COMP2221 Networks in Organisations

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Oct 23, 2013 (3 years and 8 months ago)

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COMP2221



Networks

in Organisations

Richard Henson

February 2013

Open Systems Interconnect
(OSI)


Bold aim:


produce a set of communication protocols
that would allow any system to exchange
information with any other system


Many proprietary models


IBM biggest and most influential, major say


Open systems?


Internet & TCP/IP

Open Systems Interconnect
(OSI)



Dilemma: all manufacturers wished to
have their own communication models
represented


Principle:



A new layer must be created for each
new level of abstraction




Result
:


7

software
layers!!!

OSI Model


Benefits


All manufacturers



target to aspire towards


Benefits of OSI compatible products:


other manufacturers products w
ould

be
able to communicate with their own


consumer w
ould

no longer be

locked in


to specific vendor products


vendors w
ould

be able to produce products
that work at specific layers only

»
specialise and hence produce better products

Layer Communication
(Sending)


Each layer in the OSI model considers itself to
be talking to a peer layer in another computer


adds/removes its own

header


(formatting info)


e.g. application layer


adds a header to the user data on screen


passed to the presentation layer as a single block
e.g. presentation layer


adds its header to the block of data


passed on to session layer as a single block



and so on


The OSI reference model

Physical

Layer

Data link

Layer

Network

Layer

Transport

Layer

Session

Layer

Presentation

Layer

Application

Layer

Physical

Layer

Data link

Layer

Network

Layer

Transport

Layer

Session

Layer

Presentation

Layer

Application

Layer

Transmit

Station

Receive

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Link

Layer Communication
(Receiving)


Each layer in the OSI model strips away its
own header


e.g. physical layer


removes header from data block


passed to the data link layer


e.g. data link layer


removes header to the block of data


passed on to network layer


and so on


The OSI reference model

Physical

Layer

Data link

Layer

Network

Layer

Transport

Layer

Session

Layer

Presentation

Layer

Application

Layer

Physical

Layer

Data link

Layer

Network

Layer

Transport

Layer

Session

Layer

Presentation

Layer

Application

Layer

Transmit

Station

Receive

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Link

Simplifying The OSI model


Layers can be sub
-
divided into two groups



The top 3 layers (
interworking

layers)

»
user applications and support services



The lower 4 layers (
interconnection

layers)

»
the network (and navigation of packets)



Memory aids:


PDNTSPA


Please Do Not Throw Sausage Pizza Away
!

Interworking Layers


All about servicing needs of users


support for the application layer



includes presentation layer


and session layer

Application Layer


Interface for applications to use to gain
access to network services
:


Networked file transfer


Message handling


Database query processing


Controls generalised network access
:


supports applications which exchange data


provides error & status information for applications


If network is peer
-
peer



authenticates peer partners


determines if peers are ready to communicate

Presentation Layer


Responsible (sending) for conver
t
ing data from


application
-
specific format


to a generic (machine
-
independent)

format

that can be
passed across a network


Receiving



for converting incoming data from a generic format to one
that makes sense to the receiving application


Also responsible for protocol conversion, encryption
& decryption, and graphics commands


The
redirector

(software for handling service
requests)

also

operates at this laye
r:


If a service cannot be resolved locally,
it
sends the request
out to the network resource that can offer the required
service

Session Layer


Sets up a
logical connection
between machines
called a

session

, which allows networked resources
to communicate


Manages the set
ting

up

of a user

session

, exchange
of information, and

tear down


as
the session ends


M
anages issues such as who may transmit data at a
certain time, and for how long, also ensuring that the
system doesn

t

time out


after inactivity


Ensures data is routed to the correct application on
the local machine


Synchronises services between tasks at each end of
the communications channel in
half duplex
communications

Interconnection Layers


Concerned with packets of data


and navigating them through the network



Transport


Network


Data Link


Physical

The Four Layers Model


Introduced with Unix (mid
-
1970s, pre
-
OSI)


based on Internet protocols…

“application”

“transport”

“network”

“physical”

TCP/IP


Evolved with the Unix four layers…

TCP

Application,

presentation, session

IP

Connecting with

physical medium

Transport Layer (from Unix)


Manages the transmission of level 4 data
from sender to corresponding layer in
receiver


segments data streams into chunks of a given
packet size for the medium being used


checks for errors due to corruption, requests
retransmission etc.


