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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Computer Networks -1-Part 2,*,/44,COMPUTER NETWORKS,Chapter 01,Introduction (2),Shoubao,Yang,Computer Science Department,Tel: 3601540,E-mail:,syang,http:/202.38.64.11/syang,2024/10/5,1,/44,Computer Networks -1-Part 2,1.4 Reference Models,1.4.1 OSI Reference Model,1983, International Standards Organization (ISO) proposed the “ISO OSI (Open Systems Interconnection) Reference Model”, OSI-,rm,The principles of layering,A layer should be created where a different abstraction is needed,Each layer should perform a well-defined function,The function of each layer should be chosen towards defining internationally standardized protocols,The layer boundaries should be chosen to minimize the information flow across the interfaces,The number of layers should be large enough that distinct functions and small enough that the architecture does not become unwieldy,2024/10/5,2,Computer Networks -1-Part 2,The OSI Model,2024/10/5,3,Computer Networks -1-Part 2,The Physical Layer,Essence:,Describes the transmission of raw bits in terms of mechanical and electrical issues:,Example:,Connect two computers by means of a wire:,Setting -3V to -12V on the wire corresponds to a binary 1; +3V to +12V is a binary 0,The wire is not to be longer than 15 meters,You may change the voltage at most 20,000 times per second (,Question:,whats the transfer rate?),2024/10/5,4,Computer Networks -1-Part 2,The Data Link Layer,Data link layer,is to transform a raw transmission facility into a line that appears free,of undetected transmission errors to the network layer,Observation:,We need to at least,detect bit transmission errors,send bits in,frames,that add redundancy to detect something went wrong,Examples:,Add a parity bit to every 7 transmitted bits: 1 says there were odd number of 1s; 0 says there were an even number of 1s,Add a checksum (cyclic redundancy check) that should match the bits before it,Also:,Provide the mechanisms so that fast senders dont overwhelm slow receivers (flow control),2024/10/5,5,Computer Networks -1-Part 2,OSI Data Link Layer,Observation:,We also need to specify how a number of computers can,share,a common channel (e.g. wire), that is,medium access control,sublayer,(MAC):,1. Specifies how one out of several competing senders, is eventually allowed exclusive access to the wire,2. Common approach 1: listen to each other; retreat when you hear someone else, and try again later,3. Common approach 2: wait your turn by passing a,token,between all stations,Well-known protocols:,Ethernet, token ring, token bus, FDDI,2024/10/5,6,Computer Networks -1-Part 2,The Network Layer,Essence:,Describes how routing (and congestion) is to be done. Mostly needed in subnets. Network layer controls subnet operations.,1.,How do we find out which computers/routers are in the network?,2.,How do we calculate the best route from A to B?,3.,What happens when a computer/router goes down?,4.,Should multicasting/broadcasting be supported?,5.,What happens if a router becomes overloaded,and starts dropping packets?,6.,Can we detect and avoid “hot spots?”,2024/10/5,7,Computer Networks -1-Part 2,The Transport Layer,Observation:,The transport layer is to accept data from above, split it up into smaller units if need be,pass these to network layer, and ensure that the pieces all arrive correctly at the other end.,Generally offers connection-oriented as well as connectionless services, and varying degrees of reliability. This layer provides the actual network interface to,applications,1.,Often provides network interface through,sockets,(UNIX, Windows),2.,Allows to set up a connection to another application, and subsequently deliver data,reliably, and in the order that it was,sent,3.,Often also support for,secure connections,4.,Also support for,datagrams,: unreliable message,passing on a per-message basis,2024/10/5,8,Computer Networks -1-Part 2,OSI Session and Presentation Layers,Session:,The dumbest one (and ill-defined) tells how applications can set up “long-lasting” communications, offers various services, including dialog control, token management, synchronization.,Presentation:,Describes everything