Content
• Structure of Computer Networks
• Network Architecture Models (OSI, TCP/IP)
• TCP/IP Model
• ISO/OSI versus TCP/IP
• Computer Network Structure – stack levels
Functionality :
• Interface: ensure communication between two consecutive levels
• Service: functionality provided by a level Result: reducing design complexity
The principle of communication: transmitter sends at n level what the recipient receives at the n level
Computers Networks Structure
• Protocol – Rules and conventions through which the communication takes place
Example: link among - levels, protocols and interfaces
Romanian Philosopher
Translater1
Secretary(a)1 Secretary(a)2
Translater2
Chinese Philosopher Interfata
Protocol
Figure: Architecture philosophe – translator - secretary Postal System
Interfata
Frumusetea este in ochii privitorului.
Beauty is in the eye of the beholder.
La beauté est dans l'œil de l'observateur.
Beauty is in the eye of the beholder.
The message
The message for Translator2
The message for Secretary(a)2
Virtual communication Physical communication Legend:
Aspects regarding levels design
• Specifying the service is performed by a set of
primitives (operations) available to the one who uses the service
• Service!= Protocol
Levela1
Levela2 Levelb2
Levelb1
Protocol
Service offered by Level2
Protocol
Aspects regarding levels designs
• Services types
– Connection-oriented
• Communication requires a connection
• Similar to a telephone service – Connectionless
• Communication does not require a connection
• Similar to postal service
Aspects regarding level design
• Network architecture: the set of levels and protocols – Architecture specification must provide sufficient
information for programs or equipment intended, in order to offer the specific protocols
• Protocols stack: list of protocols (on all levels) used by a
particular system
Aspects regarding levels design
• Each level must identify transmitters & receivers through an addressing mechanism
• Data transfer rules identification – simplex communication
• Example: TV
– half-duplex communication
• Example: "walkie-talkie"
– full-duplex communication
• Example: telephone
Aspects regarding levels design
• In general, communication channels do not preserve the order of the sent messages => need for a protocol that provides a mechanism to reconstruct the correct messages order
• Sometimes the receiver cannot manage the variable length messages => it must be a mechanism to divide/assemble
• High costs in the allocation of separate connections? =>
Multiplexing - use the same connection for independent conversations
• In general, there are several ways between source and destination => routing mechanism
• Physical communication circuits are not perfect => it asks for an
Reference models for network architecture
• ISO/OSI (International Standard Organization/ Open System Interconnection)
• TCP/IP (Transmission Control Protocol/ Internet Protocol)
[Computer Networks, 2010 – Andrew S. Tanenbaum, et.al.]
Network Architecture - Equipment
Figure: Devices and appropriate levels
OSI Model- motivation
• The need for a different level of abstraction => to create a new level
• Obs. The number of levels must be optimal, therefore each level has different functions and the whole architecture is functional
• A level has a clear role; a level function must take into account protocols that are standardized at international level
• Minimizing the flow of information between levels is accomplished through good boundary levels => Levels can be modified and
implemented independently
• Each level offers services for superior level (using services from previous levels)
• “peer” levels of different systems communicate via a protocol
OSI Model
[conform Computer Networks, 2010 –
OSI Model – message structure
[Retele de calculatoare – curs 2007-2008, Sabin Buraga]
OSI Model – structure
• Physical Level
• Data Link Level
• Network Level
• Transport Level
• Session Level
• Presentation Level
• Application Level
OSI Model
Physical Level: data transmission medium
Role: ensure that the sequence of bits transmitted from the transmitter reaches the receiver
– Transmission media:
• Wired (twisted pair, coaxial cable, optical fiber)
• Wireless (electromagnetic spectrum - radio,
microwave, infrared, ...) -> next course
OSI Model
• Physical Level:
Data transmission:
• Analog (continuous values) – Example: telephone
systems
– Digital (discrete)
– Example: computers Data conversion from analog to
digital and vice versa:
• Modem: digital data are
transmitted in analog format
• Codec (coder/decoder): analog data are transmitted in digital format
Figure. Analog Signal
Figure. Digital Signal
OSI Model
• Physical Level - aspects
– Bandwidth: the number of bits that can be transmitted over the network in a given
period of time (data transfer speed)
•Usually expressed in bits/seconds
– Latency: represents the maximum time required for a bit to propagate in a network, from one end to another and it is expressed in units of time
•RTT(Round Trip Time) – the necessary
time for a bit to cross from one end to the other and back to the environment
Basic parameters to ensure network
performance
OSI Model
• Physical Level - aspects
Modification suffered by signals during propagation:
• Attenuation: energy loss during signal propagation through a transmission medium
• Noise: signal change caused by external factors (e.g. lightning, other electronic equipment, etc.)
