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Communication Networks Winter Term 2019/20
Prof. Jochen Seitz 1
Communication Networks
Chapter 2 – Fundamentals
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Overview
1. Communication Network
2. Standardization
3. Communication Association
4. Communication Architectures
5. Telecommunication Service and Protocol
6. ISO/OSI Basic Reference Model
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Example of a Telecommunication Network (Repetition)
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1. Communication Network
LAN 1
Wireless LAN 3
MobileUsers
LAN 2
Backbone
URL Web Page
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1. Communication Network
Components
• Terminal Equipment (End Nodes)
▪ User interface to communication service
▪ Host for applications / server
▪ Source and/or sink of information flows
• Switching Equipment (Intermediate Nodes)
▪ Forwarding of information
▪ Routing decision
• Physical Media
▪ Links between
❖ terminal equipment and switching equipment
❖ switching equipment and switching equipment
Need for international standards
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Communication Service
1. Communication Network
NetworkTerminal
EquipmentTerminal
Equipment
Tele Service
Bearer Service
User Network Interface (UNI)
Application / User Interface
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1. Communication Network
Main Task
• Transmission of information from one terminal equipment to another
▪ Information transmission for different applications / use cases
▪ Information transmission on demand to an arbitrary terminal equipment
▪ Possibly, transformation of information required
Communication Networks: 2. Fundamentals 35
Network
Speech Communication
Transmission of Bits
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1. Communication Network
Fully Meshed Network
• Direct links between all end nodes of the network
• No switching equipment needed
• For communication simply select the correct link
• For n end nodes: ½ (n * (n-1)) links required
Communication Networks: 2. Fundamentals 36
End Node1
End Node2
End Node3
End Node4
End Node5
End Node6
End Node7Example:
• Network of 7 end nodes• 21 links
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1. Communication Network
Partially Meshed Network
• At least one path through the network for all pairs of end nodes
• Switching equipment on the path forwards information accordingly
▪ Communication association between end devices / users
▪ Control of communication association required
▪ Resources in the network might be limited
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Network
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2. Standardization
Standardization Bodies (I)
• International Standardization Organizations
▪ International Organization for Standardization (ISO)
❖ ISO Reference Model for Open Systems Interconnection (OSI)
▪ International Telecommunication Union (ITU)
❖Telecommunication Standardization Sector, ITU-T
❖Radiocommunication Sector, ITU-R
❖Telecommunication Development Sector, ITU-D
▪ Institute of Electrical and Electronics Engineers (IEEE)
• Regional / National Standardization Organizations
▪ European Telecommunication Standards Institute (ETSI)
▪ American National Standards Institute (ANSI)
▪ Deutsches Institut für Normung (DIN)
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2. Standardization
Standardization Bodies (II)
• Standardization of the Internet
▪ Internet Engineering Task Force
❖ large number of working groups and informal discussion groups (BoF – Birds of a Feather)
❖working groups organized into areas by subject matter
❖area directors, together with the IETF Chair, form the Internet Engineering Steering Group (IESG)
• Special Interest Groups (SIGs)
▪ Bluetooth-SIG
▪ Ecma International (former European Computer Manufacturers Association)
▪ The Open Group (former OSF and X/Open)
▪ ZigBee Alliance
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2. Standardization
Standardization by ISO
• Working Groups
▪ Meetings each 6 to 9 months
▪ National bodies need to agree on concepts to be standardized
• Process
▪ Draft Proposal (DP)
▪ Draft International Standard (DIS)
▪ International Standard (IS)
• International Reconcilement
Communication Networks: 2. Fundamentals 40
ISO
TechnicalCommittee (TC)
SubCommittee(SC)
Working Group(WG)
… … … …
… … … …
… … … …
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2. Standardization
Standardization in the Internet
• IETF responsible for standardization
• IESG controls standardization process
• Two results
▪ Request for Comments (RFC)
▪ For Your Information (FYI)
• For each draft standard, two interoperable and independent implementations must exist
