Presentation on theme: "Chapter 7 Local Area Networks : The Basics"— Presentation transcript:
1Chapter 7 Local Area Networks : The Basics Data Communications andComputer Networks: ABusiness User’s ApproachChapter 7Local Area Networks : The Basics
2Last time Types of errors and their prevention Error detection Parity – 50%CRC – can detect nearly all errorsError correction1. Do nothing2. Return an error message to the transmitter3. Fix the error with no further help from the transmitter
3Three Major Types of Networks Local Area Network (LAN)Serves users within a confined geographical area (usually within a mile).Metropolitan Area Network (MAN)Covers a geographic area the size of a city or suburb. The purpose of a MAN is often to bypass local telephone companies when accessing long-distance service.Wide Area Network (WAN)Covers a wide geographical area, such as a state or a country. Examples: Tyment, Telenet, Uninet, and Accunet.
4Emphasis Definition of a Local Area Network (LAN) Physical and Logical LAN TopologiesDifferent Medium Access Control ProtocolsCommon LAN SystemsEthernet (1st commercially available LAN)Token ringFDDI (fiber data distributed interface) ring
5IntroductionA local area network is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data athigh data transfer ratesvery low error rates.(WANs now do this too)Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.
6Functions of a Local Area Network Data Communications and Computer NetworksChapter 7Functions of a Local Area NetworkFile server - A large storage disk drive that acts as a central storage repository.Print server - Provides the authorization to access a particular printer, accept and queue print jobs, and provides a user access to the print queue to perform administrative duties.Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks.
7Functions of a Local Area Network Data Communications and Computer NetworksChapter 7Functions of a Local Area NetworkVideo transfers - High speed LANs are capable of supporting video image and live video transfers.Manufacturing support - LANs can support manufacturing and industrial environments.Distributed processing - LANs can support network operating systems which perform the operations of distributed processing.
8Advantages of Local Area Networks Data Communications and Computer NetworksChapter 7Advantages of Local Area NetworksAbility to share hardware and software resources.Individual workstation might survive network failure.Component and system evolution are possible.Support for heterogeneous forms of hardware and software.Access to other LANs and WANs (Figure 7-1).Private ownership.Secure transfers at high speeds with low error rates.
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10Disadvantages of Local Area Networks Data Communications and Computer NetworksChapter 7Disadvantages of Local Area NetworksEquipment and support can be costly.Level of maintenance continues to grow.Private ownership?Some types of hardware may not interoperate.Just because a LAN can support two different kinds of packages does not mean their data can interchange easily.A LAN is only as strong as it weakest link, and there are many links.
11Basic Network Topologies Data Communications and Computer NetworksChapter 7Basic Network TopologiesTopology: the geometric configurationSeparate from protocols!Local area networks are interconnected using one of four basic configurations:1. Bus/tree2. Star-wired bus3. Star-wired ring4. Wireless
12Basic Network Topologies Logical vs physical design (topologies)1. Logical design: how the data moves around the network from workstation to workstation2. Physical design: how the network physically appears if drawn on a sheet of paper (laid out)Names of topologies usually refer to the physical design.
13Bus/Tree Topology Data Communications and Computer Networks Chapter 7Bus/Tree TopologyThe original topology – 1970’sWorkstation has a network interface card (NIC) provides a physical connection to a networkData can be transferred using eitherbaseband digital signalsbroadband analog signals.
14Bus/Tree Topology Data Communications and Computer Networks Chapter 7Bus/Tree TopologyWorkstation has a network interface card (NIC) provides a physical connection to a networkAttaches to the bus (a coaxial cable) via a tap.NIC is an electronic device that performs the necessary signal conversions and protocols operations so that the workstation can send and receive data on the network.Tap is a passive deviceDoes not alter the signalDoes not require electricity to operate
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16LAN Topologies: BusConsists of a single wire to which individual stations are attachedEach end has a terminator attached to itData travels the entire length of the cableTransmission from any stations travels entire medium (both directions)Inexpensive and easy to installEthernet is the common form of a bus topology system
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18Bus/Tree Topology Baseband signals Digital signals – 10 Mbps Bidirectional and more outward in both directions from the workstation transmitting.Easy to install and maintainFewer than 100 workstationsBuses can be split and joined, creating trees.