that is needed to exchange data in a platform-independent way, concerned with the syntax and semantics of the information transmitted.,Example: think of byte ordering in different computers, or passing “binary” data through e-mail,2024/10/5,9,Computer Networks -1-Part 2,The Application Layer,Essence:,Contains the rest,Traditional:,Name services (DNS), security, e-mail(SMTP), News (NNTP), Web (HTTP),Modern:,All types of,middleware protocols,to support,distributed systems,:,New transfer protocols for object systems like Java (RMI), CORBA (IIOP), DCOM (propriety),Special protocols to handle replication, fault tolerance, caching, data persistence, etc.,High-level protocols:,Special application-level protocols for e-commerce, banking, EDI, etc.,2024/10/5,10,Computer Networks -1-Part 2,Data Transmission in the OSI Model,2024/10/5,11,Computer Networks -1-Part 2,1.4.2 The TCP/IP Reference Model,TCP/IP is a result of protocol research and development conducted on the experimental packet switched network, ARPANET, funded by the Defense Advanced Research Projects Agency (DARPA), and is generally referred to as the TCP/IP protocol suite.,This protocol suite consists of a large collection of protocols that have been issued as Internet standards by the Internet Activities Board (IAB).,There is no official TCP/IP protocol model. However, based on the protocol standards, we can organize the communication task for TCP/IP into four relatively independent layers, from bottom to top: host-to-network (physical,layer+network,access layer), internet layer, transport layer (host-to-host), application layer.,2024/10/5,12,Computer Networks -1-Part 2,1.4.2 The TCP/IP Reference Model (2),2024/10/5,13,Computer Networks -1-Part 2,2024/10/5,14,Computer Networks -1-Part 2,1.4.2 The TCP/IP Reference Model (3),The bad thing is that TCP/IP did not make a clear distinction between services, interfaces, and protocols. That makes it much harder to re-implement certain layers.,TCP/IP protocol suite is successful because,(1) it was,there,when needed (OSI implementations were terrible),(2) freely distributed with the UNIX operating system.,2024/10/5,15,Computer Networks -1-Part 2,1.4.2 The TCP/IP Reference Model (4),The Internet Layer,A packet-switching network based on a connectionless,internetwork,layer, permits hosts to inject packets into any network and have them travel independently to the destination.,The internet layer defines an official packet format and protocol called IP (Internet Protocol), delivers IP packets where they are supposed to go.,The Transport Layer,Allows peer entities on the source and destination hosts to carry on an conversation. TCP (Transmission Control Protocol) is a reliable connection-oriented protocol, while UDP (User Datagram Protocol) is an unreliable connectionless protocol.,2024/10/5,16,Computer Networks -1-Part 2,1.4.2 The TCP/IP Reference Model (5),The Application Layer,Contains all the higher-level protocols: Telnet, FTP, SMTP, DNS, HTTP, ,The Host-Network Layer,The TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network.,2024/10/5,17,Computer Networks -1-Part 2,1.4.3 Comparing OSI and TCP/IP Models,Concepts central to the OSI model,Services,Interfaces,Protocols,The TCP/IP model did not originally clearly distinguish between service, interface, and protocol.,TCP/IPs protocols came first, and the model was really just a description of the existing protocols.,Different numbers of layers,Another difference is in the area of connectionless versus connection-oriented communication.,2024/10/5,18,Computer Networks -1-Part 2,1.4.4 A Critique of the OSI Model and Protocols,Why OSI did not take over the world,Bad timing,Bad technology,Bad implementations,Bad politics,2024/10/5,19,Computer Networks -1-Part 2,1.4.5 A Critique of the TCP/IP Reference Model,Problems,Service, interface, and protocol not distinguished,Not a general model,Host-to-network “layer” not really a layer,No mention of physical and data link layers,Minor protocols deeply entrenched, hard to replace,The hybrid reference model to be used in this book.,2024/10/5,20,Computer Networks -1-Part 2,1.5,Example Networks,Novell Netware,The,ARPAnet,2024/10/5,21,Computer Networks -1-Part 2,1.5.1 