Transcript No Slide Title

Computer Networks
Chapter 2 – Network Models
Communication Tasks
 The tasks given below need to be taken care of in
any kind of communication system.





Signal generation
Transmission
Interfacing
Synchronization
Error detection and
correction
 Flow control
Summer 2006
 Addressing and routing
 Recovery from
malfunctioning
 Message formating
 Security
 Network management
 Applications
Computer Networks
2
Dealing with Communication Tasks
 Communication system is complicated and need to
solve a large number of tasks
 Solution
 Use the devide and conqure technique
 This technique splits the problem in managable pieces
and is therefore convinient for dealing with complex
systems
Summer 2006
Computer Networks
3
Design Principle
 Communication tasks are divided into series of
layers or levels
 Each layer is responsible for particular task and act
on them by using one or more protocols
 Each layer is built upon one bellow it
 The number and name of the layers differ from
network to network
Summer 2006
Computer Networks
4
The Concept of Layers
 Layer n on one machine communicates with layer n on
the other machine via layer n protocol.
 The communication is virtual
 Peers are entities comprising the corresponding layers
on different machines.
 There is an interface between each pair of adjacent
layers for communication with the layer above and the
layer below.
Summer 2006
Computer Networks
5
An Example of Five Layers Network
Machine 1
Machine 2
Layer 5 protocol
Layer 5
Layer 4/5 interface
Layer 4
Layer 3/4 interface
Layer 3
Layer 2/3 interface
Layer 5
Layer 4 protocol
Layer 3 protocol
Layer 2
Layer 2 protocol
Layer 1/2 interface
Layer 1
Layer 1 protocol
Layer 4
Layer 3
Layer 2
Layer 1
Physical medium
The path through which the actual transmission take place
Summer 2006
Computer Networks
6
Peer-to-peer Processes
 The processes on the two machines that
communicate at a given layer are called peer-to-peer
processes
 At the physical layer communication is direct
 At the upper layers the communication has to go
down through the layers on the sender machine,
than to be transmited through the physical layer and
than to go back up to the same layer at the receiving
machine
Summer 2006
Computer Networks
7
Protocols
Construction versus Reduction
DATA
Construction
Layer 5
H5 DATA
Layer 5
Layer 4
Layer 3
H4 DATA UNIT
Layer 4
Layer 3
Layer 2
Physical
H3
H2
DATA UNIT
DATA UNIT
BITS
T2
Layer 2
Physical
Reduction
H – header (control data added at the front end of the data unit)
T – trailer (control data added at the back end of the data unit)
Trailers are usually added only at layer 2
Summer 2006
Computer Networks
8
Illustration of the Construction and
Reduction Process
 Observe how headers and trailer are added at the
sender and removed at the receiver
Annimation of Figure 2.4 in the book
Summer 2006
Computer Networks
9
Messages and Protocol Stacks
 On the sender machine, each layer:
 Accepts an outgoing message from the layer above
 Adds a header and does other processing
 Passes resulting message to next lower layer
 On the receiver, each layer:
 Receives an incoming message from the layer below
 Removes the header for that layer and performs other
processing
 Passes the resulting message to the next higher layer
Summer 2006
Computer Networks
10
Drawbacks in the layered architecture
 The resources necessary for the user’s data
(payload) are also used for transfering information
in the headers and trailers.
 This cannot be avided.
 However, well designed protocols should have as litle as
possible additional information carried, yet provide in
the best way the service required.
Summer 2006
Computer Networks
11
Service vs. Protocol
 Service
 Treat the protocol as a black box. what service is
provided to the upper layer
 Protocol
 How do the peer entities cooperate with each other and
the lower layer to provide the service to the upper layer
Upper layer
Upper layer protocol Uper layer
Interface
Interface
Service
Lower layer
Summer 2006
Lower layer protocol
Computer Networks
Lower layer
12
Network Architecture
 A set of layers and protocols
 Enough information to allow implementers to
write a program or build the hardware for each
layer, so that it obeys the appropriate protocol
 Protocol stack or protocol suit – a list of protocols
used by a certain system
Summer 2006
Computer Networks
13
Protocol Suites
 Open Sysytem Interconnection (OSI)
 Used mostly as a reference model
 Internet (TCP/IP)
 Most popular suite today