– Diaphony = noise from the signal transmitted by a neighbouring transmission medium
• Distortion- is a deterministic change of a signal
OSI Model
• Physical Level - conclusions
Offers transportation services, on which we can identify a number of possible problems
– Data can be altered / destroyed due to the noise
– If the destination cannot process the data in the right time, some will be lost
– If the same transmission medium is used by multiple transmitters, packages may alter each other
– It is less expensive to build logical connections to share the same physical medium, than create independent physical links
A new level?
OSI Model
• Data Link Level:
• Offers:
• mechanisms to detect and correct errors
• regulatory mechanisms for dataflow
• control mechanism for media access
• Services at the network level
• The data unit used at this level is called frame
OSI Model
• Data Link Level:
– The data are encapsulated in frames – Analogy: frame= digital envelope
Figure: The relationship between packets and frames
[conform Computer Networks, 2010 – Andrew S.
Tanenbaum, et.al.]
Control Information for
Data Link Level
OSI Model
• Data Link Level:
– It provides services at the network level
• Unconfirmed connectionless service
» The transmitter sends independent frames to the receiver without waiting for any confirmation
» A lost frame is not recovered
• Confirmed connectionless service
» The sent frames are confirmed
» The frames are not sent in order
• Confirmed connection-oriented services
» A connection is established before the transmission
» Frames are numbered to keep the right order
OSI Model
• Data Link Level:
– Divided into two sublevels:
• LLC (Logical Link Control)
– Role: Provides an independent view of the medium at a superior level
• MAC (Medium Access Control)
– Role: Used to determine who is to transmit into multi-access channel
OSI Model
• Data Link Level:
MAC (Medium Access Control)
– Context of the problem: the same physical environment is used by more emitters (uniquely identified by a
physical address or MAC address) operating simultaneously, for example:
– Half-duplex transmission between entities that use the same physical environment for both directions – communication by radio when there are stations that
emit on the same wavelength (Wireless Ethernet - IEEE 802.11, Bluetooth, etc.)
OSI Model
• Data Link Level:
MAC (Medium Access Control) – Strategies:
– Static allocation
» FDM (Frequency Division Multiplexing)
» TDM (Time Division Multiplexing)
– Accepting the possibility of collisions and retransmitting packets affected by collisions - dynamic allocation
Collision = data is simultaneously transmitted
General mechanism: a station that has data to send, transmit them immediately; if the collision appears, the resend action is performed
OSI Model
• Data Link Level:
Medium Access Control – protocols:
– ALOHA
• Pure ALOHA : “send whenever you want”
– CSMA (Carrier Sense - Multiple Access): protocol with transmission detection (“free channel before
sending?”)
• 1-persistent CSMA
• …
• p-persistent CSMA
OSI Model
• Data Link Level:
– Medium Access Control – protocols:
• CSMA (Carrier Sense - Multiple Access)
– CSMA/CD (CSMA with Collision Detection)
» “free channel while transmit?”
» Based on Ethernet LAN (IEEE 802.3)
• MACA (Multiple Access with Collision Avoidance) – The basis for wireless networks (IEEE 802.11)
• MACAW
– Improves MACA
Standard IEEE
Description
802 Group standards for LAN and MAN 802.2 LLC (Logical Link Control)
802.3 Ethernet (Carrier Sense Multiple Access with Collision Detect (CSMA/CD))
802.3u Fast Ethernet 802.3z Gigabit Ethernet 802.11
a/b/g/n/ac
Wireless (WLAN)
802.15 Wireless PAN ( 802.15.1 Bluetooth, ...) 802.16 Wireless WAN
OSI Model
• Data Link Level - equipment
• Bridges
– Resend frames between two networks (LAN)
– It doesn’t change the frame content and only headers can be modified
– Improve safety and performance transmission – Can provide flow control and congestion data
– Retransmission is done via static routes or using a spanning tree
STP (IEEE 802.1D) – Spanning Tree Protocol – Other equipment? (Course 1)
OSI Model
• Network Level:
– Retrieves packages from the source and transfer them to the destination
– It provides services to transport level
• What services?
