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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
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
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:
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
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
• 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
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
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
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
• 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)
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
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
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