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Proposal
Proposed Standard
Draft Standard
Full Standard
Experimental
Informal
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2. Standardization
Standards of the Internet – Examples
RFC Title Date
RFC 768 User Datagram Protocol (UDP) August 1980
RFC 791 Internet Protocol (IP) September 1981
RFC 792 Internet Control Message Protocol (ICMP) September 1981
RFC 793 Transmission Control Protocol (TCP) September 1981
RFC 959 File Transfer Protcol October 1985
RFC 1945 Hypertext Transfer Protocol – HTTP/1.0 May 1996
RFC 2460 Internet Protocol, Version 6 (IPv6) Specification December 1998
RFC 8615 Well-Known Uniform Resource Identifiers (URIs) May 2019
RFC 8649 Hash Of Root Key Certificate Extension August 2019
Communication Networks: 2. Fundamentals 42
see http://www.ietf.org
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3. Communication Association
Communication Association
• Temporary service provided by the network to transfer information from one UNI to some freely selected UNI
• Requirements:
▪ Addressing of the UNI / user
▪ Fulfillment of user / application requirements:
❖ throughput / goodput
❖delay / delay variation (jitter)
❖ reliability (bit errors / packet loss / communication breakdown)
❖ security / authenticity / trustworthiness
❖availability
❖…
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3. Communication Association
Communicating Entities
• Man–to–Man
▪ one-way (e-mail) or dialogue (telephony, chat)
▪ text (e-mail, chat) or speech (telephony)
▪ synchronous (telephony) or asynchronous (e-mail, chat)
• Man–to–Machine
▪ client/server-based applications (www, online banking, …)
▪ peer-to-peer applications (gnutella, …)
• Machine–to–Machine
▪ distributed processes for smart home, industry 4.0, internet of things…
▪ pervasive computing
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Number of Communication Entities
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3. Communication Association
Number ofSenders
Number ofReceivers
CommunicationAssociation
Example
1 1 Unicast lecture (?)
1 1 Dialogue telephony
1 1 < n < all Multicast pay TV
1 all Broadcast “normal” TV
m > 1 1 Concast tele-voting
m > 1 n > 1 Multipeer video conference
1 n > 1 Anycast DHCP
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Communication Links
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3. Communication Association
Universal Serial Bus (USB)(serial transmission)
Centronix(8 bis in parallel)
▪ Serial Communication
▪ Parallel Communication
01000001
01000001
1 0 0 0 0 0 1 0
1 bit per time interval,1 physical link
8 bits per time interval,8 physical links
t
t
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Direction of Communication
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3. Communication Association
Simplex Duplex Half Duplex
- Radio / TV Broadcast- Fire Detector- Sensor Node
- Telephony- Internet- Video Game
- Citizen‘s Band- Walkie-Talkie
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3. Communication Association
Order of Delivery
• According to sending sequence (First In, First Out (FIFO))
• FIFO + Priorities
• Random delivery order
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Network3 2 1 3 1 2
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Quality ofService▪ Many criteria
▪ Optimization of all criteria at the same time not possible
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3. Communication Association
Suitability:• Complexity• Scalability• Usability
Performance:•Signal Runtime•Response Time•Throughput
Costs:• Investment Costs• Operating Costs
Reliability:• Transmission Errors• Comm. Breakdown
• Service Reliability
Security:•Information Integrity•Confidentiality•Authenticity•Non-Repudia-
bility
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4. Communication Architectures
Principles
• Different components in a communication architecture with dedicated functionality:
▪ Interface to the physical medium
▪ Interface to the application / user
▪ Several communication entities
❖ to realize the required communication service
❖ to provide the required quality of service
• Structured in a layered architecture
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Interface to thePhysical Medium
Interface to theApplication / User
CommunicationEntities
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4. Communication Architectures
The Layered Approach
• Refinement of „Communication Entities“ layer arbitrary number of layers
• Each layer
▪ offers service to the upper layer
▪ utilizes service of the lower layer
• Advantages
▪ Concentration on a specific communication aspect in each layer
▪ Exchange of a layer without side effect on the other layers
