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Chapter 2 Computer Networks

Chapter 2 Computer Networks. Computer Networking: A Top Down Approach Featuring the Internet , 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. Chapter 2: roadmap. 2.1 Network edge 2.2 Network core 2.3 Access Network and physical media

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Chapter 2 Computer Networks

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  1. Chapter 2Computer Networks Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.

  2. Chapter 2: roadmap 2.1 Network edge 2.2 Network core 2.3 Access Network and physical media 2.4 Delay & loss in packet-switched networks 2.5 Protocol layers, service models

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

  4. end systems (hosts): run application programs e.g. Web, email at “edge of network” 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. Gnutella, KaZaA, Skype The network edge:

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

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

  7. Chapter 2: roadmap 2.1 Network edge 2.2 Network core 2.3 Access Network and physical media 2.4 Delay & loss in packet-switched networks 2.5 Protocol layers, service models

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

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

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

  11. The Circuit in link is implemented FDM or TDM

  12. The Circuit in link is implemented FDM or TDM Example: FDM( Frequency-division multiplexing ) 4 users frequency time TDM( Time-division multiplexing) frequency time Circuit Switching: FDM and TDM

  13. Numerical example • How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? • All links are 1.536 Mbps • Each link uses TDM with 24 slots/sec • 500 msec to establish end-to-end circuit Let’s work it out!

  14. Solution: • Each Circuit has transmission rate of:(1.536 Mbps) / 24 =64 Kbps. • So it take (640,000 bits)/ 64 kbps = 10 sec • To transmit the file 10 + 0.5 (establish time)=10.5 sec

  15. Network Core: Packet Switching • Message can contain anything the protocol designer wants. • The source brakes long message into smaller chunks of data (packets). • Store-and-forward transmission: The switch must receive the entire packet before it can begin onto the outbound link.

  16. Bandwidth division into “pieces” Dedicated allocation Resource reservation Network Core: Packet Switching resource contention: • aggregate resource demand can exceed amount available • congestion: packets queue, wait for link use • store and forward: packets move one at a time • Node receives complete packet before forwarding 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

  17. Is packet switching Packet switching versus circuit switching • Great for bursty data • resource sharing • simpler, no call setup • Excessive congestion: packet delay and loss • protocols needed for reliable data transfer, congestion control • Q: How to provide circuit-like behavior? • bandwidth guarantees needed for audio/video apps • still an unsolved problem (chapter 7) Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

  18. Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec Packet-switching: store-and-forward L • Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps • Entire packet must arrive at router before it can be transmitted on next link: store and forward • delay = 3L/R (assuming zero propagation delay) R R R more on delay shortly …

  19. Packet-switched networks: forwarding • Goal: move packets through routers from source to destination • we’ll study several path selection (i.e. routing) algorithms (chapter 4) • datagram network: • destination address in packet determines next hop • routes may change during session • analogy: driving, asking directions • virtual circuit network: • each packet carries tag (virtual circuit ID), tag determines next hop • fixed path determined at call setup time, remains fixed thru call • routers maintainper-call state

  20. Packet-Switching Networks • Advantages: • flexibility, resource sharing, robust, responsive • Disadvantages: • Time delays in distributed network, overhead • Need for routing and congestion control

  21. Circuit-Switching • telecom network designed for voice • Network resources dedicated to one call • Shortcomings when used for data: • high idle time • Constant data rate

  22. Packet-Switching • Data transmitted in short blocks, or packets • Packet length < 1000 octets • Each packet contains user data plus control info (routing) • Store and forward

  23. Figure 4.1 The Use of Packets

  24. Figure 4.2 Packet Switching: Datagram Approach

  25. Advantages over Circuit-Switching • Greater line efficiency (many packets can go over shared link) • Data rate conversions • Non-blocking under heavy traffic (but increased delays)

  26. Disadvantages relative to Circuit-Switching • Packets incur additional delay with every node they pass through • Jitter: variation in packet delay • Data overhead in every packet for routing information, etc • Processing overhead for every packet at every node traversed

  27. Figure 4.3 Simple Switching Network

  28. Switching Technique • Large messages broken up into smaller packets • Datagram • Each packet sent independently of the others • No call setup • More reliable (can route around failed nodes or congestion) • Virtual circuit • Fixed route established before any packets sent • No need for routing decision for each packet at each node

  29. Figure 4.4 Packet Switching: Virtual-Circuit Approach

  30. Packet-switched networks Circuit-switched networks FDM TDM Datagram Networks Networks with VCs Network Taxonomy Telecommunication networks • Datagram network is not either connection-oriented or connectionless. • Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.

  31. Chapter 2: roadmap 2.1 Network edge 2.2 Network core 2.3 Access Network and physical media 2.4 Delay & loss in packet-switched networks 2.5 Protocol layers, service models

  32. Access networks and physical media Analog Modem Digital Modem Q: How to connect end systems to edge router? • residential access nets • institutional access networks (school, company) • mobile access networks Keep in mind: • bandwidth (bits per second) of access network? • shared or restricted?

  33. Residential access: point to point access Poor quality of twisted-pair line • Dialup via modem • up to 56Kbps direct access to router (often less) • Can’t surf and phone at same time: can’t be “always on” • ADSL: asymmetric digital subscriber line • up to 1 Mbps upstream (today typically < 256 kbps) • up to 8 Mbps downstream (today typically < 1 Mbps) • DSL use FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone

  34. Residential access: cable modems • HFC: hybrid fiber coax • asymmetric: up to 30Mbps downstream, 2 Mbps upstream • Requires special modem called Cable Modems. • HFC shared broadcast medium.

  35. Company access: local area networks • company/univ local area network (LAN) connects end system to edge router • Ethernet: • shared or dedicated link connects end system and router • 10 Mbs, 100Mbps, Gigabit Ethernet • LANs: chapter 8

  36. router base station mobile hosts Wireless access networks • shared wireless access network connects end system to router • via base station ( wireless access point) • wireless LANs: • 802.11b (WiFi): 11 Mbps • Ch8 • wider-area wireless access • provided by telco provider. • Serves users within a radius of 10 Km.

  37. Home networks Typical home network components: • ADSL or cable modem • router/firewall/NAT • Ethernet • wireless access point wireless laptops to/from cable headend cable modem router/ firewall wireless access point Ethernet

  38. Physical Media • Bit: traveling from one end system to another.

  39. Physical Media • Bit: propagates betweentransmitter/rcvr pairs • physical link: what lies between transmitter & receiver • guided media: • signals propagate in solid media: copper, fiber, coax • unguided media: • signals propagate freely, e.g., radio, wireless LAN

  40. Twisted Pair (TP) • two insulated copper wires • Category 1: Less expensive and most commonly used • Category 2 : traditional phone wires, 10 Mbps Ethernet • Category 3: 100Mbps Ethernet

  41. Physical Media: coax, fiber Fiber optic cable: • glass fiber carrying light pulses, each pulse a bit • high-speed operation: • high-speed point-to-point transmission (e.g., 10’s-100’s Gps) • low error rate. • Ex: Backbone of the Internet Coaxial cable: • two concentric copper conductors (not parallel) Ex: TV Cable • bidirectional • baseband: • single channel on cable • broadband: • multiple channels on cable

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