– Internet community proposes:
» Connectionless services: SEND PACKET, RECEIVE PACKET
» Packages (called datagrams) are independent and are managed individually
» Datagram services are similar to a typical post system
OSI Model
• Network Level:
– Retrieves packages from the source and transfer them to the destination
– It provides services to transport level
• What services?
– Telephone companies propose:
» Connection-oriented service – safe services
» Before the transfer some negotiations are initiated to establish a connection (VC-virtual circuit)
» These services are similar to the telephony system
OSI Model
• Network Level:
– Used Protocols
• X.25 (Connection-oriented)
• IP
– Problems
• Protocol conversions and addresses
• Error control (flow, congestion)
• Dividing and recomposing packages
• Security – encryption, firewall
OSI Model
• Transport level: it offers safe and cost-effective data transport from the source machine to the destination machine, independent of physical network or networks currently in use
Services: provides connection-oriented and connectionless services
Differences between the transport and network layer?
OSI Model
• Transport level:
– Primitives:
• LISTEN – it’s a blocking operation until a process tries to connect
• CONNECT – trying to establish a connection
• SEND – send data
• RECEIVE – it’s a blocking operation until data is received
• DISCONNECT – connection release
– Performance - quality of service (QoS - Quality of Service):
establishing /releasing the connection, error rate, protection, priority, resilience (the probability that a connection shut down
OSI Model
• Session level: refers to problems linked to session settings (dialogue control services, synchronisation etc.)
• Presentation level: handle data presentation, codified them into standard format
• To ensure communication among computers with different representations, the presentation level ensures the conversion of internal data in
standardized network representation and vice
versa
Modelul OSI
• Application level:
manage network services: virtual
terminal, file transfer, electronic mail,
remote execution of
applications, ...
TCP/IP Model
• Terms:
– end-system – host
– network - provides support for data transfer between end systems
– internet - collection of networks (interconnected)
– Sub-network - part of the internet
– intermediate system - connects two sub-
networks
OSI versus TCP/IP
Figure: Overview of models OSI and TCP / IP TCP/IP Model TCP/IP - Protocols OSI Model Application FTP, Telnet, HTTP,… Application
Presentation
Transport TCP, UDP, … Session
Transport
Internetwork IP, … Network
Host to Network Ethernet, … Datalink Physical
TCP/IP Model
[Retele de calculatoare – curs 2007-2008, Sabin Buraga]
TCP/IP Model
Figure. The TCP / IP - protocols
• It provides the ability to interconnect
multiple network types
• Network and Transport levels are the kernel of this model
• Successfully
implemented over Ethernet (IEEE 802.3) - supported by many
implementations of the physical layer (coaxial cable, twisted pair, fiber optic)
TCP/IP Model
• “Physical” level
– Ensure the connection between host and the network
Ethernet
• It provides multiple access (shared transmission medium) in a network
• Collision Detection: CSMA / CD (Carrier Sense Multiple Access with Collision Detection)
• Each Ethernet interface has a unique address 48 bits: hardware address (MAC) - e.g. C0: B3: 44: 17: 21: 17
• Addresses are assigned to NIC (Network Interface Card) producers by a central authority
Figure. TCP/IP Model
802.4 802.5
802.3
TCP/IP Model
Ethernet
• Each interface (board) network has a unique MAC address (some operating systems allow it to be modified by software)
• The first 24 bits identify the manufacturer
TCP/IP Model
Ethernet
• A frame format:
Preambul Adresa MAC destinatie
Adresa MAC a sursei
Date (payload) Camp de completare
Suma de control
Delimitator de inceput de
cadru
Lungimea campului de
date
7 bytes 1 6 6 2 0-1500 0-46 4
• Broadcast: the address has all bits set to 1
• Each network interface inspecting the destination address in each frame