Gateways

can operate at this layer

Transport Layer (from Unix)


Other roles:


managing flow control


providing acknowledgement of successful
transmission of chunks of data


software multiplexing


routing in an Internetwork


Manages OSI levels 1
-
4 so messages travel
between network nodes via pairs of

sockets


socket A

(sender)

socket B

(receiver)

Transport layer

Physical

Layer

Data link

Layer

Network

Layer

Transport

Layer


Upper

OSI

Layers


Physical

Layer

Data link

Layer

Network

Layer

Physical

Layer

Data link

Layer

Network

Layer

End User

Physical

Layer

Data link

Layer

Network

Layer

Transport

Layer


Upper

OSI

Layers


End User

Peer
-
to
-
Peer communications

Network A

Network B

Socket

A

Socket

B

End
-
end v logical neighbour
communications


Top four OSI layers
communicate logically
with
remote peer




regardless of topology or distance



The lower layers all communicate physically
with their nearest neighbour in a network


dependent on topology and routing to get the
packets through

Network Layer


Responsibilities:


packet (IP) addressing and sequencing


determining to route from source to
destination computer


Routers

operate up to this level

Network Layer Functions


Provides
messages with an address for
delivery (e.g. IP address)


Translates logical network addresses/names
into physical equiv
alents


Handles packet switching and routes packets
to their destination on the local network


Controls network packet congestion


Ensures packets conform to the network's
format


Network

Layer


Network Layer


Transport

Layer


User Specifies

Service

Network

Service

Network provides

Service

Network layer service definitions

Data Link Layer


Responsible for error free transmission, using data
frames


A frame is a basic unit for network traffic, and has a
highly structured format


Mechanism:


d
ata from the upper layers (ie the network layer) is converted
by the data link layer into frames


g
roups raw data bits received via the physical layer into
frames, for passing on to the upper layers


m
ay include an error recovery mechanism and also a flow
control mechanism, although this may be done at the
transport layer


B
ridges

operate up to this leve
l

Physical Layer


Responsible for communicating with the network
media


Bits are converted into electrical signals and vice
versa


Issues include modulation of signals and timing


Manages the interface between a computer and the
network medium, but cable type and speeds of
transmission are deliberately omitted to allow future
technology to be easily

included


Repeaters

work only at this level

Standards and the

OSI reference model


OSI designed to promote the
development of protocols



that support open systems interconnection


Become an agreed standard in 1984


ISO 7498 (the International Standard)


BSI 6568 (the identical British Standard)


CCITT recommendation X.200

How Apple complied with OSI

Level

7….
AppleShare

(files, printers, PCs)

AppleTalk Filing Protocol (
AFP
), Printer Access Protocol (
PAP
)

AppleTalk Session Protocol (
ASP
), Zone Protocol (
ZIP
), AppleTalk Data Stream Protocol (
ADSP
)

AppleTalk Transport Protocol (
ATP
), AppleTalk Echo Protocol (
AEP
), Name
-
Binding Protocol (
NBP
), Routing Table
Maintenance Protocol (
RTMP
)

Level 3…. Datagram
Delivery Protocol (
DDP
)

LocalTalk Link
-
Access Protocol (
LLAP
), Ethernet Link
-
Access Protocol (
ELAP)
, Token ring Link
-
Access Protocol
(
TLAP)
,
Other Link
-
Access Protocols

LocalTalk
,
EtherTalk
,
TokenTalk
, or other Network Topologies

Level 1…. STP
, UTP, Coax,
Fiber

The IEEE 802 Specifications

and the OSI model


Emerged from IEEE
-
OSI meeting: February

80


applied

mainly to
lower level OSI layers (1/2)


found it necessary to

extend the data link layer into
two parts


Essential for development of LANs


IEEE layer 1/2 definitions used by manufacturers
when
defining specifications for hardware such
as network interface cards

Effect of IEEE 802

on the OSI model


To cover all different types of interfaces,
IEEE d
ivide
d

the Data Link Layer into two
sub
-
layers:


Layer 2 (upper) Logical Link Control

-

used by 802.1 & 802.2


Layer 2 (lower) Media Access Control

-

802.3, 4, 5
, 11,
12
, etc.


Layer 2 (upper) Logical Link
Control


Focuses on IEEE 802.1 & 802.2



C
ontrol
s

transfer of data to the network layer



U
s
es

logical interface points called SAPs
(service access points)

Layer 2 (lower) Media
Access Control


Di
rect communication with the network card


provides packets with MAC address


Focuses on IEEE 802.3, 4, 5
, 11,
12



Provides shared access for multiple network
interface cards to the physical layer


Responsible for ensuring error
-
free
communication across the network

OSI layer software and
Network cards


Layer 1 and 2 software supplied with the
network card


card itself should contain software (on ROM)
that conforms to one of the
sixteen

IEEE 802
specifications


Cards for wired connections have connectors
for cables:


usually IEEE 802.3


more rarely


IEEE 802.5


Wireless Cards


usually based on IEEE802.11


Binding


Network Card Software


Data received by the
network card needs to be
passed on to level 3
software


Normally held on the
computer hard disk


Configuration:


level 2 software needs to
combine with level 3


achieved through

binding


OSI Level

1/2

software

OSI Level 3

software

binding

Now for the practical


Group A: after break

Group B: in one hour


Self
-
Study: check out all the IEEE
802.x standards and decide which
are most important in 2013…
feedback next session so you’d
better do this!