... Linear array. Tree. Near-neighbor mesh. Systolic array. Ring

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1 Computer Networks Architecture and Protocols Contents Supplementary Class Notes Operating System CS CS Network Standardization Network Protocols æ A set of nodes èhostsè and a set of links ètransmission linesè. æ Two extreme conægurations: í Fully connected networks with N nodes. number of connections = NèN, 1è 2 í Star networks: with N nodes and an intermediate switch. Linear array Bus... number of connections = N Tree æ Most real networks have arbitrary topology. Near-neighbor mesh Topological Classiæcation æ Static networks í 1-D èbusè í 2-D ètree, stars, rings, mesh, etc.è í Multidimensional ècube, hypercubesè æ Dynamic networks í One or more switches are used. Ring Systolic array Operating Systems 1 of 20 Operating Systems 2 of 20

2 0 1 S Star Completely connected 2 3 S S S An example of an 8 x 8 single stage dynamic network Chordal ring 3-cube Operating Systems 3 of 20 Operating Systems 4 of 20 í Single node, data base management èlibraryè. Technological Classiæcation Computer Networks æ Circuit-switching networks í A dedicated path is used between a source and a destination. í No queuing í Example: telephone systems. æ Packet Switching í The message is divided into a number of slices called packets of certain æxed size. í Each packet has its destination address. í Queuing involved í Routing is needed í Errors involved æ Message Switching í The network receives the entire message, stores it in a secondary storage and then transmits it. í It provides a long term storage even after the message has been delivered. æ Non-Switching Networks. í Broadcast networks í Single node, data processingè1-800è. æ Wide Area Networks í Bandwidth is scarcest resources í Storeèforward used to ensure optimal bandwidth utilization í Burden on hosts and nodes to conserve bandwidth í Most WANs use point-to-point links except for satellite networks. æ Metropolitan Area Networks í Intended for interconnection of LANs í MANs typically span entire country. í Owned by multiple organizations. í Covers the entire city but uses LANs technology ècatvè. æ Local Area Networks í Unlimited bandwidth í Hosts are simple machines í Lighter burden on hosts í Most LANs use a multiaccess channel. í A diameter é a few km. í Total data rate of at least several Mbps. í Complete ownership by a single organization. Operating Systems 5 of 20 Operating Systems 6 of 20

3 Diæerence between LANs and WANs Network Standardization æ Designers of WANs are always forced by legal, economic or political reasons to use PSN regardless of it technical suitability. æ LAN cables are highly reliable. æ Error rate is 1000 times lower than WAN. æ In WANs, error handling must be done in each layer. æ In the early days, diæerent vendors had diæerent networks. æ Standards fall into two categories: í De facto standard æ Have just happened without formal plan. æ IBM PC, UNIX, DOS. í De Jure standard èby lawè æ Formal legal standards æ De Jure standards are two classes. æ A network consists of a series of levels called layers. æ A protocol is the rule of communication; each layer has its own protocol. æ Each computer andèor each application program in the computer may require a diæerent communication access method and protocol. í They setup a session through the network. í They must agree on the format. í Terminals must be able to regulate data rates í Packets may arrive out of order. í Those established by treaty among national governments, í Voluntary non-treaty organizations. Transmission Standards æ Voice transmission is still the most common mode of communications. æ All projections indicate that voice will continue to be the heaviest communications. æ AT&T in early 1960s introduced digital carrier system T1 í Consists of 24 channels at 64 Kbps per channel. í Mbps. Operating Systems 7 of 20 Operating Systems 8 of 20 Network Standardization í US, Canada and Japan. æ CCITT has 30-voice channel at Mbps èrest of the worldè. æ The great interest is transmitting packetized voice in real time. æ IBM SNA was one of the ærst layered architecture. æ OSI of ISO rapidly became an international standard. IEEE Standard 802 for LANs and MANs æ In layered architecture, Protocols must appear in every network node. æ The bottom 3 layers of OSI provide network services and the upper 4 layers provide services to the end users. OSI Layers æ IEEE 802 has been adopted by ANSI, NIST, and ISO æ IEEE gives and introduction to the set of standards and deænes the interface primitives. æ IEEE describes the upper part of the data link layer èllcè æ IEEE describes CSMAèCD with its protocol. æ IEEE describes token bus with its protocol. æ IEEE describes token ring with its protocol. æ IEEE 802.6: Distributed Queue Dual Bus for MANs 1. Physical Layer æ Performs direct transmission of logical information into physical phenomena èelectronic pulsesè. æ Modulatorsèdemodulators are used at this layer. 2. Data Link Layer æ Makes sure that the message indeed reach the other end without corruption èsignal distortion and noiseè. æ Acknowledgments æ Detect duplications. æ Timers for retransmission. Operating Systems 9 of 20 Operating Systems 10 of 20

