1. CSE401N1 CSE401N Computer Networks Lecture-2 Network Structure[KR-1.2+1.3+1.4] S. M. Hasibul Haque Dept. of CSE BUET

2. CSE401N2 A closer look at network structure: r network edge: applications and hosts r network core: m routers m network of networks r access networks, physical media: communication links

3. CSE401N3 The network edge: r end systems (hosts): m run application programs m e.g., WWW, email m at “edge of network” r client/server model m client host requests, receives service from server m e.g., WWW client (browser)/ server; email client/server r peer-peer model: m host interaction symmetric m e.g.: Gnutella, KaZaA

4. CSE401N4 Network edge: connection-oriented service Goal: data transfer between end sys. r handshaking: setup (prepare for) data transfer ahead of time m Hello, hello back human protocol m set up “state” in two communicating hosts r TCP - Transmission Control Protocol m Internet’s connection- oriented service m Why not connected? TCP service [RFC 793] r reliable, in-order byte- stream data transfer m loss: acknowledgements and retransmissions r flow control: m sender won’t overwhelm receiver r congestion control: m senders “slow down sending rate” when network congested

5. CSE401N5 Network edge: connectionless service Goal: data transfer between end systems m same as before! r UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service m unreliable data transfer m no flow control m no congestion control App’s using TCP: r HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP: r streaming media, teleconferencing, Internet telephony

6. CSE401N6 The Network Core r mesh of interconnected routers r the fundamental question: how is data transferred through net? m circuit switching: dedicated circuit per call: telephone net m packet-switching: data sent thru net in discrete “chunks”

7. CSE401N7 Network Core: Circuit Switching End-end resources reserved for “call” r link bandwidth, switch capacity r dedicated resources: no sharing r circuit-like (guaranteed) performance r call setup required

8. CSE401N8 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” r pieces allocated to calls r resource piece idle if not used by owning call (no sharing) r dividing link bandwidth into “pieces” m frequency division m time division r dividing link bandwidth into “pieces” m frequency division m time division

9. CSE401N9 Network Core: Circuit Switching

10. CSE401N10 Circuit Switching: TDMA and TDMA FDMA frequency time TDMA frequency time 4 users Example:

11. CSE401N11 Circuit Switching: Resources (Frequency and Time) r Divide link bandwidth— the resource--into “pieces” m frequency division multiplexing (FDM) m time division multiplexing (TDM)

12. CSE401N12 Circuit Switching: The Process r Three phases 1. circuit establishment 2. data transfer 3. circuit termination r If circuit not available: “busy signal”

13. CSE401N13 circuit establishment DATA data transmission circuit termination Host 1Host 2 Node 1Node 2 propagation delay from Host 1 to Node 1 propagation delay from Host 2 To Host 1 processing delay at Node 1 Timing Diagram of Circuit Switching

14. CSE401N14 Delay Calculation in Circuit-Switched Networks Transmission delay: r R = bandwidth (bps) r L = packet length (bits) r time to send a packet into link = L/R Propagation delay: r d = length of physical link r s = propagation speed in medium (~2x10 5 km/sec) r propagation delay = d/s r Propagation delay: delay for the first bit to go from source to destination r Transmission delay: time to pump data onto link at reserved rate

15. CSE401N15 An Example r Propagation delay m suppose the distance between host 1 and host 2 is 4000 km, then one-way propagation delay is: r Transmission delay m suppose we reserve one slot of a T1 line, which has a bandwidth of 1.536 Mbps is divided into 24 slots, and thus each reserved slot has a bandwidth of 64 Kbps m then the transmission delay of a file with 6.4 Kbits is r Suppose the setup message is very small, and the total setup processing delay is 200 ms

16. CSE401N16 An Example (cont.) r Then the delay to transfer a 6.4 Kbits file from host 1 to host 2 (from the beginning until host receives last bit of the file) is: DATA 20 + 200 20 100