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CommunicationEntities
Layer X
Layer X-1
Layer X-2
Layer X-n+1
Layer X-n
…
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Point-to-Point Physical Medium
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4. Communication Architectures
Physical Medium
Physical Transmission Channel
Medium Access Point Medium Access Point
Sender Receiver
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Shared Physical Medium
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4. Communication Architectures
Physical Medium
Physical Transmission Channel
Sender A Receiver A2Receiver A1 Receiver B1 Sender B
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Layer i
Abstract Medium for Layer i
The Layers
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4. Communication Architectures
Application Application
Physical Medium
Service Access Point (SAP)
Horizontal Communication
Horizontal Communication
Ver
tica
lCo
mm
un
ica
tio
n
Ver
tica
lCo
mm
un
ica
tio
n
Interface
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Service and Protocol (I)
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5. Telecommunication Service and Protocol
Service Provider 1 Service Provider 2
Service User 1 Service User 2
Protocol
Serv
ice
Serv
ice
Layer i
Layer i+1
System A System B
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5. Telecommunication Service and Protocol
Service and Protocol (II)
• Peer entities on one layer
▪ utilize the service of the layer below (if there is one)
▪ offer their service to the layer above (if there is one)
▪ only know the interface to the service beneath, but not its implementation
▪ communicate with each other according to specific rules
❖ receive input
❖ generate events
• Service offered by a layer is based on the cooperation of the communicating peer entities in the layer
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Telephony Service (I)
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5. Telecommunication Service and Protocol
Telephony Service
Pick Up
Dial Tone
Enter theTelephone Number
Ringing Tone
End of Ringing Tone
Hang UpConversation
Pick Up
Ringing
Hang Up
Busy Tone
Time
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Telephony Service (II)
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5. Telecommunication Service and Protocol
Telephony Service
Pick Up
Dial Tone
Enter theTelephone Number
Ringing Tone
End of Ringing Tone
Hang UpConversation
Pick Up
Ringing
Hang Up
Busy Tone
Time
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Telephony Service (I)
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5. Telecommunication Service and Protocol
Pick Up
Dial Tone
Enter theTelephone Number
Ringing Tone
End of Ringing Tone
Hang UpConversation
Pick Up
Ringing
Hang Up
Busy Tone
Time
Network
Disconnect
Response
Disconnect
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5. Telecommunication Service and Protocol
Definition of Service
• Service = set of functions in one layer
▪ delivered by cooperating entities of the layer communicating according to a specific Protocol
▪ offered at the Services Access Point (SAP) of the layer
▪ defined by a set of Service Primitives and rules for their usage
• Service Primitive types
▪ Request
▪ Indication
▪ Response
▪ Confirmation
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Illustration of a Telecommunication Service
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5. Telecommunication Service and Protocol
Abstract Medium Mi
Communi-catingEntity
Ii+11
Communi-catingEntity
Ii+12
Communi-catingEntityIi+1
N-1
Communi-catingEntityIi+1
N
…
ServiceInterfaceof Layer i
ServiceAccessPoint (SAP)
Exchange ofServicePrimitives
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Layer i
5. Telecommunication Service and Protocol
Service of Layer i
• Set of functions offered by layer i at its Service Access Point vertical communication
• Delivered by peer entities in layer i communication with each other utilizing the service of layer i-1 horizontal communication
• Entities of layer i+1 only know the service interface of layer i
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Entity 1of Layer i+1
Ii+11
Entity 2of Layer i+1
Ii+12
Entity 1of Layer i+1
Ii1
Entity 2of Layer i+1
Ii2
SAP of Layer i SAP of Layer i
Protocol
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Different Kinds of Services
Confirmed Service
• Confirmed by service user
Unconfirmed Service
• Initiated by the service user
• Initiated by the service provider
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5. Telecommunication Service and Protocol
Request
Indication
Response
Confirmation
Indication
Request
IndicationIndication
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5. Telecommunication Service and Protocol
