2 Primary Function of a LAN File serving – large storage disk drive acts as a central storage repositoryPrint serving – Providing authorization to access a particular printer, accept and queue print jobs, and user access to print queue to perform administrative dutiesVideo transfers – High speed LANs are capable of supporting video image and live video transfersManufacturing support – LANs can support manufacturing and industrial environmentsAcademic support – In classrooms, labs, and wirelesssupportInterconnection between multiple systems
3 Advantages of LAN Ability to share hardware and software resources Individual workstation might survive network failureComponent and system evolution are possibleSupport for heterogeneous forms of hardware and softwareAccess to other LANs and WANsPrivate ownershipSecure transfers at high speeds with low error rates
4 Disadvantages of LAN Equipment and support can be costly Level of maintenance continues to growPrivate ownership?Some types of hardware may not interoperateJust because a LAN can support two different kinds of packages does not mean their data can interchange easilyA LAN is only as strong as it weakest link, and there are many links
5 Basic LAN TopologiesBus/treeStar-wired busStar-wired ringWireless
6 Bus/Tree Topology The original topology. Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap.Data can be transferred using either baseband digital signals or broadband analog signals.Baseband signals are bidirectional (broadcast) and move outward in both directions from the workstation transmitting.Broadband signals are usually uni-directional and transmit in only one direction. Because of this, special wiring considerations are necessary.Buses can be split and joined, creating trees.
8 Star-wired Bus Topology Logically operates as a bus, but physically looks like a starStar design is based on hub. All workstations attach to hubUnshielded twisted pair usually used to connect workstation to hubHub takes incoming signal and immediately broadcasts it out all connected linksHubs can be interconnected to extend network sizeModular connectors and twisted pair make installation and maintenance of star-wired bus better than standard busHubs can be interconnected with twisted pair, coaxial cable, or fiber optic cableBiggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium
10 Star-wired Ring Topology Logically operates as a ring but physically appears as a starBased on MAU (multi-station access unit) which functions similarly to a hubWhere a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashionLike hubs, MAUs can be interconnected to increase network size
12 Wireless LANs (I) Not really a specific topology Workstation in wireless LAN can be anywhere as long as within transmitting distance to access pointSeveral versions of IEEE standard defines various forms of wireless LAN connectionsTwo basic components necessary:Client Radio - usually PC card with integrated antenna installed in a laptop or workstationAccess Point (AP) - Ethernet port plus transceiverAP acts as bridge between wired and wireless networksCan perform basic routing functionsSingle-cell - Workstations reside within a basic service setMultiple-cell - Multiple basic service sets create an extended service setAd-hoc - Wireless LANs configured without access point
14 Wireless LANs (II)IEEE – The original wireless standard, transmitting data at 2 MbpsIEEE b – The second wireless standard, transmitting data at 11 MbpsIEEE a – One of the more recent standards, transmitting data at 54 Mbps using 5 GHz frequency rangeIEEE g – The other recent standard, also transmitting data at 54 Mbps but using the same frequencies as b (2.4 GHz)Backwards compatible with bIEEE n (100 Mbps) is last standard that has been widely implemented.Available at both 2.4 & 5 GHzLatest wireless Ethernet is using MIMO technology (multiple input multiple output)Sender and receiver have multiple antennas for optimum receptionIEEE ac is the latest standard that is gaining momentumOperates only on 5 GHz band with data rate up to 6.9 Gbps
16 Medium Access Control Protocols How does a workstation get its data onto the LAN medium?Medium access control protocol - software that allows workstations to “take turns” at transmitting dataTwo basic categories:Contention-based protocolsRound robin protocols
17 Contention-Based Protocols (I) Essentially first come first servedMost common example:Carrier sense multiple access with collision detection (CSMA/CD)If no one is transmitting, a workstation can transmitIf someone else is transmitting, workstation “backs off” and waitsIf two workstations transmit at same time, collision occursWhen two workstations hear collision, they stop transmitting immediatelyEach workstation backs off a random amount of time and tries againHopefully, both workstations do not try again at exact same timeCSMA/CD is an example of a nondeterministic protocol
19 Contention-Based Protocols (II) Wireless CSMA/CA (Collision avoidance)Protocol does not listen and detect collisionsInstead, tries to avoid collisions before they happenHow does CSMA/CA do this?All devices, before they transmit, must wait an amount of time called an interframe space (IFS)Some applications have a short IFS, while others have a long IFSIf two applications want to transmit at same time, the application with shorter IFS will go first. If medium is idle after IFS, a random backoff counter is selected and transmission starts after the countdown.
20 Round Robin ProtocolsEach workstation takes turn transmitting: turn is passed around the network from workstation to workstationMost common example is token ring LAN in which a software token is passed from workstation to workstationToken ring is an example of a deterministic protocolToken ring more complex than CSMA/CDWhat happens if token is lost? Duplicated? Hogged?Token ring LANs are losing the battle with CSMA/CD LANs
22 IEEE 802To better support local area networks, data link layer of the OSI model was broken into two sublayers:Logical link control sublayerMedium access control sublayerMedium access control sublayer defines the frame layoutMore closely tied to specific medium at physical layerThus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT
23 IEEE 802 Frame FormatsIEEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3), CSMA/CA (IEEE ), and token ring (IEEE 802.5)Each frame format describes how data package is formedIf a CSMA/CD network connects to a token ring network, frames have to be converted from one to another
25 LAN Systems Ethernet or CSMA/CD IBM Token Ring FDDI (Fiber Distributed Data Interface)
26 Ethernet Originally, 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.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).Cabling can be either UTP or optical.Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queuing.Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps is limited only by media.10 Gbps is now beginning to appear.
