Transcript presentation2

COMP2221
Networks
in
Organisations
Richard Henson
February 2014
Week 2: Standards and
Computer Networks

Objectives
– Explain functions of client-server networks
and network services
– Define networking standards
– Explain how digital networks standards
have helped make world-wide digital
communications more effective
Requirements of
organisational networks

The server would be expected to offer
the following to its users:
– Network Access
– Access to “restricted” files
» users with permission directly access files on
the server
– Applications
– Printing
– Access to email & The Internet
The Client-Server Model

Centralisation of organisational resources
– client can still hold resources
» a lot (fat client)
» Not much (thin client)

Microsoft model: called a domain
Request and response
1. All network users use clients
2. Client requests information…
2. Server processes the request, sends a response
back to the client
CLIENT
Client
Program
SERVER
REQUEST
RESPONSE
Send Request
Read Results
Server
Program
Process Request
Send Back Results
Servers in Larger Networks

Larger networks have MANY servers
– University ITS network: at least 50

Functions can be distributed around
different individual servers. Examples:
– Login Server
– File and Print server
– Applications Server
– Internet Gateway
Login Servers
(the most crucial!)

Dedicated to logging on users
– database of usernames/passwords

Only allows a potential user to access the
network if both username and password
exactly correspond with entries in the
database
– In Windows networks known as Domain
Controllers
Problems with this Scenario?

Discussion in Groups…
Possible Solutions?

Further Discussion…
Peer-Peer networks
Also known as workgroups
 No central server
 Computer nodes can act as both clients
and servers
 No expensive powerful machine
dedicated to providing services

Peer-Peer networks

All users have their own local storage
capacity

Bears the following responsibilities…
– local security & network administration
– granting access to their computer’s services
and resources via the network
Advantages & disadvantages
of Client-Server, compared to
Peer-peer

In groups…

Don’t look at next
slides!
Advantages of a client-server
network, compared to a
workgroup
Centralised security
 Centralised access to resources
 Centralised network administration
 With more than about 10 users, much
easier to manage than a workgroup.
Can handle up to thousands of users

Disadvantages of client-server,
compared to a workgroup
Expensive dedicated computer(s) not
accessible to users
 Expensive server operating system
needed
 Network management required
 Reduces user autonomy
 If one server, and it goes down, the
network ceases to function!!!

Windows Networks

Peer-peer networks:
– workgroups
– limited resource sharing ability

Client-server networks:
– domains
– access to domain via domain controller(s)

Enterprise networks
– multiple domains logically linked in a hierarchy
Virtual (client) and Cloud
(server) Networks
Extension of client-server model…
 Client-end less resource intensive
 Most of resources & processing at
server end
 Popular because clients need less
CPU power & less maintenance

– therefore lower cost…
Thin Client/Cloud
Advantages and disadvantages?
Another 3 minutes….
More about Standards

Definition:
– “A standard is an established or accepted model”

Communication protocols…
– “Elements of a communication system that are
defined by an agreed set of rules, conditions,
parameters or methods”
Type of Standards

De Facto
– A product or service that is a standard by
virtue of its widespread use by interested
users

De Jure
– The standard devised by a committee of the
organisation or, a working group of a
subcommittee of a committee of the
organisation
Communication Protocols
in 1977

Lots of “proprietary standards” had arisen
–
–
–
–
–

IBM
Honeywell
ICL (UK)
Bull (France)
DEC
Each corporation thought theirs was best…
Historic Geneva Meeting (1978)
All stakeholders in International
communications protocols invited to
conference by the Lake…
 Had to agree to a hypothetical International
communications protocol
 No expectation that it would be
implemented….

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: they agreed only by developing
a model based on… 7 software layers!
OSI Model – Benefits

All manufacturers…
– target to aspire towards

Benefits of OSI compatible products:
– other manufacturers products would be
able to communicate with their own
– consumer would no longer be “locked in”
to specific vendor products
– vendors would 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
Transmit
Station
AH DATA
Application
Layer
Application
Layer
DATA
AH
PH AH DATA
Presentation
Layer
Presentation
Layer
DATA
AH PH
Session
Layer
Session
Layer
DATA
AH
PH
SH
TH SH PH AH DATA
Transport
Layer
Transport
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Network
Layer
Network
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Data link
Layer
Data link
Layer
DATA
AH
PH
SH
Physical
Layer
Physical
Layer
SH PH AH DATA
LH
Receive
Station
LT
Link
LT
TH
TH
TH
NH
NH
LH
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
Transmit
Station
AH DATA
Application
Layer
Application
Layer
DATA
AH
PH AH DATA
Presentation
Layer
Presentation
Layer
DATA
AH PH
Session
Layer
Session
Layer
DATA
AH
PH
SH
TH SH PH AH DATA
Transport
Layer
Transport
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Network
Layer
Network
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Data link
Layer
Data link
Layer
DATA
AH
PH
SH
Physical
Layer
Physical
Layer
SH PH AH DATA
LH
Receive
Station
LT
Link
LT
TH
TH
TH
NH
NH
LH
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!
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…
Application,
presentation, session
TCP
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
Socket
A
End User
End User
Upper
OSI
Layers
Upper
OSI
Layers
Transport
Layer
Peer-to-Peer communications
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data link
Layer
Data link
Layer
Data link
Layer
Data link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Network A
Network B
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
User Specifies
Service
Transport
Layer
Network
Service
Network
Layer
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:
– data from the upper layers (ie the network layer) is converted
by the data link layer into frames
– groups raw data bits received via the physical layer into
frames, for passing on to the upper layers
– may include an error recovery mechanism and also a flow
control mechanism, although this may be done at the
transport layer

Bridges operate up to this level
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
IEEE 802 Specifications and
Layers 1&2 of 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

Definitions used by manufacturers for hardware
and software of network interface cards
– origin of the MAC address
Effect of IEEE 802
on the OSI model

To cover engineering issues, IEEE
divided the Data Link Layer into two sublayers:
– Layer 2 (upper): Logical Link Control –
IEE 802.1 & 802.2
– Layer 2 (lower): Media Access Control –
IEEE 802.3, 4, 5, 11, 12, etc.
Layer 2 (upper) Logical Link
Control

Focuses on IEEE 802.1 & 802.2

Controls transfer of data to the network layer

Uses logical interface points called SAPs
(service access points)
Layer 2 (lower) Media
Access Control

Direct 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

OSI Level 3
software
binding

OSI Level
1/2
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”
Now for the practical…
Group A: after break (10.25)
Group B: in one hour (11.25)
Self-Study: check out all the IEEE 802.x
standards and decide which of these are
most important in 2014… feedback next
session so you’d better do this!