A closer look at network structure:
       network edge: hosts: 
       •   run application programs             mobile network
           e.g. Web, email, …
       •   … based on network 
           services available                                      global ISP
                                                     home 
                                                     network     regional ISP
  access networks:
  •     connect end-hosts to the Internet                                  network core: 
        (edge routers)                                                     •    interconnected 
  •     through physical media: wired,                                          routers
        wireless links                                                     •    network of 
                                                                                networks
                                            institutional
                                                   network
     Marina Papatriantafilou –  Introduction to computer communication, partB –edge&core                  2
       A closer look at network structure:
    network edge: hosts:          application
                                   transport            access networks:
    •  run application programs                         •   connect end-hosts to the Internet 
       e.g. Web, email, …          network                  (edge routers)
    •  … based on network             link              •   through physical media: wired, 
       services available          physical                 wireless links 
                                                                      link
                                                                   physical
   Types of communication service available by the transport layer @ 
   Internet:                                                                     switch
   •  connection-oriented: reliable, in-order data delivery (TCP)
   •  connectionless: “best effort”, arbitrary order data-delivery (UDP) 
   Q: How & based on what “core” functionality? 
        (…main Q for the course….)                                network
                         application                                link
                          transport                               physical
             UDP TCP       network
                             link                           network core:        router
                           physical                         •   interconnected 
                                                                routers
                                                            •   network of networks3
     Marina Papatriantafilou –  Introduction to computer communication, partB –edge&core 
       Roadmap
          1. Zooming into core
               – Ways of data transfer
               – Routing
               – Performance: delays (& loss)
               – …. throughput
          2. Network/Internet structure complemented:
               – access net, physical media
               – backbones, NAPs, ISPs
          3. Security prelude
     Marina Papatriantafilou –  Introduction to computer communication, partB –edge&core           3a-4
       The Network Core
     •   mesh of interconnected routers
     •   fundamental question: how is data 
         transferred through net? 
     •   packet-switching: data sent thru 
         net in discrete “chunks”
     Marina Papatriantafilou –  Introduction to computer communication, partB –edge&core            1-5
       Network Core: Packet Switching
                 10 Mbs                                                  C
     A           Ethernet
        B                                1.5 Mbs
                       queue of packets                            45 Mbs
                      waiting for output
                             link
                                        D                              E
              Application messages divided into packets
              •   packets share network resources 
              •   resources used as needed 
              store and forward:
              •   packets move one hop at a time
                   – transmit over link; wait turn at next link
     Marina Papatriantafilou –  Introduction to computer communication, partB –edge&core             6