Connection-oriented Service
• Three phases:
▪ Connection Establishment
❖Creation of context in terminal and switching equipment
▪ Data Transmission
❖Based on established context
▪ Connection Release
❖Deletion of context
❖Deallocation of resources
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DisconnectRequest Disconnect
IndicationC
on
nec
tio
nR
elea
se
DataRequest Data
Indication
DataRequest Data
IndicationDa
taTr
an
smis
sio
n
ConnectRequest Connect
Indication
ConnectConfirmation
ConnectResponseC
on
nec
tio
nEs
tab
lish
men
t
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5. Telecommunication Service and Protocol
Connection End Points
• Connection End Point (CEP) = Identification of a connection within a SAP
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Layer i
Entity 1of Layer i+1
Ii+11
Entity 3of Layer i+1
Ii+13
Entity 1of Layer i+1
Ii1
Entity 2of Layer i+1
Ii2
SAP of Layer i SAP of Layer i
Protocol
Entity 2of Layer i+1
Ii+12
Entity 4of Layer i+1
Ii+14
Connection End Point
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5. Telecommunication Service and Protocol
Datagram Service
• Connectionless service:
▪ „shoot and pray“
▪ no phases for connection setup/release required
▪ no creation of context in the network
▪ no guarantee for ordered delivery
▪ unconfirmed service
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Data.Indication2
Data.Request1
Data.Indication1
Data.Request2
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Protocol
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5. Telecommunication Service and Protocol
Abstract Medium Mi
Entity 1of Layer i+1
Ii+11
Entity 2of Layer i+1
Ii+12
Entity 1of Layer i
Ii1
Entity 2of Layer i
Ii2
SAP of Layer i SAP of Layer i
Protocolof Layer i
Abstract Medium Mi-1
ServiceInterfaceof Layer i
ServiceInterface
of Layer i-1
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5. Telecommunication Service and Protocol
The Idea Behind Protocols
• Rules for concurrently running cooperating entities
• Enhancement of the quality (QoS) and the functions of the layer below
• Distributed algorithm
• Important problem:
▪ Failures and faults in the lower layer must be taken into account
• Specification of protocols:
▪ In most cases only for two entities
▪ (Extended) Finite State Machine ((E)FSM)
▪ Message Sequence Chart (MSC)
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5. Telecommunication Service and Protocol
Protocol Function
• Elementary atomic function to be included
▪ in different communication architectures
▪ in different protocols
▪ in different layers
• Cannot be further divided into parts
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Protocol Function “Encapsulation”
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5. Telecommunication Service and Protocol
Abstract Medium Mi
Entity Ii+11 Entity Ii+1
2
Entity Ii1 Entity Ii
2Protocolof Layer i
Abstract Medium Mi-1
User Information User Information
User Information User Information
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Protocol Function “Segmentation and Reassembly”
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5. Telecommunication Service and Protocol
Abstract Medium Mi
Entity Ii+11 Entity Ii+1
2
Entity Ii1 Entity Ii
2Protocolof Layer i
Abstract Medium Mi-1
User InformationUser Information
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5. Telecommunication Service and Protocol
Protocol Function “Connection Management”
• Connection establishment
• Connection maintenance → resource allocation
• Connection release
• Handling of connection breakdowns
• Handling of “orphaned” connection
• Further tasks related to connection management
▪ Addressing
▪ Address mapping
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5. Telecommunication Service and Protocol
Protocol Function “Routing”
• Selection of the “optimal” physical medium interface and of the next node
• Different criteria
▪ (monetary) cost
▪ data rate
▪ availability / reliability
▪ delay / number of intermediate hops to the destination
▪ current load
▪ …
• Optimum might vary according to current network state
• Probably duplication of sent information
▪ flooding
▪ multicast
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5. Telecommunication Service and Protocol
Protocol Function “Data Transfer”
• Desired: ordered data transfer
▪ data is delivered in the same order as it was sent
▪ data might have to be reordered after receiving
• Desired: different priorities for different data
▪ “normal” versus “expedited” data transfer
▪ no overtaking of important data by normal data
▪ data might have to be reordered before forwarding
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Protocol Function “Load Control”
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5. Telecommunication Service and Protocol
