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Chapter 6: Topologies and Access Methods,Network+ Guide to NetworksThird Edition,Network +,2,Objectives,Describe the basic and hybrid LAN physical topologies, and their uses, advantages, and disadvantagesDescribe the backbone structures that form the foundation for most LANsCompare the different types of switching used in data transmission,Network +,3,Objectives (continued),Understand the transmission methods underlying Ethernet, LocalTalk, Token Ring, FDDI, and ATM networksDescribe the characteristics of different wireless network technologies, including the three IEEE 802.11 standards,Network +,4,Simple Physical Topologies,Physical topology is the physical layout, or pattern, of the nodes on a networkPhysical topologies are divided into three fundamental geometric shapes: bus, ring, and star,Network +,5,Simple Physical Topologies (continued),BusA bus topology consists of a single cable connecting all nodes on a network without intervening connectivity devicesThe single cable is called the bus and can support only one channel for communicationMost bus networks use coaxial cable as their physical mediumAt the ends of each bus network are 50-ohm resistors known as terminators,Network +,6,RingIn a ring topology, each node is connected to the two nearest nodes so that the entire network forms a circleData is transmitted clockwise, in one direction (unidirectional), around the ringThe fact that all workstations participate in delivery makes the ring topology an active topologyA ring topology also differs in that it has no “ends” and data stops at its destination and, twisted-pair or fiber-optic cabling is used as the physical medium22,Simple Physical Topologies (continued),Network +,7,In a star topology, every node on the network is connected through a central device, such as a hub or switchStar topologies are usually built with twisted-pair or fiber-optic cablingStar topologies require more cabling than ring or bus networksEach node is separately connected to a central connectivity device, they are more fault-tolerant,Simple Physical Topologies (continued),Network +,8,Star-Wired RingThe star-wired ring topology uses the physical layout of a star in conjunction with the ring topologys data transmission methodData is sent around the star in a circular patternThis hybrid topology benefits from the fault tolerance of the star topology,Hybrid Physical Topologies,Network +,9,In a star-wired bus topology, groups of workstations are star-connected to hubs and then networked via a single busWith this design, you can cover longer distances and easily interconnect or isolate different network segments,Hybrid Physical Topologies (continued),Network +,10,More expensive than using either the star or, especially, the bus topology alone because it requires more cabling and potentially more connectivity devicesThe star-wired bus topology forms the basis for modern Ethernet and Fast Ethernet networks,Hybrid Physical Topologies (continued),Network +,11,Backbone Networks,A network backbone is the cabling that connects the hubs, switches, and routers on a networkBackbones usually are capable of more throughput than the cabling that connects workstations to hubs,Network +,12,Backbone Networks (continued),In networking, the term enterprise refers to an entire organization, including its local and remote offices, a mixture of computer systems, and a number of departmentsThe backbone is the most significant building block of enterprise-wide networks,Network +,13,Serial BackboneThe simplest kind of backboneIt consists of two or more internetworking devices connected to each other by a single cable in a daisy-chain fashionIn networking, a daisy chain is simply a linked series of devicesHubs and switches are often connected in a daisy chain to extend a network,Backbone Networks (continued),Network +,14,Distributed BackboneConsists of a number of connectivity devices connected to a series of central connectivity devices such as hubs, switches, or routers, in a hierarchyThis kind of topology allows for simple expansion and limited capital outlay for growth, because more layers of devices can be added to existing layers,Backbone Networks (continued),Network +,15,A more complicated distributed backbone connects multiple LANs or LAN segments using routersProvides network administrators with the ability to segregate workgroups and therefore manage them more easily,Backbone Networks (continued),Network +,16,Collapsed BackboneUses a router or switch as the single central connection point for multiple subnetworksA single router or switch is the highest layer of the backbone6The router or switch that makes up the collapsed backbone must contain multiprocessors to handle the heavy traffic going through itThis arrangement allows you to interconnect different types of subnetworks,Backbone Networks (continued),Network +,17,Parallel BackboneThe most robust type of network backboneThe most significant advantage of using a parallel backbone is that its redundant (duplicate) links ensure network connectivity to any area of the enterpriseParallel backbones are more expensive than other enterprise-wide topologiesThey make up for the additional cost by offering increased performance and better fault tolerance,Backbone Networks (continued),Network +,18,Logical Topologies,Logical topology refers to the way in which data is transmitted between nodesThe most common logical topologies are bus and ringIn a bus logical topology, signals travel from one network device to all other devices on