" The Wise Way to The World
of Communications"

NETWORKING UNDERLYING

CONCEPTS

 

 

Table of Contents  COMMUNICATION TOOL  NETWORK ARCHITECTURES  TIME DIMENSION  NETWORK PROTOCOLS  USER& SPACE DIMENSIONS  SUMMARY

 

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 In this WWWnet section, we will address the networking 4*W questions:

• The What: Information contents and users of networks
  

• The When: "Time" dimension of networks
  
• The Where and Who: "Space" and "User" dimensions
  
• The hoW : Network architectures and protocols

• NETWORK: UBIQUITOUS COMMUNICATION TOOL

  We spend most of our time to communicating at home with our family and our friends, with our society, at work with our colleagues, our managers, our employees.
  We also communicate with non-human beings: answering machines, mail and voice boxes, computers, information servers (data, audio, video), interactive games, and so on. It is just incredible how many communication tools we are using (with quite often awkward user interfaces). Indeed we can't stay passive with information. We need to interact with it and exercise our own control. We need also to communicate it.

  The network drivers are:

  • distributed processing : In a company there is a tight relationship between a company network, its information system and its human organisation.
     
    It's common place that the companies organisation is flattening with less hierarchical levels and the day to day decision delegated at the lowest level to increase efficiency and customer reactivity. Work is carried out more in groups rather than individually.

    

    This trend is reflected in network architectures evolving from centralised (around the host computer) to partially distributed "client/server" and eventually to fully distributed ("peer to peer").
    Also increasingly networks are tuned to work-groups with the group members being interconnected locally or remotely through their network which becomes then a vital group resource.

   
  • Multimedia communication applications. The number of communication applications grow rapidly (see the following table)they use different information media and progressively multimedia. A good example is the Web where information was initially represented textually and is moving to pictural, sound and video representations. The famous HTML information descriptive language has evolved a lot accordingly since its creation but is about to be complemented with the VRML (Virtual Reality Mark up Language) to address the new multimedia "Virtual Reality" applications.
    

    Telephony	Distributed data access	Public services
    Facsimile	Remote data base access	Home shopping
    Email	Remote computing	Entertainment imaging
    Voice mail	Collaborative computing	Video on Demand
    Distance learning	Telecommuting	Audio CD on Demand
    Audioconferencing	Industrial Engineering	Interactive games
    Videoconferencing	Distance customer training	Virtual reality
    Medical Imaging	Electronic Fund Transfer	Education
    Electronic publishing	Electronic commerce	Home study
    Desktop publishing	Financial data and trading

     

    Communication Applications

  Each media (voice, data, video) has had and still has its own dedicated network. Multimedia and integrated multiservice networks tend to appear with the Internet (Voice over Internet is not yet practical for everyone but it will come) and the future Broadband ISDN. This issue will be addressed at length later in the document.

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•  NETWORK - The "TIME" DIMENSION
   
   

  Communication quality and efficiency relies upon latency, that is the time to transmit information end to end, and on transmission bandwidth or Transfer rate.

  • Latency : the latency issue will be addressed in other WWWnet Web chapters but it's critical to transmission quality, especially for media such as voice or video which requires real time transmission. On the other data usually tolerates transmission delays, property that is used in data networks to concentrate multiple data traffics.

    Don't confuse latency and bandwidth. High bandwidth doesn't imply necessarily low latency, especially when you share bandwidth with other users. Currently high speed packet switching networks (IP or X25) cannot carry properly voice traffic. They have no control of the transmission delay.
    

  • Bandwidth : two main factors are driving up the bandwidth requirement of communication networks:
    • distributed computing (rather than centralised as in the past) with endless increasing processing power available at the desktop and now at home, which implies increasing communication bandwidth needs as shown below.
       
      1 Mips of processing power needs more than 1 Mbps of network bandwidth ("Mbps/Mips" factor increasing over time), even if media processing such as compression can reduce the amount of required bandwidth.

      

     
    • multimedia communication : computer information is more practical to understand, to use and to convey when it's a mixture of words, pictures, sound, and even video. The same applies for communication between individuals.
       
      It is not only a question of comfort; it does improve efficiency in communicating and processing information. It is a factor of competitiveness.
      Remember the proverb: " a picture is worth a thousand words "

      But audio, pictural or video communication requires much more bandwidth than text as underlined in the following figure showing, for a given communication media the number, of simultaneous communications carried by transmission channels of different transfer rates.
      With a 2.4 Gbit/s channel no more than 40 HDTV communications can take place concurrently.

