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Lecture 2Network Layers and Physical Layer
Dr. Thaier Hayajneh
Department of Computer Engineering
The Hashemite University
408450 Computer Networks, Fall 2011/2012
Characteristics of Protocols
A rotocol is a set of mutuall a reed u on rules that
regiment the interactions between the communicatingentities.
e ey e ements o a protoco are:
Syntax defines the structure of informationcommunicated, including the data format, the coding, and
signal representations.
Semantics defines the meaning of the exchanged signals,includin control information for coordination and error
handling.
Timing defines the time at which data should be
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.
Characteristics of Protocols
,
WHAT is comunicated,
,
WHEN it is communicated.
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Standards
Defined by public organizations (de juro) or by widespread use (de facto)
Open standards
Avaliable to everyone (but not necessarily for free)
Everyone has the possibility to propose, criticize, and influence
tan ar s organ zat ons
ISO: International Organization for Standardization
IETF: Internet Engineering Task Force
IEEE: Institute of Electrical and Electronics En ineers
ANSI: American National Standards Institute
ETSI: European Telecommunications Standard Institute
ITU: International Telecommunication Union
ITU-T: International Telecommunication UnionTelecommunication Standards
Sector
EIA: Electronic Industries Association
...
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A La ered Model: Sendin a Letter
Modify one without affecting the other
Services and A lications
Service
What a network operator offers
to customers (subscribers)
Application
the service for
Examples
Telephone connection
Service: voice transmission
Application: conversation between two parties
Computer communication via modem
erv ce: same as a ove p us nternet connect v tyApplication: Web browsing, e-mail, file transfer, chat, etc
Network Architectures
complex to be handled as a monolithic unit.
An alternative a structured a roach.
Divide the communication task into manageable parts.
Need to describe the communication functions in terms of an
architecture.
The architecture defines the relationship and interactions
interfaces and protocols.
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La ered Architecture
v
.
Each layer performs a related subset of functions
re uired for communication and adds value to the
services provided by lower layers.
Layer Nrelies on services of layer N-1 to provide a service to
layer N+1
Service required from lower layer is independent of how that
Information and complexity hiding
Chan es in la er Ndo not affect other la ers
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La ered Model Interfaces
are called interfacesInterface protocols define the interaction between adjacent
layers in the same system
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Network Architecture
ys em es gn erpec ve
v v
Can the network be totally trusted?
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End-to-End Ar ument
functionalities in a complex system is what system
design is all about
End-to-End argument states that a function, such as
reliability or ordered delivery, should not be provided in
the lower levels of the system unless it can be
completely and correctly implemented at that level
unct ona re un ancy can e a owe per ormance
optimization is soughtExam le: error control on a ho -b -ho basis
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Statefull vs. Stateless
management
Establishment of a new connection causes a state change inthe switch
A direct side effect: fate sharing
ate o t e en -to-en connect on epen s on t e state o
intermediate nodes
Simple and more robust
Onl task re uired is to maintain end-to-end routin tables
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Statefull vs. Stateless
state complexity for more routing and processingrequirements at intermediate systems?
Connectionless networks have to perform address processing
on a packet basis
Not necessarily true, as successive pac ets usually refer
to the same address
ac ng rou ng compu a on grea y re uces process ngrequirements
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ISO/OSI Communication Model
v
the International Standard Organization (ISO) indefinining its Reference Model.
The model referred to as the Open System
Interconnection (OSI) defines a framework for the
specification of protocol standards for connecting
heterogenous computers.
e mo e prov es t e as s or two open systems t atconform to the reference model and the associated standards
to exchange information.
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ISO/OSI Communication Model
Define the rules and conventions for various functions within
each layer.
Specify the general relations among these functions.
Determine the constraints on the types of functions and their
.
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ISO/OSI Communication Model
Application Layer
Presentation Layer
H Data
Data
Session Layer
H Data
Network Layer
H Data
Data Link Layer
H Data
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Physical layer
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ISO/OSI Communication Model
Presentation PresentationUpper Layers:End Nodes
Trans ort
Session
Trans ort
SessionOnly
Network Network Network
Physical
Data L n
Physical
Data L n
Physical
Data L nLower Layers:All Nodes
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Communication Medium
ISO/OSI Communication Model Architecture
v
Application layer,
Presentation layer,
Session layer,
Transport layer,
Network layer,
Data link layer, and
ys ca ayer.
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ISO/OSI Communication La ers
Process to process communication
All layers exist to support this layer
Applications include e-mail, teleconferencing, WWW, ftp,
distributed databases
Presentation Layer
Conversion of data to conform to a common format (e.g. little
, ,
representation)
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ISO/OSI Communication La ers
Bind two communicating entities into a relationship,
Session setup (authentication), failure recovery (brokensession), synchronization services
Transport Layer:
End-to-end data delivery
End-host to end-host communication
u t p ex ng o mu t p e ata streams rom g er ayers
(upward multiplexing)
Flow control between sender and receivers
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ISO/OSI Communication La ers
Data transmission and delivery between hosts
Controls access to the network
Provides routing of packets within the network
Manages contention and bottlenecks within the network
Data Link Layer:
Error free communication over a single link between adjacent
no esSpeed matching between senders and receivers
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ISO/OSI Communication La ers
Transmission of raw bits (0/1) over physical wires
Interfacing, data encoding, clock recovery, etc.