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20Bus/Tree Topology Broadband signals Usually uni-directional and transmit in only one direction.Analogy and FDM for multiple channels (amplification necessary). Because of this, special wiring considerations are necessary.100 to 1000 workstations over larger distances due to easy amplificationBuses can be split and joined, creating trees.
21Data Communications and Computer Networks Chapter 7Twice the propagationdelay of baseband
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23Advantages/disadvantages of a bus Data Communications and Computer NetworksChapter 7Advantages/disadvantages of a busDifficult to add new devices if no tap exists.No tap existing means cutting into the lineAs such, this topology is loosing popularityPlenty still around; Ethernet uses this.
24Star-wired Topologies Data Communications and Computer NetworksChapter 7Star-wired TopologiesStars versus a single lineTwo types:Star-wired bus(often call the star topology)Star-wired ring
25Star-wired Bus Topology Logically operates as a bus, but physically looks like a star.Star design is based on hub. All workstations attach to hub.Hub is an unintelligent device that immediately transmits whatever data it receives to all connectionsUnshielded twisted pair usually used to connect workstation to hub.Hub takes incoming signal and immediately broadcasts it out all connected links.Hubs can be interconnected to extend size of network.Very popular!
27Interconnection of two hubs in a star-wired bus LAN
28Star-wired Bus Topology Data Communications and Computer NetworksChapter 7Star-wired Bus TopologyModular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus.Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable.Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium.
29Star-wired Ring Topology Data Communications and Computer NetworksChapter 7Star-wired Ring TopologyLogically operates as a ring but physically appears as a star.Star-wired ring topology is based on MAU (multi-station access unit) which functions similarly to a hub.Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion in one direction.Like hubs, MAUs can be interconnected to increase network size.
31NIC Repeaters for Star-wired Ring Data Communications and Computer NetworksChapter 7NIC Repeaters for Star-wired RingAll data must pass through a NIC repeaterPerforms basic functions:Bypass: data does not copy to the workstation, used for inactive devicesCopy: data from ring copied to workstationWrite: data from the workstation copied to the ringRegenerates: recopies data back onto ring
32Three possible operations of the workstation repeater on a ring topology
33Physical organization of a ring topology Looks like a star!
34Multi-station access unit on a ring topology The MAU replaces the hub for this topology
35Wireless Topology Data Communications and Computer Networks Chapter 7Wireless TopologyNot really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point.Range varies from 50 to 800 ft with speeds of 2 to 11 MbpsMost wireless LANs include a wired LAN backboneUsually requires a wireless NIC
37Wireless Topology – different forms Data Communications and Computer NetworksChapter 7Wireless Topology – different formsNewer IEEE and b (Wi-Fi) standard defines various forms of wireless LAN connections.Speeds up to 11 Mbps with b standard.Workstations reside within a basic service set, while multiple basic service sets create an extended service set.
38Wireless Topology Data Communications and Computer Networks Chapter 7Wireless TopologyAcceptable transmission ranges broken up into areas:Basic service set – that surrounding an access pointExtended service set – collection of basic service setsWorkstations reside within a basic service set, while multiple basic service sets create an extended service set.
39Wireless Topology Data Communications and Computer Networks Chapter 7Wireless TopologyTwo basic components necessary: the client radio, usually a PC card with an integrated antenna, and the access point (AP), which is an Ethernet port plus a transceiver.The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions.Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set.
43Wireless Topology Data Communications and Computer Networks Chapter 7Wireless TopologyWith directional antennae designed for point-to-point transmission (rare), b can work for more than 10 miles.With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet.Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.
44Wireless Topology Data Communications and Computer Networks Chapter 7Wireless TopologyIn actual tests, 11 Mbps b devices managed 5.5 MbpsTo provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. (how good?)What will Bluetooth’s impact be on b (Wi-Fi)?
