Network Architectures, Interconnecting NetworksTik-110.250 Fundamentals of Network Media, 25.1.-1.2.2001, slide 1 © Arto Karila Tik-110.250 Fundamentals of Network Media (3 cr) Spring

Tik-110.250 Fundamentals of Network Media, 25.1.-1.2.2001, slide 1 © Arto Karila

Tik-110.250 Fundamentals of Network Media (3 cr)Spring 2000

Network Architectures,Interconnecting Networks

Professor Arto KarilaHelsinki University of Technology

E-mail: [email protected]


Contents Lecture on January 25• Network architectures• ProtocolsLecture on February 1• Interconnecting networks• Internetworking


Network architectures • The architecture of a large system means its division into

smaller elements and the relationships between these elements

• Large and complex structures are best understood and handled by humans when divided into smaller pieces

• Telecommunication networks are among the largest and most complex systems ever designed and implemented by man, so they are best understood as structures consisting of various architectural elements

• We will start by briefly presenting layered reference models and the Internet architecture

• Networking is then discussed within this framework• The purpose is to help the student see the forest for the trees


Applications and the real world • People have invented various applications that use communi-

cations services, such as E-mail, WWW and digital multimedia• Applications have varying communications requirements:

• Burst/stream mode of operation• Large/small capacity requirements• Strict/loose delay requirements

• There are a number of transmission media, such as:• The ”ether”• Copper cables• Fiber optics

• These physical media suffer from limitations, such as:• Noise and interference• Attenuation• Limited bandwidth• Limited speed of signal


Layered telecommunication architectures • The purpose of telecommunication is to fill the gap between

the applications and the real world• Layered telecommunication architectures divide the gap

between the characteristics of the physical media and the needs of the applications into layers

• The lower layers deal with transferring information over the physical media

• The higher layers support various applications• Each layer uses the service of the layer immediately below,

adds its own value and offers refined service to the layer immediately above

• Layers are as independent from each other as possible only communicating through the service interfaces between them


OSI reference model • The Open Systems Interconnection (OSI) reference model of

the International Standardization Organization (ISO)• Divides telecommunications into seven layers• Defines the layers, their functions and basic concepts• Does not define the actual telecommunications protocols• Does not take a stand to implementational issues• The purpose of the OSI model was to:

• offer a generic structure for telecommunications systems• act as a framework for standards and requirements• facilitate the interconnection of various types of equipment• ease the deployment of new technologies

• OSI is analogous to the System Networks Architecture (SNA) of the IBM world (whose significance has rapidly decreased)

• OSI still is a useful and generally accepted reference model


Basic principles of the OSI model • OSI-model is based on the concept of peer-to-peer

communications, where two layer (N) protocol entities communicate using a (N) protocol

• Layer (N) is based on the service provided by layer (N-1), it adds value to the (N-1) service and provides more refined (N) service to layer (N+1)

• The layers are independent and only ”see” each other through the service interfaces between the layers

• (N+1) entity uses (N) service through (N) service primitives• (N) entity communicates with its peer entity by exchanging

(N) Protocol Data Units (or (N) PDUs)


OSI model

media

Application

Presentation

Session

Transport

Network

Data Link

Physical

applicationprocess

Application

Presentation

Session

Transport

Network

Data Link

Physical

applicationprocess

Network

DL

Phys.

DL

Phys.

7.

6.

5.

4.

3.

2.

1.