17. CSE401N17 Network Core: Packet Switching each end-end data stream divided into packets r user A, B packets share network resources r each packet uses full link bandwidth r resources used as needed, resource contention: r aggregate resource demand can exceed amount available r congestion: packets queue, wait for link use r store and forward: packets move one hop at a time m transmit over link m wait turn at next link Bandwidth division into “pieces” Dedicated allocation Resource reservation

18. CSE401N18 Packet Switching Each end-to-end data flow divided into packets r Packets have the following structure: Header and Trailer carry control information (e.g., destination address, check sum) (where is the control information for circuit switching?) r At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks) r Each packet is passed through the network from node to node along some path (Routing) HeaderData Trailer

19. CSE401N19 Packet Switching

20. CSE401N20 Inside a Packet Switching Router r A node in a packet switching network incoming linksoutgoing links node Memory

21. CSE401N21 Packet Switching: Resources r Each packet waits for its turn at the output link r On its turn, a packet uses full link bandwidth r Resources used as needed r Aggregate resource demand can exceed amount available r Congestion: packets queue, wait for link use Bandwidth division into “pieces” Resource reservation Dedicated allocation

22. CSE401N22 A Taxonomy of Packet-Switched Networks According to Routing r Goal: move packets among routers from source to destination m we’ll study several routing algorithms later in the course r Two types of packet switching m datagram network each packet of a flow is switched independently m virtual circuit network: all packets from one flow are sent along a pre-established path (= virtual circuit)

23. CSE401N23 Datagram Packet Switching r Example: IP networks r Each packet is independently switched m each packet header contains complete destination address m receiving a packet, a router looks at the packet’s destination address and searches its current routing table to determines the next hop m routes may change during session m routers do not keep any state about a flow An example of datagram-style routing in daily life?

24. CSE401N24 Host A Host B Host E Host D Host C Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7 Datagram Packet Switching

25. CSE401N25 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 processing and queueing delay of Packet 1 at Node 2 Host 1Host 2 Node 1Node 2 propagation delay from Host 1 to Node 1 transmission time of Packet 1 at Host 1 Timing Diagram of Datagram Switching

26. CSE401N26 Virtual-Circuit Packet Switching r Example: Asynchornous Transfer Mode (ATM) networks r Hybrid of circuit switching and datagram switching m each packet carries a short tag (virtual-circuit (VC) #), tag determines next hop m fixed path determined at Virtual Circuit setup time, remains fixed thru flow m routers maintain per-flow state r What advantages do virtual circuit have over datagram? m Guarantees in-sequence delivery of packets m However: Packets from different virtual circuits may be interleaved Incoming Interface Incoming VC# Outgoing Interface Outgoing VC# 112222 11631 212322 …

27. CSE401N27 Host A Host B Host E Host D Host C Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7 Virtual-Circuit Switching

28. CSE401N28 Virtual-Circuit Packet Switching r Three phases 1. VC establishment 2. Data transfer 3. VC disconnect

29. CSE401N29 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1 VC establishment VC termination data transfer Timing Diagram of Virtual-Circuit Switching

30. CSE401N30 Discussion: Datagram Switching vs. Virtual Circuit Switching r What are the benefits of datagram switching? r What are the benefits of virtual circuit switching?

31. CSE401N31 Delay at a Router in Packet Switching r A packet experiences delay at each hop r Four types of delay at each hop m nodal processing delay: check errors & routing m queueing: time waiting for its turn at output link m transmission delay: time to pump packet onto a link at link speed m propagation delay: router to router propagation

32. CSE401N32 Delay in Datagram Networks Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 nodal processing and queueing delay of Packet 1 at Node 2 propagation delay between Host 1 and Node 2 transmission time of Packet 1 at Host 1 Host 1Host 2Node 1Node 2

33. CSE401N33 Network Core: Packet Switching Packet-switching versus circuit switching: human restaurant analogy r other human analogies? A B C 10 Mbs Ethernet 1.5 Mbs 45 Mbs D E statistical multiplexing queue of packets waiting for output link