the networkIn a ring logical topology signals follow a circular path between sender and receiverLogical topologies is useful when troubleshooting and designing networks,Network +,19,Switching,A component of a networks logical topology that determines how connections are created between nodesThere are three methods for switching: circuit switching, message switching, and packet switching,Network +,20,Circuit SwitchingA connection is established between two network nodes before they begin transmitting dataBandwidth is dedicated to this connection and remains available until the users terminate communication between the two nodesWhile the nodes remain connected, all data follows the same path initially selected by the switch,Switching (continued),Network +,21,Message SwitchingEstablishes a connection between two devices, transfers the information to the second device, and then breaks the connectionThe information is stored and forwarded from the second device once a connection between that device and a third device on the path is established,Switching (continued),Network +,22,This “store and forward” routine continues until the message reaches its destinationMessage switching requires that each device in the datas path have sufficient memory and processing power to accept and store the information before passing it to the next node,Switching (continued),Network +,23,Packet Switching is the most popular method for connecting nodes on a networkBreaks data into packets before they are transportedPackets can travel any path on the network to their destinationWhen packets reach their destination node, the node reassembles them based on their control informationDoes not waste bandwidth by holding a connection open until a message reaches its destination,Switching (continued),Network +,24,Ethernet,Carrier Sense Multiple Access with Collision Detection (CSMA/CD)The access method used in Ethernet The term “Carrier Sense” refers to the fact that Ethernet NICs listen on the network and wait until they detect (or sense) that no other nodes are transmitting data over the signal (or carrier) on the communications channel before they begin to transmit,Network +,25,Ethernet (continued),The term “Multiple Access” refers to the fact that several Ethernet nodes can be connected to a network and can monitor traffic, or access the media, simultaneouslyThe last part of the term CSMA/CD, “collision detection,” refers to the way nodes respond to a collisionWhen two transmissions interfere with each other; this is known as a collision,Network +,26,The NIC will issue a special 32-bit sequence that indicates to the rest of the network nodes that the its previous transmission was faulty and that those data frames are invalid which is called jammingA collision domain is the portion of a network in which collisions occur if two nodes transmit data at the same timeA data propagation delay is the length of time data takes to travel from one point on the segment to another point,Ethernet (continued),Network +,27,Switched EthernetTraditional Ethernet LANs, called shared Ethernet, supply a fixed amount of bandwidth that must be shared by all devices on a segment, and all nodes on that segment belong to the same collision domainSwitched Ethernet enables multiple nodes to simultaneously transmit and receive data over different logical network segmentsUsing switched Ethernet increases the effective bandwidth of a network segment because fewer workstations must vie for the same time on the wire,Ethernet (continued),Network +,28,Ethernet FramesEthernet networks may use one (or a combination) of four kinds of data frames: Ethernet_802.2 (“Raw”), Ethernet_802.3 (“Novell proprietary”), Ethernet_II (“DIX”), and Ethernet_SNAPEach frame type differs slightly in the way it codes and decodes packets of data traveling from one device to another,Ethernet (continued),Network +,29,Using and Configuring FramesYou can use multiple frame types on a network, but you cannot expect interoperability between the frame typesFrame types are typically specified through a devices NIC configuration softwareMost NICs can automatically sense what types of frames are running on a network and adjust themselves to that specification which is a feature is called autodetect, or autosense,Ethernet (continued),Network +,30,The preamble signals to the receiving node that data is incoming and indicates when the data flow is about to beginThe start-of-frame delimiter (SFD) identifies where the data field begins,Ethernet (continued),Network +,31,Each Ethernet frame also contains a 14-byte header, which includes a destination address, a source address, and an additional field that varies in function and size, depending on the frame typeThe extra bytes are known as padding and have no significance other than to fill out the frameEthernet_II (“DIX”) and Ethernet_SNAPAn Ethernet frame type developed by DEC, Intel, and Xerox (abbreviated as DIX) before the IEEE began to standardize Ethernet,Ethernet (continued),Network +,32,Ethernet_II frame type contains a 2-byte type field. This type field identifies the Network layer protocol (such as IP,ARP, RARP, or IPX) contained in the frameThe Ethernet_SNAP standard calls for additional control fieldsEthernet_SNAP frames allow less room for data,Ethernet (continued),Network +,33,Power over EthernetRecently, IEEE has finalized a new standard, 802.3af, that specifies a method for supplying electrical power over Ethernet connections, also known as Power over Ethernet (PoE)The PoE standard specifies two types of devices: power