Download Network Architecture - Electrical Engineering and Computer Science

Network Architecture
• NETWORK ARCHITECTURE
• PROTOCOLS
• STANDARDS
• STANDARD ORGANIZATIONS
• ISO OSI REFERENCE MODEL
 PHYSICAL LAYER
 DATA LINK LAYER
 NETWORK LAYER
 TRANSPORT LAYER
 SESSION LAYER
 PRESENTATION LAYER
 APPLICATION LAYER
Network Architecture
• The term architecture means the formation of a
structure, or an orderly, interconnected, complex
arrangement of parts.
• An architecture encompasses hardware, software, data
link controls, standards, topologies, and protocols.
Protocols
• The term protocol defines how network components
establish communications, exchange data, and terminate
communications.
• Protocols are sets of rules and agreements.
• Modern networks are implemented using the concept of
layered protocols.
• The OSI model was created in an attempt to unify
network development, but many protocols were already in use
(and quite popular) before the OSI model was developed.
Protocol
• Since these existing networks were already functional,
compliance with the model necessitated retrofitting.
• Some vendors did this, some did not. Many are still working on
the problem.
• Although the introduction of the layered protocol did have a
profound influence on newer protocols, the collection of
protocols available today includes some that conform well to
the OSI and some that do not.
Standards
• Today's protocols demonstrate various degrees of
conformance to the OSI model.
• As many protocols exist, the standard organizations have to do
something. In the context of networking, some protocols are
standards.
• For example, TCP/IP is the Department of Defense Standards.
• This means that the Department of Defense has legislated the
use of these standards in their environments.
• Legislated standards are often referred to as de jure (by law)
standards.
Standard
• Standards that come into common use, are called de facto
(by fact) standards.
• TCP/IP and related protocols are also de facto standards
by virtue of their widespread commercial and educational use.
• IBM's System Network Architecture (SNA), which defines
how computers and terminals and printers can talk to each
other, is an example of de facto standard.
• MS-DOS 6.x, Novell's NetWare are de facto standards
because they hold important positions in the market.
De jure and de facto
• Proprietary standards are those that are invented and controlled
by a single, private commercial organization.
• Examples are SNA from IBM, DECnet from Digital and
NetWare from Novell.
• Standards developed by standard bodies and other committees
are non-proprietary.
• Examples include TCP/IP, IEEE 802.3, and OSI protocols
Standards Organizations
The major international bodies actively producing standards for
computer communications are CCITT, ISO, and IEEE.
CCITT
Consultative Committee for International Telegraphy and
Telephony
ISO
International Organization for Standardizing
IEEE
Institute of Electrical and Electronics Engineers
CCITT & ISO
• CCITT is the best-known standard organization in the world of
telecommunications.
• CCITT makes technical recommendations on telegraphy,
telephone, and data communication interfaces.
• One popular CCITT standard is V.24 in Europe (which is the
RS-232-C standard in North America)
• ISO is an international standard organization that defines and
develops standards on vast variety of topics. i.e Networking.
• The OSI protocols are well-known ISO standards.
IEEE
• The IEEE is the largest professional organization in the world.
• It sponsors a group that develops computing and electrical
engineering standards.
• The widely used IEEE802 networking series of standards are
examples of IEEE products.
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IEEE 802.3 Ethernet (CSMA/CD)
IEEE 802.4 Token bus
IEEE 802.5 Token ring
IEEE 802.6 MAN
IEEE 802.7 Broadband CATV
IEEE 802.8 Fiber-Optics
OSI Reference Model
• In the network world today (and the next decade), the OSI
RM is one of the most powerful architectures in use.
• It provides a complete model of the functions of
communications system so that two computers using OSI
RM would be able to communicate, even if they were based
on different hardware and software platforms.
• Do all people use the OSI model?
The answer is some do and some don't.
• OSI does not fit everyone's needs and it isn't available on all
manufacturer's computers.
• Conformance refers to meeting the standards.
• Conformance testing is the way conformance is
established.
• The conformance testing process is complicated and
expensive.
OSI reference Model
• Different computer platforms exist, different networking
products on the rise, networking is pervasive, so governments
must do something.
• They decided to select this model and issue a Government
OSI Profile, usually called a GOSIP.
• This means that if a vendor wants to sell communications
system to, for example, U.S., UK. French, or German
government department, the vendor must adhere to the parts
of OSI model specified by the country's GOSIP specification.
• This is very important because it helps ensure that the
communication systems from any vendor will communicate
with products from any other vendor.