       

      Media compression reduces network load. Typical compression factors range from 10 to 100 but at the expense of less quality and less content transparency.
      So there is an exponential growing need of network bandwidth, with a rapid change from Low/Medium speed networks to High/Very High speed ones. The Time (throughput and latency) is indeed a shaping dimension of networks

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• NETWORK - THE "SPACE & USER" DIMENSION

Space Dimension : Communication takes place in a space where the communication actors (human beings, machines) are present. There are varieties of networks with different technologies that fit to different communication spaces (see the table below).

 


NETWORK TYPE	DEFINITION	DISTANCE RANGE*	COMMUNICATION SPACE
HAN	Home Area Network	0.1 km	home
LAN	Local Area Network	0.1 to 1 km	building, floor, room
CAN	Campus Area Network	1 to 10 km	campus, company site
MAN	Metropolitan Area Network	10 to 100 km	city
WAN	Wide Area Network	100 to 10000 km	region, nation
GAN	Global Area Network	around the earth	multinational zones, world

 

• User Dimension : Communication can be public (open to everyone) or private (restricted to a special user community, a company for instance). Accordingly, there are public and private networks.

  Moreover networks are dependent on the user community (Research, University, Enterprise, on their activity and the computing means they use (IBM, DEC, Microsoft, Novell,...) .

  The user and space dimensions are indeed closely related: the LAN network connects the people in the same company building, the MAN in the same city.
  But with telecommuters or home workers, the relationship between space and user community (Engineering or Sales in a company) is broken.
  A LAN work-group network should also link remote group members. A new type of network addresses this need: the VIRTUAL LAN, more generally VIRTUAL network which is space independent but user dependent.

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• NETWORK ARCHITECTURES

   

  They reflect the Time, Space, User dimensions.

  • Organisation or geography driven network architectures : large communication networks, with many connected users, are organised in (sub)networks in order to optimise the traffic flows.
     
    Network users of a same "space" or a same "group" (including a "computer application community" such as IBM) will be agglomerated within a sub-network to keep the traffic local (in space in usage) as much as possible. The traditional way of network traffic partitioning is by the geography (city, region, state, country). A good example is the US telephone network split into intra and inter LATA zones with different network carriers but also the Internet that agglomerates hundreds of networks.
    The sub-networks are interconnected by the backbone network, usually also provided by a different carrier.

    In the Data communication field, usually the Enterprise networks are divided into:

    • Access and Backbone networks in the WAN
      
    • Work-group and Backbone networks in the LAN.
    The Enterprise network layered architecture reflects the flows of information and tends to optimise the overall traffic. A complete large LAN/WAN Enterprise network is depicted below.

    

    The users are connected to the LAN work-group sub-networks, themselves to the LAN backbone network.
    LAN networks are interconnected through the WAN via the WAN access and backbone networks. Isolated users, such as home workers, are connected directly to the WAN to access the Enterprise network.

    Companies with many dispersed agencies, close to the customers, (as banks, insurance companies, post offices,..), have a lot of WAN connections hierarchically organised in a "three tier" configuration with "Branch Offices", "Regional Offices "Headquarters".

    

    The traffic flows increase from the branch offices to the headquarters by roughly a factor of 10 at each level.
    With this tendency to more geographical dispersion and with the : "small offices" with a very few employees and "home offices".

   
  • Media driven network structures : As seen earlier, the different media (sound, data, video) carrying information have different transmission requirements :
   

   voice, sound and video : real time continuous transmission is a must . They can't suffer too much delay because of echo phenomenon that degrades sound quality. The transmission delay has to be fixed or with very small variations. The traffic is usually continuous and constant overtime, except for compressed video with a variable coding bit rate.

   data : error free transmission is usually a request. Variable delays are well tolerated because of their "message" format. Large delays can be also annoying in interactive data applications. Data has a bursty traffic characteristics with long idle times.

   

  Due to those differences, currently different networks are used:

  Voice networks : Inside a company they are structured around the PABX with a dedicated cabling (wired or wireless) to connect the telephone sets. Outside, it is the PTT telephone network.

  Data networks : structured around LAN and WAN data networks. Although the voice network is used to carry data signals, there are also more efficient dedicated PTT data networks: PSDN (Packet Switched Data Network), CSDN (Circuit Switched Data Network). INTERNET is in that category of dedicated data networks.

  Video networks: They are broadcast networks with little inter-activity and use satellites and cables.