• If the destination address does not match with the hardware address or the broadcast address, then the frame is ignored
minim 64 bytes
TCP/IP Model
Ethernet – standards (examples):
• 10 BASE5: 10 Mbps using thick coaxial cable (Thick Ethernet)- 1980
• 1BASE5: 1 Mbps using two Ethernet cables (Unshilded Twisted Pair)
• 10BASE-T: 10Mbps using 2 pairs UTP– 1990
• 10BASE-FL: 10 Mbps optical fiber with point-to-point link
• 10BASE-FB: 10Mbps backbone with optical fiber
• 100BASE – FX: 100MBps CSMA/CD with two optical fiber, full duplex
• … etc
TCP/IP Model
Ethernet versus Fast Ethernet
[conform Retele de calculatoare – curs 2007-2008, Sabin Buraga]
TCP/IP Model
• Gigabit Ethernet
• Implementations for both copper wires (802.3ab), and fiber (802.3z)
• The difference from other
Ethernet implementations is at physical level
• 10 Gigabit Ethernet
• Implementations for fiber (802.3ae)
• Operates at distances of 40km (useful for MAN and WAN)
• Frame format is similar to other implementations of Ethernet
TCP/IP Model
[https://www.computernetworkingnotes.com/networking- tutorials/ethernet-standards-and-protocols-explained.html]
https://www.ieee802.org/3/db/P802d3db_Updated_Objectives_Approved_November_2020.pdf
TCP/IP Model
• Network Level
– It allows hosts to emit a packet in any network; packages travel independently up to destination
• Highlights:
routing packets
congestion avoidance
TCP/IP Model
• Network Level
– Level design aimed at achieving the following objectives:
• The services offered are independent from the technology used (e.g. routers)
• Provide transport level services, which allow it to operate independently of number, type and topology
• It provides a unique mechanism to address LANs and WANs
TCP/IP Model
• Network Level – IPv4
– IPv6 – Routing
• OSPF(Open Shortest Path First) – RFC 1131
• BGP(Border Gateway Protocol) – RFC 1105 – Multicast:
• IGMP (Internet Group Management Protocol) – RFC 1112, 1054
– Control:
• ICMP (Internet Control Messages Protocol) - RFC 792,777
• SNMP (Simple Network Management Protocol) – RFC 1157
• ICMPv6
TCP/IP Model
• Transport level
– Ensures the realization of communication between the source host and destination host
– Protocols
• TCP (Transmission Control Protocol) - RFC 793,761
• UDP (User Datagram Protocol) – RFC 768
• Other Protocols: SCTP (Stream Control Transmission Protocol) – RFC 4960, 3286 (2960, 3309);DCCP
(Datagram Congestion Control Protocol) – RFC 4340, 4336;
TCP/IP Model
• Application Level:
– Contains high level protocols
– SMTP (Simple Mail Transfer Protocol) – RFC 5321 (821) – POP3(Post Office Protocol) – RFC 1081
– TELNET – RFC 854,764
– FTP (File Transfer Protocol) – RFC 454 – NFS (Network File System) – RFC 1095
– DNS (Domain Name System) – RFC 1034,1035 – HTTP (HyperText Transfer Protocol) – RFC 2616
– RTP (Real-time Transport Protocol) – RFC 3550 (1889) – SIP (Session Initiation Protocol) – RFC 3261
TCP/IP Model
• Organizations involved in standardization:
• ISOC – Internet Society
• IAB – Internet Architecture Board
• IETF – Internet Engineering Task Force
• IRTF – Internet Research Task Force
• InterNIC – Internet Network Information Center
• IANA – Internet Assigned Number Authority
• RFC (Request For Comments) documents
• Edited by Network Working Group (IETF)
• RFC 1800 (Internet Official Protocol Standards)
• More details -> www.ietf.org
OSI versus TCP/IP
• Similarities:
• Both are based on a protocol stack
• The layer
functionalities are somehow similar
• Both have an
application layer on top
• Are based (directly or not) on transport level
[conform Computer Networks, 2010 –
OSI versus TCP/IP
• Differences:
• ISO/OSI is a theoretical model; TCP/IP is effective in implementation
• OSI makes explicit the distinction between service, interface and protocol; TCP / IP does not
• ISO / OSI provides protocols that ensure reliable communication (detection and treatment of errors at each level); TCP/IP verifies communication at transport level
• OSI support both types of communication at network level (connectionless and connection oriented); TCP/IP has connectionless services at network level and both types at transport level
[conform Computer Networks, 2010 – Andrew S.
Tanenbaum, et.al.]