Layers of the OSI model and their functions • Application processes reside outside the OSI system and use the

communication services provided by it7. Application layer acts as an interface between the application process

and the telecommunications world6. Presentation layer negotiates a transfer syntax and performs

transformations between the local and transfer syntax5. Session layer provides organized and synchronized data transfer to the

presentation layer4. Transport layer raises the service provided by the network layer to the

level required by the session layer providing reliable end-to-end transport service

3. Network layer is responsible for routing messages through the network2. Data Link layer transfers data between two neighboring nodes forming

frames out of bits, it may also perform flow and error control1. Physical layer transfers bits (and possibly non-data symbols) over the

physical media from one network node to another• Physical media (such as copper cable, optical fiber or the ”ether”) act as

the medium through which information is transferred


OSI terminology • System• (N) Layer• (N) Protocol• (N) Service• (N) Connection• (N) Association• (N) Protocol Data Unit, (N)PDU• (N) Primitive• (N) Interface Data Unit, (N)-IDU• Interface Control Information, ICI• (N) Service Data Unit, (N)SDU• (N) Service Access Point, (N)SAP• Connection End-Point, CEP• CEP Identifier, CEPI


Critique of the OSI model • Too heavy - too many layers with overlapping functionality• Too connection oriented• Overly heavy and slow standardization process• The standards produced tend to be rather theoretical and

rarely provide solution to real-life problems• The standardization of OSI protocols (such as X.400 and

FTAM) and OSI profiles (such as GOSIP) has been a complete flop

• The main function of the OSI model today is to server as a generic framework and terminology, not as a protocol family

• The TCP/IP protocol suite has fulfilled all the promises made by OSI when it was conceived


General properties of protocols A protocol shall be: • Completely and unambiguously defined• Free of dead-locks and live-locks• Able to recover from all error conditions

Some possible functions of protocols:• Addressing• Connections• Error detection• Error correction• Flow control• Prioritization• Multiplexing / splitting• Segmentation / concatenation


Different types of network services Circuit switched services• Connect, communicate, disconnect• Fixed capacity reserved from the network for each connection• Based on Time Division Multiplexing (TDM)• E.g. the Public Switched Telecommunications Network

(PSTN) and the Nordic Public Data Network (X.21 Datex)Packet switched services• Based on virtual connections which are set up and discon-

nected but do not reserve fixed capacity from the network• Enable statistical multiplexing and more efficient use of

network capacity• E.g. X.25 type networksConnectionless (datagram) services• No connection• Each datagram is routed separately through the network• For example the Internet and most local area networks (LANs)


PSTN • The Public Switched Telecommunications Network (PSTN)

has developed over the past 100 years into a network with:• global coverage• high availability• well working and flexible billing system• some degree of information security• in many countries (almost) fully digitalized

• The PSTN is circuit switched• Capacity can be reserved for data (leased lines)• The network was designed for and works best with speech• Today most of the transatlantic traffic is data (mainly faxes)• Among the shortcomings of the PSTN are:

• slow standardization process• complicated signaling and network management• high cost (especially in data transfer)


X.25 packet networks • Compatible with the ITU-T X.25 recommendation• A packet switched international network service• Facilitates closed user groups (Virtual Private Network, VPN)• In Finland DataPak (Sonera) and DigiPak (Finnet)• Protocol layers:

• X.25 packet layer protocol• LAPB (HDLC)• X.21 (physical layer)

• A heavy protocol that does not adapt well to high transmission speeds

• Expensive and complex technology• Not well suited for LAN interconnection• Still widely used, especially in Central Europe and military

organizations


Frame Relay • A modern, light-weight interface to data networks• Internationally standardized (ITU-T I.233)• ”The X.25 of the 1990’s”• A connection-oriented data link layer service• Based on the same paradigms as LANs:

• Relatively high transmission speed• Reliable transmission systems => "unreliable" protocol

(that is, no acknowledgements)• Small protocol overhead

• Enables service differentiation (quality of service)• A LAN-like service in public data networks• Frame structure modified from LAPD• Detects transmission errors • No error correction• Can support a variety of physical interfaces• Mainly used at 64 kb/s to 2 Mb/s transmission speeds


Local Area Networks (LANs) • Local Area Network (LAN) is a fast (at least ~ Mb/s), geo-

graphically limited (~ km) digital communications network, which is owned and operated by the user organization

History of LANs• Packet radio networks (Aloha and Slotted Aloha) in Hawaii• Ethernet