sourcing equipment (PSE) and powered devices (PDs)Power sourcing equipment (PSE)Powered devices (PDs),Ethernet (continued),Network +,34,LocalTalk,LocalTalk is a network access method designed by Apple Computer, Inc. specifically for networking Macintosh computersIt provided a simple, cost-effective way of interconnecting Macintosh devicesLocalTalk uses a transmission method called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)LocalTalk relies on the AppleTalk protocol, but it may also support the Macintosh version of TCP/IP called MacTCP,Network +,35,Token Ring,A network technology first developed by IBM in the 1980sToken Ring networks have traditionally been more expensive to implement than Ethernet networksThe 100-Mbps Token Ring standard, finalized in 1999, is known as High-Speed Token Ring (HSTR)In token passing, a 3-byte packet, called a token, is transmitted from one node to another in a circular fashion around the ringThe active monitor maintains the timing for ring passing,Network +,36,Token Ring SwitchingToken Ring networks can take advantage of switching to better utilize limited bandwidthA Token Ring switch can subdivide a large network ring into several smaller network ringsToken Ring technology does not allow collisions,Token Ring (continued),Network +,37,Token Ring FramesToken Ring networks may use one of two types of frames: the IEEE 802.5 or the IBM Token Ring frameEvery Token Ring frame includes Start Delimiter (SD), Access Control (AC), and End Delimiter (ED) fieldsToken Ring frames use a Frame Status (FS) to provide low-level acknowledgment that the frame was received whole,Token Ring (continued),Network +,38,A network technology whose standard was originally specified by ANSI in the mid-1980s and later refined by ISOFDDI (pronounced “fiddy”) uses a double ring of multimode or single mode fiber to transmit data at speeds of 100 MbpsFDDI is more reliable and more secure than transmission methods that depend on copper wiringFDDI works well with Ethernet 100BaseTX technologyFDDI technology has a high cost relative to Fast Ethernet,Fiber Distributed Data Interface (FDDI),Network +,39,Asynchronous Transfer Mode (ATM),An ITU networking standard describing Data Link layer protocols for both network access and signal multiplexingATM may run over fiber-optic or CAT 5 or higher UTP or STP cableIn ATM, a packet is called a cell and always consists of 48 bytes of data plus a 5-byte headerATM technology is that it relies on virtual circuitsATM a connection-oriented technology using virtual circuits,Network +,40,ATM (continued),Establishing a reliable connection allows ATM to guarantee a specific Quality of Service (QoS) for certain transmissionsQoS is a standard that specifies that data will be delivered within a certain period of time after it is sentATM networks can be integrated with Ethernet or Token Ring networks through the use of LAN Emulation (LANE),Network +,41,Wireless Networks,Each wireless technology is defined by a standard that describes unique functions at both the Physical and the Data Link layers of the OSI ModelThese standards differ in their specified signaling methods, geographic ranges, and frequency usages, among other things.The most popular wireless standards used on contemporary LANs are those developed by IEEEs 802.11 committee,Network +,42,802.11 Another name for Wireless Local Area Networks (WLAN) standards committeeAccess Method802.11 standards specify the use of Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to access a shared mediumUse of ACK packets to verify every transmissionRTS/CTS enables a source node to issue an RTS signal to an access point requesting the exclusive opportunity to transmit,Wireless Networks (continued),Network +,43,AssociationIn the context of wireless networking, communication that occurs between a station and an access point to enable the station to connect to the network via that access pointAs long as a station is on and has its wireless protocols running, it periodically surveys its surroundings for evidence of an access point, a task known as scanning,Wireless Networks (continued),Network +,44,There are two types of scanning: active and passiveIn active scanning, the station transmits a special frame, known as a probe, on all available channels within its frequency rangeIn passive scanning, a wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access pointService Set Identifier (SSID), a unique character string used to identify an access pointA station might choose a different access point through a process called re-association,Wireless Networks (continued),Network +,45,FramesFor each function, the 802.11 standard specifies a frame type at the MAC sublayerThese multiple frame types are divided into three groups: management, control and dataManagement frames are those involved in association and re-association, such as the probe and beacon framesControl frames are those related to medium access and data delivery, such as the ACK and RTS/CTS framesData frames are those that carry the data sent between stations,Wireless Networks (continued),Network +,46,802.11bAlso known as “Wi-Fi,” for Wireless FidelityUses direct sequence spread spectrum (DSSS) signaling802.11b was the first to take hold and remains the most popularIt is also the least expensive of all the 802.11 WLAN technologies,Wireless Networks (continued),Network +,47,802.11a802.11as high throughput is attributable to its use of higher frequencies, its unique method of encoding data, and more available bandwidthHigher frequency signals require more power