1、1: Introduction,1,Packet switching versus circuit switching,Great for bursty data resource sharing 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/
2、video apps still an unsolved problem (chapter 6),Is packet switching a “slam dunk winner?”,1: Introduction,2,Packet-switched networks: routing,Goal: move packets among routers from source to destination well study several path selection algorithms (chapter 4) datagram network: destination address de
3、termines next hop routes may change during session(分时系统用)对话期间 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 maintain per-call state,1: Introd
4、uction,3,Access networks and physical media,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 dedicated?,1: Introduction,4,Residential acc
5、ess: point to point access,Dialup via modem up to 56Kbps direct access to router (conceptually) ISDN: integrated services digital network: 128Kbps all-digital connection to router ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router up to 8 Mbps router-to-home ADSL deployment - 2 mil
6、lion lines in U.S. and Canada,1: Introduction,5,Residential access: cable modems,HFC: hybrid fiber coax asymmetric: up to 10Mbps downstream, 1 Mbps upstream network of cable and fiber attaches homes to ISP router shared access to router among homes issues: congestion, dimensioning deployment: availa
7、ble via cable companies, e.g., MediaOne,1: Introduction,6,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 Mbs, 100Mbps, Gigabit Ethernet Deployment（配置）: institutio
8、ns, home LANs soon LANs: chapter 5,1: Introduction,7,Wireless access networks,shared wireless access network connects end system to router wireless LANs: radio spectrum replaces wire e.g., Lucent Wavelan 10 Mbps wider-area wireless access CDPD: wireless access to ISP router via cellular network,1: I
9、ntroduction,8,Physical Media,physical link: transmitted data bit propagates across link guided media: signals propagate in solid media: copper, fiber unguided media: signals propagate freely e.g., radio,Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps etherne
10、t Category 5 TP: 100Mbps ethernet,1: Introduction,9,Physical Media: coax, fiber,Coaxial cable: wire (signal carrier) within a wire (shield) baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet,Fiber optic cable: glass fiber carrying light
11、pulses high-speed operation: 100Mbps Ethernet high-speed point-to-point transmission (e.g., 5 Gps) low error rate,1: Introduction,10,Physical media: radio,signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects in
12、terference,Radio link types: microwave e.g. up to 45 Mbps channels LAN (e.g., waveLAN) 2Mbps, 11Mbps wide-area (e.g., cellular) e.g. CDPD, 10s Kbps satellite up to 50Mbps channel (or multiple smaller channels) 270 Msec end-end delay geosynchronous versus LEOS,1: Introduction,11,Delay in packet-switc
13、hed networks,packets experience delay on end-to-end path four sources of delay at each hop,nodal processing: check bit errors determine output link queuing time waiting at output link for transmission depends on congestion level of router,1: Introduction,12,Delay in packet-switched networks,Transmis
14、sion delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R,Propagation delay: d = length of physical link s = propagation speed in medium (2x108 m/sec) propagation delay = d/s,Note: s and R are very different quantities!,1: Introduction,13,Queuing delay (revisited),R
15、=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate,traffic intensity = La/R,La/R 0: average queueing delay small La/R - 1: delays become large La/R 1: more “work” arriving than can be serviced, average delay infinite!,1: Introduction,14,Protocol “Layers”,Networks are complex!
16、 many “pieces”: hosts routers links of various media applications protocols hardware, software,Question: Is there any hope of organizing structure of network?Or at least our discussion of networks?,1: Introduction,15,Organization of air travel,a series of steps,1: Introduction,16,Organization of air
17、 travel: a different view,Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below,1: Introduction,17,Layered air travel: services,Counter-to-counter delivery of person+bagsbaggage-claim-to-baggage-claim deliverypeople transfer: loading g
18、ate to arrival gaterunway-to-runway delivery of plane,airplane routing from source to destination,1: Introduction,18,Distributed implementation of layer functionality,ticket (purchase)baggage (check)gates (load)runway takeoffairplane routing,ticket (complain)baggage (claim)gates (unload)runway landi
19、ngairplane routing,Departing airport,arriving airport,intermediate air traffic sites,1: Introduction,19,Why layering?,Dealing with complex systems: explicit structure allows identification, relationship of complex systems pieces layered reference model for discussion Modularization（模块化） eases mainte
20、nance, updating of system change of implementation of layers service transparent to rest of system e.g., change in gate procedure doesnt affect rest of system layering considered harmful?,1: Introduction,20,Internet protocol stack,application: supporting network applications ftp, smtp, http transpor
21、t: host-host data transfer tcp, udp network: routing of datagrams from source to destination ip, routing protocols link: data transfer between neighboring network elements ppp, ethernet, ATM physical: bits “on the wire”,1: Introduction,21,Layering: logical communication,Each layer: distributed “enti
22、ties” implement layer functions at each node entities perform actions, exchange messages with peers,1: Introduction,22,Layering: logical communication,E.g.: transport take data from app add addressing, reliability check info to form “datagram” send datagram to peer wait for peer to ack receipt analo
23、gy: post office,transport,transport,1: Introduction,23,Layering: physical communication,1: Introduction,24,Protocol layering and data,Each layer takes data from above adds header information to create new data unit passes new data unit to layer below,source,destination,message,segment,datagram,frame
24、,1: Introduction,25,Internet structure: network of networks,roughly hierarchical national/international backbone providers (NBPs) e.g. BBN/GTE, Sprint, AT&T, IBM, UUNet interconnect (peer) with each other privately, or at public Network Access Point (NAPs) regional ISPs connect into NBPs local ISP, company connect into regional ISPs,NBP A,NBP B,regional ISP,regional ISP,1: Introduction,26,National Backbone Provider,e.g. BBN/GTE US backbone network,