The Internet,(a) Structure of the telephone system.,(b),Barans,proposed distributed switching system.,The ARPANET,2024/10/5,22,Computer Networks -1-Part 2,The ARPANET (2),The original ARPANET design.,2024/10/5,23,Computer Networks -1-Part 2,The ARPANET (3),Growth of the ARPANET (a) December 1969. (b) July 1970.,(c) March 1971. (d) April 1972. (e) September 1972.,2024/10/5,24,Computer Networks -1-Part 2,NSFNET,The NSFNET backbone in 1988.,2024/10/5,25,Computer Networks -1-Part 2,Internet Usage,Traditional applications (1970 1990),E-mail,News,Remote login,File transfer,2024/10/5,26,Computer Networks -1-Part 2,Architecture of the Internet,Fig. 1.29 Overview of the Internet.,2024/10/5,27,Computer Networks -1-Part 2,ATM Virtual Circuits,Fig. 1.30 A virtual circuit.,Fig. 1.31 An ATM cell.,2024/10/5,28,Computer Networks -1-Part 2,The ATM Reference Model,The ATM reference model.,2024/10/5,29,Computer Networks -1-Part 2,The ATM Reference Model (2),The ATM layers and,sublayers,and their functions.,2024/10/5,30,Computer Networks -1-Part 2,1.5.3 Ethernet,Fig. 1.34 Architecture of the original Ethernet.,2024/10/5,31,Computer Networks -1-Part 2,1.5.4 Wireless LANs: 802.11,(a) Wireless networking with a base station.,(b) Ad hoc networking.,2024/10/5,32,Computer Networks -1-Part 2,Wireless LANs (2),The range of a single radio may not cover the entire system.,2024/10/5,33,Computer Networks -1-Part 2,Wireless LANs (3),A,multicell,802.11 network.,2024/10/5,34,Computer Networks -1-Part 2,1.6 Network Standardization,Whos Who in the Telecommunications World,Whos Who in the International Standards World,Whos Who in the Internet Standards World,2024/10/5,35,Computer Networks -1-Part 2,ITU,Main sectors,Radio communications,Telecommunications Standardization,Development,Classes of Members,National governments,Sector members,Associate members,Regulatory agencies,2024/10/5,36,Computer Networks -1-Part 2,IEEE 802 Standards,The 802 working groups. The important ones are marked with *. The ones marked with, are hibernating. The one marked with gave up.,2024/10/5,37,Computer Networks -1-Part 2,1.7 Metric Units,Fig. 1-39. The principal metric prefixes.,2024/10/5,38,Computer Networks -1-Part 2,Carrier Data Communication Services,Observation:,As long as carriers rule the world (i.e.they have the cables), and competition increases, carriers will increase and improve their data communication services for the public:,SMDS:,Switched,Multimegabit,Data Service primarily intended to connect a number of LANs through long-haul networks (owned by the carrier),2024/10/5,39,Computer Networks -1-Part 2,X.25:,The OSI network protocol (also covering,datalink,and physical layer), intended to offer a,data,network on top of an (existing) cable infrastructure. Pretty old.,Frame relay:,Extremely simple facility that allows a customer to hire a single high-bandwidth link. There is hardly any support for error detection, routing, flow control, etc.,Carrier Data Communication Services,2024/10/5,40,Computer Networks -1-Part 2,Broadband ISDN,Observation:,The carriers are getting too many networks to maintain, and are missing one important type (i.e. those owned by cable-TVs),Solution:,Lets invent a completely new network that should integrate data and telephony, and should be able to accommodate a large range of bandwidth requirements.,2024/10/5,41,Computer Networks -1-Part 2,Broadband ISDN (2),Problem:,How do you implement a network that can handle 622 Mbps?,Use small fixed-sized,cells,instead of relatively large packets and frames (that can also vary in length): this allow you to build low-latency switches,Do the least you can (i.e., dont provide too many services): let the applications handle it all,2024/10/5,42,Computer Networks -1-Part 2,ATM for B-ISDN,Solution:,Asynchronous Transfer Mode (ATM), by which cells of 53 bytes (!) are sent along,virtual circuits,from sender to destination.,Observation:,This model does not fit into OSIs layered approach: B-ISDN assumes theres a separate signaling network to setup a connection,2024/10/5,43,Computer Networks -1-Part 2,Assignments,Chapter 01,Please see separate sheet.,2024/10/5,44,Computer Networks -1-Part 2,
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