Xerox Networking Sysytems (XNS)
System Network Architecture (SNA – IBM)
Digital Network Architecture (DNA – DEC)
NetBIOS (Software interface)
Summer 2006
Computer Networks
14
Open System Interconnection (OSI)
 Developed by International Standard Organization
(ISO) as a first step towards international
standardization
 De jure protocol
 Deals with interconnecting systems that are open for
communication with other systems
 Open protocol suite
 Good as theoretical model, but not widely
implemented in practice
Summer 2006
Computer Networks
15
The Layers of OSI Model
End
System
R
Application
Presentation
Intermediate
System
Application
Presentation
Session
Session
Transport
Network
Transport
Network
Data Link
Physical
Summer 2006
Network
Data Link
Physical
Computer Networks
Data Link
Physical
16
The OSI layers
 Physical layer
 Transmission of unstructured bit stream
 Deals with the mechanical, electrical, functional and procedural
characteristics to access the physical medium
 Data link layer
 Provides reliable transfer across the physical link between
two ends connected via single link
 Sends blocks of data (frames) with the necessary synchronization,
error control and flow control
 Can add header and trailer
Summer 2006
Computer Networks
17
The OSI layers
 Network layer
 Provides upper layers with independence from the data
transmission and switching technologies accross
internetwork
 Responsible for source-to-destination delivery, addressing and
routing in the internetwork
 Transport layer
 Provides transparent transport of data between end points
that might not be connected via single link
 Provides source-to-destination connection, error recovery and
flow control
Summer 2006
Computer Networks
18
The OSI layers
 Session layer
 Provides the control structure for communication
between applications (dialog control)
 Establishes, manages and terminate connections (sessions)
between cooperating applications
 Presentation layer
 Provides independence to the application processes from
differences in data representation
 Application layer
 Provides access to the OSI environment for users and
provides distributed information services
Summer 2006
Computer Networks
19
Summary of OSI Layer Functions
Application
To translate, encrypt and
compress data
Presentation
Session
To provide reliable end-toend message delivery and
error recovery
To organize bits into
frames, to provide nodeto-node delivery
Summer 2006
To allow access to network
resources
Transport
Network
Data Link
Physical
To establish, manage and
terminate sessions
To move packets from source
to destination; to provide
internetworking
To transmit bits over a
medium; to provide
mechanical and electrical
specifications
Computer Networks
20
Illustration of the Source-to-end Delivery at
the Network Layer
 Observe how data are sent from node to node to
reach the final destination.
Animation of Figure 2.11 in the book
Summer 2006
Computer Networks
21
TCP/IP Protocol Suite
 De facto (and after that de jure) standards
 Open (All modification and newly proposed
protocols are published in a form of RFC (Request
for Comments)
 RFC as well as drafts are published on the Internet
 can be found on many URL (one is www.rfceditor.org)
 RFC becomes a standard when it is:
 Stable and well understood
 Technically competent
 Implemented on multiple independent places
Summer 2006
Computer Networks
22
The TCP/IP Protocol Suite (Cont.)
 Allows computers of many sizes, vendors and
operating systems to communicate with each other
 History:
 Developed as de facto standard before OSI
 1960’s: started as goverment financed research project
 1990’s: most widely used form of networking
 Forms the basis for the Internet (capital “I”)
(a WAN that spans the globe)
Summer 2006
Computer Networks
23
TCP/IP Layers and Relation to OSI
SMTP
FTP
HTTP
Presentation
TELNET DNS
Session
Application
Transport
Network
TCP
ICMP
Transport
N
UDP
e
t
w
o
IGMP
r
k
Network
IP
ARP
Physical
Protocols defined by the
underlying networks
Summer 2006
Application
Computer Networks
RARP
Data Link
Physical
24
Some Standard Organizations
 ITU: International Telecommunication Union
 CCITT: International Telegraph and Telephone
Consultative Committee
 ISO: International Standards Organization
 IEEE: Institute of Electrical and Electronic Engineering
 IRTF: Internet Research Task Force
 IETF: Internet Engineering Task Force
Summer 2006
Computer Networks
25