Load Control
Flow ControlNetwork Overload
Control
Congestion Control Traffic Control
on terminalenvironments
in the network
preventivelyreactively
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Protocol Function “Error Detection and Correction”
Error Detection
• Redundancy
▪ Parity
▪ Checksum
• Acknowledgements
▪ Stop & Wait
▪ Credit-based
• Timeout
• Numbering of packets
▪ Sequence control
Error Correction
• Redundancy
▪ Forward Error Correction (FEC)
• Retransmission
▪ Selectively
▪ Go-back-N
• Reset
• Abort
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5. Telecommunication Service and Protocol
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Layer i
Protocol Function “Multiplex and Bundling”
Multiplex
• One entity serves several entities on the layer above
Bundling
• On entity utilizes the services of several entities in the layer below
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5. Telecommunication Service and Protocol
Entity Ii+11
Entity Ii1
Entity Ii+12
Layer i
Entity Ii+11
Entity Ii1 Entity Ii
2
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Protocol Function “Shared Medium Access”
Controlled Medium Access
• Alternatives
▪ centrally controlled
▪ without central control(e.g. token based)
• No collision
• Problem: Part of bandwidth might be unused
Arbitrary Medium Access
• No central control
• Each nodes decides whether it may send
• Problem: Collisions possible
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5. Telecommunication Service and Protocol
Combination: Hybrid Medium Access
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5. Telecommunication Service and Protocol
Protocol Function “Synchronization”
• Different contexts:
▪ Signal level
❖ clock, sensing, …
▪ Medium Access
❖ time slots, start of sending after last transmission
▪ Transmission
❖ start and end (bits, octets, frames, packets, …)
▪ Connection
❖establishment, release
▪ Data transfer
❖progress of data transfer in case of restart
▪ …
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6. ISO/OSI Basic Reference Model
The ISO/OSI Basic Reference Model
• Standardization of protocols and services to implement open systems(OSI = Open Systems Interconnection)
• Base for interoperability of components of different companies
• Nota bene: ISO/OSI Basic Reference Model is mainly a framework to explainand classify communication architectures
• Implementation especially in public networks, but not relevant any more
• Important standard:
▪ ISO/IEC IS 7498: Information Processing Systems – Open Systems Interconnection –
Basic Reference Model, International Standard, October 15, 1994
▪ Adopted by CCITT resp. ITU-T in recommendation X.200
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6. ISO/OSI Basic Reference Model
Principles
• OSI End System▪ Terminal system that adheres to the ISO/OSI standard
• (N)-Layer▪ All entities/instances in the layer N
• (N)-Entity▪ Implementation of a process providing the service of the layer N
▪ Might be instantiated several times in one open system in one layer
• Peer Entities▪ Cooperating entities in one layer to deliver a certain service
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End System A End System B
(N)-Layer(N)-Entity (N)-Entity
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Connection and Association
(N)-Connection
• Relation between two or more (N+1)-Entities
• Connection established between (N)-CEPs in
the (N)-SAPs
• (N)-Entities offer connection-oriented service
(N)-Association
• Cooperative relation between two or more
(N)-Entities
• Can be based on
▪ (N)-Connection
▪ connectionless service of (N)-Layer
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6. ISO/OSI Basic Reference Model
Entity IN+11 Entity IN+1
2
Entity IN1 Entity IN
2
(N+1)-Association
(N)-Connection
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Generic Communication over SAP
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6. ISO/OSI Basic Reference Model
(N-1)-Layer
(N+1)-LayerSAP
(N)-Layer
SAP
(N)-SDU (N)-ICI
(N-1)-SDU (N-1)-ICI
(N)-IDU
(N-1)-ICI
(N-1)-IDU
(N)-PCI
(N)-PDU
(N )-PCI (N)-SDU
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Data and Information Units
• (N)-IDU Interface Data Unit▪ exchanged between (N+1)- and (N)-Entities
via an (N)-SAP
▪ composed of (N)-ICI and (N)-SDU
• (N)-ICI Interface Control Information▪ exchanged between (N+1)- and (N)-Layer
▪ used to control service delivery
▪ contains e.g. addresses
• (N)-SDU Service Data Unit ▪ consists of information to be transparently
transferred between (N)-SAPs
• (N)-PCI Protocol Control Information▪ exchanged between Peer Entities of the
(N)-Layer
▪ used to control protocol functions
▪ contains e.g. sequence numbers, check
sums etc.