• The first LAN was the early version of Ethernet in 1976, about 3 Mb/s

• Digital-Intel-Xerox “DIX” specification in 1979, 10 Mb/s• Ethernet 2 in 1982

• Token Ring• IBM Token Ring in 1985, 4 Mb/s• Later also 16 Mb/s version

• Fiber Distributed Data Interface (FDDI), 100 Mb/s• High-speed Ethernet: 100 Mb/s (FE) and 1 Gb/s (GE)


LAN topologies LANs can be classified by their topologies:• Bus - e.g. the original Ethernet• Ring - e.g. Token Ring and FDDI• Star - e.g. twisted pair Ethernet and ATM LANs• Wireless LAN (WLAN) - radio or infra red

bus ring star wireless


Ethernet 2 MAC frame format

Preamble

Start Frame Delimiter

Destination Address

Source Address

Protocol Type

Data

Pad

Frame Check Sequence

7 octects

1 octect

6 octects

6 octects

2 octects

4 octects

min. 64octects

n octects


Switched LANs • LAN switches multiply the performance of Ethernet

• In traditional Ethernet the computers of a segment share a 10 Mb/s half duplex channel

• In switched Ethernet every work station has a 10 Mb/s full-duplex channel of its own

• 100 Mb/s and 1 Gb/s Ethernet is normally switched leading into performance beating the current ATM networks

• A modern LAN Switch facilitates the interconnection of various types of LANs, such as:

• 10 Mb/s Ethernet• 100 Mb/s Ethernet• 155 Mb/s ATM (LAN Emulation) • 622 Mb/s ATM• 1 Gb/s Ethernet


LAN switches and VLANs • A modern LAN switch:

• 10/100 Mb/s full duplex Ethernet port per work station• 1 Gb/s Ethernet to backbone and servers• Virtual LAN (VLAN) support (IEEE 802.1Q)• Prioritization and quality of service (IEEE 802.1p etc.)• Wire-speed on all ports• Cost under $100 per work station

• Lots of products already on the market• Port and protocol based VLANs for work stations (& servers)• Tagged VLANs on trunk lines (& servers)• VLANs make it possible to define a LAN per work group

independently of geography• VLANs can be used to enhance the manageability and

security of the network and simplify routing


Gigabit Ethernet and its significance• Gigabit Ethernet is the most powerful off-the-shelf LAN

technology and is already priced quite competitively• Lots of products are on the market and their compatibility is

already proven• According to the standard, the maximum segment length is

550 m in multimode and 5 km in single mode fiber• Commercial products normally work up to 10 km but special

long-reach versions can work up to over 100 km• A copper version has recently arrived to the market:

• It uses all the four pairs of a UTP cable• Requires cabling that clearly exceeds Cat. 5 requirements• Especially the RJ-45 connectors used have to be of quality

• Gigabit Ethernet already is a significant network technology


A modern local area network

100/10 Mb/s EthernetUTP5

Workstations and servers

Optical fiber

LANswitch

…

LANswitch

…

LANswitch

…

LANswitch

…

…

Gigabit EthernetBack-boneR/X

Backbone network

Wiring closets


In-house cabling • Cabling is the IT investment with the longest life time• Structured cabling

• One or several wiring closets on every floor• Enough space for routers and switches• Enough fiber in the backbone cabling between them• Cross connections for fibre and copper in the closets• UTP cat. 5 (currently, in the future probably something

else) cabling from the closets to the outlets (max. 90 m)• RJ-45 connectors, 4 per office desk• Adequate power outlets to the closets and office rooms

=> A network that can adapt to changing needs:• Data and telephony in the same network• Traditional LANs: Ethernet, Token Ring etc.• Fast LANs: 100 Mb/s - 1 Gb/s Ethernet• ATM 25 – 155 – 622 Mb/s


Wireless LANs • Wireless LANs (WLANs) appear in too configurations:

• infrastructural - wireless transmission replacing cables• ad-hoc - networks formed without prior arrangements

• Most WLANs use spread spectrum technologies:• Frequency Hopping Spread Spectrum (FHSS) - uses a

narrow-band carrier changing frequencies in a pre-defined manner known to transmitter and receiver

• Direct Sequence Spread Spectrum (DSSS) - generates a redundant bit pattern ("chip" or "chipping code") for each bit transmitted

• Spread spectrum technologies use a wider frequency band than narrow-band technologies but are more robust

• Other users of the same frequency band hear spread spectrum traffic as background noise


WLANs • The IEEE 802.11 WLAN standard is already widely

implemented in commercial mass products• Current products operate at 2.4 GHz frequency at 11 Mb/s• New products at 5 GHz frequency an 54 Mb/s are coming to

the market soon• 100 Mb/s and faster WLANs already operate in laboratories• Blue tooth offers limited low-cost low-capacity WLAN

technology for a variety of devices• Blue tooth’s main purpose is to replace wires in telco devices• WLANs promise high capacities at low cost within buildings• Their greatest strength is that they operate at unregulated

frequencies• Outside buildings this may also be their greatest weakness


ATM basics• ATM stands for Asynchronous Transfer Mode• ATM was designed as the technology of Broadband-ISDN• ATM is based on switching fixed-sized cells• ATM operates in the Data Link layer of the OSI model• Connection oriented• Small, fixed-size (53 octets) cells make hardware implementa-

tions fast, reliable and (in large quantities) inexpensive• ATM does not define the MAC or Physical layer =>

it can easily adapt to new transmission speeds and systems• The same technology in LAN, MAN and WAN =>

seamless integration of services• European ATM systems in WANs will mainly be based on

SDH (Synchronous Digital Hierarchy) infrastructure• ATM is already becoming obsolete


Interconnecting networks• The main trend of internetworking continues to be the

interconnection of ever faster LANs at ever higher speeds• Devices used in interconnection of networks (LANs)

• Repeater – physical layer, handles bits(and possibly non-data symbols)

• Bridge – data link layer, handles frames• Switch – data link layer, handles frames• Router – network layer, handles packets• Gateway – upper layers, used to interconnect totally

different systems (such as a DECNet-SNA gateway)• Internetworking is based on routers and network layer

addresses (IP addresses) • All the devices connected into the network must use the

same network protocol - the Internet Protocol (IP)


• The main trend in telecommunications since 1980• Increasing capacities in the local and wide area• Internet = network of networks

WAN

MAN

LAN

LANLAN

MAN

LAN

LAN

LAN

LAN

LAN

LAN LAN

LAN

LAN

LAN

Internetworking


Terminology • Internetworking = the interconnection of separate

(and often different) communications networks into an internetwork (or internet, a network of networks)

• An internetwork typically consists of a large number of Local Area Networks (LAN), Metropolitan Area Networks (MAN) and Wide Area Networks (WAN) interconnected via routers

• The Internet = a global open internetwork• Internet technology can also be used to implement closed

corporate networks known as intranets• Two or more interconnected intranets are called an extranet• An internetwork provides only an unreliable connectionless

datagram service between any two computers• It is the services and applications implemented in the

computers that make the internetwork interesting


Growth of the Internet


The internet abstraction • For the connected machines an internetwork appears to be a

uniform network with each host identified by its IP address• Any machine can directly communicate with any other

machine

Internetwork

Work station

Server

IP addresses

130.233.1.65

130.233.2.63

130.233.2.1

130.233.1.1

130.233.4.244 130. 233.3.3


Real structure of an internet • An internetwork consists of different physical (sub)networks

interconnected via routers as illustrated by the example below

130.233.1.65

130.233.2.63

130.233.2.1

130.233.1.1

130.233.4.244130. 233.3.3

R R

RR

R

Router

Physical network


Internet architecture • In an internetwork separate networks are interconnected into

a larger virtual network• All networks are treated as “equal”, such as:

