1. Lecture 2 1-1 Internet Overview: roadmap 1.1 What is the Internet? (A simple overview last week) Today, A closer look at the Internet structure! 1.2 Network edge  end systems, access networks, links 1.3 Network core  circuit switching, packet switching 1.4 Delay, loss and throughput in Internet 1.5 Protocol layers, service models 1.6 Networks under attack: security

2. Lecture 2 Recap: What are the components of Internet?  End-users (Hosts)  e.g. computers  access networks, physical media:  wired, wireless communication links  network core:  interconnected routers  network of networks 1-2

3. Lecture 2 End-users (Hosts)  End-users (hosts):  run application programs  e.g. Web, email  Hosts further divided into  Client Hosts  Server Hosts  Two different models of networking  client/server model client host requests, receives service from always-on server e.g. Web browser/server; email client/server  peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent client/server peer-peer 1-3

4.  Client/server model is the dominant design for Internet applications  server - is the information provider  client - is the information consumer  example  web server and a client running web browser  a CNN web server simultaneously serves thousands of clients. Lecture 2 The Client/Server Model 1-4

5. Lecture 2 Hosts are not sufficient for networking!  End-users (hosts):  run application programs  e.g. Web, email  But, hosts alone would not be enough  We need to connect the hosts  HOW? 1-5

6. Lecture 2 Access networks and physical media Q: How to connect end systems to edge router? 1. residential access nets 2. institutional access networks (school, company) 3. mobile access networks 1-6

7. Lecture 2 Residential access: point to point access  Dialup via modem  up to 56Kbps direct access to router (conceptually)  ADSL: asymmetric digital subscriber line  up to 1 Mbps home-to-router  up to 8 Mbps router-to-home  ADSL deployment: happening 1-7

8. Lecture 2 Residential access: cable modems  HFC: hybrid fiber coax  asymmetric: up to 10Mbps upstream, 1 Mbps downstream  network of cable and fiber attaches homes to ISP router  shared access to router among home  issues: congestion  deployment: available via cable companies, e.g., MediaOne, CableVision 1-8

9. Lecture 2 Institutional access: local area networks  company/univ local area network (LAN) connects end system to edge router  Ethernet:  shared or dedicated cable connects end system and router  10 Mbps, 100Mbps, Gigabit Ethernet  deployment: institutions, home LANs happening now 1-9

10. Lecture 2 Wireless access networks  shared wireless access network connects end system to router  wireless LANs:  radio spectrum replaces wire  e.g., 802.11b/g (WiFi): 11 or 54 Mbps  wider-area wireless access  next up (?): WiMAX (10’s Mbps) over wide area base station mobile hosts router 1-10

11. Lecture 2 1-11 Internet Overview: roadmap 1.1 What is the Internet? (A simple overview last week) Today, A closer look at the Internet structure! 1.2 Network edge  end systems, access networks, links 1.3 Network core  circuit switching, packet switching 1.4 Delay, loss and throughput in Internet 1.5 Protocol layers, service models 1.6 Networks under attack: security

12. Lecture 2 1-12 The Network Core  Internet: mesh of interconnected routers  How is data transferred through net?  circuit switching: dedicated circuit per call: telephone net  packet-switching: data sent thru net in discrete “chunks”

13. Lecture 2 1-13 Network Core: Circuit Switching  Telephone call like mechanism  End-end resources reserved for “call”  dedicated resources: no sharing (link bandwidth)  circuit-like (guaranteed) performance  call setup required

14. Lecture 2 1-14 Network Core: Circuit Switching  Total network resources (e.g., bandwidth) divided into “pieces”  pieces allocated to calls  resource piece idle if not used by owning call (no sharing)  dividing link bandwidth into “pieces”…HOW?  frequency division multiplexing (FDM) Users use different frequency channels  time division multiplexing (TDM) Users use different time slots

15. Lecture 2 1-15 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:

16. Lecture 2 1-16 Numerical example 1  You need to send a file of size 640,000 bits to your friend. You are using a circuit-switched network with TDM. Suppose, the circuit-switch network link has a bit rate of 1.536 Mbps (1Mb = 10 6 bits) and uses TDM with 24 slots. How long does it take you to send the file to your friend? Let’s work it out!

17. Lecture 2 1-17 Disadvantages of Circuit-Switching  Only static number of users  This number must be fixed before the actual operation  Each user gets only a “piece of the pie” even if the other users are possibly idle  Prev. example: I get only 1/24 th of the entire time  Resource wastage  Impossible to admit new user in the middle of the operation

18. Lecture 2 1-18 Packet Switching A B C 100 Mb/s Ethernet 1.5 Mb/s D E queue of packets waiting for output link

19. Lecture 2 1-19 Network Core: Packet Switching each end-end data stream divided into packets  user A, B packets share network resources  each packet uses full link bandwidth  resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation

20. Lecture 2 1-20 Packet switching versus circuit switching  Adv: Packet switching allows users to use the network dynamically!  resource sharing  simpler, no call setup  New user can enter or leave inside the operation  Is there any downside of packet switching?  With excessive number of users packet delay and loss  Efficiency of the system (measured in throughput) drops!

21. Lecture 2 1-21 How do delay and loss occur? packets queue in router buffers  store and forward: packets move one hop at a time  Router receives complete packet before forwarding  packets queue, wait for turn…DELAY A B packet being transmitted (delay) packets queueing (delay)

22. Lecture 2 1-22 Four sources of packet delay  1. nodal processing:  check bit errors  determine output link A B propagation transmission nodal processing queueing  2. queueing  time waiting at output link for transmission  depends on congestion level of router

23. Lecture 2 1-23 Delay in packet-switched networks 3. Transmission delay:  R=link bandwidth (bps)  L=packet length (bits)  time to send bits into link = L/R 4. Propagation delay:  d = length of physical link  s = propagation speed in medium (~2x10 8 m/sec)  propagation delay = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantities!

24. Lecture 2 1-24 Total delay  d proc = processing delay  typically a few microsecs or less  d queue = queuing delay  depends on congestion  d trans = transmission delay  = L/R, significant for low-speed links  d prop = propagation delay  a few microsecs to hundreds of msecs

25. Lecture 2 1-25 Numerical example 2  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero propagation delay. Let’s work it out! R R R L A B

26. Lecture 2 1-26 Packet loss  queue (aka buffer) preceding link in buffer has finite capacity  packet arriving to full queue dropped (aka lost)  lost packet may be retransmitted by previous node, by source end system, or not at all A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area)

27. Lecture 2 1-27 Throughput  throughput: rate at which information bits transferred between sender/receiver RsRs RsRs RsRs RcRc RcRc RcRc R

28. Lecture 2 1-28 Numerical example 3: Throughput RsRs RsRs RsRs RcRc RcRc RcRc A B  Example: A has requested for a packet (size 640,000 bits) from server B. The packet will come through an intermediate router C. It takes 0.1 second for the packet from B to C and 0.4 seconds from C to A. (Note: 1Mb=10 6 bits). Assume zero propagation delay.  What is the throughput from B to C?  What is the throughput from C to A?  What is the average throughput from B to A? Let’s work it out! C