• (N)-PDU Protocol Data Unit▪ exchanged between (N)-Entities on
utilizing the service of the (N-1)-Layer
▪ composed of (N)-PCI and (N)-SDU
▪ corresponds to the (N-1)-SDU
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6. ISO/OSI Basic Reference Model
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Exchange of Data Units
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6. ISO/OSI Basic Reference Model
(N+1)-Layer
SAP
(N)-IDU
SAP
(N-1)-Layer
SAPSAP
Entity 1of Layer N
IN1
Entity 2of Layer N
IN2
Protocol of Layer N
(N)-IDU
(N-1)-IDU (N-1)-IDU
(N)-PDU
(N)-Service
(N-1)-Service
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Layer 7 (A Layer)
Layer 6 (P-Layer)
Layer 5 (S-Layer)
Layer 4 (T-Layer)
Layer 3 (N-Layer)
Layer 2 (DL-Layer)
The OSI Layers
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6. ISO/OSI Basic Reference Model
Layer 1 (Ph-Layer) Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application LayerApplication-
orientedLayers
Transport-oriented
Layers
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Service Primitives
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6. ISO/OSI Basic Reference Model
Layer Service Primitive
Ph -Connect .Req (Request)
DL -Data .Ind (Indication)
N -Disconnect .Rsp (Response)
T -UserAbort .Cnf (Confirmation)
… -…
(N)-Service.Primitve
Note: Service Primitives in Application Layer named according to the relatedApplication Service Element (ASE)
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Communication Example
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6. ISO/OSI Basic Reference Model
DL-Connect.ReqDL-Connect.Ind
DL-Connect.RspDL-Connect.Cnf
DL-Data.Req
DL-Data.IndDL-Data.Req
DL-Data.Ind
DL-Disconnect.Req
DL-Disconnect.Ind
Co
nn
ect
ion
Esta
blis
hm
en
tD
ata
Tran
sfe
rC
on
ne
ctio
nR
ele
ase
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6. ISO/OSI Basic Reference Model
Transport-oriented Layers
• Do not consider the contents of the transported information
• Only deal with bit sequences
• Apply the same procedures to all transported information
• Consist of the following layers
▪ Transport Layer
▪ Network Layer
▪ Data Link Layer
▪ Physical Layer
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Layer 4 (T-Layer)
Layer 3 (N-Layer)
Layer 2 (DL-Layer)
Layer 1 (Ph-Layer)
Transport-oriented
Layers
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6. ISO/OSI Basic Reference Model
Physical Layer
• Transfers (unstructured) bit sequences in form of physical signals
• Implements channel coding
• Defines the physical interfaces (connectors, jacks, sockets, …)
• Might not be able to deal with transmission errors
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Physical Medium
Physical Transmission Channel
Physical Layer Entity 1
Physical Layer Entity 2
Bits Bits
Physical Signals Eventually Disturbed Physical Signals
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Transmission over PhysicalLayerTaken from F. Halsall (2000)
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6. ISO/OSI Basic Reference Model
Transmitted Data
Transmitted Signal
Sampling Instants
Received Data
0 1 1 110 0 0
0 1 0 110 0 0
+V
-V
Bit Error
Typical Received Signal
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Data Link Layer
• Responsible for detection and correction of bit errors
▪ For detection: adding redundancy (parity bits, cyclic redundancy check, …)
▪ For correction: several mechanisms
❖ Forward Error Correction (FEC)
❖Acknowledgements
❖ Sequence Numbers
❖Timeout & Retransmission
❖ Flow Control
❖Reset
• Medium Access Control (MAC) for shared medium
• Addressing receiver (especially in case of a shared medium) e.g. MAC address
• Control information and user information frame structure:
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Header User Data Trailer
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Intermediate System End System AEnd System A
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Network Layer
• Links several point-to-point associations (of the Data Link Layer) to an and system association
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• Is responsible for finding the optimal path through the network (routing) and forwarding the information on this path
• Should control network load
• Works connection-oriented or connectionless
• Is based on world-wide unique addresses (e.g. telephone number or IP address)
Network LayerEntity 1
Data Link LayerEntity 1
Network LayerEntity 2
Network LayerEntity 3
Data Link LayerEntity 3
Data Link Association Data Link Association
Network Association
DL LayerEntity
2.1
DL LayerEntity
2.2
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End System AEnd System A
Transport LayerEntity 1
Transport LayerEntity 3
6. ISO/OSI Basic Reference Model
Transport Layer
• Data transfer between different pairs of applications on the end systems
• Abstracts from the underlying networking technologies
• Adds Quality of Service (QoS) to the service provided by the Network Layer as required by the applications
• Works connection-oriented or connectionless
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Intermediate System
Network LayerEntity 1
Data Link LayerEntity 1
Network LayerEntity 2
DL LayerEntity
2.1