• LANs• Frame Relay connections• Leased lines

• Routing is based on Internet Protocol (IP) and its addresses• An IP address consists of a network prefix and the number of

the node in the network• A routers forwards packets from one network into another

based on the recipient’s IP address

router 1 router 2network 1 network 2 network 3


Internet layer model The Internet model only has four layers:

higher-level protocols

TCP / UDP

Internet Protocol

communication networks


Critique of the Internet model • No well defined terminology (inconsistencies and

ambiguities)• No clear distinction between protocols, services and

interfaces• Protocol definitions are often closely tied up with

implementational issues• Cannot describe all telecommunications systems• No distinction between physical, data link and lower part of

the network layer (just one "physical network")• Nevertheless, Internet has proved its power and continues to

be the leading network architecture• The OSI model with layers 5 and 6 combined into layer 7

can be used to discuss Internet and other protocols• In this course we will use the OSI reference model as a

generic framework for both telecom and datacom systems


Internet and OSI layer structures • The figure below shows the layers of Internet and those of OSI• In some cases (such as X.25) the ”physical network” of

Internet reaches up to the network layer of OSI• The Internet Protocol (IP) can be run on virtually any network,

e.g. LAN, leased line, ATM, Frame Relay, X.25 or packet radio

1. Physical layer

2. Data link layer

3. Network layer

4. Transport layer

5. Session layer

6. Presentation layer

7. Application layer

IP

TCP UDP

Application level protocols:

etc.

IEEE 802.3, FDDI, E1, ATM, Frame Relay etc.

application processes

media

OSI Internet

Telnet FTP SMTP HTTP SNMPDNS ISAKMP


How layering works in practice • Each protocol entity sends its PDUs using the service

provided by the layer immediately below• (N)PDU (including its header) is packed into the data field of

an (N-1)PDU• At the receiving end, the headers (and possible trailers) are

removed and decoded in reverse order• Example: sending a UDP message over IP and Ethernet

UDP-header UDP data

IP-header IP data

E-net-header Ethernet data CRC


A brief history of the Internet • In 1957 the successful launching of the Soviet ‘Sputnik’

lead into the founding of the US Department of Defense (DoD) Defense Advanced Research Projects Agency (DARPA)

• In 1968/1969 DARPA started the development of ARPANET• The purpose was to develop an information network for the

use of the R&D projects of DoD• Telecommunications software turned out to be much larger

and more complex than anticipated• In 1974 Robert Kahn and Vinton Cerf published a paper

specifying the TCP/IP protocol• Around 1975 DARPA started the massive development of

modern internet technology• By 1979 so many scientists were involved in this work that

the Internet Control and Configuration Board (ICCB) was founded to coordinate it


A brief history of the Internet • In 1980 DARPA started converting their machines into TCP/IP

and ARPANET became the backbone of the new Internet• In 1981 the National Science Foundation (NSF) provided the

seed money for CSNET (Computer Science NETwork) to inter-connect the computer science departments of US universities

• In 1982 DoD defined the TCP/IP protocol as their standard• TCP/IP was included in Berkeley Unix (BSD) which was used

in about 90% of all university machines in those days• In 1985 NSF started to develop the TCP/IP based NSFNET as

an access network for its super computers• In 1986 the NSFNET backbone operated at 56 kb/s • Around 1986 commercial TCP/IP implementations started to

proliferate


A brief history of the Internet • In 1988 TCP/IP was specified as the preferred protocol set to

be used by the Finnish public sector (”Toimisto -90”)• In 1988 the Finnish University Network (FUNET) was con-

nected to the Internet (together with its Swedish, Norwegian and Danish counterparts) via the Nordic University Network (NORDUNET)

• In 1989 Telecom Finland (now Sonera) started its commercial IP service DataNet

• DoD had published its plans to replace TCP/IP by OSI protocols in the early 1990’s