45Other Wireless Standards IEEE (older 2 Mbps)IEEE b (11 Mbps, 2.4 GHz)Also called Wi-Fi (wireless fidelity)IEEE a (54 Mbps, 5 GHz, in 2002)IEEE g (54 Mbps, 2.4 GHz, in 2002)HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)
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47Summary of topologies Logical vs physical topologies Bus and star-rings - old technologiesStill some aroundYou’ll probably useStar-wired bus (star)With bus or routers (now much easier to use)Wireless network (Wi-fi)
48ProtocolsLast time topologiesNow how the data actually flows
49Medium Access Control (MAC) Protocols Data Communications and Computer NetworksChapter 7Medium Access Control (MAC) ProtocolsHow does a workstation get its data onto the LAN medium?A medium access control protocol is the software that allows workstations to “take turns” at transmitting data.Since a LAN is a broadcast network, it’s imperative that only one workstation at a time be allowed to transmit data onto the network. (for multiple channels, one channel per workstation)Switches have changed that, but we’ll take about that in Ch8.
50Medium Access Control Protocols Data Communications and Computer NetworksChapter 7Medium Access Control ProtocolsThree basic categories:1. Contention-based protocols (1st come; 1st serve)2. Round robin protocols (take your turn!!)3. Reservation protocols (requests please!)
51Contention-Based Protocols Data Communications and Computer NetworksChapter 7Contention-Based ProtocolsEssentially first come first served.Most common example is Carrier Sense Multiple Access with Collision Detection (CSMA/CD).If no one is transmitting, a workstation can transmit.If someone else is transmitting, the workstation “backs off” and waits.Half duplex protocol.
52Contention-Based Protocols If two workstations transmit at the same time, a collision occurs.When the two workstations hear the collision, they stop transmitting immediately.Each workstation backs off a random amount of time and tries again. Workstations use a persistence algorithm to decide when to resubmit.Hopefully, both workstations do not try again at the exact same time.CSMA/CD is an example of a non-deterministic protocol (cannot calculate time at which a workstation will transmit)
53Collisions Data Communications and Computer Networks Chapter 7CollisionsCollision window is the interval during which a workstation’s signal can propagate down the bus and back. During this period a workstation might falsely hear no one transmitting and retransmit a signal.Changes for collisions go up when every workstation wants to send something – heavy traffic!
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55CollisionsBusy CSMA/CD networks can waste over half their time dealing with collisions.
56Round Robin Protocols Data Communications and Computer Networks Chapter 7Round Robin ProtocolsEach workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation.Most common example is token ring LAN in which a software token is passed from workstation to workstation.Token ring is an example of a deterministic protocol.Token ring more complex than CSMA/CD. What happens if token is lost? Duplicated? Hogged?Token ring LANs are losing the battle with CSMA/CD LANs.
57Token Passing Protocols Before workstation can transmit it must possess the one and only tokenTwo types:Token ringToken bus
58Data Transmission on a token ring LAN Station A has just released the tokenStation M copies the data meant for it from BB removes the data it sent and releases the token
59Why CSMA/CD more popular than Token Ring Data Communications and Computer NetworksChapter 7Why CSMA/CD more popular than Token Ring1st LAN MAC method, more installations and equipment.Token rings have almost always lagged behind in transmission speed.CSMA/CD less expensive to implement because of widespread marketing and acceptance.CSMA/CD is a simpler protocol.Wireless CSMA/CD now exists
60Token Bus Data Communications and Computer Networks Chapter 7Token BusJust like the token ring, but a bus insteadEach workstation maintains a list of neighbors that one passes the token to (logical not physical) neighborDesigned primarily for manufacturing plants since a non-deterministic protocol is not acceptable there. Why?
61Reservation Protocols Data Communications and Computer NetworksChapter 7Reservation ProtocolsWorkstation places a reservation with central server.Workstation cannot transmit until reservation comes up.Under light loads, this acts similar to CSMA/CD.Under heavy loads, this acts similar to token ring.Powerful access method but again losing out to CSMA/CD.Most common example of reservation protocol is demand priority protocol.