OSI RM
The OSI RM is divided into seven layers. (7-layers figure) Each
layer builds upon the last so that each layer adds functionality to the
services of the previous layer.
OSI RM
• The important thing about the OSI RM is that it doesn't
actually say how the layers will be built--it doesn't care
what goes inside them, it concentrates on how the layers
work with each other.
• It means that the model is concerned with communication
(the structure of passing messages) rather than
implementation (what passes the messages).
• Building a complex communication system in a structured
way means if you have to make changes after you have
built your system well, you only have to change the details
in a layer instead of redesigning the entire system.
Physical layer
• Physical layer corresponds to basic network hardware. For
example, the specification of RS-232.
• The purpose of the physical layer is to deliver data from one
computer to another.
• Specifically, the physical layer translates bits of data into a
format suitable for transmission or receives a transmission and
translates it back into bits.
• This layer sees all data as a stream of bits.
• There are four areas covered by the physical layer:
Electrical: what voltages and currents are used.
Mechanical: the physical shape and size of the connectors.
Functional: the significance of a connector pin and the
voltage on that pin.
Procedural: the sequence of functional changes that indicate
event occurrences.
Physical layer
• A very important concept to remember is that the physical
layer is not the same as the physical media (the wires that
connect the computers are not part of the physical layer).
• Conceptually, they sit below the physical layer, and they
aren't part of the physical layer specification. It is a network
dependant layer.
Data Link layer
• Data link layer protocol specifies how to organize data into
frames and how to transmit frames over a network. For
example, the discussion of frame format, bit or byte stuffing,
and checksum computation are classified as layer 2.
• Responsible for the reliable transfer of information across the
physical link.
• Sends blocks of data (frames) with the necessary
synchronization, flow control and error control.
• It is a network dependant layer.
• Examples of standards at this layer are HDLC, LAPB, LLC,
and LAPD.
Network layer
• Network layer protocol specifies how addresses are assigned and
how packets are forwarded from one end of the network to
another
• It relies higher layers of the need to know anything about the
underlying data transmission and switching techniques used to
connect systems.
• Responsible for establishing, maintaining and the routing of data
between addresses, assembling incoming frames into blocks of
data and segmenting blocks of data to be sent into frames, and
terminating connections.
• A network dependant layer.
Transport Layer
• Network independent layer.
• It has three major functions that support the transparent
transfer of data to and from the session layer above it:
 Provides reliable transfer of data between end points.
 Provides end-to-end error recovery and flow control
 Releasing the connection.
The reason for this apparent duplication of effort is that the data
link layer deals with only a single, direct link, whereas the
transport layer deals with a chain of network nodes and links.
Session layer
• This layer protocols specify how to establish a
communication session with a remote systems (e.g. how to
login to a remote timesharing computer). Specifications for
security details such as authentication using passwords
belong in this layer.
• The session layer provides the mechanism for controlling
the dialogue between the two end systems.
• Example
FTP session with discipline of half-duplex, TALK is also
Full-duplex.
Presentation layer
• Layer 6 protocols specify how to represent data. Such protocols
are needed because different brands of computers use different
internal representations for integers and characters. Thus, layer
6 protocols are needed to translate from representation on one
computer to the representation on another.
• The presentation layer defines the format of the data to be
exchanged between applications.
• Offers application programs a set of data transformation
(presentation, syntax) services, such as graphics, binary or
ASCII data. Data compression or data encryption could occur at
this level.
Application layer
• Each layer 7 protocol specifies how one particular application
uses a network. For example, the specification for an application
that transfer files from one computer to another belongs in this
layer. This protocol specifies the details of how an application
program on one machine makes a request (e.g. how to specify the
name of the desired file) and how the application on another
machine responds.
• This layer provides support for the applications (processes)
that use the Network.
• Example: telnet grail.cba.csuohio.edu, the telnet process
needs help from the application layer to resolve
grail.cba.csuohio.edu into an address to pass it down the
stack.
• The Application layer has a function to go after some sort of
name resolution procedure to service the telnet process. and
so on, for other processes like FTP, http, and e-mail.
Stacks: Layer Software
• When protocols are designed according to a layering model, the
resulting protocol software follows the layered organization.
• The protocol software on each computer is divided into
modules, with one module corresponding to each layer.
• Layering determines the interactions among modules.
• In theory when protocol software send or receives data, each
module only communicates with the module for the next
highest layer and the module for the next lowest.
• Thus, outgoing data passes down through each layer, and
incoming data passes up through each layer.