• This lead into intensive development of OSI profiles which turned out to be a total flop


Recent history • In 1991 NSF lifted the restrictions on the commercial use of

the Internet• In 1991 World Wide Web (WWW) software was released by

CERN (the European Nuclear Research Center)• The Mosaic browser, developed by the National Center for

Supercomputer Applications (NCSA), was released in 1993 • In 1995 US Internet traffic was carried by commercial Internet

Service Providers (ISPs)• In 1998 the number of Internet hosts passed 50 million• Internet has established itself as the leading network

architecture that even public telco networks have to adapt to


Official IT standardization bodies • International Telecommunication Union (ITU) -

a suborganization of the United Nations (UN)• ITU-T (former CCITT)

• International Organization for Standardization (ISO)• ANSI (American National Standards Institute)• BSI (British Standards Institute)

• IEC (International Electrotechnical Commission)• JTC1

(Joint Technical Committee 1 on Information Technology)• CEN/ISSS (European Committee for Standardization /

Information Society Standardization System)• CWA (CEN Workshop Agreement)• TIEKE (Tietotekniikan Kehittämiskeskus,

Finnish Information Technology Development Centre)


Official IT standardization in Europe

IEC ISO CEN

JTC1 ISSS

TIEKE

Vienna Convention


Official IT standardization bodies • International Telecommunication Union (ITU) -

a suborganization of the United Nations (UN)• ITU-T (former CCITT)

• International Organization for Standardization (ISO)• ANSI (American National Standards Institute)• BSI (British Standards Institute)

• IEC (International Electrotechnical Commission)• JTC1

(Joint Technical Committee 1 on Information Technology)• CEN/ISSS (European Committee for Standardization /

Information Society Standardization System)• CWA (CEN Workshop Agreement)• TIEKE (Tietotekniikan Kehittämiskeskus,

Finnish Information Technology Development Centre)


Other standardization organizations • ETSI (European Telecommunications Standards Institute)

• ETSI's main achievement is GSM• NIST (National Institute of Standards and Technologies,

former NBS, National Bureau of Standards)• IEEE (Institute of Electrical and Electronics Engineers)

• IEEE 802 LAN/MAN standards• EIA (Electronics Industries Association)

• EIA-232D• ECMA (European Computer Manufacturers' Association)• 3GPP (3rd Generation Partnership Project)• UNICODE• OMG (Object Management Group)• W3C (WWW Consortium)• ATM Forum (ATMF)• IETF (Internet Engineering Task Force)


Internet standardization • The Internet standardization process has always been very

pragmatic aiming at ”a rough consensus and working code”• This has led into a considerably faster standardization

process than is possible at for example ISO, ITU-T or ETSI• All Internet standards are RFCs but not all RFCs are standards• Some bodies participating in Internet standardization:

• Internet Engineering Task Force (IETF) is divided into working groups developing RFCs

• Internet Architecture Board (IAB) is responsible for defining the overall architecture of the Internet and providing guidance for IETF

• Internet Engineering Steering Group (IESG) is responsible for technical management of IETF activities and the Internet standardization process

• Internet Society (ISOC) is a professional membership orga-nization commenting on policies and overseeing activities


Standards vs. de facto standards • Technological discontinuities:

• Automization and digitalization of the PSTN• Routers• Mobile phones and networks

• Networked products -the product only has value with other products

• Ever shorter technology and product cycles• Globalization of the market and increasing competition• Time to market is the key to success• Market-driven product/service development

• Choices of the market are replacing standards• “Dominant Designs” are replacing standards

(PC, Windows, intel x86, mobile phone...)• Learning together with the customer

• The role of standardization is changing

Documents

Network Architectures, Interconnecting NetworksTik-110.250 Fundamentals of Network Media, 25.1.-1.2.2001, slide 1 © Arto Karila Tik-110.250 Fundamentals of Network Media (3 cr) Spring