as it is able to handle any network,
whereas the Novell protocols suites were
generally considered to be lightweight in so
much as they were efficient and quick.
Apple went their own way and developed
(Apple Transfer Protocol).
These days, the trend tends to be towards
TCP/IP, and indeed is offered now by Novell,
as well as IPX/SPX.
Protocols and the OSI Model
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Whilst each protocol has its own approach
to communications, they must all cover the
ISO layer 4 for functionality, as well as layer
3 for addressing.
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With TCP/IP this is split off into two parts
with the first part, layer 3 (the Internet
Protocol (IP)), dealing with addressing
issues, and the Transmission Control
Protocol (TCP) part dealing with functionality
- layer 4.
Transport Control Protocol &
IP (Internet Protocol)
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IP – moves the packet of data from
node to node
TCP – verifies the accurate delivery of
data from client to server. TCP can
detect errors or lost data and controls
re-transmission if the data is not
accurate or entirely received
IP Addressing
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Transmission of messages between
two machines within the same network
is different between each technology
E.g. MAC addresses are used on an
Ethernet LAN
TCP/IP assigns a singular “IP” number
to each existing node in addition to the
local end network address
Packets and Frames
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The high-level protocol packets are
carried between computers in the data
field of Ethernet frames (MAC address)
The system of high-level protocols
carrying application data and the
Ethernet system are independent entities
that cooperate to deliver data between
computers
The protocol that provides that
cooperation is called “ARP”
Address Resolution Protocol (ARP)
ARP is used to help a host to locate a
hardware (MAC) address for the destination
target in a given transmission, when the
destination IP address is known.
ARP accomplishes this by transmitting
broadcast messages on it’s local network, with
the IP address of the destination contained
within the broadcast’s header.
ARP – Address Resolution Protocol…
A
192.2.2.1
D
I need the
Ethernet
address of
192.2.2.4
192.2.2.4
That broadcast
message was for
me. Here’s my
Ethernet (MAC)
address for you
192.2.2.4 = what?
192.2.2.4 – Ethernet =
0000.8000.1118
192.2.2.2
B
192.2.2.3
C
TCP/IP - Background
It’s history lies
within the US
military
One of the main
benefits of TCP/IP is
that it is royalty free.
Introduced as a protocol suite in the 1970’s to support ARPAnet
(American Research Projects Agency), who are the US defence
network.
After installing TCP/IP on ARPANET in 1983 all devices were
configured to use the new protocol. Using this protocol a hybrid
network known as the Internet was born.
The format of an IP address
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IP addresses are 32 bits long
They consist of two parts :–
The Network number and Host number
The 4 byte value or IP number converts
each byte into a decimal number (1-255)
and separates each byte with a period
e.g. 192.100.75.123
They are divided into classes
CLASS A – 1 to 126
CLASS B – 128 to 191
CLASS C – 192 to 223
TCP Port numbers
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“Port” numbers are used by TCP to advance
information to the upper layers of the OSI model
Application FTP
layer
Telnet SMTP DNS
TFTP
HTTP SNMP
Port
Number
23
69
80
21
25
53
161
Summary
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There are many protocols involved in
networking
CSMS/CD is one “access method” protocol
discussed operating at layer 2
High level protocols to transfer data operate at
layer 3 and 4 of the OSI Model, ARP is used to
match these to the MAC address at layer 2
TCP/IP is the de-facto protocol now used for
LAN’s and the Internet
Finally, we looked at the format of an “IP”
address and TCP port numbers