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Network Architecture


Academic year: 2022

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Network Architecture

Lenuta Alboaie

[email protected]



• 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


Secretary(a)1 Secretary(a)2


Chinese Philosopher Interfata


Figure: Architecture philosophe – translator - secretary Postal System


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


Levela2 Levelb2



Service offered by Level2



Aspects regarding levels designs

• Services types


• 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


– 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



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:


• Pure ALOHA : “send whenever you want”

CSMA (Carrier Sense - Multiple Access): protocol with transmission detection (“free channel before


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)


– Improves MACA


Standard IEEE


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


Wireless (WLAN)

802.15 Wireless PAN ( 802.15.1 Bluetooth, ...) 802.16 Wireless WAN


OSI Model

Data Link Level - equipment


– 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:


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



Modelul OSI

Application level:

manage network services: virtual

terminal, file transfer, electronic mail,

remote execution of

applications, ...


TCP/IP Model


– 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-



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


Transport TCP, UDP, … Session


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


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


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



TCP/IP Model


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


A frame format:

Preambul Adresa MAC destinatie

Adresa MAC a sursei

Date (payload) Camp de completare

Suma de control

Delimitator de inceput de


Lungimea campului de


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]



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


ICMP (Internet Control Messages Protocol) - RFC 792,777

SNMP (Simple Network Management Protocol) – RFC 1157



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


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


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.]



• Computer Networks Structure

• Network Architecture Models (OSI, TCP/IP)

• TCP/IP Model

• ISO/OSI versus TCP/IP





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