34. CSE401N34 Packet switching versus circuit switching r 1 Mbit link r each user: m 100Kbps when “active” m active 10% of time r circuit-switching: m 10 users r packet switching: m with 35 users, probability > 10 active less than.0004 Packet switching allows more users to use network! N users 1 Mbps link

35. CSE401N35 Packet switching versus circuit switching r Great for bursty data m resource sharing m no call setup r Excessive congestion: packet delay and loss m protocols needed for reliable data transfer, congestion control r Q: How to provide circuit-like behavior? m bandwidth guarantees needed for audio/video apps m still an unsolved problem (chapter 6) Is packet switching a “slam dunk winner?”

36. CSE401N36 Access networks and physical media Q: How to connection end systems to edge router? r residential access nets r institutional access networks (school, company) r mobile access networks Keep in mind: r bandwidth (bits per second) of access network? r shared or dedicated?

37. CSE401N37 Residential access: point to point access r Dialup via modem m up to 56Kbps direct access to router (conceptually) r ISDN: integrated services digital network: 128Kbps all- digital connect to router r ADSL: asymmetric digital subscriber line m up to 1 Mbps home-to-router m up to 8 Mbps router-to-home m ADSL deployment: happening

38. CSE401N38 Residential access: cable modems r HFC: hybrid fiber coax m asymmetric: up to 10Mbps upstream, 1 Mbps downstream r network of cable and fiber attaches homes to ISP router m shared access to router among home m issues: congestion, dimensioning r deployment: available via cable companies, e.g., MediaOne

39. CSE401N39 Residential access: cable modems Diagram: http://www.cabledatacomnews.com/cmic/diagram.html

40. CSE401N40 Institutional access: local area networks r company/univ local area network (LAN) connects end system to edge router r Ethernet: m shared or dedicated cable connects end system and router m 10 Mbs, 100Mbps, Gigabit Ethernet r deployment: institutions, home LANs happening now r LANs: chapter 5

41. CSE401N41 Wireless access networks r shared wireless access network connects end system to router r wireless LANs: m radio spectrum replaces wire m e.g., Lucent Wavelan 11 Mbps r wider-area wireless access m CDPD: wireless access to ISP router via cellular network base station mobile hosts router

42. CSE401N42 Home networks Typical home network components: r ADSL or cable modem r router/firewall r Ethernet r wireless access point wireless access point wireless laptops router/ firewall cable modem to/from cable headend Ethernet (switched)

43. CSE401N43 Physical Media r physical link: transmitted data bit propagates across link r guided media: m signals propagate in solid media: copper, fiber r unguided media: m signals propagate freely, e.g., radio Twisted Pair (TP) r two insulated copper wires m Category 3: traditional phone wires, 10 Mbps Ethernet m Category 5 TP: 100Mbps Ethernet

44. CSE401N44 Physical Media: coax, fiber Coaxial cable: r wire (signal carrier) within a wire (shield) m baseband: single channel on cable m broadband: multiple channel on cable r bidirectional r common use in 10Mbs Ethernet Fiber optic cable: r glass fiber carrying light pulses r high-speed operation: m 100Mbps Ethernet m high-speed point-to-point transmission (e.g., 5 Gps) r low error rate

45. CSE401N45 Physical media: radio r signal carried in electromagnetic spectrum r no physical “wire” r bidirectional r propagation environment effects: m reflection m obstruction by objects m interference Radio link types: r microwave m e.g. up to 45 Mbps channels r LAN (e.g., WaveLAN) m 2Mbps, 11Mbps r wide-area (e.g., cellular) m e.g. CDPD, 10’s Kbps r satellite m up to 50Mbps channel (or multiple smaller channels) m 270 Msec end-end delay m geosynchronous versus LEOS

46. CSE401N46 Thank YOU