  Multimedia networks : The current network infrastructures do not address the multimedia and multiservice communications where voice, data, fax, video signals are simultaneously transmitted. An attempt has been made with ISDN that can transport voice, fax and data but not video which will be taken into account by the future Broadband ISDN.
  To day considerable efforts are devoted to set up experimental new multimedia networks, so called information highways. INTERNET is a good candidate but the embedded protocols need to evolve to carry real time signals as voice and video.

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• NETWORK PROTOCOLS

  Communications involve terminals, communication applications and the linking network. They all interact to assure proper transportation (Link set up, transfer, Link release) and optimal use of the network transport services.
  In order for this interaction to take place, a communication protocol is necessary between the end user and the network but also within the network, between the different network nodes. It will be in charge of establishing end to end routes according to the user's request, of transferring information reliably and safely, monitoring and controlling the network.

  To structure these communication interactions and implied services, a reference model has been defined: the Open Systems Interconnect Reference Model (OSIRM).

  

   The seven layers OSI model

   

  Other models exist such as the IBM, DEC, Novell and Microsoft ones to structure communication exchanges. They are quite similar and tend to converge to the OSI model at the network level with IP being the ubiquitous data gram protocol
   
   

  • OSI layer services : they are summarised in the following table:
   
  

  LAYER	SERVICE	NETWORKS
  Physical	Transparent bit stream transmission	Analog: Leased Lines & PSTN - Digital: DLL, SONET/SDH, ISDN - LAN: ETH, TR, FDDI
  Data Link	Error detection/correction- Multiplexing-Traffic management-Media access (MAC)	LAN, FR, ATM, SNA
  Network	End to end addressing-Routing-Switching -Multiplexing-In sequence delivery-Flow control-Traffic management	X25, IP, SMDS, SNA Switched FR, ATM, ISDN, PSTN
  Transport	segmentation,reassembly-multiplexing-Flow control-Error correction	Terminal , Computer Domain
  Session	connection management-flow management-address translation
  Presentation	user's data formatting
  Application	Mail, Directory services, File transfer, Remote terminal, Audioconferencing,...
  NOTES: 1 Acronyms: PSTN : (Public Switched Telephone Network) - DLL : (Digital Leased Lines) ETH: Ethernet - TR: Token Ring - MAC: Media Access Control - FR: Frame Relay - ATM: Asynchronous Transfer Mode - SNA: System Network Architecture (IBM) - X25: Packet switching - IP: Internet Protocol - SMDS: Switched Multimegabit Data Service - 2. A layer can only provide a subset of the services listed here. IP does not assure an "in sequence delivery" of messages.

   
  • communication between OSI layers: the seven independent layers interact at two levels:
    • between consecutive layers via a service interface, so called SAP (Service access point), through which layer services are provided,
      
    • within a layer, between the local and remote ends, via a layer protocol
    Information is exchanged through messages so called layer Protocol Data Units (PDU). Each layer will send its PDU to the next one which will then create its own PDU containing the received message and add to it its own information to interact with the remote layer end.
   

  

  Network Communication protocols

   

  After jumping through these seven layers, the initial user message will have a lot of overhead added by the different layers it went through.
  But the big advantages are their open (making possible any to any communication) and perennial attributes, thanks to the independent layer structuring, hiding communication applications from the variety and constant changes of the communication networks. Indeed the same computer application (File transfer, for instance) can run on Ethernet, Token Ring, X25, Frame Relay, ATM, ISDN networks.

  So, networks are also structured by service layers with dedicated layer networks. It's a fifth dimension to the network structuring, in addition to time, space, user and media, although all of them are not independent, which makes networks complicated to understand.

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• SUMMARY

  After this long digression on the underlying concepts of networks, let make it straight and simple.

  Two fundamental elements shape networks: their architecture and their embedded protocol.

  • Architecture depends on key factors such as the time (latency, bandwidth), the geography, the user organisation (including its community of communication usage), the transported information media and also the communication services.
     
    They are optimised according to those dimensions in different ways: to minimise transmission cost, throughput time, to insure reliability and safety, to integrate various media, and so on, this to fit to the customer requirements, with the everlasting dilemma of dedicated or universal or ubiquitous networks.
    
  • Protocols do provide the network services, in an orderly way as modeled by the OSI layer model. They structure the communication exchanges and do contribute to the quality of service provided to the customer.
  In the following chapters, will be addressed the architectures and protocols of Backbone and Access Networks

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