62Problems with the OSI model for LANs Problems with broadcast networks!Routing (no decisions have to be made)Data link and physical link tightly coupledNeed to specify hardware! Ugh!Broadcast networks need to be have a medium access control to decide who talks
63Medium Access Control Sublayer Data Communications and Computer NetworksChapter 7Medium Access Control SublayerTo better support local area networks, the data link layer of the OSI model was broken into two sublayers:1. Logical link control sublayer2. Medium access control (MAC) sublayerMedium access control sublayer defines the frame layout and is more closely tied to a specific medium at the physical layer.Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT.
64Modification of OSI model to split data link layer into two sublayers
65IEEE 802 Frame Formats Data Communications and Computer Networks Chapter 7IEEE 802 Frame FormatsThe IEEE 802 suite of protocols defines the frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5).Each frame format describes how the data package is formed.Note how the two frames are different. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another.
68Local Area Network Systems Data Communications and Computer NetworksChapter 7Local Area Network SystemsThe entire system with protocol and topology
69Local Area Network Systems Data Communications and Computer NetworksChapter 7Local Area Network SystemsEthernet or CSMA/CDMost common form of LAN today.Star-wired bus is most common topology but bus topology also around.Ethernet comes in many forms depending upon medium used and transmission speed and technology.
70Ethernet Data Communications and Computer Networks Chapter 7EthernetOriginally, CSMA/CD was 10 Mbps.Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps.Then 1000 Mbps (1 Gbps) was introduced.10 Gbps is now available.
71Ethernet Data Communications and Computer Networks Chapter 7Ethernet1000 Mbps introduces a few interesting wrinkles:Transmission is full duplex (separate transmit and receive), thus no collisions.Prioritization is possible using 802.1p protocol.Topology can be star or mesh (for trunks).
72Ethernet Data Communications and Computer Networks Chapter 7EthernetCabling can be either UTP (unshielded twisted pair) or optical (but 10 Gbps Ethernet may not work over UTP due to radio frequency interference).Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queueing.Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps limited only by media.
74Ethernet Standards Key XXXBase or XXX BroadXXX 10 means 10Mbps
75Local Area Network Systems Data Communications and Computer NetworksChapter 7Local Area Network SystemsIBM Token RingDeterministic LAN offered at speeds of 4, 16 and 100 Mbps using star-ring topology.Very good throughput under heavy loads.More expensive components than CSMA/CD.Losing ground quickly to CSMA/CD. May be extinct soon.
76Local Area Network Systems Data Communications and Computer NetworksChapter 7Local Area Network SystemsFDDI (Fiber Distributed Data Interface)Based on the token ring design using 100 Mbps fiber connections.Allows for two concentric rings - inner ring can support data travel in opposite direction or work as backup.Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.
78Local Area Network Systems Data Communications and Computer NetworksChapter 7Local Area Network Systems100VG-AnyLANDeterministic LAN based on demand priority access method.Similar to hub topology (star design).Two levels of priority - normal and high.Supports a wide-variety of media types.Losing ground quickly to CSMA/CD. Will be extinct soon?
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80LANs In Action : A Small Office Solution Data Communications and Computer NetworksChapter 7LANs In Action : A Small Office SolutionWhat type of system will interconnect twenty workstations in one room and 15 workstations in another room to a central server, which offers:InternalA database that contains all customer informationHigh quality printer access
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83LANs In Action : A Home Office Local Area Network Solution Data Communications and Computer NetworksChapter 7LANs In Action : A Home Office Local Area Network SolutionWhat if you have two computers at home and want both to share a printer and a connection to the Internet.Some type of “network-in-a-box” solution might solve this problem.Essentially a LAN with a 2- or 3-port hub, connecting cables, and software.In some models the hub also acts as a router to the Internet.
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85What we covered Topologies Protocols Systems Star-busWirelessProtocolsCSMA/CDSystemsEthernetNext time: wireless networks?