28 Power & EthernetWhat if you have a remote device that has an Ethernet connection?It will require a power connectionWhat if you don’t have an electrical outlet nearby?Use PoEPower to drive Ethernet NIC is sent over wiring along with usual Ethernet signalsEthernet over power lineUses existing power lines in the buildingNo new wiring neededSlower
29 IBM Token RingDeterministic LAN offered at speeds of 4, 16 and 100 Mbps.Very good throughput under heavy loads.More expensive components than CSMA/CD.Losing ground quickly to CSMA/CD. May be extinct soon.
30 FDDI 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.
31 InterconnectionNecessary to connect a local area network to another local area network or to a wide area network.LAN-to-LAN connections are often performed with a bridge-like device.LAN-to-WAN connections are usually performed with a router.A switch can be used to interconnect segments of a local area network.
32 Why Segment or Interconnect? To separate / connect one corporate division with anotherTo connect two LANs with different protocolsTo connect a LAN to the InternetTo break a LAN into segments to relieve traffic congestionTo provide a security wall between two different types of users
33 Hubs Interconnects two or more workstations into a local area network. When a workstation transmits to a hub, the hub immediately resends the data frame out all connecting links.A hub can be managed or unmanaged.A managed hub possesses enough processing power that it can be managed from a remote location.
34 Hub issues Maximum distance between devices (100m in 10Base-T) Must avoid loops between connected hubsmessage would circulate endlesslyNumber of devices on network increases collision riskscollisions during peak traffic periods can crash the network (200 devices)
35 Bridges Connect two similar LANs, such as two CSMA/CD LANs. Connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN.Examines the destination address in a frame and either forwards this frame onto the next LAN or does not.Examines the source address in a frame and places this address in a routing table, to be used for future routing decisions.
38 Transparent BridgeDoes not need programming but observes all traffic and builds routing tables from observation.The observation is called backward learning.Each bridge has two connections (ports) and there is a routing table associated with each port.Observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table.Found with CSMA/CD LANs.Can also convert one frame format to another.Sometimes refereed to as a gateway or sometimes a router.Removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format.
40 Remote BridgePassing a data frame from one LAN to another when the two LANs are separated by a long distance and there is a wide area network connecting the two LANs.Takes the frame before it leaves the first LAN and encapsulates the WAN headers and trailers.When the packet arrives at the destination remote bridge, that bridge removes the WAN headers and trailers leaving the original frame.
41 Switches (I) Combination of a hub and a bridge. Can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns.When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection.Workstations that connect to a hub are on a shared segment.Workstations that connect to a switch are on a switched segment.
42 Switches (II)The backplane of a switch is fast enough to support multiple data transfers at one time.A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch.Multiple workstations connected to a switch use dedicated segments.This is a very efficient way to isolate heavy users from the network.A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server.Using a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network
43 Virtual LANsLogical subgroup within a LAN that is created via switches and software rather than by manually moving wiring from one network device to anotherEven though employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network”A relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LANInstead of sending technician to a wiring closet to move a workstation cable from one switch to another, an 802.1Q-compliant switch can be remotely configured by a network administrator
44 Full Duplex SwitchesAllows for simultaneous transmission and reception of data to and from a workstationThis full duplex connection helps eliminate collisionsTo support a full duplex connection to a switch, at least two pairs of wires are necessaryOne for the receive operationOne for the transmit operationMost people install four pairs today, so wiring is not problem
45 Link AggregationCombining multiple physical connection into one logical connectionIncrease connection speedFault toleranceIEEE 802.3ad-2000
46 Spanning Tree Algorithm In large network, a loop can be created where a frame can circle through the network and back to the originating deviceThe spanning tree algorithm (used in Spanning Tree Protocol and now Rapid Spanning Tree Protocol) runs in switches and can identify loops and remove themIdentify a switch as the root switchVisit each switch and identify the one port (RP) that has the shortest path back to the root switch.Visit each LAN and identify the port (DP) that provides the shortest path back to the root switch.Mark the remaining unidentified ports as Removed in the forwarding tables.
48 Quality of Service (QoS) Set priority for each frameThe 802.1p adds a 3-bit field (PCP) to each Ethernet framePCP Value Traffic Type0 Best effort1 Background (lowest priority)2 Excellent effort3 Critical applications4 Video5 Voice6 Internetwork control7 Network control (highest priority)
49 Routers Router - device that connects a LAN to a WAN or a WAN to a WAN Accepts outgoing packetRemoves any LAN headers and trailersEncapsulates necessary WAN headers and trailersBecause router has to make wide area network routing decisions Ú router has to dig down into the network layer of the packet to retrieve network destination addressRouters are often called “layer 3 devices”Operate at the third layer, or OSI network layer, of the packetOften incorporate firewall functions