Network LayerEntity 3
Data Link LayerEntity 3
Data Link Association Data Link AssociationDL LayerEntity
2.2
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6. ISO/OSI Basic Reference Model
Application-Oriented Layers
• Depend on the application and its semantics
• Are aware of the kind of information to be transferred (not just bits)
• Must be directly controlled by the application
• Consist of the following layers:
▪ Application Layer
▪ Presentation Layer
▪ Session Layer
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Layer 7 (A Layer)
Layer 6 (P-Layer)
Layer 5 (S-Layer)
Application-oriented
Layers
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Session Layer
• Allows organizing and synchronizing the dialogue of two applications
• Is built on sessions which abstract from network connectivity
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• Controls the exchange of information (which application entity might send information at a specific point in time)
• Provides synchronization points for restarting whenever the connectivity is interrupted during transmission
t
Session
Transport Layer Connections
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Presentation Layer
• Provides coding of information so that the receiver understands the syntax and the semantics of the transmitted bits
• Is based on abstract data syntax forms which the application can choose from
• Transfers information according to the associated transfer syntax
• Overcomes the ambiguity and heterogeneity of different syntaxes in various operating systems or processor architectures
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End System A End System B
Presentation Layer
Entity 1
Presentation Layer
Entity 2
0 1 1 0 0 0 1 0 1 1 0 0 1 0 1 0 0 0 0 1 1 0 1 1 1 1 0 0 0 0 1 0 1 1 0 0 1
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6. ISO/OSI Basic Reference Model
Application Layer
• Provides a set of different services suitable for different applications
▪ File Transfer, Access and Management (FTAM)
▪ Message Handling System (MHS, ISO/OSI E-Mail)
▪ Remote Operations (ROSE)
▪ Association Control (ACSE)
• Consists of several Application Service Elements (ASEs) forming an application layer entity
▪ Common Application Service Element (CASE)
▪ Specific Application Service Element (SASE)
• Single Association Control Function (SACF) to control the cooperation of ASEs
• Multiple Association Control Function (MACF) to control several SACFs
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The Seven Layers
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6. ISO/OSI Basic Reference Model
Layer 7 Entity
Layer 6 Entity
Layer 5 Entity
Layer 4 Entity
Layer 3 Entity
Layer 2 Entity
Layer 1 Entity
Layer 3 Entity
Layer 2 Entity
Layer 1 Entity
Layer 2 Entity
Layer 1 Entity
Layer 7 Entity
Layer 6 Entity
Layer 5 Entity
Layer 4 Entity
Layer 3 Entity
Layer 2 Entity
Layer 1 Entity
NetworkEnd System A End System B
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Encapsulation
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Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Sen
dR
eceive
Data
Data
Data
Data
Data
Data
Data
Bits
AH
PH
SH
TH
NH
DLTDLH
AH Application Header NH Network HeaderPH Presentation Header DLH Data Link HeaderSH Session Header DLT Data Link TrailerTH Transport Header
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Comparison with the Internet Protocol Suite
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Application Layer
Transport Layer
Internet Layer
Network-to-HostLayer
Network to host interface implemented on the network cardin the device (subsumes Data Link and Physical Layer)
Routing and forwarding in the Internet –layer of the Internet Protocol
Data transfer end-to-endbetween two applications on two computers
Application specific functions concentrated in one layer only(subsumes Session, Presentation and Application Layer)
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6. ISO/OSI Basic Reference Model
Theoretic Model: Three Layer Architecture
• Less processing overhead
• Prevention of redundant functions
• High performance networking
• But: not compatible to current architectures / best practice
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Application-orientedLayer
Transport-orientedLayer
Network AccessLayer
Corresponds to OSI layers 5 – 7
Corresponds to OSI layers 2b – 4
Corresponds to OSI layers 1 – 2a
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References
• Halsall, Fred (2005): Computer Networking and the Internet. 5th edition. Harlow, England: Addison-Wesley.
• Halsall, Fred (2000): Data Communications, Computer Networks and Open Systems. 4th edition, reprint. Harlow: Addison-Wesley.
• Kurose, James F.; Ross, Keith W. (2017): Computer Networking. A Top-Down Approach. 7th edition. Hoboken, New Jersey: Pearson.
• Nutt, Gary J. (1992): Open Systems. Englewood Cliffs, N.J.: Prentice-Hall (Prentice-Hall Series in Innovative Technology).
• Stallings, William (2014): Data and Computer Communications. 10th edition. Upper Saddle River, N.J.: Pearson.
• Tanenbaum, Andrew S.; Wetherall, David J. (2011): Computer Networks. 5th edition. Boston: Pearson Prentice Hall.
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