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ISO/OSI Communication Model
different machines is only achieved at the physical
layer.
The interlayer interface defines a framework for
standards
No details of the implementation, no definition of the
interlayer interfaces.
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ISO/OSI Communication Model Interfaces
Peer-To-Peer
Layer N Layer Nrotoco
SAP SAPInterface Interface
Service Access Point
rotoco rotoco
Layer N-1 Layer N-1
Peer-To-Peer
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End System End System
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Interfaces
interlayer interfaces, since these are not visible fromoutside.
The interfaces is used to access services provided by a
lower layer to a higher layer.
The point at which service is provided is called Service
Access Point (SAP)
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Model Limitations
Focus entirely on listing of details, with no motivation
for techni ues ado ted.
Every reasonable suggestion becomes an option.
This results in an excessive number of options, for a protocol
architecture that is supposed to be an international standard.
The layering structure of the model is arbitrary
Appropriate placement of features in layers is not alwaysobvious.
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Model Limitations
v
communication mentality.
Techniques from communication fields have been sometimesinappropriate.
Use of interrupts, when implementation calls for use of a
.
Initial focus entirely on connection-oriented, rather than
connectionless service.
Connectionless service is currently being provided, but as
an afterthought.
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Model Limitations
The model does not allow ex licit b assin of ad acentlayers when not needed.
May lead to redundancy of functionalities.May result in inefficiency.
In general, layering has many conceptual advantages, .
Fanatical ahderence to layering as a religion may beproblematic.
Layer Nmay duplicate lower layer functionality,
Different layers may need the same information,-
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.
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Internet Model
Communication Between La ers
- -
Betwen protocols at the same layer in different devices
Logical connection
Interfaces between layersBetween adjacent layers in the same device
down (sending) or up (receiving)
Communication takes place over well-defined interfaces
Communication Between La ers
Two types of communication between layersPeer-to- eer communicationsame la er different devices ,
Communication over interfaces between layersdifferent layers, same device
Interfaces Between La ers
v
can access it
Each la er has its own format for the Protocol DataUnit (PDU)
A layer in the sending device may add more rotocol
information to the data unit from the layer aboveHeaders and trailers
A layer in the receiving device may strip offprotocol
information
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Data Exchan e
A layer in the sender adds protocol information to the data
Headers and trailers
A layer in the receiver strips off protocol information
Function of La ers
Data Link Layer
Transport Layer
Ph sical La er
Transmission of bits between nodes
signals
Unguided medium (free space) for radio andoptical signals
Protocols
Mechanical and electrical interfaces
Connectors, cables, voltage levels
Transmission and reception of signals
digital modulation, line coding
Synchronous and asynchronous transmission
Standards, for example EIA RS-232, ITU-T SDH (ANSI SONET)
Data Link La erHo -to-Ho
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Data Link La er
Transmission offrames between nodes
Framing
Divides bit stream into larger data units, framesEthernet frame up to 12,144 bits (1518 bytes), including control information
Flow control
Prevent receiver from being overrunError control
Detect and (perhaps) retransmit damaged framesAccess control
c ev ce may sen on a s are nAddressing
Data Link Layer Frame Example
Header
address and control Data
Trailer
error control
8-bit
flag
8-bit
flag
Bit sequence for frame synchronization
AddressesSource and destination
ontroSequence number
Transmitted and expected
Link connect and disconnect
TrailerBit sequence for detecting bit errors
Data Link La er Standards
Ethernet
Family of protocols
Ethernet (10 Mb/s), Fast Ethernet (100 Mb/s), Gigabit
Ethernet 1 Gb/s
IEEE 802.11 Wireless LAN
IETF: Point-to-Point Protocol (PPP)
: u t -protoco a e w tc ng )
ISO: High-level Data Link Control (HDLC)
Link Access Procedure Balanced (LAP-B), ITU-T X.25
Normal Response Mode (NRM), SDLC
Network La erSource-to-Destination
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Network La er
Deliver of ackets from source to destination
possibly across multiple linksAddressing
Logical addresses
Unique within the network
RoutinCalculation of paths between pairs of nodes
IETF: Internet Protocol (IP)
Trans ort La erProcess-to-Process
Trans ort La er
Delivery between end users (processes)
Addressed by ports
Reliable (connections) or non-reliable (datagrams)Flow control
Reactive traffic control (prevent congestion)
rror an ng
Connection set-up and tear-down
Se mentation and reassembl
IETF: Transport Control Protocol (TCP), User Datagram Protocol (UDP)
ISO: Transport Protocol Class 0 4
Application Layer
Provides services to the end user
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Future Trends of Communication Networks
In recent ears, a lar e technolo ical ro ress occurred both in
the field of electronics and the field of opticsTechnological progress has opened the door to a new class of
applications: ea - me pp cat onsThese applications create the need for:
Greater flexibilit in the transfer mode
Transport of services other than pure data,
Transport of high bit rate services,
eve opmen o g er spee sys ems.
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