4 3. Network Layer 6. Presentation Layer æ Controls routes for individual message through the actual topology. æ Finds the best route. æ Finds alternate routes. æ Buæering and deadlock handling. 4. Transport Layer æ Locates the other party æ Creates a transport pipe between both end-users. æ Breaking the message into packets and reassembling them at the destination. æ Applies æow control to the packet stream. 5. Session Layer æ Is responsible for the relation between two end-users. æ Maintains the integrity and controls the data exchanged between the end-users. æ The end-users are aware of each other when the relation is established èsynchronizationè. æ It uses naming and addressing to identify a particular user. æ Makes sure that the lower layer guarantees delivering the message èæow controlè. æ It translates the language used by the application layer. æ It makes the users as independent as possible, then they can concentrate on conversation. 7. Application Layerèend usersè æ Where they process information that is being exchanged. æ The users don't want to be aware of the mechanism of the network. æ The users shouldn't be bothered by each other's language. Advantages of æ Any given layer can be modiæed or upgraded without eæecting the other layers. æ Modulazition by means of layering simpliæes the overall design. æ Diæerent layers can be assigned to diæerent standards, committees, and design teams. æ Diæerent mechanisms èpacket-switching, circuit-switchingè may be used without eæecting more than one layer. æ Diæerent machines may be plugged in at diæerent layers. æ The relation between diæerent control functions can be better understood. Operating Systems 11 of 20 Operating Systems 12 of 20 æ Common lower levels may be shared by diæerent higher levels. æ Functions èespecially at lower levelsè may be removed from software to hardware and microcodes. æ Increases the compatibility of diæerent machines. Disadvantages of Network Protocols TCPèIP æ Total overhead is higher. æ Two communicating machines may have to use certain functions which they could do without layers. æ As technology changes, the functions may not be in the most cost-eæective layer. æ Was developed to interconnect heterogeneous networks. æ IP resides between the Transport and Network layers of OSI model. æ In this position, IP remains hardware independent. Orientations Host A Host B æ Connection-oriented services Process Process í The sender pushes objects in at one end and the receiver collects them in the same order at the other end. í It was modeled after the telephone system. Request TCP Response Request TCP Response æ Connectionless-oriented services Send Deliver Send Deliver í It was modeled after the postal service IP IP í Packet could take independent routes í Packet could be received out of order. Reordering may be required. NAP-1 INternet NAP-1 í Datagrams NAP= Network Access Protocol Use of TCP/ and IP services primitives Operating Systems 13 of 20 Operating Systems 14 of 20

5 Network Protocols Network Protocols æ IP provides connection-less service between nodes or hosts. æ The order of datagrams received is not necessarily in the same order. æ Each datagram is individually routed æ IP can be operated across disjoint network services èconnection oriented or connection-lessè. æ TCP in coordination with IP provide routing, æow control, fragmentation, addressing, and error correctionèdetection. æ Each datagram has a life-time. A datagram can loop indeænitely. æ During its journey, a datagram can be continually downsized. æ Defragmentation is not performed at intervening gateways. æ IP does not deæne a standard packet size. Transmission Control Protocol, TCP æ Since IP is not reliable, TCP assumes that: í Segments may be lost or arrive somewhere with errors. í Segments may not be received in transmitted order. í Segments may be delayed at a variable rate which slows communications. æ TCP sends ACK for each segment it receives. æ If the sending TCP doesn't receive the ACK within a reasonable amount of time, it presumes the lost and res-send. æ TCP establishes a connection èsessionè between two sockets ènodesè, with two purposes: í To deæne all connection characteristics including security. í To allow for each TCP socket to maintain state information about the connection èlast sequence number used and received, and last sequence number acknowledgedè. æ TCP has been designed to service multiple processes' requests within a single node. æ Developed originally for use in Arpanet. æ TCP in conjunction with IP is a de facto standard for heterogeneous node communications. æ TCP resides in the Transport layer of the OSI model. æ TCP is a connection-oriented protocol. æ TCP operates as an intermediary between applications and the internetwork. Operating Systems 15 of 20 Operating Systems 16 of 20 æ So far, we assumed a single homogeneous network, with each machine using the same protocol in each layer. This is not realistic. æ Controversy: whether problems associated with heterogeneous networks will go away or we have to live with it. í LAN-WAN-LAN æ These connection use one or more of the following black boxes. í Repeaters at layer 1 í Bridges at layer 2 í Multiprotocol routers at layer 3 í Transport gateways at layer 4 í Application gateways allows internetwork above layer 4. æ For the time being variety of diæerent networks will be around for the following reasons. í The installed base of diæerent networks is large and growing. í All Unix shops run TCPèIP. í Many large businesses still have mainframes running SNA, DEC, etc. í Personal computer LANs often use Novell NCPèIPX or AppleTalk., etc. í ATM systems are starting to be widespread. í Specialized protocols are often used in satellite, cellular, and infrared networks. æ Within an organization èuniversityè, diæerent networks interact. í LAN-LAN í LAN-WAN í WAN-WAN Operating Systems 17 of 20 Operating Systems 18 of 20

6 Internet Protocol Hierarchy Asynchronous Transfer Mode èatmè HTTP FTP NV TFTP TCP UDP IP Ethernet FDDI ATM Fiber Channel æ Connection-oriented packet-switched network æ Used in both WAN and LAN settings æ Signaling èconnection setupè Protocol: Q.2931 æ Speciæed by ATM Forum æ Packets are called cells: 5-byte header + 48-byte payload æ Commonly transmitted over SONET èbut not necessarilyè Physical Medium: Fiber, Coax, Satellite links, etc. Plane Management Function æ There is no real structure, but several major backbones exist. æ Attached to the backbones are regional èmidlevelè networks. æ Attached to regional networks are the LANs at many places. æ The glue that holds the Internet together id the IP protocol. æ Its job is to provide a best way to transport datagrams from source to destination transparently. æ The transport layer takes data streams, breaks them into datagrams. æ Datagrams can be up to 64 KB, but in practice they are usually around 1500 bytes. Control Plane Higher Layers: Protocols and Functions Adaption Layer ATM Layer User Plane Higher Layers: Protocols and Functions Physical Medium Dependent Layer B-ISDN protocol Model for ATM (CCITT I. 121) Operating Systems 19 of 20 Operating Systems 20 of 20